[Closed] Bonding plastic

Keying the surface is important. Use sandpaper to rough it up, should give a better bond.

Try a more flexible epoxy, some can be too brittle.

Depending on the plastic Tensol (plastic cement) gives a pretty permanent bond. Will only work on certain plastics like acrylic or hips though.

The first thing you should do is key the surfaces before bonding them. Clean the surfaces of any remains of the glue, then score across with a sharp knife blade, like a Stanley blade, and rub over with really coarse sandpaper. Degrease with some of the alcohol hand cleaner you’ve got around the house as well.
Looking at the photo, it doesn’t help that you’ve filled the threaded hole with glue as well.
If you’ve got too thick a layer of glue between the two surfaces, that increases the chance of the joint failing as well.

A heavy duty sealant like Sikaflex might do it, or a more flexible resin like g-flex, but both those are sold in much bigger quantities than you need, so would be expensive.

I would try again with Araldite, but thoroughly scratch the plastic with something like the tip of a knife first. Another thing that's often recommended for making plastic accept glues is to oxidise the surface by quickly flashing a flame over it, such as lighter. You don't want to melt it or visibly burn it, it's just a very quick scorching of the surface.

I'd be trying to do everything I could to avoid glue and find a mechanical fix tbh, is there any way you can get a slim nut on the rear side of the light housing, then find a slightly longer bolt to thread onto it?

Or an ugly fix with a big jubilee clip and a section of old inner tube to put between the light and the clip for it to tighten on to, you could probably get it to bond but a whack on the wrong point of the light could crack the bond and leave you riding home holding the light in your mouth or something 🙂

Unfortunately where screw (metal) goes into the light body (metal) has threaded

I don't really understand how it all mounts together but if the only problem is that a metal screw to a metal body has stripped, can't you glue the screw in place with the stripped thread providing a key?

I have had very good results gluing aluminium to aluminium with PU glue, so I would try that, although PU glue is extremely messy if care is not taken.

Retap the hole with a larger thread and use a larger scree to attach?

Loctite do an All Plastics Superglue

https://www.amazon.co.uk/gp/product/B0001GRVPU/ref=ppx_yo_dt_b_asin_title_o04_s00?ie=UTF8&psc=1

I epoxied mine on about 2 years ago with some old West system thickened with micro balloon.
The thread was fine the bracket was just a tad flexible/loose for my liking without the adhesive.

It's still holding on but has definitely cracked away near the edges in places, the Aluminium has a sort of bead blasted finish I roughed it up locally with some wet and dry paper, but isn't a great substrate for it to key into generally.

You could probably helicoil the thread
Or perhaps 3D print/fab up a wrap around/clamp style bracket...

From the look of things the araldite has stuck well to the plastic but not stuck well to the metal (or more importantly to whatever the finish is on the metal?). You can get incredibly strong adhesion/bonds between things in the right circumstances but unless you really know what those surfaces are and are prepared to spend ££ on the right adhesive then scratch is probably right - a mechanical rather than a chemical solution is probably best. That said, if it was mine and that wasn't easy to do I'd clean the threads and wrap some PTFE tape round them to see if I could get enough to stick! (I've also done this with locktite thread lock and standard superglue in the past as "get you home" fixes that have lasted much longer than intended). Superglue may even work to stick the bracket to the light.

Superglue is not appropriate for that sort of application, it’s far too brittle.
Clearing the glue out of the thread and re-tapping with a slightly larger bolt would be the best option, then using thread lock.

Thanks all
Retapping isn't really an option as the metal is paper thin.
Looks like a combo of better surface prep, different bond and perhaps a physical option is still worth a shot.
Jubilee clip is a good shout, if I can bend it to shape.
Will keep trying.

I would also try araldite in the thread and then screw the bracket back in

3d print or use Sugru of with a 'flat top', then use heavy duty elastic or velcro to mount the light to it?

A couple of zip ties around the bracket and the whole light - not pretty but might keep everything together?

^^^ what he said

Cable ties and some strong double sided adhesive tape / foam, or something like a go pro adhesive pad

Clean all the old glue off, drill a small hole in the bracket above where the existing mount hole is, stick double-sided foam pad / tape on the bracket then one zip tie through the hole you drilled and another just behind the mounting bracket thing, if it's sloping in that area file a small notch to hold the zip tie in place.

I love zip ties, my car / bike / house are held together with them I think!

A clever production engineer where I used to work questioned our practice of using a file to roughen the surface we were going to araldite. Trials showed that the bond that stayed strongest for longest, through temperature cycling and damp heat, was on a smooth, clean surface.

Plastics generally need a primer for the best bond, but which one depends on the type of plastic.
If you Google 'Identify type of plastic', you should find some good guides, like this one from The Welding Institute:
https://www.twi-global.com/technical-knowledge/faqs/faq-how-can-i-easily-identify-a-plastic
If you can find out what the plastic is, again Google should help you find the best primer.

Mechanical support as well, like the Jubilee clamp or cable tie suggested above, is a good call.

Sorry it isn't easier, but that's plastics for you.

Evo Grip type stuff? You've probably got some floating around in the shed/garage. I've been amazed at the bond it gets on things - latest one was a ceramic surface mount sink onto a varnished oak work top. Its in a very busy commercial bathroom in a pub and has had all kinds of idiots swinging around on it and hasn't budged at all..

Thanks all.
Have tried the roughing and cleaning surfaces, but also bond in the screw hole as well.
Also the old araldite from the first attempt set in the screw hold and gave the bolt something to thread into.
Also...zip tie now.

Will see what happens on tomorrow's night ride!

Fill the old screw hole with Araldite then pilot hole and drive a self tapper in while it's going off. Undo it once the new threads are cut and use that to hold the bracket on. I'd also use a 3M foam (acrylic)  double sided tape as a gasket for the joint between bracket and light body (gopro pad).

The joint will need to flex slightly or it will vibrate itself to bits if it's too stiff.

Metal should been smooth and clean (alcohol wipe clean) and the bracket will need cleaning but probably not scuffing, think it's ABS. You only really need to oxidise poly prop to get good surface adhesion. Primer is only really for initial grab, if the surfaces are clean then after 24/48 hours the tape will have stuck, don't try and use it straihht away!

I've worked in automotive manufacture for 20 plus years, we've stuck a lot of different bits of plastic together over the years!

Sikaflex for sticking plastic panels to cars/caravans/motorhomes should do the trick. It's amazing what it's holding together in my campervan.

This is a slightly cheaper, slightly more available version from a sub-company of Sika.
https://www.toolstation.com/puraflex-40-pu-sealant-adhesive/p85566?searchstr=puraflex

But @fettlin above sounds like he knows what he's doing