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:Klocke, F. et al.: "Manufacturing of glass diffractive optics by use of molding process", American Society for Precision Engineering (Annual Meeting) 21, 2006, Monterey/Calif.Klocke, F.; Dambon, O.; Hünten, M.:"Integrative Fertigung von Mikrooptiken" erschienen in "WT Werkstatttechnik Online" Heft-Nr.6/99
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.Klocke, F.; Dambon, O.; Wang, F.: "Referate der Vorträge und Poster der 81. Glastechnischen Tagung der Deutschen Glastechnischen Gesellschaft", 04. - 06. Juni 2007, Aachen
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.”
Ground and Polished Lenslet Preform “The Lenslet preform is primarily for lenses with positive power, biconvex, planoconvex, and meniscus: where the convex side is the strongest surface. The use of this type of preform allows for molding of the largest volume of glass at any given time in the molding machines. The Lenslet is traditionally ground and polished to a near net shape of the final lens, and then pressed. [...] The cost associated with the manufacturing of the lenslet preform is the highest of all preform types.”
Gob Preform Precision gobs can be used as preforms for the production of aspherical lenses in a precision molding process. They are manufactured from a continuous glass melting process. The resulting precision gobs exhibit a very smooth firepolished surface with an excellent surface roughness and high volume accuracy.
Dimensions The dimensions of the optical elements that can be moulded depend on the size of the moulding machine. The precision glass moulding process is not limited to small optics. For the right element geometry, it can enable economical production of aspheric lenses up to 60 mm in diameter and more than 20 mm thick.Vogt, H.: Precision moulding provides compact consumer optics. Laser Focus World, July 2007, pp.115-118. To view the article online click here
General design recommendations:
Size: Diameter range: 0.5–70 mm depending on the applicationBraunecker, B.: Advanced Optics Using Aspherical Elements. SPIE Press, Bellingham, 2008, p.264 Flank angle: Sometimes <60 degree due to limited metrology but higher angles are possible by some manufacturers that have expanded metrology capabilities (e.g. Panasonic UA3P or similar). Edge thickness preferably > 1.0 mm, alternatively 0.5 to 2.0 x center thickness Clear aperture should be smaller than the lens diameter, preferably at least 1.0 mm (per side) less Optical surfaces:
Radius: Base radius no less than 3.0 mm
Optical Surfaces: Sags no greater than 8 mm on both concave and convex surfaces Transition from the optical surface to the lens outside diameter requires a minimum radius value of 0.3 mm.
Volume: Volume of the lens (including flanges), V <= 4/3 π r3, where r is the smallest local convex radius.
Tolerances Although the form, dimensional and positional tolerances that can be achieved in precision glass moulding are subject to a natural border, the values being achieved in practice strongly depend on the degree of control and experience in mould making and moulding. The table below gives an overview of achievable manufacturing tolerances in precision glass moulding at different companies.Klocke, F.; Dambon, O.; Sarikaya, H.; Pongs, G.:"Investigations to the Molding Accuracy of Complex Shaped Glass Components" from "Proceedings of the euspen International Conference – Zurich - May 2008"
Parameter Rochester Precision OpticsDeegan, J. et al.:RPO Precision Glass Moulding Technical Brief 6/28/2007 BrauneckerBraunecker, B.:
Advanced Optics Using Aspherical Elements. SPIE Press, Bellingham, 2008, p.255 Lightpath TechnologiesLightpath Technologies: http://www.lightpath.com/ IngenericFISBA Diameter +0/-0.010 mm +/-0.005 mm 0,005 mm +/- 0.005 mm Center thickness +/-0,012 mm +/- 0.010 mm 0,010 mm +/- 0.01 mm Alignment 2,5 min (axis) < 1.5min (angular) 5 μm - 5 μm Scratch-Dig 20-10 20-10/10-5 - 20-10 Abbe-Number +/-0.5% +/-1% 1% - Surface Roughness - <3 nm <3 nm 5 nm 4 nmIndex of Refraction +/-0.0003 +/-0.001 0,001 -Wedge 0.01mm +/-1 arcmin 1 arcmin 3'RMS WFE Diffraction Limited
For aspherical lenses, the design should be able to tolerate 0.010 mm of lateral shear between surfaces plus 5 micrometres Total Internal Reflection of wedge (across the part without considering the lateral shear) to be considered robust.
Specifications for aspheres: Surface roughness (Ra): < 3 μm depending on diameter Form error (PV): < 1 μm depending on diameter
Index drop Due to the fast cooling after moulding, the part retains a small amount of residual stress. Consequently, the glass exhibits a small change in the refractive index which has to be considered in the optical design. A higher cooling rate corresponds to a larger decrease of the refractive index. A lower cooling rate could circumvent the index drop, but would be less cost-efficientDeegan, J. et al.: RPO Precision Glass Moulding Technical Brief 6/28/2007. Download here
Glass material Many glasses can be used with PGM. However, there are some limitations:
The glass transition temperature Tg must not exceed the maximum heating temperature of the mould Many lead oxide flint glasses are not compliant with the RoHS EU directive (Restriction of certain Hazardous Substances) The glass composition influences moulding tool life Chalcogenide materials require certain preform shapes Glass expansion/contraction is highly temperature and rate dependent phenomenon. The coefficient of thermal expansion (CTE) of mould and glass should match. High CTE difference means high deviation between the moulded glass and the mould. High CTE glasses are also critical in terms of non-uniform temperature distribution in the glass. This means that especially fast cooling can not be applied. In addition to this, the temperature difference between the warm lens directly after moulding and the surrounding air can easily cause cracks.