Steps in creating a Two part silicon mold Sometimes called a two piece block mold, this type of silicon mold making involves enclosing your model in silicon as you would with a one piece block mold.
The different with two piece block molds is that you create the mould in two parts.
Models that have detail on all sides instead of a flat back require this form of silicon mold making.
(1) Choose a molding box which fits your model and will allow for the silicon to pour around it.
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) Draw a half way line around your model with an marker pen to show the parting line position of the two halves of the mold.
Using a non-hardening modelling clay embed your model up to this line in the mold box.
Make sure the clay is formed right up close to the model and mold box walls so the silicon doesn't leak.
(3) Push a few indentations into the clay 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 clay at the head or base of the model to act as a feeder channel for the casting medium.
Make sure the tube is closely touching the model and the wall.
(5) Using wax or petroleum jelly, coat the clay, model and the mold box walls to prevent the silicon from sticking.
(6) The silicon mold making rubber can now be mixed being careful not to beat any air into it.
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 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 clay.
(9) Now put the half completed mold back into the mold box.
Make sure the completed side of the mold goes in first.
Seal any cracks between the silicon and the box wall with clay.
Coat all surfaces of the box, silicon and model with wax or petroleum jelly release.
(10) You can now mix and pour the remaining half of the mold in a similar way to the first half.
Let the silicon mold making rubber cure.
(11) Remove both halves of the mold from the mold box and separate.
Take the model and pouring channel tube out of the rubber.
Trim any flashing from the mold.
(12) Put both pieces back together ensuring the registration keys made with the pencil are locked firmly together.
Secure both halves with a rubber band or tape.
(13) Now mix your casting material and pour a small amount into the mould.
Block off the pouring hole with your finger and rotate the mold to let the material run over all surfaces in the mold.
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) Once the resin has set, open the mold.
Remove the casting which will be a faithful reproduction of the original model.
Any flash or extra casting material from the casting channel can be cut off and sanded.
If trimming is left to do when the model is fully cured you will have difficulty in cleaning the casting.
By Stan Alderson Copyright Aldax Enterprises Pty Ltd