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. Silicone rubber is a broad material family, and the cure chemistry is the first fork in the road when selecting a product.
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)
Smooth-On’s technical documentation covers inhibition testing procedures and lists known problem materials across their product line — worth bookmarking before you start a new project.
Shore A Hardness Selection
Shore A hardness determines how flexible and tear-resistant the cured mold is. The mold making process generally rewards choosing the softest hardness that still gives you an acceptable mold life for your production volume.
| 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.
Frequently Asked Questions
What is the difference between tin-cure and platinum-cure silicone rubber? Tin-cure (condensation-cure) silicones use a tin catalyst, cure through atmospheric moisture reaction, cost 30–50% less than platinum-cure, have slight 1–3% shrinkage, a shorter shelf life of 12–18 months, and are less inhibition-sensitive. Platinum-cure silicones have no shrinkage, better tear resistance, longer shelf life up to 24 months, but are sensitive to inhibition from sulfur, latex, and certain other materials.
What Shore A hardness range is recommended for most B9 robot and prop work? Shore A 25–35 is described as ’the prop-building sweet spot’ for most B9 components, figurative heads, and small props. Shore A 5–20 is for extreme undercuts and complex figurative molds, while Shore A 40–60 is for flat molds, tiles, and architectural elements where long mold life is the priority.
What inhibition-causing materials must be kept away from platinum-cure silicone? Sulfur-containing modeling clays (Roma Plastilina No. 1 and 2), latex gloves, some wood treatments and stains, and some epoxy resins especially when freshly mixed can all prevent the platinum catalyst from working. Chavant NSP Medium and Hard are sulfur-free and safe to use, while NSP regular contains sulfur.
What Smooth-On products are recommended for standard prop mold work? Mold Max 20 (Shore A 20, platinum-cure) is described as the most popular prop-building silicone with a 30-minute pot life, 24-hour cure, and excellent detail capture. Mold Max 30 (Shore A 30) is better for flat-faced molds and larger production runs. OOMOO 25 and 30 are budget tin-cure options for practice and low-precision molds.
How is silicone volume calculated for a mold project? Silicone volume is calculated by measuring the mold box interior volume in cubic inches, subtracting the master’s estimated volume, then converting to pounds at approximately 1.1–1.2 lbs per cubic inch depending on the product. A 10–15% safety margin is added because running short during a pour means a ruined mold with no recovery option.
What are brush-applied silicones and when should they be used? The Rebound series (Smooth-On Rebound 25 or 40) are thixotropic brush-applied silicones used for very large molds where pouring a block mold would require impractical amounts of rubber. They are built up in 2–3 layers on the master and require a rigid mother mold (fiberglass or hard plaster) to support the thin silicone skin during casting.