Method for predicting void of resin molding, method for reducing void of resin molding, program for predicting void of resin molding, and program for reducing void of resin molding

Abstract

To provide a method for predicting occurrence behaviors of voids generated inside an injection molding obtained by injection molding a thermoplastic resin into a mold.SOLUTION: A method includes: creating a model for analysis in which an injection molding is divided into a plurality of elements; determining temperature distribution and pressure distribution of the model for analysis in a step of molding a thermoplastic resin; calculating temperature distribution in a cooling step after mold release, using the temperature distribution; calculating elastic constant distribution and temperature load distribution of the model for analysis, using previously measured temperature dependency data of the elastic constant, data of a coefficient of thermal expansion and PVT data of the thermoplastic resin, from the temperature distribution and the pressure distribution; calculating a strain generated in each element of the model for analysis, by structural analysis using the elastic constant distribution and the temperature load distribution in the cooling step after the mold release; and predicting a generation place of voids and / or a void amount from the strain.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present disclosure relates to prediction of molding defects in resin molded products. [Background technology]

[0002] Injection molding is used to manufacture parts with complex shapes using thermoplastic resins. Depending on the molding conditions or product shape, molding defects called "sink marks (depressions on the surface of the molded product)" or "voids (cavities inside the molded product)" may occur in the resin molded product.

[0003] Sink marks and voids occur during the injection molding of thermoplastic resins, when the thermoplastic resin injected in a molten state cools and solidifies. In particular, in the case of crystalline resins, the molecular chains, which were random immediately after filling the mold, become oriented (folded and aligned) through crystallization, resulting in a decrease (shrinkage) in volume compared to the volume immediately after filling the mold (mold dimensions), resulting in sink marks or voids.

[0004] When these molding defects occur, it can lead to a decrease in the dimensional accuracy of the product (for example, in airtight parts, a dent can cause a gap to form on the sealing surface that comes into contact with the mating member) or a decrease in strength (voids can cause breakage). Therefore, there is a demand for improved technology to suppress sink marks and voids.

[0005] Measures to suppress sink marks and voids include checking the actual molded product and changing the molding conditions, or changing the design of the gate or wall thickness of the molded product, etc. However, since these require a huge amount of time and cost, in recent years, investigations have been conducted into whether it is possible to predict the occurrence of sink marks and / or voids through injection molding simulations using flow analysis software and optimize the product shape and molding conditions. In Patent Document 1, data obtained by flow analysis software is applied to distortion analysis using structural analysis software to predict the occurrence of voids. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2009-233882 A Summary of the Invention [Problem to be solved by the invention]

[0007] With existing methods, it was sometimes difficult to predict with sufficient accuracy the occurrence of voids in molded products after cooling during the injection molding process using resin materials. The purpose of the present disclosure is to provide a method for accurately predicting the occurrence behavior of voids in a resin molded product produced by an injection molding process using a resin material. By solving this problem, it becomes possible to anticipate the product shape design, mold design, molding condition setting, and molding material for obtaining a molded product without voids in advance at the design stage, thereby enabling efficient commercialization. [Means for solving the problem]

[0008] The present disclosure includes the following aspects.

[0009] [1] A method for predicting the behavior of voids occurring inside an injection-molded product obtained by injection molding a thermoplastic resin into a mold, comprising: A step (S1) of creating an analysis model in which the injection molded product is divided into a plurality of elements; A step (S2) of determining a temperature distribution and a pressure distribution of the analysis model in a process of molding a thermoplastic resin; A step (S3) of calculating a temperature distribution of the analysis model during a cooling process after demolding using the temperature distribution and the pressure distribution; A step (S4) of calculating an elastic constant distribution and a temperature load distribution of the analysis model from the temperature distribution and the pressure distribution, using temperature dependency data of elastic constants, thermal expansion coefficient data, and PVT data of the thermoplastic resin that have been previously measured; A step (S5) of calculating a strain generated in each element of the analysis model by a structural analysis using the elastic constant distribution and the temperature load distribution during a cooling process after the demolding; and (S6) predicting the location of void occurrence and / or the amount of voids from the strain.

