Recently I have been playing around with 2 part foam. Basically is comes in 2 cans, and when you mix it it foams up, quadruples in size and then goes hard. I have been using it to make seats for single seat race cars. Basically you fit a bin bag inside the car, pour the mixed 2 part mix into the bag, than sit on the bag ad hey presto, you have a set that "grows" around you creating the perfect fit between man and machine....
Anyway enough about cars...
Ever since playing with this stuff I have been thinking about using it for making moulds for a composite UAV fuelage and/or wing. In some places on the aircraft I would use an internal wooden skeleton as an aid to giving the desired shape (following the crurves of the skeleton when cutting and sanding the foam) These foam moulds would be discarded once the outer shell has set.
In other areas (like the wing) I would just leave just foam within the structure to give a little support and add a little extra stiffness.
This has left me with a question.
Seeing as though the little bubbles are closed cells and effectively "sealed" at atmosperic pressure within the structure, would they try to expand at altitude? Would this be enough stress on the outer composite skin to cause delamination of the fibreglass or carbon layers of the outer skin?
Obviously this probably wouldn't have an effect on small FPV aircraft that go up to 500ft, but what about on something that climbs to 5000ft?
I remember seing what happens to an empty water bottle in an airliner if you seal the lid when in cruise @ ~8000ft and then when you land it is quite crumpled?
I guess it would be the reverse!
I would think that if there were any issues, it would just by the destruction of the cellular structure of the foam, with some cells going before others, kind of cascading such that the failure of one makes room for the expansion of the next, and so on. As long as whatever the foam is inside of isn't airtight, then I would guess the air from inside the foam cells would simply exit and nothing more would happen.
But this is just a guess!!
I wouldn't think this would be a problem but it's pretty easy to test.
-Sea level is ~14.7psi if memory serves. Space is a vacuum. Higher in altitude the closer you approach a vacuum.
-If you put a sample of the material in a vacuum jar, you can use a vacuum pump to bring the sample to "altitude" and then perform destructive testing of the material to determine any loss in compressive strength resulting from burst air cells.
My instinct is that any decently thick fiberglass or other composite shell should be able to withstand a 14.7 psi internal pressure without failure. Since the actual max altitude shouldn't be anywhere near the vacuum of space, and other forms of foam such as EPS have similar properties with no issues, I don't expect you'll see any harm to the strength of the material or even much expansion for that matter.
could you tell me what are the name of the two foams you mix and where is it possible to purchase them.
If you Google
2 part Polyurethane modelling foam you should find lots of different brands.
They all do the same thing. Unfortunately I can't remember the brand name i used.
Best of luck!
Depending on the type of foam, it can be disolved using alcohol, when no longer needed.