Plexiglass and Lexan

Discussion in 'Scratch Built' started by nukemmcssret, Jan 3, 2011.

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  1. Jan 3, 2011 #1

    nukemmcssret

    nukemmcssret

    nukemmcssret

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    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 :cheers:
     
  2. Jan 3, 2011 #2

    ClayD

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    Last edited: Jan 3, 2011
  3. Jan 3, 2011 #3

    bytesiz

    bytesiz

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    Lexan is just a brand of polycarbonate... GE I beleive developed Lexan. Plexiglass is a brand of PMMA which is different from polycarb and Lexan in that it is shatter proof. Lexan is more flexibile and is stronger typically than PMMA.
     
  4. Jan 3, 2011 #4

    nukemmcssret

    nukemmcssret

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    :D:D Looks Like it is Lexan I will be using. I will start with 1/8" and see how that works. Thanks for the information.
     
  5. Jan 3, 2011 #5

    Micromeister

    Micromeister

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    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.

    192a-sm_Crayon 2D_Clu-Odd-Roc_08-17-96.jpg
     
    Last edited: Jan 3, 2011
  6. Jan 3, 2011 #6

    nukemmcssret

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    :confused: I would never have thought of the plastic not being to be glued! I know what a rivet is and I know what epoxy is but how do you epoxy rivet? Drill holes and epoxy a plastic rivet into it? Have any pictures or drawings? Also would through the wall mounts be better. Being two dissimilar materials? Do you rivet the centering rings also?
    Now I have to think. Thanks I would not want every thing I do fall off. LOL nukemmcssret :cheers:
     
  7. Jan 3, 2011 #7

    georgegassaway

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    Attached below left is how I mounted the clear fins to my shuttle model (Mounted to the SRB Aft Skirts). The image shows a mock-up test, done to see if it was strong enough (it was). The fins were .062” (1/16”;) Lexan/Polycarbonate.

    What is not shown is the fact that the fins had tabs to fit into slots that I cut into the SRB Aft Skirt (Vac-formed). The tabs also had some 1/16” diameter holes drilled into them. With the fin tab put into place inside the slot, then I applied a Casting resin inside of the Aft Skirt, to anchor the tab securely inside. The holes drilled into the tabs were critical to holding them in, because the resin did not “glue” anything together.

    I am attaching a drawing of a shuttle fin to show the tab. (The shuttle fin had a short tab due to the limitations of making it work with an SRB aft Skirt). Also a drawing of a generic fin as I would use for a longer slot.

    Key to that is to apply the epoxy inside of the tube before adding the engine mount. Well, you could glue in the front centering rig first (above the slots), then attach the fins, applying epoxy inside to anchor the tabs. Then finally add the engine mount tube and aft centering ring.

    Fourth pic is a view of the contest-quality shuttle model’s fins (At NARAM-41, where it won). I have better pics of the shuttle model, but unfortunately not any close-ups showing the fin joint region.

    Micromeister is right, .062" Lexan/polycarbonate would be plenty for this. I have one of those crayons myself. I have not gotten around to converting one, but if I did it with clear fins attached to the Crayon, I would use .062" Lexan.

    Actually, if I ever do get around to it, I may just add 6" or so of 30mm tube sticking out of the back end, attach balsa fins, and paint the tube/fin assembly flat black. Sort of like what I did with the Halloween rocket in the final pic.

    - George Gassaway

    Clear%20Fin%20attach.jpg

    -shuttleFin.jpg

    -GenericFin.jpg

    dcp00428.JPG
     
    Last edited: Jan 3, 2011
  8. Jan 4, 2011 #8

    Micromeister

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    Some folks like TTW fins, Personally I find for Model and most LMR's up to 3.3lbs I just don't see the need, it just makes finishing the model all that much more of a pain.

    It's been my very well flown experience that any well done epoxy rivet, butt-on fin application with good epoxy fillets will survive some rather rough landings without failure. If you don't mind the additional work, TTW is always an option.

    The Trick is to use a 3 line epoxy rivet attachment. For this we mark the centerline of each fin location. Then the Outside thickness of the fin material on both sides of the centerline. Starting on whichever line you like (I generally go with the center line) drill 1/16" holes about 3/4" apart staggered down the three lines trying hard NOT to put more the one in any one parallel location. If using thin fin material like .063 Polycarbonate drill a line of holes up about 1/8" from the root edge with the same spacing (more or less as in Georges drawing without the tab). If using Thicker material or wood it's possible to drill hole in the end grain as well (See pics 1,4 & 5 below). Run a good line of 30 minute epoxy along a fin line and tape or fixture the fin in position until the epoxy sets. When all fins are attached run fillets along the joining edges which should completely conceal the root edge holes in the fins as will.
    Here are a couple photos from a class taught on "Working with Plastics" a year ago. They may help get a handle on the process, If your interested the text of the Class handout is available as a free download from the narhams.org library section under Tech-Tip-017 "Working with Plastics" I don't believe the photos have been posted as yet.
    Hope this helps.

    Plastics-5c_Polycarb-Cardboard epoxy rivet joining_01-02-10.JPG

    Plastics-6a_Epoxy rivet materials & tools_01-02-10.JPG

    Plastics-6c_Styrene-Polypropylene epoxy rivet fin_01-02-10.JPG

    Plastics-6e1_Best polycarb-anything epoxy rivet joint_01-02-10.jpg

    Plastics-6e2_Polycarb-cardboard epoxy rivet & fillet joint_01-02-10.JPG
     
    Last edited: Jan 4, 2011

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