Platinum RTV Catalyst Ratios for Large-Scale Mold Pours

Large platinum RTV pours fail from heat, inhibition, and trapped air more often than from “not enough catalyst.” The mix ratio on the TDS (1:1, 10:1, 100:2, whatever the grade is) is the stoichiometry of that product. It is not a speed knob you invent for a 20 kg block.

If you're buying a custom RTV mold — a jacketed gummy sheet, a texture block, a keyed two-part resin glove — you can, on some two-part systems, work within a supplier-allowed band of Part B to shorten or extend work time. Outside that band you under-network, over-crosslink, or leave a sticky face that looks like inhibition. Temperature, vacuum degas, and a clean master move demold more than a secret extra drop of platinum.

This is poured addition-cure RTV-2 mold rubber. It is not LSR meter-mixed into a hot cavity (cold-runner on that quote, never a LIM hot tool). It is not tin-cure (a condensation catalyst with a different ratio logic). Unmix them.

The ratio on the can is the network, not a suggestion

Platinum RTV is vinyl-functional polymer plus Si–H crosslinker plus a platinum complex, split between A and B so the kit is stable. The published weight or volume ratio sets Si–H to vinyl and the platinum concentration the formulator intended.

What happens when shops freelance:

• Starved catalyst / starved Si–H: long gel, tacky surface, poor tear, a mold that keeps shrinking as it tries to finish.
• Excess catalyst side: shorter pot life, possible brittleness or modulus rise, more platinum to poison later if you ever pour platinum on that bench, and no guarantee of a faster through-cure in a thick block. The outside still gels first.
• Eyeballed 10:1: a 10% weigh error on the small side is a large chemistry error. 1:1 kits forgive less than people think if one cup is aerated.

If the supplier publishes a range, stay inside it and record the weigh. If they do not, do not invent one. Buy the fast or slow grade instead. That is how professionals customize cure: they change SKU and temperature, not folklore.

Common mix-up: extra B versus a long-pot grade

A 15 kg texture block needs pot life more than a heroic demold. Adding B shortens both clocks. The honest tools are a slow or long-pot platinum grade, vacuum degas of mixed rubber until the foam rises and collapses, a pour path that does not re-entrain air, and heat after the face has wet and gelled if the grade allows a mild warm cure.

Through-cure in deep sections follows heat and time, not extra platinum. A 40 mm section is a bake/wait problem. Layered pours (gel, then next lift) — or a thin high-tear skin plus mother mold — beat a single under-catalyzed brick.

What to ask when you're buying

• Shot mass and max section thickness?
• Fast / regular / slow grade, mix ratio as TDS?
• Master patch-tested 24 h for inhibition?
• Jacket vs solid cube (skin plus mother mold shortens through-cure)?
• Who keeps the master if the first pour skins sticky and we recast?

Worked example: 12 kg gummy-sheet pour

A 12-cavity bear sheet, ~12 kg platinum 15A, 8 mm pocket, jacket poured separately. Winter shop 16 °C versus summer 30 °C moved gel more than a 2% B tweak. We weighed 10:1 to the TDS, degassed to about −0.095 MPa until the foam collapsed, poured, then a grade-allowed warm finish after gel. A witness coupon of the same thickness tore clean before demold. Extra B would have gelled in the bucket mid-pour.

Inhibition beats every ratio trick

If the master is dirty, extra Part B will not save you. Sulfur clay, tin soap on a brush, amine primer, undercured SLA acrylate: the face stays wet while the cup cures. That is catalyst death at the interface, not a ratio error.

For large, expensive pours:

• Patch-test the worst pocket of the master for a full 24 h at shop temperature.
• Barrier-coat or fully post-cure photopolymer masters.
• Keep tin-cure tools and sticks off the platinum bench.
• If the patch fails, copy in tin, then pour platinum on the copy.

A large mold also sees more container surface and more chance of a contaminated paddle. Scale the cleanliness with the kilograms. Paying a tooling charge does not mean the shop may skip the patch.

