Highly precise infrared lenses are used in a broad range of optical systems such as night visions, thermal imaging or gas sensing. As most infrared materials (e.g. Germanium, Chalcogenide glass) suffer from high Fresnel reflection losses, the use of anti-reflective coatings is state of the art to overcome this issue. An alternative approach is the implementation of anti-reflective microstructures into molded infrared lenses. This shortens the process chain and enables many advantages for example regarding the monolithic optic’s design. Precision Glass Molding (PGM), a replicative manufacturing technology, allows the macroscopic lens molding and the replication of surface microstructures to be carried out simultaneously. While PGM is an established process for manufacturing glass optics in general, there is a lack of knowledge regarding the replication of microstructures. This leads to the necessity to further investigate the PGM process chain for molding microstructures. The current paper addresses the process chain of manufacturing anti-reflective optics by precision glass molding. Process simulations are presented by a multiscale approach. In order to prevent wear, a suitable anti-adhesive coating system for molding tools with regard to the special requirements of microstructured surfaces is introduced. The results of the molding experiments highlight the importance of a multiscale simulation approach and demonstrate the stability of the anti-reflective microstructure.
The 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.
Infrared (IR) optic holds a key element over a broad range of advanced optical systems such as thermal imaging, night visions or laser-based sensing. Most infrared optical materials like chalcogenide glasses, however, suffer great transmission losses due to their high refractive index. Therefore, antireflective (AR) surfaces are necessary to enhance the optical performance of the IR optics by suppressing undesirable reflection at the optical surfaces and thus increasing the transmission. The AR-coatings commonly used for IR lenses in the contemporary optic market are expensive and environmentally critical. Instead, Precision Glass Molding (PGM), a replicative manufacturing method for the production of highly precise glass optics, becomes a promising solution to fabricate the AR-nanostructures on the chalcogenide glasses in a cost-efficient manner. The PGM process development starts out a multiscale modeling of the molding process, by which the form accuracy of the molded glass lenses is predicted at macroscale while the replication of the AR-structure is visualized at nanoscale simulation. This simulation necessitates a newly developed thermal-mechanical constitutive model to represent thermo-viscoelastic behaviors of the chalcogenide glass. Experimental validations of the form accuracy and the replicated AR-structure of the molded lenses demonstrate essential benefits of the simulation model. This paper focuses on the process simulation as well as the subsequent steps of mold manufacturing and glass molding itself. The success of molding AR-structures by precision glass molding promisingly satisfies the increasing demands for the high volume production of inexpensive IR optical elements in today’s optics and photonics markets.
The continuously rising demands in the today’s photonic market towards increasing precision, high surface finish, geometrical complexity yet low cost for thin microstructure glass optics require advanced fabrication technologies. The conventional method via grinding and polishing is limited to those applications mainly due to the unavoidable deformation caused by mechanical stress between lenses and clamping parts. Over the last decade, replication technology such as thermal slumping has become an advanced method in manufacturing complex and precision thin lenses. However, the technology efficiency is strongly diminished by incomplete glass flow into mold cavity and extremely long processing time. In contrast, such deficits can be avoided by a novel molding process with vacuum assistance, recently developed at Fraunhofer IPT. The vacuum-assisted molding promises an effective and reliable technology in the fabrication of high precision thin glass optics for mass production. In this paper, the newly developed molding concept is firstly presented. Besides, this research introduces a numerical implementation based on an enhanced material model characterizing glass behavior at high temperature near softening point, which is crucial to study the internal stress, surface tension and form accuracy of the thin glass. With the help of simulation, the influences of process parameters will be discussed. Experiments were performed for the validation, and the accuracy of the molded glass with microstructure features is discussed in detail. Finally, the experiment results of both thermal slumping and vacuum-assisted molding are compared to illustrate the process efficiency and guidance for industrial applications is delivered.
The advantages of LED lighting, especially its energy efficiency and the long service life have led to a wide distribution of LED technology in the world. However, in order to make fully use of the great potential that LED lighting offers, complex optics are required to distribute the emitted light from the LED efficiently. Nowadays, many applications use polymer optics which can be manufactured at low costs. However, due to ever increasing luminous power, polymer optics reach their technological limits. Due to its outstanding properties, especially its temperature resistance, resistance against UV radiation and its long term stability, glass is the alternative material of choice for the use in LED optics. This research is introducing a new replicative glass manufacturing approach, namely non-isothermal glass molding (NGM) which is able to manufacture complex lighting optics in high volumes at competitive prices. The integration of FEM simulation at the early stage of the process development is presented and helps to guarantee a fast development cycle. A coupled thermo-mechanical model is used to define the geometry of the glass preform as well as to define the mold surface geometry. Furthermore, simulation is used to predict main process outcomes, especially in terms of resulting form accuracy of the molded optics. Experiments conducted on a commercially available molding machine are presented to validate the developed simulation model. Finally, the influence of distinct parameters on important process outcomes like form accuracy, surface roughness, birefringence, etc. is discussed.
Glass molding has become a key replication-based technology to satisfy intensively growing demands of complex precision optics in the today’s photonic market. However, the state-of-the-art replicative technologies are still limited, mainly due to their insufficiency to meet the requirements of mass production. This paper introduces a newly developed nonisothermal glass molding in which a complex-shaped optic is produced in a very short process cycle. The innovative molding technology promises a cost-efficient production because of increased mold lifetime, less energy consumption, and high throughput from a fast process chain. At the early stage of the process development, the research focuses on an integration of finite element simulation into the process chain to reduce time and labor-intensive cost. By virtue of numerical modeling, defects including chill ripples and glass sticking in the nonisothermal molding process can be predicted and the consequent effects are avoided. In addition, the influences of process parameters and glass preforms on the surface quality, form accuracy, and residual stress are discussed. A series of experiments was carried out to validate the simulation results. The successful modeling, therefore, provides a systematic strategy for glass preform design, mold compensation, and optimization of the process parameters. In conclusion, the integration of simulation into the entire nonisothermal glass molding process chain will significantly increase the manufacturing efficiency as well as reduce the time-to-market for the mass production of complex precision yet low-cost glass optics.
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