Glue for plastic
 

Hi All, I am restoring an RGD R130 transistor radio. Internally, the PCB support pillars have all broken off. I was thinking of gluing with epoxy resin and then adding a reinforcing fillet. But the epoxy glue says "not suitable for polyethylene or polypropylene".

I don't have much knowledge of plastic types but I think the case may possibly be one of the above. Any recommendations for a suitable glue? Obviously it has to take the weight of the PCB so needs to be a good strength.

Thanks, John.

Re: Glue for plastic
 

Might be worth taking a look at (say) Loctite's website. I cannot recall ever having dealt with this particular problem myself.

B

Re: Glue for plastic
 

I'd suggest perhaps using a small amount of any glue you intend to use, to bond the head of a flathead screw to some not-normally-visible part of the inside of the plastic case. Leave it for a few days then see how easy it is to remove the screw.

That way you'll at least get to see if the glue bonds successfully, or whether it turns the plastic into a quivering, slimy gelatinous mess.

Re: Glue for plastic
 

Polyethene needs a solvent adhesive, such as that sold for solvent welding polythene waste pipes.

Re: Glue for plastic
 

I'm about to embark on the same repair of mounting pillars on a 1980s Soviet radio (discussed on another current thread). Knowing exactly what formulation of plastic you're working with is pretty well impossible. I'm about to try "Araldite Standard" two-part epoxy (which gives the same warning as yours). I will let you know how it goes. If you get to try G6Tanuki's test before I do, please do let us know the result

Mike

Re: Glue for plastic
 

I'm not 100% sure, but I think the pipes you can join using solvent welding are PVC and ABS. I think those adhesive are based on acetone and basically liquefy the surface for a while. Acetone doesn't react with polyethylene.

B

Re: Glue for plastic
 

The Pipe adhesive that I use is MEK based and very good it is too.

Re: Glue for plastic
 

I'm pretty sure that, like acetone, MEK doesn't dissolve PE or PP

Re: Glue for plastic
 

I'd be surprised if a radio case were polythene or polypropylene. They're usually something more brittle polystyrene perhaps or ABS.

Re: Glue for plastic
 

Ah, well spotted that man.
B

Re: Glue for plastic
 

My recollection is that polythene/polyethylene and polypropylene have a soapy feel and by their nature cannot be glued or bonded. Something to do with the molecular structure. I think that plastics of these types were used in some of my children's toys, and when they broke I had to mend them using mechanical fastenings such as wire, small rivets or screws as nothing would stick to them. Possibly the radio case would be ABS or Polystyrene in that era, which can be mended using solvents or epoxy adhesives. I remember that the Polyclens paint brush cleaner that was around circa 1960 could be used for assembling Airfix kits as it dissolved polystyrene. Where a pillar needs some extra support, or part of it is missing, I use Plastic Padding. Plastic Padding seems hard to come by these days, so I use Ronson wood filler, which seems to be much the same product.

Edit: crossed with Herald!

Re: Glue for plastic
 

I haven't tried it, but have picked up the impression that two-part epoxy can be used for replacing small mouldings and for filling gaps in broken parts (though a rather expensive way of doing it). Am I misinformed?

Mike

Re: Glue for plastic
 

two part epoxy can, also the resin and hardener for glassfibre work is a cheaper alternative available in larger quantities.

If beefing up or repairing a pillar of a moulding, you can get a big boost in strength by including threads or a wrapping of threads in the wet stuff before it sets.

David

Re: Glue for plastic
 

Plastic Padding sets quickly ( often too quickly) , which can be useful. It is also stiff and will stay in place better than epoxy adhesives like Araldite. For runny epoxy, strategically-placed pieces of masking tape can be used to stop the epoxy running away from where it is needed.

I have mended things like plastic knobs for D-shaped shafts where the boss with the D-shaped socket is cracked, by winding stainless steel wire around the boss and then embedding the wire in Araldite. When "Proops" was stlll in Tottenham Court Road, they once had a consignment of government surplus medical wire. I bought some coils of s/s dental wire in various gauges and hardnesses, and some brass ligature wire, all of which have been useful for making various repairs. The fine stainless steel wire is very flexible and I have mended broken unglueable plastic parts by sewing them together with the wire, first drilling rows of small holes in both parts and threading the wire through the holes using a sewing needle , thereby pulling the broken parts into engagement.

Re: Glue for plastic
 

I have successfully used “plastic weld” on such pillars. Available from model shops etc, it produces a solvent welded joint. I think it is MEK. If bits are missing, then epoxy can be used to build it up but mechanical reinforcement such as binding wire, metal pegs etc are useful.

Re: Glue for plastic
 

Emeritus's suggestion of using wire is a good one. Steel guitar strings are a good source of wire in controlled diameter. And for something-like-Araldite-but-a-bit-thicker-so-it-doesn't-run-away - try a methacrylate glue like Plexus (expensive) or scigrip (cheaper). But watch out - it goes off in about 2 mins.

Re: Glue for plastic
 

Thanks for all replies. I'm currently trying the test glueing suggestion and so far no disasters!

Re: Glue for plastic
 

Thanks Chris, it's certainly a hard plastic which will break with a big enough impact or stress, but I don't have the knowledge to say what it is.

Hope you are keeping well these days, I take it you are not working at OMT these days?

John.

Re: Glue for plastic
 

I remember Polyclens. I was using it as a brush cleaner back in 1973 and had it in a margarine tub or similar. I left it while I went out, on return the tub had dissolved away!

Re: Glue for plastic
 

Loctite 'All Plastics' superglue is specifically designed to bond 'soapy' plastics such as polypropylene, polystyrene, polyethylene, PTFE, etc. It's a two part glue with an 'activator' and also glues china, rubber, metal and wood. I have used it to successfully repair all sorts of cracked plastic, including the gears on an HP8640B. Don't use it if you just want to surround a weak point with something that will harden off and provide some additional support. Expect to pay at least £5 for a small tube and buy it only when needed as it does not have a particularly long shelf life and tends to bung up the nozzle.