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MouldingAccording to the method of removal of CastingExpendable moulding
Multiuse moulding
According to the material usedSand moulding
Investment or Precision mouldingCeramic moulding
Plaster moulding
Metallic mouling
Loam moulding
According to the method usedHand moulding
Machine moulding
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Expendable mold Mold is sacrificed to remove part
Advantage: more complex shapes are possible
Disadvantage: production rates often limited by
time to make mold rather than casting itself
Multiuse Mold Mold is made of metal and can be used to make many
castings
Advantage: Higher production rates
Disadvantage: Geometries limited by need to open
mold
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Sand Molding
The cavity in the sand mold is formed by packing sand
around a pattern, then separating the mold into two
halves and removing the patternThe mold must also contain gating and riser system
If casting is to have internal surfaces, a core must be
included in mold
A new sand mold must be made for each part produced
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Sand Moulding
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Types of Sand Mold
• Green-sand molds
Mixture of sand, clay, and water;
– “Green" means mold contains moisture at time of pouring
• Dry-sand molds
Organic binders rather than clay – And mold is baked to improve strength
• Skin-dried
Mold Drying mold cavity surface of a green-sandmold to a depth of 10 to 25 mm, using torches or
heating lamps
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Types of Sand Mold
• Core-Sand molds
Mixture of sand, binders, water, and additives.
• Shell molds
Moulding process in which the mold is a thin shell of sand held
together by thermosetting resin binder
• Miscellaneous Molds Pit & Floor molds
Loam molds
Hot box molds CO2 process mold
Vacuum molds
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Sand Casting
cope: top half
drag: bottom half
core: for internal cavities
pattern: positive
funnel sprue
runners gate
cavity
{risers, vents}
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Shell Molding
Casting process in which the mold is a thin shell of sand held together by thermosetting resin binder
STEP 1 A match-plate or cope-and-drag metal pattern isheated and placed over a box containing sand mixed with
thermosetting resin.
Steps in Shell Molding
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Shell Molding
STEP 2 : Box is inverted so thatsand and resin fall onto the hot
pattern, causing a layer of the
mixture to partially cure on the
surface to form a hard shell
STEP 3 box is repositioned so
that loose uncured particles
drop away
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Shell Molding
; Step 4: Sand shell is heated inoven for several minutes to
complete curing
STEP 5: Shell mold is stripped from the pattern;
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Shell Molding
STEP 7: The finished casting with sprue removed.
STEP 6: Two halves of the
shell mold are assembled,
supported by sand or metalshot in a box, and pouring is
accomplished.
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Shell molding is also known as croning process
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Shell Moulding – Sand Mixture
Sand Mixture = Fine grained Quartz sand (92%)
+ Thermosetting resin (8%)
Coating Method
Fine grained Quartz sand (92%)+
Bakelite (5%)+Ethyl aldehyde (3%)
Thermosetting Resin
Phenolformaldehydes, Urea formaldehydes,
Ethyl aldehyde, etc.
Mold release agent
Silicone solutions
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Advantages and Disadvantages
Advantages of shell molding:
Smoother cavity surface permits easier flow of molten
metal and better surface finish
Good dimensional accuracy - machining often notrequired
Mold collapsibility minimizes cracks in casting
Can be mechanized for mass production
Disadvantages:
More expensive metal pattern
Difficult to justify for small quantities
Shell Molding
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Investment Moulding
A pattern made of wax is coated with a refractorymaterial to make mold, after which wax is melted
away prior to pouring molten metal
"Investment" comes from a less familiar definition of "invest" - "to cover completely," which refers tocoating of refractory material around wax pattern
It is a precision casting process - capable of producingcastings of high accuracy and intricate detail
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Investment Casting
Steps in investment casting:STEP 1: Wax patterns are produced,
STEP 2: Several patterns are attached to a sprue to form apattern tree
I C i
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STEP 3: The pattern tree is coated with a thin layer of refractory material
STEP 4: The full mold is formed by covering the coated tree
with sufficient refractory material to make it rigid
Investment Casting
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STEP 5: The mold is held in an inverted position andheated to melt the wax and permit it to drip out of thecavity,
STEP 6: The mold is preheated to a high temperature,
the molten metal is poured, and it solidifies
Investment Casting
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STEP 7: The mold is broken away from the finished casting
and the parts are separated from the sprue
Investment Casting
