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2013-01 EM315_EM311 CH02 Casting Processes
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Transcript of 2013-01 EM315_EM311 CH02 Casting Processes
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CHAPTER 02
CASTING PROCESSES
Manufacturing Processes
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Introduction
Solidification of Metals Fluid Flow
Fluidity of Molten Metal
Heat Transfer
Defects
PART I : Fundamentals of Casting Processes
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Learning Outcomes
Mechanisms of solidification in metals and their alloys
Significance of solidification patterns in casting
Characteristics of fluid flow and heat transfer in molds and their effects
Role of gases and shrinkage in defect formation in casting
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Introduction
Casting process basically involves:a) Pouring molten metal into a mold patterned after the part to be
manufactured
b) Allowing it to solidify
c) Removing the part from the mold
Important considerations in casting operations:
Flow of the molten metal into the mold cavity
Solidification and cooling of the metal in the mold
Influence of the type of mold material
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Solidification of Metals
Pure metals
T as a function of time Density as a function of time
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Solidification of Metals
Pure metals
When temperature of the molten metal drops to its freezing point,latent heat of fusion is given off
Solidification frontmoves through the molten metal from the moldwalls in toward the center
Metals shrink during cooling and solidification Shrinkage can lead to microcracking and associated porosity
Grains grow in a direction opposite to heat transfer out through themold
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Solidification of Metals
Pure metals
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Solidification of Metals
Alloys
Solidification in alloys startswhen below liquidus andcomplete when it reaches thesolidus
Alloy in a mushyorpastystate consisting ofcolumnardendrites
Dendrites have interlocking3D arms and branches
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Solidification of MetalsAlloys
Width of the mushy zone is described in terms offreezing range,TL - TS
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Solidification of Metals
Alloys
Effects of cooling rates
Slow cooling rates result in coarse dendritic structures with largespacing between dendrite arms
For higher cooling rates the structure becomes finerwith smallerdendrite arm spacing
Smaller the grain size, the strength and ductility of the cast alloyincrease, microporosity in the casting decreases, and tendency forcasting to crack decreases
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Solidification of Metals
Alloys
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Solidification of Metals
Structureproperty relationships
Under the faster coolingrates, cored dendrites areformed
Surface of dendrite has ahigher concentration of
alloying elements, due tosolute rejection from the coretoward the surface duringsolidification of the dendrite(microsegregation)
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Solidification of Metals
Structureproperty relationships
Macrosegregation involves differences in composition throughoutthe casting itself
Gravity segregation is the process where higher density inclusionsand lighter elements float to the surface
Dendrite arms are not strong and can be broken up by agitation ormechanical vibration during solidification results in finer grain sizewith equiaxed nondendritic grains distributed uniformly
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Fluid Flow
Two basic principles of fluid flow are relevant to gating design:1. Bernoulllis theorem
2. Law of mass continuity
Bernoullis theorem
Mass continuityQ =A1v1 =A2v2
Sprue designA1/A2= h2/h1
Modelingv= c2gh v= c2gh-x
Flow characteristicsRe = vD/
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Fluidity of Molten Metal
Viscosity fluidity
Surface tension fluidity
Inclusions fluidity
Solidification pattern
Mold design
Mold material and its
surface characteristics
Degree of superheat
fluidity Pouring rate fluidity
Heat transfer
Characteristics of molten metal Casting parameters
Fluidityconsists of two basic factors:1) Characteristics of the molten metal2) Casting parameters
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Heat Transfer
Solidification time
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Solidification time = C(Volume/Surface area)n
C= a constant that reflects
a) mold materialb) metal properties (including latent heat)c) temperature
n = a value between 1.5-2
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Heat Transfer
Shrinkage
Shrinkage, which causes dimensional changes and (sometimes)cracking, is the result of the following three sequential events:
1. Contraction of molten metal as it cools prior to its solidification
2. Contraction of metal during phase change from liquid to solid (latent heatof fusion)
3. Contraction of the solidified metal (casting) as its temperature drops toambient temperature
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Defects
Standard nomenclature for casting defects:AMetallic projections (fins, flash, projections)
BCavities (blowholes, pinholes, shrinkage cavities)
CDiscontinuities (cracks, cold or hot tearing, cold shuts)
DDefective surface (folds, laps, scars, adhering sand layers, oxidescale)
EIncomplete casting (misruns, runout)
FIncorrect dimensions or shape (improper shrinkage allowance,pattern-mounting error, irregular contraction, deformed pattern,warped casting)
GInclusions
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Defects
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FIGURE 2.? Examples of common defects in castings
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Defects
Porosity
Porosityis caused byshrinkage and/or dissolvedgases
Porosity can cause ductilityto a casting and surface
finish
Shrinkage can be reducedby:
Adequate liquid metal
Internal or external chills With alloys, mold materials
with high thermalconductivity may be used
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FIGURE 2.? Various types of (a) internal and(b) external chills used in castings to eliminate
porosity caused by shrinkage (chills are
placed in regions where there is a larger
volume of metal as in (c))
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Defects
Porosity
Because liquid metals havegreater solubility for gasesthan do solid metals, when ametal begins to solidify, thedissolved gases are expelled
from the solution Gases may also result from
