Tricky plastics gluing problem

[dom]

I've had the door mirror on my van clonked. Not only did it break the glass, it also broke part of the electric adjust mechanism. Every other bit is fine.

I want to glue this bit of the mechanism back together.

It's a very thin area on a little flat connecting rod which has sheared through - about 1mm thick and 4mm wide.

It's a very smooth white plastic - is this nylon6-6 ?

I need to get the best possible glue repair - what will really work?

I tried araldite but it barely bonded to the plastic, and fairly easily detached.

[Dave Plowman (News)]

Some of these bendy plastics are impossible to glue together.

[dom]

It's a (very) rigid plastic.

[Grunff]

I gave up trying to do this type of repair many years ago. I used to try and try, and it would always fail, either immediately or after a few weeks.

The tensile strength of most structural plastics is at least an order of magnitude greater than that of any adhesive bond you're likely to achieve.

I would say trying to locate a replacement piece of plastic from a scrap yard will be both quicker and cheaper.

[Stuart Noble]

But it's only 1mm thick so end to end fixing probably isn't possible unless you use splints and the extra thickness might affect the mechanism.

[Andrew Gabriel]

If you can find a solvent (depends on the plastic -- PVC is probably the easiest for this), a solvent weld can work well.

If it's a thermosoftening plastic then melting the two broken edges and pushing together might work. I did this suprisingly successfully with a piece of plastic inside the freezer which broke, and is flexed each time you open and close one of the compartment doors.

In this case, could you connect the two ends together with an electrical chocolate block connector, possibly just the metal part extracted from the plastic covering? I repaired a snapped clock pendulum for a neighbour this way once. The little brass part looked like it had been made for exactly the purpose!

[Dorothy Bradbury]

More likely to be PET (polythene) or ABS or similar. Nylon is an extremely tough material by comparison.

Your problem is the surface energy of polypropylene & polythene is very low - adhesives have nothing to bite on. Recently 3M "solved" this to some extent by a primer that is used with their glues/superglues - but it is not cheap.

As mentioned solvent weld can work - look on the piece of plastic for a triangle with a code like PE PET PP etc. It will tell you what the plastic is - PVC, ABS and so on.

If not successful check recyclers for replacement part.

Car door mirrors which are pre-painted, heated & electric are about 59-89ukp for the entire assembly. So a van may be somewhat less - 3M glue/primer would be £12-35 and if it is a plastic that can be solvent welded £3-9 or so.

[dom]

Hi thanks for the comprehensive advice.

As a little experiment, I tried burning another part of the assembly with a lighter. It both melts and burns, and continues to burn with a bluish near invisible flame. No perceptible smell on the tiny bit I burnt. No smoke either.

So that means it's a thermoplastic?

No triangle code on any of the components btw.

To tell the full story, I first glued the component with araldite (in situ), and it appeared to hold - then tried to attach a replacement mirror (=A325) - which put too much strain on it and broke it. So I gave up and bought a whole mirror assembly (=A360).

But now I've taken off the old mirror and disassembled the motorised mirror components, I'm hoping to have a better chance of a fix.

If I do, I have saved myself =A360 for next time somebody can't drive round a parked car (though no doubt they'll get the other side).

The way the other part (which looks and feels identical) melted, does give me some hope of a thermal weld - although it's a very small thing to try to dab with a soldering iron.

[Skipweasel]

Can you splint it? If you can roughen the surface a bit to give the glue a key and then bind a splint to it - soaking it in glue when it's bound. Don't make the coils of the bindings too close or the glue won't get in.

[Pete C]

If it's a thin part I'd try a strand of wire wool as a hot wire, clamp it between the two faces and apply power for long enough to fuse them.

cheers, Pete.

[Mathew Newton]

I'd recommend trying Chemical Metal - aside from making a good adhesive for the (limited) mating surface area you can also build it up beyond the joint to make a bigger joint which overlaps the other faces of the pieces if you see what I mean? Dries to full strength within minutes so you'll soon know how good it is.

Mathew

[Harry Bloomfield]

Dave Plowman (News) used his keyboard to write :

One of my repairs methods for delicate broken plastic parts, is to push thin steel splints in with a hot soldering iron. Glue the parts with what ever will hold them temporarily, then find some suitable thin metal pins which can be pushed in across the break.

I sometimes find individual paper staples, or paper clips of use. The method is to cut the metal to length, put a sharp bend at each end to help anchor it in the plastic, then lay it on top of the plastic and apply an hot soldering iron to push it deep into the plastic. Let it cool then add more splints if necessary.