When any of the above listed materials are mixed with the required additives, accelerators, filler etc., then air bubbles can become trapped within the mixture. If not removed before the material cures the air bubbles can cause defects such as nodules, cavities and hollows in the finishes cast. With electrical and electronic encapsulation then these air bubbles can cause electrical breakdown. On intricate castings additional work will be required to correct the defects caused by the air bubbles.
All Epoxy Resins, Araldites, Silicone Rubbers as well as any other mixed materials to be degassed will expand during the degassing process. It is essential that a container of sufficient volume is used. It is not uncommon for materials to expand two to six times their initial volume while degassing. As the bubbles burst at the surface the expansion decreases. This process can take several minutes depending on the viscosity of the material, the speed of the vacuum pump and the volume of the vacuum chamber.
Do take heed of both the viscosity and the pot life of the materials to be degassed. Materials with a very high viscosity will take a lot longer. Some very high viscosity materials will be very difficult to degas. If the pot life is short then speed could be essential.
If you are unsure of the properties of your materials then a few simple tests will help:
Once you are satisfied and confident the materials you are using are not gong to cause problems then proceed.
Remove the vacuum chamber lid. Ensure that both the vent and vacuum isolation valves are closed and switch on the vacuum pump to warm up. A warm pump is more efficient and also handles condensable vapours better.
Measure out the required amount of material together with any hardeners or fillers into a clean bucket of adequate size to allow for expansion. Mix well, either by hand or with an electric mixer.
Place the bucket in the vacuum chamber and replace the lid. Gently open the vacuum isolation valve. Some light hand pressure on the lid may be required to establish a good seal – you will see on the gauge when the pressure starts to fall indicating a seal has been made.
Observe the surface of the material. Once bubbles start to come to the surface and break close the vacuum isolation valve. If the bubbles subside after a short time open the valve again. Repeat that process so that you progressively pump the chamber down.
If there is a rapid rise in the level of the degassing material close the vacuum isolation valve and allow a little air back into the chamber with the vent valve. That will ‘collapse’ the degassing material and will break a lot of the surface air bubbles. Continue pumping down and by balancing both the vacuum isolation and vent valves it will be possible to degas quite volatile mixes.
Once you are satisfied the material has been degassed close the vacuum isolation valve, open the vent valve and remove the chamber lid. You are now ready to use your degassed material.
This degassed mixture is then poured into the mould taking care to minimise any trapped air. When the mould has been filled it can be placed into the vacuum chamber and evacuate as before. This time there should be minimum air bubbles breaking the surface which was trapped in undercuts or from the surface of the mould. Please again beware of boiling off the volatile components of the mixture and only a few seconds should be required at full vacuum.
Below are a few .pdf files and downloads for system advice and guidance notes.