View Full Version : Polyurethane foam troubleshooting help

07-23-2013, 11:40 AM
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...


07-24-2013, 08:08 PM
I have occasional collapse issues even on big props... i got a few funky looking full size gators. Two counts, and I am guessing the factors were, one was age of chemicals and the other was room temp. I think I also had one where it seemed like a gas pocket trap. But just guessing....

07-24-2013, 08:53 PM
I have found that when it gets to hot, it starts eating itself before it cures. Or it heats the release to much .
Less hardner and a clear coat over the release and a cooler place seems to be the trick for me so far...


Allen H
07-24-2013, 09:16 PM
Im gonna be the odd man out and say your issue sounds like humidity. Do you have a hydrometer to measure humidity? Moisture in the air plays havoc with my foam. having a dehumidifyer in the shop helps greatly, I dont even run foam when the humidity is higher than 70% outside.
Allen H

Allen H
07-24-2013, 09:19 PM
Another answer to your back pressure theory is to use sheet plastic and run the foam in layers, pushing down on the sheet plastic to create pressure. I have done that in the past with success.
Allen H

07-25-2013, 07:45 AM
Thanks for the help.

Well, at first I was suspect of the age of the materials, but after having the same problem with numerous different batches I gave up on that theory for the most part. I hope thats not it- how would you be able to tell?!

Then I was thinking about the release- maybe it was reacting, but I switched from 2831 to PVA and still got collapse. 2831 seems to be the way to go with urethane foam- I can get several pulls between coats.

I'd love to try the plastic sheet/ back pressure idea, but I can't get my hands inside the mold cavity.

As for the humidity- well I did ask the smoothon tech about it- but in regards to low not high. The very first pour we did was in the winter- warm but dry and got substantial collapse. Anyway he didn't give much input in regards to humidity, but I can definitely see how it may play a part. Damn... my workspace is NOT air conditioned and has a pretty large volume that is not well sealed. Dehumidifying it is going to be a biotch.

I guess I could drag it into my office area...

Thanks again everyone.


Allen H
07-25-2013, 08:03 AM
Be careful working in a smaller space, most foams off gas syanide as they expand, so ventalation and all that. just covering our butts lol. good luck with it and keep us posted.

07-30-2013, 02:52 PM
Well, thanks to the weather going bonkers and cooling down to the 60's (!) I can't say with 100% certainty that humidity was a factor, BUT I will say since I moved into a climate controlled area I have had zero collapse.

To further confuse things I was also using a different foam... so... yeah...

Anyway, my experience has led me to believe that BITY foams are somehow better. They're probably all the same, but somehow I get less collapse (if it happens) and much better expansion.

So far, for making body forms, their 3lb flexible has worked the best.

Another thing I think is important in the learning curve is to give up on trying to cut the amounts used to absolute minimums to save a buck or ten. It seems better to overfill slightly- just make sure your molds are strong enough to handle back pressure.

Thanks everyone.

Attack mannequins are now part of our doll attic.....

Here's a few of the dozen we made.

08-03-2013, 02:05 AM
Please tell me your wearing a vapor regulator?

08-06-2013, 10:16 AM
Yup, sure do. I'm curious- since you're here- what your opinion about foaming is. Would appreciate the pro advice!

08-06-2013, 11:35 AM
I live in california about 14 blocks from the beach. So moisture is present but not an issue.
I have never had any foam collapse on me ever.
I mainly use SP-200-4 from SILPAK. It works great.

08-07-2013, 12:40 PM
Thanks Adam- I appreciate the info. I am still at a loss as to why I'm getting collapse on occasion. We just poured a few pieces in pretty high humidity with zero issues. Maddening.... Since we do so very little its really hard to get any practical experience.