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
I'd almost bet the Crayon you're talking about is my 4" dia 5 D12 clustered crayon.
The Fins are .063 clear Lexan. Not anywhere near 1/8".
Plexiglass is the Brand name and Common name for ALL ACRYLIC plastics. Acrylic plastic is either continuous cast or cell cast in thicknesses from about .030" to more the 6". Acrylic Plastic really has NO place in rocketry other then possibly making display stands as it is FAR to brittle to be useful on flying models. Flight Stress and Landings would make exceedingly Short work of Acrylic or even modified Acrlyic plastic fins.
As Someone posted eariler GE developed the Polycarbonate with the Trade name Lexan. it is the ONLY Clear materal I recommend for use on model and mid power rockets as it is Shatter Resistant.. to correct the misstatement that lexan or any other polycarbonate are Shatter proof. it is NOT, at temperatures below 32°f Lexan and other polycarbonates WILL CRACK and BREAK. If subjected to temps below 0°f and struck with a shape blow it will shatter like any other plastic or glass.
That said at most ambient air temps we generally fly in. LEXAN or other polycarbonates make great fin material. I use it often on many Odd-roc designs in thicknesses from .010" to about .080" but NOTHING thincker, more often then not on Odd-rocs and clusters .063" it the limit. To date flying is all weather, year round i've only snapped one polycarbonate fin on a very cold FEB. launch coupled with a plastic wad near no deploy on the chute.
Choose your Plastics well, be sure you SEE the word LEXAN or POLYCARBONATE on the liner before you buy. Many hardware and home improvement stores sell a modified acrylic with an unmarked blue line that remembles Tufak another tradename Polycarbonate but it is NOT. If a store clark or owner tells you it's a Polycarbonate ask them to prove it, either with paperworks stating clearly it is, or bending a piece 90° without snapping. If it will not cold from over 90° without breaking it is not polycarbonate.
AS for material thickness. I rarely use anything thicker then .063" (1/16"), Why? Polycarbonates and other plastics as well are HEAVY. It pays to use 4 smaller span thinner fins the 3 largers thicker ones. As mentioned before I've been flying these models an awful long time with 0 problems in these thicknesses. 1/8" Polycarbonate is Way to heavy for model or mid power models.
A couple other nice things about Polycarbonates: they can be cold formed (Bend without heating) to from some interesting fin shapes, DO NOT score polycarbonate as it will also, Break along the score line. Use a soft lead pencil to draw your line, clamp in a vise or bewteen to hard surfaces and fold away. It can be coldfolded well over 120° without breaking. I can be attached to itself with a material called Weld-on 16. it can be joined to dissimilar materials with epoxy rivets and Epoxy fillets. it can be drilled, taped and machine screw fastened or pop rivited. All in all some pretty KEWL stuff.
Ps: Don't try to heat form it as it MUST be low heat dried for a specific time and to a particular temperature before it can be heat formed.
How do I know all this? I'm in the Sign Business, we use the materal EVERY Day in many outdoor Illuminated sign face applications.
Hope this helps.
Pss: Thought of something else that may be helpful:
Most, not all of the Crayon Banks out there are a combination of Cardboard and Polypropylene, or Polyethylene tubing and Polypropylene ends and cap. Polypropylene is one of the nearly impossible to glue "plastics". It's best to use the epoxy rivet method for attaching whatever to these materials.
