MANUFACTURING TECHNOLOGY - I

Transcription

1 MANUFACTURING TECHNOLOGY - I (Production Technology) For III Semester BE, Mechanical Engineering Students As per Latest Syllabus of Anna University - TN With Short Questions & Answers and University Solved Papers New Regulations 2017 Dr S Ramachandran, ME, PhD, Professor - Mech Sathyabama Institute of Science and Technology Chennai Dr S Ramesh, BE, MTech, PhD, FIE, LM-ASME, LM-PMA Professor / HOD Mechanical Engineering KCG College of Technology, Chennai - 97 Dr G Nallakumarasamy, BE, MTech (IITM), Ph,D, MISTE, MSAE, Professor & Head Mechanical Engineering Excel Engineering College, Namakkal (Near All India Radio) 80, Karneeshwarar Koil Street, Mylapore, Chennai Ph: , aishram2000@gmailcom, airwalk800@gmailcom wwwairwalkbookscom, wwwsrbooksorg

2 Fourth Edition: June / wwwairwalkbookscom wwwsrbooksorg

3 ME8351 MANUFACUTRING TECHNOLOGY - I L T P C UNIT I METAL CASTING PROCESSES Sand Casting: Sand Mould Type of patterns Pattern Materials Pattern allowances Moulding sand Properties and testing Cores Types and applications Moulding machines Types and applications; Melting furnaces; Blast and Cupola Furnaces; Principle of special casting process; Shell investment Ceramic mould Pressure die casting Centrifugal Casting CO 2 process Stir casting; Defects in Sand casting UNIT II JOINING PROCESSES Operating principle, basic equipment, merits and applications of: Fusion welding processes: Gas welding Types Flame characteristics; Manual metal arc welding Gas Tungsten arc welding Gas metal arc welding Submerged arc welding Electro slag welding; Operating principle and applications of: Resistance welding Plasma arc welding Thermit welding Electron beam welding Friction welding and Friction Stir Welding; Brazing and soldering; Weld defects: types, causes and cure UNIT III METAL FORMING PROCESSES Hot working and cold working of metals Forging processes Open, Impression and closed die forging forging operations Rolling of metals Types of Rolling Flat strip rolling shape rolling operations Defects in rolled parts Principle of rod and wire drawing Tube drawing Principles of Extrusion Types Hot and Cold extrusion UNIT IV SHEET METAL PROCESSES Sheet metal characteristics shearing, bending and drawing operations Stretch forming operations Formability of sheet metal Test methods special forming processes Working principle and applications Hydro forming Rubber pad forming Metal spinning Introduction of Explosive forming, magnetic pulse forming, peen forming, Super plastic forming Micro forming UNIT V MANUFACTURE OF PLASTIC COMPONENTS Types and characteristics of plastics Moulding of thermoplastics working principles and typical applications injection moulding Plunger and screw machines Compression moulding, Transfer Moulding Typical industrial applications introduction to blow moulding Rotational moulding Film blowing Extrusion Thermoforming Bonding of Thermoplastics

4 Index I1 INDEX A Clay Content Test 131 Additives and Fillers in Plastics55 Closed Die Forging (or) Impression Die Additives 114 Forging (or) Precision Coal dust114 Adhesiveness 120 Cohesiveness 119 Air-Acetylene Welding 236 Cold chamber die casting 1125 Air Furnace or Reverberatory Furnace181Cold forming 570 Anisotropy 44 Cold Working Of Metals 33 Arc Welding (or) Manual Metal arc Cold Spinning 365 Welding (or) Shielded arc Arc Cold Working Processes welding 25 Collapsibility 120 Arc Welding Equipment 238 Combustion zone 177 B Compression Moulding 514, 529 Backing sand 117 Continuous Casting 1138 Bench Moulding120 Convertor 187 Bending operations 368 Cope and drag pattern 141 Bending 34 Core sand 117, 155 Binder 113 Core Making 156 Blanking 46 Cores 154 Blast Furnace 172 Corn flour and Dextrin 114 Blow Moulding 514, 536 Cracking 2105 Blowholes 2109 Cross-linked polymers 53 Bonding of Thermoplastics 560 Crucible Furnaces 189 Bottom gates 1103 Cupola Furnace 174 Branched polymers 52 Cupping Test 426 Brazing 295 Cutting off 48 C Carbon arc Welding 245 Cast Iron 147 Casting 515 Centrifugal Casting 1127 Centrifuge Casting 1131 Ceramic Mould Casting 1121 Cereal binder 156 Chemical dip brazing296 D Deep drawing (or) Cupping 356 Defects in Sand Casting 1146 Dextrin 156 Die materials 331 Die design features 330 Dielectric welding 565 Diffusion Welding 278 Dimensional inspection 1156

5 I2 Manufacturing Technology I - wwwairwalkbookscom Dip Soldering 2100 Flame Cutting 290 Dip Brazing 296 Flames Characteristics 233 Direct Arc furnace 192 Flasks 127 Distortion (or) Camber allowance153 Flat Strip Rolling 339 Distortion 2106 Floor Moulding 121 Draft (or) Taper allowance 152 Flowability Test 133 Draft Angles 329 Flowability (or) plasticity 119 Draw spike 125 Flux Material 298 Drawing 34 Flux Cored Arc Welding (FCAW)254 Drives for Extrusion 363 Follow board pattern 143 Drop Forging 319 Forge Welding 270 Dry strength 119 Forging 319 Dry sand 116 Forging Tests 327 Dry sand moulding 122 Forging Processes 36 E Elastomers 511 Forging under sticking condition319 Forging Defects 325 Electric Furnaces 192 Forming Limit Diagram (FLD)427 Electric arc Welding Processes237 Four High rolling mill 336 Electro Hydraulic Forming 431 Friction Stir Welding (FSW)274 Electro Slag Welding250 Friction Welding 272 Electrode Standards 223 Furnace (or) Forge brazing 296 Electrodes 212 Fusion bonding 567 Electron Beam Welding 285 G Elongation 43 Gaggers 127 Excessive Penetration2109 Gas Welding Processes 224 Explosive Forming 438 Gas Welding 25 Explosive Welding 276 Gas welding equipments 228 Extrusion defects 363 Gas Tungsten Arc Welding (GTAW) Extrusion 551 (or) Tungsten Inert Gas Welding F (TIG) 248 Facing sand 116 Gas Metal Arc Welding (GMAW) or Filler materials 297 Metal Inert Gas Welding (MIG)249 Filler Materials and Fluxes in Brazing297Gate cutter 126 Fillers 56 Gate 1103 Film Blowing and Sheet Blowing546 Gated pattern 143

6 Index I3 Gating System 1100 Grain Fineness Test (GFT) 131 Grain size 44 Green strength 119 Green sand 115 H Hand riddle 123 Hand forging 313 Hard Soldering 2100 Heat Affected Zone (HAZ) 211 High rolling mill 335 Horizontal core 164 Hot Box Core 164 Hot twist test 328 Hot gas welding566 Hot Rolling: Hot Working Of Metals32 Hot chamber die-casting 1123 Hot Extrusion 359 Hot-platen welding 565 Hot Spinning 365 Hydro Mechanical Forming 431 Hydro Forming 429 Hydrostatic Extrusion361 I Impact extrusion (or) Cold Extrusion360 Induction brazing 296 Initiators 56 Injecting Moulding 517 Injection Blow Moulding 537 Inspection Method 1156 Investment casting (Lost wax casting) 1118 J Jolt-squeezer machine 169 Jolt machine 168 L Lancing 49 Laser Beam Welding 283 Laser brazing and electron beam brazing 297 Lifters 125 Linear polymers 52 Liquid (Dye) Penetrant Test 1163 Loam Moulding 121 Loam sand 116 Loose piece pattern 140 M Machine Moulding 121 Machining allowance 152 Magnetic Particle Inspection 1160 Magnetic Pulse Forming 440 Mallet 124 Match plate pattern 141 Mechanical Fastening562 Mechanical Testing 1157 Melting Furnaces 171 Melting zone 177 Metal Pattern 147 Metal Spinning 435 Micro-Forming 446 Modern Welding Processes 283 Modifiers 56 Moisture Content Test 130 Moisture / Water 113 Molasses 156 Molten metal bath process 297 Mould Hardness Test135 Moulding Sand Testing 129 Moulding Boxes 127 Moulding Machines 167

7 I4 Manufacturing Technology I - wwwairwalkbookscom Multiple Roll Mill/Cluster roll mill337 N Network polymers 53 Nibbling 49 Notching 48 O One piece (or) solid pattern 139 Open Hearth Furnace185 Open Die Forging (or) Smith Die Forging (or) Flat-die Forging Operations 310 Overlays 2109 Oxy - Hydrogen Welding 236 Oxy-Fuel Gas Welding (OFW)224 Oxy Acetylene Welding 225 P Parting sand 117 Parting 48 Pattern Materials 146 Pattern 138 Pattern Allowances 151 Pattern draw machines 171 Peen Forming 442 Percussion Welding 267 Perforating 410 Permeability Test 136 Permeability 118 Piercing or Seamless Tubing348, 364 Piercing 47 Pit Furnace 189 Pit Moulding 121 Planetary rolling mill339 Plasma arc Welding 287 Plaster-mould casting 1135 Plasticizers 55 Plunger moulding 534 Polymerisation Process 55 Polymers 51 Poor Weld Bead Appearance (Fig 251) 2108 Porosity (Fig 245) 2104 Pot Transfer Moulding 533 Potting & Encapsulation 569 Power hammers 314 Power shearing 47 Power forging 314 Preheating zone 178 Preshaping 331 Press forging 320 Pressure Die Casting 1123 Principle Of Rod And Wire Drawing352 Principles Of Extrusion 358 Protein binder 156 Punching47 R Rammers 124 Rapping (or) Shake allowance153 Reaction-Injection Moulding (RIM)568 Reducing zone 177 Refractoriness 118 Refractoriness Test 132 Resistance Spot Welding 259 Resistance Welding 25 Resistance brazing 297 Ring rolling process: 346 Riser of Casting 1106 Rod drawing process 354 Roll Forging 323 Rolling Of Metals 333 Rotary Melting Furnace 183

8 Index I5 Rotational Moulding 514, 542 Rubber Pad Forming 433 Runner 1102 S Sand Casting 15 Sand Mould 17 Sea coal and Pitch 114 Segmental (or) part pattern 145 Semi-Centrifugal Casting 1130 Shape Rolling Operations 345 Shatter Index Test 134 Shaving 410 Shear Spinning 437 Shearing processes 366 Shearing 35 Sheet Metal Characteristics 42 Shell mould casting 1114 Shell Core 163 Shovel 124 Shrinkage Cavity 2109 Shrinkage allowance 151 Silica flour 115 Silica sand 112 Skeleton pattern 145 Skin-dried moulding 123 Slag Inclusion (Fig 247) 2106 Slicks 126 Slinging machines 170 Slitting 48 Slush-moulding 569 Smoothers 126 Soft Soldering 2100 Solder Fluxes 2102 Soldering 299 Solid State Welding 26 Solidification of Weld Metal210 Solvent Bonding562 Solvents 56 Spin Welding 562 Spirit level 126 Spray-gun 127 Spruce: 1101 Sprue pin 124 Squeezer machine 168 Squeezing 35 Squeezing operations 369 Stack 178 Step gate 1104 Stir casting 1134 Strength Test 134 Stretch Blow Moulding (SBM)539 Strike off bar 124 Structural foam moulding 514 Stud welding 265 Submerged arc Welding 247 Sulphite binder 156 Super Plastic Forming 444 Surface Defects 348 Swab 126 Sweep pattern 144 System sand 117 T Thermit Welding Thermoforming 514, 556 Thermoplastics 57 Thermosetting resin 156 Thermosetting plastics 59 Thread rolling 347 Three-piece or multi- piece pattern142 Three High rolling mill 336

9 I6 Manufacturing Technology I - wwwairwalkbookscom Torch brazing 296 Transfer Moulding 515, 532 Transfer moulding types 533 Trimming 410 Trowels 125 True Centrifugal Casting 1128 Tube Drawing 355 Tube spinning 438 Two High reversible 335 Two piece (or) split pattern 139 Typical Shearing 44 U Ultrasonic Welding 280 Ultrasonic Inspection 1164 Undercutting and Overlapping (Fig 252) 2108 Universal rolling mill 338 Unpressurized gating system:1106 Upset forging (Heading) 321 Upsetting test 328 V Vacuum casting 1135 Vent rod 125 Vertical core 165 Vibration welding 567 Visual Inspection 1156 W Wave Soldering 2100 Weld Defects - Types and Causes2104 Weldability 26 Welding 242 Welding Terminology 27 Welding Positions 29 Well 176 White Metal 148 Wire drawing 354 Wood 146 Wood flour 115 Y Yield Point Elongation 43

10 Contents C1 Table of Contents Unit 1: METAL CASTING PROCESSES 11 Introduction to Solidification Process Sand Casting Sand Mould Features of Sand mould Desirable Mould Properties and Characteristics Steps/ Procedure for making sand mould Constituents of Moulding Sand Silica sand Binder Moisture / Water Additives Corn flour and Dextrin Coal dust Sea coal and Pitch Wood flour Silica flour Types of Moulding Sands Green sand Dry sand Loam sand Facing sand Backing sand System sand Parting sand Core sand Moulding Sand Properties 118

11 C2 Manufacturing Technology I - wwwairwalkbookscom 161 Refractoriness Permeability Cohesiveness Green strength Dry strength Flowability (or) plasticity Adhesiveness Collapsibility Classification of Moulding Processes Bench Moulding Floor Moulding Pit Moulding Machine Moulding Loam Moulding Dry sand moulding Skin-dried moulding Hand Tools Used In Foundry Shop Moulding Sand Testing Need for sand testing Moisture Content Test Clay Content Test Grain Fineness Test (GFT) Refractoriness Test Flowability Test Shatter Index Test Strength Test Mould Hardness Test 135

12 Contents C Permeability Test Pattern Functions of the Pattern Types of Pattern Design Considerations for a good Pattern Pattern Materials Selection of pattern material Pattern Allowances 151 (a) Shrinkage allowance 151 (b) Machining allowance 152 (c) Draft (or) Taper allowance 152 (d) Rapping (or) Shake allowance 153 (e) Distortion (or) Camber allowance 153 (f) Mould wall movement allowance Cores Functions (or) Objectives of core Core Sand Considerations in Selecting Core Sand Binders for core sand Core Making Types of Cores and Applications Moulding Machines Types and Applications of Moulding Machines 167 (a) Squeezer machine 168 (b) Jolt machine 168 (c) Jolt-squeezer machine 169 (d) Slinging machines 170

13 C4 Manufacturing Technology I - wwwairwalkbookscom (e) Pattern draw machines Melting Furnaces Factors responsible for the selection of furnace Types of Furnaces Blast Furnace Cupola Furnace Air Furnace or Reverberatory Furnace Rotary Melting Furnace Open Hearth Furnace Convertor Pit Furnace Crucible Furnaces Electric Furnaces Overall Comparison Of Melting Furnaces Gating System Riser of Casting Principle of Special Casting Processes Advantages of Special casting techniques over conventional sand casting Classification of Special Casting Processes Shell mould casting Investment casting (Lost wax casting) Ceramic Mould Casting Pressure Die Casting Centrifugal Casting Carbon-dioxide Moulding Principle Stir casting 1134

14 Contents C Expendable-pattern casting (lost foam process) Continuous Casting Design Considerations of Castings Defects in Sand Casting Inspection Method Visual Inspection Dimensional inspection Mechanical Testing Flaw detection by Non destructive testing Radiography Test (X-ray (or) -ray) : Magnetic Particle Inspection Fluorescent Penetrant Inspection (Zyglo Process) Liquid (Dye) Penetrant Test Ultrasonic Inspection 1164 Unit - 2: JOINING PROCESSES 21 Introduction to the Joining Processes Welding - Operating Principle Classification of Welding Processes Types of Welding Processes 24 (i) Gas Welding 25 (ii) Arc welding 25 (iii) Resistance Welding 25 (iv) Solid State Welding 26 (v) Thermit Welding 26 (vi) Modern Welding Processes 26 (vii) Related Process Weldability 26

15 C6 Manufacturing Technology I - wwwairwalkbookscom 23 Welding Terminology Filler Metals Fluxes Welding Positions Solidification of Weld Metal Heat Affected Zone (HAZ) Electrodes Selection of electrodes Electrodes and Their Uses Electrode Coating and Specifications Electrode Classification (as per AWS A51) Electrode Standards Gas Welding Processes Oxy-Fuel Gas Welding (OFW) Oxy Acetylene Welding Control in oxy-acetylene welding Gas welding equipments Flames Characteristics Oxy - Hydrogen Welding Air-Acetylene Welding Electric arc Welding Processes Arc Welding Equipment Arc Welding (or) Manual Metal arc Welding (or) Shielded arc Welding Merits and Demerits of Arc Welding Applications of Arc Welding Carbon arc Welding 245

16 Contents C7 211 Submerged arc Welding Gas Tungsten Arc Welding (GTAW) (or) Tungsten Inert Gas Welding (TIG) Gas Metal Arc Welding (GMAW) or Metal Inert Gas Welding (MIG) Electro Slag Welding Flux Cored Arc Welding (FCAW) Resistance Welding - Operating Principle Solid State Welding Processes (pressure Welding Processes) Forge Welding Friction Welding Friction Stir Welding (FSW) Explosive Welding Diffusion Welding Ultrasonic Welding Thermit Welding Modern Welding Processes Laser Beam Welding Electron Beam Welding Plasma arc Welding Flame Cutting Brazing Soldering Weld Defects - Types and Causes Porosity Cracking Slag Inclusion 2106

17 C8 Manufacturing Technology I - wwwairwalkbookscom 4 Distortion Incomplete Fusion and Penetration Poor Weld Bead Appearance Undercutting and Overlapping Overlays Blowholes Burn Through Excessive Penetration Shrinkage Cavity Highlights 2111 Unit - 3: METAL FORMING PROCESSES 31 Hot Working and Cold Working of Metals Hot Rolling: Hot Working Of Metals Cold Working Of Metals Comparision Between Hot Working And Cold Working Forging Processes Tools used in forging Classification of Forging Processes/Methods Open Die Forging (or) Smith Die Forging (or) Flat-die Forging Operations Hand forging Power forging Forging under sticking condition Closed Die Forging (or) Impression Die Forging (or) Precision Forging Drop Forging Press forging 320

