Method of molding inner thread on neck portion of preform

A method for forming a preform having a body portion and a neck portion, said preform neck portion having an inner thread comprising the steps of:

• providing an injection mold, a barrel spacer placed onto said injection mold, a neck mold slidably disposed over said barrel spacer, a core member holding an injection core and inserted into said neck mold, said injection core having a base portion and a neck portion, said injection core neck portion having a thread pattern, said injection mold, said barrel spacer, said neck mold and said core member together defining an outer surface of a mold cavity, said injection core defining an inner surface of said mold cavity, said injection core being positioned in said mold cavity so that said injection core base portion is within said injection mold and said injection core neck portion is within said barrel spacer;
• injecting resin to fill said mold cavity and thereby form said preform, said thread pattern on said injection core neck portion forming said inner thread of said preform neck portion;
• forming an expansion space proximate to said injection core neck portion by separating said neck mold and said core member holding said injection core from said barrel spacer and said injection mold, and
• expanding said preform neck portion fully in said expansion space by blowing air between said injection core neck portion and said preform neck portion while said preform neck portion is at a temperature for deforming so that a gap for removal of said injection core from said preform is formed between said injection core neck portion and said preform neck portion.