RTV Silicone Mould — Room-Temperature Mould-Making for Casting Masters
RTV Silicone Mould — Room-Temperature Mould-Making for Casting Masters
The flexible mould medium that enabled studio casting from delicate or non-vulcanisable masters
An RTV silicone mould — room-temperature vulcanising silicone — is a flexible mould produced by mixing a liquid silicone base with a curing agent and pouring or injecting the mixture around a master model, where it cures at ambient temperature without heat or pressure. The technique has become standard in jewellery casting where the master cannot tolerate the temperatures and pressures of conventional vulcanised rubber moulding: wax models, resin masters, gem-set masters, and any model with delicate or heat-sensitive components. RTV moulds are described in detail in Oppi Untracht's Jewelry: Concepts and Technology and in the technical literature published by Castaldo, Zero-D, and other industry suppliers.
Materials and chemistry
RTV silicones for jewellery casting are typically two-part addition-cure or condensation-cure systems. Addition-cure silicones (platinum-cure) provide low shrinkage, excellent dimensional accuracy, and resistance to inhibition by most cleaning solvents, and are the standard for high-precision jewellery moulding. Condensation-cure silicones (tin-cure) are less expensive but show slightly higher shrinkage and have a shorter shelf life. The choice between them depends on the precision required, the durability target, and the cost sensitivity of the application.
Hardness ranges from approximately Shore A 20 (very soft, flexible) to Shore A 70 (firm, dimensionally stable), with Shore A 30 to Shore A 50 being the common range for jewellery moulding. Softer silicones release complex undercuts more readily; harder silicones hold dimensional accuracy on production runs. Pot life — the working time after mixing before the silicone begins to cure — varies from a few minutes to several hours depending on formulation, with longer pot life favoured for complex masters that require time to position and pour around.
Process
The basic RTV mould-making process is straightforward: the master model is positioned within a containing frame (a mould box, typically aluminium or plastic), the silicone is mixed in the prescribed ratio (commonly 10:1 base to catalyst by weight), degassed under vacuum to remove air bubbles, poured around the master, and allowed to cure. After the first half is cured, the master is left in place, the cured surface is treated with a release agent to prevent silicone-to-silicone bonding, and the second half is poured. Once the full mould has cured, it is cut open along a parting line, the master removed, and the mould is ready for use.
For complex masters, more elaborate procedures apply. Multi-part moulds with three or more sections handle deep undercuts and complex geometry. Brushed-on silicone, applied in successive layers from a thicker mixture, is used for very large or irregular masters where pouring is impractical. Vacuum injection of silicone is used in production environments to ensure complete fill of fine detail. The choice of method depends on the master's complexity and the production volume target.
Applications in jewellery casting
The principal application of RTV silicone in jewellery is the moulding of wax masters, resin printed masters, and metal masters with gem-set or other heat-sensitive components. Conventional vulcanised rubber moulds, formed at temperatures around 150 to 165 degrees Celsius and at substantial pressure, would melt wax, soften resin, and damage delicate stones. RTV silicone, curing at room temperature without pressure, leaves these masters intact and produces a serviceable mould directly from the original.
The technique has been particularly important in the spread of resin-printed master technology in the contemporary jewellery industry. CAD-designed pieces, printed in photopolymer resin, can be moulded directly in RTV silicone without the intermediate step of casting a metal master, accelerating the development cycle from design to production. The combination of CAD, resin printing, and RTV moulding has substantially shortened the lead time from concept to first cast piece in modern studio practice.
Mould life and limitations
RTV silicone moulds have shorter useful lives than vulcanised rubber moulds, typically yielding between fifty and several hundred wax injections before tear or distortion compromises the mould. The platinum-cure addition silicones produce longer mould life than the tin-cure condensation silicones, and harder formulations outlast softer ones in production use. For long production runs of a fixed design, vulcanised rubber moulding from a metal master remains the more economical choice; for short runs, prototypes, and gem-set masters, RTV is the appropriate technology.
Other limitations include sensitivity to certain inhibitors. Sulphur-bearing materials, some clays, and certain release agents can prevent silicone cure or cause incomplete cure at the master interface, producing surface defects in the mould. Test cures on small samples of any new master material are recommended before committing to a full mould.
In the trade
For studio casters and small-production workshops, RTV silicone moulding is one of the standard techniques in the working repertoire alongside conventional vulcanised rubber moulding, sand casting, and direct lost-wax casting. The technique allows the workshop to produce moulds from a wide range of master types and to extend the design vocabulary beyond what conventional moulding allows. Suppliers such as Castaldo, Smooth-On, and Zero-D offer formulations targeted to the jewellery market, with documentation and technical support oriented to the specific demands of jewellery production.