reactions of the molten metalwith mold materials
Gases either accumulate inregions of existing porosity(interdendritic regions) orcause microporosity
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FIGURE 2.? Solubility of hydrogen in aluminum
(note the sharp decrease in solubility as the
molten metal begins to solidify)
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Defects
General defects
Misruns: Solidification ofcasting before completely fillingmold cavity
Cause:
Insufficient fluidity of moltenmetal
Low pouring temperature
Pouring too slowly
Small cross section within
mold cavity
Remedy:
Proper casting design
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Defects
General defects
Cold shuts: Lack of fusionwhen two streams of moltenmetal meet from oppositedirection in the pouring ofcasting
Cause:
Insufficient fluidity of moltenmetal
Low pouring temperature
Pouring too slowly Small cross section within
mold cavity
Remedy: Proper casting design
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Defects
General defects
Cold shots: Formation of smallsolid metal globules entrappedin but not entirely fused with thecasting
Cause:
Metal splatters duringpouring
Remedy:
Proper pouring procedures
and gating system
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Defects
General defects
Shrinkage cavity: Internal voidor surface depression in casting
Cause:
Uncontrolled solidification
Remedy: Proper riser design
Adequate risers and feeders
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Defects
General defects
Microporosity: A network ofsmall voids distributedthroughout casting, usuallyassociated with alloys
Cause:
Localized solidificationshrinkage of the final moltenmetal in the dendriticstructure
Remedy:
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Defects
General defects
Hot tearing/cracking:Separation of metal at a point ofhigh tensile stress
Cause:
Casting is restrained fromcontraction aftersolidification or early stagesof cooling
Remedy:
Remove part from moldimmediately after freezing
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Defects
General defects
Fin: A thin metal projection atthe parting of mold or coresections
Cause:
Incorrect assembly of coresand molds
Improper clamping andsealing
Remedy:
Proper clamping of coresand mold
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Defects
General defects
Warped casting: Deformationin casting
Cause:
Large cross sections orintersections are prone towarping
Remedy:
Proper casting design
Use of ribs
Allowances can be givenalong with machiningallowance to remove bymachining
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Defects
General defects
Inclusions: Unwanted particlescontained within the materialact as stress raiserscompromising the castings
strength
Cause: Interaction of molten metal
with the environmentincluding the atmosphere(chemical reactions with
oxygen), and the mold itself
Remedy:
Good mold maintenance
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Defects
Defects (sand casting)
Sand blow: A balloon-shapedgas cavity at or below castingsurface near the top of casting
Cause:
Release of mold gasesduring pouring
Low permeability, poorventing, and high moisturecontent of the sand mold
Remedy: Provide sufficient
permeability and vent holes
Minimum quantity of water
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Defects
Defects (sand casting)
Pinholes: Formation of manysmall gas cavities at or slightlybelow the surface of casting
Cause:
Release of mold gasesduring pouring
Low permeability, poorventing, and high moisturecontent of the sand mold
Remedy: Provide sufficient
permeability and vent holes
Minimum quantity of water
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Defects
Defects (sand casting)
Sandwash: Irregularity in thesurface of casting
Cause:
Erosion of sand mold duringpouring
The contour of erosion isimprinted into surface of thefinal cast part
Remedy:
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Defects
Defects (sand casting)
Scabs: A rough area of thecasting due encrustations ofsand and metal
Cause:
Portions of the mold surface
flaking off duringsolidification and becomingembedded in the castingsurface
Remedy:
Reduce clay content
Use of additives to reducethermal expansion of sand
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Defects
Defects (sand casting)
Penetration: Surface of castingconsists of a mixture of sandgrins and metal
Cause:
When the fluidity of the
liquid metal is high, it maypenetrate into the sand moldor sand core after freezing
Remedy:
Harder packing of sandmolds
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Defects
Defects (sand casting)
Mold shift: A step in the castproduct at the parting line
Cause:
Sidewise displacement ofthe cope with respect to the
drag caused by buoyancy ofthe molten metal
Remedy:
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Defects
Defects (sand casting)
Core shift: A similar movementwith the core
Cause:
Vertical displacement of thecore caused by buoyancy of
the molten metal
Remedy:
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Defects
Defects (sand casting)
Mold crack: Formation of fin onfinal casting
Cause:
If mold strength isinsufficient a crack may
develop into which liquidmetal can seep
Remedy:
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Defects (sand casting)
Swell: Localized or overallenlargement of castings
Cause:
Enlargement of mold cavityby metal pressures
Insufficient ramming of thesand
Rapid pouring of moltenmetal
Insufficient weighting ofmold
Remedy: Avoid rapid pouring
Provide sufficient ram onsands
Proper weighting of molds
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Introduction
Expendable-Mold Casting Processes
Permanent-Mold Casting Processes
Inspection of Castings
Melting Practice and Furnaces
PART II : Metal Casting Processes
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Learning O tcomes
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Characteristics of expendable-mold and permanent-mold processes Applications, advantages, and limitations of common casting processes
Inspection techniques for castings
Brief review of melting practice and furnaces
Learning Outcomes
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Introduction
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Introduction
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Introduction
Introduction
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Introduction
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Introduction
Expendable mold processes - mold is sacrificed to remove part Advantage: more complex shapes possible
Disadvantage: production rates often limited by the time to make moldrather than casting itself
Permanent mold processes - mold is made of metal and can beused to make many castings
