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

A synthetic resin bottle having a gradation pattern in color, obtained by biaxially drawing and blow molding a preform that has been injection molded in a shape of a bottomed cylinder, the synthetic resin bottle comprising:

• a portion of a cylindrical body of the synthetic resin bottle having a laminar structure in which a main resin layer made of a main resin is laminated with a colored resin layer made of a colored resin of a different color than a color of the main resin;
• a color-decorated portion defined by the colored resin layer in the portion of the cylindrical body;
• a color-gradated portion defined by a thin boundary area which is an end portion of the colored resin layer and where the colored resin layer has a gradually reduced thickness in a downward direction;
• and
• a non-decorated portion,
• wherein:
• the color-gradated portion is located between the color-decorated portion and the non-decorated portion,
• the thickness of the colored resin layer gradually decreases from the color decorated portion until it reaches the non-decorated portion,
• the colored resin layer is not present in the non-decorated portion, and
• the end portion of the colored resin layer is formed using a gradual decrease in the thickness of the colored resin layer caused by the flow behavior of the main resin acting on the colored resin during injection molding of the preform,
• the colored resin layer is embedded in the main resin layer as an intermediate layer,
• the preform has a gate imprint at a center of a bottom of the preform, a preform colored resin layer forms a preform intermediate layer in a predetermined height range of the preform and the preform has a color-gradated portion defined by a layer thickness of the preform intermediate layer that is reduced in the downward direction,
• a change in the layer thickness of the preform intermediate layer in the color-gradated portion is shown by a condition of L/t1≧30 where L is a length of the color-gradated portion and t1 is the layer thickness at a base of the color-gradated portion, and
• the preform colored resin layer and the preform intermediate layer form the colored resin layer and the intermediate layer, respectively.