Method for predicting mold stress and aspherical product structure deformation based on thermal-mechanical coupling simulation

Abstract

The invention provides a method for predicting mold stress and aspherical product structure deformation based on thermal-mechanical coupling simulation, which adopts digitization to improve aspherical product design defects. The invention discloses a method for predicting mold stress and aspherical product structure deformation based on thermal-mechanical coupling simulation. The mold stress prediction comprises mold design optimization, lens residual stress analysis and double optical path difference analysis; the aspherical product structure deformation comprises the steps of center thickness compliance, product surface profile correction and prefabricated part lens design optimization. According to the method, a finite element simulation iterative calculation optimization method is adopted, design defects such as mold stress distribution, aspheric surface shape precision and product appearance in mold pressing thermal forming are simulated, predicted, analyzed and optimized, the aspheric contour of a product can be digitally simulated, relevant assembly part design is continuously corrected according to a simulation result, and the product quality is improved. Finally, the difference value between the simulation value and the design value is within the required tolerance range.

Description

TECHNICAL FIELD

[0001] The present application relates to a design defect improvement method of aspheric products, ultra-precision molds and the like, and in particular to a method for predicting mold stress and aspheric product structural deformation based on thermal-mechanical coupling simulation. BACKGROUND

[0002] Aspheric lenses have advantages over traditional lenses in terms of eliminating spherical aberration, reducing optical system weight, improving imaging quality, and are widely used in camera lenses, laser systems, optical measuring instruments, medical equipment and other fields. Compared with traditional spherical lenses, aspheric lenses can effectively reduce the number of lenses in the optical system, thereby reducing the complexity of the system and improving the light transmittance.

[0003] Optical aspheric elements are traditionally completed using cold working processes, including roughing, roughing, fine grinding, milling and grinding, turning, and polishing. These methods have obvious defects: high processing error, difficult manufacturing, and high production cost.

[0004] To simplify the processing technology, an aspheric element hot pressing forming process route is adopted. This technology can achieve high-precision, batch production of products by heating the glass preform to the softening temperature and then pressing it in a precision mold, greatly improving processing efficiency and reducing production cost. However, existing mold pressing hot forming technology still faces many technical challenges: 1) aspheric surface profile precision control problem: due to the geometric complexity of concave-convex aspheric products, the forming precision is difficult to guarantee, and it is difficult to accurately control the micron-level surface profile; 2) mold thermal fatigue failure: under high temperature and high pressure environment, stress concentration and alternating stress type lead to micro-crack propagation, significantly shortening the service life of the mold.

[0005] Current mainstream simulation technology is mostly based on finite element analysis (FEA) to simulate material flow and stress distribution during mold pressing. Mold design is usually designed using aspheric empirical formula, and there is a lack of intuitive visualization means to monitor possible design risk points during mold pressing. The preform design efficiency is low, and it relies on traditional trial-and-error method to adjust the preform and mold size, the related design iteration cycle is long, and the initial design volume is determined according to the theoretical thermal expansion law of the material, lacking closed-loop verification feedback with surface error. Product surface design: most models do not consider the relaxation characteristics of glass materials, resulting in high-temperature forming precision far from the expected design.

[0006] In view of this, the present application aims to provide a digital solution to the aspheric product profile precision and design defect identification optimization scheme, to achieve the goal of mold stress and aspheric product structural deformation prediction and improvement. SUMMARY

[0007] In order to achieve the above-mentioned purpose, the technical problem to be solved by the present application is to provide a method for predicting mold stress and aspheric product structural deformation based on thermal-mechanical coupling simulation by adopting digitalized improved aspheric product design defects.

[0008] The technical solution adopted by the present application to solve the technical problem is:

[0009] The method for predicting mold stress and aspheric product structural deformation based on thermal-mechanical coupling simulation, wherein the mold stress prediction includes mold design optimization, lens residual stress analysis and double optical path difference analysis; the aspheric product structural deformation includes center thickness compliance, product surface profile correction and preform lens design optimization.

[0010] The method for predicting mold stress and aspheric product structural deformation based on thermal-mechanical coupling simulation, the method comprising the following steps:

[0011] Step 1: according to the product shape profile, initially construct a three-dimensional geometric model of the center module, determine the basic forming process, including time, temperature, pressure, establish a primary simulation model, calculate the cavity radiation temperature in the two-dimensional model, and export the product aspheric profile node data, stress, displacement, temperature and other related data in the simulation results. Compare the simulation calculation value with the theoretical design value, and adjust the assembly design according to the difference between the two. Modify the three-dimensional space parameters of the local parts in the assembly model, and import the cavity thermal radiation temperature data calculated for the first time to establish a second simulation model;

[0012] Step 2: analyze whether the product center thickness data is qualified. If the product center thickness is unqualified, return to step 1. According to the difference between the actual simulation calculation and the theoretical design, adjust the mold design scheme, modify the simulation model, and calculate again until the product center thickness and the theoretical design are within the required tolerance range, then enter the product appearance prediction link;

[0013] The analysis step of the product center thickness is:

[0014] 1) export the product simulation calculation surface profile node position data, fit the data set, and compare the fitted data with the theoretical design value;

[0015] 2) according to the difference between the fitted value and the design value at the center, calculate the assembly design height that needs to be modified, substitute the modified assembly design into the simulation model, calculate again until the difference between the final fitted data and the theoretical data is within the allowable error range, that is, the aspheric product with qualified center thickness is obtained.

