ch11_more_metal_casting

7/31/2019 ch11_more_metal_casting

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METAL CASTINGPROCESSES1.Sand Casting2.Other Expendable Mold Casting

Processes

3.Permanent Mold Casting Processes4.Foundry Practice5.Casting Quality6.Metals for Casting7.Product Design Considerations

2007 John Wiley & Sons, Inc. M PGroover, Fundamentals of Modern

Manufacturing3/e


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Two Categories of CastingProcesses1. by need to open moldExpendable mold

processes - mold is sacrificed toremove parto

Advantage: more complex shapes possibleo Disadvantage: production rates often limited by

time to make mold rather than casting itself

2.Permanent mold processes - mold is

made of metal and can be used tomake many castingsoAdvantage: higher production rateso Disadvantage: geometries limited

2007 John Wiley & Sons, Inc. M PGroover, Fundamentals of Modern

Manufacturing3/e


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Overview of Sand Casting

Most widely used casting process, accounting for asignificant majority of total tonnage cast

Nearly all alloys can be sand casted, including metalswith high melting temperatures, such as steel, nickel,and titanium

Castings range in size from small to very large

Production quantities from one to millions


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Making the Sand Mold

The cavityin the sandmold is formed by packingsand around a pattern,then separating the mold

into two halves andremoving the pattern

The mold must alsocontain gating and riser

system If casting is to have

internal surfaces, a coremust be included in mold

A new sand mold must be


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Steps in Sand Casting

1.Pour the molten metal into sand mold2.Allow time for metal to solidify3.Break up the mold to remove casting4.Clean and inspect casting

oSeparate gating and risersystem

5.Heat treatment of casting is sometimes

required to improve metallurgical

properties


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The PatternA full-sized model of the part, slightly enlarged to account forshrinkage and machining allowances in the casting Pattern materials:

o Wood - common material because it is easyto work, but it warps

o Metal - more expensive to make, but lastsmuch longer

o Plastic - compromise between wood andmetal


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Types of Patterns

Figure 11.3 Types of patterns used in sand casting:(a) solid pattern(b) split pattern(c) match-plate pattern(d) cope and drag pattern


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CoreFull-scale model of interior surfaces of part It is inserted into the mold cavity prior to pouring The molten metal flows and solidifies between the mold

cavity and the core to form the casting's external and internalsurfaces

May require supports to hold it in position in the mold cavity

during pouring, called chaplets

2007 John Wiley & Sons, Inc. M P

Groover, Fundamentals of ModernManufacturing3/e


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Desirable Mold Properties

Strength - to maintain shape and resisterosion

Permeability - to allow hot air and gasesto pass through voids in sand

Thermal stability - to resist cracking oncontact with molten metal

Collapsibility - ability to give way and

allow casting to shrink without crackingthe casting Reusability - can sand from broken mold

be reused to make other molds?2007 John Wiley & Sons, Inc. M P



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Foundry Sands

Silica (SiO2) or silica mixed with other minerals Good refractory properties - capacity to endurehigh temperatures

Small grain size yields better surface finish on

the cast part Large grain size is more permeable, allowinggases to escape during pouring

Irregular grain shapes strengthen molds due to

interlocking, compared to round grainso Disadvantage: interlocking tends to reduce

permeability




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Types of Sand Mold

Green-sand molds - mixture of sand, clay,and water;o Green" means mold contains moisture at time of

pouring

Dry-sand mold - organic binders ratherthan clayo And mold is baked to improve strength

Skin-dried mold - drying mold cavity

surface of a green-sand mold to a depthof 10 to 25 mm, using torches or heatinglamps




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Buoyancy in Sand CastingOperation

During pouring, buoyancy of themolten metal tends to displace thecore, which can cause casting to be

defective Force tending to lift core = weight of

displaced liquid less the weight ofcore itself

Fb= Wm- Wc

where Fb= buoyancy force; Wm= weightof molten metal displaced; and Wc=

weight of core2007 John Wiley & Sons, Inc. M P



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Expanded PolystyreneProcess

Figure 11.7 Expanded polystyrene casting process: pattern ofpolystyrene is coated with refractory compound;

Uses a mold of sand packed around a polystyrene foam pattern whichvaporizes when molten metal is poured into mold Other names: lost-foam process, lost pattern process,

evaporative-foam process, and full-mold process Polystyrene foam pattern includes sprue, risers, gating system,

and internal cores (if needed)

Mold does not have to be opened into cope and drag sections


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Investment Casting

Figure 11.8 Steps in investment casting: (5) the mold is held in aninverted position and heated to melt the wax and permit it to drip out ofthe cavity, (6) the mold is preheated to a high temperature, the molten

metal is poured, and it solidifies


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Advantages andDisadvantages Advantages of investment casting:

o Parts of great complexity and intricacy can becast

o Close dimensional control and good surface

finisho Wax can usually be recovered for reuseoAdditional machining is not normally

required - this is a net shape process

Disadvantageso Many processing steps are requiredo Relatively expensive process


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Plaster Mold Casting

Similar to sand casting except mold is madeof plaster of Paris (gypsum - CaSO4-2H2O) In mold-making, plaster and water

mixture is poured over plastic or metalpattern and allowed to seto Wood patterns not generally used due to extended

contact with water

Plaster mixture readily flows around

pattern, capturing its fine details and goodsurface finish




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Advantages andDisadvantages

Advantages of plaster moldcasting:o Good accuracy and surface finisho Capability to make thin cross-sections

Disadvantages:o Mold must be baked to remove moisture,

which can cause problems in castingo Mold strength is lost if over-bakedo

Plaster molds cannot stand hightemperatures, so limited to lower meltingpoint alloys




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Ceramic Mold Casting

Similar to plaster mold casting exceptthat mold is made of refractory ceramicmaterial that can withstand higher

temperatures than plaster Can be used to cast steels, cast irons,

and other high-temperature alloys Applications similar to those of plaster

mold casting except for the metalscast

Advantages (good accuracy and

finish) also similar2007 John Wiley & Sons, Inc. M P



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Die Casting

A permanent mold casting process in whichmolten metal is injected into mold cavityunder high pressure Pressure is maintained during

solidification, then mold is opened andpart is removed

Molds in this casting operation are called

dies; hence the name die casting Use of high pressure to force metal intodie cavity is what distinguishes this fromother permanent mold processes




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Die Casting Machines

Designed to hold and accurately closetwo mold halves and keep themclosed while liquid metal is forced into

cavity Two main types:

1.Hot-chamber machine2.Cold-chamber machine




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Molds for Die Casting

Usually made of tool steel, mold steel,or maraging steel

Tungsten and molybdenum (good

refractory qualities) used to die caststeel and cast iron Ejector pins required to remove part

from die when it opens

Lubricants must be sprayed intocavities to prevent sticking




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Advantages and Limitations

Advantages of die casting:o Economical for large production quantitieso Good accuracy and surface finisho Thin sections are possibleo Rapid cooling provides small grain size and

good strength to casting

Disadvantages:o Generally limited to metals with low metal

pointso Part geometry must allow removal from die




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THANK YOU



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