Platinum Cure Silicone Rubber for Concrete & Building Mold Making: 2026 Spec, Hardness & Cost Guide

Platinum cure (addition cure) RTV-2 silicone is the standard mold material for concrete, gypsum and building-product casting because it shrinks less than 0.1%, releases cleanly from cement slurry and reproduces stone texture down to fine grain. For most building molds, use Shore A 20–35, mix viscosity 8,000–20,000 cps, 1:1 mix ratio by weight, 3–4 hours cure at 25 °C, and expect 50–200+ demolding cycles per mold depending on abrasive load.

Building molds are abused: abrasive cement, alkaline slurry, heavy stone blanks dropped in and out every cycle. Three properties decide whether a mold survives 30 cycles or 300.

• Shrinkage: addition cure 0.1% or less vs 0.3–0.6% for tin cure (condensation cure). On a 600 mm culture stone mold that difference is 2–4 mm of drift — enough to lose the interlocking tolerance between panels.
• No by-product: tin cure releases alcohol during cure, which leaves a film that transfers to the first casts. Platinum cure has no by-product, so the first pull is already clean.
• Tear and tensile strength: platinum cure typically delivers 25–40 kN/m tear strength, which is what keeps thin undercut edges on brick-face and wood-grain textures alive.

The trade-off is inhibition sensitivity: sulfur, amine-cured epoxy, certain clays and latex gloves will stop platinum silicone from curing. Sections 6 and 7 cover the fix.

1. Seal the master. Concrete, gypsum and wood positives are porous. Apply a sealer, then a release agent. Unsealed gypsum will absorb silicone and produce a chalky, short-lived mold.
2. Check for inhibitors. Sulfur-based clay, tin-cure residue, amine epoxy, some 3D-print resins and latex gloves block platinum cure. Test a 50 g cup pour for 4 hours before committing a full batch.
3. Weigh 1:1 by weight. Part A is pigment-filled and Part B is the catalyst — volume mixing drifts the ratio and leaves tacky spots.
4. Degas. Vacuum to 1–3 mbar for 2–3 minutes. Air bubbles at the texture face become pinholes in every cast.
5. Pour in a thin stream from one corner so air is pushed ahead of the flow, not trapped under it. For vertical faces, use thixotropic or high-viscosity grade.
6. Demold at 3–4 h, use at 24 h. Early demolding is fine for a thin skin mold but full tear strength needs a full day.

Volume of silicone (cm³) = mold length * width * average wall thickness (typically 5–10 mm for backed molds, 15–25 mm for free-standing block molds). Mass (g) = volume * 1.1–1.2 g/cm³.

Worked example: a 500 * 300 * 20 mm block mold = 3,000 cm³ ≈ 3.3–3.6 kg of silicone. At 150 demolding cycles per mold and 4 casts per hour, silicone cost per cast usually lands between USD 0.15 and USD 0.45 — which is why the hardness and tear-strength choice matters more than the raw material price.

Yes. Cured platinum silicone is chemically inert to cement slurry and gypsum. The limiting factor is abrasion and the alkalinity of the mix, not chemical attack. Expect 30–80 cycles for high-abrasion dry-cast concrete, 150–300 cycles for gypsum decorative work.

Inhibition, in this order of likelihood: sulfur or latex contamination, amine-cured epoxy or 3D-print resin master, wrong mix ratio, or workshop below 15 °C. Warm the material to 25 °C and re-test on a clean cup sample.

Platinum, unless the master contains sulfur clay or tin residue. Platinum gives a longer service life and a cleaner first cast; tin cure remains useful for polyester and resin casting where platinum is inhibited.

At an average 3.5 kg per 500 * 300 mm block mold, a 20 kg kit yields roughly 5–6 molds.

Yes — clean with isopropyl alcohol, roughen the area, and patch with fresh 1:1 silicone. Repaired molds typically retain 70–90% of original cycle life at the patch.

Hanast has manufactured RTV-2 platinum cure silicone since 2010 at its 40,000 m² plant in Shenzhen, exporting 70–80% of output to industrial mold makers. Grades cover Shore A 0–50 and mix viscosity 800–20,000 cps, with 3–4 hour cure and free samples for mold trials. Send your mold dimensions, texture depth and substrate (concrete / gypsum / resin) and we will return a hardness and viscosity recommendation with a per-mold consumption calculation.