How to make latex moulds
Latex moulds are the cheapest and most common way of producing small to medium size castings from materials such as plaster of Paris, concrete and polyester resin. They’re also used in the catering industry to produce cake decorations from chocolate and sugar-crafts and are reusable many times over.
The brush on method
Used for small moulds and non-porous masters. Several coats are usually necessary to build up enough mould thickness to accommodate the casting material without causing distortion of the mould. Each coat should be allowed to dry before application of the next, otherwise the latex won’t cure evenly. Liquid latex is pre-vulcanised and doesn’t require a curing agent. Individual coats will take approximately 20 minutes to dry at room temperature. Gentle heat from a hair dryer can reduce drying time by half. It’s imperative that any tiny air bubbles are removed from the first coat before the latex begins to cure otherwise small air holes will be visible on the finished castings. To help reduce air bubbles it's recommended the latex is left to settle for a few hours before using. This will allow air to dissipate resulting in a smoother liquid. To avoid air bubbles, shaking or stirring liquid latex before use is not recommended. A can of compressed air can also be used to remove any remaining air bubbles.
The dipping method
Used in large scale production of many latex products using porous masters created from materials such as plaster of Paris or concrete. The master is suspended in a container of liquid latex for several hours or overnight. This will allow the thickness of the mould to build naturally as the porous surface draws moisture from the latex. Further applications usually aren't necessary when using the dipping technique. Usual practice is to dip the master briefly then expose to air for a few minutes allowing any air bubbles to burst. The master is then dipped for a second time which would fill any small cavities created by trapped air.
Larger moulds, such as garden ornaments, will require a support to prevent distortion under the weight of casting material. Mould supports can be made from plaster, modroc or resin and should be applied over the dried latex before removal from the master. Use of polyester resin will require the application of a mould release agent before use. A mould support isn’t necessary for small ornaments or decorations that don’t require a heavy amount of casting material.