Precision Glass Molding Process Research Articles

Machine learning for aspherical lens form accuracy improvement in precision molding of infrared chalcogenide glass

Precision glass molding (PGM) is an effective approach to manufacturing infrared chalcogenide glass (ChG) aspherical lens with complex shapes. However, infrared ChG aspherical lens often experiences form error in the designed profile and the final profile obtained by PGM. To reduce the form error of infrared ChG aspherical lens in the PGM process, a form error compensation model based on the random forest (RF) algorithm is proposed. The infrared ChG aspherical lens profile was first machined on an electroless nickel-phosphorus (Ni–P) plating to serve as the mold for PGM. After molding, the profile data of the lens was extracted, and a compensation model based on RF was constructed to optimize the model parameters using the evaluation parameters such as root mean square error (RMSE), coefficient of determination (R2), and out-of-bag (OOB). Finally, the generated compensated profile based on the compensation model was used for the compensation machining of the mold. Through this compensation approach, we have demonstrated a substantial 63.5 % reduction in the form error of the fabricated infrared ChG aspherical lens, decreasing the Peak-to-Valley (PV) value from 1.04 μm to 0.38 μm.

Analysis on the instability of the surface profiles of precision molding chalcogenide glass aspherical lenses in mass production.

Chalcogenide glass lenses have been widely applied in infrared optical systems for their outstanding optical performance. It is a tendency for complex optical glass elements to be mass-produced with precision glass molding (PGM) technology, of course including chalcogenide glass aspheric lenses. But there is a problem that sometimes the surface profiles of the molded lenses are unstable which leads to a low pass-yield. Precision glass molding experiments and finite elements simulations are carried out to study the reasons for the mentioned problem in this paper. The results reveal that the laying error of the ball chalcogenide glass preform does not have a significant effect on the surface profile of the molded lens. However, in mass production the control of the temperature after forming stage in the PGM process is very important for obtaining the molded lenses with very similar surface profiles. The research results could help relevant researchers design the PGM processing parameters to overcome some errors in the mass production and manufacture precision glass molding machines. The increase in the yield of complex optical glass elements fabricated by PGM technology will further promote the application of such elements in various fields.

Studies on Protective Coatings for Molding Tools Applied in a Precision Glass Molding Process for a High Abbe Number Glass S-FPM3

Precision glass molding (PGM) is an efficient process used for manufacturing high-precision micro lenses with aspheric surfaces, which are key components in high-resolution systems, such as endoscopes. In PGM, production costs are significantly influenced by the lifetimes of elaborately manufactured molding tools. Protective coatings are applied to the molding tools to withstand severe cyclic thermochemical and thermomechanical loads in the PGM process and, in this way, extend the life of the molding tools. This research focuses on a new method which combines metallographic analysis and finite element method (FEM) simulation to study the interaction of three protective coatings—diamond-like carbon (DLC), PtIr and CrAlN—each in contact with the high Abbe number glass material S-FPM3 in a precision glass molding process. Molding tools are analyzed metallographically using light microscopy, white light interferometry, scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDX). The results show that the DLC coating improved process durability more than the PtIr and CrAlN coatings, in which the phenomenon of coating delamination and glass adhesion can be observed. To identify potential explanations for the metrological results, FEM is applied to inspect the stress state and stress distribution in the molding tools during the molding process.

Study of Interfacial Adhesion and Re-Ir Alloy Coating in Chalcogenide Glass Molding.

Precision glass molding (PGM) has become an efficacious technique to fabricate high-precision optics. Chalcogenide (ChG) glass is increasingly used in thermal imaging, night vision, etc., because of its excellent infrared optical properties. Nevertheless, glass-mold interfacial adhesion has emerged as a pivotal issue within the PGM process. The interfacial adhesion during PGM has the potential to significantly undermine the performance of molded optics and reduce the longevity of molds. It is important to investigate interfacial adhesion behaviors in the PGM. In this study, the interfacial adhesion mechanism between ChG glass and the nickel-phosphorus (Ni-P) mold is analyzed using the cylindrical compression test. The effect of ChG glass internal stress on physical adhesion is investigated by finite element method (FEM) simulation. The spherical preform is proven to be capable of reducing the stress concentration and preventing physical adhesion. More importantly, a rhenium-iridium (Re-Ir) alloy coating is deposited on the Ni-P mold surface by ion sputtering to prevent atomic diffusion and resolve the problem of chemical adhesion. Finally, ChG glass microstructures with high accuracy are fabricated using the spherical ChG glass preform and the Re-Ir-coated Ni-P mold by PGM.

