Heating tools that simultaneously heat both pipe ends are used to accomplish this operation. These heating tools are normally furnished with thermometers to measure internal heater temperature so the operator can monitor the temperature before each joint is made. However, the thermometer can be used only as a general indicator because there is some heat loss from internal to external surfaces, depending on factors such as ambient temperatures and wind conditions. A pyrometer or other surface temperature-measuring device should be used before the first joint of the day is made and periodically throughout the day to ensure proper temperature of the heating tool face that contacts the pipe or fitting ends. Additionally, heating tools are usually equipped with suspension and alignment guides that center them on the pipe ends. The heater faces that come into contact with the pipe should be clean, oil-free and coated with a nonstick coating or covered with a non-stick fabric as recommended by the fusion equipment manufacturer to prevent molten plastic from sticking to the heater surfaces. Remaining molten plastic can interfere with fusion quality and must be removed according to the tool manufacturer's instructions. Never use chemical cleaners or solvents to clean heating tool surfaces.
The surface temperatures must be in the temperature range 400 - 450°F (204 - 232°C). Install the heater in the butt fusion machine and bring the pipe ends into full contact with the heater at fusion pressure to ensure that full and proper contact is made between the pipe ends and the heater. After holding the pressure very briefly, it should be released without breaking contact. On larger pipe sizes, fusion pressure must be maintained until a slight melt is observed around the circumference of the pipe before releasing pressure. Continue to hold the components in contact with each other, without force, while a bead of molten polyethylene develops between the heater and the pipe ends. A minimum of 4.5 minutes per inch of pipe wall is recommended as the minimum heat soak time. Then continue the heat soak cycle until the minimum bead size is formed against the heater on both sides (see table).
Ref: Polyethylene Piping Systems Field Manual for Municipal Water (Irving, TX: The Plastics Pipe Institute)