How to use molding silicone and high temp silicone

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.

You can find molding silicone and many other materials you need for making moulds and casting on our web store. You can enter the store by clicking here.

By Stan Alderson

Copyright 2010 Aldax Enterprises Pty Ltd