Heated tool-butt welding of polyethylene-pipes — Welding parameters and testing technique

Pipes in semi-crystalline thermoplastics are frequently joined by heated tool butt welding. The quality of the connection is essential dependent upon the set process parameters, the pipe material and the surrounding conditions.

The paper first looks into the welding parameters of temperature, pressure, displacement and time. A matching displacement equation is set out, which links all the essential parameters of the matching phase. A model law for matching is derived from this and illustrated with an example. The pressureless heating phase is described by an equation for the temperature distribution in the part being joined. The thickness of the molten layer present after heating is calculated and tallies well with experimental results. A theoretically derived joining displacement equation is shown for the connection process. This allows the influence of temperature and pressure on joining displacement to be estimated.

The known short-time test methods are not suitable for optimising the welding parameters. Two test methods modified for testing weld seams are thus presented. These are the low temperature bending test and the tensile test on hole-notched weld specimens. Both methods show that above a minimum welding temperature the temperature has no essential influence on seam quality.