Are you planning your first mold-making project using silicone—or looking for a better solution for your mass production? This guide explains how to choose the right silicone, avoid mistakes, and achieve professional results.
Why use silicone for mold making?
Silicone is the dominant material in mold making—and for good reason. It offers a unique combination of flexibility, precision, temperature resistance, and durability that no other mold-making material can match.
Compared to polyurethane molds, plaster molds, or 3D-printed molds, silicone offers significant 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 castings with hot metals, wax, and reactive resin systems.
Addition cross-linking or condensation cross-linking?
The most important decision in mold making is choosing the curing system. Both have clear advantages and disadvantages:
Addition-curing silicones (platinum-catalyzed)
- Schrumpf: Minimal (<0,1 %). Höchste Masstreue – entscheidend für Präzisionsteile
- Food contact: Permitted (compliant with FDA and EU Regulation 1935/2004)
- 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-curing systems
- Reliability: No inhibition issues – works on almost all substrates
- Shelf life: Lower long-term stability (continues to shrink 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 a factor.
Choosing the Right Shore Hardness
Shore hardness determines the flexibility of the mold and, consequently, which geometries can be demolded:
| Shore A | property | Application | recommendation |
|---|---|---|---|
| 00–10 | Very soft, highly elastic | Deep undercuts, body molding | SILISIL MF-Soft 00 |
| 12–20 | Soft, flexible | Standard shapes, 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 a PVA release agent. Caution regarding addition-curing silicones: Models made of sulfur-containing modeling clay or latex can inhibit curing. If in doubt, test on a small area first.
2. Build the formwork
Molds made of wood, MDF, acrylic glass, or Lego. Seal the edges with hot glue or modeling clay. The gap between the model and the mold determines the wall thickness of the mold—at least 10–15 mm for sturdy molds.
3. Mix the silicone
Strictly adhere to the mixing ratio (usually 10:1 or 1:1 by weight, depending on the product). Mix thoroughly (2–3 minutes), scraping down the sides and bottom of the container. For bubble-free results: vacuum degassing (2–5 minutes at –0.9 bar) or apply a thin coat and pour from a height.
4. Pouring and Curing
Pour in a thin stream into the deepest part. Pouring slowly allows air bubbles to rise. Curing time varies by product: 4–24 hours at room temperature. Heat (40–60 °C) accelerates cross-linking in addition-curing systems.
5. Removing from the mold
Carefully remove the mold from the model. Allow the silicone to cure for 24 hours before use. For best results, “condition” the mold before the first casting—make a test cast and discard it.
Common Mistakes and How to Avoid Them
Silicone does not cure (inhibition)
The most common problem with addition-curing silicones. Causes: Sulfur (plasticine, latex), amines (certain epoxies, fresh PU parts), tin (residues from condensation-curing silicones). Solution: Seal the surface with shellac or acrylic lacquer, or switch to a condensation-curing silicone.
Air bubbles in the mold
Causes: Pouring too quickly, insufficient degassing, viscosity too high. Solution: Vacuum degassing, choose a thinner silicone type, or use the “brushing method”—first brush a thin layer of silicone onto the model, then fill it.
The mold cracks after just a few castings
Causes: Insufficient tear strength, Shore hardness of the casting material too low, demolding too aggressive. Solution: Select a higher Shore hardness, use a release agent (especially with PU resins), reconsider the mold design.
SILISIL Molding Silicones: Our Product Range
Our own brand, SILISIL, was developed specifically for professional users—with six product lines designed to meet a variety of needs:
- MF Series (Mold making, addition-curing): Shore 00 to 70, for all mold making applications
- PC Series (Condensation-curing): Economical, robust, for large molds
- PP Series (Platinum, food-safe): 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 available from our Swiss warehouse in Gümligen near Bern. Technical advice, samples, and custom packaging are available upon request.