Antireflective structured surface of chalcogenide glass fabricated using glass to glass molding

Chalcogenide (ChG) glasses with micro-nano structure arrays are increasingly demanded in infrared optical systems owing to their excellent antireflective properties, and can be mass-produced by precision glass molding (PGM). Nickel-phosphorus (Ni–P) plated molds are commonly used in the PGM process, but the Ni atoms tend to react with the ChG glass at high temperatures. In this study, we propose a novel two-step PGM process. A Ni–P mold is used to transfer antireflective micro-nano structure array to silicate glass. Then the silicate glass, which has a higher glass transition temperature (Tg) than the ChG glass, is used as the mold to replicate the antireflective structured surface to the ChG glass. The results show that the ChG glass micro-nano structure array has high forming accuracy and favorable infrared optical transmission characteristics. The silicate glass is proved to be an appropriate mold material for the ChG glass molding process. This work provides a novel type of the PGM techniques for ChG glass materials.

Study on gas trapping during precision glass molding of microlens array in a nitrogen atmosphere

AbstractMicrolens arrays will suffer from filling defects due to trapped gas when molded in a nitrogen atmosphere by precision glass molding (PGM). In this paper, a multistep molding method is proposed to avoid gas trapping and improve the accuracy of a microlens array. The defect formation mechanism of the microlens array caused by the trapped gas is investigated, and the effect of the molding pressure on the defect formation is analyzed. A numerical model of the mold‐nitrogen‐glass interface at high temperature is established to evaluate the defect evolution, and the minimum number of PGM steps required to greatly reduce defects caused by the trapped gas is predicted. The numerical model is validated by a multistep PGM experiment of D‐K59 glass material. The results show that a three‐step PGM process significantly reduced the height of the defect. The difference between the height of the microlens unit and the depth of the mold is less than 0.4%. The molded microlens array has a peak‐to‐valley value of 0.38 μm and a surface roughness Ra of 3.5 nm. This work is instructive for the fabrication of high‐precision glass microlens arrays.

Simulation of the Refractive Index Variation and Validation of the Form Deviation in Precisely Molded Chalcogenide Glass Lenses (IRG 26) Considering the Stress and Structure Relaxation.

Precise infrared (IR) optics are core elements of infrared cameras for thermal imaging and night vision applications and can be manufactured directly or using a replicative process. For instance, precision glass molding (PGM) is a replicative manufacturing method that meets the demand of producing precise and accurate glass optics in a cost-efficient manner. However, several iterations in the PGM process are applied to compensate the induced form deviation and the index drop after molding. The finite element method (FEM) is utilized to simulate the thermomechanical process, predicting the optical properties of molded chalcogenide lenses and thus preventing costly iterations. Prior to FEM modelling, self-developed glass characterization methods for the stress and structure relaxation of chalcogenide glass IRG 26 are implemented. Additionally, a ray-tracing method is developed in this work to calculate the optical path difference (OPD) based on the mesh structure results from the FEM simulation. The developed method is validated and conducted during the production of molded lenses.

Prediction model of residual stress during precision glass molding of optical lenses.

Precision glass molding (PGM) is an important processing technology for aspheric lenses that has the advantages of low complexity, high precision, and short processing time. The key problem in the PGM process is to accurately predict the residual stress of aspheric lenses. In this paper, we examine the residual stress relaxation model for aspheric lenses, including a creep experiment of D-K9 glass, calculating shear relaxation function, and predicting residual stress of aspheric lenses with the finite element method. Validations of the proposed model are conducted for three different process parameters, including molding temperature, molding pressure, and molding rate. The experimental and simulation results show that the errors of the residual stresses of the three process parameters are within 0.358 Mpa, which proves the validity of the model. The model can be used to predict the residual stress of the optical glass lens fabricated by PGM and analyze the processing parameters.

