Thinking about your project a little more. I'd said self polymerizing
castable plastic don't shrink much. That isn't accurate. But they do
shrink through out the part uniformly because they solidify through
chemical means, not by shedding off heat, which would work from the
outside in, creating problems.

My point is, without the specialized high pressure equipment used to
process thermoplastics, you're going to have some difficulties casting
this plastic. Uncontrolled shrinkage is the root of the problem.

I believe you will be able to create a casting with some usable
properties, so I hope you'll try. You should encourage the melt to cool
as evenly as possible. Do this by cooling very slowly. Some plastics
suffer badly due to prolonged exposure to high heat. POM isn't one of
them. Preheat your mold before pouring the plastic in. Then set them in
an oven at 150 F to cool overnight. Your finally casting will be more
even than if you let it cool quickly.

Sorry to be long winded. This happens to be an interesting subject to
me.