2 Permanent Mold Casting Processes Students’ presentations Economic disadvantage of expendable mold casting: a new mold is required for every castingIn permanent mold casting, the mold is reused many timesThe processes include:Basic permanent mold castingDie castingCentrifugal casting
3 The Basic Permanent Mold Process Uses a metal mold constructed of two sections designed for easy, precise opening and closingMolds used for casting lower melting point alloys are commonly made of steel or cast ironMolds used for casting steel must be made of refractory material, due to the very high pouring temperatures
4 Permanent Mold Casting Figure Steps in permanent mold casting: (1) mold is preheated and coatedFigure Steps in permanent mold casting: (2) cores (if used) are inserted and mold is closed, (3) molten metal is poured into the mold, where it solidifies.
5 Advantages and Limitations Advantages of permanent mold casting:Good dimensional control and surface finishMore rapid solidification caused by the cold metal mold results in a finer grain structure, so castings are strongerLimitations:Generally limited to metals of lower melting pointSimpler part geometries compared to sand casting because of need to open the moldHigh cost of mold
6 Applications of Permanent Mold Casting Due to high mold cost, process is best suited to high volume production and can be automated accordinglyTypical parts: automotive pistons, pump bodies, and certain castings for aircraft and missilesMetals commonly cast: aluminum, magnesium, copper‑base alloys, and cast iron
7 Slush CastingSlush Casting is a special type of permanent mold casting, where the molten metal is not allowed to completely solidify. After the desired wall thickness is obtained, the not yet solidified molten metal is poured out. This is useful for making hollow ornamental objects such as candlesticks, lamps, statues etc.
8 Slush CastingLow-melting-point metals such as lead, zinc, and tin are used.The exterior appearance is important, but the strength and interior geometry of the casting are minor considerations.
9 Low Pressure CastingInstead of using gravity to assist in the metal pour and flow in the mold, a low pressure of up to 0.1 MPa (15 psi) gas is applied to the molten metal.This maintenance of pressure on the melt causes complete fill of the mold and compensates for any shrinkage on cooling.Thin wall castings can be made.
10 Low Pressure Casting (Just for your info) Mechanical properties are superior.Since no riser is used (unlike a regular casting), the yield is generally higher since the metal in the pressurized feed tube is still molten and the mold is ready for the next shot right away.Molten metal is always cleanerGas porosity and oxidation defects are minimized.
11 Vacuum Permanent Mold Casting Similar to the low-pressure permanent mold casting, where a vacuum is used instead of a pressure.Reduced air pressure from the vacuum in the mold is used to draw the liquid metal into the cavity (rather than forcing it by pressure)Thin wall castings can be made as in the low-pressure permanent mold casting. In addition, the yields are high since no risers are used.Advantages: Reduced air porosity, greater strength.
12 Die CastingA permanent mold casting process in which molten metal is injected into mold cavity under high pressurePressure is maintained during solidification, then mold is opened and part is removedMolds in this casting operation are called dies; hence the name die castingUse of high pressure to force metal into die cavity is what distinguishes this from other permanent mold processes
14 Die Casting MachinesDesigned to hold and accurately close two mold halves and keep them closed while liquid metal is forced into cavityTwo main types:Hot‑chamber machineCold‑chamber machine
15 Hot-Chamber Die Casting Metal is melted in a container, and a piston injects liquid metal under high pressure into the dieHigh production rates parts per hour not uncommonApplications limited to low melting‑point metals that do not chemically attack plunger and other mechanical componentsCasting metals: zinc, tin, lead, and magnesium
16 Hot-Chamber Die Casting Figure Cycle in hot‑chamber casting: (1) with die closed and plunger withdrawn, molten metal flows into the chamber
17 Hot-Chamber Die Casting Figure Cycle in hot‑chamber casting: (2) plunger forces metal in chamber to flow into die, maintaining pressure during cooling and solidification.
18 Cold‑Chamber Die Casting Machine Molten metal is poured into unheated chamber from external melting container, and a piston injects metal under high pressure into die cavityHigh production but not usually as fast as hot‑chamber machines because of pouring stepCasting metals: aluminum, brass, and magnesium alloysAdvantages of hot‑chamber process favor its use on low melting‑point alloys (zinc, tin, lead)
19 Cold‑Chamber Die Casting Figure Cycle in cold‑chamber casting: (1) with die closed and ram withdrawn, molten metal is poured into the chamber
20 Cold‑Chamber Die Casting Figure Cycle in cold‑chamber casting: (2) ram forces metal to flow into die, maintaining pressure during cooling and solidification.
21 Molds for Die Casting Usually made of tool steel, mold steel, etc Tungsten and molybdenum (good refractory qualities) used to die cast steel and cast ironEjector pins required to remove part from die when it opensLubricants must be sprayed into cavities to prevent sticking
22 Advantages and Limitations Advantages of die casting:Economical for large production quantitiesGood accuracy and surface finishThin sections are possibleRapid cooling provides small grain size and good strength to castingDisadvantages:Generally limited to metals with low metal pointsPart geometry must allow removal from die
23 Centrifugal CastingA family of casting processes in which the mold is rotated at high speed so centrifugal force distributes molten metal to outer regions of die cavityThe group and some examples:True centrifugal casting => Cast iron pipes, tubes, bushings, rings.Semicentrifugal casting => Pulleys and wheelsCentrifuge casting => Parts with non-radial symmetry
