What is Addition Cure Silicone?

  Addition cure silicone is a rubber that is good for most mould making and casting techniques. It is suitable for many special effects and prothetic applications including paints. Addition silicones also include food safe and skin safe versions. Catalyst ratios vary but we mainly use an addition cure silicone that uses 10% catalyst. More importantly it is available in a transparent version.

Addition silicones use a platinum-based catalyst.

Addition Cure Silicone

How do I use Addition Cure Silicone?

1.Ensure the vessel is dry and your master is free from moisture before you begin. Also make sure that the mixing vessel you are using is clean and free from any condensation cure silicone, even a residue of condensation silicone will affect the curing process.

2.Remember, after you put the mixing vessel on the scales, you must zero the scales. This will cancel the weight of the vessel, simplifying the calculations you will do when adding your catalyst.

3.Work out how much catalyst you will need and add it to the mix. To mix silicone effectively, mix the top layer with the catalyst slowly for a minute or so until the catalyst is stirred into the surface layer of silicone. Now, mix this into the main body of the silicone for 3-5 minutes. Take extra care when using transparent silicone, as both catalyst and silicone are clear, it is difficult to tell when they are mixed properly.

4.When mixing, remember to scrape all silicone from the sides and bottom of mixing vessel and stir it into the mix. Avoid folding the silicone when mixing; this will only add air to the mix.

5.Once you are happy that the mix is thorough, ask your technician to check it. If it is OK, you can de-gas it. Place the mixing vessel into the vacuum chamber and wait until most of the air has been removed. This usually takes between 5-10 minutes depending on the amount you are de-gassing.

6.When the bubble popping has slowed and clear patches begin to appear at the surface, the silicone is about done.

7.As always, when you pour your material start from the lowest point and allow the level to rise gradually. This will force the air out as you fill your mould/cast. Try not to pour the silicone directly onto the master if you are making a mould.

8.Once cured the master/cast may be removed. Although addition cure silicone is very strong and pliable, it may split or tear if over stretched or twisted too much.

Tip: Post-curing the freshly cured silicone in the oven for 4-6 hours at 70 degrees centigrade will improve the mechanical properties of the mould and give it a longer working life.

Avoiding mistakes with addition cure silicone

Any cured addition silicone may cause inhibition on any uncured addition silicone surface, so it is essential an even coat of a suitable release agent forms a barrier between any adjoining surfaces. Addition cure silicones are also intolerant of sulphur, tin and amines, many materials contain these elements and should be avoided. Always ask your technician for help identifying safe materials to use if you are unsure. However, it is generally not recommended that silicone moulds be used to cast silicone parts, because it is difficult to ensure a complete seal of the surface especially with when dealing with complex shapes and fine detail.

If a rubber component is required from a silicone mould, a polyurethane rubber should always be the preferred casting material.

Note: Many addition cure silicones are incompatible with polyurethane rubbers.




Addition silicone spiral-cut split mould

Skin-safe, addition silicone skin mould

Addition silicone casting from a plaster mould


Addition silicone skin over a flexible plastic core lighting unit


Addition silicone skin over a vacuum formed plastic core

Silicones are available from Bentley Advanced Materials

Prices vary: £15.00-£30.00 p/kg for most types