RTV-2 Mold Making Silicone Rubber: Condensation vs Addition Cure, Hardness & Viscosity, Process, and Troubleshooting
Choosing the wrong RTV-2 system is the single most expensive mistake in a mold shop: a blocked platinum cure ruins a master in minutes, and a too-soft mold tears on the twentieth demold. This guide gives resin-craft workshops, concrete/gypsum decor factories, and OEM replication shops an engineering-level framework to pick between condensation and addition cure, match hardness and viscosity to the casting material, run a repeatable pouring process, and fix the three most common failures (inhibition, bubbles, and tearing). Use it as a bench reference before you mix the next batch of mold making silicone rubber.
1. Condensation vs Addition Cure: Pick the Right RTV-2 Chemistry
Both systems are room-temperature-vulcanizing two-component silicone rubber (Component A base + Component B catalyst) poured over a master. They differ in catalyst, byproducts, shrinkage, and chemical sensitivity, so the casting material and the master determine the choice.
| Property | Condensation Cure (Tin) | Addition Cure (Platinum) |
|---|---|---|
| Catalyst | Dibutyltin dilaurate; mix ratio 100:2-3 by weight | Platinum complex; mix ratio 1:1 or 10:1 |
| Cure byproduct | Alcohol (must vent) | None — closed-mold friendly |
| Linear shrinkage | 0.2-0.5% | <0.1% |
| Deep-section cure | Limited; thick sections may stay tacky | Yes — uniform through-thickness cure |
| Typical mold life (resin casting) | 20-50 casts | 100-300+ casts |
| Inhibition risks | Not inhibited by sulfur, amines, or tin-cured silicone | Inhibited by sulfur clay (plasticine), tin silicone, amines, many SLA resins |
| Food-contact / chocolate / sugar | Not suitable (tin, alcohol) | Required — use food-grade platinum systems |
Rule of thumb for buyers sourcing RTV-2 silicone rubber: choose addition (platinum) cure for resin replication, food-grade molds, and masters cast in non-sulfur clays; choose condensation (tin) cure for plaster, concrete, wax, and for any master made from sulfur-containing plasticine where inhibition would otherwise block the cure.
2. Hardness and Viscosity: Match the Rubber to the Casting Material
Hardness controls undercut release and detail reproduction; viscosity controls how the rubber flows around fine features or stays on a vertical face. The table below maps typical casting materials to a workable Shore A and pourable viscosity window. For deeper silicone material selection reasoning, keep these three drivers in mind: undercut depth, part size, and surface detail.
| Casting Material | Recommended Shore A | Viscosity (mPa·s) | Cure System & Notes |
|---|---|---|---|
| Epoxy / polyester / PU resin (resin craft) | 20-30 (all-round); 10-15 for deep undercuts | 10,000-25,000 pourable | Addition cure preferred; self-releasing, 100-300+ casts |
| Gypsum / plaster decor | 15-25 | 8,000-15,000 | Condensation cure is fine; seal porous master to avoid surface bubbles |
| Concrete / cement decor | 25-40 (abrasion resistance) | 15,000-30,000 or thixotropic brush-on | Higher hardness extends mold life against abrasive aggregate |
| Wax / soap / candles | 10-20 | 5,000-15,000 | Addition or condensation; very long mold life; food-grade if skin-contact |
| Chocolate / sugar (confectionery) | 15-25 | 10,000-20,000 | Addition cure only — food-grade platinum system, no tin, no alcohol byproduct |
For large vertical masters (friezes, columns, life-size figures) choose a thixotropic, non-sag grade and brush it on in 3-4 layers instead of pouring; this avoids air pockets and controls exotherm in thick builds.
3. Step-by-Step Mold Making Process for RTV-2 Silicone
- Prepare the master. Seal porous masters (wood, plaster, concrete, unglazed ceramic) with shellac or a compatible primer; a porous surface will outgas and create bubbles stuck to the model surface. Position the master in a mold box with 10-20 mm clearance on the sides and at least 10 mm above the highest point.
- Weigh the components. Stir Component A (base) to homogenize fillers, then weigh A and B on a precision scale at the ratio specified on the TDS — 100:2-3 for tin systems, 1:1 or 10:1 for platinum. Off-ratio mixing is the #1 cause of tacky or soft molds.
