Transcript Chapter 9 Change of Form - Southern Illinois University

Thermal Methods of
Change of Form
Chapter 9
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Competencies
 Identify the unique characteristics for each
casting process
 Describe the Investment Casting process
 List the advantages of Investment Casting over
Sand Casting
 Describe how to select a casting process
 Identify a given casting process as either
expendable mold or permanent mold
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Metal Casting: Solidification of molten metal
poured into a mold or injected into a die.
4 Most Popular Casting Processes:
• Sand casting
• Investment casting
• Permanent mold casting
• Die casting
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SAND CASTING (Expendable Mold classification) – mold
is produced around a pattern that is withdrawn to leave a
cavity. Used only once and must be broken up to free the
solidified casting
There are two significant restrictions on the shape of an
object that can be sand cast.
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There must be a plan that will become the plane of
separation for the two halves of the mold
The pattern should slip cleanly out of the sand without
disturbing the sand.
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Important factors in casting operations:
• The flow of the molten metal into the mold
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cavity
Heat transfer during solidification and cooling
of the metal in the mold (The metal grain
sizes are smaller near the mold than in the
center)
Influence of the type of mold material
Solidification of the metal from its molten state
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Parts of a Sand Casting:
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Pattern is the shape that is to be cast, about which
the Mold is made.
Mold is the container into which liquid metal, plastic,
or other material is poured in making a casting.
The Cope is the top half of the casting flask. The
Drag is the bottom part of the flask.
Pouring Basin - Molten metal is poured through a
pouring basin (cup) and flows through the gating
system
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Sprue - vertical channel through which the molten
metal flows downward into the mold
Runners - channel to carry molten metal from sprue
to gate
Gates - portion of the runner through which the
molten metal enters the mold cavity, traps
contaminate, prevents gas buildup and provides for
ease of separation
Riser (Feeder head) – let out trapped air, reservoirs
to supply the molten metal necessary to prevent
shrinkage during solidification.
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Procedure
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Sand is Mulled
The pattern is place flat side down on a smooth surface
The empty drag is placed around it
Pattern is dusted with dry parting sand
Mulled sand is riddled into the drag and tamped about the pattern
After complete packing around pattern, the bottom of drag is
screeded
Packed drag is carefully lifted from the patter and inverted
The cope is similarly made with cores forming the sprue and riser
for filling
Cope is placed atop drag and secured if necessary
Mold is ready to pour.
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Melting and pouring
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Charge - the mixture of alloying elements that will be
at the specified composition upon melting
Fluxes - various compounds that can be mixed with
the metal to react with the melt
Melt - the molten metal that will be used to produce
the casting
Tapping - when melt reaches proper temperature,
method used to extract the metal from furnace
Pouring - transferring the tapped material to the
casting
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Casting techniques:
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Mulling is the mixing of the sand with the bonding
agent.
Riddling is the sifting of the sand into the flask.
Green-sand casting uses water as the bonding
agent for the sand. When the water dries out, the
mold falls apart.
Shell molds use a resin or other organic bonding
agent and are baked to harden the mold. Since they
do not dry out, they last a long time.
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Casting Alloys
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Cast steels - readily welded to build up components of
unusually large size and complexity
Railroad equipment (wheels, truck, frames, couplers)
Construction and mining equipment
Metal working machinery
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White cast irons - hard and brittle, wear resistant parts
(virtually unmachineable except by grinding)
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Grinding balls
Liners for ore-crushing mills
Some agricultural machinery parts
Gray Iron - (low cost) is preferred choice in all fields where
ductility and high strength are needed
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Weights
Motor gears
Pump and pipe fitting
Machine tool bases (damping capacity)
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INVESTMENT CASTING (Expendable Mold)
 Lost wax– Pattern made of wax or polystyrene. Dipped
into refractory material, then wax is melted out and
metal is poured in.
Advantages of investment casting versus sand are
greater detailed parts, better surface finish, less
machining, less tool wear.
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Full-mold – lost-form/expanded polystyrene/evaporative
patter – pattern is made of expanded polystyrene rather
than wax. Stryrofoam is vaporized and molten metal
takes its place.
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Minimum wall thicknesses - are 2.5 mm
Tolerances - can be held to .3 % on dimensions
Surface finish - can be held from 2.5µm to 25µm (0.1µin to
1.0 µin) rms.
Size limits- are from 400 g (1 lb) to several tons. No draft
allowance is required.
Typical materials - aluminum, iron, steels, nickel alloys,
copper alloys.
Types of parts - pump housings, manifolds, and auto brake
components.
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Plaster Mold – investment material is plaster, works only
with plastics or low-melting-point nonferrous metals or
alloys. In plaster mold casting, a plaster, usually gypsum
or calcium sulfate, is mixed with talc, sand, asbestos, and
sodium silicate and water to form a slurry.
 This slurry is sprayed on the polished surfaces of the
pattern halves (usually brass). The slurry sets in less than
15 minutes to form the mold. The mold halves are
extracted carefully from the pattern, and then dried in an
oven.
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The mold halves are carefully assembled, along with the
cores. The molten metal is poured in the molds. After the
metals cools down, the plaster is broken and the cores
washed out.
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Minimum wall thicknesses- section thickness can be as
small as 0.6 mm (0.025 in)
Tolerances - 0.2 % linear
Surface finish- is 1.25 µm to 3 µm (50 µin to 125 µin) rms.
Size limits - usually small to medium size, ranging in weight
from 30 g (1 oz) to 7 kg (15 lb) the draft allowance is 0.5-1.0
degree.
Typical materials - Low temperature melting materials such
as aluminum, copper, magnesium and zinc.
Types of parts -used to make quick prototype parts as well
as limited production parts.
