CASTING. Dept. of Mech & Mfg. Engg. 1

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1 CASTING 1

2 CASTING It is the process of producing metallic parts by pouring a molten metal in to the mould cavity and allowing the metal to solidify. 2

3 Casting Process Pattern making Mould making Metal is Heated Above Melting point Liquid Metal Poured into Mould cavity Metal Solidifies into Shape of Mould Solidified Part is Removed from Mould Post Casting Operations Clean Up Machining 3

4 Mould making The process of making void or cavity of required size and shape in a refractory material is called mould making. The mould made of sand is called sand mould and is widely used mould type for manufacturing parts by casting process. 4

5 Molding Sand Composition The main ingredients of any molding sand are: Base sand, Binder, and Moisture Base Sand Silica sand is most commonly used base sand. Other base sands that are also used for making mold are zircon sand, Chromite sand, and olivine sand. Silica sand is cheapest among all types of base sand and it is easily available. 5

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7 Binder Clay binders are most commonly used binding agents mixed with the molding sands to provide the strength. The most popular clay types are: Kaolinite and Bentonite Of the two the Bentonite produce strongest bonds between sand particles. Moisture Clay acquires its bonding action only in the presence of the required amount of moisture 7

8 Molding sand classification They can be classified as: 1. Natural molding sand - Taken from river beds & contain major amount of clay - easily available, low cost 2. Synthetic sand - Natural sand without clay, water. Required proportion of clay-water is mixed - strength & bonding property can be easily controlled - more expensive than natural sand 3. Special sand: - ideal in getting special characteristics. 8

9 PATTERN A pattern is an replica of a casting or object to be made. Pattern is a model around which sand is packed to give rise to a cavity known as mould cavity. 9

10 Components of a gating system 10

11 Components of a gating system Pouring cup or basin: used to minimize splash and turbulence as the molten metal flows into downsprue. Down sprue: Vertical passageway through which molten metal enters a runner leading to the mould cavity Runner: The runner will receive metal from the basin through downsprue and distribute to several gates around the mould cavity. Gate: one of the horizontal channels which leads molten metal into the mould cavity. Riser: Reservoir in the mould which is a source of liquid metal to compensate for shrinkage during solidification. 11

12 Cores in the Mold Cavity The mold cavity provides the external surfaces of the cast part In addition, a casting may have internal surfaces, determined by a core, placed inside the mold cavity to define the interior geometry of part In sand casting, cores are generally made of sand. 12

13 Core Obstruction or part placed in mould cavity so that molten metal does not fill the space occupied by this part. 13

14 Green sand mold Pattern Drag Molding board FIRST OPERATION Vents Riser pin Sprue pin Cope Riser Pouring basin Parting line Parting line Runner Core Gate SECOND OPERATION COMPLETE MOLD 14

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16 Green sand bench molding procedure using split piece pattern Place moulding board. Place drag box in inverted position and place the one half(drag half) of pattern at the centre of drag box. Sprinkle parting sand or parting powder on pattern and fill and ram sand around the pattern. Turn the drag box upside down and place it on moulding board. Place cope box on top of drag box and fix other half of the pattern(cope pattern) on drag pattern inside drag box. Sprinkle parting sand or parting powder on pattern. Position riser and sprue pins at suitable locations in cope box and fill and ram sand around the pattern. Remove the riser and sprue pins which gives sprue hole(down sprue) and riser hole in the cope box. Cut(or make) funnel shaped cavity at the top of sprue hole.

17 Green sand bench molding procedure Separate cope box from drag box. Remove the patterns from each box by tapping(hitting) the pattern using wooden piece and lift the pattern. By removing the pattern the cavity obtained is called mould cavity. Cut horizontal passageways called runners to make connection between sprue hole and mould cavity. Place core in position.assemble cope box with drag box. Thus, mould is ready and molten metal can be poured into the cavity and allowed to solidify. The object after solidification obtained is called casting.

