Three-Dimensional Modeling of Glass Lens Molding

The required accuracy for the final dimensions of the molded lenses in wafer-based precision glass molding as well as the need for elimination of costly experimental trial and error calls for numerical simulations. This study deals with 3D thermo-mechanical modeling of the wafer-based precision glass lens molding process. First, a comprehensive 3D thermo-mechanical model of glass is implemented into a FORTRAN user subroutine (UMAT) in the FE program ABAQUS, and the developed FE model is validated with both a well-known sandwich seal test and experimental results of precision molding of several glass rings. Afterward, 3D thermo-mechanical modeling of the wafer-based glass lens manufacturing is performed to suggest a proper molding program (i.e., the proper set of process parameters including preset force-time and temperature-time histories) for molding a wafer to a desired dimension and quality. Moreover, the effect of some important process parameters such as cooling rate and pressing temperature on the final size and residual stress inside the wafer is evaluated. Finally, it is noted that the suggested molding program minimizes the costly empirical efforts and raises the process efficiency.     

Simulation Strategy of the Final Dimension in the Glass Pressing Process

The simulation of the final dimension in the glass pressing process is presented in this paper, in which the part contraction and mold deformation are considered as important factors. A thermorheologically simple thermoviscoelastic material model is used to calculate the residual stress. The model for the analysis of contraction is proposed based on the theory of shells, as an assembly of flat elements. The numerical model for the mold deformation is based on a three-dimensional thermoelastic boundary element formulation. Finally, an application example of the picture tube panel is used to verify the presented models and simulation methods.     

Birefringence measurement for validation of simulation of precision glass molding process

During 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.     

High-precision molding simulation prediction of glass lens profile for a new lanthanide optical glass

Precision glass lens molding (PGLM) is a recently developed method for fabricating glass optical components with high precision in large volumes. Lanthanum optical glasses are extensively used as optical materials owing to their superior optical properties, such as high refractive index, low dispersion, and high transparency. However, the transformation temperature of currently available high refractive index glass is generally above 650 °C and poses a challenge in manufacturing ultra-hard molds, durable coatings, and high-temperature molding equipment using PGLM. In this study, a preparation method for obtaining high refractive index, low -melting -point lanthanide optical glass (B-ZLaT198) used in PGLM was developed to reduce the transformation temperature. The developed method also characterizes the glass refractive indices and thermal-mechanical properties. To achieve the high-precision prediction of a molding shape in a simulation, a viscoelastic constitutive model of glass was established based on a micro-deformation uniaxial compression creep test. Moreover, by solving the Tool-Narayanasway-Moynihan model parameters based on the specific heat capacity fitting of optical glass at different heating and cooling rates, the input parameters of the structural relaxation model (SRM) for simulation prediction of aspheric glass lens profile deviation in the annealing stage were obtained. Finally, the profile deviation of the aspheric lens was predicted using a finite element model simulation. The results showed that the simulation’s predicted profile of an aspheric lens using the SRM model was in good agreement with that of experimental molding profile. In addition, using the SRM provided a higher prediction accuracy than that of the thermal expansion model in the annealing stage. Adopting the SRM was necessary for the annealing simulations of molding pressing and also verified the accuracy of the proposed viscoelastic characterization method for calculating the thermomechanical parameters of optical glasses.     

Simulation of the residual stresses in the contour laser welding of thermoplastics

This article presents the influence of the process parameters in laser transmission welding for plastics on the residual stress in the welded part. The contour welding process is modeled by means of finite element (FE) simulation. In this process, the weld seam is only partially heated, i.e., only part of it melts. The calculations are performed using a material model that describes the time‐dependent temperature and stress development in a plate geometry, making allowance for the material's asymmetric compressive‐tensile behavior. Experimental data were measured under different load cases to present the time‐dependent material behavior, and then implemented in numerical terms by formulating the necessary constitutive equations. The calculations to simulate the influence of process parameters on the residual stress behavior were performed using a finite element model that was developed. The simulation covers the entire welding process, including the heating and cooling stages. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

精密注塑成型PVT最佳保压控制

阐述了PVT最佳保压控制的概念、基本原理及方法,并以精密塑料透镜注射成型为例,通过PVT状态图讨论注射成型工艺过程并制定工艺路线,选择注射-压缩工艺成型精密光学透镜,通过该工艺有效地解决了注塑中的应力问题,在制品的变形、尺寸精度及光学特性等方面大幅提高产品质量.     

