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I want to create a custom two-part epoxy resin sculpture of a robot, with Lego Technic inside as a skeleton. Will the resin get so hot it will melt the plastic?

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Lego is ABS, which can have a range of melting points melts up to 200°C*. We should consider the lower end of the range just in case, which is often quoted as 105°C. Your resin shouldn't get that hot but you could test (a similar volume in a mould made of scrap, with one brick inside if you're really worried).

Generally, slower epoxies are cooler, and large volumes get hotter, so you can make it work by choosing a resin that works at low temperatures, and keeping the volume involved in any single cure down to a minimum. Depending on what you're aiming for this may mean multiple pours.

* Strictly speaking we shouldn't talk about melting points as it's amorphous, instead "glass transition temperatures"

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  • According to this answer on Bricks SE, ABS melting temperature is much lower at 105°C (221°F). Also, casting under lower temperatures might help, I think.
    – Joachim
    Aug 31 at 10:13
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    @Joachim I must admit I was surprised when I found a reference to 200C online as my first hit, especially as it's common for 3d printing. I'll check further, but the conclusion is unaffected
    – Chris H
    Aug 31 at 12:26
  • Casting at too low a temperature can also negatively affect the resin and increase the risk of trapping bubbles. I've had resin cure for almost 2 weeks (that should only take 24 hours) because it was too cold, I've had resin get a cloudy surface because air humidity was too high, and I've had resin bubble up on me because it got too hot, even though the volume of the mold was low. If it overheats, even putting the mold in cold water won't help because silicone insulates and the cold can't reach the middle of the mold as fast as the thing heats up. The safest option is to cast in several layers
    – Elmy
    Sep 1 at 4:31
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    @Joachim the 200°C came from a research paper and didn't appear to refer to a special formulation, but in this case I'll prefer Wikipedia's lower figure for caution
    – Chris H
    Sep 2 at 8:46

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