Question about expanding foam...

[gunnerk19]

For my pulse rifle build, I was planning to use spray can expanding foam to fill the shroud for support, instead of having a hollow/weak cavity; does that kind of foam normally expand in such a cavity in all directions when curing, or does it follow a path of least resistance (like a sealable hole), so to speak?

 

[Hangar18Studios]

It will "generally" follow the path of least resistance, however, if your pulse rifle shroud is exceptionally thin you will most certainly have the danger of warpage from foam expansion. My suggestion would be to spray in a little bit at a time and allow it to cure. You'll need to clean the spray tube with a solvent such as acetone after each use, but there is much less danger of the pulse rifle looking like an overinflated tire when your done.

You can also try to jig your pulse rifle in a setup where it is firmly clamped to it's proper width between some plywood, and then spryfoam it while it is clamped in place.

 

[PHArchivist]

Great Stuff is pretty nasty and hard to work with.

 

[eltee]

There are a variety of expanding foams available inc. high and low expansion, insulating vs. solid gap filling (soft/porous vs. dense), etc. so you may want to experiment.

 

[SurferGeek]

Great Stuff is a bit weird in the fact that it expands rapidly and then when fully cured it will have voids as during the curing process it kind of falls back on itself.

If you could support the piece on all sides and leave areas for the foam to expand out of you should be fine.

 

[Invicta]

Be careful with the foam!! I had a guy in the store the other day who used triple expansion foam to fill a crack between the doorframe and wall...it bent the frame out as it cured. If you want to use the foam try minimal expansion foam in small quantities, but as already said you'll have to clean out the nozzle between applications.

 

[Harry Harris]

I'd reccommend the two part liquid mix as opposed to the stuff in the can. For a start you can mix as much or as little as you like and there's no nozzle to bung up - plus it cures much quicker and more uniformly. Obviously experiment with a little of it before you tackle the actual job too.I bought the stuff I use from a fibreglass supplier as it's used to fill the buoyancy cavities in boats etc.Hope that helps.Harry

 

[Noeland]

As long as you set the shrouds on a table in halves, and spray the foam inside them, you won't have any warping from the foam expanding.

You will have to trim the foam that expands up over the sides, but I did it in stages and had zero problems with mine, and it was pretty easy to trim away.

Then I cleared the foam out with a knife where the thompson parts fit in, and installed the shroud in halves once it fit snug, securing them to each other with long screws where they go, and was done.

Njc------------

 

[Rhett J Martin]

I'd reccommend the two part liquid mix as opposed to the stuff in the can. For a start you can mix as much or as little as you like and there's no nozzle to bung up - plus it cures much quicker and more uniformly. Obviously experiment with a little of it before you tackle the actual job too.I bought the stuff I use from a fibreglass supplier as it's used to fill the buoyancy cavities in boats etc.Hope that helps.Harry

That's a solid recommendation right there. Steer clear of Great Stuff! It just plain sucks. I'm a Smooth On man myself, and love my 'Foam It' stuff! I've used three different kinds, and they're all great. It's also very cheap, and if you do a run of something, this stuff helps hugely in cutting the cost. (one pint would be enough for about 20 Pulse rifles, depending on the resin thickness of course)
The best part, is that it can also take a beating, but is very light, where Great stuff stay kinda soft, and can be brittle. I've used this stuff for about a year in my stunt ghost traps.

Check their website, and pick the right foam for you. Just be sure that the stuff has an exit point on the mold, and that you've properly mold-released the OUTER mold/part. Have some good cover on your hands, as its sticky. Also, lung protection is vital if you plan on sawing or sanding it.
Best of luck!

 

[gunnerk19]

As long as you set the shrouds on a table in halves, and spray the foam inside them, you won't have any warping from the foam expanding.

You will have to trim the foam that expands up over the sides, but I did it in stages and had zero problems with mine, and it was pretty easy to trim away.

Thanks for the help guys!
Noeland, your idea sounds like the best for my particular project :cool

The stuff Hotshot recommended sounds good too, but one pint to last for 20 PR's? I don't expect I'd ever make that many in a lifetime... That, and there is no molding involved... Top, sides, front, rear etc. of the shroud are being cut piece by piece and epoxied into place seperately :eek I just need a way to stiffen the shroud so it doesn't collapse when handled...