Abstract: An improved injection/compression molding process for the manufacturing of high quality concave optical lenses of thin thickness. A thermoplastic material is injected into a mold cavity of an injection/compression molding machine, the mold cavity is designed to have adjustable thickness and is initially set at a thickness greater than the lens to be produced. After the amount of the plastic injected fills the enlarged cavity, the thickness of the cavity is gradually reduced by exerting a compressional force on the thermoplastic material inside the mold cavity. During the compression stage, the injection continues and the excess thermoplastic material is squeezed into an overflow pocket inside the injection/compression mold. The injection apparatus can be switched into a pressure holding mode during and after the compression stage to prevent any backflow and post-injection shrinkage.