Dr. Duane Priddy, Plastic Expert Group
The valve failed on the molding weld-line because of hoop stress from the threaded connection. The lack of glass fiber reinforcement across the weld-line due to orientation results in a weak location for the failure to occur. The tee failed on the molding weld-line, because of hoop stress from a fitting that was threaded in until bottom contact (over tightened). The lack of glass fiber reinforcement across the weld-line due to orientation, and poor adhesion of the fiber to the Nylon resin resulted in a weak location for the failure to occur. In general, inside tapered pipe threads are a poor choice for thermoplastic materials as the joint relies on a tight fit between the threads to create a seal. The result is a significant amount of hoop stress on the fitting from tightening the male thread. Straight threads with a gasket seal or external threads are the better choice. If inside pipe threads are necessary, increases in wall thickness or metal reinforcement bands are potential remedies to consider.