Moulding and Casting Methods 1. General methods of moulding and casting

Transcription

1 MME 345, Lecture 21 Mulding and Casting Methds 1. General methds f mulding and casting Ref: [1] Heine, Lper & Rsenthal, Principles f Metal Casting, McGraw-Hill, 1976 [2] Beeley, Fundry Technlgy, Butterwrth-Heinemann, 2001 Tpics t discuss tday 1. Mulding and casting methds 2. Sand system 3. Nn-sand system 4. Special methds

2 1. Mulding and casting methds Gd castings cannt be made withut gd mulds. Casting prcesses ften described by the materials and methds emplyed in mulding. Mulding prcess methds f making muld and the material used. Casting methds mulding prcess, design f gating and feeding systems. Mulding methds and materials encmpass a wide range f ptins. The selectin f the crrect ne t use in a specific casting applicatin depends n 1. the ally, 2. the dimensinal tlerances desired, 3. the quality f the casting, and 4. the price the custmer is willing t pay. 3/ Classificatin f mulding and casting methds Based n the mulding medium 1. sand muld, 2. ceramic muld, 3. metal muld, etc Based n the type f muld 1. expendable destryed after each casting (sand casting) 2. permanent reused many times (die casting) Based n the type f pattern 1. expendable destryed fr each muld (investment casting, fam casting) 2. permanent reused many times (wd/metal fr sand casting) Based n the type f filling (metal flw in muld) 1. gravity sand casting, gravity die casting 2. pressure lw and high pressure die casting 3. vacuum vacuum investment casting Mst metals and allys can be cast using any f the methds. There are, hwever, limitatins in sme cases. 4/25

3 Classificatin system fr fundry prcesses based n muld type Mulding and Casting Prcesses Expendable muld Permanent muld Sand system Bnded sand system Greensand mulding Dry sand mulding Resin-bnded sand mulding Sdium silicate prcess Investment casting Un-bnded sand system Vacuum mulding Lst fam prcess Nn-sand system Plaster mulding Ceramic mulding Graphite mulding Gravity die casting Pressure die casting Lw pressure die casting High pressure die casting Centrifugal casting Hybrid prcesses Squeeze casting Semi-slid metal casting (rhecasting) Cntinuus casting 5/25 2. Sand-based mulding and casting methds Silica sand, either bnded r unbnded, is the mst ppular mulding media. Cmmn advantages: widely available inexpensive able t withstand the elevated temperatures invlved in casting mst allys, and easily recycled Des nt react with mst metals and allys, and can be bnded with clay, resins, r silicate cmpunds. Permeable, the air displaced by the metal when it enters the muld can escape easily thrugh the muld. 6/25

4 Sand-based mulding prcesses have certain features in cmmn: the use f pattern and/r cre bxes an aggregate mixture, cmprising a granular refractry material and binder a means f frming the aggregate mixture arund the pattern hardening f the aggregate (r develping bnd) while in cntact with the pattern withdrawal f pattern frm the hardened aggregate muld assembly f muld and cre pieces t make a cmplete muld t pur the liquid metal The varius mulding prcesses differ primarily in the methd f frming the muld, the type f granular refractry material, and the methd f bnding the granular refractry material 7/ Bnded sand mulding methds In bnded sand mulds, the sand particles are held tgether with electrstatic/chemical bnds. The bnds that are develped must be easily brken when the casting is slid s that the casting can be remved frm the muld. Cmmn bnded sand mulding prcesses Green sand mulding Dry sand mulding Resin bned sand mulding Sdium silicate bnded sand mulding (CO 2 prcess) Cement bnded sand mulding prcess Investment (r precisin) casting prcesses 8/25

