Mould Rubber — Vulcanised Rubber for Lost-Wax Production
Mould Rubber — Vulcanised Rubber for Lost-Wax Production
The flexible mould material that enables multiple wax patterns from a single master model
Mould rubber is the natural or synthetic rubber compound used in the jewellery manufacturing process to create vulcanised flexible moulds for lost-wax casting. The mould rubber is built around a master model — typically a hand-fabricated or CAD-printed precursor of the intended jewellery piece — vulcanised under heat and pressure into a durable flexible mould, then cut open and used to produce wax patterns by injection. The vulcanised rubber mould is the central enabling technology of contemporary production jewellery manufacture, allowing a single master model to produce hundreds or thousands of identical wax patterns and, through subsequent casting, identical metal pieces.
Materials and types
Conventional jewellery mould rubber is a natural rubber compound supplied as flat sheets in various thicknesses (typically 1.5 to 3 millimetres), sometimes with sulphur-based vulcanising agents incorporated. Silicone-based moulding materials are an alternative that supports cold or low-temperature curing and is used for masters that cannot tolerate the heat of natural rubber vulcanisation (for example, masters incorporating heat-sensitive components or stones that should remain in the master). High-temperature silicone rubbers extend the working temperature range for unusual applications. Each material class has trade-offs in tear strength, dimensional stability, thermal tolerance, and mould life.
Process
The standard vulcanised natural-rubber mould-making process involves several stages. The master model is mounted on a sprue former that defines the pour channel for wax injection. The master and sprue are sandwiched between layers of unvulcanised rubber sheet inside a steel mould frame. The frame is placed in a vulcaniser — a heated press operating at approximately 150 degrees Celsius and several hundred pounds of clamping pressure — for the time required to fully cross-link the rubber, typically 30 to 90 minutes depending on the rubber thickness. The vulcanised rubber forms a unified solid block enclosing the master.
The cured rubber block is removed from the vulcaniser and carefully cut open along a parting line that allows the master to be removed without damaging either the master or the mould. The cut surfaces typically incorporate registration features (interlocking irregular profiles, alignment pins, registration cuts) that allow the two halves of the mould to be reassembled in precise alignment for wax injection. The completed mould is then ready for production use.
Wax injection and mould life
Wax injection is performed with a heated wax injection machine that forces molten investment wax (typically a low-temperature wax in the range of 70 to 80 degrees Celsius) into the mould cavity through the sprue. The wax solidifies in the mould, the mould halves are separated, and the wax pattern is removed. Each cycle takes a few minutes; a single mould can produce hundreds to several thousand wax patterns over its working life depending on the complexity of the master, the care of operation, and the rubber type.
Worn moulds can be repaired by partial recasting or replaced with new moulds taken from the original master (which is preserved against this requirement) or from a new master. Master models in valuable jewellery production are themselves stored carefully and treated as intellectual property assets.
In the trade
Mould rubber technology is the foundation of contemporary production jewellery manufacture and underlies essentially all multi-piece jewellery production from the major commercial brands. The reference texts in the field include Murray Bovin's Jewelry Making, Oppi Untracht's Jewelry: Concepts and Technology, and the various technical references published by the Society of North American Goldsmiths and equivalent organisations. The major mould-making suppliers — Castaldo, Pierre, R&R Industries — supply the rubber, vulcanisers, and supporting equipment to the jewellery manufacturing industry worldwide.