18 Contents C Upset forging (Heading) Roll Forging Comparison between press forging and drop forging (Hammer forging) Forging Defects Defects Elimination/removal in forgings Annealing & Normalizing of Forgings Forging Tests 327 (i) Upsetting test 328 (ii) Hot twist test Design Considerations In Forging Rolling Of Metals Types of rolling mills 334 (a) High rolling mill 335 (b) Two High reversible 335 (c) Three High rolling mill 336 (d) Four High rolling mill 336 (e) Multiple Roll Mill/Cluster roll mill 337 (f) Universal rolling mill 338 (g) Planetary rolling mill Flat Strip Rolling Shape Rolling Operations Defects In Rolled Parts Applications Principle Of Rod And Wire Drawing Applications of drawing Equipment used in drawing Classification of drawing operations 354

19 C10 Manufacturing Technology I - wwwairwalkbookscom 313 Principles Of Extrusion Equipment used in extrusion Types of extrusion process Hot Extrusion Impact extrusion (or) Cold Extrusion Hydrostatic Extrusion Drives for Extrusion Extrusion defects Piercing Or Seamless Tubing Hot Spinning Cold Spinning Cold Working Processes Shearing processes Drawing operations Bending operations Squeezing operations Equipment used in extrusion 371 Unit - 4: SHEET METAL PROCESSES 41 Introduction Sheet Metal Characteristics Typical Shearing Factors Affecting Shearing Operation Stages in Shearing Action Shearing Operations Bending Operations Typical Drawing Operations In Sheet Metals Stretch Forming Operations 423

20 Contents C11 47 Formability Of Sheet Metal Test Methods For Formability Of Sheet Metals Cupping Test Forming Limit Diagram (FLD) Special Forming Processes Hydro Forming Hydro Mechanical Forming Electro Hydraulic Forming Rubber Pad Forming Metal Spinning Shear Spinning Tube spinning Explosive Forming Magnetic Pulse Forming Peen Forming Super Plastic Forming Micro-Forming 446 Unit - 5: MANUFACTURING OF PLASTIC COMPONENTS 51 Introduction Classification of Organic Materials Polymers The Structure of Polymers Polymerisation Process Additives and Fillers in Plastics Types And Characteristics Of Plastics Thermoplastics Thermosetting plastics 59

21 C12 Manufacturing Technology I - wwwairwalkbookscom 5163 Elastomers Differentiate between Thermoplastic and Thermosetting plastics Advantages and disadvantages of plastics Characteristics Of Forming And Shaping Processes Moulding Of Thermoplastics Injection Moulding Compression Moulding Transfer Moulding Blow Moulding Working Principle Classification of Blow Moulding Strech Blow Moulding (SBM) Advantages of Blow Moulding Common plastics for blow moulding Manufacture of plastic bags Rotational Moulding Film Blowing and Sheet Blowing Extrusion Thermoforming Bonding of Thermoplastics Other Plastic Processes Reaction-Injection Moulding (RIM) Slush-moulding Casting Potting and Encapsulation Cold forming 570

22 Unit 1 METAL CASTING PROCESSES Sand Casting: Sand Mould Type of patterns Pattern Materials Pattern allowances Moulding sand Properties and testing Cores Types and applications Moulding machines Types and applications; Melting furnaces; Blast and Cupola Furnaces; Principle of special casting process; Sheel investment Ceramic mould Pressure die casting Centrifugal Casting CO 2 process Stir casting; Defects in Sand casting 11 INTRODUCTION TO SOLIDIFICATION PROCESS In manufacturing processes, the raw material is in either a liquid or is in a highly plastic condition, and a part is created through solidification of the material Solidification processes can be classified according to the engineering material being processed as: Metal casting process Ceramics, specifically glass working process Polymers and polymer matrix composites (PMCs) process Fig 11 Shows the Classification of solidification processes Sand Casting Shell Moulding Solidification Process Metal casting Glass working Polymers & PMC Process Expendable-mould Casting Permanent-mould Casting Extrusion Injection Moulding Other Moulding Special Moulding for PM C Fig 11 Classification of Solidification Processes Vacuum M oulding Expanded Polystyrene Investment Casting Plaster-Mould Casting Ceramic-Mould Casting Permanent- Mould Casting Variations of Permanent-Mould Casting Die Casting Centrifugal Casting

23 12 Manufacturing Technology I - wwwairwalkbookscom 111 Casting of metals Casting is a manufacturing process by which a liquid material is usually poured into a mould, which contains a hollow cavity of the desired shape, and then allowed to solidify The solidified part is also known as a casting, which is ejected or broken out of the mould to complete the process Casting materials are usually metals or various cold setting materials that cure after mixing two or more components together; examples are epoxy, concrete, plaster and clay Casting is a 6000 year old process The oldest surviving casting is a copper frog from 3200 BC In comparison to other fabrication processes, casting is the most economical Casting techniques are used when: The finished shape is so large or consists of complex internal and external part geometries A particular alloy is so low in ductility that forming by either hot or cold working would be difficult Some casting processes can produce parts to met shape (no further manufacturing operations are required) Can be used with any metal that can be heated to its liquid phase Some types of casting are suited to mass production Casting is usually performed in a Foundry Foundry is a factory equipped for making moulds, melting and handling molten metal, performing the casting process, and cleaning the finished casting Workers who perform casting are called foundrymen 112 Important factors in casting operations Important factors in casting operations are Solidification of the metal Molten metal into metal cavity Heat transfer during solidification and cooling of the metal in the mould Influence of the type of the mould material

24 Metal Casting Processes Classification of Casting processes Casting processes are classified as 1 Expendable mould processes Permanent Pattern (eg Sand Casting) Expendable Pattern (eg Investment Casting) 2 Permanent mould processes (eg Die, Centrifugal & continuous Castings) Semi Permanent core (eg Sand core) Permanent core (eg Metal core) Expendable mould process uses an expendable mould which must be destroyed to remove casting Mould materials: sand, plaster, and similar materials, plus binders Advantage: more complex shapes are possible Disadvantage: production rates often limited by time to make mould rather than casting itself Permanent mould process uses a permanent mould which can be used over and over to produce many castings Made of metal or a ceramic refractory material Advantage: Higher production rates Disadvantage: Part geometrics are limited in this process as the mold needs to open and close 114 Capabilities and Advantages of Casting Can create complex part geometries Can create both external and internal shapes Some casting processes are net shape; others are near net shape Can produce very large parts Some casting methods are suited to mass production 115 Disadvantages of Casting Limitations on mechanical properties Poor dimensional accuracy and surface finish for some processes; eg, sand casting

25 14 Manufacturing Technology I - wwwairwalkbookscom Safety hazards to workers due to hot molten metals Environmental problems 116 Parts made by Casting Big parts -Engine blocks and heads for automotive vehicles, wood burning stoves, machine frames, railway wheels, pipes, bells, pump housings Small parts-dental crowns, jewelry, small statues, frying pans All varieties of metals can be cast - ferrous and nonferrous 117 Mould in Casting Mould contains a cavity whose geometry determines part shape Actual size and shape of cavity must be slightly oversized to allow for shrinkage of metal during solidification and cooling Moulds are made of a variety of materials, including sand, plaster, ceramic Moulds are in two forms namely (a) open mould, simply a container in the shape of the desired part; and (b) closed mould, in which the mould geometry is more complex and requires a gating system (passageway) leading into the cavity (Fig 12) Moulds are of the following types Expendable moulds These moulds are mixed with various types of binders or bonding agents eg Sand, Plaster, Ceramic Moulds These moulds are able to withstand high temperatures and mould is broken up to remove the casting Permanent moulds-are moulds made of metal These moulds are subjected to a higher cooling rate and affects grain size These are used repeatedly and casting can be removed easily Composite moulds - are moulds made of two or more materials like sand, graphite, metal etc, These moulds combines advantages of each material and are used to control cooling rates, improve mould strength and optimize economics of the process

26 Metal Casting Processes SAND CASTING Cast m etal Sand mould Flask Sand Fig 12 (a) Open mould Fig 12 Two forms of mould Sand casting method involves pouring a molten metal into sand mould Sand casting, is a metal casting process characterized by using sand as the mould material It is relatively cheap A suitable bonding agent (usually clay) is mixed with the sand The mixture is moistened with water to develop

27 16 Manufacturing Technology I - wwwairwalkbookscom strength and plasticity of the clay and to make the aggregate suitable for moulding The term sand casting can also refer to an object produced via the sand casting process Sand castings are produced in specialized factories called foundries Sand casting consists of following basic steps Placing of the pattern having the shape of the desired casting in sand to make an imprint Incorporating of gating, runner and riser systems Filling the resultant cavity with molten metal Allowing solidification and cooling Breaking the sand mould and removing the casting Heat treating the casting to relieve the stresses Cleaning and finishing the casting Inspecting for the casting defects A typical sand casting operation for production of castings is shown in the Fig 13 - Pattern making - Core m aking - Gating system Sand Casting Furnaces Solidification Shakeout Removal of risers and gates Additional Heat treatm ent Defects Pressure tightness Dimensions Fig 13 Sand Casting operation for production of castings

28 Metal Casting Processes Advantages and disadvantages of sand casting Advantages of sand casting Low cost of mould materials and equipment Large casting dimensions may be obtained Wide variety of metals and alloys (ferrous and non-ferrous) can be cast (including high melting point metals) using this method Disadvantages of sand casting Rough surface Poor dimensional accuracy High machining tolerances Coarse Grain structure Limited wall thickness: not higher than 25 to 5 mm 13 SAND MOULD 131 Features of Sand mould A typical sand mould is shown in the Fig 14 with the following parts/features: Fig14 A Typical sand mould

29 18 Manufacturing Technology I - wwwairwalkbookscom Cope / Drag: The mould is made of two parts, the top half is called the cope, and bottom part is the drag Mould cavity: The liquid flows into the gap between the two parts, called the mould cavity Pattern: The geometry of the cavity is created by the use of a wooden shape, called the pattern The shape of the pattern is (almost) identical to the shape of the part we need to make Sprue: A funnel shaped cavity at the top of the funnel is the pouring cup; the pipe-shaped neck of the funnel is the sprue the liquid metal is poured into the pouring cup, and flows down the sprue Runners: The runners are the horizontal hollow channels that connect the bottom of the sprue to the mould cavity The region where the runner joins with the cavity is called the gate Risers: Some extra cavities are made connecting to the top surface of the mould Excess metal poured into the mould flows into these cavities called risers They act as reservoirs; as the metal solidifies inside the cavity, it shrinks, and the extra metal from the risers flows back down to avoid holes in the cast part Vents: Vents are narrow holes connecting the cavity to the atmosphere to allow gas and the air in the cavity to escape Cores: Many cast parts have interior holes (hollow parts), or other cavities in their shape that are not directly accessible from either piece of the mould Such interior surfaces are generated by inserts called cores Cores are made by baking sand with some binder so that they can retain their shape when handled The mould is assembled by placing the core into the cavity of the drag, and then placing the cope on top, and locking the mould After the casting is done, the sand is shaken off, and the core is pulled away and usually broken off Chaplets: Chaplets are metal distance pieces inserted in a mould either to prevent shifting of mould or to locate core surfaces The distance pieces in form of chaplets are made of parent metal of which the casting is These are

30 Metal Casting Processes 19 placed in mould cavity suitably which positions core and to give extra support to core and mould surfaces Its main objective is to impart good alignment of mould and core surfaces and to achieve directional solidification Chills: Chills are pieces of copper, brass or aluminium and are inserted into the mould s inner surface Water passages in the mould or cooling fins made on outside the mould surface are blown by air otherwise water mist will create chilling effect A chill is used to promote directional solidification 132 Desirable Mould Properties and Characteristics The desirable mould properties and characteristics are Strength - to maintain shape and resist erosion Permeability - to allow hot air and gases to pass through voids in sand Thermal stability - to resist cracking on contact with molten metal Collapsibility - ability to give way and allow casting to shrink without cracking the casting Reusability - can be reused to make other moulds Size and shape of sand : Small grain size - Better surface finish Large grain size - To allow escape of gases during pouring Irregular grain shapes - Strengthen moulds due to interlocking but reduces permeability 133 Steps/ Procedure for making sand mould for a two piece pattern The steps involved in making a sand mould is discussed below: (Fig 15) Selection of Mould box / Flask: Select a suitable size of moulding box for creating suitable wall thickness for a two piece pattern The moulding box must be of proper size to adjust mould cavity, riser and the gating system (sprue, runner, and gates etc) Preparation of Drag: Place the drag portion of the pattern with the parting surface down on the bottom (ram-up) board as shown in Fig 15 (a)

31 110 Manufacturing Technology I - wwwairwalkbookscom Fig 15 (a) The facing sand is then sprinkled carefully all around the pattern so that the pattern does not stick with moulding sand during withdrawal of the pattern The drag is then filled with loosely prepared moulding sand and ramming of the moulding sand is done uniformly in the moulding box around the pattern Fill the moulding sand once again and then perform ramming Repeat the process three four times The excess amount of sand is then removed using strike off bar to bring moulding sand at the same level of the moulding flask height to complete the drag Drag Fig 15 (b)

32 Metal Casting Processes 111 The drag is then rolled over by 180 and the parting sand is sprinkled over on the top of the drag [Fig 15(b)] Preparation of Cope: Now the cope pattern is placed on the drag pattern and alignment is done using dowel pins Then cope (flask) is placed over the rammed drag and alignment is done using the aligning pins Then the parting sand is sprinkled all around the cope pattern Sprue (runner) and riser pins are placed in vertical position at suitable locations using the support of moulding sand It will help to form suitable size cavities for pouring molten metal etc [Fig 15(c)] They should not be located too close to the pattern or mould cavity otherwise they may chill the casting Now fill the cope with moulding sand and ram uniformly Sprue pin Riser pin Cope Pattern Lug Aligning Pin Drag Fig 15 (c) Strike off the excess sand from the top of the cope Remove sprue and riser pins and create vent holes in the cope with a vent wire The basic purpose of creating vent holes in cope is to permit the escape of gases generated during pouring and solidification of the casting Sprinkle parting sand over the top of the cope surface and roll over the cope on the bottom board

33 112 Manufacturing Technology I - wwwairwalkbookscom Cutting of Gate & Pouring of Metal Rap and remove both the cope and drag patterns and repair the mould suitably if needed and dressing is applied The gate is then cut connecting the lower base of sprue basin with runner and the mould cavity Apply mould coating with a swab and bake the mould in case of a dry sand mould Set the cores in the mould, if needed and close the mould by inverting cope over drag The cope is then clamped with drag and the mould is ready for pouring, [Fig 15(d)] Pouring basin Fig 15 (d) Gate Fig 15 Steps/procedure for making a sand mould 14 CONSTITUENTS OF MOULDING SAND The main constituents of moulding sand are Silica sand, Binder, Moisture content and Additives 141 Silica sand Silica sand (SiO 2 ) in the form of granular quartz is the main constituent of moulding sand having enough refractoriness which can impart strength, stability and permeability to moulding and core sand

34 Metal Casting Processes 113 Along with silica small amounts of iron oxide, alumina, lime stone, magnesia, soda and potash are present as impurities The presence of excessive amounts of iron oxide, alkali oxides and lime can lower the fusion point to a considerable extent which is undesirable The silica sand can be specified according to the size (small, medium and large silica sand grain) and the shape (angular, sub-angular and rounded) 142 Binder The binders are added to bind the silica sands and can be either inorganic or organic substance The inorganic group includes clay, sodium silicate, port land cement etc Organic groups are dextrin, molasses, cereal binders, linseed oil and resins like phenol formaldehyde, urea formaldehyde etc Organic binders are mostly used for core making In foundry shop, the clay acts as binder which may be Kaolonite, Ball clay, Fire clay, Limonite, Fuller s earth and Bentonite (most common) However, this clay alone cannot develop bonds among sand grains without the presence of moisture in moulding sand and core sand 143 Moisture / Water The amount of moisture content in the moulding sand varies generally between 2 to 8 percent This amount is added to the mixture of clay and silica sand for developing bonds This is the amount of water required to fill the pores between the particles of clay without separating them This amount of water is held rigidly by the clay and is mainly responsible for developing the strength in the sand The effect of clay and water decreases permeability with increasing clay and moisture content

35 114 Manufacturing Technology I - wwwairwalkbookscom 144 Additives For increasing the moulding sand characteristics some other additional materials besides basic constituents are added which are known as additives Additives are the materials generally added to the moulding and core sand Some commonly used additives for enhancing the properties of moulding and core sands are discussed below 1441 Corn flour and Dextrin It belongs to the starch family of carbohydrates and is used to increase the collapsibility of the moulding and core sand It is completely volatilized by heat in the mould, thereby leaving space between the sand grains This allows free movement of sand grains, which finally gives rise to mould wall movement and decreases the mould expansion and hence defects in castings Corn sand if added to moulding sand and core sand improves significantly strength of the mould and core 1442 Coal dust Coal dust is added mainly for producing a reducing atmosphere during casting This reducing atmosphere results in any oxygen in the pores becoming chemically bound so that it cannot oxidize the metal It is usually added in the moulding sands for making moulds for production of grey iron and malleable cast iron castings 1443 Sea coal and Pitch Sea coal is the fine bituminous coal powder which occupies the pores of the silica sand grains in moulding sand and core sand It can be added from 002 % to 2% in mould and core sand When heated, it changes to coke which fills the pores and is unaffected by water and does not allow the sand to move Thus, sea coal reduces the mould wall movement and the permeability in mould and core sand and hence makes the mould and core surface clean and smooth

