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One of the biggest problems facing many small scale mould makers is air bubbles in their silicone moulds. The bubbles inside the silicone rubber mould itself are generally not a big problem. The problem is when the air bubbles form at the casting surface making the mould unusable.
There are several methods to use to ensure bubble free silicone moulds:
1) Special pouring techniques
2) Applying compressed air
3) Using Vacuum to remove bubbles
4) Using pressure to force air into solution
Special pouring techniques.
The surface where casting materials meet silicone moulds is the most important part. It is therefore essential that special care is given to ensure no bubbles form when pouring the silicone rubber into the mould box and onto the model. It is important not to simply pour the rubber over the top of the model in one go. Start by pouring a small amount of rubber into the mould box and start tilting it in different angles to make the rubber slide over all surfaces. This wets the surface of the model with a fine film of rubber and any air bubbles can be easily seen and popped with a pin or paint brush.
You can also use a paint brush to apply this first fine film of rubber to the surface of the model. Choose the method which works best for your silicone moulds.
Once you are satisfied that all the bubbles have been popped you can pour the rest of the mould making rubber into the mould box. Pour from as high a point as possible in a fine stream so that any bubbles in the mixture will pop on the way down. Aim this stream into the corner of the mould box and not on top of the model.
Applying compressed air to your silicone moulds.
If you have access to an air compressor you have an effective way of popping air bubbles. Ensure that you have about 30psi pressure and a small nozzle tip and after pouring the first small amount of rubber into the mould box blow the material over the model to wet the surface in a similar way to the first method. The first film of rubber should be bubble free.
Continue with the remainder of the rubber as described in method one.
Using Vacuum to remove air from your silicone moulds
Bubbles are usually a result of air beaten into the rubber during the mixing process. Ensure that you stir the mixture in a steady and slow action. There will still be air in the mixture however and this can be removed with the aid of a vacuum chamber.
These vacuum chambers use a vacuum to suck out any air bubbles in the mixture and bring them to the surface where they can be easily removed. Each vacuum chamber has their own instructions on how to use it but the following is a guide:
1) Use an oversized container for mixing. Allow for the mixture to double in size when placed in the chamber.
2) Mix both part A and part B in the container and place into the chamber. Close the chamber and start the vacuum pump.
3) The mixture will start rising to the top of the container. All the air bubbles in the mixture will rise to the surface. As the mixture approaches the top of the container release the valve which breaks the vacuum and the mixture will drop to the bottom and the air bubbles will pop. Do this two to three times to ensure all bubbles are removed.
4) Release the vacuum one final time and take the mixture out of the chamber. It is now ready for pouring.
The use of a vacuum chamber is usually only possible for mould rubbers which take a fair amount of time to set. Some of the quicker setting silicone rubbers will set too quickly and you won’t have the time to vacuum the bubbles out. Make sure you check the label on the product you are using.
Using pressure to force air into the solution.
This technique is useful for quicker setting mould making rubbers. It works on the principal of forcing air into a pressure vessel with the mixed moulding rubber. The pressure of the air will press down on the material and squeeze the air bubbles out.
The pressure vessels are freely available and are usually used by painters for spray painting. Here is the procedure:
1) Choose a mould box which fits inside your pressure vessel. Also check the setting times for your mould making rubber to know how long you have to work.
2) Mix and pour the mould making material into the mould box over the model.
3) Seal the pressure vessel and pressurise it to 60-100psi.
4) Let the mould making material set while under pressure in the vessel. Most mould making rubbers work really well with this technique.
If you are still getting air bubbles after trying some of these techniques get in contact with your supplier for further advice about the product you are using.
By Stan Alderson
write by Roger