2 on the frame 1 and the second workpiece support device 23 is in the top part of the interior space 2 of the frame 1 its contact travel 129 being substantially vertical the heating plate 21 is mounted on a mobile heating plate support bracket 28 in turn movable in vertical translation along said welding travel shown by the arrow 29 in fig1 and 4 perpendicularly to the plane of the heating plate 21 sliding on vertical guides 30 and 31 of a frame 32 in turn carried by a heating plate support carriage 33 the heating plate support carriage 33 has a carriage top main crossmember 34 the ends of which have skids 35 and 36 which slide on longitudinal guides carried by the intermediate longitudinal members 15 and 16 the heating plate support carriage 33 also has two vertical side flanges 37 and 38 shaped to retain the frame 32 and joined at their top ends by a horizontal connecting plate 133 retained by the top main crossmember 34 the horizontal connecting plate 133 is preferably mounted to slide longitudinally in the direction of the welding travel 29 its two lateral edges being engaged in lateral slides 134 and 135 fastened.