a packaging machine comprises a strip feeder and tube former for heat - welding plastic film strip into a tube . first welding apparatus weld between them the side edges of the strip while a feed line inputs into the tube thus formed products that are to be packaged . second weld apparatus welds transversely of the tube at intervals to isolate each product in a section of the tube , thus realizing the packaging . characteristically the first and second welding apparatus are pressed against the strip parts to be welded by an actuator controlled by a control device to exert a pressure which is a function of the speed of travel of the strip .