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Degassing

Principle

After pouring elastomer into a mold, there will almost always be air bubbles present, which can lead to holes or other defects in your cured part. Degassing is a way to get rid of air bubbles in your elastomer before curing. There are two main methods of degassing: vacuum degasification and pressure ovens.

Both methods take advantage of Henry’s Law: p=kHc, where p is the partial pressure of the solute in the gas above the solution, c is the concentration of the solute and kH is a constant.

Basically, increasing pressure increases solubility of gas, and decreasing pressure decreases solubility of gas.

In vacuum degassing, molds with poured elastomer are placed in a vacuum chamber. Applying vacuum causes the pressure to drop, and by Henry’s Law this lowers the solubility of air in the elastomer. As a result, bubbles come out of solution and float to the surface where they pop. (Think about what happens when you open a new bottle of soda!)

This process removes bubbles from the body of the mold and improves the material properties of the final cured part.

Note: for very small/thin applications with features on the millimeter scale, the bubbles will often get stuck and lead to holes in your molded part, and your elastomer itself may potentially be sucked out by the vacuum as well. Pressure ovens are a better alternative in this case.

Using a vacuum chamber

  1. Place your mold+elastomer inside the vacuum chamber so that it is level.
  2. Put the lid on the vacuum chamber and connect the hose leading to the vacuum pump.
  3. Switch on the vacuum pump, wait a few seconds, then tug on the lid to verify that the seal is good (the lid should not budge).
  4. Once the biggest bubbles have stopped coming to the surface, switch off the pump.
  5. Next, the hose has to be disconnected, but this has to be done very carefully:
    1. Grip the hose firmly with one hand.
    2. Pull the hose slightly to one side until a small part of the vacuum hole is exposed. Air will start rushing in.
    3. Maintain a steady stream of air by exposing more and more of the hole, making sure it is not too turbulent inside the chamber.