Method for forming hollow partially crystalline biaxially oriented heat set polyethylene terephthalate articles

A method of forming a hollow biaxially oriented heat set partially crystalline polyethylene terephthalate container having side walls and a closed base opposite an open neck area, said closed base being of outwardly convex, hemispherical shape and comprising a major portion and a central portion surrounded by said major portion, the ratio of the diameter of said central portion of the base of the container to the diameter of the base of the container ranging from about 0.2 to 0.4, said method comprising the steps of:

• closing a mold around a biaxially oriented hollow polyethylene terephthalate container, said mold having side walls and a mold base portion comprising a major mold base portion corresponds to said major portion of the base of said container and a central mold base portion corresponding to the central portion of the base of said container,
• maintaining the side walls and major portion of the base of the mold at a temperature in the range of form 120.degree. C. to 250.degree. C., which corresponds to a temperature which effects heat setting of polyethylene terephthalate, while maintaining only said central portion of the base of the mold at a temperature substantially not greater than 150.degree. C., said central portion of the base of the mold being maintained at a temperature substantially less than the temperature of the side walls of the mold,
• introducing fluid pressure internally of the hollow container to maintain contact between the container and internal mold surfaces for a time sufficient to heat set and partially crystallize the side walls of the container and the major portion of the base of the container while also crystallizing the central portion of the base of the container a substantially lesser amount than the sidewalls and major portion of the base of the container, and quenching the container to preclude shrinkage thereof, said heat setting of side walls of said container and of said central and major portions of the base of the container being sufficient to prevent the container from sticking to said internal mold surfaces and to eliminate deformation of or blow out of the central portion of the base of the container upon opening of said mold while maintaining internal pressure within said container.