22. Casting

• In casting we essentially start with an amorphous material, and hold it in shape while the material solidifies.

• Some of the typical casting processes and materials are listed below,

 

• A relative comparison of some casting techniques is given below,

 

22.1 Sand Casting

• Sand casting is one of the older techniques. In this form a mold is made from sand, and the part is cast into it. When the metal has hardened and cooled the part is removed, and the sand removed.

 

• Typical stages of operation include,

1. Patterns are made. These will be the shape used to form the cavity in the sand.

2. Cores may also be made at this time. These cores are made of bonded sand that will be broken out of the cast part after it is complete.

3. Sand is mulled (mixed) thoroughly with additives such as bentonite (clay) to increase bonding and overall strength.

4. Sand is formed about the patterns, and gates, runners, risers, vents and pouring cups are added as needed. A compaction stage is typically used to ensure good coverage and solid molds. Cores may also be added to make concave, or internal features for the cast part. Alignment pins may also be used for mating the molds later. Chills may be added to cools large masses faster.

5. The patterns are removed, and the molds may be put through a baking stage to increase strength.

6. Mold halves are mated and prepared for pouring metal.

7. Metal is preheated in a furnace or crucible until is above the liquidus temperature in a suitable range (we don’t want the metal solidifying before the pour is complete). The exact temperature may be closely controlled depending upon the application. Degassing, and other treatment processes may be done at this time, such as removal of impurities (i.e. slag). Some portion of this metal may be remelted scrap from previously cast parts: 10% is reasonable.

8. The metal is poured slowly, but continuously into the mold until the mold is full.

9. As the molten metal cools (minutes to days) the metal will shrink. As the molten metal cools the volume will decrease. During this time molten metal may backflow from the molten risers to feed the part, and maintain the same shape.

10. Once the part starts to solidify small dendrites of solid material form in the part. During this time metal properties are being determined, and internal stresses are being generated. If a part is allowed to cool slowly enough at a constant rate then the final part will be relatively homogenous and stress free.

11. Once the part has completely solidified below the eutectic point it may be removed with no concern for final metal properties. At this point the sand is simply broken up, and the part removed. At this point the surface will have a quantity of sand adhering to the surface, and solid cores inside.

12. A bulk of the remaining sand and cores can be removed by mechanically by striking the part. Other options are to use a vibrating table, sand/shot blaster, hand labor, etc.

13. The final part is cut off the runner gate system, and is near final shape using cutters, torches, etc.. Grinding operations are used to remove any remaining bulk.

14. The part is taken down to final shape using machining operations. And cleaning operations may be used to remove oxides, etc.

22.1.1 Molds

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• The basic components found in many molds are shown below,

 

• The terms for the parts of a mold are,

pouring cup: the molten metal is poured in here. It has a funnel shape to ease pouring accuracy problems.

runner/sprue: a sprue carries metal from the pouring cup to the runners. The runners distribute metal to the part.

gate: a transition from the runner to the cavity of the part

riser: a thermal mass where excess metal will remain in a liquid state while the part cools. As the cooling part shrinks, the molten metal in the riser will feed or fill in the shrinkage. Risers can also be used to collect impurities that rise in molten metal.

mold cavity: this is the final shape of the part.

vent: a narrow escape passage for gases that would otherwise be trapped in the mold.

parting line: a line of separation that allows the mold (made in two pieces) to be put together to make a full cavity. Note that this line does not have to be a straight line, and is often staggered to make the mold making easier.

cope: the upper part of a casting mold

drag: the lower part of a casting mold

• There are a number of interesting points about patterns,

molds are made by compacting sand around the shape of the pattern.

patterns are made of wood, metal and plastics: the material must be stronger if a large number of molds are to be made.

a parting agent can be used on a pattern to allow easy removal after the mold is made.

pattern types include

one piece patterns (loose or solid patterns): low quantity simple shapes

split patterns: for complex shapes made in two patterns for each half of the part.

match plate: the split patterns are mounted in a single plate. This allows gating on the drag side to match up with the runners on the cope.

design of the patterns should include consideration of shrinkage

a slight taper should be added to the sides all patterns this will make them easy to remove from the completed mold. i.e. a cone is easier to remove than a cylinder.

