Butt Fusion (sometimes called Heat Welding or simply Heat Fusion) is a welding process used to join two different pieces of a thermoplastic. This process involves heating both pieces simultaneously and pressing them together. The two pieces then cool together and form a permanent bond. When done properly, the two pieces become indistinguishable from each other.

Dissimilar plastics, or untrained installers can result in improper bonding and later failure of the joint resulting in leakages.

This can be a cost saving method on the traditional fusion coupler method of jointing, as no fittings are required on the straight joints and certainly on the larger sizes of pipe these can be very costly.