Advantage: higher production rates
Disadvantage: geometries limited by need to open mold
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Sand casting
Most prevalent form of casting process, accounting for a significantmajority of total tonnage cast
Nearly all alloys can be sand casted, including metals with highmelting temperatures, such as steel, nickel, and titanium
Castings range in size from small to very large
Production quantities from one to millions
Application: machine bases, large turbine impellers, propellers,plumbing fixtures
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Sand casting
Sand casting weighing over 680 kg (1500 lb) for an air compressor
frame (photo courtesy of Elkhart Foundry)
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Sand casting
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FIGURE 2.? Outline of production steps in a typical sand casting operation
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Sand casting
Sands Sand-casting operations use silica sand (SiO2) as mold material;
naturally bonded (bank sand) orsynthetic (lake sand)
Inexpensive and good refractory properties (high-temperaturecharacteristics and high melting point)
Fine and roundstrength ,permeability, surface finish Coarsecollapsibility, permeability , surface finish
Irregularstrength , permeability
Sand making: Sand (90%) + Clay (7%) + Water (3%)
Other bonding agents: Organic resins (e.g. phenolic resins)
Inorganic binders (e.g. sodium silicate and phosphate)
+ Additivesstrength , permeability EM315/EM311 MANUFACTURING PROCESSES Nur Azyyah
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Sand casting
Types of sand molds1. Green-sand mold
Green means mold is moist/damp at time of pouring
Skin-dried method: mold surfaces are air dried or usingtorches/heating lamps to a depth of 10-25mm
2. Cold-box mold Organic and inorganic binders are blended into the sand to bond the
grains chemically
3. No-bake mold
Synthetic liquid resin is mixed with the sand and allow to harden atroom temperature
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Sand casting
Desirable mold properties Strength to maintain shape and resist erosion
Permeability to allow hot air and gases to pass through voids insand
Thermal stability to resist cracking on contact with molten metal
Collapsibility ability to give way and allow casting to shrink withoutcracking the casting
Reusability
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Sand casting
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FIGURE 2.? Schematic illustration of a sand mold showing various features
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Sand casting
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Feature Function
Flask Support mold
Cope Top-piece mold
Drag Bottom-piece mold
Parting line Seam between two-piece molds
Cheeks Additional parts when more than two piecesPouring basin/cup Into which molten metal is poured
Sprue Through which molten metal flows downward
Runner system Channels that carry molten metal from sprue to mold cavity
Gates Inlets into mold cavity
Risers Supply additional molten metal to casting as it shrinks
Cores Inserts to form hollow regions
Vents Exhaust gases and air from mold cavity
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Sand casting
Patterns
Patterns (full-sized model of a part, slightly enlarged to account forshrinkage and machining allowances) are used to mold the sandmixture into the shape of the casting
Pattern materials: Wood, metal, plastic
Selection of a pattern material depends on:1. Size and shape of the casting
2. Dimensional accuracy
3. Quantity of castings required
4. Molding process
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Sand casting
Patterns
Patterns can be designed with a variety of features to fit specificapplications and economic requirements
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FIGURE 2.? Types of pattern used in sand casting; (a) solid pattern, (b) split pattern,
(c) match-plate pattern, and (d) cope and drag pattern
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Sand casting
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Sand casting
Patterns
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FIGURE 2.? A typical
metal match-plate
pattern used in sand
casting
FIGURE 2.? Taper on
patterns for ease of
removal from the sand
mold
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Sand casting
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Sand casting
Cores
Cores (full-scale model of interior surfaces of a part) are placed inthe mold cavity to form the interior surfaces of the casting
Made of sand aggregates for strength, permeability, refractory,collapsibility
Placed in mold cavity prior to pouring Anchored by core prints, which are recesses added to the pattern to
support the core and to provide vents for the escape of gases
Metal supports (chaplets) may be used to anchor the core in place
Molten metal flows and solidifies between the mold cavity and the core
to form the castings external and internal surfaces Removed from the finished part during shakeout and further
processing
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Sand casting
Cores
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FIGURE 2.? Examples of sand
cores showing core prints and
chaplets to support cores
FIGURE 2.? (a) Core held in
place in the mold cavity by
chaplets (b) possible chapletdesign (c) casting
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Sand casting
Sand-molding machines
In vertical flaskless molding, the halves of the pattern form avertical chamber wall against which sand is blown and compacted
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FIGURE 2.? Vertical flaskless molding (a) Sand is squeezed between two halves
of the pattern (b) Assembled molds pass along an assembly line for pouring (c)
A vertical flaskless molding line
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Sand casting
The sand-casting operation
The cavity in sand mold is formed by packing sand around apattern, then separating the mold into two halves and removing thepattern
After the mold has been shaped and the cores have been placed in
position, the two halves (cope and drag) are closed, clamped, andweighted down
After solidification, the casting is shaken out of its mold, and thesand and oxide layers are removed by vibration or sand blasting
Castings are cleaned by shot blasting
Risers and gates are cut off by oxyfuel-gas cutting, sawing,shearing, and abrasive wheels, or trimmed in dies
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Sand casting
The sand-casting operation
Castings are cleaned further by electrochemical or pickling
Castings may subsequently be heat treatedto improve certainproperties required for its intended service use