[0016] Step 3: analyze whether the product has obvious appearance problems. If the appearance result is not ideal, return to step 1 to adjust the design parameters and optimize the preform until the product appearance is qualified, then enter the "stress analysis" and "product surface profile analysis" links;

[0017] The product appearance problem analysis step is:

[0018] 1) Monitor the entire molding process to observe whether there is a gap between the preform and the mold, overlap, product "overflow", product edge shaping fullness, and other appearance quality problems;

[0019] 2) According to the appearance quality problems in the model, modify the design scheme of the relevant components in the assembly, and calculate again in the simulation model until the product appearance is qualified.

[0020] Step 4: Analyze whether the product has obvious stress problems, including aspheric product stress analysis and mold stress analysis.

[0021] "Non-spherical product stress analysis" is Figure 1 "lens residual stress analysis" in the analysis of the "mold stress" part of the calculation results. The stress distribution of the entire assembly (including the aspheric product) can be observed, so the stress of the aspheric product can be directly analyzed. The same as the "mold stress analysis" step, it can be directly analyzed according to the "lens residual stress analysis" part in the "mold stress" part.

[0022] The aspheric product stress analysis step is:

[0023] 1) According to the simulation results, extract the residual circumferential S11 and height S33 direction stress data of the glass after forming. After data processing, the aspheric product double optical path difference calculation result is obtained;

[0024] 2) Analyze the residual stress concentration of the design scheme, and verify the difference between the simulation and the actual measurement results. If the stress result is not ideal, return to 1) to adjust the design parameters and optimize the mold or preform until the product stress is compliant;

[0025] 3) Determine whether mold stress analysis is needed according to the actual situation. If needed, proceed to the mold stress analysis link; if not needed, proceed to the mold processing and molding verification link, and finally obtain an aspheric product with high imaging quality.

[0026] The mold stress analysis step is:

[0027] 1) Through the simulation results, monitor the stress change of each component of the assembly during the molding process, identify the stress type, stress concentration area, and analyze the stress level in the stress alternating / concentrating area;

[0028] 2) judge whether the material yield strength or fatigue limit strength is exceeded, wherein the stress concentration area mainly includes the junction positions of the mold and the mold and the mold and the glass, if the stress level is unqualified, return to 1) to adjust the design parameters, optimize the mold or the preform, until the product external stress is qualified, then enter the mold processing and mold pressing verification link, and finally obtain the aspheric product with high imaging quality;

[0029] 3) according to the stress concentration area and the deformation large part, the material and the assembly design scheme are evaluated in the temperature cycle, the material service life under the working condition, the stress level and the deformation are combined, the feasibility of the design scheme is predicted, and the design scheme is modified. Step 5: analyze the aspheric product surface profile problem. Extract the product simulation forming data, compare the deviation between the aspheric product surface fitting profile node data and the theoretical design curve, give a certain coefficient correction according to the deviation of each point, adjust the assembly design scheme of the related parts, return to step 1, until the simulated aspheric profile and the theoretical design value difference is within the allowable machining error range, then enter the mold processing and mold pressing verification link, and finally obtain the aspheric product with high imaging quality. Figure 1

[0031] The product surface profile analysis step is:

[0035] 1) according to the deviation between the actual calculation simulation value and the theoretical value design aspheric profile, the deviation between the upper and lower molds and the theoretical design aspheric profile at each node is calculated;

[0036] 2) give a certain surface correction coefficient, increase, decrease, reduce and compensate the value on the basis of the initial design mold, modify the design mold aspheric shape, obtain the second simulation upper and lower mold aspheric three-dimensional modeling profile data;

[0037] 3) considering the viscoelasticity of the glass material, the second modified upper and lower mold aspheric profile is three-dimensionally constructed into a finite element simulation model again, and the surface profile result is simulated and calculated again under the same process condition. Repeat 2), until the finally calculated product aspheric profile shape is within the allowable tolerance range, so as to determine the mold surface shape, and finally obtain the aspheric element with the conforming surface profile.

[0038] The beneficial effects of the present application are:

[0039] ​1) can identify stress concentration area and non-spherical product deformation of large parts, realize intuitive visualization and accurate find design defect position, and realize accurate regulation and control of assembly design scheme, so that the non-spherical product after mold forming can realize micron level precision regulation and control of surface profile, and the precision is improved by more than 50%. The technology is suitable for large-scale industrial manufacturing field, and the simulation iterative analysis calculation method is used instead of traditional industrial processing "trial and error" experience, the design cycle is shortened by 60%, and the industrial production efficiency is improved.

[0037] 2) can simulate the interaction between structure deformation and temperature field, the structure response behavior under high temperature environment, the thermal stress / tensile stress concentration distribution, and then avoid the premature failure or excessive deformation problem caused by mold due to thermal fatigue (high temperature stress concentration, thermal stress and tensile stress concentration / alternating action leading to micro cracks), and the mold life is improved by 3-5 times.

[0038] The method of the present application adopts the method of finite element simulation iterative calculation optimization, through the simulation, prediction, analysis and optimization of design defects such as "stress distribution" of mold, "non-spherical surface shape precision" of glass preform and "product appearance" in mold hot forming, the digital simulation of product non-spherical profile can be realized, then the related assembly component design is corrected according to the simulation results, finally the difference between simulation value and design value is within the required tolerance range, and the actual hot pressing forming verification is carried out. The implementation effect of the technology is good, and the non-spherical optical product with batch production, light weight and high imaging precision can be obtained, which has significant economic effect. BRIEF DESCRIPTION OF DRAWINGS

[0039] Figure 1 is a step schematic diagram of the method of the present application.

