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1、2007 John Wiley&Sons,Inc.M P Groover,Fundamentals of Modern Manufacturing 3/e,METAL CASTING PROCESSES,Sand CastingOther Expendable Mold Casting ProcessesPermanent Mold Casting ProcessesFoundry PracticeCasting QualityMetals for CastingProduct Design Considerations,2007 John Wiley&Sons,Inc.M P Groover
2、,Fundamentals of Modern Manufacturing 3/e,Two Categories of Casting Processes,Expendable mold processes-mold is sacrificed to remove part Advantage:more complex shapes possibleDisadvantage:production rates often limited by time to make mold rather than casting itselfPermanent mold processes-mold is
3、made of metal and can be used to make many castings Advantage:higher production rates Disadvantage:geometries limited by need to open mold,2007 John Wiley&Sons,Inc.M P Groover,Fundamentals of Modern Manufacturing 3/e,Overview of Sand Casting,Most widely used casting process,accounting for a signific
4、ant majority of total tonnage cast Nearly all alloys can be sand casted,including metals with high melting temperatures,such as steel,nickel,and titanium Castings range in size from small to very large Production quantities from one to millions,Figure 11.1 A large sand casting weighing over 680 kg(1
5、500 lb)for an air compressor frame(photo courtesy of Elkhart Foundry).,2007 John Wiley&Sons,Inc.M P Groover,Fundamentals of Modern Manufacturing 3/e,Making the Sand Mold,The cavity in the sand mold is formed by packing sand around a pattern,then separating the mold into two halves and removing the p
6、attern The mold must also contain gating and riser system If casting is to have internal surfaces,a core must be included in mold A new sand mold must be made for each part produced,2007 John Wiley&Sons,Inc.M P Groover,Fundamentals of Modern Manufacturing 3/e,Steps in Sand Casting,Pour the molten me
7、tal into sand moldAllow time for metal to solidifyBreak up the mold to remove casting Clean and inspect castingSeparate gating and riser systemHeat treatment of casting is sometimes required to improve metallurgical properties,Figure is from,2007 John Wiley&Sons,Inc.M P Groover,Fundamentals of Moder
8、n Manufacturing 3/e,The Pattern,A fullsized model of the part,slightly enlarged to account for shrinkage and machining allowances in the castingPattern materials:Wood-common material because it is easy to work,but it warps Metal-more expensive to make,but lasts much longer Plastic-compromise between
9、 wood and metal,Top center is the clay original,then the two part plaster mold used for casting the lead at above,and wax cast from mold,sprued for better brass casting,not yet cast.2008-01-12.,2007 John Wiley&Sons,Inc.M P Groover,Fundamentals of Modern Manufacturing 3/e,Types of Patterns,Figure 11.
10、3 Types of patterns used in sand casting:(a)solid pattern(b)split pattern(c)matchplate pattern(d)cope and drag pattern,2007 John Wiley&Sons,Inc.M P Groover,Fundamentals of Modern Manufacturing 3/e,Core,Fullscale model of interior surfaces of part It is inserted into the mold cavity prior to pouringT
11、he molten metal flows and solidifies between the mold cavity and the core to form the castings external and internal surfaces May require supports to hold it in position in the mold cavity during pouring,called chaplets,Figure 11.4(a)Core held in place in the mold cavity by chaplets,(b)possible chap
12、let design,(c)casting with internal cavity.,2007 John Wiley&Sons,Inc.M P Groover,Fundamentals of Modern Manufacturing 3/e,Desirable Mold Properties,Strength to maintain shape and resist erosion Permeability to allow hot air and gases to pass through voids in sand Thermal stability to resist cracking
13、 on contact with molten metal Collapsibility ability to give way and allow casting to shrink without cracking the casting Reusability can sand from broken mold be reused to make other molds?,2007 John Wiley&Sons,Inc.M P Groover,Fundamentals of Modern Manufacturing 3/e,Foundry Sands,Silica(SiO2)or si
14、lica mixed with other minerals Good refractory properties capacity to endure high temperatures Small grain size yields better surface finish on the cast part Large grain size is more permeable,allowing gases to escape during pouring Irregular grain shapes strengthen molds due to interlocking,compare
15、d to round grainsDisadvantage:interlocking tends to reduce permeability,BindersSand is held together by a mixture of water and bonding clay Typical mix:90%sand,3%water,and 7%clay Other bonding agents also used in sand molds:Organic resins(e g,phenolic resins)Inorganic binders(e g,sodium silicate and
16、 phosphate)Additives are sometimes combined with the mixture to increase strength and/or permeability,2007 John Wiley&Sons,Inc.M P Groover,Fundamentals of Modern Manufacturing 3/e,Types of Sand Mold,Greensand molds-mixture of sand,clay,and water;“Green means mold contains moisture at time of pouring
