Bending Polypropelene - Woodworking Talk - Woodworkers Forum
 
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post #1 of 11 Old 06-16-2019, 11:39 PM Thread Starter
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Bending Polypropelene

I have a cover for a GPS that doesnít stay on well because the sides have splayed out a bit.

I want to bend them back in a bit, but I want to be careful not to crack it, since itís very hard to get another if I screw it up.


Any tips for how to bend it and make it take a set without messing it up?
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post #2 of 11 Old 06-17-2019, 06:37 AM
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Gently warm it with a hair dryer.
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post #3 of 11 Old 06-17-2019, 11:04 AM
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Polypropelene melts at about 325 to 350 degrees F. It is the stuff they make "living hinges" from. My guess is that it is some other form of plastic.

But I agree with Johnep that heat is the way to form this. Approach the temperature slowly and check for pliability often. A hair dryer might not supply enough heat.


You might be able to dip the offending bend in boiling water. Leave it for a while and then try bending it.

My optician has a pan filled with sand and he heats that up to some prescribed temperature and then places the arms of the glasses in the sand for a few minutes and then makes the adjustments. It applies a uniform amout of heat. The device is specially made for opticians.

I would try the hair dryer first, but I don't think it will get hot enough.
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post #4 of 11 Old 06-17-2019, 01:15 PM
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An inexpensive 2- temp heat gun should work and rather quickly. Heat in front of and behind the area you wish to bend. Use gloves as you want to hold it in place a few seconds after it's where you want it and allow it to cool.

Takes less time to get to a 'bending' temperature than you think with a heat gun. I'm not sure a consumer-grade hair dryer will get to the desired temp as quickly and efficiently as a heat gun.

Plus, you can use a good heat gun for making dust collection fittings from inexpensive plumbing parts.

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post #5 of 11 Old 06-17-2019, 10:40 PM Thread Starter
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Thanks for all the suggestions.

The part I need to bend was originally bent to 90 degrees, but has “relaxed” to 95 degrees or so.

I need to get it back to 90 degrees and keep it there.

The heat gun I have is pretty hot, so I was afraid of melting a hole in it. This part is impossible to find, so if I ruin it, I’ll be very sad. 😔
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post #6 of 11 Old 06-18-2019, 07:41 AM
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OK, first adjust the sides to required angle and secure with twine.
The gently warm it at first with your wife's hair dryer. You could check the temperature with a suitable thermometer such as a sugar one at the nozzle.
then allow to cool and recheck angle. Myself, I would probably just use an elastic band to hold the cover on.
Good luck.
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post #7 of 11 Old 06-18-2019, 11:33 AM
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Correction on my earlier post. Nowadays they don't use sand, they use glass beads.

In any case if you have a decent high temp thermometer (meat thermometer), then get some fine sand from anywhere, but for small quantities, an aquarium supply store.

Place the sand in a pan on the stove top, or in the oven and heat to about 225 degrees. Once it is at temperature it will hold that temperature for a while. It may require more heat than that. Go up incrementally.

Bury the offending angle in the sand and let it sit for a while. Remove it and shape to the desired angle.

I would try boiling water first. The 212 degrees is probably not hot enough, so add some salt to raise the boiling temp. It might not work, but you won't melt the plastic. It is probably worth a shot.
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post #8 of 11 Old 06-18-2019, 12:24 PM Thread Starter
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You guys are geniuses!

I never thought of stretching a band around it. That could work nicely.

I could also take it to my optometrist and ask them to give it a bend for me.
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post #9 of 11 Old 06-19-2019, 10:05 AM
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Quote:
Originally Posted by Quickstep View Post
I never thought of stretching a band around it. That could work nicely.



I could also take it to my optometrist and ask them to give it a bend for me.
Another idea is to plaster cast the exterior of the cover which will then match the shape that needs correction. From this mold you can then buildup the areas needing adjustment. Now using a heat gun or low temp oven, reshape and let it cool. This will relax the plastic and take the stress out. The mold of plaster retains the shape. Once the mold shape is corrected, hang on to it as you may need it again, or it may serve to make a new one with vacuum forming . Tom




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post #10 of 11 Old 06-19-2019, 10:13 AM
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Your optician should be able to help.

A diamond is how coal reacts under pressure.
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post #11 of 11 Old 06-19-2019, 02:01 PM
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Quote:
Originally Posted by Pineknot_86 View Post
Your optician should be able to help.
Yeah, if they have that heated pan, it would be a 2 minute job, with one minute and 55 seconds just waiting for the plastic to warm up and 5 seconds to make the bend.
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