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.     

Study on shape deviation and crack of ultra-thin glass molding process for curved surface

The demands towards high precision and surface quality of ultra-thin glass for curved screens are continuously rising in the field of smart mobile terminals. Although the ultra-thin glass molding process (UTGMP) has the advantage of the shorter production cycle and higher efficiency, there are still typical forming defects in the molding process, namely crack, shape deviation, and large surface roughness. This paper aimed to investigate the influence mechanism of UTGMP molding temperature and pressure on the shape deviation, crack area, and surface quality of ultra-thin glass. In this study, a finite element model (FEM) was established to study typical forming defects of curved surfaces, and the effects of molding temperature and pressure on the shape deviation and crack area for ultra-thin glass were studied by the FEM simulation method. The simulation results revealed the molding temperature has a significant effect on the shape deviation, crack area and surface quality, while the molding pressure is only strongly correlated with shape deviation and crack area. In addition, the reliability of the model was verified by a series of five-level single factor experiments, and the shape deviation and crack area of ultra-thin glass were discussed in detail. Under the appropriate molding pressure and temperature range (0.45 MPa, 802–806 °C), the accuracy of curvature was improved by 33%, the roughness was reduced by 21%, and the probability of crack was also reduced. Thus, this study contributes to improving UTGMP's molding accuracy and reducing molding defects, and plays a positive role in reducing production costs and improving production efficiency.     

A non‐isothermal finite element model for injection stretch‐blow molding of PET bottles with parametric studies

A non‐isothermal finite element (FE) model for the injection stretch‐blow molding (ISBM) process of polyethylene terephthalate (PET) bottles is presented in this paper. The constitutive behavior of PET is modeled by the physically based Buckley glass‐rubber model in form of UMAT in ABAQUS. The heat transfer between the stretch rod, the preform, and the mold is modeled. Particular attention is paid to thermal and contact modeling, material model, and selection of proper element types. Extensive FE simulations are carried out to model ISBM of a 20 g‐330 ml bottle made in plant tests. Comparisons of numerical results with the measurements demonstrate that the model can satisfactorily predict the bottle thickness and material distributions. Significant nonlinear differentials are found in strain, temperature, and temperature reduction rate in both bottle thickness and length direction during the process. A volume approach is therefore necessary for accurate predictions of final bottle properties because they are governed by orientation and crystallinity, which are highly temperature and strain dependent. Parametric studies on contact modeling and heat transfer coefficient are also conducted and the results are discussed. Polym. Eng. Sci. 44:1379–1390, 2004. © 2004 Society of Plastics Engineers.     

变曲率ICM聚碳酸酯制品残余应力分布

结合注射压缩成型（injection compression molding,ICM）工艺特点,运用平面偏振和数值仿真方法,对变曲率聚碳酸酯ICM制品残余应力的分布进行分析,研究不同压缩工艺下残余应力分布特点以及随曲率变化的规律。结果发现：除了浇口和末端小部分区域外,光弹应力条纹环绕制品形状分布;顺序式ICM残余应力条纹有较规整的对称分布结构,同步式ICM充填末端区域残余应力在厚度方向与传统注塑成型区别较为明显,呈现“压-拉”两层应力分布状态;同一平面内,变曲率ICM制品厚度方向残余应力随曲率的减小而递减。除平板制品外,其余四类不同曲率制品的平均残余应力与对应曲率均呈反比例变化关系。研究对优化变曲率透明聚合物制品的设计有一定指导意义。     

Reducing residual stresses in molded parts

Means of reducing the flow-induced residual stresses in injection molded parts through optimization of the thermal history of the process are presented. An approach through the use of a passive insulation layer with low thermal inertia on the cavity surface was investigated. The passive insulation layer prevents the polymer melt from freezing during mold filling and allows the flow-induced stresses to relax after the filling. The criteria for the optimal thermal properties and the required thickness of the layer are presented. A numerical simulation model of non-isothermal filling and cooling of viscoelastic materials was also used to understand the molding process and to evaluate this approach. This model predicts the stress development and relaxation in the molding cycle. Both simulation and experimental results show that the final stresses in the molded parts can be reduced significantly with the use of an insulation layer. This technique can also be applied to other molding or forming processes in order to decouple the material flow and cooling process for minimum residual stresses in the molded parts.     