[0040] Figure 2 is a stress comparison schematic diagram of the mold component design scheme of the assembly before and after optimization.

[0041] Figure 3 is a comparison schematic diagram of the center thickness of the concave-convex non-spherical lens before and after optimization.

[0042] Figure 4 is a simulation calculation result schematic diagram of the internal residual stress and birefringence optical path difference of the non-spherical glass after optimization by the method of the present application.

[0043] Figure 5 is a simulation non-spherical lens surface deviation schematic diagram after optimization by the method of the present application.

[0044] Figure 6 is a profile surface deviation curve schematic diagram of the mold lens relative to the target design lens after optimization by the method of the present application. DETAILED DESCRIPTION

[0045] Example 1: Mold stress prediction and structure deformation optimization of Φ15mm concave-convex aspherical lens

[0046] Step 1: Establish the initial simulation model.

[0047] 1. Simulation model establishment: Establish a thermal-mechanical coupling model using ANSYS Workbench

[0048]

[0049]

[0050] According to the shape of the Φ15mm concave-convex aspherical lens, a three-dimensional geometric model of the center module is initially constructed, and the basic molding process is determined, mainly including time, temperature, pressure, etc.

[0051] a. Initial process parameters:

[0052] Heating temperature 550-580℃ Soaking time 900-1200s Pressing pressure 0.4-0.7 Mpa

[0053] 2. Initial parameter setting

[0054] b. Material parameters:

[0055] (1) Mold material: tungsten carbide (elastic modulus 550GPa, thermal expansion coefficient 5.2×10 -6 / ℃)

[0056] (2) Glass material: D-LaK6 (transition temperature Tg = 552℃, sag temperature Ts = 560℃, annealing point T 10 13 = 513℃, thermal expansion coefficient 9.5×10 -6 K)

[0057] Calculate the cavity radiation temperature in the two-dimensional model, and export the simulation results of the product aspherical profile node data, stress, displacement, temperature and other related data. Compare the simulation calculation value with the theoretical design value, and adjust the assembly design according to the difference between the two. Modify the three-dimensional space parameters of the local parts in the assembly model, and import the cavity thermal radiation temperature data calculated for the first time to establish the second simulation model.

[0058] 3. Iterative optimization process: Center thickness correction. The center thickness of the product needs to be corrected before each simulation.

[0059] Step 2: Analyze whether the product center thickness data is qualified. If the product center thickness is not qualified, return to step 1. According to the difference between the actual simulation calculation and the theoretical design, adjust the mold design scheme, modify the simulation model, and calculate again until the product center thickness and the theoretical design are within the required tolerance range, then enter the product appearance prediction link.

[0060] The analysis step of the product center thickness is:

[0061] The product simulation calculation surface profile node position data is exported, the data set is fitted, and the fitted data is compared with the theoretical design value; according to the difference between the fitted value and the design value at the center, the assembly design height that needs to be modified is calculated, the modified assembly design is substituted into the simulation model, and the calculation is performed again until the difference between the final fitted data and the theoretical data is within the allowable error range, that is, the non-spherical product with the center thickness is obtained.

[0062] Design product specifications:

[0063] Center thickness 2.8±0.010 mm Preform parameter specification Φ8.6±0.010 mm

[0064] Actual simulation results:

[0065]

[0066]

[0067] Step 3: Analyze whether the product has obvious appearance problems. If the appearance result is not ideal, return to step 1 to adjust the design parameters and optimize the preform until the product appearance is qualified, then enter the "stress analysis" and "product surface profile analysis" links.

[0068] The analysis step of the product appearance problem is:

[0069] Monitor the entire molding process, observe whether there is a gap between the preform and the mold, overlap, whether the product will produce "overflow", product edge forming fullness, and other appearance quality problems; according to the appearance quality problems in the model, modify the design scheme of the related components in the assembly, and substitute it into the simulation model for calculation again until the product appearance is qualified.

[0070] Problem detection:

[0071] The edge thickness deviation is 0.015mm.

[0072] Optimization measures:

[0073]

[0074] Step 4: Analyze whether the product has obvious stress problems, including non-spherical product stress analysis and mold stress analysis.

[0075] The step of non-spherical product stress analysis is:

[0076] 1) According to the simulation results, extract the stress data of the residual circumferential S11 and height S33 directions of the formed glass, and after data processing, obtain the double optical path difference calculation results of the non-spherical product.

[0077] 2) Analysis of the design scheme residual stress concentration, and verify the simulation and the difference between the measured results, if the stress results are not ideal, then return to step 1) to adjust the design parameters, optimize the mold or preform, until the product stress compliance.

[0078] 3) According to the actual situation to determine whether to need to carry out mold stress analysis, if necessary, then like mold stress analysis link; If not, then into the mold processing, mold verification link, ultimately get a higher imaging quality of aspherical products.

[0079] Problem detection:

[0080] Glass maximum residual stress 9.5 MPa (located in R2 transition zone) Mold maximum equivalent stress 980 MPa (convex mold shoulder)

[0081] The simulation results show that the stress change of each component of the assembly in the mold pressing process is mainly tensile stress and compressive stress at the chamfer position, and the stress is more concentrated at this position. Due to the alternating action of stress, micro cracks are easily produced in the stress concentration area, and mold failure is caused.

[0082] According to the stress concentration area and the deformation of the large part, the failure risk of the material and assembly design scheme under the working condition of temperature cycle and material service life is evaluated. Combined with the stress level and deformation, the feasibility of the design scheme is predicted and the design scheme is modified. Determine whether it exceeds the yield strength or fatigue limit strength of tungsten carbide mold material.

