SPECIAL MOULDING PROCESSES
Hareesha N G
Lecturer
Dept. of Aeronautical Engineering
Special molding Processes
A. Sand moulds
1.
2.
3.
4.
5.
6.
7.
8.
Green sand mould
Dry sand mould
Core sand mould
Carbon dioxide mould (CO2 mould)
Shell mould
Investment mould
Sweep mould
Full mould
B. Metal moulds
9.
10.
11.
12.
13.
14.
Gravity die casting or Permanent mould casting
Pressure die casting
Continuous casting
Centrifugal casting
Squeeze casting
Thixocasting process
1. GREEN SAND MOULDS
1. GREEN SAND MOULDS
Procedure involved in making green sand moulds
• Suitable proportions of silica sand (85 - 92 %), bentonite binder (6-12 %), water
(3-5 %) and additives are mixed together to prepare the green sand mixture.
• The pattern is placed on a flat surface with the drag box enclosing it as shown in
figure (a). Parting sand is sprinkled on the pattern surface to avoid green sand
mixture sticking to the pattern.
• The drag box is filled with green sand mixture and rammed manually till its top
surface. Refer figure (b). The drag box is now inverted so that the pattern faces
the top as shown in figure (c). Parting sand is sprinkled over the mould surface
of the drag box.
• The cope box is placed on top of the drag box and the sprue and riser pin are
placed in suitable locations. The green sand mixture is rammed to the level of
cope box as shown in figure (d).
• The sprue and the riser are removed from the mould. The cope box is lifted and
placed aside, and the pattern in the drag box is withdrawn by knocking it
carefully so as to avoid damage to the mould. Gates are cut using hand tools to
provide passage for the flow of molten metal. Refer figure (e) and (f).
• The mould cavity is cleaned and finished. Cores, if any, are placed in the mould
to obtain a hollow cavity in the casting. Refer figure (g).
• The cope is now placed on the drag box and both are aligned with the help of
pins. Vent holes are made to allow the free escape of gases from the mould
during pouring. The mould is made ready for pouring. Refer figure (h).
Advantages
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–
Green sand molding is adaptable to machine molding.
No mold baking or drying is required.
There is less mold distortion than in dry sand molding.
Time and cost associated with mold baking or drying is eliminated.
Green sand molds having smaller depths permit the escape of mold gases without
any difficulty.
– In green sand molding, flasks are ready for reuse in minimum amount of time.
– Being soft, green sand molds do not restrict the free contraction of the solidifying
molten metal.
– Green sand molding provides good dimensional accuracy across the parting line.
Disadvantages
– Green sand molds possess lower strengths.
– They are less permeable.
– There are more chances of defects (like blow holes etc.) occurring in castings made by
green sand molding.
– In green sand molding, sand control is more critical than in dry sand molding.
– Mold erosion is very common especially in the production of large sized castings.
– Surface finish deteriorates as the weight of the casting increases.
– Dimensional accuracy of the castings decreases as their weight increases.
2. DRY MOLDING SAND
• Dry molding sand differs from the green molding sand in the sense
that it contains binders (like clay, bentonite,. molasses etc.) which
harden when the mold is heated and dried.
• A typical dry sand mixture (for making non-ferrous castings) consists
of floor sand 40%, new silica sand 30%, coal dust 20% and bentonite
10%.
• A dry sand mold is prepared in the same manner as a green sand
mold; however, it is baked at 300 to 700°F for 8 to 48 hours
depending upon binders used and the amount of sand surface to be
dried.
Advantages
– Dry sand molds possess high strength.
– They are more permeable as compared to green sand molds.
– Castings produced from dry sand molds possess clean and smooth surfaces.
– As compared to green sand molding, dry sand molding turns out castings with less
defects.
– Dry sand molding imparts better overall dimensional accuracy to the molds and
castings as compared to green sand molding.
Disadvantages
– Dry sand molding involves more labour and consumes more time in completing
the mold. Mold baking is an extra work as compared to that required in green
sand molding.
– Dry sand molding is more expensive as compared to green sand molding.
– Dry sand molding involves chances of hot tears occurring in the castings.
– Because of baking, a mold may distort.
– Dry sand molding involves a longer processing cycle as compared to green sand
molding.
– Dry sand molding gives a slower rate of production as compared to green sand
molding.
4. CARBON DIOXIDE (CO2) MOLDING
• Carbon dioxide moulding also known as
sodium silicate process is one of the widely
used process for preparing moulds and
cores.
• In this process, sodium silicate is used as the
binder. But sodium silicate activates or tend
to bind the sand particles only in the
presence of carbon dioxide gas. For this
reason, the process is commonly known as
C02 process.
