Silicones for polyester casting resin

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Silicones for polyester casting resin

Polyester casting resin is hard on a silicone mold. The styrene in the resin penetrates the silicone: the mold swells, the surface turns dull and over time the mold becomes hard and brittle. On top of that comes the heat polyester generates as it cures, especially in thick castings. According to Wacker, the higher the styrene content and the curing temperature, the stronger the attack. Every silicone mold therefore has a limited mold life with polyester. The type of polyester, the volume and the time the resin stays in the mold determine how many castings you get. 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 polyester?

Condensation-cure silicones (tin) are the classic choice for polyester. The catalyst really makes a difference here: with Catalyst T 51, Wacker offers a catalyst specifically tailored for resistance to polyester resins. Tin-cure silicones do not suffer from inhibition, so you can also use them to mold a polyester or filler model that has not yet fully cured. Drawbacks: slightly more shrinkage (0.2 to 0.4%), demolding only after 8 to 24 hours, and the mold becomes stiffer after long storage. Not suitable for food contact.

  • Wacker Elastosil M 4514 set with catalyst T 51: Shore A 25, tear strength above 25 N/mm and 450% elongation. Wacker positions this combination specifically for casting polyester resins. If you already have catalyst, you can also order the separate A component with T 51. Our first choice for polyester.
  • Silicone Condensation 20 Polyester: affordable tin-cure silicone, Shore A 20, supplied with catalyst T 51. Good for anyone who casts polyester regularly and keeps an eye on the cost per mold.
  • Dow Silastic RTV 3481 R: Shore A 19, tear strength 26 N/mm, 622% elongation and excellent release. We supply it with the R catalyst (RTV-3081 R), which Dow developed for longer mold life with polyester casting resin.
  • Wacker Elastosil M 4440 with catalyst T 37: Shore A 37, for affordable molds with few undercuts. Due to its low tear strength (4.5 N/mm), less suitable for complex shapes.

With tin-cure silicones, the catalyst determines the resistance. With the same base, one catalyst gives you a mold that lasts a long time with polyester, and another gives you a mold that mainly cures quickly. The Dow Silastic RTV 3481 shows this clearly: with RTV-3081 R (resin resistant) you get Shore A 19 and a mold that Dow has specifically tailored for longer mold life with polyester casting resin. With RTV-3081 F (fast) the mold can be demolded after approx. 6 hours, but that catalyst is designed for speed and not tailored to resins. So for polyester, choose the 3481 R and not the 3481 Fast. Wacker works the same way: T 51 for polyester, T 21 for epoxy and PU. You can read more about this under silicones for aggressive resins.

Private or business customer? All Wacker tin catalysts, including T 51 and T 37, contain dioctyltin compounds. Because of the European REACH rules for these compounds in two-component casting rubbers, the M 4514 set, the M 4440 set, Catalyst T 51 and our own Silicone Condensation 20 Polyester 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 polyester 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-cure silicones (platinum) are shrink-free (less than 0.1%), cure faster, remain stable on the shelf for years and are available in transparent versions. If you choose platinum, watch out for inhibition when molding your model: sulfur, latex, some 3D prints and not fully cured polyester or filler, among others, can disrupt curing. When in doubt, do a test first.

  • 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 polyester, epoxy 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 polyester and epoxy. For hard, dimensionally accurate molds such as moldings and architectural elements. Available on quotation.
  • PlatSil HTS-25 and HTS-40: very high tear strength and elongation, made for production molds. Polytek explicitly names polyester 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. Here too, Polytek recommends a release agent with polyester.
  • 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. Handy if you want to see through the mold.
  • Wacker Elastosil M 4641: transparent, Shore A 43. Wacker describes it as resistant to casting resins, with the emphasis on PU and epoxy and vacuum casting of prototypes. If you mainly cast polyester, M 4514 with T 51 is the more logical choice.
  • Silicone Addition Green 30 (Shore A 27) and Yellow 25 (Shore A 22): our affordable platinum silicones for small series. Yellow 25 contains silicone oil that can bleed out slightly, so clean castings thoroughly before painting or bonding them.

How to get more castings from your mold

  • Use a release agent, especially for the first castings and with platinum silicones.
  • Do not dose the polyester's catalyst higher than prescribed. Too much catalyst produces more heat, and heat accelerates the attack on the mold.
  • Pour thick pieces in layers so the resin gets less hot.
  • Let the mold cool down and rest between pours. Wacker recommends regularly heating molds to 100 to 150 °C to remove absorbed styrene, as this makes the mold swell. If you work in series, rotate between several molds.
  • Let the silicone cure fully before you pour for the first time.
  • Check the mold regularly. If the surface becomes dull, rough or stiff, replace the mold in good time.

Related

Not sure which silicone suits your resin and application? Get in touch with us and we will be happy to help.