[0083] Tungsten carbide mold material performance index Typical value range Yield strength 1800-2800 MPa Fatigue limit strength 800-1200 MPa

[0084] The results show that the fatigue limit strength of the convex mold shoulder exceeds that of the tungsten carbide mold material, and the stress level is unqualified. Then go back to step 1 to adjust the design parameters and optimize the mold or preform.

[0085] Optimization scheme:

[0086] Modification scheme Before design After design Increase mold chamfer 0.05-0.15 mm 0.20-0.40 mm Increase transition fillet R 1.96-2.04 R 2.98-3.02

[0087] Optimization of mold design, the calculation results after simulation iteration:

[0088] Design scheme / contrast item Time Miss total stress maximum value Upper chamfer stress Lower chamfer stress Design scheme 1 t=113 s 624 MPa 8.5 MPa (tensile stress) 162 MPa (tensile stress) Design scheme 2 t=113 s 603 MPa 8.2 MPa (tensile stress) 156 MPa (tensile stress)

[0089] After modifying the mold assembly scheme, the tensile stress value at the chamfer position is reduced, which helps to alleviate the stress concentration phenomenon at the chamfer position due to the alternating action of tensile stress and compressive stress, thereby reducing the generation of micro cracks at the chamfer position of the mold, and avoiding mold failure. As shown in Figure 2 , wherein, Figure 2 The left side is the stress schematic diagram before optimization, Figure 2 The right side is the stress schematic diagram after optimization.

[0090] Step 5: Analyzing the aspherical product surface profile problem. Extract the product simulation data after forming, compare the deviation between the aspherical product surface fitting node data and the theoretical design curve, and correct it according to the deviation of each point. According to this, adjust the assembly design scheme of related parts, return to step 1, until the simulated aspherical profile and the theoretical design value difference is within the allowable machining error range, then enter the mold processing and mold verification link, and finally get the aspherical product with high imaging quality.

[0091] The analysis steps of the product surface profile are:

[0092] 1) According to the deviation between the actual calculation simulation value and the theoretical value design aspherical profile, the deviation between the upper and lower molds and the theoretical design aspherical profile at each node is calculated respectively;

[0093] 2) Give a certain surface correction coefficient, increase or decrease the compensation value on the basis of the initial design mold, and modify the design mold aspherical shape to get the second simulation upper and lower mold aspherical three-dimensional modeling profile data;

[0094] Optimization scheme:

[0095]

[0096]

[0097] 3) Considering the viscoelastic properties of glass material, the second corrected upper and lower mold aspherical profile is three-dimensionally constructed into a finite element simulation model again, and the surface profile result is simulated again under the same process conditions. Repeat step 2), until the final calculated product aspherical profile shape is within the allowable tolerance range, so as to determine the mold surface shape, and finally get the aspherical element with regular surface profile.

[0098] 4. Actual verification result

[0099] Aspheric product technical index Measured parameter Center thickness 10.002 mm Surface accuracy PV 0.25 μm, RMS 0.04 μm Double optical path difference <3 nm / cm

[0100]

[0101] From example 1:

[0102] 1) The actual product center thickness: the center thickness technical deviation is controlled within 18um in the first simulation calculation, and after iterative calculation, the center thickness deviation value can be controlled within 3um. After the center thickness optimization, the edge of the product is more full. As shown in Figure 3 .

[0103] 2) Appearance: after iterative optimization of the preform diameter in simulation, the "overflow" problem is solved, and the mold edge gap can be controlled within 20um.

[0104] 3) Aspheric product stress: the maximum residual stress of the glass is detected at the R2 transition zone, and the maximum equivalent stress of the mold is at the "convex shoulder", as shown in the following figure. Figure 4 The stress overlarge area is accurately and intuitively monitored, the assembly design is modified in a targeted manner, and the visualized modification is more targeted. After simulation optimization, the maximum residual stress value of the glass is reduced, and the possibility of "cracking" of the product is reduced.

[0105] 4) Aspheric product surface profile accuracy: the calculated surface profile accuracy is closer to the design value, and the accuracy is higher, as shown in the following figure. Figure 5-6 The left figure of the following figure is a schematic diagram of the upper surface profile deviation result, Figure 5 The right figure of the following figure is a schematic diagram of the lower surface profile deviation result. Figure 5 The longitudinal coordinate is the upper surface profile deviation, the lower surface profile deviation, and the like from bottom to top. Figure 5 Figure 6 Example 2: Mold stress control and forming optimization of Φ10mm micro-convex aspheric lens

[0106] Step 1: Establish a primary simulation model.

[0107] Step 1: Establish a primary simulation model.

[0108] 1. Simulation model establishment: a thermal-mechanical coupling model is established by using ANSYS Workbench:

[0109] Contact pair setting Face-face contact Friction coefficient 0.25 Boundary condition Fixed constraint of lower edge and central axis of heating plate

[0110] According to the shape of the Φ10mm concave-convex aspheric lens, a three-dimensional geometric model of the center module is initially constructed, and the basic forming process is determined, mainly including time, temperature, pressure, etc.

