The document describes the rapid vacuum investment casting process which combines rapid prototyping and vacuum casting. It involves using rapid prototyping to create a master pattern, then making a silicone rubber mold and wax pattern from the master. The wax pattern is invested in ceramic powder and burned out to form a ceramic mold cavity. Finally, molten metal is poured into the mold cavity under vacuum conditions and allowed to solidify to form the cast part. The process allows for casting of metal prototypes and parts directly from 3D CAD models.

1. Metal casting
Shell & investment method
by
Aruna c p

2. Rapid vacuum investment casting
• The Rapid vacuum investment casting is one of the recent
technologies in the Rapid Prototyping (RP) field for producing
metallic components
• It is a combination of Rapid prototyping and Vacuum casting
technologies.
• Here the master patterns are made using rapid prototyping
techniques followed by investment casting process.
• Although this technology has been subjected to numerous
improvements to enhance the mould filling, casting accuracy,
casting quality and alloys range, some intrinsic constraints of the
process limits it’s application

3. Steps involved in Rapid Vacuum Investment
Castings
3D CAD model is the Input for this process .This needs to be provided
by the customer in STL, IGES or STEP format.
Step 1:Producing Rapid Prototype Master using SLA or CNC
machining.

4. Step 2:Preparation of Silicon Rubber Mould (SRM) using the RP
Master pattern in Vacuum

5. • Step 3:Produce wax pattern
(replica of the master pattern)
in the SRM.
• Step 4:Construction of wax
pattern tree by adding sprue,
runners and gates. The wax
tree is placed in a casting flask

6. Step 5: Investment pouring
• Wax pattern is dipped in pre
coat slurry
 Pre coat slurry - fine grained,
heat resistant, costly
• The slurry coated pattern is
covered with fine ceramic
(alumina of silica) powder.
• Drying of 8 to 10 hours in room
temperature (slow cooling is
done else cracking may occur)
• After drying secondary coating
is done by dipping in secondary
slurry and coating with silica
powder.
• The secondary coating Is done
until the shells attain optimum
thickness, usually 7-8 times by
allowing 8 to 10 hours in
between each coating for drying.

7. Step 6: Burn out in furnace
• After the coating, dewaxing is
done by steam heating the
shell for 10 minutes at 10 bar
pressure, 175°C
• After dewaxing shell is kept in
oven at 850-900°C for 2-3
hours for complete burnout of
the wax.
• Shell is ready

8. Step 7:The flask with the ceramic
cavity is placed in the casting
machine.
Step 8: Metal pouring
• Raw alluminium metal is melted
in burner at 680°C.
• Nitrogen gas (inert gas) is passed
and stirred using degassing
machine, to make slag and
impurities to float.
• The shell and the filter is
preheated to 400°C
• Liquid metal is transferred to
graphite crucible, the floating
impurities are removed.
• Then the liquid metal is poured to
shell.
• Allowed it to cool for 30 minutes
at room temperature

9. Step 9:Flask is removed . The
ceramic mould is broken. The
metal tree will be fettled and
cleaned to get the final
castings. Shell at complex
places are removed by dry
sand blasting.
• Step 10:Heat treatment
• Step 11:Machining

10. Investment casting using Crystalline silica powder
• Mould making
• Wax pattern is fitted in the
cylindrical casing.
• Investment powder and chilled
water (at the ratio 100:28 by
weight) are stirred in vacuum
for 4 minutes.
• The mixture poured to the
casing in vacuum in 6 minutes.
• Vacuum condition is
maintained to eliminate the air
bubbles.

11. • Investment mould is air cooled
for 2 hours.
• Then the mould is kept in
heating chamber for 3 days.
• In first 2 days the temperature
is increased slowly to 750°C,
for hardening of the mould
and dewaxing.
• On 3rd day temperature of
mould is decreased to 250°C.

12. • Metal casting
• Measure Al ingot weight based
on weight f wax pattern.
• Preheat ingots to 100 to 120°C
for 15 minutes and loaded in
vacuum casting machine.
• Flask is taken out of the
heating chamber and flask
inside temperature is checked.
(it should be around 250°C)
• melt the ingots up to 700°C at
0.9 bar vacuum

13. • Open the top chamber, clean the
slug
• Load the flask in bottom
chamber
• Supply nitrogen gas to top
chamber.
• Vacuum is created in bottom
chamber.
• metal is poured to the flask at
vacuum for 6 to 10 minutes.
• Unload the flask and air cooled
for 2 hours
• Then the flask is immersed in
water to loose the hardness of
the mould.

14. • The metal part is removed
by breaking the mould.
• Then the remaining powder
is removed with help of
pressurized water.
• Unload the flask and air
cooled for 2 hours
• Then the flask is immersed in
water to loose the hardness of
the mould.

15. metal finishing
• Supports and gates are cut, and polishing is done with grinding
wheels.
• Emery is done for surface polishing.
• Wet sand blasting is done for the final finishing of the products.