Major fractures

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Swftfox

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Jul 19, 2016
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Location
Tonganoxie, KS
I have been casting a while now in purchased silicone molds but yesterday I made 8 blanks in PVC pipe. I'm using poly-resin and love it but I let these cure in the tubes about 8 hours then popped them out. Without exception every ones of them had major fractures. Any clues or hints?
 
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Hello I am casting poly resin in pvc and have had no fractures or problems.
Are you using 10 drops of catalyst per 2 oz. ?
 
Do you have any photos to show the issue?

I'm only speculating, but it sounds like they overheated.
This can happen in a hot building during the summer when temps are high.
It would take an extreme amount of catalyst to make a small blank fracture. Within a few drops, it shouldn't cause a problem.

Are you sure it's fractures and not the paper lifting, or a reaction of the paint?

A photo might help determine the root cause.
 
Just guessing as well. If it was working fine in silicone and not causing problems and in PVC is and you didn't change any thing other than the mold I'd start with the mold. I Know that PR shrinks when it's hardening. I'll also assume the silicone mold was a horizontal one. If that's the case I think when the resin is shrinking the silicone flexes and helps with separation. When you use the rigid PVC it's probably sticking the mold and pulling away from the tube. I had that problem before switching to horizontal molds. If not make sure your using plenty of good mold release. If you used a different resin that has very little shrinkage you might not be having these problems. Without pictures it hard to tell what might be going on. If the resin is actually cracking you might be using too much MEPK which would be building up heat and the PVC isn't allowing it to dissipate like the silicone making it crack.
 
I'm guessing the horizontal silicone molds are rectangular in cross section and the PVC pipes being round means a lot less wasted resin being turned off the tubes ....

I would suggest that if you want to make the investment and switch over to round blanks you could make your own vertical round molds. Turn something plastic or plastic coat something that's the right size for what you want and then pour a silicone mold of it .... once the mold is cured, remove the object and you should have the shape and size mold you need.

Don't forget to also turn some stub ends to hold the ends of the brass tube from something that your material WILL NOT stick to, in order to keep your item centered in the mold.
 
I use PVC pipe molds quite often with PR. I also am guessing but heat would be my first question. Perhaps excessive catalyst? Did they come out of the molds easily or was hammering necessary? I would also let them cure more than 8 hours, perhaps the center was still soft and the removal process caused cracks.
 
I'm guessing they got too hot in the cure. You only need 2-3 drops per ounce of catalyst to get a nice blank. Let them set up for about 12-15 hours in a room temp place. If they get too hot they crack.
 
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