5 2.2 Un-bnded sand mulding methds It is nt always necessary t use a binder with sand. In un-bnded sand mulds, the pattern is held either by the pattern itself during puring, r by the use f a vacuum. Use f un-bnded sand mulds is advantageus because they need n binders, mulling equipment, r muld additives. Cmmn un-bnded sand mulding prcesses Lst fam mulding Vacuum mulding 9/25 Greensand mulding prcess The mst widely used f all sand mulds fr small t medium size metal castings. Made f silica sand, bnded by a mixture f clay and water. Other materials may be added t cntrl its strength and prevent metal penetratin. The sand mixture is cmpacted in a remvable flask that is attached t the pattern. After the sand is cmpacted the pattern is remved revealing a cavity in the sand muld. It is called "green sand" because f the presence f misture in it and is nt baked r fired. Its actual clur is usually black. Principal advantages Flexible; can be reused Rapid and repetitive Can be mechanized Cheaper Disadvantages Single-time use Create defects Bad dimensinal accuracy and surface finish Sme casting designs (e.g., large castings; castings having thin, lng sectins; intricate designs, etc.) require ther methds. 10/25

6 Resin-bnded sand muld 1 2 wt.% chemical binder (plymer resin) and dry sand are mixed tgether and frmed arund a pattern r placed in a cre bx. The resin may be cured by heat, r it may be cured by the setting f the resin at rm temperature. T accelerate the curing f the resin, slid/liquid/gaseus catalysts may be added. Gd surface finish Greater lngevity and higher strength f the muld; strength which varies with (1) amunt f resin, (2) type f resin, (3) misture level in cre r muld, and (4) time after setting has begun Limited bench life due t water absrptin 11/25 Shell mulding prcess very fine sand cated with a thermsetting resin placed in cntact with a heated pattern t prduce a shell f nly abut 10 mm thick prduces an excellent surface finish

7 Investment (r Precisin) mulding Very gd surface finish Cmplex gemetry Lng prductin cycle 13/25 14/25

8 Lst fam r full muld casting eliminates the use f sand binders, additives, and cres capable f making sme shapes that culd be made n ther way care must be taken in gating lst fam parts t be certain that, the mlten metal flws thrugh the pattern in ne directin 15/25 2. Permanent mulding and casting methds Mulds used many times; made f grey irn, steel, brnze. Muld cavity and gating system are machined; generate very gd surface finish and dimensinal accuracy in the castings. Cmmn allys t cast: Al-, Mg-, Zn-, Pb-, Cu-base allys, and cast irns; mst steel casting nt suitable due t extremely high temperatures cnsequent muld ersin. Limited t relatively simple shapes and vlume prductin nly ; autmatin used t btain a cntinuus cycle. Gravity die casting prcesses metal fed by gravity, r by air pressure f 3 t 10 psi. Pressure die casting prcesses mlten metal is frced int muld cavity under high pressures, 1000 t psi. Cmmn mdes f die failure: (1) Thermal fatigue, and (2) Muld ersin Muld life is extended and ejectin made easier by using muld cat (carbn sht, refractries suspended in liquids). 16/25

9 Permanent muld (r, Gravity die) casting prcess better mechanical prperties hmgeneus structure and cmpsitin lw shrinkage and gas prsity better surface finish and dimensinal accuracy lw-melting-pint allys simple-shaped casting high muld cst cstly mulds simpler shapes nly 17/25 (Pressure) Die casting prcess highly prductive methd f simple-shaped casting with lw dimensins tlerance and high surface quality expensive die simple-shaped small castings cmplex and large expensive machinery ht-chamber prcess (2) cld-chamber prcess 18/25

