Choosing the wrong silicone rubber for a mold project is an easy mistake to avoid once you understand the variables. The wrong Shore hardness means you can’t demold your master, or your mold tears after a few pulls. The wrong chemistry means the silicone never cures properly against your master material. This guide covers the selection criteria and gives product-level recommendations for prop and replica work.
The Two Chemistry Types
All silicone rubbers are either tin-cure (condensation cure) or platinum-cure (addition cure). The chemistry affects cure properties, compatibility, and shelf life.
Tin-Cure Silicone
Tin-cure silicones (also called condensation-cure) use a tin catalyst. They cure through a reaction with atmospheric moisture. Key characteristics:
- Less expensive — typically 30–50% cheaper than comparable platinum-cure products
- Slight shrinkage — 1–3% dimensional shrinkage during cure. For precise reproduction work this matters; for most prop molds it’s acceptable
- Shorter shelf life — typically 12–18 months from manufacture
- Less inhibition-sensitive — fewer materials interfere with cure
Tin-cure silicone is fine for simple molds where dimensional precision isn’t critical and you’re working on a budget. For detailed prop work where you’re spending significant time on the master, platinum-cure is worth the cost difference.
Platinum-Cure Silicone
Platinum-cure silicones use a platinum catalyst in a two-part system (Part A + Part B). They cure through a chemical addition reaction.
- No shrinkage — excellent dimensional accuracy
- Longer shelf life — typically 12–24 months
- Better tear resistance — at equivalent Shore A, platinum-cure silicones generally tear less easily than tin-cure
- Inhibition-sensitive — certain materials prevent the platinum catalyst from working (sulfur, some amines, tin compounds, latex). Uncured or sticky spots on a finished mold are usually inhibition
The inhibition issue is the main gotcha with platinum-cure. Always test new master materials against your silicone before committing to a full mold — press a small amount of your silicone against the master material and check for cure after 24 hours.
Inhibition-causing materials to watch for:
- Sulfur-containing modeling clays (Roma Plastilina No. 1 and 2, Chavant NSP contain sulfur — Chavant NSP Medium and Hard are sulfur-free)
- Latex gloves — cure inhibition from hand contact is a real problem
- Some wood treatments and stains
- Some epoxy resins (especially when freshly mixed or partially cured)
Shore A Hardness Selection
Shore A hardness determines how flexible and tear-resistant the cured mold is.
| Shore A | Character | Best For |
|---|---|---|
| 5–10 | Very soft, extremely stretchy | Masters with extreme undercuts; small jewelry pieces |
| 15–20 | Soft, very flexible | Complex figurative molds, significant undercuts |
| 25–35 | Medium, the prop-building sweet spot | Most B9 components, figurative heads, small props |
| 40–60 | Firm, minimal stretch | Flat molds, tiles, architectural elements, long mold life |
For the B9 robot parts and similar prop pieces, Shore A 20–30 is the working range for most applications.
Product Guide
Smooth-On Products
Mold Max 10 (Shore A 10, platinum-cure) — Very soft. Use for extreme undercuts on small masters. Tears easily; not suitable for production molds.
Mold Max 20 (Shore A 20, platinum-cure) — The most popular prop-building silicone. Mix ratio 100A:10B by weight. Pot life 30 min, cure 24 hours. Excellent detail capture, manageable flexibility for most prop geometries.
Mold Max 30 (Shore A 30, platinum-cure) — Firmer than MM20. Better for flat-faced molds and large production runs. Longer mold life.
Dragon Skin 10 Medium (Shore A 10, platinum-cure) — Soft and stretchy. Used for skin-effect molds and extreme undercut situations.
Dragon Skin 20 (Shore A 20, platinum-cure) — Similar to Mold Max 20 in hardness but with a different handling character. Some builders prefer its slightly longer working time.
OOMOO 25 and 30 (tin-cure) — Budget-friendly entry-level silicones. Good for practice and for molds where dimensional precision isn’t critical. Shelf life is shorter.
Polytek Products
Poly 74-20 (Shore A 20, platinum-cure) — Comparable to Mold Max 20. Mix ratio 1:1 by weight.
Poly 74-30 (Shore A 30, platinum-cure) — Firmer, for flat molds and longer runs.
Rebound Series (Brush-Applied)
For very large molds where pouring a block mold would require impractical amounts of rubber, brush-applied silicones (Smooth-On Rebound 25 or 40) let you build up the mold in layers. Apply by brush in 2–3 layers, allowing each to gel before the next.
Brush-on molds require a rigid mother mold (fiberglass or hard plaster) to support the thin silicone skin during casting.
How Much Silicone Do You Need?
Calculate the volume of the mold box (length × width × height in inches), subtract the volume of the master (estimate for irregular shapes), and convert to pounds: silicone weighs approximately 1.1–1.2 lbs per cubic inch depending on the product.
Add 10–15% safety margin. Running short during a pour means a ruined mold.
For practical guidance on using these materials, see the complete mold-making guide and the two-part mold walkthrough.