2013-01 em315_em311 ch02 casting processes

7/29/2019 2013-01 EM315_EM311 CH02 Casting Processes

1/133

CHAPTER 02

CASTING PROCESSES

Manufacturing Processes


2/133

Introduction

Solidification of Metals Fluid Flow

Fluidity of Molten Metal

Heat Transfer

Defects

PART I : Fundamentals of Casting Processes

EM315/EM311 MANUFACTURING PROCESSES Nur Azyyah


3/133

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



4/133


5/133

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



6/133


Solidification of Metals

Pure metals

T as a function of time Density as a function of time


7/133


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



8/133



Pure metals


9/133


Alloys

Solidification in alloys startswhen below liquidus andcomplete when it reaches thesolidus

Alloy in a mushyorpastystate consisting ofcolumnardendrites

Dendrites have interlocking3D arms and branches



10/133

Solidification of MetalsAlloys

Width of the mushy zone is described in terms offreezing range,TL - TS



11/133


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



12/133


Alloys



13/133


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)



14/133


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



15/133

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/



16/133

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



17/133

Heat Transfer

Solidification time


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


18/133

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



19/133

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



20/133

Defects


FIGURE 2.? Examples of common defects in castings


21/133

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


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))


22/133

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


FIGURE 2.? Solubility of hydrogen in aluminum

(note the sharp decrease in solubility as the

molten metal begins to solidify)


23/133

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


D f


24/133

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


D f


25/133

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


D f t


26/133

Defects

General defects

Shrinkage cavity: Internal voidor surface depression in casting

Cause:

Uncontrolled solidification

Remedy: Proper riser design

Adequate risers and feeders


D f t


27/133

Defects

General defects

Microporosity: A network ofsmall voids distributedthroughout casting, usuallyassociated with alloys

Cause:

Localized solidificationshrinkage of the final moltenmetal in the dendriticstructure

Remedy:


D f t


28/133

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


Defects


29/133

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


Defects


30/133

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


Defects


31/133

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


Defects


32/133

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


Defects


33/133

Defects


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


Defects


34/133

Defects


Sandwash: Irregularity in thesurface of casting

Cause:

Erosion of sand mold duringpouring

The contour of erosion isimprinted into surface of thefinal cast part

Remedy:


Defects


35/133

Defects


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


Defects


36/133

Defects


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


Defects


37/133

Defects


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:


Defects


38/133

Defects


Core shift: A similar movementwith the core

Cause:

Vertical displacement of thecore caused by buoyancy of

the molten metal

Remedy:


Defects


39/133

Defects


Mold crack: Formation of fin onfinal casting

Cause:

If mold strength isinsufficient a crack may

develop into which liquidmetal can seep

Remedy:


Defects


40/133


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



41/133

Introduction

Expendable-Mold Casting Processes

Permanent-Mold Casting Processes

Inspection of Castings

Melting Practice and Furnaces

PART II : Metal Casting Processes


Learning O tcomes


42/133

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


Introduction


43/133

Introduction


Introduction


44/133


Introduction

Introduction


45/133

Introduction


Introduction


46/133

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




47/133

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




48/133

Sand casting

Sand casting weighing over 680 kg (1500 lb) for an air compressor

frame (photo courtesy of Elkhart Foundry)




49/133

Sand casting


FIGURE 2.? Outline of production steps in a typical sand casting operation



50/133

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



51/133

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



S d


52/133

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



S d i


53/133

Sand casting


FIGURE 2.? Schematic illustration of a sand mold showing various features


S d i


54/133

Sand casting


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


S d ti


55/133

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



S d ti


56/133

Sand casting

Patterns

Patterns can be designed with a variety of features to fit specificapplications and economic requirements


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


Sand casting


57/133

Sand casting

Patterns


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


Sand casting


58/133

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



Sand casting


59/133

Sand casting

Cores


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


Sand casting


60/133

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


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


Sand casting


61/133

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



Sand casting


62/133

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



Sand casting


63/133


Sand casting


Sand casting


64/133


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


Sand casting


65/133


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)


Shell molding


66/133

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



Shell molding


67/133

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



Shell molding


68/133

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



Shell molding


69/133

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



Vacuum molding


70/133

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



Vacuum molding


71/133

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



Evaporative-pattern casting (lost-foam process)


72/133

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





73/133

(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





74/133

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





75/133





76/133


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.


