A flexible heat-sealable web is shaped continuously into a tubular cross-sectional shape, e.g. as an intermediate stage in the production of infusion bags comprising severed and sealed lengths of the tubular shape. The web is drawn along a shaped shoe and side margins of the web are folded over opposite sides of the shoe by rollers and fingers to place the opposite side edges of the web in overlapping relationship against the shoe. The overlapped edges pass between a roller mounted in the shoe and an external heated roller to be pressed between the two rollers in order to lap weld the edges together as the web travels along the shoe. The shoe continues to form the web downstream of the welding station into a flattened tube.