This type of silicon mold making is usually called 2
piece block molds as you are totally enclosing your model in silicon,
as you do with a normal block mold. However the mold is made in 2
parts for easy release of the model and later the casting.
The model that requires this form of silicon mold making is usually one that has detail on all sides and no flat back.
(1) Select a mould box to contain the
Craftsil silicon that you are going to pour around your model. For small pieces
this can be a simple plastic container found in any $2 shop or a wood
base on which you can build a box wall surround made of Lego blocks,
Perspex, mat board or something similar.
(2) Trace a half way line around your model
with an Artline pen to show the parting line position of the two halves
of the mold. Embed your model up to this line in a non hardening clay
such as Plastelina within the mould box. Make sure that the clay is in
close contact with the walls of the box and the model to prevent
silicon leaking into any intertisces.
(3) Push a few indentations into the Plastelina around your model with
the round head of a pencil, or something similar, to make the
registration keys that will enable both halves of the mold to fit
together exactly.
(4) Press a small plastic tube about as
thick as a pencil halfway down into the plastelina at the head or base
of the model to act as a feeder channel for the casting medium. Ensure
that the tube is in contact with the model.
(5) Coat all surfaces of the box, plastelina and model with a thin
layer of wax or petroleum jelly (Vaseline)
(6) Mix the Craftsil silicon mold making rubber with a figure
action being careful not to beat any air bubble into the mix. Pour into
the mold beginning at the lowest part from as high a position as you
can manage. This very thin stream of silicon mold making rubber has the effect of
breaking any air bubbles that may be in the mix.
(7) Wait for the Craftsil silicon mold making rubber to cure.
(8) Remove the model and the half poured
silicon mold from the mould box. Turn the mold over and remove the
plastelina clay.
(9) Reposition the mold half already poured
back in the mold box. Seal any cracks between the silicon and the box
wall with plastelina clay. Coat all surfaces of the box, silicon and
model with wax or Vaseline release.
(10) Now pour the second half of the mold
in a similar manner to the first pour and wait for the Craftsil silicon mold making rubber to cure.
(11) Remove both halves of the mold from the mold box and separate.
Lift out the model and the tube use to form the pouring channel. Trim
any flashing from the mold.
(12) To cast position both halves of the mold together so that the
registration nodes are fully engaged and secure together with a rubber
band or masking tape.
(13) Pour in a small quantity of casting
medium, place a finger over the pouring channel and carefully rotate
mould to ensure that all model detail is covered with the medium.
Continue pouring the casting medium, tilting the mold occasionally to
permit air to escape. Note: Small air channels can be cut into the
silicon mould halves if you find there are any undercuts or areas not
covered by the medium. When you notice casting medium coming from these
channels simply plug with clay and continue pouring.
(14) Open the mold after casting has set
but is still a little soft and not completely cured. Remove the
casting which will be a faithful reproduction of the original model.
Cut off the pouring channel, flashing and any air channels. If trimming
is left to do when the model is fully cured you will difficulty in
cleaning the casting.