• Cores are typically used for more complex shapes. Some point of interest,

Cores allow features that could not be easily formed into a sand core.

Cores are made with techniques similar to those for making sand molds.

The cores may need structural support in the mold: these metal supports are called chaplets.

The cores are added when the cavity are made, and they act as part of the mold during casting, but they are rigid enough to allow internal features on parts.

Cores can be made easily in automated settings.

 

• A mold might undergo a hardening process,

green sand: no hardening, just moist

cold-box: binders are mixed with the sand to increase dimensional accuracy

no-bake: liquid resin binders harden the sand at room temperature

skin-dried: the sand is hardened by drying in an oven or air. Higher strength, but distortion and lower collapsibility.

baking: the molds are baked before casting to harden the entire mass

• When the pattern and cores have been inserted into the sand it is compacted. There are a number of techniques for doing this,

Squeeze Molding Machines: automatically insert and compact sand. The processes used are designed to produce a uniform compaction. Jolting is sometimes used to help settle the sand. These molds are made in flasks.

conventional flat head

profile head

equalizing pistons

flexible diaphragm

Vertical Flaskless Molding: the molds halves are made by blowing sand against a vertical mold. High production rates are possible.

Sandslingers: A high speed stream of sand into the flask tends to pack the sand effectively.

Impact molding: an explosive impulse is used to compact the sand. The mold quality with this technique is quite good.

Vacuum molding: an envelope of plastic is created about the sand using plastic sheeting. Air is drawn from the sand, and the vacuum leads to compaction.

22.1.2 Sands

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• The sands used tend to fall into the following categories,

naturally bonded (bank): less expensive

synthetic (lake): this sand can have a variety of controlled compositions.

• Types of sand include,

Zircon (ZrSiO4): low thermal expansion

Olivine (Mg2SiO4): low thermal expansion

Iron Silicate (Fe2SiO4): low thermal expansion

Chromite (FeCr2O4): high heat transfer

• The sand effects the following aspects of the casting,

granule shape: smaller and rounder grains produce a better casting surface.

granule size: a coarse grained sand will be porous and allow gases to escape during casting. a fine grained sand leads to a stronger mold.

collapsibility: if the sand can shift during cooling of the part it will reduce stress tears and cracks

• Green sand molding refers to a slightly wet condition of the sand (much like ‘green wood’). At the right level of humidity the moisture will increase sand binding. But in excess this moisture expand when heated during pouring and blow metal back out of the mold (i.e. explosion). This is one of the least expensive molding techniques).

22.2 Single Mold Use Methods

22.2.1 Shell Mold Casting

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• The basic process for these molds is,

1. Create two mating patterns of desired shape.

2. Coat the molds with a shell (sand and binders, such as a resin) until desired thickness and other properties are obtained.

3. Cure the molds and remove the patterns.

4. The mold halves are mated and held firm while metal is poured.

5. The final part(s) is removed.

 

• This technique can be very economical.

• Special care must be taken to assure venting for gasses, as the mold media is less porous.

• This method can easily use cores and chills to make complex molds.

• Graphite molds can be used for materials that would normally react with other materials used for the molds.

22.2.2 Lost Foam Casting (Expandable Pattern)

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• This process has a number of basic steps,

1. Make a mold for producing styro-foam patterns.

2. Make styro-foam patterns using inject molding of expanded polystyrene foam beads (or another low density monomer foam) This process can be automated.

3. Glue the parts foam patterns together, and glue to sprue/runner/gate systems as required.

4. For high quality surface finish the parts may be coated with a ceramic slurry and hardened in a drying oven.

5. Place the pattern in sand, taking care to compact the sand about the pattern.

6. Cast metal into the pattern. The foam will evaporate, and escape through the normal routes gas evacuates through.

7. Wait until the part is hard, and remove from the sand.

8. If a ceramic coating was used this can be removed using impact, vibration, or abrasive techniques as appropriate.

 

• This process can be automated, and can be very inexpensive in quantities.

