Bending Plexiglass: I need a jig


I need to make a jig for bending plexiglass so that I can get perfectly straight bends.
I'm mounting some 4" wide x 6" long strips of 1/8" plexiglass in my vise and bending them by heating them up with my heat gun. The problem I'm having is that the bend isn't always straight because I'm having trouble keeping constant pressure and constant temperature across the 4" wide bending area. Even when I use a 5" wide piece of wood to bend the plexiglass, my bends tend to come out a little "twisted". In other words, the bend isn't always at a right angle to the edge of the stock.
The problem gets worse if I don't bend it to the correct angle the first time. It's next to impossible to put it back in the vise in the exact same position so that the stock will continue to bend along the original bend.
Does anyone have any suggestions to help me get consistantly straight bends everytime?
Thanks!
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use a strip heater rather than a heat gun. also, you can set up a jig out of wood that has the correct angle in it. after you heat the plastic, place in the jig to let cool and it should keep the bend aligned correctly.
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Thanks, but...
I don't have a strip heater and not sure I want to invest in one for the few times I need to bend plexiglass. Funds are tight right now.
Angles are not always the same, so I would need to build multiple cooling jigs. I guess I'm just looking for a jig to help do the actual bending. Besides, my limited experience has been that as soon as I remove the heat, the stock cools so quickly that I wouldn't be able to get it into a jig before it stiffened up. Maybe I need to experiment a little more. Thanks again.
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I think a strip heater is easier to find, but before I needed it, I bought some I guess you'd call metal tube heaters. 1/4" in diameter or a little more, with two wires coming out of one end. They were sold on Canal St. in NYC. Surplus from something. If you have a scrap laser printer or laser copier or even a regular copier that uses light and mirrors, I guess I mean if you have a scrap anything that uses toner, there's a metal tube inside that fixes the toner by heating it. It's the last step, so the heater is near the exit. I have the feeling that they're hotter than my heaters, but I really don't know. Mine took maybe 15 seconds to a minute to heat the plastic enough, starting from cold. It was consistent, but I can't remember any closer?.
I might have seen them at some hamfests also. But go to a big one to maximize your chances. They'll be with the guys who have lots of open plastic boxes on their tables. New stuff that is surplus, like Canal St. Check www.arrl.com to find hamfests in your area. The season is just starting. Buy and sell used stuff too.
Allthe ones I have and have seen are long enough for this application.

Or one, with a hinge or brackets behind it to hold it at the right angle.

I think with the hot tube you can reheat it at the same spot, but it's harder to hold the heater in place the second time. The first time, I just lay it on the plastic, iirc. 110 volts.

I think I just eye-balled it to get 90 degrees. I'd never bent plastic before, and I think I was lucky enough to get it the first time. Just two bends. Like a Z, with a smaller middle segment. I was making a vent window for a '73 Buick Centurion. One end went into the window slot, a triangular part was horizontal, and the main part, the other end, rested against the side view mirror. It worked well at up to 80 mph.
I used it for something else too, years later.
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Bend it over a heated piece of metal pipe.
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Pre-heat the material in an oven. Consider making a slotted fixture to hold the plexi so you can drop it in and use the wide board to bend it over. If it lends itself to a hinge, it would push the same way every time eliminating another variable.
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I'd like to wrap 0.010" Lexan around an 8' 1x3 in a right angle. I'm thinking about a fixture like this, viewed in a fixed font:
--------- |3/4"| 1x3 | --------- ............................... Lexan ------------ ----------------- --------------------- inset hinge 2x4 | pin | ------------ ~1.5" ----------------- ------------------------------------- plywood -----------------------------------
My plan would be to let air from a 350 F heat gun flow through the preheated 1.5"x1.5"x8' horizontal channel into the page above, with plywood below and a 2x4 on each side and Lexan screwed to the 1x3 above, with 3 hinges let in to the plywood so they are flush with the surface, with the hinge pins below the hinges so they open backwards when the right hand 2x4 is tilted up... Nick
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It's a little more expensive, but if you can use lexan instead of plex, you will get better results. Get a cheap sheetmetal brake from Harbor Freight, the $20-30 one with the clamp-on bar will do fine.
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I've made a number of small instrument cabinets and picture frames from the window sheet material I get at the building supply store. The cabinets are fabricated as two U's, the bottom unit providing the base, front and rear while the top unit forms the top and two sides. Glued blocks tapped for screws fasten them together.
My bending method has been to firmly mount a block of wood such as a short piece of 2x4 on a table top, mark the desired fold line lightly on the plastic with a fiber pen, then heat along the fold line with the slotted outlet of a heat gun. When the plastic is sufficiently warm, press the marked line into the 90 degree angle at the base of the block with a dowel. When is starting to cool and set you should give it a little extra set with hand force. A slight overbend gives a firm fit to the finished item. If you get too much, you can warm the joint a bit and hold it by hand while it cools off at a better angle.
You should practice on some scraps to get the feel the first time. Hope these concept are of some help.
SJF
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Thanks! You've given me an idea that just might work. If I mount 2 blocks to the workbench with a slot just wide enough for the plexiglass to slip into, I'll have a way to ensure that the plexiglass is at a right angle to the bench which should allow me get a bend that is at a right angle to the edge of the piece. I think that part of problem was getting the plexiglass perpendicular when using the vise. Time to go play!
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