joints between thermoplastic parts are friction welded or otherwise thermally welded in such a manner that a non - uniform temperature distribution results during welding which effectively blocks flash flow from the edge of the joint , and upon cooling , produces a joint that is at least partially stress relieved and is resistant to solvent - induced crazing and cracking . joints may employ nonparallel adjoining surfaces and / or flexural lips , which bend slightly under pressure applied during welding so as to produce a non - uniform pressure distribution and nonparallel welding surface over at least a portion of the resulting weld near the outside edge . in addition , one or more axial grooves may be used to isolate the nonparallel or lipped area of the weld from the remainder of the weld area so as to increase the lip compliance and reduce the effective pressure of molten material acting on the lip during welding .