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Dave Platt[_2_] Dave Platt[_2_] is offline
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Default Epoxy / adhesive for hard plastic (headphone hinge) ?


In article , Mike S wrote:
I have Skullcandy Crusher headphones and the hinge plastic broke. It's a
really common problem with them. I don't need a hinge, I'd like to epoxy
the hinge joint in the fully open position as it would be when being
worn. I tried using 2 part epoxy to hold it together, but it wasn't
plastic rated epoxy and it held perfectly for about 4 minutes, it was
easy to pry off of the plastic and it was clear it didn't adhere well at
all. I remember making plastic model cars and planes as a kid, the glue
seemed like it melted the plastic pieces into one another for a really
good joint, like a weld in metal. I don't know what grade the plastic
they used for the joints is and I don't know how to find that
information. Is there an all purpose adhesive or epoxy that I can use to
make one big solid mass of plastic? Or do I pretty much have to try the
various adhesives used for different grades of plastic?


Plastics can be tricky. The broken surfaces tend to be fairly smooth,
which doesn't give epoxy much opportunity to "grab" onto it and make a
good mechanical bond.

Also, some common plastics have a "low surface energy" issue - put
simplistically (and not quite correctly) this means that the surface
of the plastic has relatively few chemically-reactive sites on it, to
which the epoxy resin could create a chemical bond.

So, without good mechanical or chemical bonds, you end up with a glue
joint that's not held together very strongly, and it breaks apart
easily.

I've had good luck on plastics using a thickened epoxy called G-Flex
655. It's sold by West System (a.k.a. West Marine) specifically for
use in repairing plastic and fiberglass boats. In my experience it's
an excellent all-around repair epoxy for most materials. A friend of
mine used it to bond steel pipes and flanges to sheets of granite,
after roughing up the granite surface with some emery paper. When he
tested it by banging sideways on the pipe with a sledgehammer, the
epoxy didn't fail... instead, the granite itself "tore apart" - the
rock pulled apart into separate crystals. This stuff has become my
"go-to" repair epoxy.

There are two tricks you can/should use when bonding difficult
plastics. The first is to roughen the surfaces with some sandpaper,
giving the material some "tooth" into which the epoxy can flow and
then harden - this gives you a better mechanical bond.

The second is flaming. Take a propane and butane torch set on a
fairly low flame. *Quickly* pass the very tip of the flame over the
area to be bonded. You don't want to heat the plastic up enough that
it melts, or even starts to soften... you simply want to let the tip
of the flame oxidize the very surface of the plastic. This helps deal
with the "low surface energy" issue, by creating reactive sites to
which the epoxy resin can create a chemical bond.

If you buy the West System G-Flex 655 boat-repair kit, you get two
very generous tubes of epoxy, some sandpaper, gloves, and "plastic
boat repair" instructions which goes through the above procedures in
detail. You can buy just the epoxy (without the extras) for a bit
less money. It's a more expensive initial purchase than the little
tubes you'll find at a hardware store, but it'll be enough to last you
for years.