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PublicationSurrogate modeling for multi-objective optimization in the high-precision production of LiDAR glass optics( 2024-04-24)
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PublicationSurface Grinding of Borosilicate Crown Glass Optics via a Robotic Approach Based on Superposed Trajectories( 2023)The production of large-sized optical components with complex shapes requires several phases, including surface finishing. Currently, mainly skilled workers can correctly perform this operation, divided into the successive steps of grinding and polishing, leading to long production times, poor reproducibility of results, and exposure to human error. For this reason, the industry is trying to move towards automation involving, for example, high-precision machine tools and machining centers. However, these solutions require high investment costs and long setup times. Using robotic cells helps to reduce these expenses, manufacture larger components, and increase the flexibility in the production chain. In this research, we present an unconventional approach to the robot-assisted grinding of optical samples made of borosilicate crown glass. The samples were guided by a six-degree-of-freedom industrial robot on a rotating grinding disc while imposing to them different trajectories with complex geometry. We avoided regular grinding patterns, which are easily recognizable by human eyes and affect the quality assessment, by superposing multiple relative movements between the machined surface and the abrasive grains. The ground surfaces of the samples were characterized based on average roughness values, profile error data, and surface topography images. Finally, we selected the best robotic grinding procedure matching the trajectory and strategy with optimal surface quality, processing time, and productivity. The suggested methodology not only shortens the manufacturing sequence by eliminating manual methods but also provides components with optical properties within the required specifications for subsequent polishing steps.
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PublicationMold protective coatings for precision glass molding( 2022-10-04)Precision Glass Molding (PGM) is a replicative technology to manufacture glass lenses with complex geometries such as aspheres, freeform-optics or lens arrays. During the PGM process, a glass preform is heated until the viscous state and afterwards pressed into the desired shape using two high-precise molds. This process enables the direct and efficient manufacture of high shape accuracy and surface quality optics without any mechanic post-processing step. The efficiency of the PGM process depends primarily on the lifetime of the high-precision molds made of cemented tungsten carbide. During each molding cycle, the molds have to withstand severe thermo-chemical and thermo-mechanical loads. Using protective coatings, the lifetime of the molds can be increased. In this study, the performances of a diamond-like carbon (DLC) and a precious metal alloy coating, namely PtIr, were evaluated in an industrial glass molding machine. The degradation mechanisms of the coatings were analyzed using surface characterization such as scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX). At this, phenomena such as glass adhesion and coating disintegration were observed.
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PublicationAn Analytical Model for Robot-Based Grinding of Axisymmetric Mold Inserts Using a Rotary Unit( 2022)The grinding of mold inserts used for injection molding aims to improve the surface roughness according to precise quality standards. The insert surface must also have a surface topography that facilitates the release of the plastic material at the end of the injection process. In particular, fine machining lines must be parallel to the extraction direction from the mold to avoid the sticking of plastic material and subsequent surface damages compromising the functionality of the finished product. However, this step in the production chain is most often conducted manually. This paper presents an analytical model to grind a truncated cone-shaped mold insert for the mass production of plastic cups. The automated solution consists of a flexible robotic system equipped with a rotating external axis to improve the accessibility of the tool to the surface to be machined. The tool path programming requires the development of an analytical model considering the simultaneous mot ion of the insert and the robot joints. The effectiveness of the developed model is evaluated in terms of final surface quality, grinding lines direction, and total process time. The automated strategy developed can be easily implemented with machine tools and applied to inserts with different axisymmetric geometries.
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PublicationBio-inspired manufacturing of molded optics and optical systems( 2022)Manufacturing technology is driven by ever increasing demands of costs, quality and lead time. For many existing industries, digitization is key to obtain these goals in the future. In terms of optics production, it is rather a research topic. Meanwhile, the so called »Biological Transformation« is said to be the subsequent industrial revolution. This paper will explain this development and translate it to the optics production. Two examples of biologically transformed production scenarios will be presented. The presentation concludes with an assessment, whether »Biological Transformation« can deliver a substantial innovation push to optics manufacturing and glass molding in particular.
