Molding
silicone usually
comes in a two component form Part A base and a Part B accelerator
which after mixing in the proper ratio cures at ordinary room
temperatures to a flexible, strong RTV (room temperature vulcanizing)
rubber mould.
Our most popular general purpose molding silicone is called
CraftSil-750
and is what is known as a tin-catalyzed, condensation-cure
RTV system. After mixing it gels in one to two hours and cure
within 24 hours.
The properties that attract most users to molding silicone
is that it does not shrink as it cures. It can also be cast
quite quickly and is safe, often used in medical applications.
It has the ability to retain very fine detail. Fortunately
most solvents do not attack silicone. However one of the most
important features is that almost nothing will stick to silicone.
This simplifies the mould making procedures in that no release
agents or model pre-preparation are usually necessary.
In fact the only reason why you would not use molding silicones
for most of your mould making functions is cost. If your silicone
costs are high, examine the use of polyurethane as an alternative.
What can you cast into a silicone mould?
Silicone moulds can be used for casting polyester resins,
plaster, hot melt vinyl, cement, wax, and special high temp
silicone types can be used to cast low temperature melting
metals such as pewter or lead. For reasons of cost, plaster
and cement are almost never used with silicone moulds. Latex
and plastic is a prefered moulding material for these types
of castings.
Molding silicone is also the preferred material for casting
polyurethane plastic resins such as CraftCast
75D but should be left for a week before casting.
High temp silicone
is a popular material when making moulds for toy soldiers.
These molding silicones are designed to accept higher temperatures
without burning the rubber.
Safety
Silicones are safe to use if directions are followed. Avoid
contact with your skin or eyes as irritation may develop.
In case of eye contact flush with water for 15 minutes and
then see a doctor. Use in well ventilated areas.
How to prepare the model
Sealing a porous model:
Although silicones will not usually stick to other materials,
some porous materials such as plaster or wood need sealing
to prevent silicone embedding itself in wood pores and preventing
mould removal from the model.
Seal porous models with several coats of CraftSeal an acrylic
based aerosol spray.
Release
agents: Release agents are not usually required when
making silicone moulds from properly sealed models. However
if in doubt lightly smear the model with petroleum jelly (Vaseline)
being careful to leave no fingerprints. We use CraftRelease
20 which is petroleum jelly dissolved in a solvent that allows
us to spray on the release- which evaporates rapidly and leaves
a thin layer of petroleum jelly.
Some surfaces will inhibit
the cure of silicones such as sulphur based modeling clays.
It is always a good idea to make a test if the surface of
your model is in doubt.
Making
the mould Weigh the two parts accurately before mixing.
Slowly and thoroughly. The colour will indicate when you have
an even mix. Scrape the sides and bottom of the container.
Do not mix air into the mixture with a whipping or up and
down motion.
Pour the silicone over the model in a thin stream
from as high as is comfortable and let it flow over the model.
Air bubbles will break up if you pour in a thin stream. Break
any bubbles that surface with a pin or needle. Allow the silicone
to sit undisturbed for 12 to 24 hours.
Making
a mould by brushing silicone Silicone can be used
to brush on a skin mould over a model by thickening with CraftThix
to a non-sag viscosity. This is sometimes used as a cost saving
alternative to a solid pour.
Casting
Release agents are not usually required when using silicone
moulds. |