[0010] [2] The method described in [1], wherein the step (S2) of determining the temperature distribution and pressure distribution of the analysis model in the process of molding the thermoplastic resin is a step (S2) of determining the temperature distribution and pressure distribution of the analysis model in the process of injecting the thermoplastic resin from the gate of the mold into the cavity and then releasing it from the mold.

[0011] [3] The method according to [1] or [2], wherein in the step (S4) of calculating the elastic constant distribution and the temperature load distribution of the analysis model, the elastic constant distribution and the temperature load distribution are calculated starting from the gate seal and ending at a set time from the start of injection.

[0012] [4] The method according to any one of [1] to [3], wherein in the step (S5) of calculating the strain, the strain is calculated using the elastic constant distribution and the temperature load distribution, taking into account the temperature dependency calculated from the temperature distribution, with the start point being the time when the injection-molded product is released after being cooled under pressure, and the end point being the time when a set time has elapsed from the start of injection.

[0013] [5] The method according to [3], wherein the gate seal is determined by setting molding conditions in a flow analysis of the analytical model so that the weight of the injection molded product is maximized and the time to the gate seal is minimized.

[0014] [6] The method according to [3], wherein the gate seal is at a point where the temperature at the center of the gate reaches the flow stop temperature Ts.

[0015] [7] The method according to [6], wherein the flow stop temperature Ts is calculated using data fitting coefficients b5 and b6 of the 2-domain Tait PVT model for the PVT data, where P is the pressure, and Ts = b5 + b6 × P.

[0016] [8] The method according to [6], wherein the flow stop temperature Ts is the inflection point when cooling from 1°C / min to 50°C / min in specific heat measurement.

[0017] [9] The method according to any one of [1] to [8], wherein in the step (S2) of determining the temperature distribution and pressure distribution of the analysis model in the process of molding the thermoplastic resin, a thermal conductivity determined by a thermal conductivity measurement method called the AC steady-state method (ISO22007-6) is used for the calculation.

[0018]

[10] A method for reducing voids generated inside an injection-molded product obtained by injection molding a thermoplastic resin into a mold, comprising the steps of: comparing the amount of voids predicted by the method described in any one of [1] to [9] with the predicted results when one or more of the design, molding conditions, and molding material are changed; and repeating this process until the amount of voids is reduced to a predetermined amount or less.

[0019]

[11] A program for causing a computer to execute the method according to any one of [1] to [9].

[0020]

[12] A program that causes a computer to execute the method described in

[10] . [Brief description of the drawings]

[0021] [Figure 1] FIG. 1 is a diagram for explaining each step of injection molding. [Diagram 2] FIG. 2 is a flowchart illustrating an example of a method for predicting void generation behavior according to an embodiment. [Diagram 3] FIG. 3 is a diagram showing an example of temperature dependency data of elastic constants of a thermoplastic resin. [Figure 4]FIG. 4 is a diagram showing an example of PVT data of a thermoplastic resin. [Diagram 5] FIG. 5 is a diagram showing a three-dimensional CAD model of a shape according to the embodiment. [Figure 6] FIG. 6 is a cross-sectional view of an analytical model for the shape used in the examples. [Figure 7] FIG. 7 shows a comparison of void occurrence, where A is an X-ray CT scan of an actual molded product, B is a comparison example using a conventional method that only performs flow analysis, C is a reference example that performs a coupled analysis of flow analysis and structural analysis, and D is an example using the improved method. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] Figure 1 is a diagram for explaining each step of injection molding. Figure 1 focuses on one mold and shows the timeline of molding. Injection molding starts by injecting resin into the mold cavity through a gate. The resin delivery speed is controlled immediately after injection, but when, for example, 99% of the pre-set amount of resin is filled, the control switches to resin pressure control (holding pressure) (VP switching). After switching, injection continues while holding pressure.

[0023] Here, gate sealing is one of the indicators for obtaining stable injection molded products. Gate sealing is a phenomenon in which the resin at the gate solidifies and stops flowing, and the time it takes for the resin at the gate to solidify and stop flowing is called the gate sealing time. If the holding pressure is stopped before the gate sealing time, the molten resin will flow back through the gate into the injection molding machine, resulting in poor filling and weight loss. On the other hand, even if the holding pressure is stopped after the gate sealing time, the gate has solidified and the resin will not flow back, allowing for a stable injection molded product to be obtained, so this is an indicator that is always measured at the molding site.

