IMPORTANT NOTE : Up to 10% shrinkage may occur using this material.
INTRODUCTION
Liquid Latex Rubber is a prevulcanised emulsion, which will air dry on the surface of a master pattern to form a highly flexible thin rubber mould. This extremely versatile material is widely used in such diverse fields as the manufacture of moulds for garden ornaments to the production of theatrical masks. Moulds made of Latex have a comparatively long life, are easily made and will give good definition and accuracy of reproduction.
Natural rubber is a non vulcanised Latex substitute for non critical and surface coating application such as the manufacture of Live Role Play Weapons. (See leaflet T123). Mould making is possible using Natural rubber but as it lacks the elasticity and strength of Latex only simple small shapes should be considered and production will be limited to only a few casts per mould. Mould making methods are identical for both Latex and Natural rubber.
MASTERS
When considering making a Latex mould a suitable master must be either acquired or created and choice of materials is quite important. Plaster of Paris - Probably the most suitable material as the porosity of the plaster draws moisture from the Latex causing it to thicken. Clay Masters - are suitable but should be allowed to dry out or once fired before use. Wood Masters - Experimental work has shown that moulds can be made on wood masters, which must have a smooth surface and be of a porous nature. The dipping and curing is the same as for plaster and clay. Non-Porous Masters - For these type of masters the 'Paint On' method will be necessary. Some metals may react with Latex causing weak moulds. Plasticine Masters - Plasticine masters can be used with Latex but may result in a reduced mould life.
MOULD MAKING
Two mould making methods are used, the 'Dipping' or the 'Paint on' method.
Dipping: This is best carried out with porous masters as the porosity draws moisture from the Latex thus causing it to thicken on the surface. However, as the moisture enters the master it replaces the air already there and forces air bubbles into the Latex. To overcome this, re-dip for a few seconds, remove from the Latex and with a brush, stick or palette knife, burst the bubbles as they form and spread the Latex over the surface. As this is being done the Latex will quickly form a paste which will prevent the release of further air. Re-dip in the Latex and leave in for 15-20 minutes. Remove and allow any surplus to drip off. A mould thick enough to be used should have formed. The Latex should be touch dry in 10 minutes at normal room temperature (2OºC) turning from white to a semi-transparent creamy yellow colour, and ready to peel from the master in 2-3 hours. Drying can be speeded up by using a gentle heat up to about 70º-75º C (a hairdryer is perfect) If the master is dipped for a longer period a thicker coating will be obtained and a longer drying time needed.
Before attempting to peel the mould from the master, apply talc or washing up liquid over the surface to prevent it from sticking to itself when it is peeled.
In the event of an overthin mould being made with the first dip, the thickness can be increased by re-dipping as soon as possible but within about 12 hours to ensure the Latex bonding to itself. Best results are obtained by trying to get the correct thickness of Latex during the initial dip. Paint on: Paint a number of layers of Latex with a soft brush allowing each layer to part cure at normal room temperature before applying the next layer. If the master is non-porous the latex will tend to run so only very thin layers can be applied. Keep repeating the process until a skin of sufficient thickness has been built up. For small pieces 7 or 8 layers may be necessary giving 'high spots' extra layers. If the master shape is warmed in the oven before the initial layer is applied a thicker skin will form and subsequent painting will give a better-finished mould.
LATEX THICKENER
One of the difficulties of using Latex for mould making is its low viscosity and it's inability to form easily on non-porous surfaces. Another difficulty is the amount of air, which can be trapped on the surface both when painting and dipping.
However, by the addition of a very small amount of Latex Thickener well stirred in, the viscosity of the Latex can be adjusted to whichever method is used.
By adding enough thickener to make the Latex similar in consistency to emulsion paint, it can be painted on any surface, porous or not, without running.
Care must be taken to brush the thickened Latex on in thin layers, although after the initial coat has been painted on, it is possible to dip, either in the thickened or the original Latex, in the normal way. While the addition of Thickener at the correct rate will not reduce the strength of the cured Latex or have any adverse effect on the finished castings, excessive use will produce a very brittle mould.
The thickened Latex can be used to take moulds from non porous vertical surfaces, e.g. wood and stone carvings on walls etc., later supported by a Plaster of Paris or glassfibre case mould, before removing from the original.
As a guide about one drop of the Thickener will thicken l GM of Latex so that it can be painted on easily. This amount should be varied to give the best results on any particular job.
If your Latex mould is of a large size and liable to distort when used then a support case or jacket will be needed. This is usually made of either plaster/plaster bandage or glassflbre. No release is required when using either of these materials with Latex. A fuller description of the techniques involved is given in leaflets T122 (plaster) and T10 (Glassfibre).
COLOURING LATEX
Latex may be coloured using Trylon Latex Colourants. These are added at a rate of around 5%. The colours will darken considerably on drying and excessive use may also affect the curing of the Latex. Colourants may be intermixed to obtain different shades.
OTHER~APPLICATIONS
While this leaflet concentrates on mould making, both Latex and Natural Rubber have lots of other uses including MASK MAKING and LIVE ROLE PLAY WEAPON MAKING. These techniques are described more fully in leaflets T125 and T123 respectively.
Although Latex moulds are very popular and versatile, there will be occasions when other mould making materials will need to be considered. See leaflet T105 for user notes on Gelflex remeltable P.V.C. or leaflet T 114 for a series of mould making techniques for Silicone Rubber.
Reproduced by kind permission of TRYLON LTD
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