A process using an injection molding device to injection mold a preform for biaxial stretching and blow molding use, the preform having a shape of a test tube and having a second-resin layer laminated with main-resin layers made of a main resin that makes up a shape of the preform, the injection molding process comprising, in an injection molding device comprising:
- a nozzle section for forming a joined resin fluid by allowing a second resin (Rb) for forming the second-resin layer to join a main resin (Ra) for forming the main-resin layers, and having a mold disposed ahead of the nozzle section, the nozzle section comprising:
- three cylindrical layer-forming flow channels including an outer flow channel, a middle flow channel, and an inner flow channel in an outside-to-inside order,
- a joined flow channel connected to the three flow channels and extended to the head of the nozzle section, wherein the main resin (Ra) is passed through the outer flow channel and inner flow channel, and the second resin (Rb) is passed through the middle flow channel, and
- a rod-like shut-off pin disposed on an inner side of the inner flow channel and slidably inserted therein, wherein an outlet of the inner flow channel, which is open to the joined flow channel, is shut off or opened, and a degree of aperture can be adjusted, by controlling the position of a forefront of the shut-off pin,
- a) supplying the main resin from the outer flow channel and the inner flow channel to the joined flow channel at a predetermined supply velocity for a predetermined period of time;
- b) supplying the second resin from the middle flow channel to the joined flow channel simultaneously with the main resin at a predetermined supply velocity for a certain period of time within the predetermined period of time;
- c) joining the main resin and the second resin in the joined flow channel to form the joined resin fluid, and injecting the joined resin fluid into a cavity by way of a pin gate disposed at a position of the cavity of the mold corresponding to a center of a bottom wall of the preform in order to fill the cavity with the joined resin fluid;
- d) sliding the shut-off pin to bring the forefront of the shut-off pin to a predetermined position near an outlet of the inner flow channel, which is open to the joined flow channel, before the second resin is supplied, or during a period of time starting after a predetermined time from the start of the supply of the second resin and ending with termination of the supply of the second resin, so that the velocity of main resin supply from the inner flow channel to the joined flow channel is reduced to a predetermined level by adjusting the degree of aperture for the outlet;
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- e) setting an injection pattern, including a speed of the main resin supply, a start time, an ending time, and a speed of the second resin supply, a moment of sliding the shut-off pin, and the positioning of the forefront of the pin, associated with the degree of aperture to be opened or closed at the outlet of the inner flow channel, and thereby positioning a trailing edge of the second-resin layer within a predetermined range in the bottom, excluding a gate vestige formed at the center of the bottom, the bottom having a semi-spherical shell shape in the preform.