How to remove air bubbles from silicone rubber

You have spent hours working on your original model, but after demolding you see small holes and air bubbles everywhere on the surface of your silicone rubber. Air often gets 'trapped' on the object, especially with complex shapes with many details or textures. This has two major disadvantages:

• Ruined finish: Every air bubble on the surface becomes a bump on your final castings, completely ruining the details.
• Reduced quality: A mold full of air bubbles is structurally weaker. The rubber loses its tensile strength, causing the mold to tear more quickly during demolding or after a few castings.

The cause: why air sticks to your object

Air bubbles arise almost always during the intensive mixing of the two components. When you then pour the silicone over your object, the viscous liquid can trap air in corners, holes or against the walls of the model. Without action, these bubbles remain exactly where you don't want them: at the crucial boundary between your model and the rubber. An important rule of thumb: the higher the Shore A hardness, the greater the chance of air entrapment. Harder silicones are often thicker and more viscous, so air bubbles get 'trapped' more easily during mixing. Unlike very fluid, soft silicones, air simply does not have the force to rise to the surface on its own before the curing process begins.

 

The solution: First brush, then pour

The most effective and simple way to prevent air bubbles on the surface is to 'prepare' the object with a first coat of silicone. Use a brush to apply a thin layer of the already-mixed silicone rubber directly onto the surface of the object, pressing it firmly into all the details, corners and crevices. By working the silicone in with a brush, you manually remove the air from all the tricky spots. Only after this first coat has been applied do you pour the remaining silicone over the object in one go.

 

For professionals: vacuum degassing

Want a guaranteed perfect result without manual brushwork? For entrepreneurs and demanding makers, using a vacuum pump and vacuum chamber is the standard. By briefly placing the mixture under vacuum after stirring, the air in the silicone expands and 'boils' out of the liquid. Once the air is removed, the silicone settles back and you have a completely air-free material ready to be poured. This is the only way to make molds that are technically and optically perfect for commercial purposes.

 

Extra tips for an air-bubble-free mold

• The thin stream: Always pour the silicone in as thin a stream as possible (like a thread) from a considerable height. This causes bubbles in the stream to burst before reaching the mold.
• Tapping and vibrating: After pouring, gently tap the side of the mold housing or use a vibrating plate. This causes the last remaining bubbles to vibrate to the surface.
• Curing in a pressure pot: For a guaranteed professional result, cure the mold in a pressure pot. By placing the mold under high pressure, the remaining air bubbles are compressed so small that they are no longer visible to the naked eye.

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