An apparatus is described for moulding a plastics component onto a length of flexible web material, particularly a paper tube coated with thermoplastic synthetic material, the apparatus comprising a complete injection unit (5), a nozzle (26), an injecting head portion (12), external mould portions (11,11') and a core (14) which can be moved relative to these. As a means of increasing the output of the apparatus given a continuous supply of the web material, the invention provides that the core (14) is arranged on the end of a mandrel (23) which carries the length of material (35) with it and which is part of a rotatable mandrel wheel driven in cadence about its drive shaft (3), with its mandrel (23) projecting radially in one plane, that the external mould portion (11, 11') has at least two mould portions (11, 11') which can be moved apart and together laterally of the mandrel (23) to free the circular path of the mandrel (23), and that the mould portions can be held together by bringing them into positive and frictional engagement with a movable head plate (12) at the nozzle side.