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Plastic is welded through a variety of techniques, some of which are also used in welding metals. Because the welding processes involved use heat to soften and melt the plastic materials being welded, they can only be used on thermoplastic polymers, which do not degrade or burn. However, plastic welding generally requires far less heat than metal welding, and is easily accomplished by a variety of friction and vibration techniques.

Typical plastic welding processes include ultrasonic, friction, vibration, hot-gas and hot-plate welding, among others. Ultrasonic welding is one of the fastest techniques, but it is also one of the most expensive, and is best suited to small welds. It uses ultrasonic vibrations to produce rapid perpendicular movement between the joints; this heats the surfaces and creates a weld within seconds. Vibration welding is a similar technique, but the parts are moved across one another at lower frequencies and it takes a slightly longer period of time. Friction welding requires at least one circular part, as it entails spinning one of the welded parts at high speeds while pushing it against the other surface to be welded. The heat generated by friction causes the surfaces to melt, welding them together. Another form of friction welding involves using a probe that moves at high velocity between prepared edges. The movement of the probe against the edges causes them to melt and form a bond at the joint.

In hot-plate welding, which can be used on larger parts, two surfaces are placed in mounts separated by a heated tool, or platen. After the tool has heated the surfaces to a point ready for bonding, it is removed and the surfaces are pressed together. Hot-plate welding produces strong welds but can be a relatively lengthy process. Hot-gas welding, a familiar technique for metal welding, is also used on plastics, and uses a welding gun, often in conjunction with filler material.

Other welding techniques employed in plastic welding are laser and infrared welding, as well as contact and solvent welding. Proper selection of a welding method generally depends on the shape and size of the parts being welded, the extent of production, and the strength of the bond required.