How to glue plastic together successfully?

[beko1987]

Hi Guys.

A little project of mine at the moment is restoring an early 1980's Electrolux 412 Electricity Board Special. All going fairly well at the moment, although I have hit a slight snag.

The plastic is very brittle, and whilst I was re-assembling, I managed to snap a plastic thread off the chassis. Very upset by this, as the machine was almost in mint condition prior to this.

I tried sticking it back together with some glue from Wickes, and superglue but the join cracked again as soon as I even thought about moving it. The plastic does not seem to be taking to the glue.

The break in question is where the bag latch mechanism screws to the bag door, so when you press on the release button, it bends and releases a plastic catch, freeing the door. It is the housing that all this sits on, and flexes with that has snapped.

Can anyone recommend anything to permanently fix this together? I won't be opening and closing the machine often, to avoid further repeated damage, but it would be nice to re-assemble the machine and use it. It would also be good if the glue could fix some cracks in the rubber furniture bumper too. The wickes glue holds to this, but the glue itself flexes too much and eventually snaps.

The pictures show:

The casing before I took it apart. On the dark brown part, on the right hand side of the picture.

The snapped off thread. To the right before the hole where the button protrudes through. This has snapped clean off, so I am looking to adhere the assembly straight to the housing.

The bag catch which needs to be stuck to the housing. The button to the right protrudes through the hole as above to that orientation.

Thanks

Sam

[Lucien Nunes]

If the cross-section of the break is small and the part is highly stressed, which is often the case if it broke without excessive rough handling, even the best glue probably won't do the trick as it will never be as strong as the unbroken part, which itself wasn't strong enough. This joint is also at a disadvantage from being in tension rather than shear or compression.

Your vac parts might be ABS or PC/ABS mix, which can be bonded using high performance epoxies intended for materials with low surface energies. Even superglue might might do the job under ideal conditions maybe with the use of an olefin primer. Solvent welding would probably be better, using acetone in which some shavings of the same plastic have been dissolved. A solvent-weld can be nearly as strong as the original material if the solvent is allowed to penetrate the surface of the parts before they are clamped up firmly. Success with any of these methods will depend on the exact nature of the material and the quality of the preparation.

I work on the basis that if the fit has to be exact, with the texture of the mating parts interlocking, there will only ever be one chance to get it right. If you try one adhesive that does not work well enough, it will usually be impossible to get the surfaces chemically clean again to try another, without mechanically abrading them e.g. by grinding off the surface with a burr. Then you don't have an exact fit and the strength advantage that this confers is lost.

I did once re-attach a plastic boss to a housing by friction welding, it think it was polycarbonate. I put the boss in the chuck of a pillar drill and pressed it, spinning, against the surface until a bead of molten material appeared at the joint. After stopping the drill and waiting for 30 seconds with the pressure still applied, I had a very secure bond. You might be able to use this method to re-attach the threaded pillar, which I think will be better than attaching the catch directly. Fusion welding usually gives the strongest joint because it is homogeneous and it works on all thermoplastics including polypropylene, which has such a low surface energy that it can't normally be bonded with adhesives.

Lucien

[beko1987]

Hi Lucien.

Thanks for the tips there. I can attach the housing straight to the part and leave out the little threaded part and it will still work, and that is what I attempted to do before, although as you said, the surface area may not be great enough still.

Will have a good look at solvent welds. I may be able to shave bits from hidden plastic, to brace the join out a bit. Or, would plastic from another vacuum cleaner of similar vintage be enough? I'll have to be pretty lucky to get the colour, but it's a rare enough machine that I would rather it were working than scrap.

Sam

[ex 2 Base]

I'm not exactly sure what you are try to do, but if it's what I think you are trying to do, could you drill down the centre of the plastic thread and fit say a long 4BA nut and bolt. I've done similar things in the past when the plastic part was no longer available.Best of luck, such things can be so annoying, especially when one has been so careful with a restoration.Ted

[beko1987]

I did think of that, either a small pop rivet and glue, or a flathead nut and bolt, but they would be seen from the front, and I don't want that.

Plus, I'd be worried about the rivet cracking the main housing and causing even more damage. At least if I cannot stick it, all I have to do to empty it is manouver a screwdriver up and pop the catch from the inside.

Been looking at solvent weld products (bit quiet at work today), and think I will give that a shot, along with maybe some surplus bits of plastic to beef the join up a bit.

[Station X]

Solvent weld is sold in DIY outlets and plumber's suppliers. It's used for sticking together plastic waste pipe.

[Leon Crampin]

I am a firm believer in slow-setting Araldite, applied warm and to warm, meticulously clean components.

If you have a highly stressed area to stick, use a fine drill to make holes for reinforcing pins. Pieces of darning needle often do this job well.

Dentists often fit metal "posts" when securing a crown or large filling to an unstable area - any comments, Nick?

Leon.

[beko1987]

Just found some Araldite Fusion which looks like it may do the trick. I think there is some Solvent Weld in the garage too, so will grab that tonight and give it a go.

The next problem I can see is I need to clean the two mating surfaces up of the old glue. I'm hoping it will just peel off, or chip off without causing too much damage. Will have to get some fine sandpaper and whittle it down.

Thanks for your help guys. Will try solvent weld, if not then araldite it is

[Nickthedentist]

Originally Posted by Leon Crampin

I am a firm believer in slow-setting Araldite, applied warm and to warm, meticulously clean components.

So am I, but like Lucian, I'm not sure any repair here will be particularly durable.

Originally Posted by Leon Crampin

Dentists often fit metal "posts" when securing a crown or large filling to an unstable area - any comments, Nick?