Steps involved in making carbon dioxide mould
• Suitable proportions 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.
• The pattern is placed on a flat surface with the drag box enclosing it. Parting sand
is sprinkled on the pattern surface to avoid sand mixture sticking to the pattern.
• The drag box is filled with the sand mixture and rammed manually till its top
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.
• Figure (a) shows the assembled cope and drag box with vent holes. At this stage,
the carbon dioxide gas is passed through the vent holes for a few seconds. Refer
figure (b).
• Sodium silicate reacts with carbon dioxide gas to form silica gel that binds the
sand particles together. The chemical reaction is given by:
Na2Si03 + C02 -> Na2C03 + Si02
(Sodium Silicate)
(silica gel)
• 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 ready for
pouring. Refer figure (c).
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 or any odour while mixing and pouring. Hence, the
process is safe to human operators.
• Very little gas evolution during pouring of molten metal.
Disadvantages
• Poor collapsibility of moulds 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 flowability.
• There is a significant loss in the strength and hardness of moulds which have
been stored for extended periods of time.
• Over gassing and under gassing adversely affects the properties of cured sand.
Fig: 5. SHELL MOULDING steps involved
5. SHELL MOULDING
•
•
Shell moulding is an efficient and
economical method for producing steel
castings.
The process was developed by Herr
Croning in Germany during World war-II
and is sometimes referred to as the
Croning shell process.
Procedure involved in making shell mould
a. A metallic pattern having the shape of
the desired casting is made in one half
from carbon steel material. Pouring
element is provided in the pattern itself.
Refer figure (a).
b.
c.
The metallic pattern is heated in an oven to a suitable temperature between
180 - 250°C. The pattern is taken out from the oven and sprayed with a
solution of a lubricating agent viz., silicone oil or spirit to prevent the shell
(formed in later stages) from sticking to the pattern.
The pattern is inverted and is placed over a box as shown in figure 3.3(b). The
box contains a mixture of dry silica sand or zircon sand and a resin binder (5%
based on sand weight).
d. The box is now inverted so that the
resin-sand mixture falls on the heated
face of the metallic pattern. The resinsand mixture gets heated up, softens
and sticks to the surface of the pattern.
Refer figure (c).
e. After a few seconds, the box is again
inverted to its initial position so that the
lose resin-sand mixture falls down
leaving behind a thin layer of shell on
the pattern face. Refer figure (d).
f. The pattern along with the shell is
removed from the box and placed in an
oven for a few minutes which further
hardens the shell and makes it rigid. The
shell is then stripped from the pattern
with the help of ejector pins that are
provided on the pattern. Refer figure (e).
g.
h.
Another shell half is prepared in the
similar manner and both the shells are
assembled, together with the help of
bolts, clips or glues to form a mould.
The assembled part is then placed in a
box with suitable backing sand to
receive the molten metal. Refer figure
(f).
After the casting solidifies, it is
removed from the mould, cleaned and
finished to obtain the desired shape.
Advantages
Better surface finish and dimensional tolerances.
Reduced machining.
Requires less foundry space.
Semi-skilled operators can handle the process easily.
Shells can be stored for extended periods of time.
Disadvantages
 Initially the metallic pattern has to be cast to the desired shape, size and finish.
 Size and weight range of castings is limited.
 Process generates noxious fumes.
6. INVESTMENT MOULD
• Investment mould also called as 'Precision casting' or 'Lost
wax process' is an ancient method of casting complex
shapes like impellers, turbine blades and other airplane
parts that are difficult to produce by other manufacturing
techniques.
The various steps involved in this process are:
Step 1 Die and Pattern making
• A wax pattern is prepared by injecting liquid wax into a prefabricated die having the same geometry of the cavity of
the desired cast part. Refer figure.1.
• Several such patterns are produced in the similar manner
and then attached to a wax gate and sprue by means of
heated tools or melted wax to form a 'tree' as shown in
figure 2.
Step 2 Pre-coating wax patterns
• The tree is coated by dipping into refractory slurry which is a
mixture of finely ground silica flour suspended in ethyl silicate
solution (binder).
• The coated tree is sprinkled with silica sand and allowed to
dry. Refer figure 3 and 4.
Step 3 Investment
• The pre-coated tree is coated again (referred as 'investment')
by dipping in a more viscous slurry made of refractory flour
(fused silica, alumina etc.) and liquid binders (colloidal silica,
sodium silicate etc.) and dusted with refractory sand.
• The process of dipping and dusting is repeated until a solid
shell of desired thickness (about 6 - 10 mm) is achieved.