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Investment Casting
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Investment Casting
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Investment Casting
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Investment Casting
C i
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Steps Involved
1. A die for casting the wax is made. (Metal)
2. Wax patterns and gating systems are produced form the
metal dies by injection.3. Precoating ( Slurry- 325 mesh silica flour suspended in
ethyl silicate solution)
4. The coated assembly is placed in the mold and pouring
the investment molding mixture around the pattern.5. Dewaxing and Preheating
6. Pouring
7. Cleaning operations follow cooling of the casting
Investment Casting
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Advantages and Disadvantages
• Advantages of investment Moulding
– Parts of great complexity and intricacy can be mold
– Close dimensional control and good surface finish
– Wax can usually be recovered for reuse
– Additional machining is not normally required - this is anet shape process
• Disadvantages
– Many processing steps are required
– Relatively expensive process
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Refractory Coating MaterialsSilica (67%) + Ethyl Alcohol (33%)
Silica + Alumina + Sodium silicate + Polyvinyl Alcohol
Plaster of paris (60%) + Finer silica (25%) + Talc (15%) +
Water
Investment Molding Mixture:
Sand (95%) + Alumina Cement (5%) + Water (27-31 %)
Wax
Blends of Beeswax, Carnauba, Ceresin, Acrawax, Paraffin
and Resins
Investment Casting
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Permanent Mold Casting Processes
• In permanent mold casting, the mold is reused
many times
• The processes include:
– Basic permanent mold casting
– Die casting
– Centrifugal casting
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The Basic Permanent Mold Process
Uses a metal mold constructed of two sections designed
for easy, precise opening and closing
• Molds used for casting lower melting point alloys are
commonly made of steel or cast iron• Molds used for casting steel must be made of
refractory material, due to the very high pouring
temperatures
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Permanent Mold Casting
Steps in permanent mold casting:
(1) mold is preheated and coated
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Permanent Mold Casting
Steps in permanent mold casting:
(2) cores (if used) are inserted and mold is closed
(3) molten metal is poured into the mold, where it solidifies.
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Advantages and Limitations
• Advantages of permanent mold casting: – Good dimensional control and surface finish
– More rapid solidification caused by the coldmetal mold results in a finer grain structure,
so castings are stronger
• Limitations:
– Generally limited to metals of lower meltingpoint
– Simpler part geometries compared to sandcasting because of need to open the mold
– High cost of mold
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Applications of Permanent Mold Casting
• Due to high mold cost, process is best suited to high
volume production and can be automated accordingly
• Typical parts: automotive pistons, pump bodies, and
certain castings for aircraft and missiles• Metals commonly cast: aluminum, magnesium,
copper-base alloys, and cast iron
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Die Casting
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 die casting
• Use of high pressure to force metal into die cavity is
what distinguishes this from other permanent mold
processes
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Die Casting Machines
• Designed to hold and accurately close two mold
halves and keep them closed while liquid metal is
forced into cavity
• Two main types: 1. Hot-chamber machine
2. Cold-chamber machine
H Ch b Di C i
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Hot-Chamber Die Casting
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 chemically attack plunger and other
mechanical components
• Casting metals: zinc, tin, lead, and magnesium
H Ch b Di C i
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Hot-Chamber Die Casting
Cycle in hot-chamber casting:
(1) The die is closed and the piston rises, opening the port and
allowing molten metal to fill the cylinder.
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Hot-Chamber Die Casting
Cycle in hot-chamber casting:
• (2) The plunger moves down and seals the port pushing the moltenmetal through the gooseneck and nozzle into the die cavity, where it
is held under pressure until it solidifies.
H t Ch b Di C ti
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• 1. The die is closed and thepiston rises, opening the portand allowing molten metal tofill the cylinder.
• 2. The plunger moves downand seals the port pushing the
molten metal through thegooseneck and nozzle into thedie cavity, where it is heldunder pressure until itsolidifies.
Hot-Chamber Die Casting
Hot Chamber Die Casting
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3. The die opens and the cores, if
any, retract. The casting remains inonly one die, the ejector side. The
plunger returns, allowing residual
molten metal to flow back through
the nozzle and gooseneck.
4. Ejector pins push the casting
out of the ejector die. As the
plunger uncovers the filling hole,
molten metal flows through the
inlet to refill the gooseneck, as in
step (1).
