RTV Silicone Rubber: Prototypes, Moulds and the Honest Limits of Room-Temperature Cure

What RTV is

Room-temperature-vulcanising silicone is a two-part liquid that cures without an external heat source. It is supplied as a base and a catalyst, mixed by hand or by a dispensing machine, poured or injected into an open or closed mould, and left to cure.

Its viscosity is 5 to 50 Pa·s, low enough to pour and to reproduce fine detail, and low enough that it will flow into any cavity without pressure. That is the property that makes it useful: no press, no vacuum, no heated tool.

The price of that convenience is mechanical performance. RTV silicone has the lowest tear strength and tensile strength of the three families, and it is the most expensive per kilogram. It exists for prototypes, for tooling and for very small runs.

Table 1 — RTV against the other two families. The gap in tear strength is the one that limits its use.

Two cure chemistries, and only one is safe on a mould

Condensation-cure RTV crosslinks by reacting with moisture in the air and releases an alcohol, an acid or a ketone as a by-product. It is cheap, it adheres to many surfaces, and it is the standard for making moulds. The by-product matters: an acid-cure silicone will corrode a metal master, and an oxime-cure will inhibit the cure of addition-cure silicone poured into it later.

Addition-cure RTV crosslinks by a platinum-catalysed hydrosilylation with no by-product. It cures more completely, has better mechanical properties and does not corrode. It is also easily inhibited by sulphur, tin, amine and certain plasticisers, which means it will not cure in contact with modelling clay, some gloves or a condensation-cure mould.

The practical rule is that a mould for casting addition-cure silicone must itself be addition-cure, or must be sealed and allowed to fully outgas. A part that stays tacky for a week is almost always a cure inhibition, not a bad batch.

Pot life, cure time and the temperature that halves both

Pot life is the working time after mixing. It is 20 to 60 minutes for most RTV at 25 °C, and it halves for roughly every 10 °C of temperature rise. A workshop at 32 °C in summer will see pot life drop to 15 minutes, and a batch mixed for a large mould will start to gel before it is poured.

Cure time follows the same curve. A 25 Shore A RTV takes about 24 hours to demould at 20 °C, 13 hours at 30 °C and under an hour at 80 °C. For mould making, most shops cure at room temperature overnight and accept the wait; for parts, a warm box at 60 °C shortens the cycle usefully.

Thick sections cure from the outside in and the exotherm can be significant. A 50 mm thick block of RTV can reach 60 °C internally from its own reaction, which accelerates the cure in the middle and produces a hard core with a soft skin. For thick moulds, pour in layers.

Making moulds with RTV

RTV is the standard material for casting moulds because it reproduces detail at the micron level and releases from most materials without a release agent. A silicone mould for resin casting can produce 20 to 100 parts before the detail softens, depending on the resin and the release discipline.

The master pattern must be sealed and released. A porous master — wood, plaster, a 3D print — will absorb silicone and bond. Two coats of a shellac or polyurethane sealer and a silicone-compatible release agent are the minimum.

For a two-part mould, the parting line is planned before the first pour: the pattern is half-buried in clay, the first half is poured, the clay is removed, a release is applied to the cured silicone, and the second half is poured. A mould made this way has an accurate registration between halves and no flash line.

The limits, stated plainly

RTV will not replace a production process. Three limits define where it stops being sensible.

• Volume. Above about 1,000 parts, the labour per part exceeds the tooling saving. A compression mould in HCR pays for itself quickly at that volume.
• Tear strength. RTV tears at 5 to 15 kN/m. A part with a thin web, a sharp edge or a snap feature will fail in service regardless of how well it was cast.
• Consistency. Hand mixing produces batch-to-batch variation in hardness of two to three Shore A points. For a part that must seal, that variation is the difference between passing and leaking.

The right use of RTV is to answer a question before spending money on the answer. A cast prototype in RTV tells you whether the geometry feels right, whether the hardness is comfortable and whether the part assembles. It does not tell you whether the production part will pass a compression set test, because the material is not the same.

When to move to HCR or LSR

The transition point is usually a volume decision, but it can be a property decision. Move to HCR when the part needs tear strength, compression set or a hardness below 30 that must be consistent. Move to LSR when the part is small, the volume is high and the tolerance is tight.

The prototype does not have to be thrown away. A part designed for LSR can be cast in RTV for fit checks, then moved to an LSR tool without redesign, provided the wall thickness and radii respect LSR rules rather than RTV ones. Design for the production process from the start, and use RTV only as a fast confirmation.