In the practice of apparatus engineering and pipeline construction, wall thicknesses of 100 mm are processed and joined with classic heated tool butt welding. The welding procedure is regarded as very well-understood and has been successfully utilised in the manufacture of semi-finished products for decades. However, there are no systematic experimental investigations into the high wall thicknesses above approx. 30 mm. The parameters for high wall thicknesses of polyethylene and polypropylene are extrapolated. But cases of damage to welded pipelines show that not enough understanding of the process management and the weld properties is available for the welding of large pipe dimensions. The present study systematically investigates the welding behaviour of components with wall thicknesses up to 100 mm. The mechanical properties are tested extensively in short-term tests as well as in long-term tensile creep tests. The results relating to the fusion behaviour basically show a more or less extremely curved melt profile. The shape of the melt layer thus influences the flow behaviour during the joining as well as the pressure and the residual stress condition after the cooling. Short-term tensile tests hardly indicate any differences in the strength depending on the sampling location but, in the tensile creep test, it is possible to establish a tendency towards the premature failure of the peripheral regions. With the help of parameter variations and process optimisation it is tried to improve the strength of the weld joint.