Process for preparing saturated polyester resin bottles

A process for preparing a saturated polyester resin bottle from a tubular intermediate piece made of saturated polyester resin having a bottom wall and a top opening, comprising the sequential steps of:

• heating the intermediate piece to a temperature suitable for biaxial orientation and setting the same in a heated mold for blow molding;
• blowing compressed gas into the intermediate piece while inserting a rod downwardly into the interior of the intermediate piece to conform the intermediate piece to the inner walls of the mold and effect biaxial orientation;
• heat fixing the sidewalls of the bottle at a temperature higher than said temperature suitable for biaxial orientation to which said intermediate piece has been heated while maintaining gas pressure in the bottle to effect thermal fixing of said sidewalls heated to said higher temperature after the formation of the bottle by said biaxial orientation, the duration of the last-mentioned heating step being sufficiently short that crystallization of the polyester resin and milky white discoloration thereof do not occur;
• and
• lowering the temperature of the bottle and removing the bottle from the mold;
• wherein, during biaxial orientation of the intermediate piece and while the bottle is kept along the inner walls of the mold, the center of the bottom wall of the bottle is protruded inwardly so that the thickness and degree of orientation of a lower half of the resulting tapering rising wall of the protrusion are substantially the same as those of the trunk portion of the bottle, thereby making it possible to prevent heat deformation of the bottom wall;
• and wherein the bottom of the bottle is also thermally fixed without crystallization and milky white discoloration.