• Complex parts can be made with relative ease by gluing together foam pieces.

22.2.3 Plaster Mold Casting

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• This technique is basically,

1. Create a two part pattern.

2. A mold material is used that is a plaster of paris type mixture (fast setting) to make two cavities. This may have some additives to improve properties. Foamed plaster may be used to increase permeability.

3. After setting these cavities will be dried in an oven to remove moisture.

4. The Antioch process is optional and increases mold permeability by dehydrating in an autoclave, and rehydrating for a number of hours.

4. The mold halves are then mated and heated.

5. After reaching adequate heat levels the molten metal is poured. Mold porosity is low so pressure or vacuum must be used to encourage complete filling of the mold.

6. The final part is removed and cleaned

• This technique is known for its high level of dimensional accuracy.

22.2.4 Ceramic Mold Casting

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• Also known as ‘cope and drag investment casting’.

• The basic process is,

1. A wood or metal pattern is placed in a flask and coated with a slurry of zircon and fused silica combined with bonding agents.

2. The mold is removed, cleaned and baked. The shells may be used as given, or they may have other materials, such as clay put on as backing materials.

3. The molds are then used as normal.

• This can make high temperature material parts.

22.2.5 Investment Casting

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• The basic steps are,

1. An expendable mold of a part is made in wax, plastic, etc.

2. The part has a gate and runner attached to it, and all are dipped in a ceramic slurry.

3. The slurry is hardened, and the core is melted and/or burned out.

4. The core is burned out and the mold is preheated to the temperature of the molten metal

644°C for aluminum

1040°C for ferrous alloys

etc.

5. Molds are filled by pressure, vacuum or centrifugal force.

6. After cooling, the mold is broken off, the sprues are cut off, and stubs are ground off.

 

• Many parts can be made at the same time by attaching them to a common gating system.

• Parts can be glued together to make shapes that would normally be too complex to mold.

• Typical methods used are,

cast iron

steel

aluminum alloys

brass

bronze

magnesium

zinc

• The die used to make the mold cores can be used for thousands of parts.

• Typical large applications are,

large propellers

large frames

nozzles

cams

valve parts

• Typical small applications are,

dental

jewelry

orthopedic surgical implants

camera components

• Advantages,

fine details can be made

thin sections are possible

high accuracy

weights from <1 ounce to > 100 lb.

any castable metal can be used

no parting lines

good surface finish (60-220 μ in.)

can be automated

many parts can be made at once providing lower per piece cost

high melting point metals can be used

• Disadvantages,

less strength than die cast parts

process is slow

changes to the die are costly

more steps are involved in production

22.3 Multiple Mold Use Techniques

22.3.1 Vacuum Casting

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• The basic process is,

1. A mold is made using sand, urethane, and amine vapors to cure.

2. The mold is mounted on a moving head.

3. The head is lowered into molten metal in an induction furnace so that the lower face of the mold is submerged.

4. Vacuum is applied to the mold and metal is drawn up to fill the cavity.

• This process is relatively inexpensive and can be automated.

• Thin walls, down to 0.02” are possible.

• The process can be used effectively with reactive metals.

22.3.2 Permanent Mold Casting

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• The basic process is,

1. A metal mold is made in two halves.

2. The mold is then coated with a refractory coating, or sometimes graphite is used instead. This acts as a thermal barrier, and as a parting agent.

3. Cores are then added as required.

4. The mold halves are mated and preheated to about 300-400°F.

5. Low melting point molten metal is poured into the dies.

6. Water channels, or heat sink fins are used to cool the mold quickly.

7. The mold is opened, and ejector pins are used to force the part out of the mold: this leaves small circular depressions on the surface of the part.

