Aeropoxy delay between oven cure and post-cure?

[gizza2]

Does anyone know if there's a minimum, optimum, or maximum delay between oven curing an Aeropoxy structure and post-curing?

I'm really short on time these days and would prefer to build the part at night, cure it for 90 mins, leave it overnight, and post-cure the next day. Are there any mechanical or thermal tradeoffs with such a time gap?

Thanks in advance.

 

[MarkII]

I have used Aeropoxy 6209 Structural epoxy many times, but I'm confused by your question. What do you mean by post-curing?

I have almost always allowed it to cure at room temperature; it usually cures anywhere from 18-24 hours. (When my supply was fresh, it took longer to cure - sometimes 2 full days before the surface no longer felt "tacky.") I oven-cured a relatively thin coating (on wood) of an Aeropoxy-microballoons mixture once (at about 250° F for about 20 or 30 minutes, as I recall) and then let it cool for an hour. It was fully cured and hard as a rock after that.

I have had my current supply (quart cans) for a few years now, and I'll admit that I haven't always kept the lids tight on the cans. One of the components (the amber one - I think it's the hardener) has gotten a little condensed because of that. It still mixes up fine, even though it is a bit stiffer, but now it cures much faster, reaching full hardness in just a few hours. I'm not sure how much that has affected its strength, but I doubt that it enhanced it. I probably should get some fresh stuff.

MarkII

 

[gizza2]

I purchased the ShadowAero DVD set on composite construction in high-power rocketry. I highly recommend these videos and anyone who's reinforcing their rockets should buy/study these videos. I've watched all five hours of video twice now!

I also purchased Dave's curing oven plans that specifically recommend 1 hr Aeropoxy. In the oven the resin cures in only 90 mins and that absolutely rocks. You can then disassemble the part and put it back in the oven to post-cure. This extra duration near the Tg of the cured resin improves both mechanical strength and thermal resistance well beyond the initial cure.

What's not clear to me is whether I have the temporal latitude to wait one minute or five years between the cure and post-cure steps in the oven.

I'd ask Dave Triano directly but I've deluged him with enough questions recently and he sounds swamped. At least I've bought almost every non-kit product of his as reimbursement.

Thanks for requesting clarification.

 

[Adrian A]

In the times I have heard of post-curing, there was never a rush to minimize the time between room temperature curing and an elevated temperature post-cure. That plus some related experience tells me that the post cure will still have the same beneficial effect if it's done days or weeks after the initial cure. I was waiting for someone with more definitive information to weigh in, but that's what I know.

 

[daveyfire]

The part should be cooled to room temperature before post-cure. After that, Adrian's spot on -- 10 minutes or 10 weeks doesn't really matter. We have done parts with a variety of resins this way, including many different flavors of PTM&W blends.

One note before higher-temperature post curing for molds; if you're plug molding, pull the plug out before you ramp the temperature up. We usually do our first cure at 150 to minimize thermal expansion of the plug and let the resin set, then once the mechanical strength is there, we pull the mold out of the oven, pull the plug from the mold, and post-cure the mold halves separately to 350 (or whatever the resin calls for). Otherwise, the mold might end up out of spec as the plug reaches a higher temperature, thus expanding more and either deforming or cracking the mold. (Been there, done that, no t-shirt.)

(On an unrelated note: hey, I think I got some pictures of your L3 flight back in August of last year -- is this https://www.tdkpropulsion.com/wp-content/gallery/roc808/IMG_5084.jpg your rocket?)

 

[gizza2]

I'm feeling much more informed about my oven curing efforts now. Thanks, all!

Yes that is indeed my level 3 flight, David. Nice shot! I've queued up an M1939 for that vessel next so the good times should continue.

May your balsa survive mach 2.1.