Latex Liquid Rubber is a prevulcanised emulsion which will air dry on the surface of a master pattern to form a highly flexible thin rubber skin. 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. Latex is easy to use, is relatively strong, and will give good definition and accuracy of reproduction.
When considering making a latex mould a suitable master must be either acquired or created. The 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.
This material is suitable but should be allowed to dry out or fired before dipping in Latex.
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 process is the same as for plaster and clay.
For these type of masters the paint on method will be necessary. Some metals react with Latex resulting in weak moulds.
Plasticene masters can be used with Latex but may result in a reduced mould life.
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, it replaces the air already there, thus forcing air bubbles to form in 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 turn to a paste which will prevent the release of further air. Re-dip in the Latex and leave for 15-20 minutes, remove and allow any surplus to drip off; a mould thick enough to be used should have been formed. The Latex should be touch-dry in 10 mins. at normal room temperature (20ºC), 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 gentle heat up to about 70ºC - 75ºC. Before attempting to peel the mould from the master, apply talc or washing up liquid over the surface to prevent it sticking to itself when it is peeled. If the master is dipped for a longer period a thicker coating will be obtained, needing a longer drying time. In the event of an overthin mould being made with the first dip, 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 Method
Paint a number of layers of Latex with a soft brush allowing a part cure at normal room temperature (20ºC) 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, high spots can be given extra layers. If the master shape is warmed in an oven before the initial layers is applied a thicker skin will form and subsequent painting will give a better finished mould. Spraying the Latex may be beneficial on larger moulds.
Often moulds for garden ornaments or advertising display items are too big for the dipping process to be used. Therefore for larger moulds, the latex may be painted by brush or spray gun directly onto the mould. The number of coats and drying is the same as for the dipping method. In another technique flour is dusted onto the wet latex surface or on to the mould to give a higher viscosity thicker coating of latex. Flour may also be mixed with latex to give a viscous paste. Sometimes cotton muslin cloth is applied to the wet latex coating on the former and more latex applied over it. This is done to reinforce certain mould areas where distortion due to the weight of cement or plaster filling must be prevented. Distortion of large moulds due to weight of in-filled moulding composition is more generally avoided by making a support case. This is usually made of plaster or fibreglass and should be made before the Latex mould has been removed. Release agent is not required.
This can be achieved by adding talc or plain flour to the latex. The amount added should be varied to give the best results on any particular job. Also “wallpaper paste” (Polycell) which should first be mixed with water (as described on the packet) and then added to the latex until desired viscosity is achieved. It is particularly useful when painting on non porous surfaces but can be used on all surfaces and does allow you to build up the thickness of your mould much quicker. 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. To avoid air bubbles 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. Excessive use of thickened latex may produce a rigid 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 fibreglass case mould, before removing from the original. As a guide about one drop of the thickener will thicken 1g of latex so that it can be painted on easily.
To achieve fine detail reproduction the mould should first be coated with one or two coats of pure latex without additive. Subsequent coatings should be applied using any of the methods described.
Shringage of Moulds
Moulds made from latex will always shrink slightly. However shrinkage can be reduced by air drying the moulds slowly instead of drying in an oven. Allowing the rubber mould to fully dry on the master before removing it will prevent shrinkage.
Discolouration of Moulds
Brown, yellow blotches on moulds are usually caused by copper contamination. Check all materials and equipment thoroughly for anything that may contain copper, Items often used which may have copper in them are:a) Paint brushes used to paint on latex, i.e. copper bands and nails are often used in construction of paintbrush (copper wire sometimes used to tie bristles together).b) Dilution water- Note this will lead to a darkening of the whole mould.c) Fingermark due to hands coming into contact with coins, cigarettes, etc.d) Model or Master used to make moulds, i.e. brass ornaments etc.
Poor Strength of Moulds
Often due to the fact that rubber has not been fully dried before removal from master. Also exposure to light or heat. See mould storage section.In extreme cases the mould can be soaked in distilled water and dried out, as this will increase tensile strength of rubber.
Do not allow liquid Latex to freeze. Finished moulds should be stored in darkness or at least away from direct sunlight and in a cool place to prolong mould life.
Important Note: Up to 10% Shrinkage may occur using latex
Although Latex moulds are very popular and versatile there will be occasions when other mould making materials will need to be considered. CFS supply a range including remeltable Vinamold and Silicone rubber.