36 Metal Casting Processes Wood flour This is a fibrous material mixed with a granular material like sand; its relatively long thin fibers prevent the sand grains from making contact with one another It can be added from 005 % to 2% in mould and core sand It volatilizes when heated, thus allowing the sand grains to expand It will increase mould wall movement and decrease expansion defects It also increases collapsibility of both mould and the core 1445 Silica flour It is called as pulverized silica and it can be easily added up to 3% which increases the hot strength and finish on the surfaces of the moulds and cores It also reduces metal penetration in the walls of the moulds and cores 15 TYPES OF MOULDING SANDS 151 Green sand Green sand is tempered or natural sand It is prepared by mixing of silica sand with 18 to 30 percent clay and moisture content from 6 to 8% The clay and water furnish the bond for green sand It is fine, soft, light, and porous Green sand is damp, when squeezed in the hand it retains the shape and the impression given to it under pressure Moulds prepared by this sand do not require backing and hence are known as green sand moulds This sand is easily available and at low cost It is commonly employed for production of ferrous and non-ferrous castings

37 116 Manufacturing Technology I - wwwairwalkbookscom 152 Dry sand Green sand that has been dried or baked in suitable oven after the making mould and core is called dry sand It possesses more strength, rigidity and thermal stability It is mainly suitable for larger castings Moulds prepared in this sand are known as dry sand moulds 153 Loam sand Loam is mixture of sand and clay with water to a thin plastic paste Loam sand possesses high clay as much as 30-50% and 18% water Patterns are not used for loam moulding and shape is given to mould by sweeps This is particularly employed for loam moulding used for large grey iron castings 154 Facing sand Facing sand is just prepared and forms the face of the mould It is directly applied next to the surface of the pattern and it comes into contact with molten metal when the mould is poured Initial coating around the pattern and hence for mould surface is given by this sand This sand is subjected to the most severe conditions and therefore must possess high strength refractoriness It is made of silica sand and clay, without the addition of any used sand Different forms of carbon are used to prevent the metal burning into the sand A facing sand mixture for green sand moulding of cast iron may consist of 25% fresh and specially prepared sand 70% old sand and 5% sea coal They are sometimes mixed with 6-15 times as much fine moulding sand to make facings

38 Metal Casting Processes Backing sand Backing sand (or) floor sand is used to back up the facing sand and is used to fill the whole volume of the moulding flask Used moulding sand is mainly employed for this purpose The backing sand is sometimes called black sand because it is old and repeatedly used Moulding sand is black in color due to addition of coal dust and burning caused on coming in contact with the molten metal 156 System sand In mechanized foundries where machine moulding is employed, a so-called system sand is used to fill the whole moulding flask In mechanical sand preparation and handling units, no facing sand is used The used sand is cleaned and re-activated by the addition of water and special additives This is known as system sand Since the whole mould is made of this system sand, the properties such as strength, permeability and refractoriness of the moulding sand must be higher than those of backing sand 157 Parting sand Parting sand without binder and moisture is used to keep the green sand not to stick to the pattern and also to allow the cope and drag to separate without clinging This is clean clay-free silica sand which serves the same purpose as parting dust 158 Core sand Core sand is used for making cores and it is sometimes known as oil sand This is highly rich silica sand mixed with oil binders such as core oil which is composed of linseed oil, resin, light mineral oil and other binding materials Pitch (or) flours and water may also be used in large cores for the sake of economy

39 118 Manufacturing Technology I - wwwairwalkbookscom 16 MOULDING SAND PROPERTIES The basic properties required in moulding sand and core sand are described below 161 Refractoriness Refractoriness is defined as the ability of moulding sand to withstand high temperatures without breaking down (or) fusing thus facilitating to get a sound casting It is a highly important characteristic of moulding sands Refractoriness can only be increased to a limited extent Moulding sand with poor refractoriness may burn on to the casting surface and no smooth casting surface can be obtained The degree of refractoriness depends on the SiO 2 ie quartz content, and the shape and grain size of the particle The higher the SiO 2 content and the rougher the grain volumetric composition, the higher is the refractoriness of the moulding sand and core sand Refractoriness is measured by the sinter point of the sand rather than its melting point 162 Permeability It is also termed as porosity of the moulding sand in order to allow the escape of any air, gases or moisture present or generated in the mould when the molten metal is poured into it All the gases generated during pouring and solidification process must escape otherwise the casting becomes defective Permeability is a function of grain size, grain shape, moisture and clay contents in the moulding sand The extent of ramming of the sand directly affects the permeability of the mould Permeability of mould can be further increased by venting using vent rods

40 Metal Casting Processes Cohesiveness It is a property of moulding sand by virtue of which the sand grain particles interact and attract each other within the moulding sand Thus, the binding capability of the moulding sand gets enhanced to increase the green, dry and hot strength property of moulding and core sand 164 Green strength The green sand, after water has been mixed into it, must have sufficient strength and toughness to permit the making and handling of the mould For this, the sand grains must be adhesive, ie they must be capable of attaching themselves to another body Therefore sand grains having high adhesiveness will cling to the sides of the moulding box By virtue of this property, the pattern can be taken out from the mould without breaking the mould and also the erosion of mould wall surfaces does not occur during the flow of molten metal The green strength also depends upon the grain shape and size, amount and type of clay and the moisture content 165 Dry strength As soon as the molten metal is poured into the mould, the moisture in the sand layer adjacent to the hot metal gets evaporated and this dry sand layer must have sufficient strength to its shape in order to avoid erosion of mould wall during the flow of molten metal The dry strength also prevents the enlargement of mould cavity caused by the metallostatic pressure of the liquid metal 166 Flowability (or) plasticity It is the ability of the sand to get compacted and behave like a fluid It will flow uniformly to all portions of pattern when rammed and distribute the ramming pressure evenly all around in all directions Generally sand particles resist moving around corners (or) projections

41 120 Manufacturing Technology I - wwwairwalkbookscom In general, flowability increases with decrease in green strength and decrease in grain size The flowability also varies with moisture and clay content 167 Adhesiveness It is property of moulding sand that allows it to stick or adhere with foreign materials also with inner wall of moulding box 168 Collapsibility After the molten metal in the mould gets solidified, the sand mould must be collapsible so that free contraction of the metal occurs and this would naturally avoid the tearing or cracking of the contracting metal In absence of this property the contraction of the metal is hindered by the mould and thus results in tears and cracks in the casting This property is highly desired in cores 169 Classification of Moulding Processes Moulding processes can be classified as (i) Classification based on the method used Bench moulding, Floor moulding, Pit moulding, Machine moulding (ii) Classification based on the mould material used: Green sand moulding, Dry sand moulding, Skin dried moulding, Core sand moulding, loam moulding, Carbon-dioxide moulding, Shell moulding, Plaster moulding, Metallic moulding and Loam moulding Some of the important moulding methods are discussed below 1691 Bench Moulding This type of moulding is preferred for small jobs The whole moulding operation is carried out on a bench of convenient height In this process, a minimum of two flasks, namely cope and drag moulding flasks are necessary But in certain cases, the number of flasks may increase depending upon the number of parting surfaces required

42 Metal Casting Processes Floor Moulding This type of moulding is preferred for medium and large size jobs In this method, only drag portion of moulding flask is used to make the mould and the floor itself is utilized as drag and it is usually performed with dry sand 1693 Pit Moulding Usually large castings are made in pits instead of drag flasks because of their huge size In pit moulding, the sand under the pattern is rammed by bedding-in process The walls and the bottom of the pit are usually reinforced with concrete and a layer of coke is laid on the bottom of the pit to enable easy escape of gas The coke bed is connected to atmosphere through vent pipes which provide an outlet to the gases One box is generally required to complete the mould, runner, sprue, pouring basin and gates are cut in it 1694 Machine Moulding For mass production of the casting, the general hand moulding technique proves un-economical and in-efficient The main advantage of machine moulding, besides the saving of labor and working time, is the accuracy and uniformity of the castings and or even the cost of machining on the casting can be reduced drastically because it is possible to maintain the tolerances within narrow limits on casting by using machine moulding method Moulding machines thus prepare the moulds at a faster rate and also eliminate the need of employing skilled moulders The main operations performed by moulding machines are ramming of the moulding sand, roll over the mould, form gate, rapping the pattern and its withdrawal 1695 Loam Moulding Loam moulding uses loam sand to prepare a loam mould It is such a moulding process in which use of pattern is avoided and hence it differs from the other moulding processes Initially the loam sand is prepared with the mixture of moulding sand and clay made in form of a paste by suitable addition of water Firstly a rough structure of cast article is made by hand using bricks and loam sand and it is then given a desired shape by means of strickles and sweep patterns Mould is thus prepared It is then baked to

43 122 Manufacturing Technology I - wwwairwalkbookscom give strength to resist the flow of molten metal This method of moulding is used where large castings are required in numbers Thus it enables the reduction in time, labor and material which would have been spent in making a pattern But this system is not popular for the reason that it takes lots of time in preparing mould and requires special skill The cope and drag part of mould are constructed separately on two different iron boxes using different sizes of strickles and sweeps etc and are assembled together after baking It is important to note that loam moulds are dried slowly, completely and are used for large regular shaped castings like chemical pans, drums etc 1696 Dry sand moulding Dry moulding sand differs from the green moulding sand in the sense that it contains binders (like clay, bentonite, molasses etc) which harden when the mould is heated and dried A typical dry sand mixture (for making non-ferrous castings) consists of new silica sand 30%, coal dust 20% and bentonite 10% A dry sand mould is prepared in the same manner as a green sand mould; however it is baked at 300 to 70F for 8 to 48 hours depending upon binders used and the amount of sand surface to be dried Drying of moulds can be of two types: skin dried and complete mould drying Common methods of drying the mould are hot air and gas or oil flame Skin drying is accomplished with the aid of torches directed at the mould surface Advantages Dry sand moulds possess high strength They are more permeable as compared to green sand moulds Castings produced from dry sand moulds possess clean and smooth surfaces As compared to green sand moulding, dry sand moulding turns out castings with less defects Dry sand moulding imparts better overall dimensional accuracy to the moulds and castings as compared to green sand moulding

44 Metal Casting Processes 123 Disadvantages Dry sand moulding involves more labour and consumes more time in completing the mould Mould baking is an extra work as compared to that required in green sand moulding Dry sand moulding is more expensive as compared to green sand moulding Dry sand moulding involves chances of hot tears occurring in the castings Because of baking, a mould may distort Dry sand moulding involves a longer processing cycle as compared to green sand moulding Dry sand moulding gives a slower rate of production as compared to green sand moulding Applications Dry sand moulding is used for making medium to large size ferrous castings such as Large rolls, Housings, Gears, Machinery components 1697 Skin-dried moulding A skin-dried mould is intermediate between green sand mould and dry sand mould Whereas a dry sand mould has its entire surface dried, a skin dried mould has its (6 to 25 mm) skin dried Moisture of the skin is removed either by storing the mould for some time or with a gas torch It has some advantages of both green and dry sand moulding It is used for large moulds and moulds for pit moulding 1610 Hand Tools Used In Foundry Shop (Fig 16 (a to n)) Hand riddle Hand riddle consists of a screen of standard circular wire mesh equipped with circular wooden frame used for cleaning the sand for removing foreign material Fig 16 (a) Fig 16 (a) Hand riddle

45 124 Manufacturing Technology I - wwwairwalkbookscom Shovel Shovel consists of a steel pan fitted with a long wooden handle It is used for mixing, tempering, moving and conditioning the foundry sand by hand Fig 16 (b) Rammers Rammers are required for striking the moulding sand mass in the moulding box to pack or compact it uniformly all around the pattern The common forms are hand rammer, peen rammer, floor rammer and pneumatic rammer Fig 16 (c) Fig 16 (b) Shovel Sprue pin Sprue pin is a tapered rod of wood or iron which is placed or pushed in cope to join mould cavity while the moulding sand in the cope is being rammed Later, its withdrawal from cope produces a vertical hole in moulding sand called sprue through which the molten metal is poured into the mould using gating system Fig 16 (d) Strike off bar Strike off bar is a flat bar having straight edge and is made of wood or iron It is used to strike off or remove the excess sand from the top of a moulding box after completion of ramming Fig 16 (e) Fig 16 (c) Rammer Fig 16 (d) Sprue pin Fig 16 (e) Strike off Bar Mallet Mallet is similar to a wooden hammer and is generally used in carpentry or sheet metal shops

46 Metal Casting Processes 125 Draw spike Draw spike is a tapered steel rod having a loop or ring at its one end and a sharp point at the other It may have screw threads on the end to engage metal pattern for its withdrawal from the Fig 16 (f) Draw Spike mould It is driven into pattern which is embedded in the moulding sand and raps the pattern to get separated from the mould cavity and finally draws it out from the mould cavity Fig 16 (f) Vent rod Vent rod is a thin spiked steel rod or wire carrying a pointed edge at one end Fig 16 (g) Vent rod and a wooden handle or a bent loop at the other It is utilized to pierce series of small holes in the moulding sand called vent holes which allow the exit or escape of steam and gases Fig 16 (g) Lifters Lifters are also known as cleaners or finishing tool which are used for cleaning, repairing and finishing the bottom and sides of deep and narrow Fig 16 (h) Lifters openings in mould cavity after withdrawal of pattern They are also used for removing loose sand from mould cavity Fig 16 (h) Trowels Trowels are utilized for finishing flat surfaces and joints and parting lines of the mould The trowels are basically employed for smoothening or slicking the surfaces of moulds They may also be used to cut in-gates and repair the mould surfaces Fig 16 (i) Fig 16 (i) Trowels

47 126 Manufacturing Technology I - wwwairwalkbookscom Slicks Slicks are small double ended mould finishing tool which are generally used for repairing and finishing the mould surfaces and their edges after withdrawal of the pattern Fig 16 (j) Fig 16 (j) Slicks Smoothers Smoothers are finishing tools which are commonly used for repairing and finishing flat and round surfaces, round or square corners and edges of moulds Fig 16 (k) Swab Swab is a small hemp fiber brush used for moistening the edges of sand mould, which are in contact with the pattern surface before withdrawing the pattern It is used for sweeping away the moulding sand from the mould surface and pattern It is also used for coating the liquid blacking on the mould faces in dry sand moulds Fig 16 (l) Fig 116 (k) Smoothers Fig 116(l) Swab Spirit level Spirit level is used by moulder to check whether the sand bed or moulding box is horizontal or not Gate cutter Gate cutter is a small shaped piece of sheet metal commonly used to cut runners and feeding gates for connecting sprue hole with the mould cavity Fig 16 (m) Gate Cutter

48 Gaggers Gaggers are pieces of wires or rods bent at one or both ends which are used for reinforcing the downward projecting sand mass in the cope They support hanging bodies of sand Spray-gun Spray gun is mainly used for spray coating of facing materials etc on a mould or core surface Nails and wire pieces They are basically used to reinforce thin projections of sand in the mould or cores Bellows Bellows gun is a hand operated leather made device equipped with compressed air jet to blow or pump air when operated It is used to blow away the loose or unwanted sand from the surfaces of mould cavities Fig 16 (n) Metal Casting Processes 127 Fig 16 (n) Bellows Clamps, cotters and wedges They are made of steel and are used for clamping the moulding boxes firmly together during pouring Flasks The common flasks are also called as containers which are used in foundry shop as mould boxes, crucibles and ladles Moulding Boxes Mould boxes are also known as moulding flasks Boxes used in sand moulding are of two types: Open moulding boxes Open moulding boxes are shown in Fig 16 (p) They are made with the hinge at one corner and a lock on the opposite corner A snap moulding is made of wood and is hinged at one corner It has special applications in bench moulding in green sand work for small non-ferrous castings The size, material and construction of the moulding box depends upon the size of the casting

49 128 Manufacturing Technology I - wwwairwalkbookscom Hinge Cope Drag Lugs (p) (q) Fig16 (p) Open moulding box (q) Closed moulding box Closed moulding boxes Closed moulding boxes are shown in Fig 16 (q) which may be made of wood, cast-iron or steel and consist of two or more parts The lower part is called the drag, the upper part the cope and all the intermediate parts, if used, cheeks All the parts are individually equipped with suitable means for clamping arrangements during pouring Wooden Boxes are generally used in green-sand moulding Dry sand moulds always require metallic boxes because they are heated for drying Large and heavy boxes are made from cast iron or steel and carry handles and grips as they are manipulated by cranes or hoists, etc Crucible and ladles Crucibles are made from graphite or steel shell lined with suitable refractory material like fire clay They are commonly named as metal melting pots The raw material or charge is broken into small pieces and placed in them Metals are melted in crucibles, they are taken out and received in crucible handle Pouring of molten metal is generally done directly by them instead of transferring the molten metal to ladles But in the case of an oilfired furnace, the molten metal is first received in a ladle and then poured into the moulds Fig 17

50 Metal Casting Processes 129 Hook for crane Gear box for pouring Pouring spout Top view Turn handle Handles Front view (a) Fig17 Crucible & Ladles (a) Crane ladle, and (b) Two-man ladle (b) 17 MOULDING SAND TESTING Moulding sand and core sand depend upon shape, size, composition and distribution of sand grains, amount of clay, moisture and additives 171 Need for sand testing The increase in demand for good surface finish and higher accuracy in castings necessitates certainty in the quality of mould and core sands Sand testing often allows the use of less expensive local sands It also ensures reliable sand mixing and enables a utilization of the inherent properties of moulding sand Sand testing on delivery will immediately detect any variation from the standard quality and adjustment of the sand mixture to specific requirements so that the casting defects can be minimized It allows the choice of sand mixtures to give a desired surface finish Thus sand testing is one of the dominating factors in foundry and pays for itself by obtaining low unit cost and mould increased production resulting from sound castings Generally the following tests are performed to judge the moulding and casting characteristics of foundry sands:

