Chapter 13 Multiple-Use-Mold Casting Processes (Part I) EIN 3390 Manufacturing Processes Summer A, 2012.

Chapter 13Chapter 13

Multiple-Use-Mold Multiple-Use-Mold Casting ProcessesCasting Processes

(Part I)(Part I)

EIN 3390 Manufacturing ProcessesEIN 3390 Manufacturing ProcessesSummer A, 2012Summer A, 2012

Shape-Producing ProcessesShape-Producing Processes

Figure 11-1 The four materials processing families, with subgroups and typical processes.

13.1 Introduction13.1 IntroductionIn expendable mold casting, a separate

mold is produced for each casting and has the following disadvantages:◦Low production rate◦Quality control issues, such as dimensional

and property variation due to Mold to be crated each time Variation in mold consistency Mold strength Mold moisture content Pattern removal

13.1 Introduction13.1 IntroductionMultiple-use molds

Assets:◦Higher productivity ◦Good product quality

Liability:◦Metal molds are limited to low melting temperature nonferrous metals and alloys

◦Part size limitation◦Higher cost of dies or molds

13.2 Permanent-Mold Casting13.2 Permanent-Mold CastingAlso known as gravity die castingMold materials:

◦Gray cast iron, alloy cast iron, steel, bronze, or graphite

Most molds are made in segments with hinges to allow rapid and accurate closing◦Molds are preheated to improve properties

Liquid metal flows through the mold cavity by gravity flow

Permanent Mold CastingPermanent Mold CastingProcess can be repeated immediately

because the mold is still warm from the previous casting

Most frequently cast metals◦Aluminum, magnesium, zinc, lead, copper, and

their alloys◦If steel or iron is to be used, a graphite mold

must be used

Advantages of Permanent-Mold Advantages of Permanent-Mold CastingCastingNear- net shapesLittle finish machiningReusable moldsGood surface finishConsistent dimensionsDirectional solidificationFast cooling rate to produce a strong

structureCore can be used to increase complexity

Disadvantages of Permanent Disadvantages of Permanent Mold CastingMold Casting

Limited to lower melting temperature alloys

High mold costs◦Mold life is strongly tied to cost◦Mold life is dependent on the following factors:

Alloys being cast, especially melting temperature Mold material Pouring temperature Mold temperature Mold configuration

◦High production runs can validate high mold costsLimited mold complexity

Low Pressure Permanent-Mold Low Pressure Permanent-Mold CastingCastingMolds are not permeable

◦Venting in Permanent Casting Slight crack between mold halves Very small vent holes to permit escape of trapped

air, but not passage of molten metalDesign feature affects mold life

◦Difference in section size through moldRemoval castings immediately after

solidification

Permanent Mold CastingPermanent Mold Casting

Low Pressure Permanent-Mold CastingLow Pressure Permanent-Mold Casting

Low pressure permanent-mold (LPPM) casting◦Mold is upside down and connected to a crucible that

contains the molten metal◦Pressure difference induces upward flow◦Metals are exceptionally clean because it is fed

directly into the mold◦Little or no turbulence during flow◦No risers, yields > 85%◦Mechanical properties are about 5% better than

those of conventional permanent-mold casting.Typical metals cast using low pressure process

◦Aluminum, magnesium, and copper

Low-Pressure Permanent-Mold Low-Pressure Permanent-Mold CastingCasting

Figure 13-2 Schematic of the low-pressure permanent-mold process. (Courtesy of Amsted Industries, Chicago, IL.)

Vacuum Permanent-Mold CastingVacuum Permanent-Mold CastingAtmospheric pressure in the chamber

forces the metal upward after the vacuum is drawn

Thin-walled castings can be madeExcellent surface qualityCleaner metals than low pressure

◦Lower dissolved gas contentBetter mechanical properties than low

pressure castingFinal castings range 0.2 to 5 kg and have

better mechanical properties than LPPM.

Vacuum Permanent-Mold Vacuum Permanent-Mold CastingCasting

Figure 13-3 Schematic illustration of vacuum permanent-mold casting. Note the similarities to the low-pressure process.

13.3 Die Casting13.3 Die CastingMolten metal is forced into the mold under

high pressureHeld under high pressure during

solidificationCastings can have fine sections and

complex detailsLong mold lifeTypical metals cast

◦Zinc, copper, magnesium, aluminum, and their alloys

Advantages of Die CastingAdvantages of Die CastingHigh production ratesGood strengthIntricate shapesDimensional precisionExcellent surface qualitiesSmall-medium sized castings

Disadvantages of Die CastingDisadvantages of Die CastingHigh cost for diesLess flexibility for productsLimited to small- to –medium-sized

partsMost for nonferrous metals and alloys

Die Modifications and Die Life Die Modifications and Die Life Die complexity can be improved through

the use of◦Water cooled passages◦Moving pins to eject castings

Die life ◦Limited by erosion and usage temperature◦Surface cracking◦Heat checking◦Thermal fatigue

Die Modifications and Die Life Die Modifications and Die Life Die Materials

◦Harden tool steels, since cast iron cannot withstand casting pressures

Vary Pressure on Molten Metal during casting◦Reduce turbulence and air entrapment by lower injection pressures, and followed by higher pressures after mold has been filled completely and metal starts to solidify.

Die-Casting DiesDie-Casting Dies

Figure 13-4 Various types of die-casting dies. (Courtesy of American Die Casting Institute, Inc., Des Plaines, IL.)

Basic Types of Die-CastingBasic Types of Die-CastingHot chamber castings

◦ Gooseneck chamber for molten metal◦ Plunger to control molten metal flow

Fast cycling timesNo handling or transfer of molten metalCan’t used for higher-melting-point metals

◦Aluminum tends to pick up some iron of casting equipments

Used with zinc, tin, and lead-based alloys

Die Casting (Hot-Chamber)Die Casting (Hot-Chamber)

Figure 13-5 (Below) Principal components of a hot-chamber die-casting machine. (Adapted from Metals Handbook, 9th ed., Vol 15, p. 287, ASM International, Metals Park, OH.)

Basic Types of Die CastingBasic Types of Die CastingCold-chamber machines

◦Used for materials not suitable for hot chamber machines

◦Separated furnace◦Drive measured quantity of molten metal into

unheated chamber by hydraulic plunger◦Maintain or increase pressure until solidification

done◦Typical materials

Aluminum, magnesium, copper, and high-aluminum zinc◦Longer operating cycle than hot-chamber◦High productivity

Die Casting (Cold-Chamber)Die Casting (Cold-Chamber)

Figure 13-6 (Above) Principal components of a cold-chamber die-casting machine. (Adapted from Metals Handbook, 9th ed., Vol 15, p. 287, ASM International, Metals Park, OH.)

Summary of Die CastingSummary of Die CastingDies fill so fast with metal, only little time for

the air in the runner and die to escapeMolds offer no permeability

◦Air can become trapped and cause defectsRisers are not used because of the high

pressures usedSand cores can’t be used due to high

pressures◦Cast-in inserts can be used

High production ratesLittle post casting finishing necessary

Die CastingDie Casting

Die Cast MaterialsDie Cast Materials