Expanding foam for plug design suggestions

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#1

So I have a large 3 piece mold and I want to modify the shape. I was thinking of using an expanding foam that I could pour into the mold and when cured I could get to work shaping as i like.

I have bought a test sample of Smooth-on Foam It3 which is an expanding polyurethane foam and I tried it on a smaller mold made of similar materials as the larger one. The molds have semiperm 700nc as a release agent. Once the foam had cured I pulled it form the mold(successful release)but the mold surface of the foam was softer then the side exposed to the air, and it started to crackle and seemed to collapse with only a gentle touch. I contacted smooth on to see if they could shed some light on the situation but they didn’t know why that had happened. Also I think this foam was not dense enough for what I would have needed… Plus it was ~$40 for the small units!!

Anyone have any suggestions of a good quality higher density foam they have used that can be machined or sanded and is strong enough to stand up to handling and then finishing with bondo type fillers, paint and clear? Would you also know if the foam could be released with 700nc semiperm?

Thanks.

#2

Wow, that sounds expensive…

I have not used any type of “specialty” polyurethane foam. The only type I have used was from a boat building supply shop, just a generic 2 part polyurethane filler foam. I think they use this to fill boat hulls, etc.

This mixes at 1:1 and expands up to 30 times the volume. I poured it into a plain plastic tub, and just let it sit for a while (probably 24 hours). It pulled out of the tub fine (inside of the tub was a smooth plastic surface, just a small bit was sticking) but I was able to work with it no problems, just by sawing, filing and using bondo to smooth it out. Just need to make sure that I use a PVA release agent for the next mold I make (wax was not enough, maybe use a 2 pack paint on the plug to be safe as well).

In short - I would try a standard filler foam from a boat building supply shop. It should be in the “cheap” to “reasonable” price range as well. Not sure about the release agent though.

#3

did you find the foam dense/hard enough? Did it deform at all when fairing with the body filler?

#4

The foam mentioned has a density of some 30-35 kg/m3. For succesful plug making I would say 80 kg/m3 is more suitable, and the higher, the harder, but also more expensive per liter / gallon.

Try to find a producer of 2-part PU foam, they should be able to help you out.

#5

Thanks Herman, There is a local retalier but they have pretty steep prices. And from how it looks the higher, the harder, the less volumetric expansion, the more you actually need… They only carry the smooth on products.

#6

Smooth on is expensive. Contact people that do spray insulation. Although the spray version is totally unsuitable (cures in 8 seconds or so) they also have casting versions usually, which are a lot slower.

#7

Thanks for the sug.

#8

RC51,
This may be irrelevant depending upon why you are needing to use foam… but at times that I wanted to change a part design and to have a plug that is easy to cut but also sand on… I would lay one layer of fiberglass cloth into the mold and then once in the “green”, use an expanding pour foam behind it, so the face of the new plug has a very thin fiberglass shell. If I wanted to cut a section out and reshape it with clay or filler, a razor blade will cut through the one layer of fiberglass no problem… but if I needed to sand the face or add filler, the fiberglass provides a good surface for adhesion.

-C

#9

thanks corban

#10

Sorry for the late reply!

The foam I used was definitely tough enough, it did not deform at all when working with it, this was some really solid stuff once set. If you think you can shape it by hand by applying pressure… think again.

But as herman mentioned, it’s probably not “ideal”. The air gaps are large enough that you definitely do need to use a filler over the entire surface, it makes a fairly big mess when trimming, sanding etc. But if you’re on a budget, your options are limited…

Here’s some photos if you’re interested. This one shows the density and cell size. I think you can get more consistent results depending on how you mix & pour it, but I haven’t yet had another chance to do additional tests.

And just to show that you can sucessfully work it to a usable plug. This was not 100% shaped, but close.

I like that tip Corban… I’ll be keeping that in mind!