Finishing operations may involve machining, straightening, or
forging with dies (sizing) to obtain final dimensions
Inspection is carried out to ensure that the casting meets all designand quality-control requirements
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Sand casting
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Sand casting
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Sand casting
(a) A mechanical drawing of the part is used to generate a design forthe pattern (considerations such as part shrinkage and draft mustbe built into the drawing)
(b-c) Patterns have been mounted on plates equipped with pins foralignment
(d-e) Core boxes produce core halves which are pasted together(f) The cope half of the mold is assembled by securing the cope
pattern plate to the flask with aligning pins and attaching inserts toform the sprue and risers
(g) The flask is rammed with sand and the plate and inserts are
removed
(h) The drag half is produced in a similar manner with the patterninserted
Expendable-Mold Casting Processes
Sand casting
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Sand casting
(i) The pattern, flask, and bottom board are inverted; and the pattern iswithdrawn, leaving the appropriate imprint
(j) The core is set in place within the drag cavity
(k) The mold is closed by placing the cope on top of the drag andsecuring the assembly with pins
(l) After the metal soidifies, the casting is removed from the mold(m) The sprue and risers are cut off and recycled, and the casting is
cleaned, inspected, and heat treated (if necessary)
Expendable-Mold Casting Processes
Shell molding
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g
Casting process in which themold is a thin shell of sand heldtogether by thermosetting resinbinder
(1) A metal pattern is heated
and placed over a boxcontaining sand mixed withthermosetting resin
(2) Box is inverted so that sandand resin fall onto the hot
pattern, causing a layer of themixture to partially cure on thesurface to form a hard shell
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Shell molding
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g
(3) Box is repositioned so looseuncured particles drop away
(4) Sand shell is heated in ovenfor several minutes to completecuring
(5) Shell mold is stripped frompattern
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Shell molding
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g
(6) Two halves of the shell moldare assembled, supported bysand or metal shot in a box,and pouring is accomplished
(7) Finished casting with sprue
removed
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Expendable-Mold Casting Processes
Shell molding
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g
Advantages:
Smoother cavity surface permits easier flow of molten metal and bettersurface finish
Good dimensional accuracy
Mold collapsibility minimizes cracks in casting
Can be mechanized for mass production Disadvantages:
More expensive metal pattern
Difficult to justify for small quantities
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Expendable-Mold Casting Processes
Vacuum molding
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Uses sand mold held together by vacuum pressure rather than by a
chemical binder
The term "vacuum" refers to mold making rather than castingoperation itself
Developed in Japan around 1970
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Expendable-Mold Casting Processes
Vacuum molding
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Advantages:
Easy recovery of the sand, since no binders
Sand does not require mechanical reconditioning done when bindersare used
Since no water is mixed with sand, moisture-related defects areavoided
Disadvantages: Slow process
Not readily adaptable to mechanization
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Expendable-Mold Casting Processes
Evaporative-pattern casting (lost-foam process)
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Evaporative-pattern castingprocess uses a polystyrene pattern,
which evaporates upon contact with molten metal to form a cavityfor the casting
Used for ferrous and nonferrous metals which is applicable toautomotive industry
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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Expendable-Mold Casting Processes
Evaporative-pattern casting (lost-foam process)
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(1) Polystyrene foam pattern is
coated with refractorycompound
(2) Foam pattern is placed in moldbox, and sand is compactedaround the pattern
(3) Molten metal is poured into theportion of the pattern that formsthe pouring cup and sprue
As the metal enters the mold, thepolystyrene foam is vaporized
ahead of the advancing liquid,thus filling the mold cavity
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Expendable-Mold Casting Processes
Evaporative-pattern casting (lost-foam process)
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Advantages:
Pattern need not be removed from the mold
Simplifies and speeds mold-making, because two mold halves are notrequired as in a conventional green-sand mold
Disadvantages:
A new pattern is needed for every casting Economic justification of the process is highly dependent on cost of
producing patterns
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Expendable-Mold Casting Processes
Evaporative-pattern casting (lost-foam process)
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Expendable-Mold Casting Processes
Evaporative-pattern casting (lost-foam process)
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CASE STUDY : Lost foam casting of engine blocks
One of the most important parts of in an internal combustion engine is the engineblock that provides the basic structure that encloses the engine, pistons andcylinders, and encounters significant pressure during operation. Recognizing theindustry pressures on high-quality, low-cost and lightweight designs, MercuryCastings built a lost-foam casting line to produce aluminum engine blocks andcylinder heads. One example of a part produced through lost-foam casting is a 45-
kW 3-cylinder engine block used for marine applications. Previously manufacturedas eight separate die castings, the block was converted to a single, 10-kg lost foamcasting with a weight and cost savings of 1 kg and $25 on each block, respectively.Lost-foam casting also allowed consolidation of the engines cylinder head andexhaust and cooling systems into the block and eliminated the associated machiningand fasteners required in sand-cast or die-cast designs. Since the pattern contained
holes and these could be cast without the use of cores, numerous drilling operationswere also eliminated.
