Heat fusion (sometimes called heat welding or simply 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 can result in improper bonding.[1][2]
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This process is commonly used in plastic pressure pipe systems to join a pipe and fitting together, or to join a length of pipe directly to another length of pipe. Generally, polyolefins (such as polypropylene, polyethylene, and polybutylene) and PVC are used for these applications.
Heat fusion is usually performed using one of two methods. The first, and most common, is butt welding, which is a type of hot plate welding. This technique involves pressing two planed surfaces of the plastic material against a heated surface. After a specified amount of time, the heating plate is removed and the two pieces are pressed together and allowed to cool under pressure, forming the desired bond.[3]
The other major technique is socket fusion. It is distinguished from butt-welding by using custom-shaped and -sized heating plates rather than a basic flat surface. These heads allow for more surface contact, reducing the time needed to heat and fuse the pipe. Socket fusion joins pipe and fittings together, rather than simply joining pipe to pipe. It requires less pressure than butt-welding and is more commonly used on smaller sizes of pipe (4" or less).[4]