foundry

1. Other Expendable Mold Processes
• Shell Molding
• Vacuum Molding
• Investment Casting
• Plaster Mold and Ceramic Mold Casting
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2. Shell Molding
Casting process in which the cavity (& gating
system) is a thin shell of sand held together by
thermosetting resin binder
Steps in shell-molding: (1) a match-plate or cope-and-drag
metal pattern is heated and placed over a box containing
sand mixed with thermosetting resin.
part
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3. • Preparation of the metal match plate cope and drag
type patterns : Cu, CI, Al or steel
• Mix the investment material : dry, fine and thorughly
washed silica & 5 to 8% thermosetting resin Phenolic
resin ( phenol formaldehyde) acting as binder
• Heat the pattern : 200 to 3000 C
• Invest the pattern : dump box 30 to 45 s get 5 to 8 mm
thick shell
• Curing the shell 1 to 3 min at 250 to 4500 C
• Remove the shell
• Assemble the shells
• Pour the mould
• Remove the casting

4. Steps in shell-molding: (2) box is inverted so that sand
and resin fall onto the hot pattern, causing a layer of
the mixture to partially cure on the surface to form a
hard shell; (3) box is repositioned so that loose
uncured particles drop away;
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5. Steps in shell-molding: (4) sand shell is heated in oven
for several minutes to complete curing; (5) shell mold
is stripped from the pattern;
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6. Steps in shell-molding: (6) two halves of the shell
mold are assembled, supported by sand or metal shot
in a box, and pouring is accomplished; (7) the
finished casting with sprue removed.
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7. Advantages and Disadvantages
• Advantages of shell molding
– High precision and accuracy of casting with tolerance of ± 0.002
to ± 0.005 mm/mm is possible
– Smoother cavity surface permits easier flow of molten metal,
better surface finish and reduced machining allowance possible.
The roughness is in the range of 3.2 μ m Ra -Good dimensional
accuracy - machining often not required
– Complex parts can be made even of difficult to machine metals
and alloys
– Mould can be stored until required.
– Permeability of thin shell moulds is higher compared with other
types of moulds and thin shell does not have great chilling effect
as a sand mould resulting better quality of casting 7

8. – Less sand is used compared to sand casting
– Cleaning is considerable reduced and in some cases
eliminated
– Mold collapsibility minimizes cracks in casting
– Can be mechanized for mass production
– Saving of metal through use of smaller gates, sprues and
risers resulting in increased yield.
• Disadvantages:
– More expensive metal pattern and binder, it is
economical for mass/batch production , Difficult to
justify for small quantities
– Size of casting is limited - 10 to 13.5 kg
– Serious dust and fume problems during snad and resin
mixing
– Carbon pickup in case of steels

9. Investment Casting (Lost Wax Process)
A pattern made of wax is coated with a refractory
material to make mold, after which wax is melted
away prior to pouring molten metal
Pattern made of wax is melted out and gets destroyed –
“lost wax method”
• "Investment" comes from a less familiar definition of
"invest" - "to cover completely," which refers to
coating of refractory material around wax pattern
• It is a precision casting process (called Precision
Investment Casting) - capable of producing castings
of high accuracy and intricate detail
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10. Step-by-step Procedure
1. Making a master pattern of the part to be cast :made of
metal such as brass, Al alloy or steel or a fusible alloy
(Alloy of Sn, Pb and Bi) considering shrinkage of wax
2. Making a composite die to the master pattern for casting
the wax/plastic pattern ; die matrial : low melting point
material Bi or Al and Cast Iron. Cavities Can be machined
directly from die-blocks of steel
3. Making of wax/plastic patterns: Molten wax/plastic
injected into mater die cavity under pressure
4. Assembling the wax patterns to a wax gating system:
several small vax patterns are assembled together to a
wax gating system connected to a central sprue by wax
welding using heated tools to form a cluster or Tree of
wax pattern

11. • 5. Investing the wax patterns: Pre-coating of wax tree by
dipping it in an extremely fine refractory slurry or spraying :
slurry coating determine the surface quality of castings
• 6. Melting of wax patterns and baking the mould: After the
investment has set around the wax tree, the formed
investment mould is placed in inverted form in an oven
kept between 90 to 1750 C to dry the investment and to
melt out wax pattern
• 7. Melting the metal and pouring the mould: Mould is
further dried by heating to 800 to 1050 0C to remove wax
completely - preheated mould is filled with molten metal
and allowed to solidify
• 8. Shaking out the casting, removing gates and feeder:
fragile mould material is broken to get casting and gates
and runners are removed
• 9. Cleaning and inspecting the casting