17、 Drysand mold-organic binders rather than clayAnd mold is baked to improve strength Skindried mold-drying mold cavity surface of a greensand mold to a depth of 10 to 25 mm,using torches or heating lamps,2007 John Wiley&Sons,Inc.M P Groover,Fundamentals of Modern Manufacturing 3/e,Buoyancy in Sand Ca
18、sting Operation,During pouring,buoyancy of the molten metal tends to displace the core,which can cause casting to be defectiveForce tending to lift core=weight of displaced liquid less the weight of core itself Fb=Wm Wc where Fb=buoyancy force;Wm=weight of molten metal displaced;and Wc=weight of cor
19、e,2007 John Wiley&Sons,Inc.M P Groover,Fundamentals of Modern Manufacturing 3/e,Other Expendable Mold Processes,Shell MoldingVacuum MoldingExpanded Polystyrene ProcessInvestment CastingPlaster Mold and Ceramic Mold CastingHere is a good reference web site:http:/,2007 John Wiley&Sons,Inc.M P Groover,
20、Fundamentals of Modern Manufacturing 3/e,Shell Molding,Casting process in which the mold is a thin shell of sand held together by thermosetting resin binder Figure 11.5 Steps in shellmolding:(1)a matchplate or copeanddrag metal pattern is heated and placed over a box containing sand mixed with therm
21、osetting resin.,2007 John Wiley&Sons,Inc.M P Groover,Fundamentals of Modern Manufacturing 3/e,Shell Molding,Figure 11.5 Steps in shellmolding:(2)box is inverted so that sand and resin fall onto the hot pattern,causing a layer of the mixture to partially cure on the surface to form a hard shell;(3)bo
22、x is repositioned so that loose uncured particles drop away;,2007 John Wiley&Sons,Inc.M P Groover,Fundamentals of Modern Manufacturing 3/e,Shell Molding,Figure 11.5 Steps in shellmolding:(4)sand shell is heated in oven for several minutes to complete curing;(5)shell mold is stripped from the pattern
23、;,2007 John Wiley&Sons,Inc.M P Groover,Fundamentals of Modern Manufacturing 3/e,Shell Molding,Figure 11.5 Steps in shellmolding:(6)two halves of the shell mold are assembled,supported by sand or metal shot in a box,and pouring is accomplished;(7)the finished casting with sprue removed.,From,2007 Joh
24、n Wiley&Sons,Inc.M P Groover,Fundamentals of Modern Manufacturing 3/e,Advantages and Disadvantages,Advantages of shell molding:Smoother cavity surface permits easier flow of molten metal and better surface finish Good dimensional accuracy-machining often not requiredMold collapsibility minimizes cra
25、cks in casting Can be mechanized for mass productionDisadvantages:More expensive metal pattern Difficult to justify for small quantities,2007 John Wiley&Sons,Inc.M P Groover,Fundamentals of Modern Manufacturing 3/e,Expanded Polystyrene Process,Figure 11.7 Expanded polystyrene casting process:pattern
26、 of polystyrene is coated with refractory compound;,Uses a mold of sand packed around a polystyrene foam pattern which vaporizes when molten metal is poured into mold Other names:lostfoam process,lost pattern process,evaporativefoam process,and fullmold process Polystyrene foam pattern includes spru
27、e,risers,gating system,and internal cores(if needed)Mold does not have to be opened into cope and drag sections,From www.wtec.org/loyola/casting/fh05_20.jpg,2007 John Wiley&Sons,Inc.M P Groover,Fundamentals of Modern Manufacturing 3/e,Expanded Polystyrene Process,Figure 11.7 Expanded polystyrene cas
28、ting process:(2)foam pattern is placed in mold box,and sand is compacted around the pattern;,Figure 11.7 Expanded polystyrene casting process:(3)molten metal is poured into the portion of the pattern that forms the pouring cup and sprue.As the metal enters the mold,the polystyrene foam is vaporized
29、ahead of the advancing liquid,thus the resulting mold cavity is filled.,2007 John Wiley&Sons,Inc.M P Groover,Fundamentals of Modern Manufacturing 3/e,Advantages and Disadvantages,Advantages of expanded polystyrene process:Pattern need not be removed from the mold Simplifies and speeds moldmaking,bec
30、ause two mold halves are not required as in a conventional greensand moldDisadvantages:A new pattern is needed for every casting Economic justification of the process is highly dependent on cost of producing patterns,2007 John Wiley&Sons,Inc.M P Groover,Fundamentals of Modern Manufacturing 3/e,Expan
31、ded Polystyrene Process,Applications:Mass production of castings for automobile engines Automated and integrated manufacturing systems are used to Mold the polystyrene foam patterns and thenFeed them to the downstream casting operation,2007 John Wiley&Sons,Inc.M P Groover,Fundamentals of Modern Manu
32、facturing 3/e,Investment Casting(Lost Wax Process),A pattern made of wax is coated with a refractory material to make mold,after which wax is melted away prior to pouring molten metal Investment comes from a less familiar definition of invest-to cover completely,which refers to coating of refractory
33、 material around wax pattern It is a precision casting process-capable of producing castings of high accuracy and intricate detail,2007 John Wiley&Sons,Inc.M P Groover,Fundamentals of Modern Manufacturing 3/e,Investment Casting,Figure 11.8 Steps in investment casting:(1)wax patterns are produced,(2)