Expendable-Mold Casting Processes
Investment casting (lost-wax process)
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A pattern made of wax is invested with refractory material to make
the mold, after which wax is melted away prior to pouring moltenmetal
"Investment" comes from a less familiar definition of "invest" - "to covercompletely," which refers to coating of refractory material around waxpattern
Precision casting process
Capable of producing ferrous and nonferrous castings of intricate detailwith high accuracy
Application: office equipments, mechanical components (e.g. gears)
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Expendable-Mold Casting Processes
Investment casting (lost-wax process)
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(1) Wax patterns are produced
(2) Several patterns areattached to a sprue to form apattern tree
(3) Pattern tree is coated with a
thin layer of refractorymaterial
(4) Full mold is formed bycovering the coated tree withsufficient refractory material
to make it rigid
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Investment casting (lost-wax process)
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(5) Mold is held in an inverted
position and heated to meltthe wax and permit it to dripout of the cavity
(6) Mold is preheated to a hightemperature, the moltenmetal is poured, and itsolidifies
(7) Mold is broken away fromthe finished casting and the
parts are separated from thesprue
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Investment casting (lost-wax process)
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Also called lost-wax process Used to make office equipment, and mechanical
components such as gears
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Expendable-Mold Casting Processes
Investment casting (lost-wax process)
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One-piece compressor stator with 108 separate airfoils made by
investment casting (photo courtesy of Howmet Corp.)
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Expendable-Mold Casting Processes
Investment casting (lost-wax process)
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Advantages:
Parts of great complexity and intricacy can be cast
Close dimensional control and good surface finish
Wax can usually be recovered for reuse
This is a net shape process
Additional machining is not normally required Disadvantages:
Many processing steps are required
Relatively expensive process
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Expendable-Mold Casting Processes
Plaster mold casting
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Similar to sand casting except mold is made of plaster of Paris
(gypsum - CaSO4-2H2O) In mold-making, 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
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Expendable-Mold Casting Processes
Plaster mold casting
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Advantages:
Good accuracy and surface finish
Capability to make thin cross sections
Disadvantages:
Mold must be baked to remove moisture
Moisture can cause problems in casting Mold strength is lost if over-baked
Plaster molds cannot stand high temperatures
Limited to lower melting point alloys
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Expendable-Mold Casting Processes
Ceramic mold casting
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Similar to plaster mold casting except that mold is made of
refractory ceramic material that can withstand higher temperaturesthan plaster
Can be used to cast steels, cast irons, and other high-temperaturealloys
Applications similar to those of plaster mold casting except for the
metals cast Advantages (good accuracy and finish) also similar
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Permanent-Mold Casting Processes
Permanent-mold casting
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Two halves of a mold are made from materials with high resistance
to erosion and thermal fatigue designed for easy, precise openingand closing
Molds used for casting lower melting point alloys are commonly madeof steel or cast iron
Molds used for casting steel must be made of refractory material due
to the vey high pouring temperatures
In order to increase the life of permanent molds, the surfaces of themold cavity are coated with a refractory slurry or sprayed withgraphite
Equipment costs is high but labor costs are kept low throughautomation
Not economical for small production runs
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Permanent-Mold Casting Processes
Permanent-mold casting
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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:
1. Basic permanent mold casting
2. Die casting
3. Centrifugal casting
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Permanent-Mold Casting Processes
Permanent-mold casting
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(1) Mold is preheated and
coated for lubrication andheat dissipation
(2) Cores (if any are used) areinserted and mold is closed
(3) Molten metal is poured into
the mold, where it solidifies
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Permanent-Mold Casting ProcessesPermanent-mold casting
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Advantages:
Good dimensional control and surface finish
Rapid solidification caused by metal mold results in a finer grainstructure, so castings are stronger
Limitations:
Generally limited to metals of lower melting point
Simpler part geometries compared to sand casting because of need toopen the mold
High cost of mold
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Permanent-Mold Casting Processes
Die casting
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A permanent mold casting process in which molten metal is injected
into mold cavity under high pressure Pressure is maintained during solidification, then mold is opened and
part is removed
Molds in this casting operation are called dies; hence the name diecasting
Use of high pressure to force metal into die cavity is what distinguishesthis from other permanent mold processes
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Permanent-Mold Casting Processes
Die casting
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Designed to hold and accurately close to mold halves and keep
them closed while liquid metal is forced into cavity Two basic types of die-casting machines:
1. Hot-chamber process use a piston to forces a certain volume ofmetal into the die cavity through a gooseneck and nozzle
2. Cold-chamber process is where molten metal is poured into theinjection cylinder (shot chamber)
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Permanent-Mold Casting Processes
Die casting (hot-chamber)
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Die casting (hot-chamber)
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Metal is melted in a container, and a piston injects liquid metal
under high pressure into the die High production rates
500 parts per hour not uncommon
Applications limited to low melting-point metals that do not chemicallyattack plunger and other mechanical components
Casting metals: zinc, tin, lead, and magnesium
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Hot-chamber die casting cycle:
(1) with die closed andplunger withdrawn, moltenmetal flows into the chamber
