I am looking at ResinServices website. I saw a laminating resin that says 24 hour ambient cure at room temp, then demold and post cure in oven.
Wouldn’t the demolded part lose it’s close tolerance shape?
My part can’t be off more than .020", or 0.5mm tolerance.
I can’t answer this from a practical standpoint but from theory if the epoxy fully crosslinks in those 24 hours and in the subsequent thermal cure doesn’t stretch past it’s plastic limit i.e. the bonds are just stretched, not shifted or altered, it will cool down to the original shape. That answer probably doesn’t help you much but maybe try making a square plate, letting it cure for 24 hours, then cutting it to the desired shape and then post curing it, measuring before and after to see if you can measure any changes.
When I use VER, i postcure unmolded. Never had a problem of resin softening. Epoxy, even after the 24hr, I have had soften!!! Even if out for a week, I will bag it and post cure. We use mostly API SC15 and FCS2 resins. When in doubt, keep it in the mold, or remold it under vacuum. Do test pieces first and check your dimensions.
@canyon: a full cure is not reached in that case. If the epoxy improves after a postcure, that is because it cures further.
Warm-curing epoxy (what we use mostly) will cure at room temperature, but will only reach full cure at elevated temperatures. In general they reach 80-90% of the potential cross linking, very much depending on the type of epoxy.
A free-standing postcure is possible, but to avoid any distortion, temperature control is absolutely neccesary. Depending on the size of object, do not elevate the temperature more than 5 to 10 degrees C per hour. The Tg of the resin will in general be about 10-15 degrees C over the highest temperature achieved (until ultimate Tg is reached).
This means during a free standing postcure, you like to push the Tg forward, not run over it.
You’re right it would be. In that case theoretically if new bonds were made when the material was expanded from heat and then subsequently cooled you won’t get your old dimensions back. This is actually a problem with composites because both heat and humidity absorption cause laminates to stress and flex due to their anisotropic nature.
Fastrr,
I’ve been using Resin Services HTR-350 (clear, not filled) with the 70 min hardener. After curing a day at ambient, the parts are defintely hard and rigid, giving off a very metallic “ping” when tapped. Having said that, the difference in stiffness is very noticable after post-curing to 400°F.
One foot-long bracket I made twisted about .005" during post-cure, but I was in a hurry and did not wait the full 24 hrs on the room-temp cure. Other times, I’ve noticed no distortion. I do follow RS’s ramp/cure schedule exactly…
Thanks guys 
That’s pretty much what i was wondering Anders… i’ll be using the HTR212 from Resin Services with 386 hardener. How clear ( color ) is the HTR-350? Does it noticably yellow or is the hardener already yellow/amber to begin with?
One problem i need to avoid is yellowing of the part say after 12 months of use.
the part i want to post cure is a complex shape so measuring every dimension would be very difficult.
Have you guys ever heard of post curing a bonded part. Say you have a 5 piece part, you mold the 5 parts, trim, then bond and assemble the parts. Then post cure. Is that a recipe for disaster or?
No, that is not a recipe for disaster, actually it is the best way to do.
Before postcuring, the cross linking rate is some 80-90%. Which means there is still room for chemical bonds. If you can bond before postcuring, that is better. It is also one of the nice things about epoxy.
Fastrr,
THR-350 resin is very clear (uncolored) but the hardener is really amber, pretty much like maple syrup. The new laminate does not look amber at all, however.
A bracket I made that was exposed to sunlight everyday did yellow after 1-2 years. If I was really worried about UV exposure, I would coat the part with Simtec Polyester Clear Filler/Primer (has UV inhibitor)
Here’s the HTR-350 with Simtec Clear filler. It does not have an amber hue:
Thank you Anders and Herman
If your part is hollow with no vents you risk having the part blow up if temps get too high. keep that in mind when post curing.
Good comment.
I once prevented a guy from infusing the outer skin of the floats of his trimaran. He had the core in shape, laminated the interior skin and bonded the deck to the hull. Then he planned to wrap the thing in fabric, and infuse it in 1 piece, by sticking it in a tube vacumm film and pulling 95% vacuum. The part never would have survived.
same as postcuring a epoxy part. Get better strength. Make sure your part is fully cured first off, so it won’t soften. Post curing also leads to higher service temps in some cases.