# 2008 Budapest
Structured wall plastic pipes (double wall with corrugated external wall) are widely used for underground sewage drains. Socket joints, integrated onto the pipes themselves, are the most commonly used junction system. With PVC-U pipes, the socket is normally thermoformed in the corrugated wall by a specific belling machine. Other systems, such as coupling collars, socket formed directly by the corrugator, sockets injection-moulded onto the pipe or sockets welded onto the pipe, are not economically or technically competitive in comparison to sockets thermoformed onto the corrugated wall of the pipe. Thermoforming (also called socketing or belling) has only recently been introduced in industrial plants producing pipes in PP and HDPE, with a system that has resolved complex problems concerning the structural integrity of the wall shape and dimensional stability. In addition to the evident advantages already demonstrated with PVC pipes, the system introduces new rules for designing the geometry of the pipe wall and promises further developments towards optimized walls to obtain reduced weight, improved rigidity, strength and socketability with thermoforming.