51 130 Manufacturing Technology I - wwwairwalkbookscom Moisture content Test Clay content Test Chemical composition of sand Grain shape and surface texture of sand Grain size distribution of sand Specific surface of sand grains Water absorption capacity of sand Refractoriness of sand Strength Test Permeability Test Flowability Test Shatter index Test Mould hardness Test 172 Moisture Content Test The moisture content of the moulding sand mixture may be determined by drying a weighed amount of 20 to 50 grams of moulding sand to a constant temperature up to 100C in an oven for about one hour It is then cooled to room temperature and then the moulding sand is reweighed The moisture content in moulding sand is thus evaporated The loss in weight of moulding sand due to loss of moisture, gives the amount of moisture which can be expressed as a percentage of the original sand sample % Moisture Content of Sand Weight of wet sand Weight of heated/cooled sand 100 Weight of wet sand The percentage of moisture content in the moulding sand can also be determined in fact more speedily by an instrument known as a speedy moisture teller This instrument is based on the principle that when water and calcium carbide react, they form acetylene gas which can be measured and this will be directly proportional to the moisture content This instrument is provided with a pressure gauge calibrated to read directly the percentage of moisture present in the moulding sand CaC 2 2H 2 O CaOH 2 C 2 H 2

52 Metal Casting Processes 131 Calcium carbide Water Calcium hyrdroxide Acetylene Some moisture testing instruments are based on principle that the electrical conductivity of sand varies with moisture content in it 173 Clay Content Test The clay content of the sand is determined as follows, Take 50 gms of dry moulding sand and transfer to a wash bottle Add 475 cc of distilled water and 25 cc of 35 % NaOH solution and agitate with a stirrer for 10 minutes Fill the water bottle with water upto mark After the sand is settled down drain out the water (clay is dissolved in water and is removed) Repeat the above step for 7 times to ensure complete removal of clay Dry the settled sand and weigh it say A gms Weight of clay 50 A gms % content of clay 50 A/ Grain Fineness Test (GFT) GFT determines the grain size, distribution and grain fineness For carrying out grain fineness test a sample of dry silica sand weighing 50 gms free from clay is placed on the top most sieve bearing US series equivalent number 6 A set of eleven sieves having US Bureau of standard meshes 6, 12, 20, 30, 40, 50, 70, 100, 140, 200 and 270 are mounted on a mechanical shaker (Fig 18) The topmost sieve is coarsest and bottom most finest and the inbetween sieves are in order of fineness from top to bottom The above setup is vibrated for 15 minutes After this weight of sand retained in each sieve is obtained and percentage distribution of grains is computed To obtain the AFS (American Foundry Society) grain fineness number, each % is multiplied by a factor The resulting products are added and divided by total percentage of sand grain retained

53 132 Manufacturing Technology I - wwwairwalkbookscom Adjusting Knob Clamping strip Side flexible bar Set of sieve Spring Bumper Timer Toggle switch Indicator lam p Pannel Levelling screw Base Fig18 Grain fineness testing mechanical shaker AFS grain fineness number = Sum of products / Total sum of the % of sand retained on pan and each sieve 175 Refractoriness Test The refractoriness of the moulding sand is judged by heating the American Foundry Society (AFS) standard sand specimen to very high temperature ranges (1300C) depending upon the type of sand

54 The heated sand specimens pieces are cooled to room temperature and examined under a microscope for surface characteristics or by scratching it with a steel needle A good refractory sand retains the shape and shows very little (< 7%) expansion A less refractory specimen will shrink and distort 176 Flowability Test Flowability is the ability of sand to take up the desired shape Sand must be able to transmit the blows throughout during ramming Flowability test setup is shown in the Fig 19 A standard sand/core specimen is prepared The flowability measurement device setup consists of a ramplunger, a flow dial indicator whose stem rest on top of plunger, a standard prepared specimen and supporting stand/table Ram plunger is dropped on the standard specimen for five times The movement of the plunger between the fourth and fifth drop (x) is measured on the dial which is calibrated to give the flowability of the sand/core Flow indicator Metal Casting Processes 133 Stem Plunger Ram Stand X st 1 drop 4 th drop 5 th drop Standard specimen Table Fig 19 Flowability test

55 134 Manufacturing Technology I - wwwairwalkbookscom 177 Shatter Index Test In this test, the AFS standard sand specimen is rammed usually by 10 blows and then it is allowed to fall on a half inch mesh sieve from a height of 6 ft The weight of sand retained on the sieve is weighed It is then expressed as percentage of the total weight of the specimen which is a measure of the shatter index 178 Strength Test Green strength and dry strength is the holding power of the various bonding materials The most commonly performed test is compression test which is carried out in a compression sand testing machine (Fig 110) Generally sand mixtures are tested for their compressive strength, shear strength, tensile strength, transverse tests and bending strength For carrying out these tests on green sand sufficient rammed samples are prepared The process of preparing sand specimen for testing dry sand is similar to the process as prepared before, with the difference that a split ram tube is used The specimen for testing bending strength is of a square cross section Molding Sand Specimen Dial Gauge Peep Hole Hand wheel Lugs Adjusting Cock Low Pressure Manometer High Pressure Manometer Fig 110 Strength testing Machine

56 Metal Casting Processes 135 The various tests can be performed on strength tester as follows: The specimen is placed between the grips Hand wheel when rotated actuates a mechanism which builds hydraulic pressure The dial indicator measures the deformation occurring in the specimen There are two indicators One is meant for testing low strength moulding sand and the other relatively high strength core sand Each indicator has three scales one for reading compressive strength, the other two for recording tensile (or transverse) and shear strength respectively The compression strength of the moulding sand is determined by placing standard specimen at specified location and the load is applied on the standard sand specimen to compress it by uniformly increasing load by rotating the hand wheel of compression strength testing setup As soon as the sand specimen fractures for break, the compression strength is measured by the manometer Also, other strength tests can be conducted by adopting special types of specimen holding accessories 179 Mould Hardness Test This test is performed by a mould hardness tester shown in Fig 111 The working of the tester is based on the principle of Brinell hardness testing machine In an AFS standard hardness tester a half inch diameter steel hemi-spherical ball is loaded with a spring load of 980 gm This ball is made to penetrate into the mould sand or core sand surface The penetration of the ball point into the mould surface is indicated on a dial in thousands of an inch The dial is calibrated to read the hardness directly ie a mould surface which offers no resistance to the steel ball would have zero hardness value and a mould which is more rigid and is capable of completely preventing the steel ball from penetrating would have a hardness value of 100 The dial gauge of the hardness tester may provide direct readings

57 136 Manufacturing Technology I - wwwairwalkbookscom Plastic sleeve Metallic sleeve Needle Dial Ring Fig 111 Mould hardness tester 1710 Permeability Test Initially a predetermined amount of moulding sand is being kept in a standard cylindrical tube and then moulding sand is compressed using slightly tapered standard ram till the cylindrical standard sand specimen having 508mm diameter with 508 mm height is made and it is then extracted from the cylindrical tube This specimen is used for testing the permeability or porosity of moulding and the core sand The test is performed in a permeability meter consisting of the balanced tank, water tank, nozzle, adjusting lever, nose piece for fixing sand specimen and a manometer A typical permeability meter is shown in Fig 112 which permits to read the permeability directly Tip

58 Metal Casting Processes 137 The permeability test apparatus consists of two concentric cylinder one inside the other The space between the two concentric cylinders is filled with water A bell having a diameter larger than that of the inner cylinder but smaller than that of outer cylinder, rests on the surface of water Balanced tank Water tank Nozzle adjusting lever Nose piece for fixing sand specimen tube Bell Balanced tank Moulding sand Specimen sample tube Pressure manometer Dial meter Variable nozzle Air passage Fig 112 Permeability meter Mercury seal Standard sand specimen together with ram tube is placed on the tapered nose piece of the permeability meter The bell is allowed to sink under its own weight by the help of multi-position cock In this way the air of the bell streams through the nozzle of nosepiece and the permeability is directly measured Permeability is volume of air (in cm 3 ) passing through a sand specimen of 1 cm 2 cross-sectional area and 1 cm height, at a pressure difference of 1 gm/cm 2 in one minute In general, permeability is expressed as a number and can be calculated from the relation: Permeability (P) = vh/pat Where P Permeability; v = volume of air passing through the specimen in cc; h Height of specimen in cm; p = pressure of air in gm/cm 2 ; a = cross-sectional area of the specimen in cm 2 ; t = time in minutes

59 138 Manufacturing Technology I - wwwairwalkbookscom For AF S standard permeability meter, 2000 cc of air is passed through a sand specimen (508 cm in height and sqcm in cross-sectional area) at a pressure of 10 gms/cm 2 and the total time measured is 10 seconds = 1/6 min Then the permeability is calculated using the relationship as given below P / / App 18 PATTERN A pattern is a model or the replica of the object (to be casted) except for the various allowances It is embedded in moulding sand and suitable ramming of moulding sand around the pattern is made The pattern is then withdrawn for generating cavity (known as mould) in moulding sand Thus it is a mould forming tool When this mould/cavity is filled with molten metal, molten metal solidifies and produces a casting (product) The quality of the casting produced depends upon the material of the pattern, its design, and construction It should have finished and smooth surfaces for reducing casting defects Pattern may also possess projections known as core prints for producing extra recess in the mould for placement of core to produce hollowness in casting Pattern establishes the parting line and parting surfaces in the mould It may help to position a core in case a part of the mould cavity is made with cores 181 Functions of the Pattern A pattern prepares a mould cavity for the purpose of making a casting A pattern may contain projections known as core prints if the casting requires a core and need to be made hollow Runner, gates, and risers used for feeding molten metal in the mould cavity may form a part of the pattern Patterns properly made and having finished and smooth surfaces reduce casting defects

60 Metal Casting Processes 139 A properly constructed pattern minimizes the overall cost of the castings It establishes the parting line and parting surfaces in the mould 182 TYPES OF PATTERN The different types of pattern are given below (a) One piece (or) solid pattern Solid pattern is made of single piece without joints, parting lines or loose pieces It is the simplest form of the pattern Typical single piece pattern is shown in Fig 113 Single Piece Pattern Fig 113 It is inexpensive and is used for making a few large sized simple castings It is usually made of wood or metal depending upon number of castings It is placed either in cope or drag Example: Stuffing box of Steam engine can be cast from single piece pattern (b) Two piece (or) split pattern Dowel holes Dowel holes Fig114 Split Piece Pattern

61 140 Manufacturing Technology I - wwwairwalkbookscom Pattern of intricate shapes made by single piece pattern are difficult to withdraw from the mould cavity, hence solid pattern is split into two parts Fig 114 Split pattern is made in two pieces which are joined at the parting line by means of dowel pins The splitting at the parting line is done to facilitate the withdrawal of the pattern One part is placed in cope & the other in drag Example are taps, water stop cocks etc, (c) Loose piece pattern Loose piece pattern (Fig 115) is used when pattern is difficult for withdrawal from the mould Main pattern E Fig 115 Loose Piece Pattern F Dowel Pins Loose pieces are provided on the pattern (E & F) and they are attached to pattern by dowel pins The main pattern is removed first leaving the loose piece portion of the pattern in the mould Finally the loose piece is withdrawn separately leaving the intricate mould It requires more labour & is a time consuming process

62 Metal Casting Processes 141 (d) Cope and drag pattern In this case, cope and drag part of the mould are prepared Cope Pattern Drag Pattern Fig 116 Cope & Drag Pattern separately (Fig 116) It is another form of Split pattern The Pattern is split about a convenient and suitable Surface or line This is done when the complete mould is too heavy to be handled by one operator The pattern is made up of two halves, which are mounted on different plates by an independent moulder It is used for producing big casting (e) Match plate pattern This pattern is made in two halves and is mounted on the opposite sides of a wooden or metallic plate, known as match plate (Fig 117) A number of different size and shape pattern can be attached to one match plate

63 142 Manufacturing Technology I - wwwairwalkbookscom Runner Patterns Match plate Fig 117 Match Plate Pattern Hole for locating The match plate is clamped to drag with the help of locator holes The gates and runners are also attached to the plate This pattern is used in machine moulding It is used to produce small casting in mass scale with high accuracy and at faster rate Eg Piston rings of IC Engine (f) Three-piece or multi- piece pattern Some patterns are of complicated kind in shape and hence cannot be made in one or two pieces because of difficulty in withdrawing the pattern Therefore these patterns are made in either three pieces or in multi-pieces Multi moulding flasks are needed to make mould from these patterns (Fig 118) 1 Cope Moulding Box Cheek 2 Part/C ast 3 Drag Fig 118 Three Piece Pattern

64 Metal Casting Processes 143 (g) Follow board pattern A contour corresponding to the exact shape of one half of the pattern is made in a wooden board which is called a follow board (Fig 119) Sand Pattern Follow board Fig119 Follow Board Pattern It is used for supporting a pattern which is very thin and fragile and which may give way and collapse under pressure when the sand above the pattern is being rammed In addition to supporting a thin section a follow board forms the natural parting line of the mould or the casting (h) Gated pattern Patterns Gate Runner Fig 120 Gated Pattern

65 144 Manufacturing Technology I - wwwairwalkbookscom In the mass production of castings, multi cavity moulds are used Such moulds are formed by joining a number of patterns and gates and providing a common runner for the molten metal, as shown in Fig 120 These patterns are made of metals and metallic pieces to form gates and runners are attached to the pattern A gated casting produce many castings at one time and are used for mass production of small castings (i) Sweep pattern Sweep patterns are used for forming large circular moulds of symmetric kind by revolving a sweep attached to a spindle as shown in Fig 121 Post Sweep Green sand Fig121 Sweep Pattern Actually a sweep is a template of wood or metal and is attached to the spindle at one edge and the other edge has a contour depending upon the desired shape of the mould The pivot end is attached to a stake of metal in the center of the mould A sweep pattern can be used for both green & dry sand moulding

66 Metal Casting Processes 145 (j) Skeleton pattern A skeleton pattern is the skeleton of desired shape which may be a S-bend or a chute or something else The skeleton is made from wooden strips and is thus a wooden frame work When only a small number of large and heavy castings are to be made, it is not economical to make a solid pattern In such cases, however, a skeleton pattern may be used This is a ribbed construction of wood which forms an outline of the pattern to be made This frame work is filled with loam sand and rammed The surplus sand is removed by strickle board For round shapes, the pattern is made in two halves which are joined with glue or by means of screws etc (k) Segmental (or) part pattern Patterns of this type are generally used for circular castings, for example wheel rim, gear blank etc Such patterns are sections of a pattern so arranged as to form a complete mould by being moved to form each Pivot Fig122 Segmental (or) part pattern

67 146 Manufacturing Technology I - wwwairwalkbookscom section of the mould The movement of segmental pattern is guided by the use of a central pivot A segment pattern for a wheel rim is shown in Fig Design Considerations for a good Pattern A good pattern produces a sound casting and a poor pattern will produces poor castings The following are design considerations for a good pattern A pattern should be accurate in its dimensions and posses very good surface finish Proper material selection of pattern A pattern should carry all proper allowances In split pattern, parting surface should be such that maximum portion of pattern is in drag All sharp edges and corners should be rounded Changes in the section thickness should be smooth, gradual and uniform It reduces stresses, strains and minimizes crack formation Type of Pattern selection should be proper Jointed core should be avoided to obtain uniform holes Core prints provided with pattern should be of optimum size and suitably located All patterns for repeat orders should be coated with preservatives 184 PATTERN MATERIALS Patterns may be constructed from the following materials Each material has its own advantages, limitations, and field of application Some materials used for making patterns are: wood, metals and alloys, plastic, plaster of Paris, plastic and rubbers, wax, and resins Wood Wood is the most popular and commonly used material for pattern making The main varieties of woods used in pattern-making are shisham, kail, deodar, teak and mahogany

68 Metal Casting Processes 147 Advantages of wooden patterns Wood is cheap, easily available in abundance, repairable and easily fabricated in various forms using resin and glues It is very light and can produce highly smooth surface It can preserve its surface by application of a shellac coating for longer life of the pattern Disadvantages Wood is susceptible to shrinkage and warpage and its life is short It is highly affected by moisture of the moulding sand After some use it warps and wears out quickly as it is having less resistance to sand abrasion It cannot withstand rough handling and is weak in comparison to metal Wooden patterns are preferred only when the number of castings to be produced is less Metal Pattern Metallic patterns are preferred when the number of castings required is large The wear and tear of this pattern is very less and hence posses longer life Metal pattern is easy to be shaped with good precision, surface finish and intricacy in shapes It can withstand against corrosion and handling for longer period It possesses excellent strength to weight ratio These patterns are not much affected by moisture of the sand The main disadvantages of metallic patterns are higher cost, higher weight and tendency of rusting The metals commonly used for pattern making are cast iron, brass and bronzes and aluminum alloys Cast Iron Cast iron is cheaper, stronger, tough, and durable and can produce a smooth surface finish It also possesses good resistance to sand abrasion The drawbacks of cast iron patterns are that they are hard, heavy, brittle and get rusted easily in presence of moisture

69 148 Manufacturing Technology I - wwwairwalkbookscom Brasses and Bronzes These are heavier and expensive than cast iron and hence are preferred for manufacturing small castings They possess good strength, machinability and resistance to corrosion and wear They can produce a better surface finish Brass and bronze patterns are finding applications in making match plate patterns Aluminum Alloys Aluminum alloy patterns are more popular and best among all the metallic patterns because of their high lightness, good surface finish, low melting point and good strength They also posses good resistance to corrosion and abrasion by sand and there by enhancing longer life of pattern These materials do not withstand rough handling They have poor repairability and are preferred for making large castings Advantages Aluminum alloy patterns do not rust They are easy to cast They are light in weight They can be easily machined Disadvantages They can be damaged by sharp edges They are softer than brass and cast iron Their storing and transportation needs proper care White Metal White metal is an alloy of Antimony, Copper and Lead It is best used for lining and stripping plates Its melting point is around 260C, it can be cast into narrow cavities The disadvantages of this white metal are that it is too soft, it s storing and transportation needs proper care and it wears away by sand or sharp edges

