The butt fusion welding technique is used for welding pipes made from polyethylene (PE), for the water and gas industries, and from polypropylene (PP) and polyvinylidene fluoride (PVDF) for the chemical industries.

The pipes are mounted in the clamps of the butt fusion equipment and checked for initial alignment. The pipe ends are then planed to ensure that they are flat and square. The final alignment of the pipes is then checked.

The welding sequence begins when a flat hot plate, at a preset temperature, is positioned between the two pipe ends. The pipes are pushed towards each other until the pipe ends come into contact with the hot plate and the pressure is increased to give good thermal contact. The pipe ends melt and the interface pressure forces the molten material outwards to form 'weld beads' at the outside and inside pipe surfaces; hence the term 'bead-up' stage. At the end of this stage, the pressure is reduced to a value sufficient only to maintain the pipe in contact with the hot plate. This allows the melt depth to increase without increasing the size of the weld beads. At the end of this 'heat soak' stage, the pipe ends are pulled away from the hot plate. The hot plate is removed, and the two molten pipe ends are pushed together at the same pressure as used during the initial bead-up stage. This causes further growth of the weld bead and is called the 'bead roll over' stage. The pressure is maintained until the weld is fully cooled.