CastingChina circa 3000BC2.810Prof. Timothy GutowskiCasting since about 4000 BC…Ancient Greece; bronzestatue casting circa 450BCIron works in early Europe,e.g. cast iron cannons fromEngland circa 1543Outline• Sand Casting, Investment Casting, Die Casting• Basics and countermeasures• Phase Change, Shrinkage•Heat Transfer•Heat Transfer• Pattern Design• Variations & Developments• Environmental IssuesCastingReadings;1. Kalpakjian, Chapters 10, 11, 122. Booothroyd, “Design for Die Casting”Casting”3. Flemings “Heat Flow in Solidification”4. Dalquist “LCA of Casting”Note: a good heat transfer reference can be found by Profs John Lienhard online http://web.mit.edu/lienhard/www/ahtt.htmlCasting Methods• Sand CastingHigh Temperature Alloy, Complex Geometry, Rough Surface Finish• Investment CastingHigh Temperature Alloy, Complex Geometry, Moderately Smooth Surface Finish• Die CastingHigh Temperature Alloy, Moderate Geometry, Smooth SurfaceSand CastingSand CastingDescription: Tempered sand is packed into wood or metal pattern halves, removed form the pattern, and assembled with or without cores, and metal is poured into resultant cavities. Various core materials can be used. Molds are broken to remove castings. Specialized binders now in use can improve tolerances and surface finish.Metals: Most castable metals.Size Range: Limitation depends on foundry capabilities. Ounces to many tons.Tolerances:Tolerances:Non-Ferrous ± 1/32″ to 6″Add ± .003″ to 3″, ± 3/64″ from 3″ to 6″.Across parting line add ± .020″ to ± .090″ depending on size. (Assumes metal patterns)Surface Finish:Non-Ferrous: 150-350 RMSFerrous: 300-700RMSMinimum Draft Requirements:1° to 5°Cores: 1° to 1 1/2°Normal Minimum Section Thickness:Non-Ferrous: 1/8″ - 1/4″Ferrous: 1/4″ - 3/8″Ordering Quantities: All quantitiesNormal Lead Time:Samples: 2-10 weeksProduction 2-4 weeks A.S.A.Sand Casting Mold FeaturesVents, which are placed in molds to carry off gases produced when the molten metal comes into contact with the sand in the molds and core. They also exhaust air from the mold cavity as the molten metal flows into the mold.See Video from Mass FoundryProduction sand castingInvestment CastingDescription: Metal mold makes wax or plastic replica. There are sprued, then surrounded with investment material, baked out, and metal is poured in the resultant cavity. Molds are broken to remove the castings.Metals: Most castable metals.Size Range: fraction of an ounce to 150 lbs..Tolerances:± .003″ to 1/4″±.004″to 1/2″, ±.004″to 1/2″, ± .005″ per inch to 3″± .003″ for each additional inchSurface Finish:63-125RMSMinimum Draft Requirements: NoneNormal Minimum Section Thickness:.030″ (Small Areas).060″ (Large Areas)Ordering Quantities: Aluminum: usually under 1,000Other metals: all quantitiesNormal Lead Time:Samples: 5-16 weeks (depending on complexity)Production 4-12 weeks A.S.A. (depending on subsequent operations).Talbot Associates Inc.The investment-casting process, also called the lost-waxprocess, was first used during the period 4000-3500 B.C. The pattern is made of wax or a plastic such as polystyrene. The sequences involved in Investment Castingsequences involved in investment casting are shown in Figure 11.18. The pattern is made by injecting molten wax or plastic into a metal die in the shape of the object.Die CastingDescription: Molten metal is injected, under pressure, into hardened steel dies, often water cooled. Dies are opened, and castings are ejected.Metals: Aluminum, Zinc, Magnesium, and limited Brass.Size Range: Not normally over 2 feet square. Some foundries capable of larger sizes.Tolerances:Al and Mg ± .002″/in.Zinc±.0015″/in. Zinc±.0015″/in. Brass ± .001″/in.Add ± .001″ to ± .015″ across parting line depending on sizeSurface Finish: 32-63RMSMinimum Draft Requirements: Al & Mg: 1° to 3°Zinc: 1/2° to 2°Brass: 2° to 5°Normal Minimum Section Thickness:Al & Mg: .03″ Small Parts: .06″ Medium Parts Zinc: .03″ Small Parts: .045″ Medium Parts Brass: .025″ Small Parts: .040″ Medium Parts Ordering Quantities: Usually 2,500 and up.Normal Lead Time:Samples: 12-20 weeks Production: ASAP after approval.Die Casting – Cold-Chamber CastingCycle in cold-chamber casting: (1) with die closed and ram withdrawn, molten metal is poured into the chamber; (2) ram forces metal to flow into die, maintaining pressure during the cooling and solidification; and (3) ram is withdrawn, die is opened, and part is ejected. Used for higher temperature metals eg Aluminum, Copper and alloysDie Casting – Hot-Chamber CastingCycle in hot-chamber casting: (1) with die closed and plunger withdrawn, molten metal flows into the chamber; (2) plunger forces metal in chamber to flow into die, maintaining pressure during cooling and solidification; and (3) plunger is withdrawn, die is (3) plunger is withdrawn, die is opened, and solidified part is ejected. Finished part is shown in (4).High Melt Temperature•Chemical Activity•High Latent Heat•Handling•Off-gassing3000° C2000°CTungsten Carbide, WC, Silicon Carbide, SiCMolybdenumAlumina Al2O3Cubic Zirconia, ZrO20° C1000° C2000°CPlatinum, PtTitanium, TiIronFE, Plain Carbon Steels, low alloy, stainlessNickel, NiNickel AlloySilicon, SiCopper, Cu, Bronze, BrassAluminumMagnesiumZinc, ZnPTFE (Teflon)Tin, SnHDPENylonAcetalMold FillingBernouli’s Equation:.22Constgvpgph =++Reynold’s Number:•Short filling times•Potential Turbulence(see p. 273 … KalpakjianµvDP=RehMold Filling Example (1 of 2)Mold Filling Example (2 of 2)PhaseChange &ShrinkageSolidification of a binary alloyComposition change during solidificationPb-Sn phase diagramSolidificationCast structuresSchematic illustration of three cast structures solidified in a square mold: (a) pure metals; (b) solid solution alloys; and © structure obtained by using nucleating agents. Source: G. W. Form, J. F. Wallace, and A. CibulaForm, J. F. Wallace, and A. CibulaPop quiz; If you top fill the mold below, what will the part look like after solidification?Can you explain these features?Heat Transfer – Sand Casting2≈AVtsHeat Transfer – Die Casting1≈AVtsSteady State Conduction Heat TransferFigure 1Steady State Conduction Heat TransferFigure 2Thermal Conductivity “k” of Various Materials for Parts and Molds (W/m °K)Copper 394Aluminum 222Iron29Iron29Sand 0.61PMMA 0.20PVC 0.16dxdTkq
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