70 Metal Casting Processes 149 Plastic Patterns made of plastics materials are lighter, stronger, moisture and wear resistant, non-sticky to moulding sand, durable and they are not affected by the moisture of the moulding sand Moreover they impart very smooth surface finish on the pattern surface These materials are somewhat fragile, less resistant to sudden loading and their section may need metal reinforcement The plastics used for this purpose are thermosetting resins Phenolic resin plastics are commonly used These are originally in liquid form and get solidified when heated to a specified temperature To prepare a plastic pattern, a mould in two halves is prepared in plaster of paris with the help of a wooden pattern known as a master pattern The phenolic resin is poured into the mould and the mould is subjected to heat The resin solidifies giving the plastic pattern Recently a new material has stepped into the field of plastic which is known as foam plastic Plaster Plaster belongs to gypsum family which can be easily cast and worked with wooden tools and preferable for producing highly intricate casting The main advantages of plaster are that it has high compressive strength and is of high expansion setting type which compensate for the shrinkage allowance of the casting metal Plaster of paris pattern can be prepared either by directly pouring the slurry of plaster and water in moulds prepared earlier from a master pattern or by sweeping it into desired shape or form by the sweep and strickle method It is also preferred for production of small size intricate castings and making core boxes Wax Wax patterns are excellent for investment casting process The materials used are blends of several types of waxes, and other additives which act as polymerizing agents, stabilizers, etc

71 150 Manufacturing Technology I - wwwairwalkbookscom The commonly used waxes are paraffin wax, shellac wax, bees-wax, cerasin wax, and micro-crystalline wax The properties desired in a good wax pattern include low ash content up to 005 per cent, resistant to the primary coat material used for investment, high tensile strength and hardness, and substantial weld strength Wax patterns are made by injecting liquid or semi-liquid wax into a split die Solid injection is also used to avoid shrinkage and for better strength Wax use helps in imparting a high degree of surface finish and dimensional accuracy of castings Wax patterns are prepared by pouring heated wax into split moulds or a pair of dies The dies after having been cooled down are parted off Now the wax pattern is taken out and used for moulding Such patterns need not be drawn out solid from the mould After the mould is ready, the wax is poured out by heating the mould and keeping it upside down Such patterns are generally used in the process of investment casting where accuracy is linked with intricacy of the cast object 185 Selection of pattern material The following factors must be taken into consideration while selecting pattern materials Number of castings to be produced Metal patterns are preferred when castings are required large in number Type of mould material used & type of moulding process Method of moulding (hand or machine) Degree of dimensional accuracy, stability and surface finish required Minimum thickness required Shape, complexity and size of casting

72 Metal Casting Processes 151 Resistance to wear and abrasion, resistance to corrosion, and to chemical reactions Low cost of production 186 PATTERN ALLOWANCES Pattern may be made from wood (or) metal and its size may not be same as that of the casting A pattern is always larger in size as compared to the final casting because it carries allowances due to metallurgical reasons (shrinkage on cooling) and mechanical reasons (machining, distortion, draft, shake, sharp edges etc,) These various allowances given to pattern can be enumerated as, allowance for shrinkage, allowance for machining, allowance for draft, allowance for rapping or shake, allowance for distortion and allowance for mould wall movement These allowances are discussed below (a) Shrinkage allowance All common cast metals shrink a significant amount when they are cooled from the molten state The total contraction in volume is divided into the following parts: 1 Liquid contraction, ie the contraction during the period in which the temperature of the liquid metal or alloy falls from the pouring temperature to the liquidus temperature 2 Contraction on cooling from the liquidus to the solidus temperature, ie solidifying contraction 3 Solid contraction: Contraction that results there after until the temperature reaches the room temperature The first two of the above are taken care of by proper gating and risering Only the last one, ie the solid contraction is taken care by the pattern makers by giving a positive shrinkage allowance This contraction allowance is different for different metals

73 152 Manufacturing Technology I - wwwairwalkbookscom The Shrinkage allowances for different metals and alloys are Cast Iron-10 mm/mt, Brass-16 mm/mt, Aluminium Alloys-15 mm/mt, Steel- 21 mm/mt, Lead-24 mm/mt The Metal shrinkage depends upon the metal /alloy being casted, Pouring temperature, Casting dimensions, Casting design aspects, Moulding conditions (mould material and moulding methods) (b) Machining allowance Machining allowances is a positive allowance given to compensate for the amount of material that is lost in machining or finishing the casting If this allowance is not given, the casting will become undersize after machining Machining allowance is given due to the following reasons: 1 Castings get oxidised inside mould and during heat treatment Scale thus formed requires to be removed 2 For removing surface roughness, slag, dirt and other imperfections from the casting 3 For obtaining exact dimensions on the casting 4 To achieve desired surface finish on the casting The amount of this allowance depends on nature of metal, the size and shape of casting, methods of machining (grinding, turning milling boring etc,), casting condition, moulding process involved, number of cuts to be taken and the degree of finish In general, however, the value varies from 3 mm to 12 mm (c) Draft (or) Taper allowance Taper allowance (Fig 123) is also a positive allowance and is given on all the vertical surfaces of pattern so that its withdrawal becomes easier The normal amount of taper on the external surfaces varies from 10 mm to 20 mm/mt On interior holes and recesses which are smaller in size, the taper should be around 60 mm/mt These values are greatly affected by the size

74 Metal Casting Processes 153 Pattern No allowance Parting line Pattern with allowance Cracks in mould (a) (b) Fig123 Draft (or) taper allowance (a) Without allowance (b) With allowance of the pattern and the moulding method In machine moulding, its value varies from 10 mm to 50 mm/mt Fig 123 shows Draft allowance (d) Rapping (or) Shake allowance Before withdrawing the pattern, it is rapped and thereby the size of the mould cavity increases Actually by rapping, the external sections move outwards increasing the size and internal sections move inwards decreasing the size This movement may be insignificant in the case of small and medium size castings, but it is significant in the case of large castings This allowance is kept negative and hence the pattern is made slightly smaller in dimensions mm (e) Distortion (or) Camber allowance Required Shape of Casting Distorted Casting (i) Cambered Pattern (a) (b) (c) (ii) Fig 124 Distortion allowance (i) I-section (ii)(a) Without camber, (b) Actual casting, (c) With camber allowance

75 154 Manufacturing Technology I - wwwairwalkbookscom This allowance is applied to the castings which have the tendency to distort during cooling due to thermal stresses developed For example a casting in the form of U shape will contract at the closed end on cooling, while the open end will remain fixed in position Therefore, to avoid the distortion, the legs of U pattern must converge slightly so that the sides will remain parallel after cooling Another example is I shaped channel where to avoid distortion cambered is given (Fig 124) (f) Mould wall movement allowance Mould wall movement in sand moulds occurs as a result of heat and static pressure on the surface layer of sand at the mould metal interface In ferrous castings, it is also due to expansion due to graphitization This enlargement in the mould cavity depends upon the mould density and mould composition This effect becomes more pronounced with increase in moisture content and temperature 19 CORES Cores are compact mass of core sand prepared separately, when placed in mould cavity at required location It does not allow the molten metal to occupy space for solidification in that portion and hence help to produce hollowness in the casting The environment in which the core is placed is much different from that of the mould In fact the core has to withstand the severe action of hot metal which completely surrounds it 191 Functions (or) Objectives of core Core produces hollowness in castings in the form of internal cavities It must be sufficiently permeable to allow the easy escape of gases during pouring and solidification It may form a part of green sand mould It may provide external undercut features in casting It may be inserted to achieve deep recess in the casting It may be used to strengthen the mould It may be used to form gating system of large size mould

76 Metal Casting Processes Core Sand Core Sand is a special kind of moulding sand The main constituents of the core sand are pure silica sand and a binder Silica sand is preferred because of its high refractoriness For higher values of permeability, sands with coarse grain size distribution are used The main purpose of the core binder is to hold the grains together, impart strength and sufficient degree of collapsibility Beside these properties needed in the core sand, the binder should be such that it produces minimum amount of gases when the molten metal is poured in the mould Although, in general the binders are inorganic as well as organic, but for core making, organic binders are generally preferred because they are combustible and can be destroyed by heat at higher temperatures thereby giving sufficient collapsibility to the core sand 193 Considerations in Selecting Core Sand Keeping above mentioned objectives in view, the special considerations should be given while selecting core sand These considerations involve The cores are subjected to a very high temperature and hence the core sand should be highly refractory in nature The permeability of the core sand must be sufficiently high as compared to that of the moulding sands so as to allow the core gases to escape through the limited area of the core recesses generated by core prints The core sand should not possess such materials which may produce gases while they come in contact with molten metal and The core sand should be collapsible in nature, ie it should disintegrate after the metal solidifies, because this property will ease the cleaning of the casting 194 Binders for core sand The common binders which are used in making core sand are as follows:

77 156 Manufacturing Technology I - wwwairwalkbookscom (a) Cereal binder It develops green strength, baked strength and collapsibility in core The amount of these binders used varies from 02 to 22% by weight in the core sand (b) Protein binder It is generally used to increase collapsibility property of core (c) Thermosetting resin Thermosetting resins are popular because, it imparts high strength, collapsibility to core sand and it also evolves minimum amount of mould and core gases which may produce defects in the casting The most common binders under this group are phenol formaldehyde and urea formaldehyde (d) Sulphite binder Sulphite binder is also sometimes used in core but along with certain amount of clay (e) Dextrin Dextrin is commonly added to core sand for increasing collapsibility and baked strength of core (f) Pitch Pitch is widely used to increase the hot strength of the core (g) Molasses Molasses is generally used as a secondary binder to increase the hardness on baking It is used in the form of molasses liquid and is sprayed on the cores before baking (h) Core oil Core oil in liquid state, when it is mixed with the core sand, forms a coherent solid film holding the sand grains together when it is baked 195 Core Making Stages in core making Core making basically is carried out in five stages namely Core Sand Preparation, Core Making,

78 Metal Casting Processes 157 Core Baking Core Finishing Setting the cores (a) Core Sand Preparation Preparation of core to get better and uniform core sand properties, satisfactory and homogenous mixture of core using proper sand constituents and additives, the core sands are generally mixed with the help of any of the following mechanical means namely roller mills, core sand mixer using vertical revolving arm type and horizontal paddle type mechanisms These machines perform the mixing of core sand constituents most thoroughly (b) Core Making Process Small cores are made manually in hand rammed core boxes Cores on mass scale are rapidly produced on a variety of core making machines to name a few core blowing, core ramming and core extrusion machines I Hand making of cores Small sized cores for limited production are made manually in hand filled core boxes The steps involved in making core by hand are: Place the core box on work bench and it is filled with the already mixed and prepared core sand and rammed by hand and the extra sand is removed (weak cores are reinforced using steel wires) Core box is inverted over the core plate to transfer the core to the plate It is then baked in oven for specified period and then removed and cooled Now the core is ready II Core making machines (i) Core blowing machines The basic principle of core blowing machine comprises of filling the core sand into the core box by using compressed air (at 5 to 7 bar pressure) (Fig 125)

79 158 Manufacturing Technology I - wwwairwalkbookscom Compressed Air Magazine Sand Vent holes Core Box with Vent holes Floor Mounting Fig125 Core Blowing Machine The velocity of the compressed air is kept high to obtain a high velocity of core sand particles, thus ensuring their deposit in the remote corners of the core box On entering the core sand with high kinetic energy, the shaping and ramming of core is carried out simultaneously in the core box The core blowing machines can be further classified into two groups namely small bench blowers and large floor blowers

80 Small bench blowers are quite economical for core making shops having low production and are bench type Big core blowing machines are either vertical or horizontal floor mounted type The cartridge oriented sand magazine is considered to be a part of the core box equipment However, one cartridge may be used for several boxes of approximately the same size The cartridge is filled using hands Then the core box and cartridge are placed in the machine for blowing and the right handle of the machine clamps the box and the left handle blows the core (ii) Core drawing/extrusion machines Uniform cross section and regular simple cores are extruded by core extrusion machine Metal Casting Processes 159 Cores of square, round, hexagonal and oval section are widely produced by core extrusion machine Fig 126 shows a core extrusion machine which consists of hopper through which core sand is fed to horizontal spiral conveyor As the spiral conveyor is rotated it forces core sand through a die of specified shape (square, round etc,) Core Sand Die Core out Spiral conveyor Hopper Power or Hand Driven Bench Fig126 Core Extrusion machine

81 160 Manufacturing Technology I - wwwairwalkbookscom Long cores thus produced can be cut to the desired length The drawn core is then baked further before its use in mould cavity to produce hollowness in the casting (iii) Core ramming machines Cores can also be prepared by ramming core sands in the core boxes by machines based on the principles of squeezing, jolting and slinging Out of these three machines, jolting and slinging are more common for core making (c) Core Baking Once the cores are prepared, they will be baked in baking ovens or furnaces The main purpose of baking is to drive away the moisture and harden the binder, thereby giving strength to the core Core are baked upto 380C Core baking develops the properties of organic binders At 100C moisture is driven out and at 125C to 225C Core oil and other organic binders change chemically and molecularly from liquid to solid state The core drying equipments are usually of two kinds namely core ovens and dielectric bakers The core ovens are of two types namely continuous type oven and batch type oven Continuous type ovens: Continuous type ovens are preferred basically for mass production In these types, core carrying conveyors or chain move continuously through the oven The baking time is controlled by the speed of the conveyor The continuous type ovens are generally used for baking of small cores Batch type ovens: Batch type ovens are mainly utilized for baking variety of cores in batches The cores are commonly placed either in drawers or in racks which are finally placed in the ovens The core ovens and dielectric bakers are usually fired with gas, oil or coal

82 Dielectric bakers: These bakers are based on dielectric heating The core supporting plates are not used in this baker because they interfere with the potential distribution in the electrostatic field To avoid this interference, cement bonded asbestos plates may be used for supporting the cores The main advantage of these ovens is that they are faster in operation and a good temperature control is possible with them After baking of cores, they are smoothened using dextrin and water soluble binders (d) Core Finishing Core finishing is done after braking, before it is finally set in the mould The fins, bumps or other sand projections are removed from the surface of the cores by rubbing or filing The dimensional inspection of the cores is very necessary to achieve sound casting Cores are also coated with refractory or protective materials using brushing, dipping and spraying means to improve their refractoriness and surface finish The coating on core prevents the molten metal from entering into the core Bars, wires and arbors are generally used to reinforce core from inside (e) Setting of cores Setting of cores means positioning the cores in the mould Mould Cavity Core Seat 4 Chaplets Sand Fig 127 Chaplets in setting of cores Core Metal Casting Processes 161

83 162 Manufacturing Technology I - wwwairwalkbookscom In order to obtain correct cavities in the casting the cores must be accurately positioned Core in mould should be firmly secured so that they can withstand the buoyancy effect of the molten metal poured Small cores are set by hand while large ones by crane Cores are supported by chaplets to avoid sagging, shifting or sinking of the cores Chaplets are metal shapes which are positioned between mould and core surfaces Chaplets are made of the same materials as that of the cast Fig 127 Various forms of chaplets are commercially available 196 TYPES OF CORES AND APPLICATIONS Cores are classified according to (a) State (or) Condition of core Green sand core and Dry sand core (b) Nature of core materials employed core Oil bonded core, Resin bonded core, Shell core and Sodium silicate (c) Type of core hardening process employed CO 2 Process, Hot box core, cold set process, Oil No-Bake core (d) Shape and position of core Horizontal core, Vertical core, Hanging core, Balanced core, Drop core (i) Green Sand Cores Green sand cores are made by green sand containing moist condition about 5% water and % clay It imparts very good permeability to core and thus avoids defects like shrinkage or voids in the casting They are used in green condition and are generally preferred for simple, small and medium castings

84 Metal Casting Processes 163 Such cores possess less strength in comparison to dry sand cores and hence cannot be stored for longer period (ii) Dry Sand Cores Dry sand cores are produced by drying the green sand cores to about 110C These cores possess high strength rigidity and also good thermal stability These cores can be stored for long period and are more stable than green sand core They are used for large castings They also produce good surface finish in comparison to green sand cores They can be handled more easily They resist metal erosion These types of cores require more floor space, more core material, high labour cost and extra operational equipment (iii) Oil bonded cores Sand cores are produced by mixing silica sand with small percentage of linseed oil Oil bonded cores are based on the principle of Oxidation and Polymerisation of oils containing chemical additives which can be activated by an oxygen bearing material set in a predetermined time (iv) Shell Core Shell cores are made as follows: The core box is heated to a temperature of around 400C to 600C Sand mixed with 2-5 % of thermosetting resin (Phenolic type) is dumped / blown to the above heated core box The resin is allowed to melt to the specified thickness The resin gets cured The excess sand is removed The hardened core is extracted from box and does not require further baking Shell core possess very smooth surfaces and very close tolerances High permeability is achieved in shell cores They can be easily stored and are very costly

85 164 Manufacturing Technology I - wwwairwalkbookscom (v) Sodium silicate CO 2 Cores Clean, dry sand with sodium silicate is rammed into core box CO 2 gas is passed for several seconds through the above mixture as a result silica gel is formed which binds sand grains into strong solid form Na 2 SiO 3 CO 2 Na 2 CO 3 SiO 2 Silica gel Core thus formed does not require baking and have more strength than oil bonded / resin cores Cores formed by CO 2 process are used in production of cast iron, steel, aluminium and copper based alloys The used sand cannot be recovered and reused (vi) Hot Box Core This uses heated core boxes (125C 225C) for producing cores Core boxes are made up of cast iron, steel, aluminium and possess vents and ejectors for removing core gases Heated core boxes are used with core sand mixtures employing liquid resin binders and a catalyst (vii) Cold set Core Cold set cores are prepared by mixing binder with the accelerator This sand mixture has high flowability and can easily be rammed Curing starts immediately as soon as accelerator is added and continued until core becomes strong With little heating the core hardens completely This is preferred for jobbing operation and producing very large cores (viii) Oil No-Bake core This process employs a synthetic oil binder which when mixed with sand, chemically (Polymerisation reaction) reacts and produces core that can be cured at room temperature The sand, binder/catalyst, oil-no bake agents weight by ratio is 500kg : 7kg : 14kg The process assures better depth, fast breaking, easier core withdrawal and lower costs (ix) Horizontal core It is produced horizontally in the mould It can be of any shape based on cavity of cast It is supported in core seats at both ends These are placed at parting line These are commonly used in foundries (Fig 128)