In this context, we call them "pins", but from an engineering point of view, they're really just fine, short lengths of threaded stud. A specially-designed twist drill ~0.5mm in diameter is used to drill a hole ~3mm deep in the dentine, then the pin is screwed into the hole, leaving a similar length protruding above the dentine, onto which a filling material can be placed.

Non-threaded types (not dissimilar to your darning needle suggestion) are also available which are retained by cement. They're less securely retained but kinder to the tooth as the threaded pins can cause microcracks and subsequent problems.

But neither are generally used much nowadays, as "dentine bonding agents" (i.e. specially-formulated primers) have taken their place over the last decade or two, allowing restorative materials to be bonded directly to tooth tissue. Most dentists keep a few in a drawer "just in case" though, although I used my last one about 5 years ago and haven't felt the need to re-order.

Back to the vacuum cleaner... Do try Leon's darning needle suggestion. Regrettably, I have no suitable twist drills any more, otherwise you could have had one.

Nick.

[beko1987]

Will have a look tonight and see what can be done about bracing. We don't have any darning/knitting needles. Me thinks the missus needs another new hobby...

[brenellic2000]

The short answer is that you can't 'glue' plastic together as there is no fibrous matter for glue to bond to. You have to meld (like old fashioned forge welding) by fusing the two faces as one using a solvent. Araldite, cryo etc only work by surface tension.

The next problem is to determine what type of plastic it is. Methylchloride, dichloromethane etc are used industrially, but the solvent sold by DIY chains are rarely strong enough to achieve much. They are both nasty stuff!, so it's a case of trial and error or talk to a local plastics supplier/moulder.

Barry

[beko1987]

Well, I tried again with the wickes glue, since I think the problem before was I didn't clamp it together long enough. Reading the comments again it seems like it probably won't work, but I can't afford any araldite this week, so I thought I'd give it a try for something to do!

Few pics below of the progress, will leave it for a day or so and see how it does. If all else fails, I will get some proper glue.

[beko1987]

Few more as ran out of space on the last post. I can't see any identification marks on the plastic, will have a look around the motor when I next strip it out. From 1982/3 if it helps.

[Geoff 555]

Hello beko 1987 last time I was in Maplins and wanted some standard araldite I was quite surprised to see that araldite do a plastic adhesive. Perhaps worth a look to see if it would do.
Cheers.
Geoff.

[emeritus]

If the plastic is polystyrene, you can get small bottles of sutable solvents from shops that sell plastic kits, although as others have pointed out, you do need the correct solvent for the plastic you are using. What I am using at present is a liquid simply called "PLASTIC WELD" made by EMA Model Supplies, which does make effective welds in polystyrene by dissolving the mating surfaces. I previously used MEK [a trade name for a solvent that replaced the highly toxic Methyl Ethyl Ketone]. Never attempted to use it on Vacs, but is has been fine for fixing cracked or broken fridge drawers and doors. Humbrol liquid poly is not suitable as it does not dissolve the mating surfaces and leaves a white residue.

Where appearance is not an issue, I have used the pin technique to provide reinforcement for things such as projections that form the pivots of hinges by drilling a hole just big enough to take the wire in the main body and then building up the missing part with araldite or plastic padding and filing down to size when it has set. I usually use stainless steel dental wire that I got in a range of sizes and hardnesses many years ago from "Proops" when they were still at Tottenham Court Road.

[Glowing Bits!]

How about using cyanide glue (as I call it), Maplins sell it.
It works best when left overnight to 'fuse' the plastic together, although it can be hurried along with an activator although the end result after using this spray could well result in a weak joint.
If it works for bakelite, it'll work for most plastics.

[brenellic2000]

The clue to Wickes 'Superdooper adhesive 'is the word "Adhesive" - adhesives adhere, and that is all they do! These are usually rubber based.

(I also need to correct my earlier error: methyl chloride should of course be methylene chloride! It's old age!!)

[brenellic2000]

... back again! I see Dichloromethane in a usable strength is now banned by the EU! (We used it extensively in the 1970s in medical plastics).

Looks like MEK or Tetrahydrofuran are now the best all-rounders - still very nasty stuff though! - even so the average DIY tube will still not be that good on anything, unless it is a Loctite branded product.

Barry

[emeritus]

The product I said was called MEK was actually called MEK-PAK. I think it is still available, but different shops seem to stock different products that do much the same thing.

I see from the lable of the PLASTIC WELD I currently use that it is "Dichloromethane ( Methylene Chloride)", strength not mentioned. I bought it a couple of years ago in a 57ml bottle and is evidently the stuff mentioned in #11. Was it banned recently? I only have half a bottle left! On reading the rather faint small print on the lable for the first time, I see that it is stated to be suitable for the following plastics: Styrene, Butyrate, ABS, Acrylic (Lucite or Plexiglas), Perspex, either to each other or in any combination.

[Bazz4CQJ]

Re MEK, which is now butan-2-one, the exposure limit over 8 hours is 200ppm so I would not regard that as extremely dangerous. By comparison, the exposure limit for benzene over the same period is 1ppm; that is dangerous! I think it is the issue of flammability rather than toxicity that might be born in mind with MEK. In most applications, acetone will work pretty much as well as MEK, and being available as thinners for nail varnish is easier to get and is less toxic.

I've stuck with Araldite a good deal over the years, but it's funny that their run of the mill epoxy is so slow to set, whereas the fast version is all together a bit too fast. An American friend thinks very highly of the adhesives made the the US company 'J-B Weld', now commonly available in the UK. Anyone tried those products?