A controlled heat seal device for fastening wrappers of thermoplastic material, in particular in overwrapping machines comprising a belt conveyor looped around at least one pulley and embodied with a plurality of transverse slats by which single commodities enveloped in respective wrappers, typically packets of cigarettes, are fed positively toward the device, comprising:
- two mutually coaxial disks positioned on opposite sides of the conveyor, each consisting in at least one pair of concentric circular rings rotatable as one with the pulley and thermally insulated one from the other, disposed such that one ring of each pair operates in conjunction with the conveyor by taking up the commodities, each enveloped in a respective wrapper, from the surface of the belt;
- heat seal means associated with the rings by which the commodities are taken up from the conveyor in such a way as to afford mutually opposed heated surfaces positioned to enter into contact at least with the areas of the wrapper to be fastened when the commodity is taken up between the rings;
- and
- a diverting element located between the disks, adjustable for angular position about a given axis according to the operating speed of the wrapping machine, by which the commodities enveloped in the respective wrappers are directed between the rings not associated with the heat seal means in such a way that the duration of the heat sealing action remains unaltered whatever the operating speed of the machine.