10 Centrifugal casting large cylindrical parts gd quality expensive limited design hrizntal centrifugal casting vertical centrifugal casting 19/25 COMPARISON BETWEEN THE MORE COMMONLY USED CASTING PROCESS Specificatin 1. Sand casting 2. Investment casting 3. Permanent muld casting Prcess Shape Materials Sand mulds are prduced arund a permanent pattern that is withdrawn t leave a cavity. Mlten metal is pured int the muld and slidifies. Muld (and cre) is brken up t retrieve the casting. Mainly slid cmpnents but cmplex internal shapes prduced using friable cres. Very large and small castings pssible but thin sectins difficult. All metals excluding refractry and reactive allys (e.g. titanium). A ceramic shell (investment) is slip cast arund a wax pattern. Wax is melted and mlten metal cast int the investment that is brken up t remve the casting. Best fr relatively small, cmplex 3D cmpnents. Re-entrant angles pssible. Suitable fr mst metals. Reactive metals can be cast under vacuum. Mlten metal pured int a metallic muld where it slidifies. Mstly used fr small, simple shapes with nly simple cring. Mainly used fr light allys. Steels and cast irns als pssible. Cycle time Quality Usually lng as limited by rate f heat transfer ut f the casting. Use f multiple mulds increases prductin rate. Surface texture pr. Prsity endemic. Nnmetallic inclusins difficult t cntrl. Limited by rate f heat transfer ut f the casting. Prductin rates lw because f prcess cmplexity. Increased by using multiple mulds and patterns. Surface texture gd. Higher muld temperatures decrease prsity but prduce carse micrstructures. Flexibility Patterns cheap and easy t make. Mderately high because f the ease f prductin patterns. Material utilizatin Operating cst Up t 50% f casting in runners and feeders. Bth muld and scrap metal can be directly recycled. Very lw as pattern csts are lw and making is relatively easy. Near net shape prcess with little material cntained in feeding systems. Wax recycled, investment lst. Equipment csts can be high especially where reactive allys are cncerned. Labur csts are high due t the many stages in the prcess. Limited by rate f heat transfer acrss the interface. Prductin rates can be increased by using multiple mulds. Surface texture is gd. Prsity unavidable but can be minimized by slwer muld filling t reduce turbulence. Negligible setting up time fr manual peratin. Muld making relatively difficult. Rarely better than 60% utilizatin. Scrap in the runners and feeders can be directly recycled. Equipment cst can be limited t muld and melt preparatin apparatus.

11 Specificatin 4. Pressure die casting 5. Centrifugal casting 6. Full muld casting Prcess Shape Materials Cycle time Quality Flexibility Material utilizatin Operating cst Mlten metal is frced int a water-cated metal muld (die) thrugh a system f sprues and runners. The metal slidifies rapidly and the casting is remved with its sprue and runners. Used fr cmplex shapes and thin sectins. Cres must be simple and retractable. High fluidity requirement means lw melting temperature eutectics usually used (e.g. Al-Si). Ht chamber methd restricted t very lw melting temperature allys (e.g. Mg and Zn). Slidificatin time is typically < 1 s s cycle is cntrlled by time taken t fill muld and remve casting. Gd surface texture but tlerable muld filling prduces high degree f internal prsity. Tling dictated s limited by machine setting up time. Near net shape prcess but sme scrap in sprues, runners and flash which can be directly recycled. High, since machine and mulds are expensive. Mlten metal is intrduced int a sand- r cpper-lined, cylindrical steel muld that is rtated abut its lng axis, distributing the metal ver its inner surface. Technique used t prduce relatively lng, hllw bjects (e.g. pipes) withut the need fr cres. Metals excluding refractry and reactive metals. Determined by the rate f intrductin f metal int the muld and the rate f slidificatin f the metal. The latter is lwered fr sand-lined mulds. Prsity and nn-metallic inclusins migrate twards the inner surface because f their lw density, giving a high quality uter surface. Setting up times are relatively shrt. Absence f runners and risers leads t near 100% use f material. A refractry cating is applied t a vlatile r cmbustible pattern that is used in a sand muld. The pattern is destryed by the mlten metal. Very cmplex 3D shapes pssible. Nn-refractry metals with casting temperatures high enugh t vaprise the pattern. Lng due t prcess cmplexity. Multiple mulds increase prductin rate. Nrmal sand casting defects. Surface texture similar t that f pattern. Ideal fr the manufacture f ne-ffs. Pattern material entirely wasted. Metal usage pr due t runners etc. Equipment is relatively simple and can cst All equipment invlved is rudimentary a little. Increased cmplexity f water-cled and prcess is very cheap t perate. cpper-lined mulds mre cstly. 21/25 22/25

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13 Next Class MME 345, Lecture 22 Mulding and Casting Methds 2. Greensand mulding prcess