Thursday, July 19, 2012

Gluing Repairs

Hot Glue

And by this I do not mean "super glue" or Cyanoacrylate.. I mean the stuff that actually gets hot, in a little gun you plug in, and oozes out molten plastic.

You can buy two different melting temperature sticks as well as a two-temperature gun. You can also buy woodworking sticks which take longer to harden once heated to give you more time to position items together.

Here are some uses:

1) fixing drywall. Or plasterboard as the folks on the other side call it... If you don't have much time - not enough for a proper "wait for a deep crack fill job to dry" you can use hot glue as the initial stage and follow with a final coat of real filler - or plaster, whatever.

2) Any kind of temporary "tack-welding" of pieces that need to be held together while a stronger glue joint sets up

3) Fastening an external t-stat wire & sensor to an outside wall.

4) Use inside heat shrink tubing to create a good water-tight seal. Sure, you can buy heat shrink with the stuff already inside - like well wire splices - but that is not always convenient...

5) Amusing kids - add a handful of small junk items like old screws, cardboard, etc and they won't hardly complain about the inevitable burns they give themselves(!) making strange contraptions. Word of advice though - don't leave them unsupervised!

6) Fixing books - a quick a dirty method admittedly, I would not use this on any book I really cared about - but I have fixed a number of books that my 3 year old torn the cover of.

Super Glue

A really strong bond can be made by wrapping multiple turns of a strong cotton thread around something and then applying a drop or two of superglue, which will stop the cotton unravelling. Thin wire can also be used - and if you are using thin wire you can substitute solder for superglue depending on the situation.

Super glue sets in response to pressure, moisture and also certain chemicals - you can buy a "kicker" spray that sets it instantly. One alternative to buying "kicker" spray is to hold the glued-up item over the spout of a boiling kettle. Perhaps a combination of the moisture and the heat from the steam pretty much instantly cures at least the surface of the glue.

A good cheap source for this is the Gorilla brand at Home Depot, a 20g container for under $7 is great value. Those little tubes run out quickly and clog if they don't run out!

Solvent Glues

Use a solvent type glue when possible rather than gravitating to superglue for everything.

You can test if the glue might work by applying a little to an inconspicuous area of plastic, waiting a few seconds and then gently scraping the area with a screwdriver or something - some of the plastic (color) should come up if the plastic is dissolving. If so then chances are the glue will make an excellent repair. PVC glue obviously works on PVC. ABS on ABS. There is also a multi-purpose type glue that can be bought at the same type of stores (builder's, these are plumbing/electrical items) that works on both these plastics and probably other relatives.

Lego, and quite a few other similar toys, are made of ABS and so the real "Kraggle" would be ABS glue :)

For acrylic (some kitchen appliances - fridge shelves - display shelves that are clear) use something like IPS Weld-On plastic cement. The link is to quite a large tube!

Solvent glues have very strong fumes as you might expect.

Epoxy or Two-Part Glues

Epoxy comes in many different flavors and to some extent it does matter which you buy for which application.

Main differences:

  • time to cure. Can be from 90 seconds to 24 hours - in general the stronger bonds are from slower curing glues.
  • consistancy: you can buy thin liquid (for pouring in a mold) or the normal tube/plunger type which is fairly gooey and will need some containment or to be used in thin layers; or a stick / paste type which is good for filling large voids or making solid shapes.
  • additives. This is what makes one type good for plastic (it will have solvents added which help dissolve the surface of the plastic and increase bond strength) or metal / masonry where dust of the same material is added to increase strength plus I'm sure specific bonding agents. In a pinch they should work interchangeably though perhaps not as well!
  • Flexibility. Some are designed to cure fairly brittle and hard, others to a more plastic material.
Epoxy makes a really strong bond when done correctly. It is often worth roughening the mating surfaces before gluing.

One really great two-part glue is Plyogrip Plastic Repair. I used the 3 minute version to repair leaks and prevent leaks in some ABS tubular membrane shroud housings. This stuff is great in that it cures well and contains solvents to make it basically a part of the plastic you are trying to repair. It will be expensive to get started with because you have to buy the application gun - but with the right project, for instance repairing a car bumper, which I believe it is designed for - it would be invaluable. A really tough & pliable finished product.

Melting / Welding

Combine this method for best effect with the insertion of a metal strengthening member. You can use an old soldering iron (will hurt the tip of a new one or at least get it very dirty...) to "weld" the plastic together. Some plastics respond better than others to this. Some don't like it, "burning" and getting bubbly and brittle. Sometimes you can use scraps of plastic cut of things you don't want like buckets etc as "welding rods" to fill gaps. Once solidified the plastic can be filed into shape. It never looks very good afterwards but it often fixes something better than glue is able to.

Re the metal strengthening member: use whatever is at hand that works... Often a metal coat hanger comes in handy, snipped to the right length. Obviously it will depend on the size of what you are fixing! The soldering iron can be applied right to the metal and as it heats up it will sink into the surface of the plastic. When below the surface the plastic can be smoothed over the top and you have a finished product that may be stronger than the original if you did it right!


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