Hard surface over foam cannon.

[mross7]

I've been asked to make a cannon.  I gotten to the point where I'm gluing foam over the tube, and shortly will come to my stumbling block; how to create a hard shell over the foam.  My first thought was fibreglass, but I heard the resin will attack the foam (the pink stuff).  Fibreglass cast strips (the type used for limb casts) shouldn't get wet according to "care for your cast" sites.  Same for plaster of paris casting material, which may not be tough enough anyway 'cause I imagine it chips.  I don't know if bondo directly over foam will attack the foam, or if it alone would be tough enough.

Any ideas?  I just thought of plaster cast first, fibreglass over that, but I'd like to avoid extra work if I could, especially if it wouldn't be a good solution anyway.

Thanks-in-advance.

[Pascal]

I'm assuming you won't be firing this cannon!

You might want to look at some of the tools/techiques that model airplane folks use.  Easiest would be to use "monokote" (http://www.monokote.com/), which is a mylar-like material that you shrink-wrap to the foam with a hand-held hair-dryer-type tool.  Lots of different colors (you're probably wanting a non-reflective flat finish) to choose from.

You can find it at any hobby shop.

[Dinobabe]

I was thinking similar.  My dad works for an industrial plastics company that has one of those plastic sucking machines that you can make mold forms with.  Really cool!  They could suck black plastic to it.  May cost more than you're willing to spend though.

[mross7]

"I'm assuming you won't be firing this cannon!"   

Actually, the tube is made of PVC plumbing pipe, 3", 40 psi, same stuff as potato guns.  The back end, the breech, will unscrew so it can be emptied/loaded/cleaned.  They want a full length barrel so they can teach the kids how it was loaded in days of yore.  Swap out the back plug with something that has electrical leads, and you get a low grade potato gun, shooting tissue bags of flour for effect.  Not quite the same thing, but better than popping a brown paper bag.  THIS IS ONLY MUSING ON MY PART.  The fair did not ask for it, and wasn't even speculating about it, and I'm not building it with this in mind.

[mross7]

Thanks, the MonoKote looks real interesting.  I'll have to price it and give it a try. Hopefully it's not too expensive.

[Syrilla]

The mold making is fairly easy.   You can make your own Vacuum form from household items.  Most of the home stores will have the correct plastic to make the shapes with.
http://www.youtube.com/watch?v=hGBRiYhxRTM

[mross7]

Interesting video, I'll keep it in mind for other projects, but I don't think it'll work for my 5' cannon barrel.

[Hospitaller]

The problem with Monocote is that it will show every little bump and nook in the foam, and will not have any strength as it is a very thin mylar with color. It is fairly inexpensive. I would suggest covering the foam with a thin balsa sheeting first using 3M 76 or 77 spray glue. There are fiberglass resins available whih will not react with foam, but they are not cheap - although a little bit goes a long way. In large scale aircraft modeling we do the balsa cover over foam wings, then the glass over that. Keeps it light while giving amazing strength.  Most hobby shop owners are happy to provide techniques and specific materials. Just my 2cents.

[Butch]

What if you put paper mache over the foam form, let that dry, and then put the fiberglass over that?  That may be the cheapest route to go to end up with a reasonably nice looking end product.  Good luck!

[BubbleWright]

Back in 1999 the American Visionary Art Museum in Baltimore, MD held its first Kinetic Sculpture Race. It handed out a package of information with info and tips on building sculptural race machines which traverse land and water while being propelled by human power. One item in the package was about building foam pontoon floats. After cutting and shaping, water proof white glue was spread over the foam, followed by laying on strips of paper towels. When sufficiently dry, more glue was painted on followed by another layer of paper towels running at a 90 degree angle. This could be duplicated once or twice more with a final coating of glue. When dry, it could be sanded to a smooth finish and then painted. This was a "poor man's" fiber glass job that created light weight but durable floats without toxic fumes and glass spinters. I couldn't find the info package in my files for exact details but from memory came up with the basic ideas. For race details/history/photos, see link below.

http://www.kineticbaltimore.com/

[mross7]

Thanks to everybody for help and suggestions.  Even if I didn't use some they will be useful for later projects.  I went with fiberglass used for make limb casts, then Bondo over that.  Sadly I could not get a perfectly round & smooth gun, but multiple coats of paint can hide some of that.

http://www.flickr.com/photos/25808250@N08/5677446575/in/photostream


So now VARF has it's own artillery piece.