(57) A heavy duty collapsible container having integrally molded plastic side, end and bottom walls which are connected by integral snap hinge structures. Flanges on the side and end walls have mating grooves and ridges which prevent the walls from pivoting beyond the vertical when internally loaded and from relative longitudinal movement to prevent skewing. Interacting shoulder and lip means acting between the end, side and bottom walls in the area of the snap hinges protect the snap hinges from lateral loading. A domed or upwardly convex bottom wall transfers vertical loading into the side and end walls to enhance their engagement.