34、several patterns are attached to a sprue to form a pattern tree,2007 John Wiley&Sons,Inc.M P Groover,Fundamentals of Modern Manufacturing 3/e,Investment Casting,Figure 11.8 Steps in investment casting:(3)the pattern tree is coated with a thin layer of refractory material,(4)the full mold is formed b
35、y covering the coated tree with sufficient refractory material to make it rigid,2007 John Wiley&Sons,Inc.M P Groover,Fundamentals of Modern Manufacturing 3/e,Investment Casting,Figure 11.8 Steps in investment casting:(5)the mold is held in an inverted position and heated to melt the wax and permit i
36、t to drip out of the cavity,(6)the mold is preheated to a high temperature,the molten metal is poured,and it solidifies,2007 John Wiley&Sons,Inc.M P Groover,Fundamentals of Modern Manufacturing 3/e,Investment Casting,Figure 11.8 Steps in investment casting:(7)the mold is broken away from the finishe
37、d casting and the parts are separated from the sprue,2007 John Wiley&Sons,Inc.M P Groover,Fundamentals of Modern Manufacturing 3/e,Investment Casting,Figure 11 9 A onepiece compressor stator with 108 separate airfoils made by investment casting(photo courtesy of Howmet Corp.).,2007 John Wiley&Sons,I
38、nc.M P Groover,Fundamentals of Modern Manufacturing 3/e,Advantages and Disadvantages,Advantages of investment casting:Parts of great complexity and intricacy can be castClose dimensional control and good surface finish Wax can usually be recovered for reuse Additional machining is not normally requi
39、red this is a net shape processDisadvantagesMany processing steps are requiredRelatively expensive process,2007 John Wiley&Sons,Inc.M P Groover,Fundamentals of Modern Manufacturing 3/e,Plaster Mold Casting,Similar to sand casting except mold is made of plaster of Paris(gypsum CaSO42H2O)In mold-makin
40、g,plaster and water mixture is poured over plastic or metal pattern and allowed to set Wood patterns not generally used due to extended contact with water Plaster mixture readily flows around pattern,capturing its fine details and good surface finish,2007 John Wiley&Sons,Inc.M P Groover,Fundamentals
41、 of Modern Manufacturing 3/e,Advantages and Disadvantages,Advantages of plaster mold casting:Good accuracy and surface finish Capability to make thin crosssections Disadvantages:Mold must be baked to remove moisture,which can cause problems in casting Mold strength is lost if over-bakedPlaster molds
42、 cannot stand high temperatures,so limited to lower melting point alloys,2007 John Wiley&Sons,Inc.M P Groover,Fundamentals of Modern Manufacturing 3/e,Ceramic Mold Casting,Similar to plaster mold casting except that mold is made of refractory ceramic material that can withstand higher temperatures t
43、han plaster Can be used to cast steels,cast irons,and other hightemperature alloys Applications similar to those of plaster mold casting except for the metals cast Advantages(good accuracy and finish)also similar,2007 John Wiley&Sons,Inc.M P Groover,Fundamentals of Modern Manufacturing 3/e,Permanent
44、 Mold Casting Processes,Economic disadvantage of expendable mold casting:a new mold is required for every casting In permanent mold casting,the mold is reused many times The processes include:Basic permanent mold castingDie casting Centrifugal casting,2007 John Wiley&Sons,Inc.M P Groover,Fundamental
45、s of Modern Manufacturing 3/e,The Basic Permanent Mold Process,Uses a metal mold constructed of two sections designed for easy,precise opening and closing Molds used for casting lower melting point alloys are commonly made of steel or cast iron Molds used for casting steel must be made of refractory
46、 material,due to the very high pouring temperatures,2007 John Wiley&Sons,Inc.M P Groover,Fundamentals of Modern Manufacturing 3/e,Permanent Mold Casting,Figure 11.10 Steps in permanent mold casting:(1)mold is preheated and coated,2007 John Wiley&Sons,Inc.M P Groover,Fundamentals of Modern Manufactur
47、ing 3/e,Permanent Mold Casting,Figure 11.10 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.,2007 John Wiley&Sons,Inc.M P Groover,Fundamentals of Modern Manufacturing 3/e,Advantages and Limitations,Advantage
48、s 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 stronger Limitations:Generally limited to metals of lower melting point Simpler part geometries compared to sand casting b
49、ecause of need to open the mold High cost of mold,2007 John Wiley&Sons,Inc.M P Groover,Fundamentals of Modern Manufacturing 3/e,Applications of Permanent Mold Casting,Due to high mold cost,process is best suited to high volume production and can be automated accordingly Typical parts:automotive pist
50、ons,pump bodies,and certain castings for aircraft and missiles Metals commonly cast:aluminum,magnesium,copperbase alloys,and cast iron,2007 John Wiley&Sons,Inc.M P Groover,Fundamentals of Modern Manufacturing 3/e,Die Casting,A permanent mold casting process in which molten metal is injected into mol
