So, we've been using two-part, soft and rigid, urethane foams for a few years now- foaming heads and bodies etc.
I've used foams from Reynolds Advanced (smoothon), Monster Makers, BITY and some off of ebay, but I seem to be getting a recurring problem with collapse after expansion with the flexible type.
Needless to say- that makes for some funky feeling and expensive foam props.
I use 3 or 4 lb flexible foam to fill fairly large cavities (about 2-5 cubic feet).
I am using proper/clean mold materials- silicone and fiberglass- and I am using the correct release- either liquid or spray form.
The temperature of the work space and materials is always above the minimum and I use a scale to measure when required.
I have hand mixed and jiffy mixed- both carefully and thoroughly- with similar results.
The only time I do not have any issues is when I am mixing smaller amounts- like 2-6 oz.
I have contacted all of them for help, but none of them have been able to solve the collapse issue- they just repeat the above list.
What I THINK is happening may be related to the volume and how it reacts to a free-rise situation. I think as the foam expands to fill, it is in a free-rise state and the bubbles created are expanding beyond the materials ability to keep them trapped. The only solution to that- I THINK- would be to use more- a lot more material so that is reaches a back pressure situation before the air bubbles get to big.
I've even given this idea to the tech support guys and it gets dismissed... Perhaps I AM insane...