Platinum Catalyst Ratios: Customizing Cure Times for Large-Scale RTV Molds

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.

You can, on some two-part systems, work within a supplier-allowed band of Part B (the catalyst-containing side) 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 (metered 1:1 into a 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 (for example a small percent band on Part B), 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.

Large blocks have a thermal and a diffusion problem

A 200 g jewelry mold dumps reaction heat into the room. A 15 kg texture block stores it. Platinum addition is exothermic. In a thick dump:

• The core can run hotter than the TDS “room temperature” cure.
• The face against a cold master can lag.
• If you then heat the whole box to “speed it up,” you can overshoot in the core and still have a wet master face if inhibition is present.

Work time (pot life) is the clock to degas, pour, and let the foam collapse. Demold time is the clock to a tearable network. They are not the same, and adding catalyst shortens both. A large mold needs pot life more than it needs a heroic demold. Plan:

• A slow or long-pot platinum grade for the pour
• Vacuum degas of mixed rubber to about −0.095 MPa (28-29 inHg) until the foam rises and collapses
• A pour path that does not re-entrain air
• Heat after the face has wet and gelled, if the grade allows a mild warm cure, not a blast on mixed liquid in a bucket

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) beat a single under-catalyzed brick.

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. Ratio gymnastics will not reverse that ranking.

A large mold also sees more container surface, more liner chemistry, and more chance of a contaminated mixing paddle. Scale the cleanliness with the kilograms.

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. That is the shop lever that does not wreck stoichiometry.

Rules:

• Do not freeze the kit and expect a normal hydrosilylation later without letting it come to temperature (moisture on cold cups is a tin problem more than platinum, but condensation on the master is still a surface defect).
• Do not heat mixed RTV in a sealed bucket; it will gel in the pot.
• 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.
• Record shop temperature with the batch. Winter 16 °C versus summer 30 °C will move a large mold more than a 2% ratio tweak.

Some suppliers sell separate catalyst concentrates for their line. Those are still formulated ratios. They are not a bottle of mystery platinum for any vinyl rubber you have.

What to put on a large-mold work order

“Platinum-cure RTV-2, [slow/regular/fast] grade, mix ratio as TDS by weight, scale ± [agreed]. No off-ratio without written supplier band. Degas [vacuum spec] after mix. Master patch-tested 24 h. Target shop T [range]. Optional after-gel warm cure [T / t] per grade. Demold when a witness coupon of the same thickness tears clean, not by the clock on a 50 g TDS cup. Tin-cure excluded unless the patch fails.”

Refuse:

• “Add extra catalyst to pour today”
• Volume-guessing 10:1 on a 12 kg shot
• Heating a full liquid pail to shorten pot life
• Using LSR catalyst language on an RTV kit

If you need both a long pour and a same-shift demold, the honest tools are a long-pot grade, a warm room after gel, and a thinner tool (glove plus mother mold) instead of a solid cube. Chemistry ratio is the last lever, and the narrowest.

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 beat one heroic dump.

Does this apply to LSR injection cure times?

No. LSR is meter-mixed at the machine ratio and cured by tool temperature in seconds to minutes. Do not put extra catalyst in an LSR barrel to “run faster.” Change mold T, grade, or thickness.