Method of injection molding preform

A method of injection molding a test tube-shaped preform for biaxial stretch blow molding,

• the preform comprising:
• a transparent base layer;
• a circumferential wall formed of the transparent base layer;
• and
• a light-blocking or opaque intermediate layer formed in the base layer in a laminated manner over an area of a predetermined height of the circumferential wall,
• wherein the intermediate layer is formed in a laminated manner over an area covering an entire circumference of the circumferential wall excluding a non-laminated section, where the intermediate layer is not formed, that extends in a shape of a longitudinal strip along a central axis direction on the circumferential wall of the preform
• the method being performed with an injection molding apparatus including a nozzle, the nozzle comprising:
• three cylindrical flow paths for layer formation disposed concentrically about the same axis, the three cylindrical flow paths including an inner flow path and an outer flow path for forming the base layer composed of a major material resin, and a middle flow path for forming the intermediate layer composed of an intermediate layer resin, the middle flow path being disposed between the inner flow path and the outer flow path;
• a columnar combined flow path disposed downstream from the three cylindrical flow paths for layer formation, at the columnar combined flow path the three flow paths being combined;
• and
• a single shut-off longitudinal rib piece disposed at a downstream end of the middle flow path only in the opposite side of an inlet in the circumferential direction, the inlet supplying the middle flow path with the intermediate layer resin, the shut-off longitudinal rib piece traversing the middle flow path relative to the circumferential direction and circumferentially interrupting the middle flow path, wherein the shut-off longitudinal rib piece is the only rib piece traversing the middle flow path relative to the circumferential direction and circumferentially interrupting the middle flow path,
• the method comprising:
• supplying the major material resin from the outer flow path and the inner flow path to the combined flow path for a predetermined time period at a predetermined supplying rate;
• for a certain period of time within a range of the predetermined time period during which the major material resin is supplied, simultaneously supplying the intermediate layer resin from the middle flow path to the combined flow path at a predetermined supplying rate;
• after supplying the intermediate layer resin is finished, decreasing the supplying rate of the major material resin;
• injecting a columnar laminated molten resin into a cavity of a metal mold connected to a tip of the nozzle through a gate to fill the cavity, the columnar laminated molten resin being composed of the major material resin and the intermediate layer resin formed in the major material resin in a laminated manner that are combined into a columnar shape at the combined flow path and the gate being disposed at a position in the cavity that corresponds to a center of a bottom wall of a bottom of the preform;
• and
• forming a non-laminated section, where the intermediate layer is not formed, that extends in a shape of a longitudinal strip along a central axis direction of the preform on a circumferential wall of the preform, by causing the shut-off longitudinal rib piece to circumferentially interrupt the intermediate layer resin in the middle flow path.