[0024] The gate sealing time may be determined by setting the molding conditions such that the weight of the molded product is maximum and the time until gate sealing is minimum. The weight of the molded product and the time until gate sealing may be obtained through experiments (weighing). Instead of experiments, the weight of the molded product and the time until gate sealing may be obtained by simulation through flow analysis. Alternatively, the time when the temperature at the center of the gate reaches the flow stop temperature Ts may be defined as the gate sealing time. The time when the temperature at the center of the gate reaches the flow stop temperature Ts may be obtained through experiments or may be determined by simulation. The flow stop temperature Ts may be defined as the inflection point when cooling at a rate of 1 °C / min to 50 °C / min in specific heat measurement. Alternatively, for the flow stop temperature Ts, with respect to the PVT data, using the data fitting coefficients b5 and b6 of the 2-domain Tait PVT model, Ts = b5 + b6×P may be used for the pressure P. Here, in the 2-domain Tait PVT model, the specific volume v(T,P) at temperature T and pressure P is given by v 0 (T)[1 - C×ln(1 + P / B(T))] + v t (T,P) as shown. In particular, v 0 (T) is a linear equation of T, and for a predetermined temperature Tt, v t (T,P) is zero on the high-temperature side (T > Tt), but becomes an exponential function of T and P on the low-temperature side (T < Tt). And in this model, from the PVT data, it is fitted with a linear equation of P such as Tt = b5 + b6×P. Note that b5 is the inflection point of the PVT data (see Figure 4). The slope changes with b5 as the boundary, indicating that a solid-liquid phase transition is occurring.

[0025] Following the dwell time, the injection molded product is cooled in the mold for a predetermined cooling time (in-mold cooling). After the cooling time has elapsed, the mold is opened and the injection molded product is ejected from the mold and released. The mold is then closed again to form a mold cavity, and molding begins to obtain the next injection molded product. The ejected injection molded product can be cooled outside the mold (ex-mold cooling). Ex-mold cooling can be continued, for example, until the injection molded product reaches room temperature.

[0026] In actual injection molded products, especially when the product is thick, voids may occur after the product is removed from the mold (after demolding). Conventional flow analysis software can only calculate the temperature and pressure inside the mold, so it is not possible to calculate the temperature transition outside the mold and take into account actual phenomena. Therefore, it is thought that there are cases where the generation of voids cannot be predicted accurately using conventional flow analysis software. Furthermore, there is no known method for calculating temperature changes outside the mold and predicting the occurrence of voids from the calculated temperature changes.

[0027] In this disclosure, an analysis is performed that takes into account the cooling after release from the mold. From the temperature data obtained, the elastic constant distribution and the volumetric shrinkage distribution are obtained, and it has been found that by performing coupled analysis that applies the distribution to structural analysis (strain analysis), it is possible to predict the location and amount of voids that will be generated at a level comparable to the results in an actual product. Note that, although this disclosure provides a detailed explanation of cooling by air inside and outside the mold, the cooling means is not limited to air cooling.

[0028] Hereinafter, a method for predicting the occurrence behavior of voids according to an embodiment of the present disclosure will be described in detail. Note that the present disclosure is not limited to the following embodiment. (One embodiment)

[0029] (Method to predict void generation behavior) An example of the method for predicting the generation behavior of voids in this embodiment will be described in detail with reference to Fig. 2. This method involves performing a coupled analysis in which the results of a flow analysis are applied to a structural analysis. As shown in the flowchart of Figure 2, an example of a method for predicting the void generation behavior of this embodiment includes the steps of creating an analytical model (S1), calculating the temperature distribution and pressure distribution in the process of injecting and molding a thermoplastic resin (injection molding process) (S2), calculating the temperature distribution in the cooling process after demolding (out-of-mold cooling analysis) (S3), calculating the elastic constant distribution and temperature load distribution (S4), calculating the strain generated in each element of the analytical model (S5), and predicting the location of void generation and / or the void amount (S6).