Hot-Chamber Die Casting
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Cold-Chamber Die Casting Machine
Molten metal is poured into unheated chamber fromexternal melting container, and a piston injects metalunder high pressure into die cavity
• High production but not usually as fast as hot-chambermachines because of pouring step
•
Casting metals: aluminum, brass, and magnesium alloys
• Advantages of hot-chamber process favor its use on lowmelting-point alloys (zinc, tin, lead)
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Cold-Chamber Die Casting
Cycle in cold-chamber casting:
(1) With die closed and ram withdrawn, molten metal ispoured into the chamber
C ld Ch b i C i
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Cold-Chamber Die Casting
Cycle in cold-chamber casting:
(2) ram forces metal to flow into die, maintaining pressure duringcooling and solidification.
Die casting
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Die casting
- a type of permanent mold casting
- common uses: components for
rice cookers, stoves, fans, washing-, drying
machines,
fridges, motors, toys, hand-tools, car wheels, …
HOT CHAMBER: (low mp e.g. Zn, Pb; non-alloying)
(i) die is closed, gooseneck cylinder is filled with molten metal
(ii) plunger pushes molten metal through gooseneck into
cavity
(iii) metal is held under pressure until it solidifies
(iv) die opens, cores retracted; plunger returns
(v) ejector pins push casting out of ejector die
COLD CHAMBER: (high mp e.g. Cu, Al)
(i) die closed, molten metal is ladled into cylinder
(ii) plunger pushes molten metal into die cavity
(iii) metal is held under high pressure until it solidifies
(iv) die opens, plunger pushes solidified slug from the cylinder
(v) cores retracted
(iv) ejector pins push casting off ejector die
M ld f Di C ti
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Molds for Die Casting
• Usually made of tool steel, mold steel
• Tungsten and molybdenum (good refractory qualities)
used to die cast steel and cast iron
• Ejector pins required to remove part from die when it
opens
• Lubricants must be sprayed into cavities to prevent
sticking
Ad d Li i i
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Advantages and Limitations
•
Advantages of die casting: – Economical for large production quantities
– Good accuracy and surface finish
– Thin sections are possible
– Rapid cooling provides small grain size and goodstrength to casting
• Disadvantages:
– Generally limited to metals with low metal points
– Part geometry must allow removal from die
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Centrifugal Casting
A family of casting processes in which the mold is
rotated at high speed so centrifugal force distributes
molten metal to outer regions of die cavity
• The group includes:
– True centrifugal casting
– Semicentrifugal casting
– Centrifuge casting
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True Centrifugal Casting
Molten metal is poured into rotating mold to produce a
tubular part
In some operations, mold rotation commences after pouring
rather than before
Parts: pipes, tubes, bushings, and rings
Outside shape of casting can be round, octagonal, hexagonal,
etc , but inside shape is (theoretically) perfectly round, due to
radially symmetric forces
True Centrifugal Casting
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True Centrifugal CastingSetup for true centrifugal casting.
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Semicentrifugal Casting
Centrifugal force is used to produce solid castings ratherthan tubular parts
• Molds are designed with risers at center to supply feed
metal• Density of metal in final casting is greater in outer
sections than at center of rotation
• Often used on parts in which center of casting is
machined away, thus eliminating the portion wherequality is lowest
• Examples: wheels and pulleys
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Semicentrifugal Casting
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Centrifuge Casting
Mold is designed with part cavities located away from
axis of rotation, so that molten metal poured into mold
is distributed to these cavities by centrifugal force
• Used for smaller parts
• Radial symmetry of part is not required as in other centrifugal
casting methods
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Centrifuge Casting
PRODUCTMOULD
LIQUID
if i
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Centrifuge Casting
Advantages and Disadvantages
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Advantages and DisadvantagesAdvantages
High rate of production
Better quality and surface finish of castings.
In most castings, a core is not needed. Directional solidification of
metal is promoted that gives good strength and elongation.
Thin and intricate shapes can be easily cast.
No runners and risers are needed.
Mechanical and physical properties of castings are better.
Non-metallic inclusions and gases are minimized.
Foreign inclusions and defects generally exist on the surface and thus
inspection of castings is simplified.
Directional ro erties and stren th is so ood.
Advantages and Disadvantages
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Advantages and Disadvantages
Disadvantages
Cost of installing and maintaining spinning equipment is high.
Handling of machine requires skilled labour.