(2) plunger forces metal inchamber to flow into die,
maintaining pressure duringcooling and solidification
(3) Plunger is withdrawn, die isopened, and casting isejected
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Permanent-Mold Casting ProcessesDie casting (cold-chamber)
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Permanent-Mold Casting ProcessesDie casting (cold-chamber)
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Molten metal is poured into unheated chamber from external
melting container, and a piston injects metal under high pressureinto die cavity
High production but not usually as fast as hot-chamber machinesbecause of pouring step
Casting metals: aluminum, brass, and magnesium alloys
Advantages of hot-chamber process favor its use on low melting-pointalloys (zinc, tin, lead)
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Permanent-Mold Casting ProcessesDie casting (cold-chamber)
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(1) With die closed and ram
withdrawn, molten metal ispoured into the chamber
(2) Ram forces metal to flowinto die, maintainingpressure during cooling and
solidification
(3) Ram is withdrawn, die isopened, and part is ejected
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Permanent-Mold Casting ProcessesDie casting
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Process capabilities and machine selection
Die casting is able to produce strong and high-quality parts withcomplex shapes
Also produces good dimensional accuracy and surface details
Strength-to-weight ratio of die-cast parts increases with decreasing
wall thickness
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P biliti d hi l ti
Permanent-Mold Casting ProcessesDie casting
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Process capabilities and machine selection
Die casting dies can be:a) Single cavity
b) Multiple cavity(several identical cavities)
c) Combination cavity(several different cavities)
d) Unit dies
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Permanent-Mold Casting ProcessesDie casting
Ad t
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Advantages:
Economical for large production quantities Good accuracy and surface finish
Thin sections possible
Rapid cooling means small grain size and good strength in casting
Disadvantages: Generally limited to metals with low melting points
Part geometry must allow removal from die
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Permanent-Mold Casting ProcessesCentrifugal casting
A f il f ti i hi h th ld i t t d t hi h
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A family of casting processes in which the mold is rotated at high
speed so centrifugal force distributes molten metal to outer regionsof die cavity
1. True centrifugal casting
2. Semicentrifugal casting
3. Centrifuge casting
Permanent-Mold Casting ProcessesCentrifugal casting (true centrifugal)
Molten metal is poured into rotating mold to produce a tubular part
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Molten metal is poured into rotating mold to produce a tubular part
In some operations, mold rotation commences after pouring ratherthan before
Parts: pipes, tubes, bushings, and rings
Outside shape of casting can be round, octagonal, hexagonal, etc , butinside shape is (theoretically) perfectly round, due to radially symmetric
forces
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Permanent-Mold Casting ProcessesCentrifugal casting (semicentrifugal)
C t if l f i d t Examples: wheels and
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Centrifugal force is used to
produce solid castings ratherthan tubular parts
Molds use risers at center tosupply feed metal
Density of metal in final
casting is greater in outersections than at center ofrotation
Often used on parts in whichcenter of casting is
machined away, thuseliminating the portion wherequality is lowest
Examples: wheels and
pulleys
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Permanent-Mold Casting ProcessesCentrifugal casting (centrifuging)
Mold is designed with part
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Mold is designed with part
cavities located away fromaxis of rotation, so moltenmetal poured into mold isdistributed to these cavitiesby centrifugal force
Used for smaller parts Radial symmetry of part is
not required as in othercentrifugal casting methods
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Inspection of Castings
Castings can be inspected visually or optically for surface defects
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Castings can be inspected visuallyoropticallyfor surface defects
Subsurface and internal defects are investigated using variousnondestructive techniques
In destructive testing, specimens are removed for various sectionsto test for strength, ductility, and other mechanical properties and todetermine for the presence, location, and distribution of porosity
and defects
Pressure tightness of cast components (valves, pumps, and pipes)is determined by sealing the openings in the casting andpressurizing it with water, oil, or air
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Melting Practice and Furnaces
Electric-arc furnaces charge is melted by heat generated from an
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Electric-arc furnaces charge is melted by heat generated from an
electric arc Induction furnaces uses alternating current passing through a
coil to develop magnetic field in metal
Crucible furnaces metal is melted without direct contact withburning fuel mixture
Cupolas vertical cylindrical furnace equipped with tapping spoutnear base
Levitation melting involves magnetic suspension of the moltenmetal
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PART III
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Introduction
Design Considerations in Casting
Economics of Casting
PART III :
Metal Casting: Design
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General guidelines for successful casting
Learning Outcomes
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General guidelines for successful casting
Design considerations for expendable and permanent mold casting
Economic considerations in metal casting
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Introduction
Successful casting practice requires proper control of a large
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Successful casting practice requires proper control of a large
number of variables Characteristics of the metals and alloys cast, method of casting, mold
and die materials, mold design, and various process parameters
Flow of the molten metal in the mold cavities, the gating systems,the rate of cooling, and the gases evolved would influence the
quality of a casting
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Design Considerations in Casting
All casting operations share the characteristics of phase change
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cast g ope at o s s a e t e c a acte st cs o p ase c a ge
and thermal shrinkage during the casting cycle However, each process have its own design considerations
Sand casting mold erosion and associated sand inclusions in thecasting
Die casting heat checking of dies which reduce die life
