Silicones for aggressive casting resins: epoxy, polyurethane and polyester Epoxy, polyurethane (PU) and polyester are the three casting resins that put a silicone mold under the greatest strain. They attack the silicone chemically and generate heat as they cure, which accelerates that attack. You see the result after a number...
I want to pour an aggressive resin (epoxy, polyurethane, polyester) into the mold (23)
Silicones for aggressive casting resins: epoxy, polyurethane and polyester
Epoxy, polyurethane (PU) and polyester are the three casting resins that put a silicone mold under the greatest strain. They attack the silicone chemically and generate heat as they cure, which accelerates that attack. You see the result after a number of castings: the mold turns dull, the resin starts to stick or releases less easily, fine details disappear and eventually the silicone becomes brittle. No silicone is immune to this. With these resins every mold has a limited life, and the silicone you choose determines how many castings you get out of it. On this page you will find all the silicones that the manufacturer itself recommends for casting resins, with a dedicated in-depth page for each resin.
Why are these resins aggressive?
Each resin attacks the silicone in its own way. That is why the best choice also differs per resin.
- Epoxy: the catalyst and the heat of reaction attack the mold surface. The mold first loses its gloss and then starts to "burn" in places. According to Wacker, heat-curing and unfilled epoxies are much more aggressive than cold-curing, highly filled systems. Read more: silicones for epoxy casting resin.
- Polyurethane: the isocyanates in PU attack the silicone over time, causing the surface to turn dull and the resin to stick. According to Wacker, aggressiveness increases from casting resins to soft foams to rigid foams, and with the blowing agent content. In addition, PU reacts with moisture and then starts to foam. Read more: silicones for polyurethane casting resin.
- Polyester: the styrene in the resin penetrates the silicone, causing the mold to swell and eventually become hard and brittle. According to Wacker, the higher the styrene content and the curing temperature, the stronger the attack. Read more: silicones for polyester casting resin.
Quick selection by resin
| Resin | Tin (condensation) | Platinum (addition) | In depth |
|---|---|---|---|
| Epoxy | M 4514 with T 21, Silicone Condensation 20 Resin | M 4641 (clear), M 4642, Vario 40 | Silicones for epoxy |
| Polyurethane | Dow 3481 R, M 4514 with T 21, Silicone Condensation 20 Resin | M 4641, M 4644, M 4601, Vario 40 | Silicones for PU |
| Polyester | M 4514 set with T 51, Silicone Condensation 20 Polyester, Dow 3481 R | M 4642, M 4670, QM 270 | Silicones for polyester |
Tin or platinum?
Condensation-cure (tin) silicones are robust and do not suffer from inhibition. You can safely use them to mold resin 3D prints, sulfur-containing clay or a model that has not yet fully cured. They shrink slightly more (0.2 to 0.4%), can only be demolded after 8 to 24 hours and become stiffer after long storage. According to Wacker, tin-cure molds must first post-cure before you pour resin into them; for epoxy, Wacker recommends post-curing the mold for several hours at approx. 150 °C before first use.
Addition-cure (platinum) silicones cure without shrinkage (less than 0.1%), 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 and vacuum casting. When molding your model, however, watch out for inhibition caused by sulfur, latex, some 3D prints and not fully cured resins, among other things. If in doubt, do a test first.
With tin-cure silicones, the catalyst determines the resistance
With condensation-cure silicones, the choice of catalyst is just as important as the silicone itself. With the same base, one catalyst gives you a mold that lasts a long time with resin, and another gives you a mold that mainly cures quickly but is not designed for resins.
Dow Silastic RTV 3481 is a good example of this. The properties the mold gets are determined by the RTV-3081 catalyst you use with it:
- RTV-3481 with RTV-3081 R (resin resistant): Shore A 19, pot life 90 to 120 minutes, cured within 24 hours. Dow developed this catalyst for a longer mold life with polyester casting resin, and Dow lists polyurethane and polyester as typical casting materials. This is the combination we sell as Dow Silastic RTV 3481 R, and the one to choose if you cast resin.
- RTV-3481 with RTV-3081 F (fast): Shore A 23, pot life 30 to 45 minutes and demoldable after approx. 6 hours. This catalyst is designed for speed, not for resin resistance. Fine for plaster, concrete and wax, but if you cast epoxy, PU or polyester, choose the R version.
Wacker works on the same principle. For Elastosil M 4514, Wacker specifies:
- Catalyst T 21 for casting epoxy and polyurethane.
- Catalyst T 51 for polyester, plaster, concrete, artificial stone, wax and low-melting alloys.
- Both can be blended with Catalyst T 47 for faster demolding.
Please note: the complete M 4514 set is supplied with T 51 and is therefore intended for polyester. If you cast epoxy or PU, order the A component separately with T 21. Our own condensation-cure silicones follow the same logic: Silicone Condensation 20 Resin with T 21 for epoxy and PU, Silicone Condensation 20 Polyester with T 51 for polyester.
