BLOW MOLDING METHOD

摘要

Thermoplastic pre-forms which are obtained as a result of expansion of parisons in a pre-form blow mold are transformed into bottles or analogous hollow shaped articles in a final blow mold. The first stage of transformation involves mechanical stretching of the pre-form in the cavity of the final mold simultaneously with or prior to admission of a compressed blowing fluid to expand the major portion of the pre-form into contact with the adjacent surface of the final mold. The next stage involves admission of the blowing medium at a higher pressure to effect abrupt biaxial expansion of the closed end and of the portion intermediate the closed end and the major portion into contact with the adjacent surfaces of the final mold. The configuration of surfaces which are contacted by the bottom and intermediate portion of the shaped article is such that radial expansion of the corresponding portions of the pre-form in response to admission of blowing fluid at an elevated pressure necessarily takes place simultaneously with axial expansion. This is achieved by employing a final blow mold with a fixed internal bottom surface which causes the bottom of the shaped article to assume a concavo-convex shape, or by employing a final mold whose bottom surface is movable in a direction to reduce or increase the volume of the cavity prior to admission of blowing fluid at elevated pressure.