Investment casting (lost-wax process)


77/133

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)





78/133

(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





79/133

(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





80/133

Also called lost-wax process Used to make office equipment, and mechanical

components such as gears





81/133

One-piece compressor stator with 108 separate airfoils made by

investment casting (photo courtesy of Howmet Corp.)





82/133

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



Plaster mold casting


83/133

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



Plaster mold casting


84/133

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



Ceramic mold casting


85/133

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



Permanent-mold casting


86/133

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





87/133

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





88/133

(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


Permanent-Mold Casting ProcessesPermanent-mold casting


89/133

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



Die casting


90/133

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



Die casting


91/133

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)



Die casting (hot-chamber)


92/133



Die casting (hot-chamber)


93/133

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


Permanent-Mold Casting ProcessesDie casting (hot-chamber)


94/133

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


Permanent-Mold Casting ProcessesDie casting (cold-chamber)


95/133




96/133

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)




97/133

(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


Permanent-Mold Casting ProcessesDie casting


98/133

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


P biliti d hi l ti



99/133

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



Ad t


100/133

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


Permanent-Mold Casting ProcessesCentrifugal casting

A f il f ti i hi h th ld i t t d t hi h


101/133


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


102/133

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


Permanent-Mold Casting ProcessesCentrifugal casting (semicentrifugal)

C t if l f i d t Examples: wheels and


103/133

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


Permanent-Mold Casting ProcessesCentrifugal casting (centrifuging)

Mold is designed with part


104/133

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


Inspection of Castings

Castings can be inspected visually or optically for surface defects


105/133

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


Melting Practice and Furnaces

Electric-arc furnaces charge is melted by heat generated from an


106/133

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


PART III


107/133

Introduction

Design Considerations in Casting

Economics of Casting

PART III :

Metal Casting: Design


General guidelines for successful casting

Learning Outcomes


108/133

General guidelines for successful casting

Design considerations for expendable and permanent mold casting

Economic considerations in metal casting


Introduction

Successful casting practice requires proper control of a large


109/133

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


Design Considerations in Casting

All casting operations share the characteristics of phase change


110/133

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


Design Considerations in CastingGeneral design considerations for castings

2 types of design issues in casting:


111/133

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





112/133

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





113/133

g p

Corners, angles, and section thickness


FIGURE 2.? Examples of designs showing the importance of maintaininguniform cross-sections in castings to avoid hot spots and shrinkage cavities




114/133

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



Design of cast parts TABLE 2.1


115/133

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


TABLE 2.1




116/133

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





117/133

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



FIGURE 2.?

Suggested design


118/133


Suggested design

modifications toavoid defects in

casting


Locating the parting line


119/133

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



Locating and designing gates


120/133

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



Runner design


121/133

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



Designing other mold features


122/133

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



Establishing Good Practices


123/133

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


Design Considerations in CastingDesign for expendable-mold casting

Expendable-mold processes have specific design considerations,


124/133

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



Riser design


125/133

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



Riser design


126/133

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


Typical design guidelines similar as discussed in FIGURE 2.1

Design Considerations in CastingDesign for permanent-mold casting


127/133

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



FIGURE 2.? Examples

of undesirable (poor)


128/133


and desirable (good)casting designs


Poor Good


129/133


(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


Poor Good

(d) Function of a part, for instance, a knob is Inner periphery has unfunctional


130/133


( ) 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


131/133

, 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



Cost of each cast part (unit cost) depends on several factors,including materials, equipment, and labor


132/133

g q p

Each individual factors affects (to varying degrees) the overall costof a casting operation



Cost of product = costs of materials, labor, tooling, and equipment


133/133

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


Labor & skills vary

considerably depending

on the process and level

of automation

2013-01 em315_em311 ch02 casting processes

Documents