Mold-glass interface adhesion mechanisms in precision 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.

Precision glass moulding (PGM) enables the production of an aspherical lens and irregular optical products in a single step, but its product quality depends highly on the control of both material properties and process parameters. This paper investigates the thermoforming mechanism of a glass lens in PGM. To precisely describe the material behavior in PGM, a modulus-based constitutive model was framed and integrated with the finite element analysis. This model can be parameterized conveniently by an impulse excitation technique. Key processing parameters that influence the final profile and residual stresses of a lens were identified with the aid of dimensional analysis. The study found that the cooling stage above the glass transition temperature can bring about large geometry deviations of a lens. The residual stresses in a lens depend mainly on the temperature history in the supercooled liquid region caused by the variability and heterogeneity of thermal expansion. However, the stresses can be reduced effectively by decreasing the cooling rate from moulding temperature to glass transition temperature.

Wear of mold surfaces: Interfacial adhesion in precision glass molding

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.

Precision glass molding (PGM) technology, as an effective method for mass-producing glass lenses, is relatively mature in the molding process of aspheric lenses, but the glass molding technology for freeform optical elements is still in its infancy. For freeform optical elements, processing by conventional ultra-precision methods requires multiple processes and the resulting costs are high, while processing by PGM is efficient and inexpensive. Therefore, this paper investigates the molding technology of freeform lenses, the pre-compensation model of the freeform mold core is established, and predicts the residual stresses of freeform lenses after molding by the finite element method. Three different process parameters, molding temperature, molding rate and molding force, are verified. Experimental and simulation results show that the trends of residual stresses for the three process parameters are consistent. The optimal process parameters of the molding process are determined, under which the PV value of the molding lens is around 1.5µm. The experimental results show that the PV value of the molded lens is reduced to less than 1µm after using the pre-compensated mold core, which proves the validity of the pre-compensated model.

Glass micro lens arrays (GMLAs) have several advantages such as a high transmission rate, anti-environment, and can be used for special wavelength applications. Precision glass molding (PGM) has been used to mass produce high-accuracy aspherical glass lenses. Ultra-precision diamond grinding (UPDG) is a fundamental part of the precision glass molding process. Using UPDG, grinding spherical and aspherical to sub-micrometer form and nanometer surface roughness is simple. However, asymmetrical surface MLAs are difficult to generate using the UPDG process. UPDG, together with the wheel-forming method and a strategy used to separate the entire surface generation process into several grinding loops, were studied and developed to generate high filling factor MLAs on the mold surface. The GMLA material used was K-CSK120, made by Sumita Inc., Japan. Finally, GMLAs with an approximately 100% filling factor were generated using PGM with form accuracy and surface roughness that were respectively less than 0.3 μm and 10 nm. The tolerance of each single micro lens figure was greater than 95%.

Apparatus for high temperature friction measurement

It is costly and time consuming to use machining processes, such as grinding, polishing and lapping, to produce optical glass lenses with complex features. Precision glass molding (PGM) has thus been developed to realize an efficient manufacture of such optical components in a single step. However, PGM faces various technical challenges. For example, a PGM process must be carried out within the super-cooled region of optical glass above its glass transition temperature, in which the material has an unstable non-equilibrium structure. Within a narrow window of allowable temperature variation, the glass viscosity can change from 105 to 1012 Pa$s due to the kinetic fragility of the super-cooled liquid. This makes a PGM process sensitive to its molding temperature. In addition, because of the structural relaxation in this temperature window, the atomic structure that governs the material properties is strongly dependent on time and thermal history. Such complexity often leads to residual stresses and shape distortion in a lens molded, causing unexpected changes in density and refractive index. This review will discuss some of the central issues in PGM processes and provide a method based on a manufacturing chain consideration from mold material selection, property and deformation characterization of optical glass to process optimization. The realization of such optimization is a necessary step for the Industry 4.0 of PGM.

High-technology applications which are using high precision optic components in high and medium quantities have increased during recent years. One possibility to mass-produce e.g. such lenses is the precision glass molding (PGM) process. Especially for aspheric and free-form elements the PGM process has certain advantages. Premise is to manufacture accurate press molds, which have to feature smaller figure errors as the required lenses and may be made of materials, which are difficult to machine, like silicon nitride ceramics. These work pieces have to be machined in economical and steady process chains. However, due to the complex shapes and the corresponding accuracy an error dependent polishing is required. The Magnetorheological Finishing (MRF) as a high precision computer controlled polishing (CCP) technique is used and will further be presented in this work. To achieve the postulated demands a previous study of the material removal at selected machining parameters is needed. Changing machining parameters modify the removal, which is presented through values like the peak and volume removal rate. The value changes during the controlled variation of process parameters are described and discussed. Magnetorheological Finishing (MRF) provides one of the best methods to finish PGM molds that are relatively inaccurate to high precision in an economical, steady and efficient way. This work indicates the MRF removal selection and removal interference for the correction and finishing of precise silicon nitride molds for the precision glass molding.

