Hey all! This is my first (serious) instructable so be gentle on me as I learn how to write good instructables.
That being said, in this instructable I will be teaching you how to fix plastic parts under the following assumptions:
(a) that the part has snapped, with two precisely matching interfaces and no lost and/or various bits and pieces in-between
(b) given that the plastic is a thermoplastic, which means that it could be remelted with heat over and over without changing its properties
To test if the plastic you have in hand suitable for this repairing process, try to heat a corner and see if it melts in a straightforward fashion as opposed to changing in color, bubbling, or just softening without melting. In case the material showed any of the previous signs, however, then it is probably unsuitable for this repairing process.
On the good side, this repairing process provides you with the following advantages:
(a) Takes less than 5 minutes to repair and cure (depending on the size of the interface)
(b) Does not require you to identify the type of plastic used, as long as it proves to be a thermoplastic
(c) Does not require you to have additional mass (in the form of wire or filament) of the thermoplastic in question
(d) Gives a bonding strength equivalent to the strength of the original piece before snapping
(e) The repair can be repeated with excellent result and without making the part any less fixable (beyond the second disadvantage below)
On the bad side:
(a) A weld-like ring will form at the perimeter of the joined interface, deforming in shape a bit
(b) As the repair is repeated over and over, the dimension normal to the snapped interface will shorten by a millimeter or so at a time (depends on the type of plastic and the amount of heat applied)
So let's begin!
Step 1: Grab Your Hot Air Gun
For this repairing method, all is needed is a source of localized heat, preferably in the form of a heat gun.
Either a corded, standalone heat gun or a soldering hot air station will do. In our case, we use the soldering hot air gun as it lies around in abundance in the makerspace, and is temperature controlled.
The use of a nozzle is advised to have more control over the localization of heat, but then again, that really depends on the size of the interface to be rejoined.
Step 2: Grab Your Clamp
After the hot air gun is set, now is the time for you to figure out the best way to clamp your piece in the fixed position, where the two (matching) snapped interfaces are pressed against each other with a decent amount of force (this will help you attain the maximum strength as a result of the repair, resulting in a part as strong as the original part -before snapping).
Once that is figured out, it is the time to attempt the actual repair...
Step 3: Remelt and JOIN!
Now that you have prepared well for the repair, it is the time to start applying the heat to the two interfaces until they melt to a sufficient depth, and then clamp them with force against each other as they cool down. This will help you achieve the maximum weld strength possible. (Watch the video)
Step 4: Test and Abuse
After the clamped repaired piece has cooled down sufficiently, unclamp it, then test it within reasonable limits. Do not be scared to break it again, as the repair is totally repeatable (if it wasn't for the loss of the normal dimension).
Note that despite losing a bit of the thread depth in the case of our threaded brush handle, we were still able to fix it 3 times while retaining functionality.