Most people when they think of welding will think of heat, bright light, sparks, molten metal, and a guy in a mask, or perhaps a robotic arm. But welding doesn’t only apply to metals. Believe it or not, some plastics can be welded as well.
Thermoplastic and Thermosetting
There are two main types of plastic, thermoplastic, and thermosetting. Unfortunately, once thermosetting takes its shape, it can’t be welded; but thermoplastic can be reheated and reshaped throughout its life. Plastic welding is similar to oxy-acetylene welding, in that you have a “torch” and a filler rod. With plastic though, it’s harder and more important to pick the correct filler rod.
There are several ways to figure out what type of plastic you have. One way is to cut a small sliver of material from the base, and melt it, or burn it. Make sure and take note of the type of smoke and the color of the flame. If the material chars until it’s no more, then you have thermosetting, not thermoplastic. Another test for identifying plastic is the copper wire test. Heat a copper wire until its red, and then use it to obtain a small blob of plastic. Once again, note the smoke and or flame that issues from the plastic. These tests will help you determine what filler rod to use to create an acceptable weld. If all else fails, and you have enough of the base material, you can cut the filler rod from that.
How to Weld Plastic
The plastic welding process is a lot like oxy-acetylene torch welding, without the need for dark goggles, gloves, or leather aprons. Either a plastic welding machine or a regular heat gun can be used.
Prepare the joint to be welded like you would with steel, maintaining the correct angles and gaps. Make sure the weld area is clean by using sandpaper or a wire brush. Then, after the heat gun is hot, hold the filler rod vertically and fan the area with the heat gun. When the material becomes tacky, push the rod down into the joint firmly, but not too hard. Continue fanning the rod and joint while you push the filler almost vertically down into place. A little angle to help move forward is desirable too. When you come to the end, apply a few seconds of direct heat to the rod and then twist it off. If you require more filler to complete the weld, bevel the rod to thirty degrees, and your next filler rod to sixty, then repeat heating and pushing filler into the joint. Unlike oxyfuel welding, the filler rod will never melt off and drip into a weld puddle (unless you’ve got way too much heat).
The finished weld should be able to stand up to just as much stress as the original material, provided you’ve done it right. One quick way to check is to try and pry the filler out of the joint with your fingers after it has cooled. If it pops out, you’ve done something wrong. If it sticks in, then you’re on the right track. The only way to be sure is with destructive testing, but you probably don’t want to destroy whatever you’ve just welded together.
As with everything, practice makes perfect. If you’ve got the right materials, and the right skills, then you won’t be disappointed. Happy welding.
Written by Dustin Saunders