To obtain casts which are free of defects due to air bubbles trapped in voids cavities and undercuts, it is always desirable and often essential to fill the mould with the casting liquid material in vacuum.

This applies to art figures, electronics, patterns, moulds etc., using such materials as polyester, epoxy and polyurethane resins, RTV Silicone rubber, polysulphide rubber, casting plaster, investment plaster etc.

The first essential requirement is to degas the freshly mixed liquid material by exposing the bowl or bucket containing such liquid material to a vacuum in a vacuum chamber whereby the air entrapped in the material during the mixing process is removed.  If the mould is filled in open air with the material then unless precautions are taken, air bubbles will adhere to the mould surface, air will be trapped in voids, undercuts and in fine detail.  This air prevents the material i.e., casting liquid from entering, thus creating a defect in the finished cast.

There are four principal methods of producing casts to ensure that the air defects are eliminated.

Method 1 - The most popular technique

Fill your mould in the open air then put the whole mould with the material into a vacuum chamber and evacuate.  The air entrapped will expand as the vacuum is increased and (depending upon the geometry of the mould) rise to the material surface as bubbles to be pumped away as the bubbles burst.  After a suitable time under vacuum the atmospheric pressure is allowed to return i.e., the vacuum is released and whatever traces of air are left in the voids and cavities shrink to hopefully negligible proportions as the material flows in.

There are several difficulties with this method but it is the most widely used.

a) Some materials will foam and the bubbles of air not burst at the surface.  This can result in a mess of material all over the mould exterior and the vacuum chamber.

b) Not all air can be released, especially from the bottom, of deep moulds due to hydrostatic pressure of the material above.

c) It requires physical handling of a filled mould which can be difficult if they are large and heavy.

Reference: Island Scientific Vacuum Degassing Chambers and pumps.

Method 2

Place the mould inside the vacuum chamber together with a beaker of already degassed material arranged so that when the vacuum is reached the beaker can be tilted by a mechanism outside the vacuum chamber to pour the material into the mould.

This can often be used where several small moulds are fitted onto a turntable to be filled in turn from the beaker full of material.  It is widely in electronic components encapsulation.

Such equipment works well for small volumes of material but it is limited in practice to about 1 litre.

Reference: Island Scientific Vacuum Filling Rotary Turntable Chambers.

Method 3

For larger volumes of material to be poured into one mould under vacuum, we use a pipe through the wall of the vacuum chamber with suitable valves to conduct the material.  As most of the materials used will chemically set to a hard solid within a short time precautions must be taken to prevent this hardened material from remaining in and blocking valves and pipework used.  Otherwise only one fill can be attempted. 

Island Scientific produces a suitable system incorporating a special hand operated valve with a disposable throw away liner.  With this system a bucket of freshly mixed and vacuum degassed material is placed outside the vacuum chamber with the mould filled inside the vacuum chamber.  A flexible pipe dips to the bottom of the material bucket and passes via the special valve into the vacuum chamber and into the mould.  With the valve closed the vacuum chamber is evacuated.  When the valve is opened and the atmospheric pressure pushes the material through the pipe and valve for it to run into the mould until full.  This method is restricted to low viscosity materials at near room temperature which will flow adequately through a length of 6mm bore pipe.

However, more than one valve and pipe can be fitted to a vacuum chamber to fill either one mould faster or several moulds at one time.  The pipe and liner are used until the material begins to gel whereupon they are replaced.

Reference: Island Scientific Vacuum Chambers with Liquid Entry Valve (Technical Note 1013).

Method 4

To fill moulds under vacuum with large quantities of material which is unsuitable for Method 1, too much for Method 2, too viscous and too hot for Method 3.  We use a funnel and suitable pipework connected to the vacuum chamber.  Island Scientific produces a vacuum chamber of suitable size with a stainless steel funnel for the material.  It is connected via de-mountable stainless steel pipework connections and a level operated valve through the wall of the vacuum chamber and via suitable pipe connections allow material to flow into the mould when the system is operated.

The vacuum chamber can be fitted to take one mould or several moulds on a rotary turntable operated from outside the vacuum chamber so that moulds even of different sizes can be filled in quick succession.

In practice the funnel is filled with freshly degassed material whilst the vacuum chamber with mould(s) is evacuated.  When ready, the lever is carefully opened to allow a controlled flow of material into the first mould until full.  This is repeated until all moulds on the turntable are full.

The funnel and pipework can then be washed through with a suitable solvent for cleaning by others whilst a separate set of funnel and pipework components can be used.

In the event of partial cure of the material then the stainless steel can withstand vigorous cleaning procedures to remove unwanted material residues.

Such systems are available from Island Scientific with 12mm bore or 25mm bore as standard stock items which can be fitted to vacuum chambers of suitable size up to 500mm diameter x 1 metre deep.  Larger sizes are available to order.

By using a cylindrical vacuum chamber with appropriate connections laid horizontally then long moulds can be easily vacuum filled.

Reference: Island Scientific Vacuum Filler.

Mixing Under Vacuum

Some materials need to be mixed actually under vacuum in order to obtain an air free material.  Some need heating before or during mixing.  Possibly mixed material need constant agitation or vibration to keep them fluid so that they will actually flow as fluids to fill moulds and containers.  In such cases we would consider each application individually design and biggest suitable vacuum equipment.

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 before the material to gently vibrate the mould full of material to temporarily overcome any tendency of the material to be thixotropic, i.e., non-flowing.  For example, certain casting plaster mixes.

If the material has already begun to set whilst 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 very gassy cast.  Should this happen then vibration under vacuum and during vacuum release is needed so that the material flows in to all the cavities.

Copyright 1988

NB:     The information in this pamphlet is offered in good faith and is based on our general experience.  The methods 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 + 44 (0) 1983 855822, fax + 44 (0) 1983 852146 or E-Mail: sales@island-scientific.co.uk.

A full list of Technical Notes is available free on request.