[0030] (Creating an analysis model (S1)) In step (S1) of creating an analytical model in which an injection molded product is divided into multiple elements, the shape of the injection molded product is divided into tiny elements to create a model necessary for performing a simulation (see Fig. 6 for an example). For example, the shape of the injection molded product (which can be the design shape of the injection molded product or the shape of the mold, etc. The shape of the mold includes conditions such as the position, number, and size of runners and gates) is input into a computer using 3D shape measurement or a CAD system, etc. Next, the shape input into the computer is divided into multiple 3D elements using an element division preprocessor, etc. to create an analytical model. In addition, when performing coupled analysis of flow analysis and structural analysis, it is possible to prepare a model for flow analysis and a model for structural analysis separately, and execute the analysis. However, in this embodiment, an example in which the same analytical model is carried over from flow analysis to structural analysis will be described in detail.

[0031] (Calculation of temperature and pressure distribution in the injection molding process (S2)) In step (S2) of calculating the temperature distribution and pressure distribution of the analytical model in the process of injecting and molding the thermoplastic resin (injection molding process), a flow analysis (simulation) of the injection molding is performed using the created analytical model. The temperature and pressure of each element of the analytical model in the process of injecting and molding the thermoplastic resin (which may also include the in-mold cooling process) are calculated by the flow analysis. This allows the temperature distribution and pressure distribution of the analytical model in the injection molding process to be calculated. When performing a flow analysis, the physical property values ​​of the thermoplastic resin used in injection molding are input. The physical property values ​​used in the flow analysis include data expressing the relationship between pressure, volume, and temperature (hereinafter referred to as "PVT data." See Figure 4 for an example), thermal conductivity data, and specific heat data. The specific heat of thermoplastic resins can be measured by a differential scanning calorimeter (DSC). The thermal conductivity of thermoplastic resins can be determined by the AC steady-state method (ISO 22007-6). Thermal conductivity measurement methods include the AC steady-state method, the hot wire method, the hot disk method, etc. The AC steady-state method gives good accuracy.

[0032] Next, input the analysis conditions for thermoplastic resin flow analysis. The molding conditions include the resin (cylinder) temperature, mold temperature, injection speed, dwell pressure, and dwell time. The mold temperature is the same as the cooling water temperature or heater setting temperature. The molding conditions also include the specification of the start and end points of the flow analysis. The start point of the flow analysis can be, for example, the time when the thermoplastic resin starts to be injected from the gate of the mold into the cavity. The end point of the flow analysis can be the time from when the molten thermoplastic resin is injected into the mold cavity until the end of the cooling time and before the mold is released. In particular, it can be the time until the mold is released. The time of mold release in the flow analysis can be specified as the time when a certain set time (which may be determined by comparing with the time until mold release in the actual process) has elapsed from the start of injection.

[0033] (Outside mold cooling analysis (S3)) In the step (S3) of outside-mold cooling analysis, the calculated temperature distribution is used to calculate the temperature distribution of each element of the analysis model during the cooling process after demolding. This step can be analyzed using structural analysis software. The starting point for an outside-mold cooling analysis is the time of mold release. For an analysis inside the mold, in addition to the injection-molded part, the mold itself must be modeled and a finite element model of the mold must be used to consider the interaction between the molded part and the mold. On the other hand, for an analysis outside the mold, a model of the mold itself is not necessary, so the model of the injection-molded part itself and the boundary conditions must be significantly changed, making it extremely difficult to set the analysis conditions even as an extension of an analysis inside the mold. The end point of an out-of-mold cooling analysis can be, for example, the time when the injection molded product reaches room temperature outside the mold. The "time when the injection molded product reaches room temperature outside the mold" can be specified, for example, as the time when the injection molded product reaches room temperature outside the mold, measured in the actual process from the start of injection. Alternatively, the time when the temperature of the injection molded product reaches room temperature can be obtained during the out-of-mold cooling analysis. In general, the end point of an out-of-mold cooling analysis can be a time when a set time has elapsed since the start of injection.

[0034] (Calculation of elastic constant distribution and temperature load distribution (S4)) In step (S4) of calculating the elastic constant distribution and the temperature load distribution, the calculated temperature distribution (temperature distribution in the process of molding the thermoplastic resin and temperature distribution in the cooling process after demolding) and pressure distribution (pressure distribution in the process of molding the thermoplastic resin) are used to determine the elastic constant distribution and the temperature load distribution (distribution of temperature difference) of the injection molded product. In particular, the elastic constant distribution and the temperature load distribution of the injection molded product can be calculated from the temperature distribution and the pressure distribution by a conversion program for a computer. In addition to the temperature load distribution, the volumetric shrinkage distribution can also be calculated.

