In the packaging machines for the automatic packaging of products fed in sequence inside a continuous film of a thermoplastic material, e.g., suitably folded above and/or under said products, so that a continuous tubular element is obtained, a device is provided, which comprises a transversal welding element and counter-welding element, such as to realize a set of wrappings which enclose the products inside the thermoplastic material and separate them from one another. 
One of such devices is disclosed in Italian patent No.  1,028,456 (corresponding to U.S. Pat.  No.  4,019,307), wherein on a car, alternatively movable in the direction of running of the conveyor tape on which products fed above said film of thermoplastic material are positioned, a welding element and a counter-welding element are provided, which constitute the welding unit and are, e.g., of the bar type, positioned on opposite sides relatively to said material, and provided with a vertical reciprocating motion. 
Said welding unit, by combining the essentially horizontal movement of the car with the vertical reciprocating motion of the bars, results therefore to move according to a substantially elliptical trajectory relatively to the thermoplastic material which contains the products.  The relative distance between the two bars is fixed, independently from the height of the products to be packaged, and from the fact that the counter-welding element too is more or less movable in a vertical direction. 
In fact, if on one hand an adjustment can be carried out by modifying the position of the point at which the counter-welding element has its operating top stop point, on the other hand, by so doing, a corresponding shift upwards of the welding bar is determined, because the relative operating distance, or opening, between the two bars in their resting position must remain unchanged, and equal to the factory setting carried out when the packaging machine was manufactured. 
Such an arrangement considerably limits the speed of the machine, above all in the presence of limited-height products, because the actual stroke of both bars remains fixed, and has always its maximum value.