Injection molding process for making a preform

A injection molding process for making a preform by an injection molding device to be used molding the preform used in biaxial stretching and blow molding, the preform having a shape of a test tube and having a second resin layer or layers laminated with a main resin layer that makes up a shape of the preform, the injection molding device having a nozzle section, in which a molten resin to be formed into the main resin layer is made to join a molten second resin to be formed into the second resin layer or layers so that a joined resin mass is formed, and a mold disposed ahead of the nozzle section, the nozzle section being arranged in an order from outside to inside including:

• an outer flow channel through which the main resin flows,
• an inner flow channel through which the second resin flows,
• a cylindrical column-shaped joined flow channel where the second rein from the inner flow channel joins the main resin from the outer flow channel,
• a first confluence disposed where the main resin from the outer flow channel flows into the joined flow channel,
• a second confluence disposed where the second resin flows into the joined flow channel, wherein the first confluence is disposed downstream of the second confluence with a predetermined space between the confluence first and the second confluence, and
• a cylindrical shutoff pin inserted slidably in the joined flow channel, wherein the shutoff pin is capable of closing or opening either or both of the first confluence and second confluence based on a sliding position of the shutoff pin,
• the injection molding process comprising the steps of:
• maintaining the first confluence in an open state while controlling the sliding position of a shutoff pin, and supplying the main resin from the outer flow channel to the joined flow channel at a predetermined time span;
• supplying the second resin from the inner flow channel to the joined flow channel simultaneously with the main resin for a predetermined period of time within the predetermined time span by bringing the second confluence to an open state, and forming the joined resin mass of the main resin and the second resin in the joined flow channel;
• injecting the joined resin mass cylindrically joined at the joined flow channel into the cavity of the mold, by way of the pin gate and filling the cavity with the joined resin mass;
• and
• while the supply of the main resin is maintained, stopping supply of the second resin with the second confluence part in a shut off state such that the main resin plunges into a central portion of the second resin and breaks through a forefront portion of the second resin.