Posted by John Blazy on February 26, 2004 at 07:16:53:
In Reply to: Materials posted by Justin Hui on February 24, 2004 at 15:06:13:
My first thought is Sintra®, a cellular core PVC plastic sheet that is used in the exterior sign industry. I use it all the time for modeling, templates, and parts for my boat. Sintra bends very well, is somewhat cheap, comes in all colors (white best for exterior) and can be solvent welded and conventionally welded perfectly. I can picture double or triple layer bent laminations of, say 1/2" Sintra (solvent cement and/or polyurethane adhesive between layers) shot over plywood forms. For bonding plastic to plywood, I've discovered that when you use PL Premium Polyurethane construction adhesive (moisture-cure urethane) to bond the PVC sheet to the ply edge, AND you prime the PVC with wet PVC cement, then bed the PVC into the plywood - combining the two adhesives - you get insane joint strength. The two dissimilar materials literally WELD into each other.
If you go this route (research first) be real sure to handle the expansion/contraction issues, like kerfing the plywood every 6" x 10" deep or so on the plastic to ply edge joints so that the ply moves with the expansion of the plastic, OR use Sintra for the forms as well as the skin.
Never did this before on large scale, so this is just an idea - but I'd love to try it.
Bending plywood is what is traditionally used in these cases. "Wiggle wood" is what some call it, and three layers of 7/16" bending Lauan urethane glued together, and shot onto ply forms is a seriously strong and stable skateboard ramp. What needs to be done, is to invest in boatbuilding epoxy to seal the wood, and the edges. Not a lot of money, but will add years (literally) to the life of the plywood.
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