Welding HDPE, or high-density polyethylene, is a normal procedure for joining lengths of pipe together, whether it be for industrial or home use. One of the benefits of welding HDPE plastics in smaller applications such as you might find in home plumbing systems is that no weld filler rod is necessary. Simply heating the HDPE to the correct temperature and then fusing it together with the other piece is usually sufficient to fully weld the two pieces together. The trick is that you can't use a hair dryer to obtain the correct temperatures you'll need for the process.
Lay the HDPE pipe on a stable work bench or a surface that will not be affected by heat. In the field, plumbers might use sawhorses, a flat area of ground or the installation point as a work area. You can use a bench vise to hold one of the pieces, or if you are working in an area where you cannot access a bench vise, you can weld the HDPE pipe on a concrete floor. Avoid performing this work on linoleum or wood floors, as the extreme heat can damage such softer finishes. Performing this work on a floor or a workbench before the pipes are installed will allow you to more easily control the final shape of the pipe without having to hold it in place by hand. Welding the pipes on the floor and then leaving them alone is sufficient to obtain a strong bond.
Apply rubbing alcohol to the HDPE surfaces that you are going to weld together, then wipe each with the clean rag. This will remove surface oils and other debris that can contaminate the weld.
Lay the two pieces beside one another with the points at which they will be welded within an an inch or two of one another. Use the heat gun to heat the two surfaces that you'll be welding to a temperature of at least 325 degrees. At this point, HDPE becomes extremely soft and pliable. Do not touch the areas to test their pliability or heat, as the plastic surface at this temperature can stick to your skin if you touch it and cause extremely severe burns. Instead, use a noncontact thermometer to determine the surface temperature of the HDPE parts.
Lay the heat gun aside, and then press the heated work pieces together at the point at which they are to be joined. The HDPE will ooze slightly at the joint on both pieces. Allow the two pieces of HDPE to cool for approximately three minutes while you hold them in place, and wait another ten minutes before applying pressure to the welded joint or attempting to install the pipe. When the two pieces of HDPE have cooled to below 120 degrees, the HDPE plastic parts will be fused together. At that point you can install the new piece or continue your project as applicable.