Process for the continuous overlap welding of plastic sheets or panels

In a process for the continuous overlap welding of thermoplastic sheets or panels on the ground by heating and compressing the facing overlapped sheets, comprising the steps of placing the sheets or panels to be welded on the ground in side by side position, overlapping the edges of adjacent sheets or panels and maintaining one edge on the ground, lifting the overlapped edge of the upper sheet or panel and placing a preheating means between the sheets or panels in the region of said overlapped edges, controlling the location of the preheating means between the said overlapped edges in accordance with the unevenness of the ground in the area of preheating and preheating the said overlapped edges to the softening point thereof, extruding additional material between the preheated overlapped edges while the edges are apart, heating the additional material into the thermoplastic state at least high enough to maintain a sufficient level of heat in said overlapped edges to permit the bonding of said edges to said additional material, and controlling the placement of said additional material such that said additional material contacts both edges substantially simultaneously, and applying pressure to press the overlapped edges against the ground to weld said sheets or panels to said additional material and thereby to each other.