12. Steps in investment casting: (1) wax patterns are
produced, (2) several patterns are attached to a sprue
to form a pattern tree
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13. Steps in investment casting: (3) the pattern tree is coated
with a thin layer of refractory material, (4) the full mold is
formed by covering the coated tree with sufficient
refractory material to make it rigid 13

14. Steps in investment casting: (5) the mold is held in an
inverted position and heated to melt the wax and permit it
to drip out of the cavity, (6) the mold is preheated to a
high temperature, the molten metal is poured, and it
solidifies 14

15. Steps in investment casting: (7) the mold is broken away from
the finished casting and the parts are separated from the
sprue
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16. Advantages and Disadvantages
• Advantages of investment casting:
– Parts of great complexity and intricacy can be cast
– Close dimensional control (± 0.003 mm/mm) and good
surface finish (1.5 to 2.25 μm Ra (no parting line)
– Undercuts and other shapes difficult to withdraw
normal pattern easily obtained
– Wax can usually be recovered for reuse
– Additional machining is not normally required - this is
a net shape process (unmachinable alloys –heat
resisting steels or nimonic alloys- can be cast
• Disadvantages
– Many processing steps are required - Relatively
expensive process
– Limitation in use of and location of holes
– Parts are limited in size to a few kg 16

17. Product Applications
• Parts for Aerospace industry, in aircraft engines, frames, fuel
systems and instrument
• Computers and data processing equipment
• Food and beverage machinery
• Machine tools and accessories, scientific instruments,
sewing machine,
• Nozzles, buckets, vanes and blades for gas turbines
• Costume jewellery
• Rock drill thread chaser holder blocks
• Dentures and special metal implants for orthopedic surgery
• Parts for movie cameras and projectors
• Radar wave guide
• Parts such as reciprocates slides for cloth cutting machines

18. Vacuum Casting
• In Conventional melting, reducing and pouring metals the
molten metal comes in contact with O2, H2 and N2 of the
air – metals have gases and non-metallic inclusions in
their masses – free from contaminants metals held in
closed chamber – Ingot mould is poured within the
vacuum furnace and maintained until ingot is completely
solidified –
• produces high quality metals and alloys. – especially for Ti
having high affinity for N2 and other elements – vacuum
poured steel ingots forged to produce turbine spindles –
more uniform and free from contaminants

19. Used for thin walled (0.75 mm) complex shapes with uniform
properties ex gas-turbine components from superalloys. Process uses
slight vacuum (2/3 atm) also reduces porosity. Automated, and
production costs similar to green-sand casting.
Mold cavity is prepared in metallic dies – mould material : mixure of
fine grain sand and urethane as binder – gate provided at the bottom
of the mold – metal melted in induction furnace – mould is held with a
robot arm and is partially immersed in molten metal. When vacuum is
drawn in the mould the molten metal is drawn into mould cavity
through the bottom gate. The temp. of molten metal only about 550C
above the solidus temperature. So, it solidifies and cools very quickly
in the mould cavity – when the cavity is filled mould is withdrawn from
the furnace

20. Vacuum Casting
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22. As the metal enters the mould cavity
through the bottom gate under vacuum,
the method is called Counter gravity low
pressure process (CL)
CLA – metal melted in air, parts are made at
a high volume and low cost
CLV - Metal melted in vacuum, involve
reactive metals such as aluminum, titanium,
zirconium, and hafnium – super alloys for gas
turbines (as thin as 0.50 mm)

23. Plaster-Molding process – Slurry moulding
1) Made of Plaster of Paris (gypsum or calcium), talc, & silica
flour
2) Mix Finer grained refractory with water- slurry Mixture
poured over pattern (made of brass and phenolic resin
plastics) and allowed to harden
3) Plaster sets and is removed, the mold dried at high temps to
remove moisture and avoid steam formation
4) Mold halves assembled, preheated and molten metal is
poured.
• *Gases evolved during solidification cannot escape, so
molten metal is poured under a vacuum or under pressure.

24. • Yellow brass, Mn and Al bronzes, Al and Mg alloys
• Al pistons, locks, propellers, aircraft parts,
plumbing fixture fittings, ornaments and tire and
plastic moulds
• Low Thermal Conductivity allows casting to cool
slowly, allowing a more uniform grain structure is
obtained with less warpage
• More accurate and smooth surfaces and more
faithful reproduction of details than sand casting
• In some cases, machining and finishing operations
are eliminated
• Very close tolerances obtained ± 0.005 mm/ mm
and surface roughness 0.75 to 3 μ m.