炸药粉末压制工艺参数对药柱质量的影响

为了提高炸药粉末的压制成型质量,采用将炸药粉末视为连续体的建模方法,利用Shima-Oyane材料模型,以Φ26mm×22mm的JO-9159炸药药柱为例,采用高级非线性Msc.Marc建立了粉末压制过程仿真模型,分析了不同位置粉末位移及相对密度变化规律,研究了压制速率、初始密度对炸药粉末成型后相对密度及回弹量的影响。结果表明,Shima-Oyane材料模型可以较好地模拟粉末压制成型过程;炸药粉末流动的方向主要为轴向流动,与模具接触区域流动相对缓慢;压制速率以及初始密度影响炸药粉末成型后的质量,初始相对密度的提高有助于提高炸药粉末成型后的质量;压制速率在230~250mm/s时,粉末成型后相对密度较为均匀、回弹量较小,即粉末成型质量较好。     

Compression Molding of Aspherical Glass Lenses–A Combined Experimental and Numerical Analysis

Compression molding of glass aspherical lenses has become a viable manufacturing process for precision optics. The widespread use of this process has been hampered by the lack of its fundamental understanding. This research is a part of the ongoing effort to understand some of the issues related to the process. Simple lens molding experiments were performed on a commercial precision lens molding machine. A finite element method (FEM) program was used to create a simple numerical model and analyze the molding process. Experimental results show that this process is capable of producing precision optical components. A comparison of the experimental results with the predicted results indicates that with a more sophisticated numerical model, it is possible to use FEM as a tool for process analysis.     

Simulation of PTFE sintering: Thermal stresses and deformation behavior

A finite element model has been used to study the sintering process of polytetrafluoroethylene (PTFE) cylinders in order to predict residual thermal stresses; both solid (rods) and hollow (billets) blocks were studied. The simulation of the process was performed considering three separate stages: thermal, deformation, and stress analysis. For each stage, relevant material properties were determined experimentally. In particular, the deformation behavior of PTFE was thoroughly investigated by means of thermo‐mechanical analysis (TMA). It is shown that experimental results can be explained considering deformation recovery and orientation effects. Predictions of the model are compared with experimental measurements performed on real PTFE‐sintered cylinders. Temperature and deformation distributions determined with the model agree well with experimental data. Fair agreement between predicted and experimentally measured residual stresses is obtained, and the influence of cylinder size and applied cooling rate on residual stresses is correctly predicted. Polym. Eng. Sci. 44:1368–1378, 2004. © 2004 Society of Plastics Engineers.     

汽车A柱下饰板注射成型工艺仿真分析

采用正交试验和Moldflow数值模拟相结合的方法,对汽车A柱下饰板的注射成型过程进行了分析,研究了模具温度、熔体温度、注射时间和保压压力等工艺参数对残余应力和翘曲变形的影响。通过极差分析得到,熔体温度对翘曲变形影响最大,保压压力对残余应力影响最大,最佳工艺参数组合为模具温度40 ℃,熔体温度205 ℃,注射时间5 s,保压压力45 MPa;通过仿真分析与实际成型方案进行比较,汽车A柱下饰板的翘曲变形由3.847 mm降为3.121 mm,残余应力由66.95 MPa降为65.21 MPa。     

Integrative Approaches for Mechanical Mold Design in Injection Molding

The injection mold faces a number of different loads during the injection molding process for plastic parts. The effect on the mechanical behavior of the mold, inserts, and adjacent processes can be complex and may cause bad final parts. By using an integrative simulation approach it is possible to take the process influence into account when calculating the solid body behavior of the mold in a structural simulation. A newly developed approach at IKV uses the advantages of the integrative approach and extends it by an automatic back coupling of deformation results during the filling simulation. This way the interaction of the melt flow and the deformation of inserts or mold components can be considered during the filling phase.     

