Welding of plastics involves the creation of molecular bond between two similar thermoplastics. Welding forms a permanent joint between two parts which are joined. Welding involves three main steps pressing, heating and cooling however pressure is applied throughout the process to improve melt flow across interface and to orient the parts properly. Heating provides the intermolecular diffusion or melt mixing. Cooling is required for newly formed joint which also decide the strength of the joint. The various welding methods for plastics include ultrasonic, vibration, hot plate, spin and laser/infrared welding.

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Ultrasonic welding
This welding technique includes the generation of ultrasonic vibrational energy at ultrasonic frequencies using piezoelectric ceramics. This vibrational energy generates a frictional heating between the mating parts and due to this heat plastic part melts at the interface and forms the weld.

Vibration welding
This technique is used to form a weld over a large area. In this process one part is stationary and other vibrates on the joint plane generating heat required to weld. The vibrating part may have a linear or orbital motion.

Hot plate welding
In this process of welding the facing surface of plastic parts are heated through conduction, convection or radiation from a heated platen. The heated plate contacts the two parts or held close to them for some period of time then hot plate is removed and parts are pressed together to form a weld.

Spin welding
This process is used to weld circular parts with continuous joints. The process relies on frictional heat generated between two mating part in which one is stationary and other is spinning to melt plastic at the interface. When friction melt sufficient amount of plastic the spinning stops and pressure is applied to complete the bonding process.

Laser/Infrared welding
There are two ways of laser/infrared welding. First one is to heat the joining surface of both parts to create the melt and then pressing the parts to form the weld. The second one involves the use of a transparent part that allows laser and another that absorb it. When projected the laser will pass through the transparent part and reaches to the absorbing part which will get heated up. This heat will melt the plastic and weld is formed.