Corrosion and cracking of permanent moulds occur so often.
In production, molten metal is needed in small intervals that usually
creates trouble in controlling the quality and chemical composition.
Continuous Casting
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Continuous CastingContinuous casting is a casting process in which the operation of
pouring, solidification and withdrawal of casting from an open
mould are carried out continuously.
Continuous Casting
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Working Principle of Continuous Casting
The molten metal is continuously supplied from the ladle to the
intermediate ladle called ‘tundish’ from where it is continuously
poured into the mould at a controllable rate, keeping the level at a
constant position.
The mould usually made of copper or graphite is open at the
bottom and is water cooled so as to extract the heat of the metal
causing its solidification. The shape of the mould corresponds to
the shape of the desired casting.
The process is started placing a dummy bar at the bottom of the
mould upon which the first liquid metal falls.
Continuous Casting
Continuous Casting
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Working Principle of Continuous Casting
The molten metal from the tundish enters the mould and takes the
shape of the mould. The water cooled mould controls the cooling
rate of the metal, so that it solidifies before it leaves the mould.
The metal after coming out of the mould is further cooled by
direct water spray (or water with air) to complete solidification.
The solidified metal is continuously extracted (along with the
dummy bar) by ‘pinch rolls’, bent and fed horizontally and finally
cut to the desired length.
g
Advantages and Disadvantages
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Advantages
In Continuous Casting Sprue, runner, riser, etc. are notused. Hence, no waste metal this leads to 100% casting
yield.
Continuous Casting process is automatic.
Continuous Casting Product has good consistent
soundness.
Continuous Casting products Mechanical Properties are
high and very reproducible.
g g
Advantages and Disadvantages
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Disadvantages
Continuous casting is not suitable for small quantity
production.
Continuous and efficient cooling of moulds is required,
else, center-line shrinkage develops.
Continuous Casting process requires large floor space.
g g
Moulding
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gAccording to the method of removal of CastingExpendable moulding
Multiuse moulding
According to the material usedSand moulding
Investment or Precision mouldingCeramic moulding
Plaster moulding
Metallic mouling
According to the method usedHand moulding
Machine moulding
Types of Sand Mold
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yp
• Green-sand molds
Mixture of sand, clay, and water; – “Green" means mold contains moisture at time of pouring
• Dry-sand molds
Organic binders rather than clay – And mold is baked to improve strength
• Skin-dried
Mold Drying mold cavity surface of a green-sand
mold to a depth of 10 to 25 mm, using torches or
heating lamps
Types of Sand Mold
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yp
• Core-Sand molds
Mixture of sand, binders, water, and additives.
• Shell molds
Moulding process in which the mold is a thin shell of sand held
together by thermosetting resin binder
• Miscellaneous Molds Pit & Floor molds
Loam molds
Hot box molds
CO2 process mold
Vacuum molds
Carbon Dioxide Mould Casting (CO2 Mould Casting)
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Carbon dioxide molding also known as sodium silicate
process is one of the widely used process for preparing
moulds and cores. This process also called Sodium silicate
process.
Molds are strengthen by the application of Carbon DioxideGas instead of baking or any other process.
Sand Mixture = Silica sand+ Binder (Sodium Silicate)
+ Water
+ CO2
Steps involved in making carbon dioxide mould
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Step 1
Suitable proportion of silica sand and sodium silicate binder (3-5% based
on sand weight) are mixed together to prepare the sand mixture.
Additives like aluminum oxide; molasses etc. are added to impart
favorable properties and to improve collapsibility of the sand.
Step 2
The pattern is placed on a flat surface with drag box enclosing it. Parting
sand is sprinkled on the pattern surface to avoid sand mixture sticking to
the pattern.
p g
Steps involved in making carbon dioxide mould
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Step 3
The drag box is filled with the sand mixture and rammed manually till itstop surface. Rest of the operations like placing sprue and riser pin and
ramming the cope box are similar to that of green sand moulding
process. At this stage, the carbon dioxide gas is passed through the vent
holes for a few seconds.
Step 4
Sodium silicate reacts with carbon dioxide gas to from silica gel that
binds the sand particles together. The chemical reaction is given by:
Na2Sio3+CO2 à Na2CO3+SiO2
(Sodium Silicate) (Silica gel)
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Steps involved in making carbon dioxide mould
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Step 5
The sprue, riser and the pattern are withdrawn from the mould, and gates
are cut in the usual manner. The mould cavity is finished and made readyfor pouring.