Ductile mode grinding is a finishing process usually being applied to generate molds in brittle materials (e.g. tungsten carbide) to be used for precision glass molding (PGM). To that aim, ultra-precision machineries (UPM) are applied controlling depth of cut not to exceed a critical value, hcu,crit (e.g. 160 nm for tungsten carbide). Recent process analyses of the ductile mode grinding process of brittle materials have demonstrated that the critical indentation depth hcu,crit, that determines the transition from brittle mode to ductile mode removal, can significantly be shifted to higher values by adjusting process parameters such as the type of coolant and its pH value: e.g. for tungsten carbide up to 1600 nm and for BK7 glass up to 350 nm depth. This paper reports on a feasibility study to extend the process window of ductile mode material removal. Applying optimized ductile process parameter sets, enabling values of the critical depth of cut larger than 1 micron, single point diamond turning (SPDT) of binderless tungsten carbide molds has been successfully tested applying UPM machineries. Experimental data will be presented demonstrating that by controlling and adjusting ductile process parameters only, it is possible to extend its process window into regimes that are today not yet machinable: binderless tungsten carbide molds for precision glass molding have been processed in a ductile removal mode by SPDT generating surface roughness levels of less than 2 nm rms. An analysis of the adjustment of the critical process parameters will be presented together with a detailed description of the First Light experiments towards SPDT of binderless tungsten carbide molds.

Ductile mode grinding is a finishing process usually being applied to generate molds in brittle materials (e.g. tungsten carbide) to be used for precision glass molding (PGM). To that aim, ultra-precision machineries (UPM) are applied controlling depth of cut not to exceed a critical value, hcu,crit (e.g. 160 nm for tungsten carbide). Recent process analyses of the ductile mode grinding process of brittle materials have demonstrated that the critical indentation depth hcu,crit, that determines the transition from brittle mode to ductile mode removal, can significantly be shifted to higher values by adjusting process parameters such as the type of coolant and its pH value: e.g. for tungsten carbide up to 1600 nm and for BK7 glass up to 350 nm depth [1] Consequently, this paper reports on two experimental feasibility studies to extend the process window of ductile mode grinding of brittle materials. Applying ductile process parameter sets featuring values of the critical depth of cut larger than 1 micron depth two processes were experimentally analyzed that are up to now not applicable in industrial production environments: a) single point diamond turning (SPDT) of BK7 glass applying UPM machineries and b) ductile grinding of tungsten carbide molds applying standard CNC grinding machines featuring lower tool positioning accuracies than UPM. Experimental data of both tests will be presented demonstrating that by controlling and adjusting ductile process parameters only, it is possible to extend its process window into regimes that are today not yet machinable.

A study of Ru–Cr protective coatings for precision glass molding

Precision glass molding (PGM) can fabricate aspherical lens and irregular optical products in a single step, but its applicability is currently limited by the thermally induced residual stresses and lens shape derivation after molding. To remove this barrier, this paper develops a numerical optimization platform for PGM based on a simplex algorithm and finite element simulation. It was found that the platform can effectively reduce the residual stress in a molded lens through cooling process optimization and minimize the lens shape derivation by die shape compensation. The platform established can improve the lens quality by PMG and make molded lenses have better quality than those manufactured by ultraprecision machining processes.

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.

Ductile mode grinding is usually applied for finishing of e.g. tungsten carbide molds used for precision glass molding (PGM) by controlling depth of cut on feed controlled machines. Bifano et all. demonstrated the possibility to apply this mechanism while machining hard and brittle materials by the use of ultra-precision machines (UPM). Based on experimental investigations a formula for the transition from brittle to ductile cutting mechanism, also known as the critical depth of cut hcu,crit, relating the material specific properties Young’s-Modulus E, material hardness H and fracture toughness KC was developed and is widely used for setting up UPM machines ever since. However, the influence of cutting conditions, like tool or process characteristics, are neglected leading to discrepancies of the value of hcu,crit between the prediction and the actual machining results of up to 200%. Furthermore, previous investigations have shown that hcu,crit strongly depends on coolant fluid characteristics as well as on the compressive stress applied into the cutting zone by the use of tools with e.g. negative rank angles. In this paper, we report on a ductile grinding process analysis applying the “three wagons method”, a recently developed method for process optimization in optics fabrication. Following that trail, critical process parameters have been identified determining the process window of feed controlled ductile grinding applied on State-of-the-Art UPM machineries. The influences of the critical process parameters on the critical depth of cut hcu,crit have been tested experimentally using an ultra-precise SPDT machine. Among others, four critical process parameters could be identified determining the transition between brittle and ductile mode grinding: the critical depth of cut depends substantially on (a) the type of coolant used, (b) the pH value of the coolant, (c) the tool tip radius of the applied diamond and (d) whether ultrasonic assistance (US) is being switched on or off. Depending on the applied set of process parameters and for the experimental data collected, maximum ductile mode material removal rates could be achieved with hcu,crit, max = 1600 nm. That way, a new formula was developed, which allows the prediction of the critical depth of cut depending on critical process parameters, a.o. tool parameters and cutting fluid characteristics, while machining binderless nanocrystalline tungsten carbide molds. This formula was set up based on fundamental ruling test results and is one step towards extending Bifanos formula taking the influences of critical process parameters into account.