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Old 01-03-2011, 11:37 AM
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nukemmcssret nukemmcssret is offline
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Question Plexiglass and Lexan

Recently while cruising through the General Dollar store I found Crayola Crayon banks in the kiddies section. I snatched a few up. Here is the question; I saw some pictures of this type of rocket with clear fins so they would not interfere with the crayon. The title stated that the fins were made of Lexan. What is the difference between Lexan/Polycarbonate and Plexiglass? How thick should the fins be? The Crayons are plastic and probably weigh about 8 oz apiece and are 2 ft long. I am guessing at least a 1/8 in thick. Thanks for any help nukemmcssret
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Old 01-03-2011, 05:00 PM
soopirV soopirV is offline
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My father worked for GE Plastics during the advent of Lexan- it's a polycarbonate polymer with extensive crosslinking; the true use of the name Lexan should be: Lexan (r) brand polycarbonate.

In a nutshell, from what I remember from the time period (early '80s), it's a suped up polycarbonate (same basic ingredients but the recipe is different). This was back when he used to bring home bags of truly BLANK cd's from the extruding experiments.

Plexiglass is acrylic, which is scratch resistant, but not impact resistant (as polycarbs are). Further, polycarbonates (Lexan included) are more fire/heat resistant; acrylic (plexi) as much lower deformation temperatures.

Even without the family tie to Lexan, I'd be more inclined to use polycarbonate for fins; much less brittle than acrylic.

EDIT- Re-read your post and realized I didn't actually help at all! Fin thickness does not need to be much- take a CD and bend it between your hands over a garbage can- 9 times out of 10 you can go almost 150 degrees in the bend before the disc snaps (explodes). That's polycarbonate. Try that with Acrylic and you'll get maybe a 40 degree deflection before it cracks (less specatularly than polycarb). Point being, polycarb can deform a TON before it fails. I'm guessing (just guessing here!) that 1/8" Lexan/polycarb fins would be sufficient. However, joining polycarb is challenging- methylmethacrylate is the only suitable adhesive I have ever used (works like Testor's plastic cement on styrene), which doesn't disolve styrene (which is what the Crayon is most likely made out of). You may want to consider TTW fins with epoxy! Butt joints will probably fail.

Hope this helps!

Last edited by soopirV : 01-03-2011 at 05:06 PM. Reason: Adding more info to answer OP
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