Diamond Machining of Nitrocarburized Steel Molds for the Mass Production of Optical Components

Abstract

Optical components, e.g. lenses for cameras in mobile phones or illumination optics, are produced in high volume by repliation techniques in plastics or glass. Thus, the necessary sequence of process steps comprises all steps from optic design to mold making and the final replication process. Every single step in this process chain is accompanied by high resolution measurement techniques and quality management. An important factor in this process chain is the optical mold making by diamond cutting processes with single crystal diamond cutting tools. The drawback of diamond cutting processes so far was that steels which have very good material properties for replication (temperature resistance and hardness) cannot be machined because a catastrophic diamond tool wear occurs. If these materials are applied for optical mold making a complex process combination of grinding and polishing operations is inevitable. The substitution of this traditional process combination by diamond cutting processes can reduce costs and enhances the possibilities of optical mold making. This paper describes a possible solution for diamond machining of steel molding inserts for the replication of optical components by plastic injection molding and hot pressing of glass. Through a thermo-chemical surface treatment the chemical reactivity of steel and single crystal diamond cutting tool can be minimized. Beside the results of the diamond machining processes the occurring diamond tool wear is discussed. From these results a new process chain for the manufacturing of optical components is deduced. This novel process chain consists of nitrocarburizing the steel molding inserts and a subsequent diamond machining operation by turning or milling. Through the nitrocarburizing process the diamond tool wear can be reduced by three orders of magnitude and the machined molding inserts show an optical surface roughness Sa 10nm. These inserts can directly be applied for the replication of plastics or the pressing of glass which is shown by first application examples.