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Downland Latex Rubber

A SUGGESTED PROCEDURE FOR MANUFACTURING RUBBER MOULDS FOR CASTING POLYESTER ORNAMENTS

The basic model used for moulding with latex rubber should be made from a good quality plaster of paris. The plaster model should have a plaster base with a minimum thickness of 13mm. If the base is not made at the same time as the model then a base may be added to the model by using the following procedure:

Make a circular plaster disc to a minimum thickness of 13mm. Drill a hole through the centre of the base and fix by a screw. Gaps between the master model and the base should be filled using a cellulose filler such as Polyfilla. Plaster must not be used for this operation. The complete master model plus base should now be dried thoroughly. The model should then be placed in an oven at a temperature of approximately 80°C, until the model is just handleable. The model is then lowered BASE FIRST slowly into the latex compound. This can be achieved by standing the model on an aluminium tray supported by aluminium stainless steel or nylon rods or wire. The model is left completely immersed for a minimum, dwell time of 2 hours. The approximate thickness of latex deposited is as follows:

  DWELL TIME    THICKNESS
  2 hours                 1.6 to 2.4mm
  3 hours                 3.2 to 4mm
  4 hours                 4mm to max.

Whilst the plaster model is immersed in the latex, it is advisable to cover the container. This will help prevent skinning of the latex as well as preventing dirt or dust settling on the surface.
When the necessary dwell time has elapsed, the mould should be slowly and evenly withdrawn from the latex. As a guide the withdrawal rate should be approximately 50mm every four seconds. The mould should then be left to air dry in its natural standing position for at least two hours, when the rubber should be 'touch dry'. The mould should then be placed in a drying oven at a temperature of approximately 30°C for a minimum of three hours.
When dry the mould should be dusted with talc or a weak detergent/water solution, and stripped from the plaster model. If the mould is reasonably small, it can be immersed in a sand-filled container, to prevent any distortion when the polyester is poured in. For large moulds it is recommended that a case is made for the mould out of fibreglass or similar material. This should be constructed in two halves to facilitate removing the complete rubber mould and polyester ornament.

Storage Recommendations

Stored in well-sealed containers, Downland Latex Compound will keep satisfactorily for at least six months. The latex must be protected from frost and should preferably be stored at a temperature above 5°C (41 °F), but not above normal ambient temperatures.
The latex must never be contaminated with copper or copper-bearing alloys. Latex contaminated in this way will age rapidly and the compound's usable life will be seriously impaired. Copper or copper-alloy machine components should be chromed or exchanged for non-cuprous replacements.