Abstract: The wall of one end section of a plastic pipe is post thickened. As disclosed herein, this is accomplished by heating a predetermined length of the end of the pipe to its state of thermal deformability and thereafter causing the pipe to deform within and substantially fill a longitudinally extending, circumferential cavity which is thicker than the originally formed pipe wall. At the same time, a predetermined amount of heat is applied to the cavity from points along the length of and around the circumference of the inner and outer surfaces of the cavity. These points of heat extend from the back end of this cavity towards but stop short of its front end, a predetermined distance from the front end, thus defining a transition zone. This transition zone, among other things, prevents the remaining portion of the pipe from being heated and thus easily distorted during the thickening operation.