After mixing silicone rubber, epoxy casting resin or polyester casting resin, a lot of air can enter the material. These air bubbles can lower the quality of the final casting. So for high quality castings, it is important to vent the resin. Also, for best results, it is recommended to vent epoxy before vacuum injection.
In this information article, we explain how venting resins works!
Besides the resin used, you will need the following materials to properly vent:
The vacuum pump should be a model that can generate a deep vacuum. Our EVD-VE pumps meet this requirement. These pumps are available in different capacities, from 1.5 m3/h to 21 m3/h. This affects the speed at which sufficient vacuum is achieved to effectively vent.
A second distinction with these pumps is the final vacuum level. EVD-VE pumps are divided into single-acting (the 100 series, EVD-VE 110 to EVD-VE135) and double-acting (the 200 series, from EVD-VE215 to EVD-VE2100). The 100 series reaches a final vacuum of 0.05 mbar (abs) and the 200 series reaches a vacuum of 0.003 mbar (abs). This affects the degree of venting. The 200 series bleeds slightly better, although the 100 series will already be sufficient for normal use. In our videos, an EVD-VE115SV pump was used.
The pumps are available as bare pumps or as pumps with a pressure gauge mounted on the pump and an automatic shut-off valve. These pumps have "SV" after the type designation. For venting, the valve is not important. The pressure gauge is useful but not necessary.
The vacuum vessel should, of course, be large enough to accommodate a mixing cup with sufficient casting material. During degassing, the volume of casting material will increase. This should be taken into account when choosing the dimensions. It strongly depends on the material how much it will increase during degassing. The temperature also affects this. At a higher temperature, the increase in volume will be less because the material can release its air more easily.
Count on the following increases (at 20 degrees):
Step 1: make sure the vacuum pump and vacuum dome are fitted and working. Test this once without resin in the dome, but with a little water. The vacuum pump should be able to boil water at 20 degrees. If this doesn't work, check all connections for leaks. A little Vaseline on the seals, on the tap etc. can work wonders.
Step 2: mix enough resin for your project. Make sure this is very wel mixed, if necessary using the 2 cup method. Make sure the container is large enough to absorb the increase in volume.
Step 3: place the mixing cup in the vacuum dome and close the vacuum dome.
Step 4: switch on the vacuum pump and check that the dome is under vacuum. After a few seconds, the top of the dome is firmly attached to the underside of the dome.
Step 5: wait for the casting resin to start foaming and rising. Keep a close eye on the mixing cup or it will overflow. If this is likely to happen, allow some air in, which will reduce the volume. Allowing air in can be done with the white tap on the dome. Turn it until the (almost invisible) slot matches the tap hole. Do not pull on the white tap out on the dome!
Finally, the resin will suddenly release its air, with large bubbles rising. The volume of the resin will rapidly decrease to almost its initial volume. Depending on the cure time, the resin can be left to rest under vacuum for a further 5 to 10 minutes. Silicone rubber and epoxy resin have sufficient curing time for this. Polyurethane and polyester are often a little faster, so it's important to allow the resin to degas as quickly as possible and to use it immediately after degassing.
Step 6: switch off the pump and turn the tap on the dome so that air is admitted. Wait until the vacuum is completely eliminated and remove the resin from the dome. The resin is now fully vented and ready for use.
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