Method and device for maintaining an inner diameter of a plastic container

A method of manufacturing a plastic container having an inner undercut, comprising:

• forming a preform of plastic in amorphous state, the preform comprising a finish portion defining a mouth, the finish portion having a side wall with a first inner diameter in plan view and an inwardly projecting rib with a second inner diameter in plan view less than the first inner diameter to define a an inner undercut therebetween, the preform further comprising a hollow body portion extending from the finish portion;
• inserting a plug within the mouth of the preform, the plug having an initial condition and a deployed condition, wherein the plug comprises:
• an upper plug member and an expandable element, the expandable element having an initial outer diameter in plan view with the plug in the initial condition and a deployed outer diameter in plan view with the plug in the deployed condition, the initial outer diameter being less than the second inner diameter and the deployed outer diameter being equal to or greater than the second inner diameter,
• deploying the plug to the deployed condition with the expandable element positioned to engage the side wall proximate the inner undercut;
• crystallizing the plastic of the preform along the finish portion;
• and
• blow molding the hollow body portion of the preform to form a container body portion.

A plug for maintaining an inner diameter of a plastic container having an inner undercut during crystallization, comprising:

• an upper plug member;
• an expandable element having an initial outer diameter in plan view with the plug in an initial condition and a deployed outer diameter in plan view with the plug in a deployed condition, the initial outer diameter being less than the deployed outer diameter;
• and
• an actuator to change the plug from the initial condition to the deployed condition;
• wherein the upper plug member and the expandable element are each made of a material compatible with a temperature and a pressure used for crystallizing a plastic selected from the group consisting of polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polyethylene furanoate (PEF), or a combination thereof.