Synthetic resin bottle having a gradation pattern, and process for injection molding the preform for use in such a bottle

A process for injection molding a preform in a shape of a test tube, comprising:

• flowing molten synthetic resins through at least three layer-forming channels comprising a cylindrical inner flow channel, a middle flow channel, and a cylindrical outer flow channel;
• flowing the molten synthetic resins through a confluence disposed downstream of the at least three layer-forming channels and through a joined flow channel;
• passing the molten synthetic resins through a gate disposed at a position opposed to a center of a bottom of a preform to be molded;
• and
• injecting the molten synthetic resins into a mold to form the preform;
• supplying each of the inner flow channel and the outer flow channel with a supply of a main resin for the preform from at least one first loader at a predetermined time span, pressure, or velocity;
• supplying the middle flow channel with a supply of a colored resin from a second loader at a predetermined pressure and/or velocity during a predetermined time within the predetermined time span;
• allowing the colored resin to join the main resin from the inner flow channel and the main resin from the outer flow channel at the confluence in such a manner that a layer of the colored resin is sandwiched between two layers of the main resin;
• forming a multi-layered molten resin fluid having an intermediate layer of the colored resin between the two layers of the main resin within the joined flow channel over a certain span of time;
• adjusting an injection pattern including at least one of a time of starting and ending the supply of the colored resin, pressure and velocity profiles;
• and
• gradually reducing a thickness of the intermediate layer at an upstream or a downstream point of flow in an upstream or a downstream direction so as to form a color-gradated portion caused by a change in the thickness.