Using canned spray foam to fill vacuform parts

Another way around the old exploding vac-form kit - try spraying the foam into the fuselage halves before cutting from the sheet.  The excess can be carved away before joining the halves.  If you treat the parts with a release agent like Endust spray, you can pop the hardened foam out to cut away spaces for the cockpit, landing gear wells, etc.

HTH

I've built a few vac kits, all aircraft. For those I use plastic strips to strengthen the joints, and if I think it necessary make bulkheads for extra strength. While this is partly because I like to put in interiors, even when nothing can be seen I would never consider using foam or anything else to fill it up completely. I just don't see the need.

If you do persist in going this route, get the Great Stuff in the blue can.   It is less expanding than the red (regular) or green (no bugs) cans

I would highly recommend AGAINST using great stuff or any similar product, that stuff is messy as hell, does NOT come off of hands, clothes, wood, or anyhting else. You cannot judge the proper amount, and it will likely "over expand" destroying your model. I would use other means..... Possibly fill with epoxy or resin, but dear god not the expanding foam!!! Ive seen it used around window frames, expanding until the window inside shattered.... I would use Don's advice/method as well.

Best of luck!

  Rudy

watch out ive used it for vinyl models the stuff for sealing under sinks and baths.you can use it but if you do cover all the parts your using it with in foil.because lol it goes everywhere you can't judge the proper amount so it foams out alot of excess. it works but you need to protect your detail cause once it's stuck it you can't get it off.just be carefull ☺

I do remember an article in FSM, maybe in the late 80's or early 90's.  A guy and a vac formed model of the Star Trek USS Grissom.  He made a hole in the opposite end of the hull from where he added the foam, so the excess had a place to escape.  It might be a good idea to make several holes.

Unfortunatley, I can't remember the type or brand of foam he used.

The only suggestion I can offer is: don't use Great Stuff. I've had a couple of memorable experiences with that substance.

Memorable experience #1. Shortly after we moved into our current house, our Orkin man recommended that I seal up the holes  where the various water pipes come in. I bought a can of areat Stuff, and started under the kitchen sink. Then the valve stuck in the on position. The kids still talk about the spectacle of me running around the house, trying to get to all the pipe openings as the hideous foam squirted out in all directions. (When the Great Stuff dried, it formed enormous masses of glop all around the pipes. It looked like The Blob had taken over the kitchen and bathroom cabinets.)

Memorable experience #2. We used to have a hollow.terra cotta rabbit, about a foot long, in the back yard. One day, for some reason, my wife brought the bunny into the house and handed it to me as I sat at the kitchen table. As soon as I picked it up a three-foot water snake fell out the bottom and made a beeline for the nearest open cupboard. (It took about an hour to find the snake, get him in a pillowcase, drive him to the river, and turn him loose.) My wife thereupon demanded that I fill the rabbit up with something. Remembering Memorable Experience #1, I reached for the Great stuff and squirted what I thought was a modest amount of it into the rabbit (nowhere near filling it). The next day the rabbit was sitting on a glob of hardened foam, and the poor bunny had gaping cracks all over it. The kids made me do surgery with Milliput.

Since then, whenever I see a can of Great Stuff in a store I give it a wide berth.

I have that article somewhere, but I remember that he cut an expansion hole to keep the parts from busting open. Can't remember what he used exactly, I will try to look when I get home. 

About the only strengthening I do on vacuform models is a main wing spar, cut from 1/16 or 1/8 plywood (depending on scale and span), and I add a keel of 3/16 plywood at the join line of the fuselage halves.  The later strengthens, as well as providing a bigger gluing area so I am not butt jointing two pieces of 30 mil styrene.

I also read a FSM article some years back where the model was s 1/32 WWII Japanese fighter filled with expanding foam and some metal parts added for strength.

I do remember an article some years back about that in FSM, a C-124.

Another real problem is the heat released. If you find out anything, do post it, and experiment.

I've heard that there are different kinds of expandig foam.  Some expand and contract with environmental changes, so it's possible to make your model explode.  I think the foam that is used around window and door frames is a place to start looking.