8. the sprue is removed, and the stub is ground off.

• The mold cavity is typically coated with a refractory coating to reduce heat damage, and ease part removal after casting. The materials also help control the cooling rate of the casting. Typical materials include,

sodium silicate and clay

sprayed graphite

• Molds are machined, including the cavity and gates. Typical mold materials include,

cast iron and alloyed cast irons

steel

bronze

graphite

refractory metal alloys

• Typical core materials include,

oil-bonded sand

resin-bonded sand

plaster

graphite

gray iron: most common

low-carbon steel

hot work die steel

• Low melting point metals can be cast

aluminum

zinc

magnesium alloys

brass

cast iron

• Movable sections can be used to allow removal of cast parts.

• Can be used for thousands of parts before mold is replaced or repaired.

• Part sizes are from a few ounces to a hundred pounds.

• Typical applications are,

pistons/cylinders/rods

gears

kitchenware

• Advantages,

the mold can be chilled to speed cooling

good surface finish

good dimensional accuracy

only one mold is required

• Disadvantages,

limited numbers of alloys can be used

complex shapes cannot be cast

mold production is time consuming and costly

mold sizes are limited

22.3.2.1 - Slush Casting

• Permanent mold casting can be used to produce hollow parts without using cores.

• In this process the mold is filled as normal, and solidification begins at the outer surface and moves inwards. After a short period of time the mold can be turned over, and the molten metal inside will run out. This leaves a thin shell in the mold.

22.3.2.2 - Pressure Casting

• In this process the normal permanent mold process is used, except instead of pouring molten metal, it is forced into the die under a moderate pressure or pulled in using vacuum). This pressure is maintained until the part has solidified.

• The constant pressure allows for filling of the mold as it shrinks.

22.3.2.3 - Die Casting

• The basic process is,

1. two permanent mold halves of a die (mounted in a press) are brought together.

2. the molten metal is injected through a runner and gate with pressures up to 100 ksi: 2000-5000 psi is common.

3. air escapes into overflow wells, and out vents, and metal fills the molds

4. the mold is chilled, and the injected metal freezes

5. the mold is separated, and knockout pins eject the part

6. the parts are cut off the runners and sprues

• Used for low melting point (non-ferrous) metals such as,

zinc

aluminum

magnesium

copper

lead

tin

• Can produce complex shapes at mass production rates.

• Metal dies,

must withstand high pressures

die life is shortened by extreme temperature fluctuations

dies often made with carbon or special alloys

multiple cavities can be used in the die

• Applications,

automotive parts

appliances

office machines

bathroom fixtures

outboard motors

toys

clocks

tools

• Die casting machines can use,

hot chambers with a plunger: a reservoir of molten metal is used to directly feed the machine.

a cold chamber: metal is ladled into the machine for each shot.

• Hot chamber machines are,

good for low temperature zinc alloys (approx. 400°C)

faster than cold chamber machines

cycle times must be short to minimize metal contamination

metal starts in a heated cylinder

a piston forces metal into the die

the piston retracts, and draws metal in

• Cold chamber machines,

casts high melting point metals (>600°C)

high pressures used

metal is heated in a separate crucible

metal is ladled into a cold chamber

the metal is rapidly forced into the mold before it cools

• All die casting processes require a large press to hold mold halves together during a cycle.

• Advantages,

intricate parts possible

short cycles

inserts feasible

cycles less than 1 minute

minimum finishing operations

thin sections, high tolerances, good surface finish

• Disadvantages,

metal die is costly

porous parts

not suited to large parts

long setup times

$5000-200,000 for machine

metal melting point temperature must be lower than die

22.3.3 Centrifugal Casting

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• The basic process is,

1. a mold is set up and rotated along a vertical (rpm is reasonable), or horizontal (200-1000 rpm is reasonable) axis.

