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Mould Making With Latex

Mould Making With Latex

by Andy Slater

Craft shops are able to supply Latex as a pre-vulcanised emulsion that will air dry and can be used to make moulds in which wax, plaster or resin parts can be cast.

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 for dipping but should be allowed to dry out or be fired as wet clay will prevent the latex from drying. Plasticene can also be used if the paint on method is employed however the interaction between plasticene and latex will result in a mould with a reduced working life.

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 the 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 escaping. 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 described 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 coats will need to be applied in order to build up the required thickness with the latex being left to dry a little between coats.

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 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.

Removing the master from the mould

Before removing the mould you should apply talcum powder or washing up liquid to the outside of the mould to prevent the latex from sticking to itself. The mould can then be washed, dried and again dusted with talcum powder.

Making an external support case

If the mould is small it will be possible to use it for casting without any external support. Large moulds however will tend to be distorted by the weight of the casting material and an external support should be made.

It has been suggested that this should be done before the removal of the master from the mould however the author prefers to remove and check the quality of the mould before proceeding. The master must of course be reinserted into the mould around which the support is to be made.

The authors preferred method is to use plaster reinforced with jute scrim although glassfibre can also be employed. There is no need to use a release agent with these materials and the latex. However, an obvious requirement of this support is that it should be possible to remove it and it will therefore need to be made in two or more pieces.

Begin by deciding how many pieces the support should split into and where it is appropriate for the splits to occur. For simplicity, we will assume that a 2 piece support case is appropriate. Modelling clay or plasticine can now be rolled out and cut into strips before being applied as shuttering along the line where the support case it to split. Plaster and scrim is now applied to one half of the mould op to the clay wall and left to dry.

When dry, the clay wall is removed. Several indentations should be made in the newly exposed edge to enable the pieces of the finished support case to be matched up easily. Obviously these indentations should not be so deep or so angular that they will cause problems in separating the pieces and the edge of a small coin can be usefully employed to make appropriate dome shaped indentations.

Although it was not necessary to use a release agent between the latex and the plaster it is essential that it be used between the two halves of plaster. For this purpose the edge of casing can be painted with varnish or shellac to make it non-porous. Alternatively a coating of vaseline or washing up liquid can be applied.

The second half of the casing can now be created with more plaster and scrim.

When the whole of the casing is dry it can be carefully removed and the master removed from the inner latex mould.

Note that when casting, it is usually appropriate to put some of the casting material into the latex mould and tease it in the usual way (to release bubbles from the casting material), BEFORE it is encased in the support structure. The latex mould is then encased in the support casting with string or sticky tape bound around the outside to hold the casing together while the casting process is completed.