Repairing plastic items can be confounding for someone who’s never done it before. There’s no part to replace (usually), no nut to tighten, just something… misshapen or cracked. There’s nothing to repair but the material itself.
Plastic can be easier to work with than you might imagine. With some heat and some pressure many items can be fixed rather simply. So before you resign yourself to tossing a broken bit of plastic into your recycling bin, here are a few techniques you can use to repair plastic.
Hot Water, Cold water
This tip comes from the world of action figurine collecting. While I was researching this came up on various fan forums with very little variation from one source to the next. Plastics (particularly small or thin pieces) don’t need extreme heat to become soft enough to manipulate. 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 Delboy’s Garage 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
Friction welding 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.
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 best addressed by kayakers. 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 3D printed surfaces, bond pieces together, and troubleshoot misprints. 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 some handy recipes for making ABS slurry 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.