Re: ignorant about fiberglass techniques


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Posted by John Swensen on March 05, 2002 at 12:47:04:

In Reply to: ignorant about fiberglass techniques posted by js on March 05, 2002 at 09:29:02:

If a high strength-to-weight ratio is your desire, consider using epoxy resins, rather than the polyester resins used for boats, hot tubs, or kit-cars (I don't know if Walt Johnson cases use epoxy or polyester resin, but I have seen, first-hand, holes in Johnson cases, provided by cooperative airlines).
An very strong composite structure, used for airplanes, is a lamination of epoxy-glass on both sides of a foam core (blueboard foam insulation (sometimes pink) is not a bad choice for a structural foam). At the joints and corners (especially where hardware is attached) the foam should be tapered away to nothing so that the epoxy-glass from both sides meets.
An alternative technique, sometimes used for fuselages, involves building a shape with balsa wood strips (say, 1/8" thick), then covering the outside with one or two layers of 8 oz. glass with epoxy, then cutting the shape apart (at the hinge line in your case),
then covering the inside with one or two layers of glass with epoxy.
The use of thicker foam (say, 1/2") would provide a crush zone and more protection from outside intrusions. For that matter, polystyrene foams like blueboard are excellent for padding the insides of cases.

Areas where hardware is attached can be reinforced with additional laminations of glass-epoxy, but when installing the piano hinge for the case, I would back up the laminations with a piece of aluminum before installing the pop rivets.

I don't have any data on the shear strength of paper mache vs foam or balsa, but I would think the paper would absorb lots of resin without gaining much strength.

In all these techniques, the more glass (and less epoxy) in the matrix, the less the composite will weigh, for the same strength (assuming that the epoxy fully wets the glass); for hand layup, getting down to 40% epoxy (by weight) is doing really good.

Epoxy is more expensive than polyester resin (the standard for kit-cars, boats, and other low-strength composite structures) and many people develop contact allergies to the resin (hint: buy a box of disposable, latex gloves, change them often). It also stinks a bit, and the glass dust from sanding is a pain.

I would be happy to correspond with you, offline, for more information, or you might want to check out some homebuilt aircraft or cedar-strip kayak sites for more information on this stuff. It is fun to do, but there are many techniques to be learned to do it well.



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