crack

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?

I turned my first horn about a year ago after searching this site for tips. I had an epiphany that horn is obviously made of the same stuff as fingernails & hair. What happens to them when they dry out? CRACK... But water is not what keeps them hydrated, so to speak. Which led me to the only...