Temperature is the legitimate speed control

Within the grade's window, warmer mix and warmer master shorten gel. Cooler extends pot life for a long pour. Do not freeze the kit and expect a normal hydrosilylation later without letting it come to temperature. Do not heat mixed RTV in a sealed bucket. A mild oven after gel (grade-specific) can finish a thick platinum block. It is not an LSR 180 °C cycle and it is not a tin schedule.

Some suppliers sell separate catalyst concentrates for their line. Those are still formulated ratios.

When this article is the wrong quote

LSR injection cure times are tool temperature and grade, not extra catalyst in a barrel. Tiny jewelry molds don't need a 12 kg protocol. Geometry can override: a thin glove plus jacket is faster than a cube even at the TDS ratio.

Related reading: how we make the mold, custom RTV molds, platinum-cured silicone, what tooling means.

What to send for an RTV mold quote

• Platinum-cure RTV-2, [slow/regular/fast] grade, mix ratio as TDS by weight
• Shot mass, section, jacket vs block
• Degas spec; master patch 24 h
• Target shop T; optional after-gel warm cure per grade
• Demold when a witness coupon of the same thickness tears clean
• Tin-cure excluded unless the patch fails
• Recast of the skin if the first pour is inhibited

Send the master, kilograms, and thickness. We'll pick a long-pot grade and a glove-plus-jacket instead of extra B.

We would not recommend extra catalyst to pour today

We would not recommend volume-guessing 10:1 on a 12 kg shot. We would not heat a full liquid pail to shorten pot life. We would not use LSR catalyst language on an RTV kit. We would not raise Shore A first to “cure faster.”

RUUIPON Mold Structure Decision: large wet pour or 10A–20A? mother mold — also the through-cure fix. Flat back? open-back. Cannot peel? two-part / core. Thin skin beats a heroic cube.

Typical shop values: large platinum pours

| Item | Typical shop value | | --- | --- | | Mix | Platinum 1:1 or 10:1 as TDS; tin often 10:1 | | Degas | About −0.095 MPa (28–29 inHg) until foam rises and collapses | | Cavity Shore | 10A–20A with jacket for undercut food/resin | | Linear shrink | ~0.1–0.3% platinum; tin keeps moving in storage | | Shop T swing | Winter ~16 °C vs summer ~30 °C moves gel more than a 2% ratio tweak | | Demold clock | Witness coupon of the same thickness, not the 50 g TDS cup |

Reference values only. Confirm material-specific limits against the supplier’s technical data and agree project acceptance criteria before production.

FAQ

Can I add extra platinum catalyst to make a large RTV mold cure faster? Only inside a supplier-published band, and even then you shorten pot life as well as gel. Thick through-cure is heat and time. Off-ratio extra B risks a brittle or still-tacky network. Prefer a fast grade and a controlled warm cure after gel.

Why did the cup cure and the mold face stay sticky? Inhibition at the master, or a cold face, not a global lack of catalyst. Patch-test the master. Extra catalyst in the mix will still die at a sulfur or acrylate surface.

Is a 10:1 platinum RTV more adjustable than a 1:1? The small side is easier to get wrong. A weigh error on Part B of a 10:1 kit is a large chemistry error. 1:1 is simpler to scale on a big shot if both sides are mixed internally as the formulator intended. Neither is a free ratio playground.

How do I time demold on a 20 kg platinum block? Pour a witness coupon of the same thickness from the same batch and tear it. Do not use the 30-minute cup time on the TDS. After-gel warming, if the grade allows, helps the core. Layered lifts or a glove plus jacket beat one heroic dump.

Does this apply to LSR injection cure times? No. LSR is meter-mixed at the machine ratio and cured by cavity temperature in seconds to minutes, usually with a cold-runner. Do not put extra catalyst in an LSR barrel to “run faster.” Change mold T, grade, or thickness. That is a parts quote, not this mold.