Silicones for polyurethane casting resin Polyurethane (PU) casting resin is one of the more demanding materials to cast in a silicone mold. The isocyanates in the resin attack silicone over time: the surface turns dull, the resin starts to stick and fine details disappear. That is why every silicone mold...
Silicones for polyurethane casting resin (23)
Silicones for polyurethane casting resin
Polyurethane (PU) casting resin is one of the more demanding materials to cast in a silicone mold. The isocyanates in the resin attack silicone over time: the surface turns dull, the resin starts to stick and fine details disappear. That is why every silicone mold has a limited lifespan with PU, and the silicone you choose determines how many castings you get out of a mold. This collection only contains silicones that the manufacturer itself recommends for casting resins, or that we list with a clear note.
Tin or platinum: which one for PU?
Condensation silicones (tin-cure) are in practice often the most robust against PU and do not suffer from cure inhibition. You can safely mold resin 3D prints, sulfur-containing clay or a PU master that has not fully cured. Downsides: slightly more shrinkage (0.2 to 0.4%), demolding only after about 24 hours, and the mold stiffens after long storage. Not suitable for food contact.
- Dow Silastic RTV 3481 R: Shore A 19, tear strength 26 N/mm, 622% elongation and outstanding release. Dow lists polyurethane and polyester resins as typical casting materials; the supplied R catalyst was developed for longer mold life with polyester. A robust all-rounder for PU and polyester.
- Wacker Elastosil M 4514 with catalyst T 21: Shore A 25. Wacker specifically positions the combination with T 21 for casting PU resins.
- Silicone Condensation 20 Resin: an affordable tin-cure silicone, supplied with catalyst T 21 for PU and epoxy.
- Wacker Elastosil M 4440: Shore A 37, for cost-effective molds with few undercuts. Its low tear strength makes it less suitable for complex shapes.
Private or business customer? All Wacker tin catalysts, including T 21 and T 37, contain dioctyltin compounds. Because of the European REACH rules for these compounds in two-component casting rubbers, Catalyst T 21, the M 4440 set and our own Silicone Condensation 20 Resin can only be ordered from us by business customers. The Dow RTV-3481 sets use a different tin compound and are available to private customers. If you are a private customer casting PU and want a tin-cure mold, the Dow Silastic RTV 3481 R is the right choice. Prefer platinum? The addition-cure silicones below contain no tin and are available to everyone.
Addition silicones (platinum) cure without shrinkage (less than 0.1%), cure faster, stay stable on the shelf for years and are available in transparent grades. That makes them the standard for dimensionally accurate prototypes and vacuum casting. When molding your master, watch out for inhibition caused by sulfur, latex and some 3D prints, among others.
- Wacker Elastosil Vario 15 and Vario 40 with Catalyst Vario: hardness adjustable between Shore A 15 and 40, also available as a fast version. Wacker lists them as resistant to common casting resins, and in our own experience they hold up well against PU.
- Wacker Elastosil M 4641: transparent, Shore A 43, the classic for vacuum casting PU prototypes.
- Wacker Elastosil M 4644: transparent, Shore A 40, tear strength above 25 N/mm. It bleeds a small amount of oil, which improves release. Wacker positions it for vacuum casting PU and epoxy with a long mold life.
- Wacker Elastosil M 4642 (Shore A 37) and M 4601 (Shore A 28): high tear strength (30 N/mm) and good resistance to casting resins.
- Wacker Elastosil M 4670: Shore A 55, a firm mold for dimensionally accurate reproductions in epoxy, polyester and PU. Fairly viscous (80,000 mPa·s), so vacuum degassing is recommended.
- PlatSil FS-10 and FS-20: fast curing (demold after about 25 minutes) for short turnaround times. For very soft molds: PlatSil FS-00.
- PlatSil HTS-25 and HTS-40: very high tear strength and elongation, made for production molds that need to last many castings.
Zhermack and polyurethane
Of the Zhermack silicones, Zhermack explicitly lists ZA 10 Orange as compatible with polyurethane. ZA 13 Mould WT 45 and ZA 22 Mould were primarily developed for concrete, plaster and artificial stone. For PU, Zhermack prescribes a release agent to protect the mold. For PU, Zhermack has developed two dedicated products: ZA 43 PU, designed specifically for casting PU resin and PU foam, and XTX 45 Oil Bleeding, a transparent oil-bleeding silicone for better release of PU. Interested? Contact us. The full Zhermack range can be found under Zhermack silicones.
How to get more castings out of your mold
- Use a release agent, especially for the first castings and with aggressive resins.
- Work dry: PU reacts with moisture and starts to foam. Make sure the mold and tools are dry.
- Let the mold cool down and rest between castings. A lot of heat (thick castings, fast resins) speeds up degradation.
- Let the silicone cure fully before the first casting.
- Check the mold regularly. If the surface turns dull or tacky, replace the mold in time.
Related
- Polyurethane casting resin
- Silicones for aggressive resins (epoxy, polyurethane, polyester)
- Silicones for epoxy casting resin and silicones for polyester casting resin
- All condensation silicones and rapid prototyping
Not sure which silicone suits your resin and application? Contact us and we will help you choose.