Silicones for epoxy casting resin

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Silicones for epoxy casting resin

Epoxy is one of the most aggressive materials you can pour into a silicone mold. The catalyst and the heat that epoxy generates as it cures attack the silicone: the mold first loses its gloss, then the surface starts to "burn" in places and the silicone becomes brittle and crumbly there. Every silicone mold therefore has a limited mold life with epoxy. According to Wacker, heat-curing and unfilled epoxies are much more aggressive than cold-curing, highly filled systems. Fast resins, large casting volumes and a long contact time in the mold also cost you castings. This collection only contains silicones that the manufacturer itself recommends for casting resins, or that we list with a clear caveat.

Tin or platinum: which should you choose for epoxy?

Condensation-cure silicones (tin) are not affected by inhibition, so you can use them to mold resin 3D prints, sulfur-containing clay or a model that has not fully cured without any worries. For epoxy, the catalyst is decisive: Wacker specifies Catalyst T 21 for epoxy and polyurethane. Wacker advises post-curing tin-cure molds for epoxy for a few hours at approx. 150 °C before first use. Drawbacks: slightly more shrinkage (0.2 to 0.4%), demolding only after approx. 24 hours, and the mold becomes stiffer after long storage. Not suitable for food contact.

  • Wacker Elastosil M 4514 with catalyst T 21: Shore A 25, tear strength above 25 N/mm and 450% elongation. Wacker positions the combination with T 21 specifically for casting epoxy and PU. Please note: the complete M 4514 set is supplied with T 51, which is intended for polyester. For epoxy, order the A component with T 21 separately.
  • Silicone Condensation 20 Resin: affordable tin-cure silicone, supplied with catalyst T 21 for epoxy and PU.

With tin-cure silicones, the catalyst determines the resistance. With the same silicone, one catalyst gives you a mold that lasts a long time with resin, and another gives you a mold that mainly cures quickly. Wacker specifies T 21 for epoxy and T 51 for polyester. With the Dow Silastic RTV 3481, too, the catalyst determines the result: RTV-3081 R is tailored to polyester, RTV-3081 F to speed. Dow does not list epoxy as a casting material for the 3481, which is why it is not in this collection. You can read more about this under silicones for aggressive resins.

Private or business customer? All Wacker tin catalysts, including T 21, contain dioctyltin compounds. Because of the European REACH rules for these compounds in two-component casting rubbers, Catalyst T 21 and our own Silicone Condensation 20 Resin can only be ordered from us by business customers. If you are a private customer casting epoxy, choose one of the platinum silicones below. They contain no tin, are available to everyone and are the most commonly used choice for epoxy anyway.

Addition-cure silicones (platinum) are shrink-free (less than 0.1%), cure faster, can be used immediately after demolding, remain stable on the shelf for years and are available in transparent versions. That makes them the standard for dimensionally accurate prototypes, vacuum casting and clear epoxy. When molding your model, watch out for inhibition caused by sulfur, latex and some 3D prints, among other things.

  • Wacker Elastosil M 4641: transparent, Shore A 43. Wacker describes it as highly resistant to PU and epoxy and particularly suitable for crystal-clear casting resins, because the mold surface is oil-free. The choice if you cast clear epoxy.
  • Wacker Elastosil M 4644: transparent, Shore A 40, tear strength above 25 N/mm. Sweats out a little oil, which improves release; clean castings thoroughly before painting or bonding them. Wacker positions it for vacuum casting of 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, according to Wacker, excellent resistance to casting resins. A strong choice for molds with many undercuts.
  • Wacker Elastosil M 4670: Shore A 55, sturdy mold for dimensionally accurate reproductions in epoxy, polyester and PU. Fairly viscous (80,000 mPa·s), so degassing under vacuum is recommended.
  • QM 270: Shore A 62, very dimensionally stable and resistant to epoxy and polyester. For hard, dimensionally accurate molds such as moldings and architectural elements. Available on a quotation basis.
  • 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 describes them as resistant to common casting resins. Popular for jewelry and small decorative objects in epoxy.
  • PlatSil HTS-25 and HTS-40: very high tear strength and elongation, made for production molds. Polytek explicitly lists epoxy as a casting material and recommends a release agent for longer mold life.
  • PlatSil FS-10 and FS-20: fast curing (demold after approx. 25 minutes) for short turnaround times. For very soft molds: PlatSil FS-00. Here too, Polytek recommends a release agent with epoxy.

How to get more castings out of your mold

  • Choose a slow epoxy and pour thick pieces in layers. The less heat, the less attack.
  • Work as cool as possible and demold as soon as the casting allows. The longer the epoxy stays in the mold, the more damage.
  • Use a release agent, especially for the first castings.
  • Let the silicone cure fully before you pour for the first time. Wacker advises post-curing tin-cure molds first, see above.
  • Let the mold cool down and rest between pours. Wacker advises heating molds that are not in use to 100 to 150 °C, so that absorbed resin components are driven out. If you work in series, rotate between several molds.
  • Check the mold regularly. If the surface becomes dull, burnt or crumbly, replace the mold in good time.

Related

Not sure which silicone suits your resin and application? Get in touch with us and we will help you choose.