Private or business customer? All Wacker tin catalysts, such as T 21, T 37, T 40, T 47 and T 51, contain dioctyltin compounds. Because of the European REACH rules for these compounds in two-component casting rubbers, these catalysts, the Wacker condensation sets and our own Silicone Condensation 20 Resin and 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 and want to make a resin-resistant tin-cure mold, the Dow Silastic RTV 3481 R is the right choice. Prefer platinum? The addition-cure silicones in the table below contain no tin and are available to everyone.
All silicones in this collection
✓ = the manufacturer lists this resin or casting resins in general as an application. ○ = usable, but the manufacturer focuses on other resins.
| Silicone | Type | Shore A | Epoxy | PU | Polyester |
|---|---|---|---|---|---|
| Wacker M 4514 + T 21 | Tin | 25 | ✓ | ✓ | ○ |
| Wacker M 4514 set + T 51 | Tin | 25 | ○ | ○ | ✓ |
| Silicone Condensation 20 Resin | Tin | 20 | ✓ | ✓ | ○ |
| Silicone Condensation 20 Polyester | Tin | 20 | ○ | ○ | ✓ |
| Dow Silastic RTV 3481 R | Tin | 19 | ○ | ✓ | ✓ |
| Wacker M 4440 + T 37 | Tin | 37 | ○ | ✓ | ✓ |
| Wacker M 4641 | Platinum, transparent | 43 | ✓ | ✓ | ○ |
| Wacker M 4644 | Platinum, transparent | 40 | ✓ | ✓ | ○ |
| Wacker M 4642 | Platinum | 37 | ✓ | ✓ | ✓ |
| Wacker M 4601 | Platinum | 28 | ✓ | ✓ | ✓ |
| Wacker M 4670 | Platinum | 55 | ✓ | ✓ | ✓ |
| Wacker Vario 15 / Vario 40 | Platinum, transparent | 15 to 40 | ✓ | ✓ | ✓ |
| CHT QM 270 | Platinum | 62 | ✓ | ✓ | ✓ |
| PlatSil HTS-25 / HTS-40 | Platinum | 25 / 40 | ✓ | ✓ | ✓ |
| PlatSil FS-10 / FS-20 | Platinum, fast | 13 / 20 | ✓ | ✓ | ✓ |
For the Vario range you need Catalyst Vario, also available as a fast version. For the PlatSil ranges, Polytek recommends a release agent for epoxy, PU and polyester to extend mold life.
How to get more castings out of your mold
For all three resins:
- Use a release agent, especially for the first castings.
- Limit the heat: choose a slow resin, do not dose the catalyst higher than specified and pour thick parts in layers.
- Demold as soon as the casting allows. The longer the resin stays in the mold, the greater the attack.
- Let the silicone cure fully before you pour for the first time.
- Let the mold cool down and rest between pours. Wacker recommends 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, sticky or crumbly, replace the mold in good time.
Specific to each resin:
- Epoxy: according to Wacker, post-cure tin-cure molds for several hours at approx. 150 °C before first use. Platinum molds can be used immediately.
- Polyurethane: work dry. Moisture in the mold or tools makes PU foam.
- Polyester: styrene makes the mold swell. Regular heating removes absorbed styrene and extends mold life.
Frequently asked questions
How many castings can I get out of a mold?
That cannot be captured in a single number. It depends on the resin, the amount of heat, the thickness of the casting, the contact time, the use of release agent and the silicone itself. A small, thin epoxy pendant in a platinum mold will last much longer than a solid PU or polyester block. So always test first with your own resin and working method.
Can I use one silicone for all three resins?
Yes. Wacker M 4642, M 4601 and M 4670, the Vario range, QM 270 and the PlatSil HTS and FS ranges are recommended by the manufacturer for casting resins in general. If you work with tin-cure silicones, you can use one base, such as M 4514, and choose the right catalyst for each resin: T 21 for epoxy and PU, T 51 for polyester.
Why is my mold turning dull or sticky?
That is the beginning of the chemical attack by the resin. A release agent, less heat, faster demolding and letting the mold rest in between all slow this down. Once the surface is rough or crumbly, the mold is due for replacement.
What should I choose for clear, glossy castings?
Choose a transparent platinum silicone. Wacker M 4641 has an oil-free surface and is recommended by Wacker for crystal-clear casting resins. Beyond that, the gloss of the casting depends mainly on how smooth your model is.
Why is my platinum silicone not curing on my model?
That is inhibition, usually caused by sulfur, latex, some 3D prints or a resin on the model that has not fully cured. Do a test first or choose a tin-cure silicone, which is not affected by this.
Further reading
- Silicones for epoxy casting resin: selection guide and tips specifically for epoxy, from jewelry to vacuum casting.
- Silicones for polyurethane casting resin: selection guide for PU casting resin and PU foam, including the Zhermack options.
- Silicones for polyester casting resin: selection guide for polyester, including affordable options for small series.
- Epoxy casting resin and polyurethane casting resin
- All condensation-cure silicones, all addition-cure silicones and rapid prototyping
Not sure which silicone suits your resin and application? Get in touch with us and we will be happy to help.