1.1 APPLICATIONS
. RTV Silicone Rubber . Araldite
. Polyester Resin . Plastasols
. Casting Plaster . Polysulphide Rubber
. Polyurethane Resin . Adhesive Mixes
. Epoxy Resins . Investment Plaster
1.2 INTRODUCTION
When any of the above listed materials are mixed with the required additives, accelerator, fi ller 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, hollows in the finished cast. Sometimes such defects remain out of sight just below thesurface only to appear after a period of use. With electrical and electronic encapsulation then these cavities can give rise to electrical breakdown. On art fi gures cavities or nodules require correcting,
which increases the time and costs for a piece.Mixing can be done by hand or an electric mixer before degassing,depending upon the quantities involved. It depends on the amount andtype of fi ller and resin. The components are mixed in a container. It is essential that the resulting mixture behaves as a liquid, i.e., fl ows and can be poured.
When this mixture is put into the Vacuum Degasser and the air pressure above it reduced, i.e. evacuated, the air bubbles which were formed at atmospheric pressure now expand and rise to the surface where they burst. The air thus released is pumped away.
In practice, degassing causes the whole mixture to expand about two to six times its original volume. As the bubbles burst at the surface,the expansion decreases. This process can take up to several minutes depending upon the nature of the mixture, the volume of the vacuum chamber and the speed of the vacuum pump used.
After expansion has subsided, bubbles can still appear at the surface,which is mainly caused by the escape of traces of remaining air pluvolatile components of the mixture boiling off . The vacuum should onlybe held for a further 30 seconds to 60 seconds at this stage, otherwise the composition of the mixture will be altered and setting time will change due to volatile components being removed by the vacuum.
This degassed mixture is then poured into your mold taking care to minimize any trapped air. When your mold has been fi lled if you desire you can put the whole mold with mixture into the vacuum chamber and evacuate as before. This time only one or two bubbles should appear from air, which was trapped in undercuts or adhering to the mould surface.Again, beware of boiling off the volatile components, only a few seconds or so at full vacuum should be suffi cient.You should be able to complete the above process long before the material begins to gel or increase in viscosity. If degassing is too slow, you may need to either increase the evacuation speed, i.e., change the pump (to a larger cfm) or the decrease chamber size to reduce the pump down time, or decrease the accelerator or possibly keep the components warm in very cold weather.
1.3 COMMON CAUSES OF BUBBLES IN CASTINGS
. Air inclusion during mixing the resin and hardener together.
. Air trapped in moulds.
. Air trapped due to improper casting/moulding techniques.
. Air introduced by suction as the material shrinks or due to a leaking tool.
. Trapped residual solvent vapors from certain types of mould releases.
. Volatile components being stripped out of the materials while casting under vacuum.
. Gases generated during the reaction (hardening) process.
. Gases formed during improper storage (moisture contamination etc.)
. Gases introduced through pressurized systems.
. Air from improperly degassed resin and/or hardener.
1.4 FACTORS THAT AFFECT THE REMOVAL OF GASES
. VISCOSITY OF THE MIX.
The higher the viscosity the more diffi cult it is to remove gases
. SURFACE TENSION OF THE MATERIALS
Can be reduced by heating and the addition of surfectants
. TEMPERATURE OF THE MIX.
Heating the material will reduce the viscosity. Be sure to consider the pot life and gel time before heating any mixed materials.
. AMOUNT OF MATERIAL BEING DEGASSED AT ONE TIME.
The depth to surface ratio is important. The larger the surface that is exposed to the vacuum the better. The lower the amount of product the rising bubbles have to travel through the better.
. AGITATION
Agitation (mixing) during the degassing process (while under vacuum) will speed the removal of air.
. VACUUM PUMP SIZE
The size or pumping speed of your vacuum pump usually noted in terms of cubic feet per minute or cfm as well as the ultimate pressure or vacuum level your pump can achieve.
. CHAMBER SIZE AND PIPING
Chamber size and the diameter and length of the vacuum hose can also eff ect the time for removal of gases. It is recommended to use at least the same diameter as your pump inlet and to use the shortest length hose as possible.
1.5 MIXING UNDER VACUUM
Some materials need to be mixed under vacuum in order to obtain an air free material. Some materials may also need heating before or during mixing. Such material may need constant agitation or vibration to keep them fl uid so that they will actually fl ow as fl uids to fi ll moulds and containers. In such cases LACO can review the application and recommend suitable vacuum equipment.
6.6 COMPLETION OF PROCESS BY PRESSURE
After subjecting the liquid material to a vacuum to remove entrapped air the voids and spaces are probably still present but in vacuum. The final stage of the process is to release the vacuum in the chamber, i.e. admit the atmosphere which exerts a pressure of about 14.7 pounds on every square inch. It is this pressure that pushes the material into the voids undercuts and crevices of the mould. It is essential that the vacuum is released immediately after degassing. It may help to gently vibrate the mould full of material to temporarily overcome any tendency of the material to be thixotropic, i.e., non-fl owing such as with certain casting plaster mixes. If the material has already begun to set while in vacuum and the surface is not smooth but sponge like, then upon release of the vacuum air will enter the body of the material and create a gassy cast. Should this happen then vibration under vacuum and during vacuum release is needed so that the material fl ows in to all the cavitie
