1. Moulded part of thermoplastic, such as polyalkene, in particular a coupling piece (Fig. 8) for thermoplastic pipes, characterized in that the moulded part has a substantially constant large wall thickness and in that the wall has on the inside (15) and on the outside (14) an integrated surface layer (16) of the same plastic as the wall itself but with finer crystal distribution and in that this surface layer is smooth and closed except at the gate mark.
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