Defects are random and difficult to reproduce and consequently,troubleshooting the causes of defects is complicated
Typically, a mold design will produce mostly good parts and somedefective ones, hence, quality control procedures must be
implemented for critical applications of castings
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Design Considerations in CastingGeneral design considerations for castings
2 types of design issues in casting:
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yp g g
a) Geometric features, tolerances, etc., incorporated into the partb) Mold features needed to produce the desired casting
Design of cast parts
Corners, angles, and section thickness Avoid sharp corners, angles, and fillets as they act as stress raisers
and may cause cracking and tearing of the metal (also dies) duringsolidification
Fillet radii should be selected to reduce stress concentrations and to
ensure proper liquid-metal flow during pouring If the fillet radii are too large, the volume of material in those regions is
large, and consequently, the rate of cooling is lower
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Design Considerations in CastingGeneral design considerations for castings
Design of cast parts
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g p
Corners, angles, and section thickness Location of the largest circle that can be inscribed in a particular region
is critical
Cooling rate in these regions is lower (called hot spots), thus, can developshrinkage cavities and porosity
Cavities at hot spots can be eliminated by using small cores withoutaffecting strength significantly
Maintain uniform cross-sections and wall thicknesses throughoutcasting to avoid or minimize shrinkage cavities
Metal paddings orchills can eliminate or minimize hot spots
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Design Considerations in CastingGeneral design considerations for castings
Design of cast parts
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g p
Corners, angles, and section thickness
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FIGURE 2.? Examples of designs showing the importance of maintaininguniform cross-sections in castings to avoid hot spots and shrinkage cavities
Design Considerations in CastingGeneral design considerations for castings
Design of cast parts
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Flat areas Avoid large flat areas (plain surfaces) as they may warp during cooling
because of temperature gradients, or they develop poor surface finishbecause of an uneven flow of metal during pouring
Solution: Break up flat surfaces with staggered ribs and serrations
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Design Considerations in CastingGeneral design considerations for castings
Design of cast parts TABLE 2.1
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g p
Shrinkage Pattern dimensions should
allow for shrinkage of themetal during solidification andcooling
Allowances for shrinkage,known as patternmakers
shrinkage allowances, usuallyabout 10-20 mm/m
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TABLE 2.1
Design Considerations in CastingGeneral design considerations for castings
Design of cast parts
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Draft A small draft (taper) typically in sand-mold patterns to enable removal
of the pattern without damaging the mold
Dimensional tolerances
Dependent on casting process and size, and type of pattern used
In commercial practice, tolerances are 0.8mm for small castings and6mm for large castings
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Design Considerations in CastingGeneral design considerations for castings
Design of cast parts
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Lettering and markings Part identification; sunk into or protrude from the surface of castings
Depends on the method of producing the molds;
In sand casting, a pattern plate is produced by machining on a CNC mill,and it is simpler to machine letters into the pattern plate
In die casting, it is simpler to machine letter into the mold
Finishing operations
Consideration of the subsequent machining and finishing operations
If a hole is to be drilled, it is better to locate the hole on a flat surface thanon a curved surface to prevent the drill from wandering or a better design,
incorporate a small dimple as a starting point for the drilling operations Include feature to allow them to be clamped easily into machine tools
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Design Considerations in CastingGeneral design considerations for castings
FIGURE 2.?
Suggested design
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Suggested design
modifications toavoid defects in
casting
Design Considerations in CastingGeneral design considerations for castings
Locating the parting line
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A part should be oriented so that the large portion is low and heightis minimized
Critical surface should face downwards
Location of parting line influences mold design, ease of molding,number and shape of cores required, method of support, and the
gating system Generally, parting line should be along a flat plane rather than contour
Whenever possible, parting line should be at corners or edges ratherthan on flat surfaces in the middle so that flash will not be visible
Parting line should be low for less dense metals and at mid-height fordenser metals
Whenever practical, avoid the use of cores
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Design Considerations in CastingGeneral design considerations for castings
Locating and designing gates
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Multiple gates are preferable and necessary for large parts allowinglower pouring temperature and reducing temperature gradients
Gates should feed into thick sections of castings
A fillet should be used where a gate meets a casting, hence, lessturbulence than abrupt junctions
Place gate closest to sprue sufficiently far away for easy removal (afew mm for small castings and 500mm for large parts)
Minimum gate length should be 3-5X the diameter
Avoid curved gates
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Design Considerations in CastingGeneral design considerations for castings
Runner design
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1 runner for simple parts; 2-runner systems for complicatedcastings
Runners are used to trap dross (a mixture of oxide and metal thatforms on the surface of metals) and keep it from entering the gatesand mold cavity
Commonly, dross traps are placed at the ends of runners, and therunner projects above the gates to ensure the metal in the gates istapped from below the surface
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Design Considerations in CastingGeneral design considerations for castings
Designing other mold features
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Goal in designing a sprue is to achieve the required metal flowrates while preventing excessive dross formation
Turbulence is avoided but the mold is filled quickly compared tosolidification time
Apouring basin is used to ensure uninterrupted metal flow into the
sprue If molten metal is maintained in the pouring basin during pouring, dross
will float and will not enter mold cavity
Filters are used to trap large contaminants and to slow metalvelocity for laminar flow
