How do I bend sheet plastic perminately?

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Posted (edited) · Report post

I have to bend some .03" sheet to match the end curve on some '32 Ford fenders as shown in the photo. I want the fenders to extend across the width of the body. I want them to have a set so they won't put pressure on the joint. I'm guessing a hair dryer or hot water will soften it to bend the shape and a dunk in cold water will set it.

So, I'm asking those with some experience on this to give some insight and any recommendations you may have to accomplish this.

Thanks in advance.

rearfendersDSC_0876.jpg

Edited by Foxer

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Posted · Report post

I would use 3 pieces of .01. Cut all to size, tack the first one down with Tenax, starting at the top. Just like when filling metal in using a MIG....work the Tenax down each side while holding until it sets up. Then flow more in the seams...put aside to dry. After the shape is set, weld the sheets together...lots of solvent on a wide brush...press and hold. Heat will soften all the plastic, and without a "buck" under all you will get is a saggy middle. I use the thin styrene because it bends smooth and "welds" quick.

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Posted · Report post

I formed this one-dimensionally curved .030 firewall by simply shaping it carefully with my fingers. Polystyrene, like many materials, has an 'elastic limit' that, once surpassed, will make it tend to hold an introduced shape. All this means is that you carefully bend your styrene sheet FARTHER than you want it, and it will spring back some, but not absolutely back to flat. Practice and you can get it to bend and stay exactly where you want it.

DSCN9499.jpg

It's trickier to get it to hold a two-dimensional curved shape (a compound curve) but within limits, it's entirely possible. I made this compound-curved hard tonneau the same way....bent it carefully in my fingers PAST where I wanted it, and when it sprung-back, it stayed as I'd intended.

DSCN7073_zpsca15b3f9.jpg

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Posted · Report post

Thanks for the responses.

Using thin sheets should allow the bend pretty easily but the edges have to be exposed at the end of the fenders and they might present a problem .. doable but messy. It was a good thought.

Maybe I'll pay around the yield point .. or elastic limit :) ... and see if I can match the fender curve

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Posted · Report post

Maybe a combination of both techniques in your case, with some thinner material laminated to the unsupported edges to help hold the shape. Do it oversize and then file and sand to final contour.

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Posted (edited) · Report post

I've had pretty good luck simply boiling a pot of water, turn off the heat and holding the plastic in it for a couple, three minutes depending on the thickness. Bend to shape and hold it there for a few seconds. Conventional hair dryers never seem to get hot enough, after all...they don't set your hair on fire!

If the curve isn't quite right on the first try...stick it in the hot water again and repeat. Works for me.

Edited by GrandpaMcGurk

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Posted · Report post

I shape my styrene like Bill stated. Once I get it how I want it, I make it "set" with a hair dryer and let it cool on it's own. I have an old cheapie hair dryer that get very hot.

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Posted · Report post

I appreciate all the suggestions! This gives me something to think about now ... :)

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Posted · Report post

some stuff will bend under the (incandescent) light over the hobby bench. i also hold superglued joints up to the light to get them to kick. it's my offering gesture to the Hobby Gods!

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I want to thank you all for the suggestions and discussion. I ended up using a combination of boiling water and the old dry 'elastic limit' bend that was really all it needed and it stayed as I epoxied it in place. Sanding showed that it is flat all the way across .. nice .. and I'm a happy camper.

rearenddoneDSC_0901.jpg

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