Anyone Know A Good Way To Join Plastic?

[Jonj1611]

So the bracket for my headlight has sheered off, its a clean break and my options are replacing the whole headlight or trying to repair the break. Going to try and repair first.

There is a lot of vibration with a car so need something strong enough to resist vibration.

Not sure what automotive plastic is made from or if anything special needs to be used but definitely needs to be strong

[Saracen999]

3D printing?

Part of the problem is probably going to be that the word "plastic" is rather generic, a bit like saying "metal" bracket (well, lead, or titanium, or .... ), only much worse. Even just with 3D printing "plastic" there are, oh, a couple of dozen different types (or more) with very different characteristics, requiring different nozzle temps because of different melting points, etc. Some are 'bendy', malleable, etc, and some rigid, some strong and shock resistant, others will fracture or shatter. My guess is your 'auto' grade has to be able to withstand loads of vibration, both high and well sub-zero temperatures (and many, many cycles, at that). And, age. Some plastics can be easily glued with cyano-acrylics .... but a lot can't, and it just won't hold. And so on.

I don't envy you this one. The best bet might be a good epoxy, but I've had mixed results even with that.

P.S. I am not, by any stretch of the imagination, knowledgeable about plastics. This is just my minimal experience trying to do things like you are, often with little success..

[Jonj1611]

It's the part of the bracket I have indicated, seems more for adjustment and stabilisation. Not sure what else is holding the light in to be honest but with that bracket broken its just rocking about and being BMW I don't really want to have to buy a new one, though second hand are a lot cheaper but glue is a lot cheaper than that so going to try that route first.

[MrJim]

Araldite, perhaps? Epoxy glues can certainly provide a good bond, but clamping to two pieces together while it dries might prove problematic, given the photo you've posted...

Edit!

Making the support bracket out of plastic like that is asking for trouble, really. Many plastics become brittle with age, so using a plastic bracket in an area where there is considerable vibration is likely to fail over time. Unless it's a case of BMW building-in obsolescence, perhaps? Or maybe I'm just getting cynical in my old age!

[Jonj1611]

It's worse than that, if I want to replace the headlight, BMW in their stupid wisdom have made it so you have to take the whole bumper off!

Also it seems its both those clips that have broken, only just noticed the other one when I went to have a look at it.

Would need to repair in situ as there is no way I am taking the bumper off to take the lights out to repair, rather just buy a second light if I was going to do that and save myself the hassle of a repair that may fail and need the bumper back off again.

I think they do filler glues don't they? Where they fill small gaps as well as glue in place. Might be an option I guess

[DDY]

I could suggest two options.

1) Use superglue to glue the tab back on and reinforce it using epoxy resin, e.g. fill in the surrounding area to give it a bit more mechanical strength.

2) Identify the type of the plastic and weld it back together using the same type of plastic as filler, bonus points if you can weld on reinforcing gussets.

[filbert]

Shape a piece of metal and drill 2 holes either end and self tapping screws cut short if needed.

[Jonj1611]

Shape a piece of metal and drill 2 holes either end and self tapping screws cut short if needed.

The break is on a bend unfortunately, its too short as well to get anything behind where the break is as its right up to where the plastic joins onto the light

[Jonj1611]

I could suggest two options.

1) Use superglue to glue the tab back on and reinforce it using epoxy resin, e.g. fill in the surrounding area to give it a bit more mechanical strength.

2) Identify the type of the plastic and weld it back together using the same type of plastic as filler, bonus points if you can weld on reinforcing gussets.

Think I will try the epoxy route and see how that works out

[Saracen999]

This is probably an "of course I have" type of question, but .... have you asked on some BMW forums? i doubt you're the first to experience this and maybe someone found out what plastic it is and what might work to 'weld' it back (and a couple of supporting bits, maybe).

[Jonj1611]

I did yes, someone said Loctite 3090 is good stuff. Though only got one reply.

[Ice Tea]

Youtube search superglue and baking soda as people seem to swear by it, Obviously try it on a old scrap piece of plastic first to see if it's as strong as people claim.

I've never tried it so i'm unsure if it's a youtube windup or if it's brittle when bent?

[DanceswithUnix]

There's a specific "All Plastics" Superglue glue for this sort of thing:

https://www.amazon.co.uk/gp/product/B0001GRVPU/

Which is what I used on an MX5 rear light cluster which had a long wide locating bit which had snapped. ISTR I used a bit of paper underneath to make sure I didn't bond the light unit to the car, as it's strong glue

The alternative is a soldering iron to slightly melt and re-flow the plastic around the break, but it's really hard to get the right temperature and not just end up with a mess.

[jimborae]

I think you'll need to plastic weld it, basically solder the two peices together and feed plastic into it like you would metal welding. At least I'm pretty sure how I've seen this done before. I don't think gluing it will be strong enough unless you can add a strengthening strip to glue over the joint.

When I broke the headlight tabs on an Audi TT they were sacrifical and you could buy replacements that attached somehow, seems like BMW in their wisdom have not gone down the same route.

[Jonj1611]

I gave up on trying to do it to be honest, having a look at the car the tabs are broken on the other light as well but it went through the MOT like that so not going to worry about it. At night the lights are steady with no movement. Probably something to do with the fact that it is an over engineered BMW product. There are more bolts and screws on the headlight than there are holding the bumper on!