This invention relates to a corrugated thermoplastic blank for a container. The blank (20) has a base panel (24) with wall panels (26, 28, 30, 32) connected to the base panel (24) by fold lines (38). The fold lines (38) comprise weld lines which destroy the original memory of the thermoplastic material. The free edges of the blank are sealed by seals (38) which prevent water from entering the blank. The sealed free edges and/or the weld lines or line (38) surround and reinforce each panel (26, 28, 30, 32) and increase the strength of each panel and thereby the strength of a container erected from the blank.