Are you planning your first mold-making project with silicone – or looking for a better solution for your series production? This guide explains how to choose the right silicone, avoid mistakes, and achieve professional results.
Why silicone for mold making?
Silicone is the dominant material in mold making – and for good reason. It offers a unique combination of flexibility, detail accuracy, temperature resistance , and durabilitythat no other mold material can match.
Compared to polyurethane molds, plaster molds, or 3D-printed molds, silicone offers decisive advantages: It is self-releasing (no release agent is needed in most cases), can withstand hundreds to thousands of castings , and reproduces even the finest surface details. Its temperature resistance from -60 °C to +250 °C allows for casting with hot metals, wax, and reactive resin systems.
Addition crosslinking or condensation crosslinking?
The most important decision in mold making is the choice of meshing system. Both have clear advantages and disadvantages:
Addition-curing silicones (platinum-catalyzed)
- Shrinkage: Minimal (
- Food contact: Possible (FDA/EU 1935/2004 compliant)
- Pot life: Adjustable from minutes to hours (using inhibitors/accelerators)
- Sensitive to: sulfur, amines, tin, latex – inhibition possible!
- Typical products: SILISIL MF series, Bluesil RTV 3428, Bluesil RTV 3450
Condensation-curing silicones (tin-catalyzed)
- Shrinkage: Higher (0.3–0.8%). For non-dimensionally stable applications.
- Price: Significantly cheaper than addition-based systems
- Robustness: No inhibition problems – works on almost all substrates
- Durability: Lower long-term stability (shrinks over several months)
- Typical products: SILISIL PC series, Bluesil RTV 3325
Rule of thumb: Addition-curing for precision, food contact, and mass production. Condensation-curing for large molds, one-off castings, and when budget is the deciding factor.
Choosing the correct Shore hardness
The Shore hardness determines the flexibility of the mold and thus which geometries can be demolded:
| Shore A | Characteristic | Application | Recommendation |
|---|---|---|---|
| 00–10 | Very soft, highly elastic | Deep undercuts, body casting | SILISIL MF-Soft 00 |
| 12–20 | Soft, flexible | Standard forms, sculptures, architectural models | SILISIL MF-Flex 20 |
| 25–35 | Medium-firm | Multi-part molds, casting resins, concrete | SILISIL MF-Dura 30 |
| 40–50 | Hard, abrasion-resistant | Mass production, abrasive casting materials | SILISIL MF-Ultra 45 |
| 50–70 | Very hard | Printing plates, screen printing, rollers | SILISIL MF-Ultra 60 |
Step by step: Making a silicone mold
1. Prepare the model
The original must be clean, dry, and free of grease. For porous materials (plaster, wood, clay), seal the surface with shellac or PVA release agent. Caution with addition-curing silicones: models made of sulfur-containing plasticine or latex can inhibit curing. If in doubt, test on a small area.
2. Build the frame
Mold frame made of wood, MDF, acrylic glass, or Lego. Seal with hot glue or modeling clay. The distance between the model and the frame determines the wall thickness of the mold – at least 10–15 mm for stable molds.
3. Mix silicone
Adhere to the mixing ratio precisely (usually 10:1 or 1:1 by weight – depending on the product). Mix thoroughly (2–3 minutes), scraping the sides and bottom of the container. For bubble-free results: vacuum degas (2–5 minutes at -0.9 bar) or apply a thin coat and pour from a height.
4. Pouring and hardening
Pour in a thin stream at the lowest point. Pouring slowly will cause air bubbles to rise. Curing time varies depending on the product: 4–24 hours at room temperature. Heat (40–60 °C) accelerates the curing process for addition-curing systems.
5. Demolding
Carefully separate the mold from the model. Allow the silicone to cure for 24 hours before use. For optimal results, "condition" the mold once before the first casting – perform a test casting and discard it.
Common mistakes and how to avoid them
Silicone does not harden (inhibition)
The most common problem with addition-curing silicones. Causes: Sulfur (plasticine, latex), amines (certain epoxies, fresh PU parts), tin (residues of condensation-curing silicones). Solution: Seal the surface with shellac or acrylic lacquer, or switch to condensation-curing silicone.
Air bubbles in the mold
Causes: Pouring too quickly, insufficient degassing, excessive viscosity. Solution: Vacuum degassing, choosing a thinner silicone type, or the "painting method" – first brush a thin layer of silicone onto the model, then fill it in.
The mold cracks after only a few castings
Causes: Insufficient tear resistance, Shore hardness too low for the casting material, demolding process too aggressive. Solution: Choose a higher Shore hardness, use a release agent (especially with PU resins), reconsider the mold parting line.
SILISIL mold-making silicones: Our product range
Our own brand SILISIL was specially developed for professional users – with six series for different requirements:
- MF series (mold making, addition-curing): From Shore 00 to 70, for all mold making applications
- PC series (condensation-curing): Economical, robust, for large forms
- PP Series (Platinum, food-grade): FDA-compliant, for food molds and silicone baking molds
- PRO Series (Professional Casting): Optimized for casting applications with minimal bubble formation
All SILISIL products are from our Swiss warehouse in Gümligen near Bern . Technical advice, samples and special packaging available upon request.