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Advantages and Disadvantages
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Advantages
Instantaneous strength development. The development of strength
takes place immediately after carbon dioxide gassing is completed.
Since the process uses relatively safe carbon dioxide gas, it does not
present sand disposal problems while mixing and pouring. Hence,
the process is safe to human operators.
Very little gas evolution during pouring of molten metal.
g g
Advantages and Disadvantages
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.Disadvantages
Poor collapsibility of mould is a major disadvantage of this process.
Although some additives are used to improve this property for ferrous
metal castings, these additives cannot be used for non-ferrous
applications.
The sand mixture has the tendency to stick to the pattern and has
relatively poor flow ability.
There is a significant loss in the strength and hardness of mouldswhich have been stored for extended periods of time.
Over gassing and under gassing adversely affects the properties of
cured sand.
Plaster Mold
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Similar to sand casting except mold is made of plaster of Paris (gypsum - CaSO4-2H 2O)
•
In mold-making, plaster and water mixture is pouredover 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
Advantages and Disadvantages
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Advantages and Disadvantages
•Advantages of plaster mold – Good accuracy and surface finish
– Capability to make thin cross-sections
• Disadvantages:
– Mold must be baked to remove moisture, which can
cause problems in casting
–
Mold strength is lost if over-baked – Plaster molds cannot stand high temperatures, so
limited to lower melting point alloys
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 temperatures than plaster
• Can be used to cast steels, cast irons, and other
high-temperature alloys
• Applications similar to those of plaster mold castingexcept for the metals cast
• Advantages (good accuracy and finish) also similar
Different Moulding Processes
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Process Advantages Disadvantages Examples
Sand many metals, sizes, shapes, cheap poor finish & tolerance engine blocks,
cylinder heads
Shell mold better accuracy, finish, higher
production rate
limited part size connecting rods, gear
housings
Expendable
pattern
Wide range of metals, sizes,
shapes
patterns have low
strength
cylinder heads, brake
components
Plaster mold complex shapes, good surfacefinish
non-ferrous metals, lowproduction rate
prototypes of mechanical parts
Ceramic mold complex shapes, high accuracy,
good finish
small sizes impellers, injection
mold tooling
Investment complex shapes, excellent finish small parts, expensive jewellery
Permanentmold
Good finish, Suitable for smallsizes, High production rate
Costly mold, simplershapes only
gears, gear housings
Die Excellent dimensional accuracy,
high production rate
costly dies, small parts,
non-ferrous metals
gears, camera bodies,
car wheels
Centrifugal Large cylindrical parts, good
quality
Expensive, few shapes pipes, boilers,
flywheels
Desirable Mold Properties
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• Green Strength
To maintain shape and easy handling of mold is possible
• Dry Strength
Strength to resist erosion and metallostatic pressure
• Hot Strength
Strength of the sand at some elevated temperature. Inadequate
strength cause mold enlargement, and may break or get cracked
• Permeability
To allow hot air and gases to pass through voids in sand.
• Thermal stability
To resist cracking on contact with molten metal
Desirable Mould Properties
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• Collapsibility
Ability to give way and allow casting to shrink withoutcracking the casting
• Reusability
Can sand from broken mold be reused to make other molds
• Refractoriness
Capacity of sand to resist the higher temperature of molten metal.
• Flowability
Ability of sand to acquire a predetermined shape under pressure and
retain the same when the pressure is removed.
Expendable Mold Casting
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- Styrofoam pattern
- dipped in refractory slurry dried
- sand (support)
- pour liquid metal
- foam evaporates, metal fills the shell
- cool, solidify
- break shell part
Plaster-mold, Ceramic-mold casting
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Plaster mold, Ceramic mold casting
Plaster-mold slurry: plaster of paris (CaSO4), talc, silica flour
Ceramic-mold slurry: silica, powdered Zircon (ZrSiO4)
- The slurry forms a shell over the pattern
- Dried in a low temperature oven
- Remove pattern- Backed by clay (strength), baked (burn-off volatiles)
- cast the metal
- break mold part
Plaster-mold: good finish (Why ?) plaster: low conductivity => low warpage, residual stress
low mp metal (Zn, Al, Cu, Mg)
Ceramic-mold: good finish