Abstract: To postform the region of transition between the initial part and the end part of a section piece forming a closed frame and applied to window glass (1) by extrusion, use is made of a mould (14) which pivots about an axis parallel to the longitudinal axis of the extruded section piece and which is situated in the region of the surface of the window glass (1). The axis of pivoting of the mould (14) coincides with the cutting ridge (15) of a protrusion (16) which limits the calibrated cavity of the mould (14) on the internal face of the extruded section piece. As the mould (14) pivots, the excess polymer material is pushed back in the direction of the periphery of the window glass and beyond this periphery.