How to make a silicone mould without air bubbles

Ask any mould maker what is their biggest problem and they will tell you air bubbles. Air bubbles in the mould itself are no great problem, it is when they are next to the surface of the model and render the mould imperfect that they begin to make life difficult.

To get rid of air bubbles you have several choices:

1) Improved pouring techniques

2) Application of compressed air

3) Removal of air by vacuum

4) Forcing air into solution by pressure

Improved pouring techniques

The first coat of any liquid rubber pour is always the most important. Don’t simply pour the rubber into the mould box over the model all at once. Pour just enough of the liquid rubber that will get all surfaces wet. Then tilt the mould box so the liquid flows over the entire surface of the model. You will see any air bubbles clinging to the surface of the model and can pop them. We use a fine taklon brush size 00. Others use a brush to coat the model with this first rubber coat. Try both methods and pick the one that works best for you.

Once the bubbles have been removed pour in the rest of the mould making material. Pour from as high as possible in as thin a stream as possible into a corner of the mould and let it flow over your model.

Application of compressed air

If you have an air compressor you can achieve the same result without popping air bubbles by using compressed air delivered from a small nozzle tip at about 30psi direct to the surface of the first rubber mould pour as described previously.

You use air pressure to blow the material over the model surface and wet it out thoroughly. The air pressure is usually sufficient to break any air bubbles clinging to the surface of the model. Finish pouring your mould material after checking that the inner surface of the mould rubber is perfect.

Removal of air by vacuum

Incorrect mixing can beat air into your mould making mixture before you even pour it. Stir steadily and slowly without churning air into the mixture. If you still have air in the mixture it may be removed using vacuum. Simply mix the mould making material and place it in a vacuum chamber.

Instructions on how to make a suitable vacuum chamber and where to purchase vacuum pumps are available. To use vacuum:

1) Use a container for mixing that will allow your mixture to double in size without overflowing.

2) When part A and part B have been thoroughly mixed put it in the vacuum chamber and start the vacuum pump.

3) The material will start to rise immediately as air removal commences. Let the vacuum reach maximum and then release 2 or 3 times to ensure all air is removed.

4) Let the vacuum down slowly one final time and remove the de-aired mould making mix from the chamber and pour.

This technique is only possible with materials that do not set quickly. Check the label.

Forcing air into solution with air pressure

This may be the only technique possible with quick setting materials. Instead of removing air as in the vacuum technique you use pressure to force bubbles into solution. Pressure pots are freely available, you have seen painters use them with an air compressor for spraying paint. To use a pressure vessel:

1) Select a mould box that will fit into the paint pot you have available and double check the setting times of your material from the data sheet.

2) Mix the mould making material and pour it over the model.

3) Close the lid on the pressure vessel and presurise it to 60-100psi.

4) Leave the material in the pressure pot at that pressure until it has set. Most rubber mould making materials work like a charm with this technique. Try it.

We recommend CraftSil-750 as the best general purpose silicone for its ability to form bubble free moulds without the need for use of vacuum chambers or pressure pots. You can purchase CraftSil-750 General purpose silicone on our online Mould store. . Click here for more information