[0111] a. Initial process parameters:

[0112] Heating temperature 550-575℃ Soaking time 700-900s Pressing pressure 1.1-1.4 Mpa (two-stage type)

[0113] 2. Initial parameter setting

[0114] b. Material parameters:

[0115] (1) Mold material: tungsten carbide (elastic modulus 550GPa, thermal expansion coefficient 5.2×10 -6 / ℃)

[0116] (2) Glass material: D-LaK6 (transition temperature Tg = 552℃, sag temperature Ts = 560℃, annealing point T 10 13 = 513℃, thermal expansion coefficient 9.5×10 -6 K)​

[0117] The radiation temperature of the shape cavity in the two-dimensional model is calculated, and the node data of the product aspheric surface profile, stress, displacement, temperature and other related data in the simulation results are derived. The simulation calculation value and the theoretical design value are compared, and the assembly design is adjusted according to the difference between the two. The three-dimensional space parameters of the local parts in the assembly model are modified, and the shape cavity thermal radiation temperature data calculated in the first time are imported to establish the second simulation model.

[0118] 3. Iterative optimization process: center thickness correction

[0119] Step 2: Analyze whether the product center thickness data is qualified. If the product center thickness is unqualified, return to step 1. According to the difference between the actual simulation calculation and the theoretical design, adjust the mold design scheme, modify the simulation model, and iterate the calculation again until the product center thickness and the theoretical design are within the required tolerance range, then enter the product appearance prediction link.

[0120] The analysis steps of the product center thickness are as follows:

[0121] The product simulation calculation surface profile node position data is exported, the data set is fitted, and the fitted data is compared with the theoretical design value; according to the difference between the fitted value and the design value at the center, the assembly design height to be modified is calculated, the modified assembly design is substituted into the simulation model, and the calculation is performed again until the difference between the final fitted data and the theoretical data is within the allowable error range, that is, the non-spherical product with qualified center thickness is obtained.

[0122] Problem detection:

[0123] The first simulation center thickness deviation is ±0.022mm (design value 1.5±0.005mm).

[0124] Iterative optimization:

[0125] Iteration number Center thickness Deviation 1 1.522 ±0.022 mm 3 1.503 ±0.003 mm

[0126] Step 3: Analyze whether the product has obvious appearance problems. If the appearance result is not ideal, return to step 1 to adjust the design parameters and optimize the preform until the product appearance is qualified, then enter the "stress analysis" and "product surface profile analysis" links.

[0127] The analysis steps of the product appearance problem are as follows:

[0128] Monitor the entire molding process, observe whether there is a gap between the preform and the mold, overlap, whether the product will produce "overflow", product edge forming fullness and other appearance quality problems; according to the appearance quality problems in the model, modify the design scheme of the related parts in the assembly, and calculate again in the simulation model until the product appearance is qualified.

[0129] Problem detection:

[0130] Edge thickness is not uniform (maximum deviation 0.012 mm).

[0131] Micron-level flash appears (width about 20 μm)

[0132] Optimization measures:

[0133] Parameter Before optimization After optimization Mold gap 0.020 mm 0.015 mm Edge chamfer R0.05-0.015 mm R0.05-0.08 mm Pressing end position Down 1.0 mm Down 0.95 mm

[0134] Step 4: Analyze whether the product has obvious stress problems, including aspheric product stress analysis and mold stress analysis.

[0135] The stress analysis steps of aspheric products are:

[0136] 1) According to the simulation results, extract the stress data of the glass inside after forming in the residual circumferential S11 and height S33 directions, and after data processing, obtain the double optical path difference calculation results of aspheric products;

[0137] 2) Analyze the residual stress concentration of the design scheme, and verify the difference between simulation and actual measurement results. If the stress result is not ideal, return to step 1) to adjust the design parameters, optimize the mold or preform, until the product stress is compliant;

[0138] 3) According to the actual situation, judge whether mold stress analysis is needed, if needed, enter the mold stress analysis link; if not needed, enter the mold processing and mold pressing verification link, and finally obtain aspheric products with high imaging quality.

[0139] Problem detection:

[0140] Glass maximum residual stress 11.4 MPa (located at micro convex edge) Mold maximum equivalent stress 1250 MPa (micro convex edge)

[0141] The simulation results show that the stress distribution of each component of the assembly body during the mold pressing process is mainly concentrated at the edge of the micro convex aspheric product.

[0142] According to the stress concentration area and the deformation larger part, evaluate the failure risk of the assembly design scheme, mold pressing process, and material service life under this working condition, combined with the stress level and deformation, predict the feasibility of the design scheme and modify the design scheme accordingly. Judge whether it exceeds the yield strength or fatigue limit strength of tungsten carbide mold material.

[0143] Tungsten carbide mold material performance index Typical value range Yield strength 1800-2800 MPa Fatigue limit strength 800-1200 MPa

[0144] The results show that the stress level at the micro convex edge exceeds the fatigue limit strength of the tungsten carbide mold material, which is unqualified, then return to step 1 to adjust the design parameters and optimize the mold or preform.

[0145] In addition, the higher the temperature, the faster the stress relaxation speed, and the temperature gradient in the high-temperature section is more likely to cause permanent stress. By slowly reducing the temperature and reducing the temperature gradient, the permanent stress can be reduced, and accordingly the optimization idea is obtained.

[0146] Optimization scheme:

[0147]

[0148] After optimizing the mold design, the stress at the micro-protruding edge of the product is reduced to 860 MPa.

[0149] Step 5: Analyze the aspheric product surface profile problem. Extract the product simulation data after forming, compare the deviation between the aspheric product surface profile node data after fitting and the theoretical design curve, and give a certain correction coefficient according to the deviation of each point. According to this, adjust the assembly design scheme of the related parts, return to step 1, until the simulated aspheric profile and the theoretical design value are within the allowable machining error range, then enter the mold processing and mold verification link, and finally get the aspheric product with high imaging quality.

