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I have a small plastic torch (flashlight) with a magnet on the back. I use it to provide light in a small metal trailer. Recently I pulled the torch off the metal wall, and the magnet came away from the torch.

I've fixed this problem before, but the glue I used seems to fail in cold weather. (I'm in the UK, so by "cold", probably I mean 3°C down to -5°C.) Can someone recommend some suitable glue?

The torch and magnet Magnet close up

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Also for those that are unfamiliar with British terms, a "torch" is what is called a flashlight in the US. –  JohnP Nov 22 '13 at 21:17
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3 Answers

up vote 2 down vote accepted

Some sort of 2 part epoxy should hold up pretty well. Araldite is a common brand in the UK. For best results, try to remove the old glue so the bond is to the plastic itself, scuff both surfaces with sandpaper, then clean with something like alcohol to ensure the best bond. Allow to dry before applying epoxy. It's important to use the specified proportions of resin and hardener (usually 50:50) and that the two are well mixed. The epoxy should cure at room temperature or as specified, but once fully cured it will hold up under fairly extreme temperatures. It is also waterproof and resistant to most chemicals.

Be sure it is only applied to surfaces intended or materials disposable. It usually is impossible to remove once cured except by grinding away.

Besides epoxies, polyurethane based adhesives work well also. The two part versions are stronger but the more common one part formulas work quite well too and are more convenient to use. Such adhesives can be found in sizes anywhere from small craft quantities up to large tubes of construction adhesive by the case. Note that not all construction adhesives are polyurethane based.

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Excellent answer, thanks. I'm familiar with Araldite. I've searched for polyurethane based adhesives, and Gorilla Glue (not Gorilla Super Glue) seems to be a popular brand: I'll give that a try. –  Ashley Nov 25 '13 at 11:41
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You can use a soldering iron with a small pointed tip to heat the perimeter of the recessed area the magnet sets in. After it heats up, put the magnet in place, and the tip of the iron about 1/4" away. Then push the iron down and towards the magnet until the plastic melts and some of it overlaps the magnet, or, wedges it in place. Do this 3 or four times around the perimeter. Melting plastic like this is something I've done successfully a number of times for repairs. It creates a stronger bond than any plastic glue I've tried. If you don't own a soldering iron, you could try using a nail held in pliers and heating with a flame device like a cigarette lighter.

A simpler method would be to just cut a piece of duct tape to fit the bottom of the torch then press it down over the magnet. This assumes that it will stick to this type of plastic. You can test first by seeing if the tape sticks adequately to the plastic.

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Wow: great lateral thinking -- thanks! I do like these ideas, but I would also like to know about suitable glue... –  Ashley Nov 22 '13 at 19:16
    
Well, I've used Devcon High Strength Plastic Welder adhesive to glue cracks in an auto dashboard panel, and other things I can't recall. But it has worked good enough for me that when I run out of what I have I'll purchase more. –  getterdun Nov 28 '13 at 3:09
    
Ashley: Another plug for Devcon. I just went out and checked a test I was doing to determine the best adhesive for gluing a black plastic squeeze handle (I think it ABS). Anyway I tested Loctite epoxy, a marine epoxy, ABS solvent, and Devcon. I scraped all of them off with my fingernail except Devcon. I tried my knife and the Devcon held well. Damn impressed with how well it held. –  getterdun Nov 28 '13 at 3:25
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I have a similar torch/Flashlight and have had the exact same problem..

I fixed mine with a product called Q-Bond.

enter image description here

On my flashlight, the magnet was flush (ie not resessed) and I was able to build up a sidewall to add extra strength.

Needless to say... the magnet has not come off

(and yes, this is from years of camping in the heat/cold/rain etc.)

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