86 Metal Casting Processes 165 Core Cope Mould Core Seat Parting line Sand Drag Fig 128 Horizontal Core (x) Vertical core In vertical core, Cope side has more taper than in drag so as not to tear the sand while assembling cope and drag It is placed vertically in the mould cavity Core is supported by core seat at both ends Major portion of core remains in drag (Fig 129) Cope Core Sand Core Seat Parting line Mould Drag Fig 129 Vertical core (xi) Hanging Core or Cover core Hanging core is supported from above only and it hangs vertically in the mould cavity It is provided with a hole through which metal can flow

87 166 Manufacturing Technology I - wwwairwalkbookscom in the mould cavity It is called hanging because it hangs from above and if it is covering the mould it is called cover core (Fig 130) Core Hole Cope Core Sand Parting line Mould Drag Fig 130 Hanging Core (xii) Balanced core Balanced core is a core supported and balanced from one end only It requires a big core seat for support and does not sag or sink or fall Balanced core may be supported by chaplets (Fig 131) Chap let Cope Core Sand Parting line Mould Drag Fig 131 Balanced Core

88 Metal Casting Processes 167 (xiii) Drop (or) Stop off core A stop off core is employed to make a cavity which cannot be made with other cores A stop off core is used when a cavity is not in line with parting surface rather it is above or below (Fig 132) Cope Core Sand Parting line Mould Drag Fig 132Stop off core 110 MOULDING MACHINES Moulding machine acts as a device by means of a large number of co-related parts and mechanisms, transmits and directs various forces and motions in required directions so as to help the preparation of a sand mould The major functions of moulding machines involve ramming of moulding sand, rolling over or inverting the mould, rapping the pattern and withdrawing the pattern from the mould Most of the moulding machines perform a combination of two or more of functions 1101 Types and Applications of Moulding Machines Moulding machines can be classified as Squeezer Machine, Jolt Machine, Jolt-Squeezer Machine, Slinging Machines, Pattern Draw Machines These varieties of machines are discussed below

89 168 Manufacturing Technology I - wwwairwalkbookscom (a) Squeezer machine These machines may be hand operated or power operated The pattern is placed over the machine table, followed by the moulding box Fig 133 Squeezer head Sand Flask Pattern Mould board Tab le In hand-operated machines, the table of machine is lifted by hand operated mechanism In power machines, it is lifted by the air pressure on a piston in the cylinder in the same way as in jolt machine The table is then raised gradually The sand in the moulding box is squeezed between plate and the upward rising table thus enabling a uniform pressing of sand in the moulding box More pressure can be applied in power operated machines (b) Jolt machine This machine is also known as jar machine which comprises of air operated piston and cylinder Fig 133 Squeezer Machine The air is allowed to enter from the bottom of the cylinder and acts on the bottom face of the piston to raise it up The (platen or) table of the machine is attached at the top of the piston which carries the pattern and moulding box with sand filled in it

90 Metal Casting Processes 169 The upward movement of piston raises the table to a certain height (30 to 80 mm) and the air below the piston is suddenly released and the table drops down suddenly and strikes the guiding cylinder at bottom This sudden action causes the sand to pack evenly around the pattern Springs are used to cushion the table blows and thus reduce noise and prevent destruction of mechanism and foundation This process is repeated several times rapidly This operation is known as jolting technique (c) Jolt-squeezer machine It uses the principle of both jolt and squeezer machines in which complete mould is prepared The cope, match plate and drag are assembled on the machine table in a reverse position, that is, the drag on the top and the cope below Pattern Sand Flask Tab le Hose Plunger Channel Air opening Guiding cylinder Spring Fig 134 Jolt Machine

91 170 Manufacturing Technology I - wwwairwalkbookscom Initially the drag is filled with sand followed by ramming by the jolting action of the table After leveling off, the sand on the upper surface, the assembly is turned upside down and placed over a bottom board placed on the table Next, the cope is filled up with sand and is rammed by squeezing between the overhead plate and the machine table The overhead plate is then swung aside and sand on the top is leveled off, cope is next removed and the drag is vibrated by air vibrator This is followed by removal of match plate and closing of two halves of the mould for pouring the molten metal This machine is used to overcome the drawbacks of both squeeze and jolt principles of ramming moulding sand (d) Slinging machines These machines are also known as sand slingers and are used for filling and uniform ramming of moulding sand in moulds In the slinging operations, the consolidation and ramming are obtained by impact of sand which falls at a very high velocity on pattern A typical sand slinger consists of a heavy base, a bin or hopper to carry sand, a bucket elevator to which a number of buckets are attached and a swinging arm which Blade Fig 135 Sand Slinger Conveyor buckets Housing Mould Pattern Tab le Board carries a belt conveyor and the sand impeller head Well prepared sand is filled in a bin through the bottom of which it is fed to the elevator buckets These buckets discharge the moulding sand to the belt conveyor which conveys the same to the impeller head This head can be moved at any location on the mould by swinging the arm The head revolves at a very high

92 Metal Casting Processes 171 speed and, in doing so, throws a stream of moulding sand into the moulding box at a high velocity This process is known as slinging The force of sand ejection and striking into the moulding box compels the sand to get packed in the box flask uniformly By this way, the satisfactory ramming is automatically get completed on the mould (e) Pattern draw machines These machines enable easy withdrawal of patterns from the moulds They can be of the kind of stripping plate type and pin lift or push off type Stripping plate type of pattern draw machines consists of a stationary (platen or) table on which is mounted a stripping plate which carries a hole in it The pattern is secured to a pattern plate and the latter to the supporting ram The pattern is drawn through the stripping plate either by raising the stripping plate or by keeping the stripping plate and mould stationary by moving the ram downwards along with the pattern plate 111 MELTING FURNACES The metal to be casted has to be in the molten or liquid state before pouring into the mould A furnace is used to melt the metal A foundry furnace only remelts the metal to be casted, it does not convert ore into useful metal A blast furnace performs basic melting (of iron ore) operation to get pig iron Different furnaces are used for melting and re-melting ferrous and nonferrous materials 1111 Factors responsible for the selection of furnace Considerations of initial, repair, maintenance and operation costs Availability and relative cost of various fuels in the particular locality Melting efficiency, speed of melting Composition and melting temperature of the metal

93 172 Manufacturing Technology I - wwwairwalkbookscom Degree of quality control required in respect of metal purification of refining Cleanliness and noise level in operation Method used for pouring desired metal Chances of metal to absorb impurities during melting 1112 Types of Furnaces Furnaces for melting different materials are given below For Grey Cast Iron Cupola furnace, Air furnace (or Reverberatory Furnace), Rotary furnace and Electric arc furnace For Steel Electric furnaces, Open hearth furnace and Converter For Non-ferrous Metals (a) (b) (c) (d) (e) (f) Reverberatory furnaces (fuel fired) (Al, Cu) - Stationary and Tilting furnaces Rotary furnaces - Fuel fired & Electrically heated Induction furnaces (Cu, Al) - Low frequency & High frequency Electric Arc furnaces (Cu) Crucible furnaces (Al, Cu) - Pit type, Tilting type, Non-tilting or bale-out type & Electric resistance type (Cu) Pot furnaces (fuel fired) (Mg and Al) - Stationary and Tilting 1113 Blast Furnace Various kinds of mined iron ores are refined and converted to pig iron in the blast furnace A typical blast furnace along with its various parts is shown in Fig 136 It is large steel shell about 9 mt in diameter which is lined with heat resistant bricks It is set on the top of brick foundation There are four major parts of blast furnace from bottom to top hearth, bosh, stack and top

94 Metal Casting Processes 173 Sm all bell Top 300 o F Ore Coke Limestone Ore Coke Reduction Large bell Shell Refractory lining Stack Head absorp Blast pipe Tuyere 3000 o F Fusion Combustion Molten slag Molten iron Cinder notch Bosh Hearth Tap hole Bed Fig 136 Blast Furnace The hearth acts as a storage region for molten metal and molten slag The charge of blast furnace possesses successive layers of iron ore, scrap, coke, and limestone and some steel scrap which is fed from the top of the furnace Iron ore exists as an aggregate of iron-bearing minerals These mineral aggregates are oxides of iron called hematite, limonite, and magnetite They all contribute to the smelting process

95 174 Manufacturing Technology I - wwwairwalkbookscom It takes about 16 tons of iron ore, 065 ton of coke, 02 ton of lime-stone and about 005 ton of scrap iron and steel to produce 1 ton of pig iron For burning this charge, about 4 tons of air is requiredthe impurities or other minerals are present in the ore may be silicon, sulphur, phosphorus, manganese, calcium, titanium, aluminum, and magnesium The output from the furnace in form of pig iron is collected in large ladles from the tap hole existing at lower portion of furnace As the coke burns, aided by the air forced into the furnace, the ore melts and collects in the hearth As the melting process proceeds, the entire mass settles and thus makes room for the addition of charges at the top While the melting is going on, the limestone forms a slag with the impurities Coke supplies the heat which reduces the ore and melts the iron The iron picks up carbon from the coke and impurities from the ore The carbon becomes part of the pig iron used in the making of steel The pig iron is then processed for purification work for production of various kinds of iron and steel in the form of ingots (large sections) using different furnaces 1114 Cupola Furnace Cupola furnace is employed for melting scrap metal or pig iron for production of various cast irons It is also used for production of nodular and malleable cast iron It is available in good varying sizes The main considerations in selection of cupola s are melting capacity, diameter of shell without lining or with lining, spark arrester Cupolas are also used for melting some copper alloys also

96 Preheating zone S lack zone Metal Casting Processes Construction of cupola Spark arrester Furnace Charging door 1 Coke 2 Flux 3 Metal Stage Steel shell R efractory lining Air box Reducing zone Melting zone Air blast inlet Combustion zone Tuyeres Fettling hole Well Tapping hole Slag bottom Drop bottom Sand bottom Legs Fig 137 Cupola furnace

97 176 Manufacturing Technology I - wwwairwalkbookscom A cupola is a cylindrical shell either welded or riveted from a boiler plate of 6-10 mm thick and is open both at its top and bottom, lined with firebrick and clay supported on cast iron legs Fig 137 It has cast iron door at the bottom to open or close and used to drop the left out contents of cupola Air from blower comes through the blast pipe and enters wind box which surround the cupola and supplies air uniformly to tuyeres Tuyeres extend through the steel shell and refractory wall to the combustion zone and supply air necessary for combustion There is a tap hole at the bottom from where molten metal is taken out to pour in mould The fire in cupola is also lit through the tap hole Slag hole is present opposite and above the tap hole It is 250 mm below tuyeres Slag is removed from slag hole Cupola is provided with charging platform and charging door at suitable height to feed the charge Capacity of cupola varies from 1 to 15 tons They are 6 m in height and diameters vary from 750mm to 25mts Sometimes they are fitted with collector, filter and precipitator to minimize pollution Various Zones of Cupola Furnace Various chemical reactions taking place in different zones of cupola are: (a) Well The space between the bottom of the tuyeres and the sand bed inside the cylindrical shell of the cupola is called as well of the cupola As the melting occurs, the molten metal is collected in this portion before tapping out

98 Metal Casting Processes 177 (b) Combustion zone The combustion zone of Cupola is also called as oxidizing zone Combustion of coke takes place in this zone It is located between the upper of the tuyeres and a theoretical level above it The total height of this zone is normally from 150 mm to 300 mm The combustion actually takes place in this zone by consuming the free oxygen completely from the air blast and generating tremendous heat The heat generated in this zone is sufficient enough to meet the requirements of other zones of cupola The heat is also further evolved due to oxidation of silicon and manganese A temperature of about 1540C to 1870C is achieved in this zone A few of the exothermic reactions that take place in this zone are represented as : C O 2 CO 2 Heat Si O 2 SiO 2 Heat 2Mn O 2 2MnO Heat (c) Reducing zone Reducing zone of Cupola is also known as the protective zone which is located between the upper level of the combustion zone and the upper level of the coke bed In this zone, CO 2 is changed to CO through an endothermic reaction, as a result of which the temperature falls from combustion zone temperature to about 1200C at the top of this zone The important chemical reaction that takes place in this zone is given below CO 2 C coke 2CO Heat Nitrogen does not participate in the chemical reaction occurring in this zone as it is also the other main constituent of the upward moving hot gases Because of the reducing atmosphere in this zone, the charge is protected against oxidation (d) Melting zone The lower layer of metal charge above the lower layer of coke bed is termed as melting zone of Cupola The metal charge starts melting in this zone and trickles down through coke bed and gets collected in the well

99 178 Manufacturing Technology I - wwwairwalkbookscom Sufficient carbon content picked by the molten metal in this zone is represented by the chemical reaction given below 3Fe 2CO Fe 3 C CO 2 (e) Preheating zone Preheating zone starts from the upper end of the melting zone and continues up to the bottom level of the charging door This zone contains a number of alternate layers of coke bed, flux and metal charge The main objective of this zone is to preheat the charges from room temperature to about 1090C before entering the metal charge to the melting zone The preheating takes place in this zone due to the upward movement of hot gases During the preheating process, the metal charge in solid form picks up some sulphur content in this zone (f) Stack The empty portion of cupola above the preheating zone is called as stack It provides the passage to hot gases to go to atmosphere from the cupola furnace Operation / Working of Cupola The various steps in operating cupola are: Preparation of cupola Lightening of cupola Charging of cupola Melting Slagging and metal tapping Dropping down Preparation Bottom doors are opened and the contents (unburned coke, slag, metal) of previous melting are dumped under furnace and removed Slag, coke, iron sticking to the side walls of the furnace are chipped off Damaged bricks are replaced and damaged refractory lining is patched up and then bottom doors are closed

100 Metal Casting Processes 179 Lightening Cupola is fired 3-4 hrs before molten metal is needed Soft, dry wood are placed on the sand bed rammed above the bottom door Coke is placed above the wooden pieces till the tuyeres Wood is ignited through tap hole Charging of Cupola After the coke bed is properly ignited, the cupola is charged from the charging doorcharging of cupola involves adding of alternate layers of limestone (flux), metal (iron) and fuel (coke) up to the level of charging door Flux is a substance aiding in formation of slag for removing impurities Commonly used flux are limestone, others are sodium carbonate, fluorospar (CaF 2 ), calcium carbide and dolomite Metal charge may consist of pig iron, cast iron scrap and steel scrap Melting After charging, a soak period of minutes is given to charge for preheating At the end of soaking period the blast is turned on The coke becomes fairly hot to melt the metal charge Now molten metal starts accumulating in the hearth and appears at tap hole Tap hole is closed with a plug and metal is allowed to collect after five minutes Slagging and metal tapping After enough slag is accumulated the slag hole is opened and the slag is collected in a container and disposed off The plug is knocked off from the tap hole and the molten metal is tapped pouring into the mould

101 180 Manufacturing Technology I - wwwairwalkbookscom Dropping down As cupola heat charging is stopped, all the content of cupola is allowed to melt till one or two charge is left above coke bed Now the air blast is switch off and the prop under bottom door is knocked down and the remains in cupola are dropped down on the floor or collected in bucket The dropped cupola remains are quenched with water and then metal, coke remains are recovered for next use Applications of Cupola Cupola is most widely used for melting practices for production of grey cast iron, nodular cast iron, malleable cast iron and alloy cast iron It can be used for melting some copper-base alloys It can be used in duplexing and triplexing operations for making of steel, malleable cast iron and ductile cast iron Thermal Efficiency of Cupola Furnace Thermal efficiency of cupola furnace is the ratio of heat actually utilized in melting and superheating the metal to the heat evolved in it through various means The total heat evolved involves the heat due to burning of coke, heat evolved due to oxidation of iron, Si and Mn and heat supplied by the air blast During melting it is observed that approximately % of the evolved heat is going as waste % Thermal efficiency Heat utilized in Preheating, melting and superheating Heat evolved in the furnace Advantages of Cupola Cupola is simple and easier in construction and easy to operate Low initial cost, low operation and maintenance costs compared to other furnaces of same capacity

102 Metal Casting Processes 181 It occupies less floor space It can operate continuously for many hours Disadvantages of Cupola Close temperature control is difficult Molten iron, coke comes into contact with certain useful elements like Silicon, manganese and are lost Impurities like sulphur are picked by molten iron affecting final iron content 1115 Air Furnace or Reverberatory Furnace This furnace is also known as puddling or reverberatory furnaceair furnace is used for production of malleable cast iron and high test grey cast iron Construction Fig 138 Shows the construction of Air furnace It consists of a long rectangular structure having removable arched roof sections called Bungs over a shallow hearth made up of refractory sand damped with clay Fig 138 Air furnace or Reverberatory Furnace It resembles open hearth furnaces except that it does not have any regenerative chambers for preheating Metal is charged through bungs and temperature is less than the open hearth furnaces

103 182 Manufacturing Technology I - wwwairwalkbookscom Working The air furnace is charged (ie metal, scrap etc,) through bungs Oil or pulverized bituminous coal is placed on the fire place The air and fuel are blown through one end of furnace so that the flame passes over the metal charge The flame and hot gases heat up the air furnace roof and walls The heat reflected and radiated from the roof /walls is utilized to super heat the metal charge Acid slag protects the molten metal from direct exposure of flame The molten metal is tapped from the tap hole Difference between air furnace and cupola furnace S No Air Furnace Cupola furnace 1 Time is available to analyze the samples of molten metal and hence control of chemical composition is possible 2 Only flame not fuel comes in contact with metal and hence elements like sulphur, carbon is eliminated from molten metal 3 Melting ratio (metal to fuel) is 2:1,so high working costs 4 Air furnace supplies molten metal in large quantities in batches therefore facilitates large castings Time is not available due to continuous tapping and hence control in chemical composition is not possible Flame & fuel come in contact with molten metal and hence elements like sulphur, carbon is added in molten metal Melting ratio (metal to fuel) is 10:1, so low working costs Cupola supplies small but continuous supply of cast iron, so it can be used for large number of small castings 5 Initial costs are high Initial costs are Low 6 Capacity varies from 5 to 50 tons Capacity varies from 1 to 15 tons Advantages of Air furnaces Time is available to analyze the samples of molten metal and hence control of chemical composition is possible

