plastic snap - on hinged closures each having a bottom part and a cap that are attached to each other by a snap - on hinge tend to tear on the lateral borders . this is due to the geometry of closures having a round or oval shape when viewed from the top since only the flexibility of the outer walls have been used so far . this unresolved problem can be solved by a curvature with varying height and wall thickness extending along the joint . the curvature is elastic when pressure is exerted in the middle and when pulled from the sides .