External center pin for blow molding machine

In a machine for making a plastic bottle of biaxially oriented polyester resin by blow molding within a blow mold assembly an injection molded elongated hollow tubular preform which is shorter than said bottle and that has one end open and the other end closed, the closed end having a gate extending outwardly therefrom and being substantially centrally located and surrounded by an area that, including the gate, is more crystalline than the rest of the preform, the blow mold assembly being openable and of several individual molds which are openable and which assemble to form along an axis a cavity shaped substantially like the bottle and providing for expansion of the preform along said axis and in a direction substantially normal thereto, called the hoop direction, the open end of the preform being held substantially on said axis by suspension from a neck ring which also cooperates with the blow mold assembly to seal the neck end of the cavity, the machine also having a blow plug means for entering the open end of the preform and directing blowing fluid into the preform to form a bottle by expanding same into engagement with the mold walls defining said cavity, there being no mandrel or core supporting the preform nor any mechanical element that enters the preform to engage the interior bottom portion thereof, the improvement that comprises

• a center pin movably supported in the bottle bottom of said cavity;
• a shaped upper face on said center pin for engaging the closed end and gate of said preform and having
• a recess located approximately centrally and of a size to receive said gate, and
• an annular bearing surface surrounding said recess, said surface being of a size in the hoop direction at least equal to that of said area that is more crystalline and said surface contoured at its periphery to fair into said mold at the bottom of said mold cavity;
• support means for supporting said center pin for reciprocable motion in the direction of said axis with said recess located substantially on said axis;
• means for reciprocating said support means and center pin while keeping said recess on said axis as aforesaid between a first position where said shaped upper face annular bearing surface is in register with adjacent mold cavity surfaces to define at least a part of the bottle bottom and a second position along said axis toward said open end where said shaped upper face is engagable by the preform;
• and
• said means for reciprocating further including means to drive said center pin and support means between said first and second positions and to bias said center pin in a direction opposite to movement from said second to said first position with a force insufficient to resist expansion of said preform when blowing said bottle.