[0035] (Elastic constant distribution) The elastic constant distribution of an injection molded product can be obtained by applying previously obtained temperature dependency data of the elastic constant (generally the proportional coefficient of stress and strain) of the molding material (thermoplastic resin) to the temperature distribution obtained by flow analysis. Figure 3 shows data on the temperature dependence of the elastic constant of a certain thermoplastic resin. The horizontal axis is temperature, and the vertical axis is the elastic constant (Young's modulus). The temperature Tc on the horizontal axis indicates the transition temperature of the thermoplastic resin. The transition temperature is the temperature that divides the solidified region from the molten region, and is also called the no-flow temperature, solidification temperature, solidification temperature, or solid-liquid transition temperature. As shown in Figure 3, if data on the temperature dependence of the elastic constants of a thermoplastic resin is available, the distribution of the elastic constants can be obtained from the temperature distribution. The elastic constants can include three types of values: the elastic modulus (Young's modulus) for uniaxial stress, Poisson's ratio, and shear modulus. The use of these three types of elastic constants enables more accurate analysis.

[0036] (Poisson's ratio distribution) The Poisson's ratio distribution of an injection molded product can be obtained by applying a value calculated from the temperature dependency data of the elastic constant (Young's modulus) of the molding material (thermoplastic resin) obtained in advance and the bulk modulus obtained from the PVT data to the temperature distribution obtained by flow analysis.

[0037] (shear modulus distribution) The shear modulus distribution of an injection molded product can be obtained by applying values ​​calculated from the temperature dependency data of the elastic constant (Young's modulus) and the temperature dependency data of the Poisson's ratio to the temperature distribution obtained by flow analysis.

[0038] (Volumetric shrinkage distribution) The volumetric shrinkage distribution of an injection molded product can be obtained by applying previously obtained (actually measured) data ("PVT data") showing the relationship between the pressure, volume, and temperature of the molding material (thermoplastic resin) to the temperature distribution and pressure distribution data obtained by flow analysis. Figure 4 shows the PVT data for a thermoplastic resin. The horizontal axis is temperature (unit: °C) and the vertical axis is the inverse of density, i.e., specific volume (unit: cm 3 / g). Figure 4 shows the relationship between temperature and specific volume when the pressure (P) is 50 MPa. Assume that the relationships between temperature and specific volume have been measured in advance for several pressures other than 50 MPa. The relationship between temperature and specific volume for pressures for which there are no actual measurements can be obtained, for example by interpolation, from the relationship between temperature and specific volume for pressures for which there are actual measurements. In Figure 4, when the pressure (P) is 50 MPa, at the time when the thermoplastic resin at the gate solidifies and stops flowing (called the "gate seal time"), the temperature of the thermoplastic resin is 200°C and the specific volume is approximately 0.82 cm. 3 Similarly, when the temperature of the thermoplastic resin injection molded product reaches the mold temperature, the temperature of the thermoplastic resin is 40°C and the specific volume is approximately 0.70 cm3. 3 / g.