24 True Centrifugal Casting Figure Setup for true centrifugal casting.
25 Furnaces for Casting Processes Furnaces most commonly used in foundries:CupolasDirect fuel‑fired furnacesCrucible furnacesElectric‑arc furnacesInduction furnaces
26 CupolasVertical cylindrical furnace equipped with tapping spout near baseUsed only for cast irons
27 Direct Fuel‑Fired Furnaces Small open‑hearth in which charge is heated by natural gas fuel burners located on side of furnaceGenerally used for nonferrous metals such as copper‑base alloys and aluminum
28 Crucible FurnacesMetal is melted without direct contact with burning fuel mixtureSometimes called indirect fuel‑fired furnacesContainer (crucible) is made of refractory material or high‑temperature steel alloyUsed for nonferrous metals such as bronze, brass, and alloys of zinc and aluminum
29 Electric‑Arc Furnaces Charge is melted by heat generated from an electric arcHigh power consumption, but electric‑arc furnaces can be designed for high melting capacityUsed primarily for melting steel
30 Figure 6.9 Electric arc furnace for steelmaking
31 Induction FurnacesUses alternating current passing through a coil to develop magnetic field in metalMelting steel, cast iron, and aluminum alloys are common applications in foundry work
32 Ladles Last slide of students’ presentations Moving molten metal from melting furnace to mold is sometimes done using cruciblesMore often, transfer is accomplished by ladlesFigure Two common types of ladles: (a) crane ladle, and (b) two‑man ladle.
34 Casting QualityThere are numerous opportunities for things to go wrong in a casting operation, resulting in quality defects in the productThe defects can be classified as follows:General defects common to all casting processesDefects related to sand casting process
35 General Defects: Misrun A casting that has solidified before completely filling mold cavityFigure Some common defects in castings: (a) misrun
36 General Defects: Cold Shut Two portions of metal flow together but there is a lack of fusion due to premature freezingFigure Some common defects in castings: (b) cold shut
37 General Defects: Cold Shot Metal splatters during pouring and solid globules form and become entrapped in castingFigure Some common defects in castings: (c) cold shot
38 General Defects: Shrinkage Cavity Depression in surface or internal void caused by solidification shrinkage that restricts amount of molten metal available in last region to freezeFigure Some common defects in castings: (d) shrinkage cavity
39 Sand Casting Defects: Sand Blow Balloon‑shaped gas cavity caused by release of mold gases during pouringFigure Common defects in sand castings: (a) sand blow
40 Sand Casting Defects: Pin Holes Formation of many small gas cavities at or slightly below surface of castingFigure Common defects in sand castings: (b) pin holes
41 Sand Casting Defects: Penetration When fluidity of liquid metal is high, it may penetrate into sand mold or core, causing casting surface to consist of a mixture of sand grains and metalFigure Common defects in sand castings: (e) penetration
42 Sand Casting Defects: Mold Shift A step in cast product at parting line caused by sidewise relative displacement of cope and dragFigure Common defects in sand castings: (f) mold shift
45 Metals for CastingMost commercial castings are made of alloys rather than pure metalsAlloys are generally easier to cast, and properties of product are betterCasting alloys can be classified as:FerrousNonferrous
46 Molten Facts Metal Tin, Lead, Zinc (786F) Aluminum 1220 F Howto?Barely HotHotVery HotToo HotTempBelow 1000F1000F-1500F1500F-2250F2250F or HigherMetalTin, Lead,Zinc (786F)Aluminum 1220 FBrass, Bronze, Gold, Silver or CopperIron, SteelToolsThese alloys can be melted on the stove in a soup can. Caution: Most low-melting alloys are TOXIC, vent well and use a respirator.Aluminum can be melted in a coffee can on the BBQ, use propane, wood or charcoal for fuel.A gas or electric crucible furnace is typical.Electric Induction furnaces are used for large commercial foundries.Cupola furnaces use coke (refined coal) for smaller batches.SafetyneedsSafety GlassesGloves and GlassesThick shirt and pants.Glasses and gloves."Going into a volcano" suit !
47 Ferrous Casting Alloys: Cast Iron Most important of all casting alloysTonnage of cast iron castings is several times that of all other metals combinedSeveral types: gray cast iron, white cast iron, etcTypical pouring temperatures 1400C (2500F), depending on composition
48 Ferrous Casting Alloys: Steel The mechanical properties of steel make it an attractive engineering materialThe capability to create complex geometries makes casting an attractive shaping processDifficulties when casting steel:Pouring temperature of steel is higher than for most other casting metals 1650C (3000F)At such temperatures, steel readily oxidizes, so molten metal must be isolated from airMolten steel has relatively poor fluidity
49 Nonferrous Casting Alloys: Aluminum Generally considered to be very castablePouring temperatures low due to low melting temperature of aluminumTm = 660C (1220F)Properties:Light weightRange of strength properties by heat treatmentEasy to machine
50 Nonferrous Casting Alloys: Copper Alloys Includes bronze, brass, and aluminum bronzeProperties:Corrosion resistanceAttractive appearanceGood bearing qualitiesLimitation: high cost of copperApplications: pipe fittings, marine propeller blades, pump components, ornamental jewelryCopper + Zinc = BrassCopper + Tin + other elements= BronzeBrass,Bronze?
51 Nonferrous Casting Alloys: Zinc Alloys Highly castable, commonly used in die castingLow melting point – melting point of zinc Tm = 419C (786F)Good fluidity for ease of casting