Subject:       Removal of Air Bubbles from Liquids by Vacuum Degassing

Applicable to: RTV Silicone Rubber, Polyester Resin, Casting Plaster

Polyurethane Resin, Epoxy Resins, Araldite, Plastasols

Polysulphide Rubber, Adhesive Mixes, Investment Plaster

When any of the above listed materials are mixed with the required additives, accelerator, filler etc., then air bubbles become trapped within the mixture. If not removed before the material cures then the air bubbles will cause defects such as nodules, cavities, hollows in the finished cast. Sometimes such defects remain out of sight just below the surface only to appear after a period of use. With electrical and electronic encapsulation then these cavities can give rise to electrical breakdown. On art figures cavities or nodules require correcting by the hand of a craftsman which increases the time and costs for a piece.

Our range Vacuum Degassing Units has been developed by the writer over the past 15 years and is well established throughout Art and Industry both in the UK and overseas. A Vacuum Degassing System comprises of two main items:-

1)         The Vacuum Chamber. A strong cylindrical vessel of enamelled mild steel, fitted with vacuum control valve, vacuum release valve, vacuum dial gauge, 'L' section gasket and a clear acrylic or metal lid. Preferred and stock sizes are shown in our current price guide in diameters of 175mm, 230mm, 305mm, 460mm, 500mm and 610mm and heights up to 1500mm held in stock. Custom built units are available for your specifications. Plus:

2)         A Vacuum Pump Motorised Outfit, with connecting hose. This economy combination shows a considerable saving in capital, outlay over other systems, especially when a reconditioned Vacuum Pump Outfit (from our extensive stocks) is incorporated. The Pump can otherwise be from our range of New Edwards High Vacuum Pumps.

To select an outfit for your purpose, ensure that your mixing container will fit into the Vacuum Chamber, also that there is sufficient space in your mixing container to allow for expansion of the liquid to be degassed. Initially only fill your container one quarter with mixture, experience will show the limit of mixture level permissible. Select a pump to evacuate this Vacuum Tank to fully degas the mixture before it even beings to gel or set. You will need to know the pot lift of the material in order to select the right speed of pump.

Approximate Guide to Capacities:

The components are mixed in the container. It is essential that the resulting mixture behaves as a liquid, i.e., flows and can be poured.

When this mixture is put into the Vacuum Chamber and the air pressure above it reduced, i.e., evacuated, then 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 to about two to six times its original volume. As the bubbles burst at the surface, the expansion decreases. This process can take from one to several minutes depending upon the nature of the mixture, the volume of the Vacuum Chamber and the speed of the Vacuum Pump used with it.

After expansion has subsided, bubbles can still appear at the surface, mainly caused by the escape of minute traces of remaining air plus volatile components of the mixture boiling off. The vacuum should only be 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 mould taking care to minimise any trapped air. When your mould has been filled then you can put the whole mould 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 sufficient.

You should be able to complete the above process long before the material begins to gel or increase in viscosity. If de-airing is too slow, then you may need to either increase the evacuation speed, i.e., change the pump (to a larger model) or the chamber to reduce the pump down time, or decrease the accelerator or possibly keep the components warm in very cold weather.

Horizontal Cylindrical Vacuum Chambers for Large Rectangular Moulds

The cost of cylindrical Vacuum Chambers tend to increase far more with increases in diameter than length. Thus a Chamber 1 metre diameter which is 460mm deep is far more expensive than one which is 460mm diameter and 1 metre deep.

If your mould to be vacuum processed is rectangular say 900mm long by 350mm wide by 200mm deep then consider a horizontal Vacuum Tank 460 diameter x 1 metre long.

The 'door' can be held momentarily in position whilst the vacuum valve is opened. Once the pressure inside the Chamber has dropped below atmospheric pressure then the 'door' will stay in place held there by the pressure difference. When the vacuum is released then the door could fall away. A light support for the door is needed to prevent it falling away when the vacuum is released. A typical application would be to process long moulds, for example, wood preservation (or for rectangular moulds, rectangular chambers are now available).

Optional Extras could include:-

1)         Shelf Supports and Shelves

2)         A stand to support the Chamber at a convenient working height

3)         A set of castors fitted to the stand

4)         A trolley matching this stand in height, to assist the operator in loading and unloading the Vacuum Chamber

5)         A metal door, with glass viewing port

6)         A gantry to slide the door sideways to open and close

7)         The Vacuum Pump Outfit mounted in this stand and connected to the Chamber making a self contained system.

8)         A heating blanket fitted around the Vacuum Chamber to achieve wall temperatures up to 60^oC or so (provided the metal door in (5) is incorporated) NOT suitable for use with clear acrylic door.

Other extras; larger units and special features can be incorporated to meet your requirements.

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.