Study on the blackening phenomenon of leaded glass during microgroove molding using nickel phosphorous mold

Precision glass molding (PGM) is a recently developed method to fabricate glass microgroove components. Lead glass is commonly used as an optical material due to its high refractive index and low transition temperature. A nickel-phosphorous (Ni–P) plated mold is traditionally employed in the PGM process for microstructures optics. However, leaded glass is subject to color change and can blacken during the PGM process, reducing the light transmittance of microgrooves. In this paper, an equation for the redox reaction between Ni and Pb is proposed, which is based on the diffusion of inner Ni atoms to the surface of the mold and the standard electrode potential of the Pb ions in leaded glass. A viscoelastic constitutive model of the glass is established to simulate the compression stress distribution during molding. Finally, the effects of molding pressure, molding temperature, and mold material on glass blackening are studied. The results show that the blackening of leaded glass is caused by Pb enriching the surface. The rise in molding stress and temperature increases the deformation of Ni–P plating, which promotes the diffusion of Ni atoms. By adding a titanium incorporated diamond-like carbon (Ti-DLC) coating, the deformation of the Ni–P plating during molding is suppressed, and the diffusion of Ni atoms can be prevented. In this way, the blackening of leaded glass can be prevented.

Simulation of heat transfer in the progress of precision glass molding with a finite element method for chalcogenide glass.

Precision glass molding (PGM) has become a viable processing method for large-volume aspheric optical elements. The optimization of a PGM process is an obstacle to the realization of mass production. The current work is focused on optimizing the process parameters to gain satisfactory surface shape. But the machining cycle time is not optimized. When setting the process route in the machine interface, going to the next step after reaching the target temperature rather than reaching the target time is usually set for the heating and cooling phase. Thus, the time to complete the heating and cooling stages of the production cycle is known only in the actual production. As for chalcogenide glass, its physical and chemical properties are greatly dependent on temperature. So, it is necessary to effectively simulate these stages to obtain the cost in time for actual production. Due to the excellent availability of numerical simulation, the rapid development of computing technology, and the increase of task scale in data and information processing, the finite element method can be applied to simulate the whole molding process. In this paper, a heat transfer model is established with the partial differential equation toolbox in MATLAB software. MSC.Marc software is used to simulate the heating stage at the same time. The numerical results are consistent, indicating that the heat transfer model established in MATLAB is, at least to a certain extent, valid. The heat transfer model needs further improvement by considering temperature-dependent properties such as viscoelasticity to make it a more effective tool for process analysis and optimization.

Research on rhenium–iridium alloy coating on microgroove molds in precision glass molding

There is an increasing demand for microgrooves in optical systems with a pitch at wave length level. Precision glass molding (PGM) is one of the most efficient methods for fabricating microgrooves on glass surface. Nickel–phosphorus (Ni–P) exhibits excellent cutting properties and is developed as the mold material for glass microgrooves. However, the phosphorus in Ni–P mold tends to diffuse into the glass during the PGM process at high temperatures, which may reduce the optical performance of the molded glass microgroove component. In addition, the atomic diffusion increases the interface friction coefficient between the glass and the mold, resulting in the decrease of molding accuracy of the microgroove array. To solve these problems, a rhenium–iridium (Re–Ir) alloy coating is deposited on the surface of the Ni–P microgroove mold by ion sputtering. In this paper, the surface roughness of the mold before and after coating with Re–Ir alloy coating is investigated. The mechanical properties of Ni–P and the Re–Ir alloy coating are obtained using a combination of finite element method (FEM) and experimental tests. The mold deformation after heating to the molding temperature is analyzed. The results show that the mechanical properties of the Ni–P mold surface can be strengthened after being plated with Re–Ir alloy coating, the Re–Ir coating isolates the diffusion of phosphorus from the Ni–P mold, and improve the forming accuracy of glass microgrooves.