[0150] The analysis steps of the product surface profile are:

[0151] 1) According to the deviation between the actual calculation simulation value and the theoretical value design aspheric profile, the deviation between the upper and lower molds and the theoretical design aspheric profile at each node is calculated respectively;

[0152] 2) Give a certain surface correction coefficient, increase or decrease the compensation value on the basis of the initial design mold, and modify the design mold aspheric shape to obtain the second simulation upper and lower mold aspheric three-dimensional modeling profile data.

[0153] Optimization scheme:

[0154]

[0155]

[0156] 3) Considering the viscoelastic properties of glass material, the second corrected upper and lower mold aspheric profile is three-dimensionally constructed into a finite element simulation model again, and the surface profile result is simulated and calculated again under the same process conditions. Repeat step 2), until the finally calculated product aspheric profile shape is within the allowable tolerance range, so as to determine the mold surface shape, and finally get the aspheric element with regular surface profile.

[0157] 4. Actual verification result

[0158] Detection item Technical index Measured parameter Center thickness 1.5±0.005 mm 1.502 mm Surface accuracy PV≤0.3 μm PV≤0.22 μm Residual stress <10 MPa 8.7 MPa Mold life >5000 times Still available for 15000 times

[0159] It can be seen that: by using the above simulation process, the stress non-compliant components can be accurately predicted, and the optimization design idea can be obtained. By slowing down the cooling rate of the cooling stage and increasing the stress release groove, the phenomenon of "excessive stress" of aspheric product at the micro convex edge can be improved.

[0160] Example 3: Mold stress control and molding optimization of Φ15mm double-concave aspherical lens

[0161] Step 1: Establish the initial simulation model.

[0162] 1. Simulation model establishment: establish a thermal-mechanical coupling model by using ANSYS Workbench:

[0163] Contact pair setting Face-face contact Friction coefficient 0.25 Boundary condition Fixed constraint of lower edge and central axis of heating plate

[0164] According to the shape of the Φ10mm concave-convex aspherical lens, a three-dimensional geometric model of the center module is initially constructed, and the basic molding process is determined, mainly including time, temperature, pressure, etc.

[0165] a. Initial process parameters:

[0166] Heating temperature 540-560℃ Soaking time 900-1100s Pressing pressure 0.7-0.85 Mpa (three-stage type)

[0167] 2. Initial parameter setting

[0168] b. Material parameters:

[0169] (1) Mold material: tungsten carbide (elastic modulus 550GPa, thermal expansion coefficient 5.2x10 -6 / ℃)

[0170] (2) Glass material: D-K9 (transition temperature Tg = 496℃, sag temperature Ts = 558℃, annealing point T 10 13 = 483℃, thermal expansion coefficient 7.8x10 -6 K)

[0171] Calculate the cavity radiation temperature in the two-dimensional model, and export the simulation result node data, stress, displacement, temperature and other related data of the product aspherical surface profile. Compare the simulation calculation value with the theoretical design value, and adjust the assembly design according to the difference between the two. Modify the three-dimensional space parameters of the local components in the assembly model, and import the cavity thermal radiation temperature data calculated for the first time to establish the second simulation model.

[0172] 3. Iterative optimization process: center thickness correction

[0173] Step 2: Analyze whether the product center thickness data is qualified. If the product center thickness is not qualified, return to Step 1. According to the difference between the actual simulation calculation and the theoretical design, adjust the mold design scheme, modify the simulation model, and iterate the calculation again until the product center thickness and the theoretical design are within the required tolerance range, then enter the product appearance prediction link.

[0174] The analysis steps of the product center thickness are:

[0175] Export the product simulation calculation surface contour node position data, fit the data set, and compare the fitted data with the theoretical design value; according to the difference between the fitted value and the design value at the center, calculate the assembly design height that needs to be modified, substitute the modified assembly design into the simulation model, and calculate again until the difference between the final fitted data and the theoretical data is within the allowable error range, that is, the non-spherical product with qualified center thickness is obtained.

[0176] Problem detection:

[0177] The first simulation center thickness is 2.018 mm, the design value is 2.0±0.005 mm, and the tolerance is 0.13 mm.

[0178] Iterative optimization:

[0179] Iteration number Center thickness Deviation 1 2.018 ±0.018 mm 3 2.003 ±0.003 mm

[0180] Step 3: Analyze whether the product has obvious appearance problems. If the appearance result is not ideal, return to Step 1 to adjust the design parameters and optimize the preform until the product appearance is qualified, then enter the "stress analysis" and "product surface contour analysis" links.

[0181] The analysis steps of the product appearance problem are:

[0182] Monitor the entire molding process, observe whether there is a gap between the preform and the mold, overlap, whether the product will produce "overflow", and the appearance quality problems such as the fullness of the product edge; according to the appearance quality problems in the model, modify the design scheme of the relevant components in the assembly, and calculate again by substituting into the simulation model until the product appearance is qualified.

[0183] Problem detection:

[0184] The edge thickness is uneven (the maximum deviation is 0.015 mm), and material accumulation (about 10 μm in height) appears at the junction of the concave surface.

[0185] Optimization measures:

[0186]

[0187] Step 4: Analyze whether the product has obvious stress problems, including non-spherical product stress analysis and mold stress analysis.

