What plastic cement?

Use Polystyrene cement . (available at all good model shops)

Or if the 'Plastic' is even older (ie Bakelite) use a five minute epoxy

The viewer is probably made of ABS or something similar. If you've got some acetone, try a very little drop on the inside (ie hidden) part of the viewer using a barely-wet cotton bud. If it appears to dissolve the plastic a little by making it less shiny then you can use acetone itself as the solvent to get two pieces to stick together, or use a solvent-based plastic adhesive. If the broken bits have a smooth surface, then cyanoacrylate (instant) adhesive should work, but you'll have to be very accurate in placement.

That will be styrene. Any solvent based plastic cement designed for models will work. If you haven't a model shop buy from e.g. amazon

Always do a test first though in case its a different plastic

PVC is another one, and you can get cement for that from plumbers..e.g. screwfix.

These cements are all 'bits of the plastic I want to join, dissolved in a solvent that dissolves it'

That was polystyrene, and the glue was merely a solvent. This is not a good choice for broken stuff, since it spoils the edges as it melts the plastic. Most of the Polystyrene that breaks is old and has leeched out the plasticizer and goes crumbly or brittle, so a repair may be short lived. This sounds like a job for a 3D printing expert. Brian

I had an Airfix 8mil hand held film viewer, that could run loops of film, but the case although supposedly a styrene bas was not amenable to glue when the mechanical innards came adrift from the melted on pillars holding it together, ended up drilling it and putting in some small set screws and nuts. It may well be that if what has broken are retaining clips that a few well fashioned bits of metal and some screws might actually look better than of glue and melted plastic. Brian

I have found that whilst superglue works, it sets slowly on plastics, and it often runs everywhere and makes a right bloody mess. I have never found out why superglue sets instantly on carbon fibre tow, balsa wood, fingers and baking soda, but takes forever on anything plasticky.

If anyone knows, please enlighten me.

One trick I've found that works is to huff/blow on the join after the join has been made. The glue cures with the moisture in the air and your breath adds more moisture.

I get the feeling AirFix plastic is designed to be glued by a cement. Most everything else is a different plastic, and the AirFix type glue does not work.

I tried some of SWMBO's nail varnish remover (contains acetone). This appeared to have no effect on the plastic other than to evaporate v. quickly.

Common these days for nail varnish remover not to have acetone.

I've found that using acetone on its own doesn't work too well as it dries too fast and before the edges of what you are trying to fix dissolve and becomes sticky. Much better to dissolve some similar plastic in a small jar of acetone so it becomes a thicker substance more like airfix kit glue. This make take 30 minutes or more. Depending on what is being repaired, I've found that some reinforcement is required such as a thin sheet of the same type of plastic behind the join. It needs to be something that also partially dissolves with a acetone solvent glue allowing the bond to extend beyond the crack.

When apply things like superglue to a edge I tend to put a drop on a plastic bag (food bag) and then use a cocktail stick dipped in this drop and then using the pointy end of the stick to apply the glue only to the join edge. The same can be achieved with a shaved down match stick or a sewing needle. I use this method with a two part epoxy if what is to be joined is thin or small.

Up to a point, yes.

Not true. Every plastic has a solvent, except polyethylene, which is almost completely unglueable, and IIRC nylon.

All the other common thermoplastic ones - PVC, Styrene, ABS etc glue well.

I cant remember about polycarbonate, but the specific solvent for perspex was chloroform.

I am not sure that acetate plastics - used in transparent bubble packaging - glues well, either, with anything I have to hand

If it didnt when you wiped it craze the surface AT ALL then it is not a solvent

If you have cellulose thinners, try a tad of that. I think MEK is another very aggressive solvent

Oh, and if what is required is not massively strong bonding nor especially neat, hot glue is an option

Worth wetting ceramics prior to glue application.

Unless it lists acetone on the bottle, as was the case here :-)

However, SWMBO concedes that (1) her bottle is ancient and agrees with you about newer formulations, and (2) some of the acetone may have evaporated anyway. But something, when I applied some to the plastic, visibly evaporated like gangbusters and did not craze the surface as TNP was asking in another post. Rather, it seemed to clean it, if anything.

There are some pictures in this zip file:

Mmm. I'm not excited by that plastic. I don't think its styrene or ABS. you dont generally mould threads into that.

Try superglue but be prepared for failure. possibly using baking powder in the cracks too

Acetone doesn't work as well as a solvent when it's mixed with water (it's fully miscible). Even so, I would have expected some change in surface appearance - other than cleaning it, so I'm with TNP that it isn't ABS.

With that sort of damage, I'd go for epoxy. If there's room, I'd build up a decent layer for strength. To add to the strength, if you have any fine wire (copper, 5A fuse wire, or even a single strand from mains cable), wind it round the circular bit a few times and epoxy over that. Also, if you got the screw which goes into that threaded bit, I'd wrap a thin piece of clingfilm round it, and screw it in. That should mean that not only are the threads in the plastic still aligned, but if any epoxy does leak in it'll just stick to the clingfilm, and the screw will still be removable.

Thanks, those sound like good suggestions. It's a long bolt a few mm diameter which pushes through the rear half of the viewer (what is in the pix is the front end with the lenses) and ultimately pulls the two halves together.

It's quite lucky I had the bits, actually. The larger piece I happened to notice amongst other grot on the carpet at the FiL's house while sitting waiting to leave. That was the trip just prior to the very last visit during which the grand piano was reduced to rubble. And we completed today, so that's a load off SWMBO's shoulders.

Judging by the smell, solvent based uPVC (double glazing frame) cleaner seems to be mainly acetone.