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Say I make a fiberglass/epoxy part on a plastic mold, and it cures at room temperature. The epoxy system calls for a postcure at 250F, but the mold cannot withstand 250F.

If I take off the part and postcure it separately, am I likely to see dimensional changes in the cured part? Is there any way to postcure the part off-the-mold without losing accuracy?

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Yes it can be post cured - however, without support there is a risk of sagging or warping, particularly if it is a large and heavy part.

Thermal curing is simply a way of speeding up the chemical hardening process, which in the case of typical epoxies may normally take place over several days, even weeks. The second benefit of heating the material is the removal of dissolved gasses in the plastic (outgassing)

There is no need to cure at 250F - any temperature above ambient will hasten the curing process, so I would suggest soaking at say 125F for 12 hours instead of 250F for 4 hours.

Warping and distortions are always a possibility but in my experience if the temperature is managed and some support is supplied, (e.g. a sand bed) any distortions are imperceptible, at least in my applications.

This is a very good reference for those interested in the process: Masterbond.com

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Curing conditions depend on the resin sytem. Some systems require a minimum temperature for a certain duration to cure, others cure completely at room temperature. If in doubt, you will need to ask the manufacturer of the system.

It also depends on the requirements you have for the final part. If the final properties of the part are somehow safety relevant, you should stick to the manufacturer's instructions.

Depending on the tolerable distortion one solution may be the sand bed proposed by Donald Gibson. Another possibility could be to cure at lower temperature in the mold first and post-cure in a sand bed at higher temperatures later.

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There is a chance 250F might be above the Tg of the part if it is cured at room temperature. If this is the case then you will definitely get sagging if you postcure without a ramp at 250F. You might want to try ramping slowly up to 250F so that the temperature of the oven never shoots past the Tg of the material( which is steadily increasing with degree of cure).

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