Mould Making With Latex
The advantage of latex moulds is that they are very elastic and can therefore be used to cast parts with significant undercuts. This elasticity can also be a down side as it can result in the mould deforming under the weight of the casting material however this can be overcome with the use of additional support casing as described later.
Making a Master
The first step is of course to obtain a master from which the mould will be created. There are a range of suitable materials and the fact that the latex dries in air and does not generate heat while setting is the reason for this flexibility. The porosity of the master is a factor that will result in one of two methods being used to produce the mould. These are the "dipping" and the "paint on" method and will be described later. For sculpted pieces, plaster is probably the ideal medium as its porosity draws moisture from the latex allowing the dipping method to be employed for making the mould. Clay is another possibility allowing the dipping method to be employed for making the mould. Clay is another possibility for dipping but should be allowed to dry out or be fired first as wet clay will prevent the latex from drying. Plasticene can also be used if the paint on method is employed. Non-porous materials like wax and polyester resin can be used as masters if the paint on method is employed.
The Dipping Method
This needs to be done with a porous master which is simply dipped into the liquid latex. The porosity draws moisture from the latex causing it to thicken on the surface. Unfortunately, as this happens, air bubbles are formed in the latex. To overcome this, remove the master from the latex after a few seconds and use an old brush to stipple the surface and thus burst the bubbles. As this is done the latex will turn into a paste which will prevent further air from excaping. The master can now go back into the liquid latex for 15-20 minutes by which time a sufficiently thick coating should have formed. Remove the master from the liquid latex and leave it to dry for a few hours before removing the mould as describe later.
The Paint on Method
The dipping method will not work with non-porous masters so instead the latex is painted onto its surface with an old brush. An additional problem is that the thin liquid latex will tend to run off the non-porous master and several coasts will need to be applied in order to build up the requied thickness with the latex being left to dry a little between coasts. The author has found that this process can be made less frustrating by pouring some latex into a small dish and allowing it to begin to thicken BEFORE painting it onto the latex master. the author is unable to comment on whether this affects the life of the mould in any way as he has only ever wanted to do small numbers of casting from moulds made in this way. An "optional extra" is latex thickener which can be added to the latex. Addition of thickener at the correct rate (as specified on the bottle) should not affect the life of the mould however excessive use can result in a rigid and brittle mould.
Latex should always be stored in a cool dark place.
The ideal temperature a set piece of latex can withstand is between -55 degrees Celsius and 82 degrees Celsius.