18 Green Sand Bench Molding procedure-single piece pattern Eg2: Different shape of pattern

19 Properties of a molding sand Porosity (permeability) Permeability is the property of moulding sand which permits the escape of steam & other gases generated during pouring & solidification of metal. Sand must be porous to allow the gases/ moisture present within the molds to be removed freely when molten metal is poured. Proper venting of the mold also helps in escaping the gases that are generated inside the mold cavity. 19

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21 Adhesiveness Ability of sand particles to adhere or stick to sides of molding boxes. sand particles must be capable of adhering to sides or walls of molding boxes. 21

22 Cohesiveness / Strength Ability of sand particles to stick together. Should be sufficient to permit the mold to retain the desired shape even after the hot metal is poured into the mold 22

23 Refractoriness It is the ability of the molding material to resist the temperature of the liquid metal to be poured so that it does not get fused with the metal. Molding sands with a poor refractoriness may burn on to the casting 23

24 Flowability Ability of the sand to behave like a fluid so that, when rammed, sand will flow to all portions of a mold and around the pattern and take up the required shape To obtain good impression of the pattern in the mold. Flowability is ability of sand to get compacted to uniform density. 24

25 Properties of a molding sand Collapsibility Ability of sand to collapse as the molten metal gets solidified in it. so that during the contraction of the solidified casting it does not provide any resistance, which may result in cracks in the castings. Ability of sand to collapse while casting is separated from mold 25

26 PATTERN Mould cavity Casting 26

27 Pattern materials -Any material which will retain exact shape and size for required number of moulds. 27

28 Pattern materials Wood Readily available and cheap can be machined to required shapes Easily affected by moisture; used when small number of castings required,large size,light in wt. Metal -do not change their shape when subjected to moist condition,longer life used when large number of castings -- High surface finish,strong 28

29 Pattern materials Plastics do not absorb moisture, smooth surface, dimensionally stable,durable,light wt can be withdrawn from the mold easily, without damaging the mold Plaster High compressive strength, easy to make complicated patterns 29

30 TYPES OF PATTERNS 1. One piece pattern. 6. Sweep pattern 2. Split pattern 7. Gated pattern. 3. Loose piece pattern 8. Skeleton pattern. 4. Match plate pattern 9. Segmental pattern 5. Cope and Drag 10. Follow board pattern. 30

31 Simplest pattern. Inexpensive. Pattern is made of one piece and does not contain loose pieces or joints. For making few large size simple castings. Usually made of wood or metal. Pattern is either kept in drag or cope. One piece pattern. 31

32 Split pattern intricate shaped castings Parting line of the pattern forms the parting line of the casting. Dowel pins are used. There can be more than 2 pieces for complex shapes. Multi-piece pattern 32

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34 Match plate pattern Consists of a Match plate, on either side of which each half of a number of split patterns are fastened. A number of different sized & shaped patterns may be mounted on one match plate. Runner and gates are also attached to it. Application: Normally used in machine moulding. Preferred for producing small castings on mass scale Accurate faster rate. PISTON RINGS OF I.C. ENGINES ARE PRODUCED BY THIS PATTERN. 34

35 Cope and Drag Pattern Used for producing big castings which as a whole cannot be conveniently handled by one moulder alone. Another form of split pattern. Each half of pattern is fixed to separate plate. Provision for runner,gates in plate; Each pattern is moulded separately Cope & Drag pattern Cope plate Drag plate 35

36 Certain patterns cannot be withdrawn once they are embedded in the moulding sand. Such patterns are made with one or more loose pieces for facilitating their removal from the mould. Consume more time and labour. Application: Electric motor casing. Loose piece pattern 36

37 Sweep pattern The sweep pattern consists of a wooden board having a shape corresponding to the shape of the desired casting and arranged to rotate about a central axis. The sand is rammed in place and the sweep board is moved around its axis of rotation to give the moulding sand the desired shape. Saves time, labour and money to make 3D pattern. Application: Large kettles of Cast Iron, Bells, Large castings of circular section 37

38 Usually made of metal to increase strength and reduce warpage. Contains inbuilt gates and runners, thus saving time. Application: Used in Mass production. Employed to produce Small castings. Gated pattern. 38

39 Follow board pattern. A wooden board is used for supporting a pattern which is thin and fragile. In addition, it forms the natural parting line of the mould or casting. Application: Used for casting Master patterns for many applications. 39

40 Skeleton of the desired shape. Skeleton pattern. Made of wooden strips to and thus it is wooden framework. It is filled with sand and rammed. Extra sand is removed with a Strickle. Less material cost. Symmetrical object-using one half, two halves of pattern can be moulded Application: To produce few large castings. Turbine casings Water pipes. L bends 40

