Choosing Casting Silicone — Tin vs Platinum Cure, Shore Hardness, Cure Inhibition
The difference between tin-cure and platinum-cure silicone, how to pick a Shore A hardness, food and skin safety, and why platinum silicone sometimes refuses to cure.
Not all silicone is the same. Pick the wrong one and it will not cure, is too hard to release, or shrinks months later. Two-part casting silicone comes in two families.
Tin-cure (condensation)
- Pros: cheap, almost immune to cure inhibition (cures on nearly any surface), forgiving for beginners
- Cons: alcohol leaves as it cures so it shrinks 0.5–1 %, keeps curing and shrinking slowly over time, shorter mold life (dozens of pulls)
- Use for: practice, parts where dimensions are not critical, one-off molds
- Catalyst ratios of 10:1 to 20:1 mean a small amount of catalyst — weighing errors matter. Use a 0.1 g scale.
Platinum-cure (addition)
- Pros: negligible shrinkage (under 0.1 %), high clarity, strong tear resistance, food- and skin-safe grades exist, long mold life (hundreds of pulls)
- Cons: more expensive, sensitive to cure inhibition
- Use for: figures, food molds (chocolate, soap, candles), skin-contact products, precise replication
- Usually a 1:1 ratio, easy to measure.
Cure inhibition — why platinum silicone stays sticky
Contact with any of these leaves that area uncured and tacky:
- Sulfur — sulfur-based modelling clay, some latex gloves, rubber bands
- Tin — molds or tools that touched tin-cure silicone
- Amines — some epoxy and polyurethane hardeners, uncured 3D-printing resin
- Some PLA pigments and additives — rare, but reported with specific coloured filaments
Hardness (Shore A)
| Shore A | Feel | Use |
|---|---|---|
| 00-30 to A5 | Jelly, very soft | Squishies, stress toys, special-effects skin |
| A10–A20 | Softer than an eraser | Detailed figure molds, shapes with undercuts — the all-rounder |
| A25–A35 | Eraser | Soap, chocolate and candle molds, simple shapes, long-life molds |
| A40+ | Firm rubber | Concrete and plaster molds, large flat parts |
Molds from this site work both when the cast itself is silicone (skins, squishies) and when the mold is silicone (cast a silicone mold first, then pour resin into it). For a silicone cast, pick the feel you want; for a silicone mold, A15–A25 is a safe choice.
Food and skin contact
- Food molds (chocolate, gummies, ice) need food-contact grade platinum silicone (FDA 21 CFR 177.2600, LFGB, etc.).
- Products that touch skin (squishies, toys) need a skin-safe rated product. Selling to children may require separate safety certification in your market.
- Do not use tin-cure for food or skin applications.
Pot life and cure time
- Pot life: how long you can pour after mixing. 10–40 minutes is typical; choose longer for big or complex shells.
- Cure time: 4–24 hours. Ten degrees colder nearly doubles it — keep it warm in winter.
- Fast-cure (1–2 h) products are convenient but their short pot life leaves little time to degas.
How much silicone
When you download an STL from the mold generator it tells you the silicone weight needed (cavity volume × specific gravity 1.1). Add 10–15 % for waste.
Related: sprues and vents · troubleshooting
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