2. The mold is coated with a refractory coating.

3. While rotating molten metal is poured in.

4. The metal that is poured in will then distribute itself over the rotating wall.

5. During cooling lower density impurities will tend to rise towards the center of rotation.

6. After the part has solidified, it is removed and finished.

• There are three variants on this process,

true centrifugal casting: long molds are rotated about a horizontal axis. This can be used to make long axial parts such as seamless pipes.

semicentrifugal casting: parts with a wide radial parts. parts such as wheels with spokes can be made with this technique.

centrifuging: the molds are placed a distance from the center of rotation. Thus when the poured metal reaches the molds there is a high pressure available to completely fill the cavities. The distance from the axis of rotation can be increased to change the properties

• Centrifugal and semicentrifugal casting used for axisymmetric parts (internally).

• Parts from 6” to 5’ in diameter can be made, but typical diameters are 10’ to 30’.

• Long tubes can be made that could not normally be rolled.

• Typical metals cast are,

steel

nickel alloys

copper

aluminum

• Typical applications are,

train wheels

jewelry

seamless pressure tubes/pipes

• Advantages,

good uniform metal properties

no sprues/gates to remove

the outside of the casting is at the required dimensions

lower material usage

no parting lines

low scrap rates

• Disadvantages,

extra equipment needed to spin mold

the inner metal of the part contains impurities

22.3.4 Casting/Forming Combinations

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• These processes basically casting molten metal, but the use mechanical force to reshape.

22.3.4.1 - Squeeze Casting

• The basic process is,

1. Molten metal is poured into an open face die.

2. A punch is advanced into the die, and to the metal.

3. Pressure is applied to the punch and die while the part solidifies. This pressure is lower than normally required for forging.

4. The punch is retracted, and the part is knocked out with an ejector pin.

• This method overcomes problems with feeding the die, and produces near net, highly detailed parts.

22.3.4.2 - Semisolid Metal Forming

• The basic process is,

1. A metal is heated until it has thixotropic properties (when agitated viscosity decreases).

2. The metal is poured into a die in a semi-solid state, and the mold is filled.

3. The metal hardens.

• This can produce better metal qualities in net shape parts requiring no finishing operations.

22.3.5 Single Crystal Casting

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• The process is effectively,

1. Prepare a mold so that one end is a heated oven, and the other end chilled. The part should be oriented so that the cooling happens over the longest distance.

2. Cast metal into the mold

3. Solidification will begin at the chill plate. These dendrites will grow towards the heated end of the part as long dendritic crystals. The part is slowly pulled out of the oven, past the chill plate.

4. Remove the solidified part.

• Parts made of a single crystal can have creep and thermal shock resistance properties.

• There are two variants to this technique,

directionally solidified: in this case the dendrites grow from the chill plate towards the other end.

single crystal: a helical constriction is used so that instead of parallel dendrites, only a single crystal is formed in the blade.

22.4 Related Topics

22.4.1 Furnaces

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• Some of the types include,

coreless induction: magnetic fields induce eddy currents throughout the entire furnace, resulting in melting

core induction: magnetic fields induce eddy currents in a small section of the furnace, resulting in melting

gas fired crucible: uses ignited gas and air to heat crucible in enclosed oven

electric arc: arcs are used to heat metals

cupolas: layers of metal and ore are placed in this refractory lined vessel, and ignited to produce large volumes of metal.

22.4.2 Inspection of Castings

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• General problems with castings are,

cavities

projections

discontinuities

defective surfaces

incomplete

incorrect dimensions

inclusions

• Typical inspection methods are,

polishers & microscopes to look at microscopic structures

metal analyzer to determine chemical composition

X-rays are used to examine hidden cracks and blowholes

22.5 Design of Castings

• When designing casting the most important consideration is the effects of shrinkage during cooling. Other important factors include metal flow, and porosity.

• Some general rules of thumb are,

Avoid sharp corners: they can lead to hot tearing during cooling.

Use fillets cautiously: they lead to stresses as they shrink a radius of 1/8” to 1” are acceptable.

