Rubber Mould — Reproducing Master Models in Production Casting
Rubber Mould — Reproducing Master Models in Production Casting
The flexible mould at the heart of multiple-piece lost-wax jewellery manufacturing
A rubber mould is a flexible negative form, made from natural or synthetic rubber, used in jewellery casting to reproduce wax patterns from a master model. The technique sits at the centre of production casting workflow: a single hand-finished or CAD-grown master is enclosed in rubber, the rubber is cured around the master, the cured mould is cut open and the master removed, and molten wax is then injected into the cavity to produce as many wax patterns as the project requires. Each wax is invested, burnt out, and cast in metal by the lost-wax method.
Vulcanised natural rubber
The traditional process uses uncured rubber sheet stacked into a metal frame around the master, which is then closed in a heated vulcaniser at temperatures around 150 degrees Celsius and pressures sufficient to flow the rubber into every detail of the model. Cure times of an hour or more are typical for jewellery-scale moulds. The cured mould is removed from the frame, and the mould-cutter then opens it with a scalpel along a deliberate parting line, designed to allow the wax to release without distortion and to leave seam lines in places that finishing will easily remove.
Vulcanised natural rubber moulds are durable, dimensionally stable, and capable of capturing fine surface detail. A well-made mould can produce hundreds of wax patterns over its working life. The principal limitation is heat tolerance of the master: pieces with stones already set, or with components that cannot withstand 150-degree vulcanisation, are not candidates for traditional rubber moulding.
RTV silicone moulds
Room-temperature vulcanising (RTV) silicone moulds were developed to address the heat-tolerance limitation. RTV silicone is a two-part liquid that cures by chemical reaction at ambient temperature, allowing moulds to be made over masters that include set stones, soldered joints with low-temperature alloys, or three-dimensionally complex assemblies. The trade-off is somewhat reduced detail fidelity and shorter mould life relative to vulcanised natural rubber.
For casters working from CAD-grown wax or resin masters, RTV silicone is the standard choice because the master itself cannot survive vulcaniser heat. For traditional metal masters of robust construction, vulcanised rubber remains the workhorse.
Wax injection
Once the mould is cut, the cavity is filled by injecting molten jewellers' wax under pressure through a sprue cut into the mould face. Wax injection machines hold a reservoir of melted wax and use compressed air to push wax into the mould; the operator controls injection pressure, wax temperature, and dwell time. After a brief cooling pause, the mould is opened and the wax pattern lifted out for trimming and inspection. The process is repeated as many times as production requires.
The wax patterns are then attached to a tree, invested in plaster, burnt out in a kiln, and cast in metal — the standard lost-wax sequence. Rubber moulding is therefore the production-multiplier step that distinguishes a one-off lost-wax casting from a production run.
In the trade
For a brand or atelier evaluating production options, the rubber-mould stage is where the decision is made between bespoke and production manufacture. A piece destined for a single sale moves directly from master to investment without a rubber mould. A design that will be repeated, even at modest volumes, is moulded once and produced from the mould thereafter, with substantial savings in master-making time on each subsequent piece. Mould storage, mould renewal, and master archiving become operational considerations for any production house.