[0039] The volumetric shrinkage distribution of an injection molded product is calculated as follows. First, the temperature at a certain point before contraction (T 1 ) and pressure (P 1 By applying these to the PVT data of the thermoplastic resin obtained in advance, the volume before shrinkage (V 1 ) to In the example of gate seal time in Figure 4, T 1 = 200℃, P 1 = 50 MPa, and the specific volume is about 0.82 cm 3 / g. In other words, the volume per unit weight (1g) at this point is V 1 = approx. 0.82cm 3 It is. Similarly, the temperature at a certain point after contraction (T 2 ) and pressure (P 2 ) is calculated. By applying this to the PVT data, the volume after contraction (V 2 ) to The point in time after shrinkage can be specified as the point in time when a set time has elapsed since the start of injection (for example, this may be determined by comparison with the time it takes for the injection-molded product to reach the mold temperature in the actual process). In the example shown in Figure 4, the mold temperature is cooled to T 2 = 40℃, P 2 = 50 MPa, and the specific volume is about 0.70 cm 3 / g. In other words, the volume per unit weight (1g) at this point is V 2 = approx. 0.70cm 3 It is. And the volume of each element before and after shrinkage (V 1 , V 2 ) to calculate the volumetric shrinkage rate of each element ((volume before shrinkage - volume after shrinkage) ÷ (volume before shrinkage), that is, (V 1 -V 2 ) / V 1 In other words, if the pressure is constant at 50 MPa (P 1 =P 2 = 50MPa) and T 1 = 200℃ to T 2 The volumetric shrinkage rate when cooled to 40°C is (V 1 -V 2 ) / V 1 =(0.82-0.70) / 0.82=0.146, or 14.6%. Even if the pressure is not constant before and after shrinkage, the flow analysis can calculate the temperature of each element before shrinkage (T 1 ) and pressure (P 1 ) and the temperature of each element after shrinkage (T 2 ) and pressure (P 2 ) is calculated. 1 , P 2 ) at temperature (T 1 , T 2 If the relationship between the volumetric shrinkage rate (V 1 -V 2 ) / V 1 can be calculated. In this way, the volumetric shrinkage distribution can be calculated from the PVT data, temperature distribution, and pressure distribution.

[0040] (temperature load) The temperature load (distribution of temperature difference) is obtained by converting the volumetric shrinkage rate of each element by the previously obtained volumetric expansion rate of the thermoplastic resin to a temperature difference. The volumetric expansion rate is the coefficient β of the relational equation ΔV / V=βΔT when the original volume V changes by ΔV due to a temperature rise ΔT. Generally, the volumetric expansion rate is obtained from PVT data, but when the thermoplastic resin is isotropic, the volumetric expansion rate is three times the linear expansion rate, so it may be obtained from a previously measured linear expansion rate instead of the volumetric expansion rate obtained from the PVT data. The volumetric expansion rate and linear expansion rate are collectively called the thermal expansion rate. The thermal expansion rate (volume expansion rate and linear expansion rate) may have temperature dependency. The temperature load means the "temperature difference before and after shrinkage", and is different from the difference between the "temperature at a certain point before shrinkage" and the "temperature at a certain point after shrinkage" used in the process of calculating the shrinkage distribution. The latter temperature difference, which was calculated from the temperature data obtained by flow analysis, is simply a calculation of "temperature only". In contrast, the former temperature load is a value that is calculated by first calculating the volume from "temperature and pressure" using PVT data and then converting it back to temperature using the volume expansion coefficient, so it is calculated using "temperature and pressure" rather than "temperature only". Therefore, the temperature load is a "temperature difference" that takes into account the effect of the pressure that the resin receives in actual molding, and unlike the difference between only the temperature at a certain point before shrinkage and a certain point after shrinkage, using the temperature load will provide more accurate analysis results.

[0041] (Calculation of strain occurring in each element of the analytical model (S5)) In step (S5) of calculating the strain generated in each element of the analytical model, a structural analysis is performed using the elastic constant distribution (particularly Young's modulus distribution, Poisson's ratio distribution, and shear modulus distribution) during the cooling process after demolding, and the volumetric shrinkage distribution and / or temperature load distribution (at least one of the volumetric shrinkage distribution and the temperature load distribution), to calculate the strain generated in each element of the analytical model (distribution of the amount of strain in the analytical model). When determining the temperature distribution after demolding, it is necessary to determine the initial temperature immediately after demolding, and the initial temperature is determined from the temperature distribution inside the mold. The time start point for calculating the strain can be, for example, the time point when the mold is released after pressure-holding and cooling. The time end point for calculating the strain can be, for example, the time point when the injection-molded product reaches room temperature. Alternatively, the time end point for calculating the strain can be, for example, the time point when a certain set time (which may be set by comparison with the actual process) has elapsed since the start of injection.

