"Foaming polyurethane" Topic

It has gas.

The parts puff up a bit.

Outside of pressure, any solutions?

I thought I stumbled upon one idea when I accidentally over filled the mould and it had a heap of waste on top. I think this allowed the resin to expand outside of the mould, lessening the internal expansion.

However 2 attempts have not recreated the result.

If you decide to go with a pressure pot, check YouTube's Rybonator channel -- he makes custom resin dice, using a pressure pot. Here is a link to his video, Dice Making Basics: Supplies: "The creator discusses essential tools like silicone, resin, and dyes, as well as optional equipment like a pressure pot and vacuum chamber."

He has a lot of knowledge and expertise displayed in his videos. Perhaps his videos will offer solutions and insights? Cheers!

Not real sure this will help much:

But with polyurethane that foams, adding heat makes it foam more. Playing around with the old "Mountains in Minutes" mix it yourself kits (yeas! I am that old), I discovered you can easily triple the volume of foam with applied heat.

So, the opposite should also e true. By chilling the urethane before mixing (or shooting out of the can, if you are working with that type) should reduce the resulting volume, and thus give you a denser, stronger, result.
Pre-chilling the mold would also help. Note that the reaction of the polymer curing produces heat on its own, so there will be limits to this effect.

The original Amber colored Gorilla Glue is a polyurethane that doesn't foam nearly as much, but still foams.
I have used that to make hedgerows. Lay down a bead, and dump sand, gravel or ballast. As it foams up, it entraps the stuff on top. Repeat to get the desired height.
I've also used it in buildings, to simulate brick and rubbish debris.

I've never tried to make hills or mountains with the insulation polyurethane, though.

If you have resin that has started foaming (which you don't want) it means one of the components, probably the polyhol, has become contaiminated with water. When you mix it with the isocyanate (the dark brown liquid) a reaction takes place that creates carbon dioxide.

This is essentially how polyurethane foam works. Water is added to the polyhol as a "blowing agent". The CO2 bubbles created by the reaction expand with the heat from the reaction, making a foam. There are other additives to get a nice uniform structure by managing the expansion and curing time.

Contaminiated resin lacks the additives and so the foaming is all over the place. There isn't much can do to remove the water.