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How do you bend Lexan or polycarbonate?

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  • How do you bend Lexan or polycarbonate?

    I bought some .125" x 12"x 24" sheets to use for making chip guards. However I need to bend it for some designs. I thought boiling water might be hot enough but it just made it hot and wet. No bendie :-)

    I'm assuming a heat gun type deal?

    Rick
    Son of the silver stream ..... Bullet caster.

  • #2
    A heat gun will work, but you will not get sharp bends in material that thick. I believe that the production method used for thinner material is with a heated wire, and for thicker material it is a quartz heating element (like a laser printer fuser). I have a friend who had some good luck bending and welding using solvent. I believe he used chloroform, but several solvents should make it malleable. I would experiment with some clear PVC cleaning solvent and see if that softens it enough to be workable.

    I have used the heat gun method for .125" material, and it makes a nice rolling radius, but depending on how sharp you need it, I doubt it will be satisfactory.

    Later,
    Jason

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    • #3
      Surprisingly enough, it can be bent at room temperature in a sheet metal bender, though it might also pick up marks from the machine. At that thickness it might crack since most benders would like to bend it to a tighter radius that it would like. A test with a scrap might be in order.

      Otherwise it can be bent with heat as described, but more than boiling temperature is needed, around 275F or so. Polycarb will also absorb moisture and if it has done so to any extent it will require drying before the full heating is done, otherwise there can be crazing and bubbling, etc, and the transparency will suffer. The drying can be done at something near boiling temperature for a period of time. Moisture is more likely to be a problem the older the material is, and if the protective covering has been off.

      For a line bend, you want the heat to enter the plastic quickly to minimize the softened part for a clean tight bend, but you don't want to raise bubbles in the plastic. Again, an experiment might be in order on some scrap to determine the distance from the heater to the plastic, and the time it takes to soften enough for a good bend.

      A fully workable strip heater can be made from an old stove element, the kind that are coiled up and where the element wire is encased within the metal tube. They can be uncoiled and straightened without destroying the element inside, and are certainly powerful enough to heat even 1/4 thick plastic to the bending point. A suitably rated light dimmer can be used to control the heat.

      Where the edges of the plastic pass over the heating element, the element should be bent downwards somewhat so the edges don't heat more than the rest of the plastic. If you're using a taut wire to create the heated line, this won't be as easily done.

      In any event, avoid both under and overheating the plastic. If it's not hot enough, it obviously won't bend as expected, and if you take too long to soak the heat in, the heated area will become wider than you'd want. If you heat too fast or overheat, there will be some interesting bubbling results. Some experimenting on scrap should be done before committing the project piece.

      You can use a propane torch to do the heating if you're patient and diligent. Use two pieces of aluminum flat bar or angle spaced apart on the plastic to define the desired heated zone, then wave the torch evenly across the entire line, without pausing at any spot. Pass it right off the plastic as you pass the edges so they don't get overheated before the rest of the plastic is soft. Obviously a flame shield would be a good idea under the edges.

      It's good to have a jig set up so you can quickly make the bend to the desired angle or just past it, and hold it there while the corner is cooling.
      I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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      • #4
        Lexan is a regular thermoset plastic and can be bent by heating. There is a major problem with trapped water in the material. Lexan must be baked at a low temperature at around 250 F for several hours to dry it out. Time depends on thickness and and for 1/8 material a minimum of three hours is required. If you try to bend it without drying it first the trapped water will turn to steam and create 1000s of bubbles in the plastic making it resemble a plastic turd.

        How do I know? I use flame polishing on Lexan without drying first and it is a very tricky process. I hit it with a high temperature such as a propane bottle torch and then cool as quickly as possible with compressed air. Even then it is very easy to go too far with the heat.

        Try a test piece in the kitchen oven by drying it at 250 for several hours. Use a polished aluminum mold for the Lexan to droop on. After the drying interval ramp the temperature up to 550 F. (not a typo, we are talking pizza temps) The deflection temperature of Lexan is 539 F.
        Last edited by Evan; 08-09-2008, 04:52 AM.
        Free software for calculating bolt circles and similar: Click Here

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        • #5
          Originally posted by Evan
          Lexan is a regular thermoset plastic and can be bent by heating.
          Lexan is a termoplastic, thermosets cannot, as a rule, be bent or reformed by heat.
          Jim H.

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          • #6
            That is correct Jim. I interchanged the terms.

