Forming molecularly oriented containers from reheated preforms

In the method of forming containers from injection molded preforms comprising a major proportion of a polymerized nitrile-group-containing monomer, which method includes heating the preforms to molecular orientation temperature through temperature programming by exposing the preforms to a plurality of separate heating zones disposed along the length thereof followed by axial and radial stretching to container form in a mold,

• the improvement, therein providing reduced thickness variability in lower portions of the containers which comprises, in combination, the steps of:
• controlling shrinkage of the preforms during said heating to between about 4 to 15% of the total initial preform length;
• and then controlling said stretching according to the relation ##EQU6## wherein:
• A is at least about 30;
• B is at least about 100;
• and
• A plus B is between about 130 to 280.

In the method of forming containers from tubular, injection molded preforms comprising a major proportion of polymerized nitrile-group-containing monomer, which method includes heating the preforms to molecular orientation temperature through temperature programming by exposing the preforms to a plurality of separate heating zones disposed along the length thereof followed by axial and radial stretching in a mold to form the containers, the improvement which comprises:

• (a) providing preforms prior to heating wherein the ratio of average preform thickness to inside preform surface area is between about 0.005 to about 0.011 inch.sup.-1 ;
• (b) controlling the amount of shrinkage of the preforms of step (a) during such heating to between about 4 to 15% of the total initial preform length;
• and then
• (c) controlling said stretching according to the relation:
• ##EQU8## wherein:
• A is at least about 30;
• B is at least about 100;
• and
• A plus B is between about 130 to 280.