Technical Note No: 1011/3

Subject: Step by Step Operation of a Cylindrical Vacuum Degassing Outfit In order to remove air bubbles from resin mixes, RTV silicone and similar liquids. Degassing of a liquid mixture is the removal of air bubbles which become entrapped in the mixture when mixing the components. See the relevant notes about degassing and setting up your Vacuum Chamber and Pump. PROCEDURE 1) For moulds with large undercuts and voids which hold air, make up a temporary collar around the opening of the mould to contain the resin mix as it expands. Use paper tape or similar material. 2) close the Vacuum Valve (connected to the Vacuum Pump). 3) Open the Vacuum Release Valve (the other valve). 4) Switch on the Vacuum Pump and allow to warm up. 5) Remove the Vacuum Chamber Lid. 6) Mix the materials to be degassed in a suitable container not more that 1/3 full and place this container in the Vacuum Chamber. 7) Check that the >L= Gasket is correctly fitted. 8) Replace the lid, making sure it fully covers the >L= Gasket all over the Vacuum Chamber. 9) Close the Vacuum Release Valve. 10) Open the Vacuum Valve. 11) Watch the mixture and the Vacuum Gauge. When the Vacuum Chamber is about 3/4 evacuated then the mixture will begin to rise, slowly at first. At nearly full vacuum the mixture will have swollen up to fill the mixing vessel, air bubbles will appear and burst at the mixture surface. After a few seconds, this mixture will >collapse= back to near its original volume. Occasional bubbles will appear at the surface. 12) If the mixture rises too fast and is likely to overflow, the container then momentarily reduce the vacuum by opening and closing the Vacuum Release Valve. Repeat if necessary. For the next batch use less mixture. 13) Wait for about 30 seconds after this >collapse=. 14) Close the Vacuum Valve. 15) Open the Vacuum Release Valve. 16) Remove the Lid. 17) Fill the mould(s) carefully to minimise trapped air. 18) Place the mould(s) into the Vacuum Chamber. 19) Repeat stages 7, 8, 9 and 10 above. 20) Some bubble may appear at the mixture surface and burst. 21) Continue degassing for about 30 seconds at full vacuum. 22) Repeat stages 14, 15 and 16 above. 23) Remove the Mould(s). 24) Top up if needed and put aside to cure. Dealing with Splashes of Resin The bubbles bursting at the surface of the liquid can sometimes splash droplets onto the underside of the Vacuum Chamber Lid. These can be wiped away with a suitable material not acetone or any solvent that will NOT attack the acrylic lid. We suggest soapy water of methylated spirit and dry before re-use. Do not use any abrasive compounds. If the acrylic lid is mis-used it can develop fine cracks which will, in time, increase until breakage can occur. Inspect your lid regularly for such cracks. DO NOT use is there is a risk of such breakage. If the problem persists, e.g., on a production unit then we suggest that you use a metal lid with a glass viewing window (see our current price guide for details). This glass window can be wiped clean with acetone or similar solvent. Avoid sticking plastic film to the acrylic sheet, some of these materials are not compatible with acrylic. Dealing with Thick Mixtures If your mix is very thick and a tendency to be thixotropic, i.e., like a gel and difficult to flow, you may need to vibrate the mixture whilst under vacuum. This can be done very easily by a gentle shaking of the Vacuum Chamber. Possibly mount the Chamber on springs to aid this process. For production, you may need to mount the Vacuum Chamber on an electrically driven vibratory table. In either the bowl of material inside your Vacuum Chamber needs to be secured. Copyright 1988 NB: The information in this pamphlet is offered in good faith and is based on our general experience. The method of use of Island Scientific Ltd equipment and materials are outside our control, the responsibility to ensure that the equipment is properly used and suitable for intended application rests with the user. No responsibility will be accepted by this company nor any person or other company associated with this leaflet for loss or consequential losses as a result of them using this information. For advice, information, general literature, prices or a discussion of your needs, telephone 01983 855822, fax 01983 852146 or E-Mail: sales@island-scientific.co.uk. A full list of Technical Notes is available free on request.