Expanding foam

[Mark Wright]

How much fun is this stuff?! I found that it works great as a temporary attachment for parts that are otherwise difficult to mate. In this case I attached a rose index to the filter housing for nut plate spacing around the base. I used 4lb for this, and it’s stiff. Thinking 2lb will be better for making the plenum mold?

 

[N546RV]

I bought some 4# stuff with the intention of using it to make fiberglass molds. Did a small trial batch and wasn't very happy with the results. Was surprised how much the foam contracted as it cured, and I didn't feel like it was nearly firm enough to be effectively sculpted into a useful mold.

Maybe I just bought crappy stuff? Definitely interested in hearing contrasting opinions.

 

[Mark Wright]

I'm curious if you might have received a faulty batch. Based on the test runs I've conducted thus far, I haven't encountered any shrinkage, and the material was nearly too solid for carving.

 

[N546RV]

Hm, I might have to try another test batch. One thing that occurs to me is that environmental conditions might also be a factor. I'm guessing that Idaho isn't quite at the bananas level of heat and humidity we get here in Houston.

 

[N546RV]

Big surprise: it was user error on my part. Went out to try another batch today and immediately noticed that one bottle was quite a bit lower than the other. Seems like I managed to do something like a 2:1 ratio on the first test.

Second batch with correct proportions firmed up much better. Definitely pretty tough for sandpaper, even 40 grit removes material pretty slowly. A rasp seems to work pretty well for rough shaping.

 

[Canadian_JOY]

Keep in mind that many of these expanding foams are exothermic, giving off heat as they cure. The bigger the batch size, the more heat, the faster the cure.

Frequently the quality of the cured product suffers if the cure is too fast. Big internal bubbles, discoloration and larger amounts of post-cure shrinkage are some of the side effects of using too large a batch.

Sometimes two or more pours is the better and more reliable path forward.