Elastic‐viscoplasticity modeling of the thermo‐mechanical behavior of chalcogenide glass for aspheric lens molding

AbstractChalcogenide glass (ChG), as an alternative material in place of single‐crystal germanium, is increasingly used in thermal imaging, night vision, and infrared guidance systems, etc., owing to their excellent formability through precision glass molding (PGM). The deformation mechanisms of these glasses at the molding temperature involve elasticity, plasticity, and viscous flow, which call for a new theoretical model to assist the design of PGM process. This paper investigates the thermo‐mechanical properties of Ge22Se58As20 at the temperature above its softening point and establishes a new elastic‐viscoplasticity model to describe its thermo‐mechanical behaviors. After determining the model parameters through cylindrical compression tests, the new constitutive model is implemented in finite element method (FEM) of PGM to form an aspheric ChG lens. And the agreement of displacement‐time curves between experimental and simulation results exhibit the validity of the proposed elastic‐viscoplastic constitutive model.

Birefringence measurement for validation of simulation of precision glass molding process

AbstractDuring fabrication of glass lens by precision glass molding (PGM), residual stresses are setup, which adversely affect the optical performance of lens. Residual stresses can be obtained by measuring the residual birefringence. Numerical simulation is used in the industry to optimize the manufacturing process. Material properties of glass, contact conductance and friction coefficient at the glass‐mold interface are important parameters needed for simulations. In literature, these values are usually assumed without enough experimental justifications. Here, the viscoelastic thermo‐rheological simple (TRS) behavior of glass is experimentally characterized by the four‐point bending test. Contact conductance and friction coefficient at P‐SK57™ glass and Pt‐Ir coated WC mold interface are experimentally measured. A plano‐convex lens of P‐SK57™ glass is fabricated by PGM for two different cooling rates and whole field birefringence of the finished lens is measured by digital photoelasticity. The fabrication process is simulated using finite element method. The simulation is validated, for different stages of PGM process, by comparing the load acting on the mold and displacement of the molds. At the end of the process, the birefringence distribution is compared with the experimental data. A novel plotting scheme is developed for computing birefringence from FE simulation for any shape of lens.

Development of thermally stable and moldable chalcogenide glass for flexible infrared lenses

Abstract

Fabrication of a double-sided micro-lens array by a glass molding technique

In recent years, micro-lens arrays (MLAs) have become important elements of optical systems. One function of MLAs is to create a uniform intensity of light. Compared with one-sided MLAs, the uniformity of light intensity increases with double-sided MLAs. MLAs fabricated by glass can be used in higher temperature environments or in high-energy systems. Glass-based MLAs can be fabricated by laser machining, photolithography, precision diamond grinding process and precision glass molding (PGM) technologies, but laser machining, photolithography and precision diamond grinding process technologies are not the perfect approach for mass production. Therefore, this paper proposes a method to fabricate a mold by laser micro-machining and a double-sided MLA by a PGM process. First, a micro-hole array was fabricated on the surface of a silicon carbide mold. A double-sided MLA using two molds was then formed by a PGM process. In this paper, the PGM process parameters including molding temperature and molding force are discussed. Moreover, the profile of a double-sided MLA is discussed. Finally, a double-sided MLA with a diameter of 20 mm, and lenses with a height of 52 µm, a radius of 851 µm and a pitch of 700 µm were formed on glass.

Apparatus for high temperature friction measurement

Abstract The static and dynamic coefficient of friction between two flat surfaces at elevated temperature is under investigation. The equipment used in this study was originally designed for the precision glass molding (PGM) process and was then modified for friction measurement. The ultimate aim of this research is to study and characterize the frictional behavior between glass and a mold surface at elevated temperatures typical of the PGM process and under conditions similar to those for this process. This paper describes the design of the apparatus, and validation experiments using a steel–steel material pair at room temperature for comparison with values reported in the literature. The average friction coefficient between the steel–steel pair was found to be 0.17. Subsequent experiments using steel against steel and BK-7 glass against steel, found the stick-slip phenomena is happening at 577 °C.

The glass-molding process for planar-integrated micro-optical component

This paper presents a precision glass-molding process to fabricate planar-integrated micro-optical components (PIMOC) applied to a micro projection display system, optical interconnection system and optical storage system. The PIMOC was designed based on wave propagation theory, and fabricated using a glass molding process. Experimental results are discussed in this article. The experiment showed that the PIMOC fabricated in a vacuum environment meets the design value with precision glass-molding technology (PGMT) at a molding force of 2.75 kN, a molding temperature of 750 °C and a cooling rate of 49 °C/min.