Simply heat up some water on the stove or in the microwave until it’s just below a boil, and prepare a bowl of cold water to the side. Place the plastic piece in the water to warm it up. Take the item out of the hot water bath periodically and test it’s malleability. Eventually it will become soft enough to straighten with your fingers.
Once you have it in the shape you want, put the plastic item in the cold water to cool off. This technique isn’t just for action figures and their bent katanas. I used this method recently to fix a bit of warped plastic on my immersion blender that was preventing my blade attachment from hooking up to the motor. It took about 15 minutes and saved me the bother of replacing an otherwise functional appliance. Plastic Welding with Heat If you have two separate pieces of plastic that need to be joined or if you have a crack, then you’ll need to do some plastic welding. The basic idea is to apply heat to the edges being joined in order to melt the plastic until it is liquid enough to blend the edges together. Additional plastic can be melted into the groove to create a stronger bond, but depending on the situation this won’t always be necessary.
There are tools specifically made to do plastic welding jobs, with attachments for each step in the process, but for small or one-time repairs, a cheap, low-wattage soldering iron will be sufficient. The video above from demonstrates the technique and also shows how to embed small metal wires to “stitch” cracks in plastic together.
Important: Melting plastic can be toxic, so do this in a well-ventilated area. Plastic Welding with Friction is another way of joining plastic together. A small bit of plastic (a short segment of 3D printing filament, for example) is spun at a high RPM and pressed against the intended joint between two separate plastic pieces. The friction melts the plastic of the two pieces being joined, as well as the plastic that is spinning, which creates a sturdy bond. In the mid-1970s Mattel sold a “Spin Welder” toy that worked on this principle, and in 2012 Fran Blanche of Frantone Electronics revisited the idea to create her own friction welder using an inexpensive rotary tool.
This is an especially good technique for repairing 3D printer misprints, since you can match the plastic exactly by using the same filament. Plastic Patches If you have a large enough hole, then you’ll need to resort creating a plastic patch for it. The answer to this particular problem seems to have been. Using a heat source (preferably a heat gun), soften the edges of the hole and the section of plastic you’ll be using as a patch and carefully place it to ensure the hole is completely covered. This get’s pretty hot, so you’ll need gloves. Once, the patch is in place you can use a hot metal spoon or trowel to smooth out the edges. Important: Use the same kind of plastic for your patch.
Using Acetone for ABS Plastic Acetone is a solvent that’s pretty effective at melting ABS plastics. 3D printing aficionados have been using acetone to smooth. A concoction of acetone and ABS known as ABS slurry can be used as a glue or as a filler for smoothing grooves or filling in gaps. Matter Hackers has for yourself.
Important: This method will not work with PLA. Good Ol’ Glue Of course, there’s always the option of gluing things together. For plastics it’s best to use super glue (cyanoacrylate) or plastic model glue.