104 Metal Casting Processes 183 Only flame not fuel comes in contact with metal and hence elements like sulphur, carbon is eliminated from molten metal Air furnace supplies molten metal in large quantities in batches therefore facilitates large castings Capacity varies from 5 to 50 tons Disadvantages of Air furnaces Melting ratio (metal to fuel) is 2:1, so high working costs Initial costs are high 1116 Rotary Melting Furnace Air furnace used for making malleable cast iron is difficult to operate and lot of fuel is wasted, a rotary furnace is an improvement over the same Construction A rotary furnace consists of a horizontal cylindrical steel shell lined with refractory material and mounted on rollers (Fig 139) for rotating or rocking purposes The cylindrical shell revolves completely at about 1 RPM Burner at one end burns the fuel and resulting high temperature flame melts and reheats the metal charge lying in the shell There is a tapping spout to tap the molten metal The exhaust gas is sent to the preheater to trap the flue gas energy to the incoming cold air Refractory lining Steel shell Exhaust box Fuel & air Burner Arrangement of rotation Tap ping spout Molten metal To preheater Fig 139 Rotary Furnace

105 184 Manufacturing Technology I - wwwairwalkbookscom Working Rotary furnace is charged from one of the conical ends by removing burner or exhaust box temporarily The melting (metal-to-fuel) ratio is 5 : 1 for pulverized coal and 6 : 1 for oil fuel Due to rotation of the furnace the metal get heated from the walls and is melted more efficiently and faster The burner at one end burns the fuel and the resulting high temperature flame melts and superheats the metal charge lying in the shell Since molten metal does not come in contact with the fuel (such as coal), there is no danger of carbon or sulphur pick up The slag floating on the molten iron protects the metal and its constituent elements (like Si, Mn etc) from oxidation After the melting is complete, plugged tap hole is opened and the furnace is rotated slowly until the tap hole reaches the level of molten metal in the furnace, then the same (ie metal) can be poured into a ladle In case the tap hole is not provided in the cylindrical body, the liquid metal can be tapped from the other end of the furnace Besides tapping, this end is used for charging, slagging and evacuating the products of combustion Advantages A sample of molten metal can be drawn before pouring and analyzed as regards its chemical composition which subsequently can be accurately controlled by additions if required Charge in a rotary furnace can be super-heated to high temperatures Cheap and light scrap like steel clippings or cast iron borings which cannot be used in cupola, may be added in the charge without affecting the melt quality

106 Metal Casting Processes 185 The products of combustion may be sent to a preheater or recuperator unit where they may heat the cold air to be used subsequently in the rotary furnace A rotary furnace may have a capacity ranging from 1 to 50 tons depending upon the requirements of a consumer Applications Rotary furnaces find applications in producing high-duty cast irons having alloying elements such as Mo, Ni, Cr etc which if added in cupola will suffer a loss Metals previously melted in cupola can be held and superheated in rotary furnaces 1117 Open Hearth Furnace An air furnace does not develop tempertures enough to melt steel because a large amount of heat generated by the combustion of fuel is lost in the hot waste gases which pass up the chimney For this reason, open hearth furnaces are now widely used in large steel foundries Construction Charg ing door Hearth Bath Slag Oil burner Oil Burner (Idle) Tap h ole Checker Chambers Valve Hot flue g as to Chimney Air, nlet Fig 140 Open Hearth Furnace

107 186 Manufacturing Technology I - wwwairwalkbookscom In open hearth furnace, pig iron, steel scrap etc are melted to obtain steel The hearth is surrounded by roof and walls of refractory bricks as shown in Fig 140 Open hearth furnaces range from 5 to 100 tons capacity, the popular being a 25 ton furnace Most of the open hearth furnaces are stationary but some of the units are of tilting type also It consists of a long shallow basin called the hearth (about 45 m wide, 12 m long and half metre deep) which is lined with dolomite in case of a basic process and with silica fire brick if the process is acidic Working Scrap metal, pig iron and flux are charged into the furnace through charging doors Heating is done by burning gaseous fuel (ie natural gas, producer gas or atomised oil) Fuel is fired through nozzles (ie burners working alternatively for 20 to 30 minutes from opposite ends of the hearth One of the burners thus remains idle for all times The hot gases formed pass over the hearth to its opposite end thereby the metal charge supported on the hearth is openly exposed to the flames and is converted into molten metal Molten metal is trapped by the trap doors Besides being directly exposed to the flames, metal charge is also heated by the radiations from the walls and low hot ceiling of the furnace After passing over the hearth, the products of combustion pass through one checker chamber and heat it The process then reverses, the idle burner fires the fuel, flame passes over the hearth from the opposite direction and the initially active burner becomes idle The products of combustion after sweeping over the metal charge enter the second checker chamber and heat it up Thus each checker chamber is heated up alternatively Advantages The system of preheating air and fuel known as Regenerative system, heats air to about 100C before it reaches the furnace proper The regenerative system, because of its alternating action

108 speeds up the melting of metal and develops temperatures enough to melt steel It has high thermal efficiency Metal Casting Processes 187 Before tapping the molten metal into the ladle, a sample of the same may be tested as regards its chemical composition Applications Open hearth furnace is used for melting steels, It may also be used for melting Al, Cu and their alloys in large quantities 1118 Convertor Converters are steel-making units A Converter actually converts pig or cast iron into steel by blowing air through (bessemer) or over (side blown) molten iron Converters are of two types: Air Blast Flue gas Refractory lining Wind box Tuyeres Trunnion for Tilting Shell Molten metal Fig 141 Side-blown converter

109 188 Manufacturing Technology I - wwwairwalkbookscom Bessemer or bottom blower Converters in which a blast of cold air is blown from the bottom and, Tropellas or side-blown converter in which the blast of cold air is blown from the side of the converter A side blown converter can produce high temperature molten steels ( F) which are required to pour thin sectioned steel castings Working The metal charge is melted in a cupola and it is brought to the converter for refining the same The converter is tilted down, the molten cupola iron is given a desulfurizing treatment in the ladle and is transferred to the converter which is then tilted back to the upright position Cold air is blown through tuyeres located in the side of the converter over the molten iron at a pressure of about about 035 bar The oxygen of the cold air burns (oxidizes) Mn and Si to form slag; the carbon is oxidized out of the charge (melt) and produces CO and CO 2 These oxidizing reactions are exothermic and thus no fuel is required for heating purposes; rather the heat generated by these reactions raises the temperature of the metal to about 1700C which is sufficient for pouring purposes The cold air blow is continued (only) until carbon is reduced to about 01 to 02% The colour and length of the flame originating from the converter nose gives an indication of the rate of oxidation and the order of oxidation of the elements (like C, Si and Mn) Photoelectric devices can be used to mark the end of the air blast

110 Metal Casting Processes Pit Furnace Pit furnace is a type of a furnace bath which is installed in the form of a pit and is used for melting small quantities of ferrous and non ferrous metals for production of castings It is provided with refractory inside and chimney at the top Generally coke is used as fuel Natural and artificial draught can be used for increasing the capability towards smooth operation of the furnace Fig 142 shows the typical pit furnace Concrete lining Pit Steel shell Refractory lining cover Crucible containing metal C him n ey Sliding door coke Gate Natural Draug ht Fig 142 Pit Furnace Crucible Furnaces In a crucible furnace, the metal charge is placed and melted in a crucible A crucible is made up of silicon carbide, graphite or other refractory materials and it can withstand high temperatures Fig 143 Crucibles are available in different sizes ranging from No1 to No 400 Each number indicates the amount of metal which can be handled conveniently by that crucible

111 190 Manufacturing Technology I - wwwairwalkbookscom Cover Lift-out crucible Fuel Refractory lining Support block Fig 143 (a) Lift out crucible Cover Steel shell Fuel Refractory lining Fig 143 (b) Stationery Pot A crucible furnace is though mainly used for melting of nonferrous metals and low melting point alloys, it is being used for melting cast iron and steel also

112 Metal Casting Processes 191 Cover Tilting handle Steel shell Crucible furnace Frame Fuel A crucible furnace consists of a steel shell provided with refractory (fire brick) lining inside There are three types of crucible furnaces (a) Lift out crucible (b) Stationary Pot (c) Tilting Pot furnace (c) Tilting pot furnaces Fig 143 Three types of crucible furnaces Crucible furnaces are further classified as Gas and oil fired crucible furnaces (stationary) Coke fired crucible furnace (stationary) A crucible furnace has the following advantages Low initial cost Easy to operate Low cost of fuel Advantages of an oil or gas fired crucible furnace over a coke tired furnace Oil and gas heatup more quickly than coke and provide a fast melting rate

113 192 Manufacturing Technology I - wwwairwalkbookscom The furnace heating can be immediately stopped in oil fired furnace A coke fired furnace needs an enclosed ash pan whereas nothing like that is required in oil or gas fired furnaces Floor space can be saved by using oil or gas fired furnace It requires less floor space, less labour with improved and easier control of temperature Electric Furnaces Electric furnaces are employed for the production of high quality castings, because: the furnace atmosphere can be more closely controlled, losses by oxidation can be eliminated, alloying elements can be added without fear of (their) loss (due to oxidation) composition of the melt and its temperature can be accurately controlled Electric furnaces are used for melting steels (including alloy steel ie, tool steels and stainless steels), high-test and alloy cast iron, brasses Capacity of electric furnaces ranges from 250 kg to 10 tons Types of Electric Furnaces Direct arc furnace Indirect arc furnace Resistance heating type Coreless type (or High frequency) induction furnace Core type (or Low frequency) induction furnace Direct Arc furnace Direct arc furnace has its diameter up to 6 metres and capacity of about 125 tons It re-melts steels of widely differing compositions Rating of the transformers supplying power to the arc ranges from 800 KVA to 40,000 KVA A 50 ton direct arc furnace may require arc current of the order of Amps and arc voltage of about 250V

114 Metal Casting Processes Construction A direct arc furnace consists of a heavy steel shell lined with refractory brick and silica for acid lined furnaces and magnesite for basic lined furnaces The roof of the direct arc furnace consists of steel roofing in which silica bricks are fixed in position Depending upon whether it is a two phase or three phase electric furnace, two or three graphite electrodes are inserted through the holes in the roof into the furnace All arc furnaces rest in bearings on their two sides and bearings in turn are mounted in trunnions, thus arc furnaces can be tilted backward or forward for charging, running off the slag and pouring the molten metal into the ladle Fig phase power supply Charg ing door Molten metal E E E Electrode Roof E Slag Steel shell Spout Uprig ht position Furnace in tilted position Tap ping of metal Ladle Pit Fig 144 Direct arc furnace Working The interior of the furnace (ie refractory linings, etc) is preheated before placing the metal charge (either foundry cast iron scrap or steel scrap (Fig 144)

115 194 Manufacturing Technology I - wwwairwalkbookscom Preheating is done by alternatively striking and breaking the arc between the (vertical) electrodes and used electrode pieces (removed after pre heating) kept on the hearth Once the cold charge has been placed on the hearth of the furnace, electric arc is drawn between the electrodes and the surface of the metal charge by lowering the electrodes down till the current jumps the gap between the electrode and the charge surface The arc gap between the electrode and the charge is regulated by automatic controls Three arcs burning simultaneously produce a temperature of the order of 11000F, and readily melt flux, sand and the metal scrap The slag formed due to melting of flux, sand etc covers the molten pool of metal Slag present on the top of the molten metal bath reduces its oxidation, refines the metal, and protects roof and side walls from the large amount of heat radiated from the molten metal Before pouring the liquid metal into the ladle, the furnace is tilted backward and the slag is poured off from the charging door The furnace is then tilted forward and the molten metal is emptied into ladles Advantages of Direct Arc Furnace Direct arc furnaces undertake a definite metal refining sequence Molten metal is refined to a proper analysis and is heated to a suitable pouring temperature Analysis of melt can be kept to accurate limits High thermal efficiency as high as about 70% It is easy to control the furnace atmosphere above the molten metal Alloying elements like Cr, Ni and W can be recovered from the scrap with little losses It can make steel directly from pig iron and steel scrap Arc furnace is larger and its electrical equipment is cheaper to install

116 Metal Casting Processes 195 An arc furnace is preferred for its quicker readiness for use Limitations Heating costs are higher than for other furnaces This however can be adjusted to some extent by using low cost scrap turnings or borings as metal charge Applications In general, high quality carbon steels and alloy steels in bulk are made in electric direct arc furnace Cast iron from foundry cast iron scrap Indirect Electric Arc Furnace An indirect electric arc furnace has a capacity ranging from a few Kgs to 2 Tons and is used for smaller melts An electric arc is struck between two graphite electrodes and the metal charge does not form a part of the electric circuit The metal comes in contact with hot refractory lining and picks up heat for melting from the linings and also metal charge melts because of the radiations from the arc and the hot refractory walls of the furnace and conduction from the hot refractory (wall) linings, when the furnace rocks and molten metal rolls over the same Construction An indirect arc furnace consists of a barrel type shell made up of steel plates, having refractory lining inside There are two openings for the two graphite electrodes and the third is for the charging door for feeding the metal charge into the furnace built up with pouring spout Furnace is mounted on the rollers (Refer Fig 145) which are driven by a rocking drive unit to rock the furnace back and forth during melting While the furnace rocks, liquid metal washes over the heated refractory linings and absorbs heat from them In addition during rocking, metal charge constituents get mixed up thoroughly

117 196 Manufacturing Technology I - wwwairwalkbookscom Charging door Arc Shell Refractory lining Electrode 1 Power Lead Electrode 2 Pouring Spout Molten metal Rollers Support Fig 145 Indirect Electric Arc Furnace Rocking of furnace speeds up melting, stirs the molten metal, avoids refractory linings from getting over-heated and thus increases their life The angle of rocking of furnace is adjusted in such a manner that the liquid metal level remains below the pouring spout Operation The furnace is charged with pig iron and scrap is placed above When the electric power is on, graphite electrodes are brought nearer till the current jumps and an electric arc is set up between them The heat generated in the arc is responsible for melting the charge A number of alloys one after the other can be easily melted in this type of furnace Additions of elements like Ni, Co, Cr, W, Mo, V etc can be made easily and conveniently

118 Metal Casting Processes Limitations The initial cost of the furnace and its auxiliary equipment is high It is limited to melting high quality metals and in smaller quantities Time available for analysing the melt composition is very small, thus the melt charge should be carefully selected and it should be of required chemical composition Applications High frequency induction furnace is very useful for special alloy and high quality steels in small quantities It is used for melting Cast iron, Steel, Copper and its alloys Core Type (Or Low Frequency) / Induction Furnace Construction and working A core type induction furnace operates as an ordinary transformer The primary coil has many turns and is wound on a laminated steel core whereas the secondary coil of the transformer has one turn which is a channel or loop of liquid metal within the furnace The furnace uses a c supply of 50 cycles per second Secondary currents (having high current values at low voltage) are induced in the metal bath around the core and the heat is generated due to the electrical resistance of the metal (charge) to the flow of secondary currents Channel of molten metal around the coil connects to the main metal container above, which holds the metal charge The metal in the channel gets heated, circulates through and stirs the metal in the container and thus the melting process (of the metal charge) proceeds Once the melt reaches the required pouring temperature it can be ladled out from the pouring spout (refer Fig 146)

119 198 Manufacturing Technology I - wwwairwalkbookscom Pouring spout Molten metal Circulation of melt Steel shell Refractory lining Circulation of melt Plug for emptying Channel of molten metal Primary coil Fig 146 Core Type (Or Low Frequency) / Induction Furnace Advantages A core type induction furnace is the most efficient among the induction melting furnaces It has thermal efficiency of about 80% The furnace operation is economical Melting is rapid, clean hence no combustion products are present and oxidation losses are at a minimum Melt is accurately controlled with regards to its composition and temperature Magnetic stirring of the melt ensures uniformity of the metal

120 Limitations This Furnace cannot be operated on solid metal charge Furnace operation can be started only after filling the channels with molten metal procured from some other furnace If once by chance metal gets solidified in the channels it cannot remelted by the heat created in the secondary coil Core type furnace is more or less restricted to melt one alloy For melting another alloy, the furnace should be emptied, thoroughly cleaned and restarted with the new molten alloy A core type furnace is not suitable for intermittent operation Applications A core type furnace is used primarily for remelting non-ferrous metals and their alloys It can be used for producing malleable cast iron also Overall Comparison Of Melting Furnaces S No Furnace type Heating Method Applications 1 Crucible Pit type Solid fuel, oil or gas Most of alloys except steel Tilting type Solid fuel, oil or gas Most of alloys except steel Bale out Gas, oil Light casting (die castings) 2 Cupola Coke Cast iron, steel (duplex converter) 3 Reverberatory (air) Solid fuel gas or oil 4 Rotary Pulverized solid fuel, gas or oil 5 Open hearth Oil,Gas Steel Metal Casting Processes 199 Non-ferrous alloys, cast iron, malleable iron Non-ferrous alloys, cast iron, malleable and special Duplex holding

121 1100 Manufacturing Technology I - wwwairwalkbookscom Furnace S No type 6 Arc furnace: Heating Method Applications Direct arc Arc on metal charge Steel, Cast Iron Indirect arc Radiant arc Non-ferrous alloys, high alloy steel and special irons 7 Induction furnace: Core type Low frequency induction Cast Iron, non-ferrous alloys Coreless High frequency induction Steel and alloy steel 8 Resistance furnace Resistor Radiant resistor rod Steel, cast iron, copper alloys Resistance Elements (shroud or Non-ferrous alloys immersion) 112 GATING SYSTEM The assembly of channels which facilitates the molten metal to enter in to the mould cavity is called the gating (or) gating system Pouring Basin Gate Runner In Gates Casting Sprue Sprue Well Fig 147 Gating System In Gates