[0042] (Prediction of void occurrence location and / or void volume (S6)) In the step (S6) of predicting the location of void generation and / or the amount of voids, the amount of deformation is calculated based on the distribution of the amount of strain in the analytical model, and the generation of voids is predicted. The occurrence of sink marks is first predicted from the distribution of the amount of strain, and then the shape of the sink marks is analyzed, thereby making it possible to predict the occurrence of voids. Alternatively, without predicting the occurrence of sink marks, for example, when the distortion in each element of the analysis model exceeds a threshold value for void occurrence that is determined in advance by actual measurement for each resin material, it is possible to predict that voids will occur in that element. Conventional analysis software does not consider the temperature dependency of elastic constants (Young's modulus, Poisson's ratio, and shear modulus) and linear expansion coefficient, and therefore only allows input of the elastic constant (one value) at room temperature, which means that there is insufficient consideration of changes in the elastic constants of each part when the temperature changes, making it impossible to accurately predict the occurrence of voids. In contrast, the improved method disclosed herein can calculate the amount of strain in each part of the molded product using the temperature load calculated from the volumetric shrinkage distribution in addition to the elastic constant distribution (particularly Young's modulus distribution, Poisson's ratio distribution, and shear modulus distribution) due to temperature changes in each element based on the temperature distribution and pressure distribution obtained from the flow analysis, thereby improving prediction accuracy.

[0043] Furthermore, in the method disclosed herein, by calculating (S3) the temperature distribution and pressure distribution of each element of the analytical model during cooling outside the mold, it is possible to predict the occurrence of voids taking into account cooling outside the mold, thereby further improving the prediction accuracy.

[0044] Next, examples of the method according to the present embodiment (hereinafter, also simply referred to as the "improved method") and comparative examples of the conventional method will be shown, and prediction of sink marks and voids according to the embodiment of the present disclosure will be specifically described. Note that the present disclosure is not limited to these examples.

[0045] (Product shape and molding material) Figure 5 shows a 3D CAD model of the molded product shape with thick bolt-like ribs according to the embodiment. The molding material (thermoplastic resin) of the actual molded product is Polyplastics' Duracon (registered trademark) POM M90-44 (unfilled material). The specific heat of the molding material was measured by a differential scanning calorimeter (DSC), and the thermal conductivity was obtained by a thermal conductivity measurement method called the AC steady-state method (ISO22007-6). As shown in Figure 5, the width of the hexagonal flange is 20 mm and the thickness is 6 mm. The height of the cylinder is 15 mm. There is a gate on one side of the hexagonal flange.

[0046] For the flow analysis in the examples and comparative examples, Autodesk's Moldflow (registered trademark) Insight 2019.0.5 (3D solid model) build 20180921.0959_C70L71 (hereinafter referred to as Moldflow) was used. Adventure Cluster 2021 (hereinafter referred to as ADVC) manufactured by Allied Engineering Co., Ltd. was used for the outside mold cooling analysis and structural analysis in the examples. In the examples, the elastic constant distribution and the volumetric shrinkage distribution were calculated (S5) using a conversion program created by ourselves using Visual Basic (registered trademark).

[0047] (Example) The molding conditions of the actual molded products and the analysis conditions by this method are shown in Table 1. In the examples, the physical property values ​​of the thermoplastic resin are the same as those of the thermoplastic resin of the actual molded products. [Table 1]

[0048] FIG. 6 shows a cross-sectional view of the analysis model used in the embodiment.

[0049] In the embodiment, analysis of injection, pressure holding and cooling inside the mold (S2 in Fig. 2) was performed, and from the temperature distribution obtained by the outside-mold cooling analysis (S3 in Fig. 2), a conversion program was used to obtain temperature loads calculated from the elastic constant distribution (particularly Young's modulus distribution, Poisson's ratio distribution and shear modulus distribution) and volumetric shrinkage distribution of the injection molded product (inside and surface) (S4 in Fig. 2), and a structural analysis was performed to calculate strain and predict the location of void generation and the amount of voids (S5 and S6 in Fig. 2).

[0050] (Comparative Example) In the comparative example relating to the conventional method, voids were predicted from the volumetric shrinkage distribution calculated by flow analysis using the same analysis model as in the example, the same thermoplastic resin physical properties (excluding temperature dependence of elastic constants, etc.) and the same analysis conditions as in the example. As described above, the conventional method does not take into account the temperature dependence of the elastic constants (Young's modulus, Poisson's ratio, and shear modulus of elasticity) and the linear expansion coefficient, and therefore does not take into account changes in the elastic constants of each part when the temperature changes. In other words, the comparative example results in inferior analysis accuracy of cooling inside the mold. In the comparative example, a coupled analysis for predicting voids was not performed, but voids were predicted from the volumetric shrinkage distribution obtained from the flow analysis.

