Detail procedure of silica rubber moulding by vacuum casting

1. Vacuum casting
By
Aruna c p

2. Vacuum Casting
• Vacuum Casting is a technology in the field of Rapid Prototyping for
the fast production of plastic components that are functional and
close or identical to production parts.
• Vacuum Casting begins with Silicone Mold which is made based on
a master model, which has been built with one of the Rapid
Prototyping machines or in a conventional way. Then the Silicone
Mold (SRM) is filled with a two part Polyurethane resin under
Vacuum.
• Around 20 Polyurethane parts can be extracted from a single rubber
mold.

3. SRM Mould preparation
• An RP part is used to create a
master pattern.
• The RP part is then putty
finished to achieve the desirable
finish on the PU parts.
• The master pattern is fitted with
a sprue and gate and then
surrounded by a parting surface,
which establishes the parting
line for the mold.
• The assembly is fixtured in a
wooden box, the size of which is
determined by the size of the
master.
• Volume of the required mould is
calculated, according to mass of
silicon rubber + hardener in the
ratio 10:1 is stirred well.

4. • The liquid RTV is degassed in a
Vacuum casting machine, which
eliminates air bubbles trapped in
it. The degassed liquid RTV is
then poured over the pattern and
parting surface combination.
• Mould is kept in oven at 40°C for
10 hours for better mould life

5. • After curing mould is cut at parting line in zig-zag pattern.
• Zig-zag pattern is provided because of proper alignment and
also avoid mismatching of the two halves.
• Master is taken out, gates & risers are removed, and clean the
mould with pressurized air.
• Mould is READY

6. Vacuum casting
• The weight of the material
required is calculated
• 35% extra material (PU
liquid + hardener) is taken ,
this excess material is to
ensure that there will not be
any shortage of material.
• Mould releasing spray is
applied inside the mould.
• Male and female mould parts
are joined with tape or
staples.

7. • Material and mould is kept in
vacuum chamber and the
pouring material is stirred well
with the hardener to make the
proper mixing.
• The ratio of the liquid and the
hardener differs from material
to material

8. • After pouring keep the mould in oven at 40°C for 3 to 4 hours.
• Mould is taken out and the product is removed from the mould.
• Then the product is kept in the oven (70°C) for over curing.

10. Advantages of silicon mould & vacuum casting
• One mould can be used up to 20 times.
• The surface finish obtained is same as that of master we
used.
• Transference
• Good dimensional accuracy until 20th part
• Elimination of air bubbles, so there will not be any
discontinuities.
• Complicated structures unable to form by machining can
be done.

11. limitations
• Difficult to produce hallow structures.
• Mould releasing spray is necessary
• Leakage of material from the mould as the usage of the
mould.
• After 15 to 20 times usage of the mould, there will be
chances of defects in the product.
• Mould cannot be recycled.
• Defects due to damage in rubber parts.