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PublicationPrecision Glass Molding of infrared optics with anti-reflective microstructures( 2020)
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PublicationVibration-Assisted Face Grinding of Mould Steel( 2020)This work investigates vibration-supported, force-controlled fine machining with elastic bonded mounted points for automated fine processing of mould steel samples. The aim is to compare conventional robot- or machine-tool-based face grinding with a vibration-supported grinding process. The influence of vibration support on the surface topography is investigated primarily to minimize kinematically caused grinding traces. First, the state of the art for the production of tool moulds and vibration-supported fine machining is explained. On this basis, the potentials for the reduction of grinding marks through vibration support for an increase in the degree of automation are derived and the experimental procedure is introduced. Subsequently, robot-based grinding tests with vibration support are carried out and compared with conventional grinding tests. After the tests carried out, the results are evaluated using tactile and optical measuring methods.
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PublicationApproaches and Methodologies for Process Development of Thin Glass FormingThe steadily growing thin glass market is driven by a vast amount of applications among which automobile interiors and consumer electronics are, such as 3D glass covers for displays, center consoles, speakers, etc. or as part of optics within head up-displays. Today, glass manufacturers are suffering from challenges brought about by the increases of shape complexity, accuracy and product variants while simultaneously reducing costs. The direct manufacturing method via grinding and polishing is no longer suitable because of its limited machinability for thin glasses in respect to fracture and its cost insufficiency due to the length of the process chain. Instead, replication-based technologies or thin glass forming become promising manufacturing methods to overcome the aforementioned technical and economic challenges. For instance, thermal slumping is only able to satisfy the most basic requirements and is in particular limited regarding the deformation degree and shape complexity of thin glass products. Technologies such as vacuum-assisted slumping or deep drawing are currently in development at the Fraunhofer Institute for Production Technology IPT and promise additional cost benefits. This paper introduces all potential process variants for thin glass forming. The suitability of different methods for process development, specifically process modeling based on either experimental-, simulation- or machine learning approaches (white box and black box models), will be addressed and discussed. Furthermore, process efficiency is examined on both an economic and technical level, where molding time, suitable geometries and accuracy are the focus. The methodologies presented in this paper aim at developing a guideline for glass manufacturers on determining the optimal strategy for the process development of thin glass production.
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PublicationEvaluation of mold materials for precision glass molding( 2019)Driven by the wide range of applications in the fields of laser technology, biomedicine and consumer electronics, etc., the demand for high-quality lenses with complex geometries and small dimensions is rapidly rising. Since grinding and polishing of such lenses is neither practically nor economically viable, Precision Glass Molding (PGM) has become a popular production method. PGM is a replicative technology for producing high-precision optical lenses in medium or high volumes. During the one-cycle molding process, a glass preform is heated until the viscous state and afterwards pressed into the desired shape using two high-precise molding tools. This process permits the direct and efficient manufacture of high shape accuracy and surface quality optics without any mechanic post-processing step. The efficiency of PGM processes depend primarily on the lifetime of the high-precision molding tools. Therefore, various investigations focus on enhancing the molding tool lifetime. This work focuses on the evaluation of suitable mold materials for PGM, whereby different substrate materials as well as protective coatings are considered. At this, three different kinds of glass with varying molding temperature were investigated: common optical glass, infrared transmissive chalcogenide glass, and fused silica. The molding temperature of common optical glass ranges from 400°C to 700°C, whereas chalcogenide glass is molded at around 250°C. Fused silica requires a more challenging molding temperature of about 1400°C. Due to the varying molding temperatures, different mold materials can be evaluated for each of the investigated glasses.
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PublicationReplicative manufacturing of glass optics with functional microstructures( 2019)Microstructuring of glass optics enables a large variety of benefits for miscellaneous fields of application. From an enhancement of the performance of optical systems to the haptic improvement of coverglasses the advantages of structured glass are obvious. Especially in the field of high-precision optics, microstructured optical surfaces can carry out important functions, such as beam shaping in laser systems or the correction of dispersive color alterations. Besides enhancements regarding optics of the visible light spectrum, microstructures can compensate disadvantages of infrared(IR)-transmissive lenses such as chalcogenide glasses. As these optics suffer high transmission losses due to their high refractive index the integration of an anti-reflective (AR) function is necessary. Moth-eye-structures are a promising way to avoid the currently used AR-coatings. So far, microstructures are brought into the lens surface by lithography mainly. The therefore additional processing step follows the previous shaping. An efficient production of the structured components is the key to success for applications aside science and research. The technology precision glass molding (PGM) is able to combine the contradicting aspects of high precision and high volume production. PGM is a replicative manufacturing method that allows the macroscopic molding and the manufacturing of microscopic structures to be carried out simultaneously. Based on a representative PGM process chain, the paper at hand describes differences, challenges and current research results regarding molding microstructures.