122 Metal Casting Processes Necessity of the gating system The molten metal from the ladle is not introduced directly into the mould cavity, because it will strike the bottom of the mould cavity with a great velocity and can cause considerable erosion at the bottom of the mould cavity Because of the above mentioned reason, the molten metal is introduced into the mould cavity from the ladle through the gating system 1122 Parts of the gating system 1 Pouring basin 2 Sprue (or) down gates 3 Runner (or) Cross gates 4 Gates (or) ingates 5 Risers Pouring basin: This part of the gating system is made on or in the top of the sprue that receives the stream of molten metal poured from the ladle Sometimes the metal is directly poured into the top of the spure which is made with a funnel shaped opening However better results are usually obtained with the help of pouring basin The pouring basin should be made large and should be placed near to the edge of the moulding box to fill the mould cavity quickly Purpose: 1 To direct the flow rate of metal from ladder to the sprue 2 To help maintaining the required rate of liquid metal flow 3 To reduce turbulence and vortexing at the sprue entrance 4 Helps in seperating cross, slag etc from metal before it enters the sprue Spruce: 1 The vertical passage that passes through the cope and connects the pouring basin with the runner or gate is called the sprue or downgate 2 The cross section of a sprue may be square, rectangular or circular

123 1102 Manufacturing Technology I - wwwairwalkbookscom Positive Taper Sprue (a) Reverse Taper Sprue (b) Fig 148 Sprue design Straight (or) no taper sprue (c) 3 The sprues are generally tapered downward (2 degree to 4 degree) to avoid aspiration of air and metal damage 4 Sprues upto 20 mm diameter are round in section whereas larger sprues are often rectangular in section 5 A round sprue has a minimum surface exposed to cooling and offers the lowest resistance to the flow of metal 6 In rectangular sprue, aspiration and turbulence are minimized 7 Spures may be designed with either a positive taper, a reverse taper or with no taper at all Sprue well: It changes the direction of flow of the molten metal to right angle and passes it to the runner The sprue is tapered with its bigger end at top to receive the liquid metal and the smaller end is connected to the runner Runner The molten metal is usually carried from sprue well to several gates through a passage called runner Runners are normally made trapezoidal in shape The runner is generally preferred in the drag but for ferrous metals it is provided in the cope with ingates in the drag The runner should be streamlined to avoid aspiration and turbulence When a mould has more than one cavity the common gate supplying metal to a number of cavities is also called as runner and the branches from the runner to the respective mould cavities are referred as ingates

124 Metal Casting Processes 1103 Gate Gates is a passage through which the molten metal flows from the runner to the mould cavity The gates should break off from the casting after solidification For this, at the junction to the cavity the gates are much reduced in thickness This will also choke the flow of metal and ensure its quiet entrance into the mould cavity In actual practice, the best cross-section for gates is a trapezoidal one that smoothly passes into a rectangular section at the junction of the cavity According to their positions in the mould cavity, gates are classified as Top gates Parting gates Bottom gates Top gate This is shown in Fig 149 Generally top gates are used for small and simple moulds or for larger castings made in moulds of erosion resistant material For light and oxidisable metal like aluminium and magnesium, top gating is not advisable because of fear of entrapment due to turbulent pouring Pouring Cup Cope Drag Mould Cavity Strainer Core Fig 149 Top Gate Sand (ii) Bottom gates This is shown in Fig 150 Here, the molten metal flows down the bottom of the mould cavity in the drag and enters at the bottom of the casting and rises gently in the mould and around the cores C ope Drag Sand Mould Cavity Fig 150 Bottom Gates

125 1104 Manufacturing Technology I - wwwairwalkbookscom In the bottom gating system, turbulence and mould erosion are the least However time taken to fill the mould is more Also, directional solidification is difficult to achieve because the metal continues to lose its heat into the mould cavity and when it reaches the riser, metal becomes much cooler Bottom gates are best suited for large sized steel castings Parting gate Here, the metal enters the mould at the parting plane when part of the casting is in the cope and part in the drag (Refer Fig 151) For the mould cavity in the drag, it is a top gate and for the cavity in the cope it is a bottom gate Thus this type of gating tries to derive the best of both top and bottom gates Among all the gates, preparation of parting gate is the easiest and most economical one Parting gate is the most widely used gate in sand castings (iv) Step gate In a step gate a number of ingates are arranged in vertical steps Refer Fig 152 The metal enters through these ingates whose sizes are normally increased from top to bottom such that metal enters the mould cavity from the bottom most gate and then progressively moves to the higher gates Step gates are used for heavy and large castings Sand Cope Drag Mould Cavity Parting Sand Line Fig 151 Parting Gate Mould Cavity Fig 152 Step Gate Ingates While designing a casting, it is essential to choose a suitable gate, considering the casting material, casting shape and size so as to produce a sound casting

126 Metal Casting Processes 1105 Gating ratio The term gating ratio is used to describe the relative cross sectional areas of the components of a gating system It is defined as the ratio of sprue area to total runner area to total gate area ie, sprue area : runner area : gate area Choke Choke is a part of the gating system which has the smallest cross-sectional area Functions of choke 1 The function of choke is to control the rate of metal flow by lowering the flow velocity in the runner 2 To hold back slag and foreign material and float these in the cope side of the runner 3 To minimise sand erosion in the runner The gating ratio reveals whether the cross-section increases or decreases towards the mould cavity Accordingly, the gating system may be classified as, (i) Pressurized gating system (ii) Unpressurized gating system Pressurized gating system In pressurized gating system, the ingates serve as the choke A back pressure is maintained causing the entire gating system to become pressurized Here, the molten metal enters the mould cavity uniformly For a given metal flow rate, pressurized systems are generally smaller in volume than unpressurized ones This system is adopted for metals like iron, steel, brass, etc A typical gating ratio in the system can be 4:3:2

127 1106 Manufacturing Technology I - wwwairwalkbookscom (ii) Unpressurized gating system: Here, the sprue base serves as the choke The typical gating ratios in this system can be 1:2:2,1:2:4, 1:3:3, or 1:4:4 This gating system requires careful design to ensure them being kept filled during pouring Drag runners and cope gates aid in maintaining a full runner, but careful streaming is essential to eliminate the separation effects and consequent air separation This system is adopted for light, oxidisable metals like aluminium and magnesium where the turbulence is to be minimised by slowing down the rate of metal flow 113 RISER OF CASTING Riser is also called as feeder head Risers are reservoirs designed and located to feed molten metal to the solidifying casting to compensate for solidification shrinkage Riser is a hole, cut or moulded in the cope to permit the molten metal rise above the highest point in the casting It provides a visual check to ensure filling up of mould cavity Riser find use in casting of heavy sections (or) of high shrinkage alloys 1131 Functions of risers Provide extra metal to compensate for the volumetric shrinkage They act as a heat source so they freeze last and promote directional solidification Risers indicate to the pourer whether the metal has been completely filled up or not in the mould cavity They permit the escape of air and gases as the mould cavity is filled up with molten metal 1132 Types of risers Risers can be classified into two types (a) Open riser (b) Blind riser

128 Metal Casting Processes 1107 (a) Open riser It is the conventional riser whose top is exposed to the atmosphere The molten metal is fed into the riser through the sprue and runners under gravity Refer Fig 153 (a) Sp rue Top Riser End Riser Blind type side riser Permeable Core Sand Mould Cavity (a) Parting Line Sand Fig 153 Types of Risers Mould Cavity (b) Parting Line Advantages Can be easily moulded Since it is open to atmosphere, it will not draw metal from the casting as a result of partial vacuum in the riser Such risers serve as collectors of non-metallic inclusions floating upto the surface Limitations Their height should commensurate with the height of the cope; this reduces the yield of the casting These are the holes through which foreign matter may get into the mould cavity (b) Blind riser The blind riser is enclosed by the sand mould and is designed for a minimum surface area per unit volume A vent or permeable core at the top of the riser may be provided to have some exposure to the atmosphere Refer Fig 153 (b)

129 1108 Manufacturing Technology I - wwwairwalkbookscom Advantages Can be removed more easily from the casting than an open riser Since a blind riser is surrounded on all sides by moulding sand, therefore, it looses heat slowly which helps in better directional solidification of the casting It can be smaller than a comparable open riser, therefore, more yield is obtained Limitation As the metal in it cools, metal skins may quickly form on its walls This results in a vacuum in the riser and the riser will not actually draw metal from the casting This may be avoided by inserting a permeable dry sand core into the riser cavity, connecting it to the mould sand layers Through these sand layers air passes into the riser interior and thus the riser operates under atmospheric pressure 1133 Riser shape The metal in the riser should remain in the molten state for a longer time than in the mould cavity The heat loss in the riser should therefore be kept to a minimum level It means riser must freeze more slowly than the casting Thus their shape should be such as to give volume - to - surface ratio a maximum value From this point of view a spherical shape is ideal one as it is having the lowest surface area for a same volume But risers of spherical shape is difficult to mould Therefore, a cylindrical shape is preferred Height of a cylindrical riser 15 diameter of riser 1134 Riser size The solidification time of a casting depends upon the heat in the casting (directly) and depends (inversely) upon the surface area of the casting Based on the above facts, the following relations have been suggested for determining the riser size

130 Metal Casting Processes Chvorinov s rule (for metal casting) Chvorinov s rule states that total freezing (solidification) time for a casting is a function of the ratio of volume to surface area Where, solidifaction time or C freezing time t ie t C 2 V SA Volume V Surface area SA 2 V Volume of casting SA Surface area of casting C Constant of proportionality that depends upon composition/properties of cast metal, mould material etc Since the metal in the riser must be the last to solidify to achieve solidification V SA 2 riser V SA 2 casting Best riser is one whose 2 V SA is 10 to 15% larger than that of the casting Since V and SA for the casting are known C SA can be determined riser Assuming the height to diameter ratio for the cylindrical riser, the riser size can be determined Chvorinov s rule is not very accurate, since it does not consider solidification contraction (or) shrinkage This method is valid for calculating proper riser size for short - freezing range alloys such as steel and pure metals For non-ferrous alloys, there is no satisfactory relationship for determining riser size

131 1110 Manufacturing Technology I - wwwairwalkbookscom 2 Caine s method In this method, riser size calculation size is based on experimentally determined hyperbolic relationship between relative freezing times and volumes of the casting and the riser Caine s method states that, if the casting solidifies infinitely rapidly the riser (feeder) volume should be equal to the solidification shrinkage of the casting and if the feeder and casting solidify at the same rate, the feeder should be infinitely large Relative freezing time (or) freezing ratio R x is given by Where R x SA/V casting SA/V riser Volume ratio R y is given as R y V riser V casting Caine s formula is given by R x a R y b c a freezing characteristic constant for the metal b Contraction into from liquid to solid c relative freezing rate of riser and casting Table 11 shows the typical value of a, b and c for commonly used metals are given below Table 11 SNo Cost-metals a b c 1 Grey cast-iron Cast-iron, brass Steel Aluminium Aluminium bronze Silicon bronze R y Defective Casting R x Sound Casting Fig 154

132 Metal Casting Processes 1111 Fig 154 shows a typical hyperbolic curve In order to find the riser size for a given casting, the diameter and height of the riser are assumed After knowing the values of a, b, and c, the values of R x and R y are calculated and plotted on hyperbolic curve figure In case the values of R x and R y meet the above curve, the assumed riser size is satisfactory, otherwise a new assumption is made 1135 Location of the riser In addition to the shape and size, a riser must be properly located to obtain a sound casting Location of riser should be such that the riser ensures directional solidification Since the heaviest section of the casting solidifies last, the riser should be located to feed this section the heaviest section will now act as a riser for other sections which are not so heavy or thick 114 PRINCIPLE OF SPECIAL CASTING PROCESSES Sand moulds are single purpose moulds as they are completely destroyed after the casting has been removed from the moulding box It becomes therefore obvious that the use of a permanent mould would do a considerable saving in labour cost of mould making 1141 Advantages of Special casting techniques over conventional sand casting Greater dimensional accuracy with higher metallurgical quality High production rates and hence lower production cost (in certain cases) Ability to cast extremely thin sections Better surface finish on the castings therefore low labour and finishing costs Castings may possess a denser and finer grain structure and posses higher mechanical properties

133 1112 Manufacturing Technology I - wwwairwalkbookscom 1142 Classification of Special Casting Processes Special Casting Processes are classified as follows Based on Metal Mould Casting Gravity (or) permanent mould-casting Die casting Cold chamber process Hot chamber process Slush casting Based on Non metallic Mould Casting Centrifugal casting Carbon-dioxide moulding Investment mould casting (or) lost-wax process Shell moulding Plaster moulding Continuous casting Reciprocating moulds - Draw casting Stationary moulds Direct sheet casting 1143 Gravity (or) Permanent mould-casting (or) Metallic Moulding A gravity die (or) permanent mould casting has a permanent mould Fig 155 The mould can be reused many times before it is discarded or rebuilt Molten metal is poured into the mould under gravity and no external pressure is applied The liquid metal solidifies under pressure of metal in the risers, etc Permanent moulds are made of dense, fine grained, heat resistant cast iron, steel, bronze, anodized aluminium, graphite or other suitable refractories A permanent mould is made in two halves in order to facilitate the removal of casting from the mould

134 Metal Casting Processes 1113 Hydraulic cylinder to open and close mould Movable mould section Stationary mould section Spray nozzle Fig 155 (a) Mould is preheated and coated Cavity Core F The mould walls of a permanent mould have thickness from 15 mm to 50 mm and can remove greater amounts of heat from the casting Fig 155 (b) Cores (if used) are inserted and mould is closed Pouring cup, sprue, gates and riser are built in the mould halves itself The two mould halves are securely clamped together before pouring Simple mechanical clamps (latches, toggle,clamps etc) are adequate for small moulds whereas larger permanent moulds need pneumatic, or other power clamping methods

135 1114 Manufacturing Technology I - wwwairwalkbookscom F (c) Molten metal is poured into the mould, where it solidifies Fig 155 Permanent Mould Casting Applications Permanent mould casting process are costly and is generally limited to those applications only where an economic or engineering gain is obtained Examples are Carburetor bodies, Hydraulic brake cylinders, Refrigeration castings, Washing machine gears and gear covers, Connecting rods, automotive pistons, aircraft and missile castings 1144 Shell mould casting Working Principle Fig 156 Shows the process of Shell mould casting The 2-piece pattern is made of metal (eg aluminum or steel), it is heated to between 175C 370C and coated with a lubricant, eg silicone spray Each heated half-pattern is covered with a mixture of sand and a thermoset resin/epoxy binder The binder glues a layer of sand to the pattern, forming a shell The process may be repeated to get a thicker shell The assembly is baked to cure it The patterns are removed and the two half-shells joined together to form the mould and then metal is poured into the mould

136 Metal Casting Processes 1115 When the metal solidifies, the shell is broken to get the part Fig156 shows Steps in Shell Casting (1) A match-plate or cope-and-drag metal pattern is heated and placed over a box containing sand mixed with thermosetting resin Heated pattern Sand with resin binder (1) Dump box Fig 156 (2) (2) Box is inverted so that sand and resin fall onto the hot pattern, causing a layer of the mixture to partially cure on the surface to form a hard shell; Shell (3) Fig 156 (4) (3) Box is repositioned so that loose uncured particles drop away; (4) Sand shell mould is heated in oven for several minutes to complete curing; (5) Shell mold is stripped from the pattern

137 1116 Manufacturing Technology I - wwwairwalkbookscom Shell mould Pattern Fig 156 (5) (6) Two halves of the shell mold are assembled, supported by sand or metal shot in a box and pouring is accomplished Flask Shell moulds Metal shot (7) (6) (7) The finished casting with sprue is removed Advantages Castings as thin as 15 mm and of high definition can be casted satisfactorily Good dimensional accuracy - machining often not required

138 Metal Casting Processes 1117 Mould collapsibility minimizes cracks in casting It can be mechanized for mass production There is no surface chilling or skin hardening of castings, since shell is an excellent heat insulator Cooling rate of cast metal being slow, castings possess grain sizes larger than those obtained in green sand moulds Shell mould made castings possess excellent surface finish and high tolerance of the order 0002 to 0003 mm per mm is possible Shell moulding faithfully reproduces details with sharp clean edges thereby rendering fettling and machining unnecessary Smoother cavity surface permits easier flow of molten metal and better surface finish Disadvantages Shell moulding is uneconomical on small scale production Resin costs are comparatively high Heavy weight castings weighing greater than 10 kg may not be able to be casted by shell moulding Moulds are not normally economically recoverable Shapes in which proper parting and gating cannot be obtained are not suitable for production with shell moulding process The maximum size of the casting is limited by the maximum size of the shell which can be feasibly produced and poured Low carbon steels castings made by shell moulding may show depressions on their upper surface Applications Components cast by shell moulding are: Automotive rocker arms, valves, small pipes, camshaft, bushings valve bodies, spacers, brackets, manifolds, bearing caps, shafts and gears Shell moulding is ideal for mass production of small castings where the degree of intricacy causes high rejection rates in green sand moulding

139 1118 Manufacturing Technology I - wwwairwalkbookscom Various alloys which can be satisfactorily cast by shell moulding are: aluminium alloys, copper alloys, cast irons, stainless steels etc A number of small hydraulic castings in stainless steel and corner alloys are produced by shell moulding Shell moulding is suited to ferrous and non-ferrous alloy castings in the range 01 to 10 kg 1145 Investment casting (Lost wax casting) Working Principle Investment casting uses a wax pattern which is coated with refractory materials to form a mould The wax is then melted out and the mould cavity is filled with molten metal Cast metal is cooled and the slurry broken to get the castings It can be used for high precision complex shapes from high melting point metals that are not readily machinable Steps involves investment casting Fig 157 shows the step by step method of performing investment casting Pattern creation (Fig 157 (1),(2)) The wax patterns are typically injection moulded into a metal die and are formed as one piece Several of these patterns are attached to a central wax gating system (sprue, runners, and risers) to form a tree-like assembly The gating system forms the channels through which the molten metal will flow to the mould cavity Mould creation (Fig 157 (3),(4),(5)) This "pattern tree" is dipped into a slurry of fine ceramic particles, coated with more coarse particles and then dried to form a ceramic shell around the patterns and gating system This process is repeated until the shell is thick enough to withstand the molten metal it will encounter

140 Metal Casting Processes 1119 Wax sprue Wax pattern (1) Slurry of fine ceramic particles (2) (3) Heat Ceramic particles (4) (5) Heated in oven Wax Pouring molten metal Fig 157 Steps involved in process of Investment casting (7) Final product