Method of manufacturing plastic thin-plate article with protrusions on surface thereof

Abstract

A method of manufacturing a diffusion plate for use in a screen plate of projection TV is disclosed. The diffusion plate having a plurality of quadrangular pyramid protrusions almost over one side thereof is manufactured by means of injection molding process of plastic material. This method uses a first molding die having a first molding die surface, a pre-molding die capable of forming a first volume of molding cavity in matching with the first molding die, and a second molding die capable of forming a second volume of molding cavity smaller than the first volume in matching with the first molding die. The molding process comprises; a first step of, with matching the first molding die with the pre-molding die, injecting a molten plastic material from the injection port into the first volume of molding cavity formed between the first molding die and the pre-molding die; and the second step of, after completed the injection in the first step and before curing of the plastic material, releasing the first molding die from the pre-molding die and then matching the first molding die with the second molding die, wherein the plastic material injected in the first step is compressed in the second volume of molding cavity so as to mold the diffusion plate from the plastic material between the first molding die and the second molding die.