Precision glass moulding is a replicative process that allows the production of high precision optical components from glass without grinding and polishing. The process is also known as ultra-precision glass pressing. It is used to manufacture precision glass lenses for consumer products such as digital cameras, and high-end products like medical systems. The main advantage over mechanical lens production is that complex lens geometries such as aspheres can be produced cost-efficiently.
Process
The precision glass moulding process consists of six steps:
The glass blank is loaded into the lower side of the moulding tool. Oxygen is removed from the working area by filling with nitrogen and/or evacuation of the process chamber. The tool system is nearly closed (no contact of the upper mould) and the entire system of mould, die and glass is heated up. Infrared lamps are used for heating in most systems. After reaching the working temperature, which is between the transition temperature and the softening point of the glass, the moulds close further and start pressing the glass in a travel-controlled process. When the final thickness of the part has been achieved, the pressing switches over to a force-controlled process. After moulding has been completed, the glass is cooled down and the working environment is filled with nitrogen. When the lens has cooled to the point where it can be handled, it is removed from the tool. The process is executed on a specialized moulding machine, which precisely controls the temperature, travel, and force during the process. The tools used must withstand high temperatures and pressures, and need to be resistant to chemical interaction with the glass. The mold materials also have to be suitable for machining into the precise surface profiles.
Process chain In order to ensure the desired quality the parts are measured between each process step. Additionally, the parts are handled and transported carefully between the processing and metrology steps.
Hotforming of gobs: The precision glass moulding process yields the best results in both quality and cost if it works with precise preforms. These are usually acquired by pressing or hotforming of "gobs" of molten glass. This step is done by continuous glass melting and moulding in single-sided metal moulds. This process is only suitable for high production volumes. For smaller production volumes, the preforms have to be manufactured by mechanical material-removing steps from blocks or slices of raw glass. Precision glass moulding: In this step the preform is directly formed into an optical glass lens. It is necessary to clean the glass preform and the mould before starting the process, but there is no polishing or post-machining required. Lens coating: An antireflection coating is applied to the finished lenses. The lenses are first cleaned, and then loaded into a fixture. The fixture, containing a large number of lenses, is placed in the coating machine. After finishing the process the glass lenses are removed from the holder and the holder is cleaned by sand-blasting or other techniques. Usually the optical coating is done by one of two methods: physical vapour deposition (PVD), in which oxide materials evaporate and are deposited on the lens, and plasma-enhanced chemical vapor deposition (PECVD). Chemical reactions take place in a vacuum and the reaction product is deposited on the lens. The lenses are coated for two reasons: Manipulate or improve the optical transmission / reflection Enhance the mechanical, chemical or electrical properties
Tool and mould design
Lens shapes
Shape of optical element Precision glass moulding can be used to produce a large variety of optical form elements such as spheres, aspheres, free-form elements and array-structures. Concerning the curvature of the lens elements, the following statements can be drawn: Acceptable lens shapes are most bi-convex, plano-convex and mild meniscus shapes. Not unacceptable but hard to mould are bi-concave lenses, steep meniscus lenses, and lenses with severe features (e.g. a bump on a convex surface). In general, plano-curved lenses are easier to mould than lenses with both sides curved since matching of flat faces is easier. Moulding concave forms with small centre thickness is difficult due to sticking of the moulded part to the mould occurring as a result of the different thermal expansion coefficients. Furthermore, it is recommended to avoid undercuts and sharp edges. For the lens design it should be considered that the lens has to be mountable in measurement systems.
Shape of preforms The shape of the preform or "blank" needs to be chosen according to the geometry of the finished optical element. Possible preforms are spherical (ball), near spherical (gob), plano-plano, plano-convex, plano-concave, biconvex and biconcave blanks. Ball and gob-blanks do not have to be premachined whereas other preforms require grinding and polishing. The following section describes basic traits of preform choice:
Formed Ball Preform “Used specifically for lenses with positive power: biconvex, plano-convex, and meniscus where the convex side is stronger than the concave side, this only works for a relatively small volume of material.”
Ground and Polished Plano-Plano Preform “As a lens changes to negative in power biconcave, plano-concave, and meniscus where the concave side is stronger, an alternative preform shape, plano-plano, is required for the molding process. […] Relative to a formed preform an increase in cost is observed for the manufacturing of this type of preform.”
Ground and Polished Ball Preform “When the geometry of a lens extends beyond the volume range of a formed ball preform, a ground and polished ball preform is required. Used for lenses with positive power: biconvex, plano-convex, and meniscus: where the convex side is stronger, this geometry allows for molding of lenses with larger total volume. […] Relative to a formed preform and a plano-plano preform, an increase in cost is observed for the manufacturing of this type of preform.”
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