When you hear the word "weld," you automatically think of extreme heat and the melting and fusing of two objects into one. This describes the procedure for welding plastic, although it doesn't call for the extreme temperatures that are required to weld two pieces of metal. A common plastic welding gun, sold at many home improvement and better hardware stores, heats the two pieces of plastic that are being welded and adds a bead of new melted plastic, allowing all three pieces to mix and form a molecular bond that is as strong as the original piece.
Items you will need
Plastic welding gun
Welding rod (same type of plastic as you are welding)
Clean the plastic that you intend to weld with a degreaser to remove any waxes, grease or other potential contaminants.
Sand the edges that you intend to weld with 80-grit paper so they butt up against each other tightly.
Butt the two pieces of plastic together and hold in place with foil tape placed on the back side of the project, if possible. Be sure that the pieces you're welding are the same type of plastic. Different kinds of plastic have different molecular structures and may not bond.
Warm up the plastic welding gun for at least 5 minutes. When the gun is warm, insert the plastic welding rod. Make sure the rod is of the same kind of plastic as the pieces you are welding.
Slowly feed the welding rod into the welding gun so that it melts and is extruded onto the joint where the two pieces of plastic are being welded. The hot tip of the gun will soften the two pieces and allow them to bond with the bead of melted plastic you're applying. Keep the gun moving slowly and evenly along the weld to produce an even bead.
Allow the weld to air cool. Since the butted-together pieces of plastic are being melted into each other there is no need for clamps of any kind. Do not force or speed up the cooling time. Sand the weld with 80-grit paper until the weld is smooth. Fill in any small gaps with epoxy and allow the epoxy to harden. Sand the hardened epoxy with the 120-grit paper until the entire area is smooth, like new plastic.
The smoother you are as you move your welding gun along your weld line, the less sanding you will need to do to produce an invisible weld.
Do not breath the fumes released by the melting plastic. If you are sensitive, wear a breathing mask and safety goggles.