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42 Segmental pattern Resembles Sweep pattern. Uses part of pattern and not complete pattern. Generates circular shapes. It does not revolve continouosly, but prepares the mould in parts. Application: Big gears Wheel rims 42

43 Pattern Allowances 1. Shrinkage allowance 2. Machining or Finishing allowance 3. Draft or Taper allowance 4. Camber or distortion allowance 5. Shake or Rapping allowance 6. Mould wall movement allowance 43

44 Pattern Allowances 1. Shrinkage allowance Metals contract/shrink during cooling from pouring temperature to room temperature Provided to compensate for solidification contraction by providing positive allowances in the pattern. Pattern made oversize Amount of contraction varies with different metals 44

45 Pattern Allowances 2. Machining allowance Surface of Casting gets oxidized. Scales need to be removed. Remove surface roughness and other imperfections. To achieve exact dimensions. 45

46 Pattern Allowances 3. Draft or Taper allowance Given slight taper on the vertical surfaces of the pattern parallel to the direction of withdrawal of pattern from the mould Provided on both internal & external surfaces For easy withdrawal of pattern from the mould without damaging mold cavity 46

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48 Pattern Allowances 4. Rapping allowance Negative allowance to be provided in the pattern When pattern is withdrawn from the mould by striking over it from side to side, size of the mould cavity increases To compensate this rapping allowance is provided Pattern dimensions are kept smaller to compensate for enlargement of mold cavity due to rapping. 48

49 Pattern Allowances 6. Mould wall movement allowance Mould wall movement takes place because of excessive heat and pressure applied by the molten metal on the surface layer of the sand mold cavity Affects the size of the casting Remedy: Provide proper/negative allowance in the pattern Controlling the temperature of molten metal 49

50 Pattern Allowances 5. Distortion allowance Castings having irregular shape, will not have uniform contraction during their cooling Long flat casting,u or V shape Results in distortion of the castings Remedy: An opposite distortion is provided in the pattern 50

51 Allowance= Shrinkage allowance+machining or Finishing allowance + Draft or Taper allowance+camber or distortion allowance+ Shake or Rapping allowance+mould wall movement allowance 51

52 Green sand mold(mould) 52

53 Basic steps in making sand casting 1. Pattern making 2. Core making 3. Moulding 4. Melting and pouring 5. Cleaning 53

54 Green sand mold Pattern Drag Molding board FIRST OPERATION Vents Riser pin Sprue pin Cope Riser Pouring basin Parting line Parting line Runner Core Gate SECOND OPERATION COMPLETE MOLD 54

55 Difference between pattern and casting Size Pattern-no holes Number of pieces Material 55

56 Defects in casting Blowholes Smooth round holes appearing as a group of small holes on or below the surface of casting Due to entrapped bubble of gases Scabs Projection on the casting that occur when a portion of the face of mould lifts and molten metal flows into sand 56

57 Internal air pocket It appears as small holes inside the casting, due to rapid pouring of molten metal into the mold Shifts An external defect caused due to core misplacement or mismatching of top and bottom parts of the pattern/casting 57

58 Defects in casting Drop Occurs when upper surface of the mold cracks and pieces of sand falls in to the molten metal Shrinkage cavity Void created in the casting due to solidification of the metal 58

59 Defects in casting Fin A thin projection of the metal at parting line Pattern plates are not sufficiently rigid to keep straight during ramming Swell Enlargement of mould cavity by molten metal pressure This is caused by improper ramming, low strength Warpage Undesirable deformation in a casting that occurs during solidification due to internal stresses(uneven solidification rates) 59

60 General Casting Defects Back 60

61 Defects in casting Hot tears: Internal or external cracks Caused because of internal stresses due to insufficient collapsibility of mould. Misrun: Entire mold cavity may not be filled before metal starts solidifying due to low temp. of molten metal Cold shut: Two streams of molten metal which are at lower temperature meet in the mold cavity but do not fuse together. 61

62 Advantages Molten material can flow into very small sections so that intricate shapes can be made by this process. It is possible to cast practically any material that is ferrous or non-ferrous. There are certain parts made from metals and alloys that can only be processed this way. The necessary tools required for casting molds are very simple and inexpensive. Size and weight of the product is not a limitation for the casting process. 62

63 Limitations The metal casting process is a labor intensive process Melting of metal is a high energy process Time required for producing a casting is high 63