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Ceramic Mold – uses a refractory slurry instead of plaster
as the investing material, can withstand higher
temperatures than plaster.
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Minimum wall thicknesses- as small as 1.25 mm (.050 in)
Tolerances - can be held to 0.4 %,
Surface finish- can be better than 2 - 4 µm (.075 - .15 µin).
Add 0.3 mm (.012 in) for parting line tolerances.
Size limits - from 60 g (2oz) to a ton. Draft allowance of 1°
is recommended.
Types of parts - Typical parts made from this process
include impellers made from stainless steel, bronze,
complex cutting tools, plastic mold tooling
This process is expensive, but can eliminate secondary
machining operations.
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Shell Mold - higher throughput due to reduced cycle times.
A heated (200 ºC / 392 ºF) metal pattern is covered with a
mixture of sand and thermoset plastic.
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Minimum wall thicknesses- Minimum thicknesses can be
as low as 1.5 mm (0.062 in) to 6.25 mm (0.25 in),
depending on the material.
Tolerances - and dimensional tolerance of 0.5 %.
Surface finish- complex parts with good surface finish 1.25
µm to 3.75 µm (50 µin to 150 µin) rms
Size limits – Size limits of 30 g to 12 kg (1 oz to 25 lb).
Typical materials – cast irons, and aluminum and copper
alloys
Types of parts -connecting rods, gear housings, lever arms
etc.
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Shell Mold con’t
 A good surface finish and good size tolerance reduce the
need for machining
 A fairly high capital investment is required, but high
production rates can be achieved. The process overall is
quite cost effective due to reduced machining and
cleanup costs.
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Centrifuged- flask is rotated containing the liquid metal
where the centrifugal force drives the material into the
inner recesses of the mold cavity.
Vacuum Casting Molding – vacuum draws the material
into the mold. Can be used as the final steps in the lostwax process or with shell molds
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Production Rate: 15-25 parts each week.
Vacuum costs money and takes tooling time.
Parts are of higher quality than with Gravity Casting, with
thinner walls and fewer voids.
Materials can simulate ABS, Nylon, Polycarbonate, etc.
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PERMANENT MOLD
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Instead of using sand as the mold material, a metal is
used as a mold. Typically cast iron or Meehanite (a dense
cast iron) is used as the mold material and the cores are
made from metal or sand. Cavity surfaces are coated with
a thin layer of heat resistant material such as clay or
sodium silicate.
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Permanent mold castings permanent mold castings produce a
lower price per piece
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Minimum wall thicknesses-(such as 3mm for lengths under 75 mm),
radius (inside radius = nominal wall thickness, outside radius = 3 x
nominal wall thickness), draft angles (1 to 3º on outside surfaces, 2 to
5º on inside surfaces) etc all apply.
Tolerances - Typical tolerances are 2 % of linear dimensions.
Surface finish- ranges from 2.5 µm to 7.5 µm (100 µin to 250 µin).
Size limits – Typical part sizes range from 50 g to 70 kg (1.5 ounces
to 150 lb).
Typical materials – small and medium sized parts made from
aluminum, magnesium and brass and their alloys.
Types of parts - gears, splines, wheels, gear housings, pipefittings,
fuel injection housings, and automotive engine pistons.
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Low-pressure feed - Here, instead of using gravity to assist
in the metal pour and flow in the mold, a low pressure of
up to 1 atmosphere gas is applied to the molten metal.
This maintenance of pressure on the melt causes
complete fill of the mold and compensates for any
shrinkage on cooling.
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Vacuum feed - This is similar to the low-pressure
permanent mold casting, where a vacuum is used instead
of a pressure. Thin wall castings can be made as in the
low-pressure permanent mold casting. Castings range in
size from 200 g to 4.5 kg (6 oz to 10 lb).
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Die casting - Made of iron or steel. Molten aluminum
or magnesium is forced into die cavity by hydraulic
ram (1000-30,000 psi). Die is cooled by circulating
oil or water.
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High cost of manufacturing and maintaining a
permanent mold.
Used for high-volume production runs.
Hot-chamber die casting is used for low-melting
temperature casting materials.
Cold-chamber die cast is used for higher melting
temperature alloys.
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Other Permanent Mold Castings
Slush Casting: Slush Casting is a special type of
permanent mold casting, where the molten metal is
not allowed to completely solidify. After the desired
wall thickness is obtained, the not yet solidified
molten metal is poured out. This is useful for making
hollow ornamental objects such as candlesticks,
lamps, statues etc.
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Other Permanent Mold Castings
Corthias Casting: Corthias Casting is another
variation of the permanent mold casting, where a
plunger is used to pack down the molten metal form
the sprue hole. This allows for thinner walls and
greater details to be produced.
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Other Permanent Mold Castings
Centrifugal Casting and Semi-Centrifugal Casting.
 Centrifugal Casting: In centrifugal casting, a
permanent mold is rotated about its axis at high
speeds (300 to 3000 rpm) as the molten metal is
poured. The molten metal is centrifugally thrown
towards the inside mold wall, where it solidifies after
cooling.
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Other Permanent Mold Castings
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Minimum wall thicknesses-. 2.5 mm to 125 mm (0.1 - 5.0 in).
Tolerances - OD can be as good as 2.5 mm (0.1 in) and on the
ID can be 3.8 mm (0.15 in).
Surface finish- from 2.5 mm to 12.5 mm (0.1 - 0.5 in) rms.
Size limits –up to 3 m (10 feet) diameter and 15 m (50 feet)
length
Typical materials –iron, steel, stainless steels, and alloys of
aluminum, copper and nickel. Two materials can be cast by
introducing a second material during the process.
Types of parts - pipes, boilers, pressure vessels, flywheels,
cylinder liners and other parts that are axi-symmetric
Only cylindrical shapes can be produced with this process
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