Abstract: The disclosure relates to a method and an apparatus for producing a continuous edge thickening along the running direction of a stretched and tensioned web material of a positively and continuously transported thermoplastic film material. The web material is folded initially along its outer edge regions running in the drawing direction through 180° at least once, whereafter the web material is, for the fixing of each respective fold, moved around a cylinder or roller. The folded areas are thereafter passed through a welding device in order to provide a unifying weld between the fold/folds and the remainder of the web material.