- Mix slowly. Stir by hand or mechanical mixer at low speed for 2-3 minutes, scraping the walls and bottom. Avoid whipping air into the mix.
- Degass under vacuum. Place the mixing cup in a vacuum chamber at about -0.1 MPa for 3-5 minutes until the foam collapses; release vacuum slowly. Skip this step only for non-detail brush-on molds.
- Pour. Pour from a height of 20-30 cm in a thin stream so the rubber flows over the master and pushes air ahead of it rather than trapping it. Working time is 20-40 minutes at 25 °C — pour well before the pot life expires.
- Cure and demold. Demold in 6-24 h at 25 °C, or in 1-2 h at 40 °C for thin molds. Do not exceed 60 °C in thick sections; trapped air and alcohol (in tin systems) will expand and form voids.
- Post-cure for service. Allow 24-72 h for full property development before high-volume production casting.
4. Troubleshooting Sticky, Inhibited, and Bubbly Molds
Most RTV-2 failures fall into three buckets: cure inhibition (rubber stays tacky), entrained bubbles (surface pitting, weak spots), and mechanical defects (tears, thin sections). The table below maps the symptom to the cause and the fix.
| Symptom | Likely Cause | Fix |
|---|---|---|
| Platinum rubber stays tacky against master | Inhibition by sulfur clay (plasticine), tin-cured silicone residue, amine-cured epoxy, or many SLA 3D-print resins | Switch master to non-sulfur clay, or switch to condensation (tin) cure; barrier-coat SLA masters with a compatible clear lacquer |
| Surface pitting and small craters on the casting face | Air trapped during mixing or pouring; outgassing from porous master | Vacuum-degas mixed rubber at -0.1 MPa for 3-5 min; seal the master; pour in a thin stream from 20-30 cm |
| Tear at demold, especially on undercuts | Hardness too high for undercut depth, or thin section | Drop to Shore A 10-20; increase mold wall thickness to ≥10 mm; add a registered cut line |
| Mold life far below TDS rating (e.g. <30 casts for resin) | Abrasive casting material, high exotherm resin, or cure inhibition shortening fatigue life | Use Shore A 30-40 for concrete; pre-cure exothermic PU in thin layers; never cast resin above 80 °C into silicone |
| Bubbles in thick section of tin-cure mold | Alcohol byproduct trapped; mold overheated | Keep thickness <30 mm per pour or switch to addition cure; never heat tin-cure molds above 60 °C |
Frequently Asked Questions
How do I choose Shore A hardness for resin casting molds?
Use Shore A 20-30 as the all-round range for epoxy, polyester, and PU resin; it balances detail reproduction (above 20A) with easy demold of shallow undercuts. Drop to Shore A 10-15 when the master has deep undercuts or fine re-entrant features, and step up to Shore A 30-40 only for abrasive casting materials like concrete or for thin-walled brush-on molds that need to resist tearing.
Why is my platinum-cure silicone sticky next to the master?
The platinum catalyst is poisoned — this is called cure inhibition. Common culprits are sulfur-containing modeling clay (plasticine/plastiline), residues of tin-cured silicone, amine-cured epoxy, and many SLA 3D-print resins. Either switch the master to a non-sulfur clay, barrier-coat SLA masters with a compatible clear lacquer, or move to a condensation (tin) cure system, which is not inhibited by sulfur.
How long does an RTV-2 silicone mold actually last?
Addition-cure (platinum) molds typically deliver 100-300+ resin casts, while condensation-cure (tin) molds deliver 20-50 casts against the same resin. Wax, soap, and gypsum are much less aggressive and can extend mold life well beyond those numbers; concrete and other abrasive fillers shorten it. Store molds flat, away from UV, and avoid temperatures above 60 °C to preserve tear strength (typically 15-30 kN/m, ASTM D624 die B).
Conclusion: Specify Once, Cast Hundreds of Times
The right RTV-2 system turns molding from a recurring cost into a repeatable process: pick addition cure for resin and food-contact, condensation cure for sulfur-clay masters and concrete; match Shore A and viscosity to the casting material; vacuum-degas and pour from height; and keep an eye out for inhibition, bubbles, and tears. Send us your master description, casting material, and target part size, and our engineering team will spec a custom RTV-2 grade — hardness, viscosity, pot life, and cure system — within one business day.
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