Jochen Freund

Development of a new process for manufacturing precision gobs out of new developed low Tg optical glasses for precise pressing of aspherical lenses

Aspherical lenses or refractive elements out of optical glass can be produced either by grinding and polishing of glass or by precise molding of glass preforms. The first process is applied for lenses with larger geometries and smaller production quantities. On the other hand, precise molding is used for volume production of lenses within a diameter range between 1 mm and around 30 mm. The addressed products can be found in the consumer markets (digital imaging, digital projection and digital storage). Different preform types can be used for precise molding: polished spherical near shape preforms, polished balls, polished discs and precision gobs. The latter are made directly from the glass melt. This paper describes a newly developed process, which results in fire-polished gobs with very low surface roughness and excellent volume accuracy. Since precision gobs are mostly made for precise molding, they must meet specific process requirements apart form their optical values, such as allowing low molding temperatures and shorter process cycles times. Therefore, this paper also describes the latest results in the development of low Tg glasses, which are designed for the volume production of precision molded optical components. Beside the important parameters like nd, nd as well as Tg, other properties like chemical durability, devitrification resistance, thermal expansion and conductivity coefficients are important for optimizing the precise molding process. The characteristics of three new low Tg glasses in the FK-, PK- as well as SK-region are presented. These glasses are environmentally friendly, since they are free of lead and arsenic.