Method for forming an optical design pattern in polyethylene terephthalate articles, and method for strengthening an optical design pattern formed by said method

A method for producing and enhancing an optical design pattern in an article formed of polyethylene terephthalate which comprises:

• forming a primary molded article of polyethylene terephthalate said primary article having recessed areas on at least one surface thereof arranged to define a pattern;
• forming a wall member by injection molding polyethylene terephthalate about the recessed surface of the primary article;
• said primary article functioning as a mold surface;
• cooling and solidifying said wall member to form a composite structure with said primary article at a cooling rate at which those portions of said wall member filling recessed areas of said primary article cool more slowly and display a reduced light transmissivity than do the non-recessed areas of said wall member to thereby form an optical design pattern upon solidification of the composite structure;
• reheating said composite structure to a temperature which allows biaxial stretching of polyethylene terephthalate, and
• biaxially expanding said composite structure.