 Welding thermoplastic is very similar to welding glass . The plastic first must be cleaned and then heated through the glass transition , turning the weld @-@ interface into a thick , viscous liquid . Two heated interfaces can then be pressed together , allowing the molecules to mix through intermolecular diffusion , joining them as one . Then the plastic is cooled through the glass transition , allowing the weld to solidify . A filler rod may often be used for certain types of joints . The main differences between welding glass and plastic are the types of heating methods , the much lower melting temperatures , and the fact that plastics will burn if overheated . Many different methods have been devised for heating plastic to a weldable temperature without burning it . Ovens or electric heating tools can be used to melt the plastic . Ultrasonic , laser , or friction heating are other methods . Resistive metals may be implanted in the plastic , which respond to induction heating . Some plastics will begin to burn at temperatures lower than their glass transition , so welding can be performed by blowing a heated , inert gas onto the plastic , melting it while , at the same time , shielding it from oxygen . 