[0188] The steps of stress analysis of aspherical product are:

[0189] 1) According to the simulation results, the residual circumferential S11 and height S33 stress data of the glass after forming are extracted, and after data processing, the double optical path difference calculation results of the aspherical product are obtained;

[0190] 2) The residual stress concentration of the design scheme is analyzed, and the difference between the simulation and measurement results is verified. If the stress result is not ideal, return to step 1) to adjust the design parameters and optimize the mold or preform until the product stress is compliant;

[0191] 3) According to the actual situation, it is judged whether mold stress analysis is needed. If needed, enter the mold stress analysis link; if not needed, enter the mold processing and mold pressing verification link, and finally obtain the aspherical product with higher imaging quality.

[0192] Problem detection:

[0193] Glass maximum residual stress 13.2 MPa (located at double concave vertex center) Mold maximum equivalent stress 1380 MPa (close to tungsten carbide fatigue limit 1200 MPa)

[0194] The simulation results show that the stress distribution of each component of the assembly body during the mold pressing process is mainly concentrated at the center of the double concave vertex and the edge position of the mold.

[0195] According to the stress concentration area and the large deformation position, the failure risk of the assembly design scheme, the mold pressing process and the service life of the material under this working condition is evaluated, the feasibility of the design scheme is predicted according to the stress level and deformation, and the design scheme is modified. It is judged whether it exceeds the yield strength or fatigue limit strength of tungsten carbide mold material.

[0196] Tungsten carbide mold material performance index Typical value range Yield strength 1800-2800 MPa Fatigue limit strength 800-1200 MPa

[0197] The results show that the fatigue limit strength of the tungsten carbide mold material at the edge is exceeded, and the stress level is unqualified, so return to step 1 to adjust the design parameters and optimize the mold or preform.

[0198] In addition, the pressure in the pressing process is modified to three-stage slow pressing, which can reduce the temporary stress generated in the process of instantaneous pressing.

[0199] Optimization scheme:

[0200]

[0201]

[0202] After optimizing the mold design, the glass residual stress is reduced to 9.5 MPa, and the mold stress is reduced to 1050 MPa.

[0203] Step 5: analyze the aspherical product surface profile problem. Extract the product simulation data after forming, compare the deviation between the aspherical product surface profile node data after fitting and the theoretical design curve, and correct according to the deviation of each point. According to this, adjust the assembly design scheme of the related parts, return to step 1, and continue until the simulated aspherical profile and the theoretical design value are within the allowable machining error range, then enter the mold processing and mold verification link, and finally obtain the aspherical product with high imaging quality.

[0204] The analysis steps of the product surface profile are as follows:

[0205] 1) According to the deviation between the actual calculation simulation value and the theoretical value design aspherical profile, the deviation between the upper and lower molds and the theoretical design aspherical profile at each node is calculated.

[0206] 2) Give a certain surface correction coefficient, increase or decrease the compensation value on the basis of the initial design mold, and modify the design mold aspherical shape to obtain the second simulation upper and lower mold aspherical three-dimensional modeling profile data.

[0207] Problem detection:

[0208] The local surface profile deviation of the double-concave surface at 60% of the aperture is 1.2 μm (PV value design requirement ≤0.5 μm).

[0209] Optimization scheme:

[0210] Reverse compensation of mold surface (compensation amount 1.0 μm @ 60% aperture, 0.5 μm @ edge).

[0211] Key compensation area Compensation amount 60% caliber 1.0 μm Edge area 0.5 μm

[0212] 3) Considering the viscoelastic properties of glass material, the second corrected upper and lower mold aspherical profile is three-dimensionally constructed into a finite element simulation model, and the surface profile result is simulated again under the same process conditions. Repeat step 2), and after 2 iterations, the surface profile PV value meets the standard (0.4 μm), the mold surface shape is determined, and the aspherical element with a conforming surface profile is finally obtained.

[0213] 4. Actual verification result

[0214] Detection item Technical index Measured parameter Center thickness 2.0±0.005 mm 2.002 mm Surface accuracy PV≤0.5 μm PV < 0.4 μm Residual stress <10 MPa 9.5 MPa Mold life > 5000 times 15000 times still usable

[0215] As can be seen from Example 3:

[0216] 1) The simulation process of the present application can be applied to other glass materials;

[0217] 2) By three-stage slow pressure control, the temporary stress generated in the instantaneous pressure process can be reduced, and the stress value at the place with large aspherical structure deformation can be effectively reduced.

[0218] 3) Precise compensation correction is carried out for the biconcave curvature mutation area, and the surface type precision PV value is improved from 1.2 mu m to 0.4 mu m, which is better than the technical index (less than or equal to 0.5 mu m) requirement.

[0219] The method for predicting the deformation of an aspheric product structure based on thermal-mechanical coupling simulation of the application relates to the technical field of thermal forming of aspheric optical elements, ultra-precision molds and other aspheric products. By establishing a thermal-mechanical coupling simulation model, the temperature field and mechanical field of the assembled structure are analyzed, the stress concentration area and the position of the aspheric product with large deformation are identified, and based on the finite element simulation results, the surface profile change of the product, the thermal stress / tensile stress concentration distribution and the structural response characteristics under high temperature environment are accurately predicted. The application can realize accurate simulation analysis of the aspheric product in the thermal forming process, significantly reduce the risk of premature failure caused by mold thermal fatigue (high temperature stress concentration, alternating thermal stress and tensile stress), avoid the problem of excessive deformation of the product, and improve the reliability of the mold design and the forming precision of the product.

Claims

1. A method for predicting mold stress and aspherical product structural deformation based on thermo-mechanical coupling simulation, characterized in that, Mold stress prediction includes mold design optimization, lens residual stress analysis, and dual optical path difference analysis; aspherical product structural deformation includes center thickness compliance, product surface contour correction, and preform product design optimization.