25. Ceramic Mold casting
Similar to plaster mold process EXCEPT it uses refractory
materials suitable for HIGH TEMPS. – fine grain refractory
powders of Zircon ( Zr Si O ) or fused silica (Si O2) with
bonding agents – the slurry is applied as thin coating to the
pattern and is backed by a less expensive fire clay – mould
baked at 10000C and melt is poured while the mould is hot
Suitable for all materials due to the use of better ingredients for
the slurry.
High-temperature resistance of refractory molding materials
allows for casting of high temp alloys, stainless steel, and tool
steels.
*Expensive Process but castings have good dimensional
accuracy and surface finish

26. Permanent Mold Casting Processes
• 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:
– Basic permanent mold casting
– Die casting
– Centrifugal casting
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27. 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 (Al, Cu, Brass) 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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28. Permanent Mold Casting
Steps in permanent mold casting: (1) mold is preheated and
coated
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29. 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. 30

30. Advantages and Limitations
• Advantages of permanent mold casting:
– Good dimensional control (0.015 mm/mm to .25 mm /
mm), better appearance and surface finish – surface
roughness 2.54 to 6.35 μm
– Very economical for mass production
– Repeated use of mould (upto 25000)
– More rapid solidification caused by the cold metal mold
results in a finer grain structure, so castings are stronger -
– further by heat treatment
– Low scrap loss
– Low porosity
– Lesser floor space required
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31. • Limitations:
– Generally limited to lower melting point non-ferrous
alloys.
– Complex part geometries can not be made because of need
to open the mold (0.1 kg to 225 kg; 2.38 mm to 50 mm
thickness)
– High cost of mold and its maintenance
(3000 to 10000 castings for C.I.
10000 to 25000 castings for Al)
– Not suitable for low-volume production

32. Applications
• Automotive Al alloy pistons, cylinder heads, aircraft
and missile castings, pump bodies, cooking utensils,
refrigerator compressor cylinder blocks, head and
connecting rod, flat iron sole plates, kitchenware,
washing machine gear blanks of CI

33. 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 (7-35MPa) to force metal
into die cavity is what distinguishes this from
other permanent mold processes 34

34. 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
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35. 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
• Injection pressure: 7-35MPa
• Applications limited to low melting-point metals that do not
chemically attack plunger and other mechanical
components
• Casting metals: zinc, tin, lead, and magnesium
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36. Suresh Mayilswamy 37

37. Hot-Chamber Die Casting
Cycle in hot-chamber casting: (2) plunger forces metal in chamber
to flow into die, maintaining pressure during cooling and
solidification.
Permanent Mold Processes
Because the die material
does not have natural
permeability (like sand
has), vent holes at die
cavity needs to be made
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38. Cold-Chamber Die Casting
Molten metal is poured into unheated chamber from external
melting container, and a piston injects metal under high
pressure (14-140MPa) into die cavity
• High production but not usually as fast as hot-chamber
machines because of pouring step
• Casting metals: aluminum, brass, and magnesium alloys
Permanent Mold Processes
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39. Cold-Chamber Die Casting
Cycle in cold-chamber casting: (1) with die closed
and ram withdrawn, molten metal is poured into the
chamber
Permanent Mold Processes
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40. Cold-Chamber Die Casting
Cycle in cold-chamber casting: (2) ram forces metal to flow
into die, maintaining pressure during cooling and
solidification.
Permanent Mold Processes
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41. Molds for Die Casting
• Usually made of tool steel or mold steel
• Tungsten and molybdenum (good refractory
qualities) are used to make die for casting steel
and cast iron
• Ejector pins are required to remove part from
die when it opens
• Lubricants must be sprayed into cavities to
prevent sticking
Permanent Mold Processes
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42. Advantages and Limitations
• Advantages of die casting:
– Economical for large production quantities
– Good accuracy (±0.076mm)and surface finish
– Thin sections are possible
– Rapid cooling provides small grain size and good strength
to casting
• Disadvantages:
– Generally limited to metals with low metal points
– Part geometry must allow removal from die, so very
complex parts can not be casted
– Flash and metal in vent holes need to be cleaned after
ejection of part
Permanent Mold Processes
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43. 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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44. Centrifugal Casting
Molten metal is poured into a rotating mold to produce a tubular part
• In some operations, mold rotation commences after pouring rather
than before
• Rotational axes can be either horizontal or vertical
• 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
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45. Shrinkage allowance is not considerable factor
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46. Assignment No. 1
Propose the best suitable casting process to make an aluminum cup.
During selecting a process, keep the following points in view:
1. No of cups= 4
2. Product cost= as low as possible
3. Surface quality= good. Quality is not as important as cost
4. Defects= some defects are acceptable
5. Processing time= not important
Draw an analysis for each major type of casting process with reference to
above conditions. Then choose one casting process and write a report in its
support .
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