Avoid large masses: they will cool more slowly, and can lead to pores and cavities in the final part. Cores can be used to hollow out these large volumes. Metal padding ‘chills’ can also be placed inside the mold near large masses to help increase cooling rates.

Use uniform cross sections -this will keep the cooling rate relatively uniform and avoid stresses.

Avoid large flats: large flat areas tend to warp.

Allow some give as the part cools: by allowing the shrinkage of one part to deform another slightly, the internal stresses will be reduced. Figures of 1-2% shrinkage are common.

Put parting lines near corners: this will hide the flash.

Straight Parting Lines: where possible a straight parting line will allow easier mold making.

Use a Draft angle: A small angle of 0.5-2° on the vertical walls will make the pattern easier to remove.

Machining Allowances: allow excess material for later machining of critical dimensions

Wide Tolerances: because shrinkage occurs as the part cools it will be very hard to keep tight tolerances.

Stress Relieve When Needed: Stress relief can reduce the effects of non-uniform cooling.

Avoid thin sections: These will be very hard to fill, and will tend to harden quickly.

Avoid internal features: These will require extra steps in mold making, and may create metal flow problems.

22.6 References

22.1 Lewis, R., His previous course notes for MEC015 have basically been adapted to what is shown here.

22.2 Kalpakjian,

22.3 Krar,

22.7 PRACTICE PROBLEMS

Problem 22.1 TRUE / FALSE: Investment casting is well suited to producing many parts at once.

Problem 22.2 The part below will be referred to in a number of questions. The drawings are not to scale but they do show an axisymmetric part (i.e., round) with a hollow internal core. The drawing is not to scale, but the rough dimensions are given. You are free to make assumptions (they must be stated) where necessary.

a) Describe in details the steps required to make this part using sand casting.

b) List the steps in detailed to make this part with investment casting.

c) List other casting processes that could be used to make this part. Provide your opinion of relative ranking (e.g., 1 to 5) with a general reason for each.

d) List other casting processes that should not be used to cast this part. Provide a reason why not.

e) If the part is to be made with injection molding, what special considerations would be required?

f) List appropriate techniques for making this part using thermoplastics. Give a relative ranking (e.g., 1 to 5) with reasons.

g) List types of composite manufacturing techniques that are, and are not suitable for making this part. Give reasons why.

Problem 22.3 a) Design a sand casting mold for the jar shaped part below. Include risers, gates, runners, etc., Indicate the parting line between the cope and drag. It will be filled from the side as drawn.

b) List and explain why 2 features of this part would be hard to cast.

Problem 22.4 Suggest parts that are best suited to produce with the following casting techniques. You must briefly state why is best suited to the method.

a) Centrifugal lost wax investment casting

b) floor/pit casting

c) die casting

d) true centrifugal casting

Problem 22.5 Describe the procedures that would be involved in making a bronze statue. Which casting processes would be suitable? Why?

Problem 22.6 Why are castings normally cooled slowly?

Problem 22.7 How does the microstructure of a casting relate to the cooling rate?

Problem 22.8 What factors will result in a cast part not matching the pre-casting mold shape?

Problem 22.9 Suggest two casting methods that would be suitable for making small toy cars? Indicate which would be better and why.

Problem 22.10 Suggest two casting processes would be well suited to making a large casting of a 4 foot tall ornament? Indicate the benefits and limitations of each.

Problem 22.11 How can hot tearing be avoided in castings?

Problem 22.12 Why should risers be located near large masses in cast parts?

Problem 22.13 How can chills help deal with large masses in a mold?

Problem 22.14 List and describe 8 different casting applications.

Problem 22.15 Identify design features that will cause problems when casting.

Problem 22.16 What are the major advantages and disadvantages of casting over other manufacturing processes.

Problem 22.17 Why is a complex runner/gate system used in sand casting. Why is it important to pour slowly and continuously?

Problem 22.18 Why is it important to allow gases to escape? Are there any processes where this would be more important? Which processes eliminate this problem.

Problem 22.19 Why is moisture such a significant problem in casting?

 

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