2. A method for predicting mold stress and aspherical product structural deformation based on thermo-mechanical coupling simulation, characterized in that, The method includes the following steps: Step 1: Based on the product's shape and outline, construct the initial 3D geometric model of the central module, determine the basic molding process, establish the initial simulation model, calculate the cavity radiation temperature in the 2D model, and export the product's aspherical contour node data, stress, displacement, temperature, and other relevant data from the simulation results; compare the simulated calculation values ​​with the theoretical design values, and adjust the assembly design based on the difference between the two; modify the 3D spatial parameters of local components in the assembly model, and import the cavity thermal radiation temperature data calculated in the first step to establish the second simulation model; Step 2: Analyze whether the product center thickness data is qualified. If the product center thickness is not qualified, return to Step 1. Based on the difference between the actual simulation calculation and the theoretical design, adjust the mold design scheme, modify the simulation model, and iterate the calculation again until the product center thickness is within the required tolerance range of the theoretical design. Step 3: Analyze whether there are obvious appearance problems with the product. If the appearance result is not ideal, return to Step 1 to adjust the design parameters and optimize the prefabricated parts until the product appearance is compliant. Step 4: Analyze whether there are obvious stress problems in the product, including stress analysis of aspherical products and mold stress analysis; Step 5: Analyze the surface profile of the aspherical product, extract the data after the product simulation molding, compare the deviation between the profile node data after the aspherical product surface fitting and the theoretical design curve, and give a certain coefficient correction according to the deviation of each point. Return to step 1 until the difference between the simulated aspherical profile and the theoretical design value is within the allowable processing error range.

3. The method for predicting mold stress and aspherical product structural deformation based on thermo-mechanical coupling simulation as described in claim 2, characterized in that, The analysis steps for the center thickness of the product described in step 2 are as follows: 1) Export the surface contour node position data from the product simulation calculation, fit the data set, and compare the fitted data with the theoretical design value; 2) Based on the difference between the fitted value and the design value at the center, calculate the assembly design height that needs to be modified. Substitute the modified assembly design into the simulation model and calculate again until the difference between the final fitted data and the theoretical data is within the allowable error range, thus obtaining a medium-thickness compliant aspherical product.

4. The method for predicting mold stress and aspherical product structural deformation based on thermo-mechanical coupling simulation as described in claim 2, characterized in that, The analysis steps for the product appearance problem mentioned in step 3 are as follows: 1) Monitor the entire molding process and observe whether there are gaps or overlaps between the preform and the mold, whether the product will produce overflow, and the appearance quality of the product edge molding fullness. 2) Based on the appearance quality issues that appear in the model, modify the design scheme of the relevant components in the assembly, and re-substitute them into the simulation model for calculation until the product appearance is qualified.

5. The method for predicting mold stress and aspherical product structural deformation based on thermo-mechanical coupling simulation as described in claim 2, characterized in that, The steps for stress analysis of the aspherical product described in step 4 are as follows: 1) Based on the simulation results, extract the residual circumferential stress data in the S11 and height S33 directions inside the product after molding. After data processing, obtain the calculation results of the double optical path difference of the aspherical product. 2) Analyze the residual stress concentration in the design scheme and verify the difference between the simulation and actual measurement results. If the stress results are not ideal, return to 1) Adjust the design parameters and optimize the mold or prefabricated parts until the product stress complies with regulations. 3) Determine whether mold stress analysis is necessary based on the actual situation. If so, proceed to the mold stress analysis stage. If not required, the process proceeds to mold processing and molding verification, ultimately resulting in an aspherical product with high imaging quality.

6. The method for predicting mold stress and aspherical product structural deformation based on thermo-mechanical coupling simulation as described in claim 2, characterized in that, The steps for mold stress analysis described in step 4 are as follows: 1) By using simulation results, monitor the stress changes of each component of the assembly during the molding process, identify stress types and stress concentration areas, and analyze the stress level in stress alternation / concentration areas; 2) Determine whether the material yield strength or fatigue limit strength is exceeded. If the stress level is not qualified, return to 1) Adjust the design parameters and optimize the mold or preform until the external stress of the product is compliant. Then proceed to the mold processing and molding verification stage to finally obtain an aspherical product with high imaging quality. 3) Based on the stress concentration areas and areas with large deformation, assess the failure risk of materials and assembly design schemes under temperature cycling and material service life under this working condition. Combine the stress level and deformation situation to predict the feasibility of the design scheme and make targeted modifications to the design scheme.

7. The method for predicting mold stress and aspherical product structural deformation based on thermo-mechanical coupling simulation as described in claim 2, characterized in that, The analysis steps for the surface profile of the aspherical product described in step 5 are as follows: 1) Based on the actual calculated simulation value and the theoretical value, design the deviation between the aspherical profile and the actual value. Calculate the deviation between the upper and lower molds and each node of the theoretically designed aspherical profile. 2) By giving a certain surface correction coefficient, the compensation value is added or subtracted based on the initial mold design to correct the aspherical shape of the mold and obtain the three-dimensional modeling contour data of the upper and lower molds in the second simulation. 3) Considering the viscoelastic properties of glass materials, the aspherical contours of the upper and lower molds after the second correction are reconstructed into a 3D finite element simulation model. Under the same process conditions, the surface shape is simulated and calculated again. Repeat step 2) until the final calculated aspherical contour shape of the product is within the allowable tolerance range, thereby determining the mold surface shape and obtaining an aspherical component with a compliant surface contour.

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