Experimental investigation of contact heat transfer coefficients in nonisothermal glass molding by infrared thermography

Nonisothermal glass molding has recently become a promising technology solution for the cost-efficient production of complex precision glass optical components. During the molding process, the glass temperature and its temperature distribution have crucial effects on the accuracy of molded optics. In nonisothermal molding, the glass temperature is greatly influenced by thermal contact conductance because there is a large temperature difference between the glass and mold parts. Though widely agreed to be varied during the molding process, the contact conductance was usually assumed as constant coefficients in most early works without sufficient experimental justifications. This paper presents an experiment approach to determine the thermal contact coefficient derived from transient temperature measurements by using infrared thermographic camera. The transient method demonstrates a beneficially short processing time and the adequate measurement at desirable molding temperature without glass sticking. Particularly, this method promises the avoidance of the overestimated contact coefficients derived from steady-state approach due to the viscoelastic deformation of glass during the inevitably long period of holding force. Based on this method, the dependency of thermal contact conductance on mold surface roughness, contact pressure, and interfacial temperature ranging from slightly below-to-above glass transition temperature was investigated. The results reveal the dominance of interfacial temperature on the contact conductance while the linear pressure-dependent conductance with an identical slope observed for all roughness and mold temperatures. The accurate determination of the contact heat transfer coefficients will eventually improve the predictions of the form accuracy, the optical properties, and possible defects such as chill ripples or glass breakage of molded lenses by the nonisothermal glass molding process.     

Optimization of compression molding of stand‐alone microlenses: Simulation and experimental results

Hot compression molding is a promising method to fabricate polymer stand‐alone microlenses. A reliable theoretical as well as statistical analysis is required for the optimization of the process to minimize the residual stresses and to predict the amount of springback to achieve a better replication of the mold profile. This article in this context focuses on the finite element simulation (FES), optimization as well as experimental validation of hot compression molding of polymer stand‐alone microlenses. Three steps such as molding, cooling, and demolding, under different molding parameters, were analyzed using ABAQUS/standard solver and the results were compared with experimental results. Compression test and compression relaxation test have been conducted at different temperatures and strain rates to characterize the rheological behavior of material. Two material models, linear viscoelastic and hyperelastic–viscoelastic models, were developed and used for compression test simulations. Hyperelastic–viscoelastic model is found to predict the material behavior in low strain rates better and, thus, is used for the simulation of actual lens compression molding. Good agreement is found between the FES‐predicted curve and the lens profile molded at different molding temperatures. POLYM. ENG. SCI., 2010. © 2010 Society of Plastics Engineers     

Numerical Modeling of Cooling Stage of Glass Molding Process Assisted by CFD and Measurement of Residual Birefringence

Process parameters of Precision Glass Molding (PGM) are often sought by Finite Element (FE) simulation. Mechanical as well as thermal boundary conditions (BCs) are necessary for FE simulation in which mechanical BCs are usually known or easily determinable. However, most of the thermal BCs are generally assumed in the FE simulation as they cannot be measured directly. The focus of this article is to propose a novel method for evaluating the thermal BC of glass–N₂ gas. FE simulations as well as thermal cycling experiments are carried out for a glass disk specimen for three different cooling rates. CFD analysis of N₂ flow in the PGM machine is performed to understand the heat extraction mechanism. Based on this, adhoc values for equivalent heat transfer coefficient (h_eqv) are obtained by lumped system analysis. A novel methodology is then proposed for obtaining accurate h_eqv values by measurement of integrated residual birefringence in glass using digital photoelasticity. FE simulation is repeated for different values of h_eqv until the integrated birefringence based on simulation matches with that of the experiment. For the same cooling rates, two aspherical glass lenses are molded and their residual birefringence is measured and compared with the glass disk specimen.     

Assessing the effect of processing variables on the mechanical response of polysytrene molded using vibration‐assisted injection molding process

The present work is focused on the study of vibration‐assisted injection molding (VAIM) process, using polystyrene as a model polymeric system. This recently developed polymer processing operation is based on the concept of using motion of the injection screw to apply mechanical vibration to polymer melt during the injection and packing stages of injection molding process, to control the polymer behavior at a molecular level, which would result in improvements/alterations to the mechanical behavior of molded products. In this study, the afore‐mentioned concept was verified experimentally from monotonic tensile experiments and birefringence measurements of VAIM molded polystyrene in comparison with those of conventional injection molding process. The results of our study indicate that the actual degree of strength improvement depends on at least four parameters, namely, vibration frequency, vibration amplitude, vibration duration, and the delay time between the injection start and the vibration start. Furthermore, when these parameters were optimized, as much as a 28% strength improvement was observed, accompanied by an increase in toughness. Furthermore, birefringence measurements revealed that VAIM processing significantly altered the residual stress distribution throughout final products, but it did not, however, change the material density in the products. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci, 2006