Method for injection molding weld line free minus power lens elements

A method for injection molding of a weld line free minus power lens element made from polycarbonate with a weight average molecular weight of less than 26,000 g/mol, comprising:

• injecting a melt of thermoplastic material comprising at least one UV absorber that is a benzotriazole absorber at a temperature higher than a glass transition temperature (Tg) of the thermoplastic material and at a speed of not more than 25 mm/s in an initial molding cavity delimited by two facing mold inserts;
• during said injecting, moving said two facing mold inserts toward one another at a speed of less than 150 mm/s and at a pressure of not more than 50 US tons to define a final molding cavity whose volume is less than that of said initial molding cavity, and
• after cooling and opening of the molding cavity, recovering the weld line free minus power lens element,
• wherein one of said two facing mold inserts comprises a flat surface facing the initial molding cavity, thereby to form a flat surface on one side of the weld line free minus power lens element,
• wherein the other of said two facing mold inserts comprises a convex surface facing the initial molding cavity, thereby to form a concave surface on an opposite side of the weld line free minus power lens element, and
• wherein the center distance between said two facing mold inserts when delimiting said initial molding cavity is not more than five times a center thickness of the weld line free minus power lens element.