known to those skilled in the art the mitered surfaces forming assembly surfaces of the section pieces 1 being arranged facing each other and a determined gap between them the heating plates 10 of the welding device are inserted into said gap so as to come into contact only with the section of the main section pieces 2 particularly the double walls 8 9 after sufficient time to melt the plastic in the region of the assembly surfaces the heating plates 10 will be retracted and the section pieces 1 will be clamped together in the direction of the arrows 11 and 12 this causes the molten material along the section of the main section pieces 2 to be compressed until such point as the glazing beads 3 and 4 facing each other come into abutment against each other thus precisely determining the dimensions of the assembled frame in fig1 b the main section pieces 2 are welded together after 3 mm of material has been melted the glazing beads 3 and 4 which acted as limit stops during clamping are not themselves welded together and the glazing bead 4 can therefore be easily unclipped and moved into the open position o by pivoting about the axis.