PR cracking?

[sekach]

I worked with a friend to help him start casting his own blanks in the resinsaver molds and he uses the PR from Michaels but he has come across a problem that I saw a few times but don't anymore. When he does label casting, he allows the pour to cure overnight and then throws it in the toaster oven at 250 for 10 mins to do the final cure and do away with the lasting sticky feeling. His problem is that when he puts the molds in the toaster oven they are coming out cracked and only cracked where the seam is on the tube. I had started putting small drops of CA on the ends of the seams to keep them down but would be interested if anyone else knows the true cause of why they are cracking at the seam only.... Thanks...

 

[ldb2000]

250 degrees is way too hot for post curing . It should be done at 125 to 150 degrees to both prevent hardening the blanks (makes them very brittle) and to prevent cracking .

Edit :
The cracking is most likely due to an expanding of the brass tubes and a shrinking of the PR .

 

[sekach]

Thanks Butch...after you mentioned the heat being too high I went and checked my oven to see what I was at and I am around 175 so not sure why he went to 250. It certainly seems plausible though that the higher heat makes the tube expand slightly while shrinking the PR therefore causing it to crack. It was interesting that all the cracks happend only where the seams of the label were. Appreciate the feedback though and I have passed your gouge on to my friend....time will tell if that fixes it....

 

[PR_Princess]

Bob, I agree with Butch. Too hot. As PR is thermosetting, you need heat for it to cure. Too much heat (from over catalyzing or other means) will cause it to crack.

Cleaves will usually find a naturally weak area or some other point of imperfection. His label seam may be providing just enough....something like a score on a sheet of glass.

 

[DozerMite]

I too agree.

If the tube is expanding, the lable doesn't expand, so the seem pulls apart and since the resin is adhered to the lable... it pulls apart as well and causes the crack.