            Also, the melting temperature of Lexan depends greatly on what grade it is as there are a wide range of different grades. If the Lexan in question is the water clear optical grade without scratch resistant coating then an oven temperature of around 300F should be sufficient to cause it to droop to conform to a simple convex mold. If it does not then ramp up the temperature in 25 degree increments giving several minutes for the oven to reach the new setting. Once the material is formed turn off the heat and wait until it has cooled for 10 or 15 minutes before opening, then allow it to cool without disturbing until it can be handled.
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            • #7
              Reminded of a couple of things I saw at Oshkosh 20 years ago. Demo on pulling a bubble canopy over a male mold. demonstrator said that the heated plastic had similar elastic and strength properties as same thickness of rubber. In other words a lot. About eight guys were tugging on the edges to get it tight on the mold. The other I didn't see but sounded great. The inventor of the KR experimental aircraft formed his canopies with air pressure. The edges of the sheet was clamped down on a table with plywood with a canopy base shaped hole in it. The builder skillfully heated (torch or heat gun) and applied air pressure under it to blow the desired shape. Think I'll try some motorcycle side covers /number plates today.

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              • #8
                Light aircraft windscreens and canopies are generally made with acrylic plastic which has very different properties than Lexan polycarbonate. Acrylic is often vacuum formed in that sort of application. You can build a vacuum form system using a shop vac as the source of negative pressure. The reason for using acrylic is mainly scratch resistance. Plain polycarbonate is very easy to scratch.
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                • #9
                  Plain polycarbonate, along with some other plastics, is also not very UV resistant, you will sometimes see lexan with a coating, and the label shows which side should be "out" for windows.

                  Another thing. Lexan is very strong. You can actually hammer-forge it cold. Although it won't be useful after that, the point is that it is strong and doesn't shatter.

                  However, some solvents (and possibly oily materials with solvents in them) will change that, and apparently can dramatically weaken it, to the point that a lexan window can simply be punched out with a fist almost like glass.

                  Haven't investigated that, but a chemist of my acquaintance mentioned it with respect to a different project, suggesting that it might not be suitable due to certain conditions.
                  1601

                  Keep eye on ball.
                  Hashim Khan

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                  • #10
                    I made a protective machine guard from 1/4" thick lexan. I used the following method: Draw a line on the sheet where bend is to occur with dry erase marker. Heat along the line with a heat gun...both sides. When the lexan started to flex and before bubbles occur in the plastic, I put it in a sheet metal break and bent it. I went past 90degrees a bit...maybe 95, then let it cool. It cooled close to 90degrees.
                    Matt

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                    • #11
                      "However, some solvents (and possibly oily materials with solvents in them) will change that,"

                      Have to second that, JT.
                      Some time ago, I made a splash guard for my lathe from PC.
                      (because it was what I had on hand, no impact resistance required, just keep the suds in the tray).
                      Within weeks, it was clouded, then starred. Within months it fractured at the hinge fixing.
                      The acrylic replacement has now been in place for at least 3 years with no probs.
                      As I said, no impact resistance is required, this is just a splash stopper.
                      FWIW, the suds is just standard soluble oil "white water".
                      Just got my head together
                      now my body's falling apart

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                      • #12
                        The Lexan product by the name of Mar-Gard is craze resistant since it is coated with an entirely different material on the surface. Whatever it is it's harder than brass as a brass brush doesn't scratch it.

                        I suspect, although I haven't tried it, a wipe down using silicone oil whould protect polycarbonate quite well against the effects of various solvents. There aren't many solvents that can remove polydimethylsiloxane and once applied to a surface it is there to stay even if the film is only a few molecules thick.
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                        • #13
                          Bending Lexan

                          I have made bends in lexan using an old sheet metal pan brake we had at the shop. I had a piece .125x 12"x 24" that I creased in the middle to make a safety shield out of. I didn't heat it but I did adjust the brake so that there was a little more than 1/8" of radius (the paper was still on both sides). and it came out just fine. No bubbles or scratches either. I was amazed that it stayed bent without any springback. Try it on a small piece.
                          Jim (KB4IVH)

                          Only fools abuse their tools.

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                          • #14
                            Evan, IIRC, certain brake fluids are largely PDMS?
                            If so, are they likely to protect in the same way?
                            Just got my head together
                            now my body's falling apart

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                            • #15
                              Originally posted by J Tiers
                              However, some solvents (and possibly oily materials with solvents in them) will change that, and apparently can dramatically weaken it, to the point that a lexan window can simply be punched out with a fist almost like glass.

                              Haven't investigated that, but a chemist of my acquaintance mentioned it with respect to a different project, suggesting that it might not be suitable due to certain conditions.
                              Many of airline accessories are fitted with polycarbonate bowls, airline filters and oilers primarily. They are not resistant to some compressor oils, they usually develop a leak first, but can deteriorate to the point that they shatter.
                              Jim H.

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