Reinforced container made of synthetic materials

A container of PET material formed from a blank in predominantly amorphous state by blowing the blank with biaxial stretching at a temperature in excess of the glass transition temperature, the container being provided with a wall having inner reinforcing ribs, the improvement wherein the material of the reinforcing ribs has at least greater density or a greater density of crystallization than that of the material of the container surrounding said reinforcing ribs, said reinforcing ribs being annular in extent and obtained at grooves initially formed in said wall at one side thereof and having higher crystallinity than the remainder of said wall to provide said ribs with a cross-section shaped as humped protuberances providing increasing thickness of the container in the wall thereof at said ribs at the side thereof in which the grooves are initially formed, said container having an outer continuous surface at said ribs which is unbroken.