Chills are used to speed metal solidification in a particular region ofcasting
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Design Considerations in CastingGeneral design considerations for castings
Establishing Good Practices
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Starting with high-quality molten metal is essential for producingsuperior castings
Pouring temperature, metal chemistry, gas entrainment, and handlingprocedures can affect the quality of metal being poured
Pouring of molten metal in the mold cavity should experience a
continuous, uninterrupted, and upward advance to avoid drossentrainment and turbulence
Different cooling rates within the body of a casting cause residualstresses, thus, stress relieving may be necessary to avoiddistortions of castings in critical applications
UCSI UNIVERSITY SCHOOL OF ENGINEERING MANUFACTURING PROCESSES BY: MS. KRSHNAWATHY JAN 2011
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Design Considerations in CastingDesign for expendable-mold casting
Expendable-mold processes have specific design considerations,
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mainly attributed to the mold material, size of parts, andmanufacturing method
Mold layout
Features in the mold must be placed logically and compactly with
gates as necessary to have solidification initiate at one end of themold and progress in a uniform front across the casting with therisers solidifying last
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Design Considerations in CastingDesign for expendable-mold casting
Riser design
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Six basic rules:1. Riser must not solidify before casting by avoiding small risers and
using cylindrical risers with small aspect ratios (small ratios of heightto cross-section)
2. Riser volume must be large enough to provide sufficient liquid metal
to compensate for shrinkage3. Junctions between casting and feeder should not develop hot spot
where shrinkage porosity can occur
4. Risers must be placed so that liquid metal can be delivered tolocations where it is most needed
5. Pressure must be sufficient to drive liquid metal into locations in themold where it is needed
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Design Considerations in CastingDesign for expendable-mold casting
Riser design
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6. Pressure head from riser should suppress cavity formation andencourage complete cavity filling
Machining allowance
Because most expendable-mold castings require finishingoperations, such as machining and grinding, allowances should bemade in casting design
Machining allowances, which are included in pattern dimensions,depend on the type and increase with size and section thickness ofcastings
2-5mm for small to >25mm for large castings
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Typical design guidelines similar as discussed in FIGURE 2.1
Design Considerations in CastingDesign for permanent-mold casting
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Special considerations in designing tooling for die casting Designs may be modified to eliminate draft for better dimensional
accuracy
However, a draft angle of or is required to avoid galling(localized seizure or sticking or material) between the part and the dies
and cause distortion Die cast parts are nearly-net shaped, requiring only the removal of
gates and minor trimming to remove flashing and other minordefects
Surface finish and dimensional accuracy of die-cast parts are very
good and generally, do not require a machining allowance
UCSI UNIVERSITY SCHOOL OF ENGINEERING MANUFACTURING PROCESSES BY: MS. KRISHNAWATHY JAN 2011
EM315/EM311 MANUFACTURING PROCESSES Nur Azyyah
Design Considerations in CastingDesign for permanent-mold casting
FIGURE 2.? Examples
of undesirable (poor)
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EM315/EM311 MANUFACTURING PROCESSES Nur Azyyah
and desirable (good)casting designs
Design Considerations in CastingDesign for permanent-mold casting
Poor Good
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(a) The lower portion has a thin wall whichmay fracture under high forces or impact Eliminates problem and simplify die andmold manufacturing
(b) Large flat surfaces may warp anddevelop uneven surfaces
Break up the surface with ribs andserrations on the reverse side (does notadversely affect appearance andfunction) to reduce distortion
(c) Difficult to produce sharp internal radii orcorners
Placement of a small radius at thecorners or periphery at the bottomeliminates the problem
Design Considerations in CastingDesign for permanent-mold casting
Poor Good
(d) Function of a part, for instance, a knob is Inner periphery has unfunctional
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EM315/EM311 MANUFACTURING PROCESSES Nur Azyyah
( ) p , ,to be gripped and rotated, hence, theouter features along its periphery
p p yfeatures but save material and the die iseasier to manufacture
(e) Sharp fillets at the base of thelongitudinal grooves means the die hassharp (knife edge) protrusions and theseedges can chip off over extended use
Small radii prevents the die edges formchipping off
(f) Threads reaching the right face of thecasting, thus, molten metal canpenetrate this region forming flash andinterfering with the function of thethreaded insert
An offset on the threaded inserteliminates this problem
Casting involves complex interactions among material and processvariables and so, a quantitative study of these interactions is
Design Considerations in CastingComputer modeling of casting processes
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, q yessential to the proper design and production of high-qualitycastings
Rapid advances in computers and modeling techniques led toinnovations in modeling various aspects of casting
Heat flow, temperature gradients, nucleation and growth of crystals,formation of dendritic and equiaxed structures, impingement of grainsand movement of liquid-solid interface during solidification
Commercial software programs: Magmasoft, ProCast, Solidia, andAFSsolid
Benefits: Increased productivity, improved quality, easier planning andcost estimating, and quicker response to design changes
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Economics of Casting
Cost of each cast part (unit cost) depends on several factors,including materials, equipment, and labor
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g q p
Each individual factors affects (to varying degrees) the overall costof a casting operation
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Economics of Casting
Cost of product = costs of materials, labor, tooling, and equipment
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Producing molds and dies that require raw materials, time, and effort Making patterns (RP to reduce costs and time)
Melting and pouring molten metal into molds
Heat treating, cleaning, and inspecting castings
Equipment cost per casting will decrease as the number of parts
cast increases High production-rates can justify the high cost of dies and machinery
If the demand is small, the cost-per-casting increases rapidly
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Labor & skills vary
considerably depending
on the process and level
of automation