Note: The first coating is composed of very fine particles
that produce a good surface finish, whereas the second
coating which is referred as 'Investment' is coarser so as
to build up the shell of desired thickness.
Step 4 De-waxing
'
• The tree is placed in an inverted position and heated in a oven to about 300°F. The wax
melts and drops down leaving a mould cavity that will be filled later by the molten metal.
Refer figure 5.
Step 5 Reheating the mould
• The mould is heated to about 1000 - 2000°F (550-1100°C) to remove any residues of wax
and at the same time to harden the binder.
Step 6 Melting and Pouring
• The mould is placed in a flask supported with a backing material and the liquid metal of
the desired composition is poured under gravity or by using air pressure depending on
the requirement. Refer figure.6.
• After the metal cools and solidifies, the investment is broken by using chisels or hammer
and then the casting is cut from the gating
systems, cleaned and finished. Refer figure.7.
Advantages
• Gives good surface finish and dimensional tolerances to castings
• Eliminates machining of cast parts.
• Wax can be reused.
Disadvantages
• Process is expensive.
• Size and weight range of castings is limited
• In some cases, it is difficult to separate the refractory (investment) from
the casting.
• Requires more processing steps.
7. SWEEP MOULD
• In sweep moulding, the cavity is formed as the pattern sweeps the sand all
around the circumference.
• A thin wooden piece is attached to a spindle at one edge while the other
edge has a contour depending on the desired shape of the casting.
• The spindle is placed at the center of the mould and rotated so that the
wooden piece sweeps in the mould box generating the shape of the
required casting.
• Green sand, loam sand or sodium silicate sand can be used symmetrical
shapes.
8. FULL-MOULD PROCESS
• Full-mould casting or 'cavity less' casting is a technique
similar to investment casting, but, instead of wax,
polystyrene foam is used as pattern.
• The pattern can be hand cut or machined from pieces of
foamed polystyrene.
• Gating and risering systems are made from the foamed
material in single or multiple pieces and then assembled to
the pattern with the help of paste or glue. Refer figure (a).
• The entire pattern assembly is dipped into a water based
ceramic material, dried and positioned in a one piece sand
mould.
• Green sand or no-bake sand is preferred for moulding. Refer
figure (b).
• When the molten metal comes in contact with the foamed
pattern, the foam vaporizes (melts and burns) allowing the
molten metal to occupy and fill the cavity.
• The amount of gas produced by the foam is so small that it
can easily escape through the sand.
• Pump housing, manifolds and auto-brake components are a
few among the various products that can be made from this
process.
Advantages
• Withdrawal of pattern requires some form of design modifications
like providing draft allowance, loose pieces etc. Such complex
processes are eliminated in full-mould process through the use of
patterns that can be removed by melting and vaporization.
• No limit to size and shape of castings.
• Good surface finish.
Disadvantages
• High cost of patterns.
• More care should be taken during moulding.
• Patterns being light and low in strength can be easily distorted or
damaged.
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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9. GRAVITY DIE CASTING
• Gravity die casting or permanent mould casting is a casting process in which the
molten metal is poured into a metallic mould called die under the influence of
gravity. Hence the name 'gravity die casting'.
• The mould or die is usually made from cast iron, tool steel, graphite, copper or
aluminum alloys and the choice for a particular material depends on the type of
metal to be cast.
• Gating and risering systems are machined either in one or both the mould
halves.
• Figure shows a permanent mould made in two halves which resembles a book.
The mould halves are hinged and can be clamped together to close the mould.
Steps involved in the process
• The mould is cleaned using wire brush or compressed air to remove dust and other
particles from it.
• It is preheated to a temperature of 200 - 280°C by gas or oil flame and then the
surface is sprayed with a lubricant.
• The lubricant helps to control the temperature of the die thereby increasing its life
and also assist in easy removal of solidified casting.
• The mould is closed tightly and the liquid metal of the desired composition is poured
into the mould under gravity.
• After the metal cools and solidifies, the mould is opened and the casting is removed.
Gating and risering systems are separated from the cast part.
• The mould is sprayed with lubricant and closed for next casting. The mould need not
be preheated since the heat in the previous cast is sufficient to maintain the
temperature.
Advantages
• Good surface finish and close dimensional tolerances can be achieved.
• Suitable for mass production.
• Occupies less floor space.
• Thin sections can be easily cast.
• Eliminates skilled operators.
Disadvantages
• Initial cost for manufacturing moulds (dies) is high.
• Not suitable for steel and high melting point metals/alloys.
• Un-economical for small productions.