There are five elements in the butt fusion joining process:
Bead-up - The trimmed and aligned pipes are pressed against the heater plate using the 'initial' bead-up pressure. The pressure is maintained until the pipe begins to melt and a uniform bead is formed on each end. The optimum size of the bead depends on the material and pipe diameter.
Heat soak - After the initial bead-up, the pressure in the system is released so that the pressure gauge registers between zero and the drag pressure. The pipe ends must maintain contact with the heater plate at the soak pressure for the specified soak time, which increases with diameter and wall thickness.
Plate removal - When the heat soak time is completed, the pipe ends are retracted from the heater plate, and the heater plate removed. This is sometimes known as the 'dwell time' and should be as short as possible.
Fusion jointing - Immediately after removing the heater plate, the hot pipe ends are brought together in a smooth, controlled manner and the pressure raised to the jointing pressure. In some specifications where a large wall thickness is used, the jointing pressure is reduced after 10 seconds has elapsed.
Cooling - Finally, after the jointing phase, the weld is allowed to cool whilst the joint pressure/secondary pressure is maintained for a specified time. The cool time will depend on the pipe material and diameter.