The temperature of join partners is too low (e.g. by a heating time that is too short, relocating time that is too long etc.) so that the mobility of the molecular chains is too low and a sufficient mixing of the join partners is not given.

If the pipe walls are very thick, the temperature of the heating element is to be adjusted at the lower limit of the permitted temperature range as the huge material thickness requires a relatively long heating time in order to plastify the join partners sufficiently (i.e. not only at the contact area facing the heating element but also into the depth of the join partners).