Method for producing a biaxially oriented container from a blank of thermoplastic material

A method of forming a container from orientable thermoplastic material comprising:

• introducing into a mold with a movable mold part, a hollow blank of thermoplastic material including a side wall and a closed end,
• axially stretching the blank by an internal mandrel which is axially displaced and engages the closed end of the blank to produce axial orientation of the material of the side wall of the blank,
• expanding the blank in the mold to contact the walls thereof to form the container,
• displacing the closed end portion of the stretched blank in correspondence with the expanding of the blank such that the material of the side wall is not stretched in a plane perpendicular to the direction of expanding and thereby the material does not undergo any axial strain due to elongation and said expanding produces circumferential stretching and circumferential orientation of the material in said side wall independent of and superimposed on said axial orientation,
• the axially stretched material of the axially stretched blank tending to undergo shrinkage after the axial stretching due to internal stresses in the material,
• said expanding of said blank being effected at a controlled time and speed of expansion such that the reduction in length of the stretched material due to shrinkage is concluded before the material comes into contact with the mold walls whereby to substantially eliminate said internal stresses.