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Silicone Additives- Accelerator, Thinner, Thickener

May 1, 2017

Our tin catalyzed silicone, AeroMarine AM 128 Silicone Rubber, can be used with silicone additives that either decrease or increase the viscosity or speed up the cure time. Each of these silicone additives is added at different times in the mixing process and in different quantities to affect the silicone. These additives are the proper way to control viscosity and speed up cure time. Please do not mix silicone with both the pourable (purple) and brushable (pink) catalyst. Please select one catalyst and use the thinning or thickening additive to control the viscosity.  All of the silicone additives discussed here are for use with tin catalyzed silicone where the silicone and catalyst are separate until mixing. They are not to be used with AM 125 Silicone Rubber, a 1:1 silicone where one side contains the catalyst or our platinum catalyzed silicones, like our Food Grade Silicone or our Silicone Putty.

This silicone additive allows you to speed up the cure process from the usual 16 to 24 hours. It is added by weight to the catalyst, NOT to the white silicone base. You must use a digital gram scale for accurate weight measurement. The silicone accelerator should only be added to the catalyst at the time you are using it. The accelerator loses its potency when added to the catalyst and left to sit. It is important to remember that speeding up the cure time also shortens the pot life. So, the silicone needs to be mixed and poured in or brushed on quickly when using the accelerator. Here are some guideline weight percentages with their pot lives and cure times:

.2% by weight of accelerator to catalyst gives a 20 minute pot life and a 3 hour cure time

.6% by weight of accelerator to catalyst gives a 5 minute pot life and a 2 hour cure time

1.0 by weight of accelerator to catalyst gives a 3 minute pot life and a 1 hour cure time

You don’t want to add more than 1.0% to the mix because it can cause the finished mold to be brittle.

The silicone thinner is added to the silicone base (part A) to decrease the viscosity of the silicone. This allows the silicone to flow more easily over intricate parts and also de-air faster. It can also be used with the brushable catalyst to achieve a thinner first coat, which reduces the chance of air bubble on the working surface of your mold. Once you add the silicone thinner to the silicone base, you weigh out the catalyst to match the weight of the silicone base and silicone thinner together. If you start with 500 grams of silicone and add 50 grams of thinner, the total weight is 550 grams and you would then add 55 grams of catalyst. You don’t want to add more than 10% silicone thinner because, while it reduces viscosity, it also reduces tensile and tear strength.

For example:

5% silicone thinner reduces viscosity by 24%, the shore hardness by 13%, the tensile strength by 13%, and the tear strength by 12%

10% silicone thinner reduces viscosity by 45%, the shore hardness by 23%, the tensile strength by 18%, and the tear strength by 16%

The silicone thickener is similar to the silicone thinner in that it is added to the silicone base (part A) to control the viscosity. The thickener is used to make a pourable silicone a brushable silicone.

To use the silicone thickener, add approximately 1% by weight to the silicone base and mix well. Again, you must use a digital gram scale in order to ensure correct weight. Add the catalyst to the TOTAL weight of the silicone base and silicone thickener. Mix well and brush on to your original. 1% is the general starting point for a brushable material. You may want to adjust up or down, depending on your application. The thickener does not affect the cure time or the durability of the finished mold.

These are the basics of working with silicone additives for tin catalyzed silicone. If you have any questions about this topic or any other, please send us an email at [email protected] To see our product line and how-to videos, visit our website at









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