a container comprises an open - mouthed receptacle and a lid . the receptacle has side walls and a base , and is formed with an outwardling - extending rim which surrounds the open mouth of the receptacle . the receptacle is formed by vacuum moulding a sheet of plastics material whose external dimensions correspond to the external dimensions of the rim . the ratio of the thickness of the rim to that of the rest of the receptacle is at least 4 . 5 : 1 .