[0051] (Reference example) Furthermore, in the Reference Example, a coupled analysis was performed using the same analytical model as in the Example, the same thermoplastic resin property values ​​(including temperature dependence of elastic constants, etc.) as in the Example, and the same analysis conditions, and voids were predicted from the strain obtained by the structural analysis. The analysis conditions in the Reference Example are the same as the molding conditions of the actual molded product in Table 1. Also, in the Reference Example, the physical property values ​​of the thermoplastic resin are the same as those of the thermoplastic resin in the actual molded product. In the reference example, the analysis of cooling outside the mold was not performed, but only the analysis of cooling inside the mold was performed.

[0052] (Comparison of void predictions) FIG. 7 shows a comparison of void occurrence in an actual molded product, a comparative example using a conventional method, a reference example in which out-of-mold analysis was not performed, and an example using the improved method. Figures 7A to 7D (hereinafter referred to as Figures 7A to 7D) show X-ray CT of an actual molded product, a comparative example using a conventional method that only performs flow analysis, a reference example in which a coupled analysis of flow analysis and structural analysis is performed but no outside-mold cooling analysis is performed, and an example using the improved method, respectively.

[0053] In the comparative example, voids occurred throughout the entire interior of the molded product, and there was a large discrepancy between the locations where voids occurred in the actual molded product. In the embodiment, the locations of void occurrence in the actual molded product and the analysis are close to each other, making it possible to accurately represent the actual void occurrence phenomenon using coupled analysis.

[0054] (Methods for reducing voids) The present disclosure also includes a method for reducing voids that occur inside an injection-molded article produced by injection molding a thermoplastic resin into a mold. That is, using the method for predicting the void generation behavior described above, the predicted void amount is compared with the predicted results when one or more of the design, molding conditions, and molding material are changed, and this process is repeated until the void amount is reduced to a predetermined amount or less.

[0055] The present disclosure is not limited to the above-described embodiment, but includes various modified examples in which components are added, deleted, or converted from the above-described configuration. In addition, each embodiment can be combined in various ways. In particular, the present disclosure should not be construed as being limited to the above-described embodiments or examples in terms of shape, material, or condition.

[0056] Furthermore, the present disclosure also includes a program for causing one or more processors to execute the method for predicting the generation behavior of voids or the method for reducing voids according to the present disclosure. The program may be provided by being recorded in a computer-readable non-transitory storage medium.

Claims

1. A method for predicting the behavior of voids occurring inside an injection-molded product obtained by injection molding a thermoplastic resin into a mold, comprising: Creating an analysis model in which the injection molded product is divided into a plurality of elements; determining a temperature distribution and a pressure distribution of the analysis model in a process of molding a thermoplastic resin; calculating a temperature distribution of the analysis model during a cooling process after demolding using the temperature distribution; calculating an elastic constant distribution and a temperature load distribution of the analysis model from the temperature distribution and the pressure distribution using temperature dependency data of elastic constants, thermal expansion coefficient data, and PVT data of the thermoplastic resin that have been previously measured; calculating a strain generated in each element of the analysis model by a structural analysis using the elastic constant distribution and the temperature load distribution during a cooling process after the demolding; A step of predicting a location where a void will occur and / or an amount of voids from the strain; The method includes:

2. 2. The method according to claim 1, wherein the step of determining the temperature distribution and pressure distribution of the analysis model in the process of molding the thermoplastic resin is a step of determining the temperature distribution and pressure distribution of the analysis model in a process of injecting the thermoplastic resin from a gate of the mold into a cavity and then releasing the thermoplastic resin from the mold.

3. 2. The method according to claim 1, wherein in the step of calculating the elastic constant distribution and the temperature load distribution of the analysis model, the elastic constant distribution and the temperature load distribution are calculated starting from a gate seal and ending at a set time elapsed from the start of injection.

4. 2. The method according to claim 1, wherein in the step of calculating the strain, the strain is calculated using the elastic constant distribution and the temperature load distribution, taking into account temperature dependency calculated from the temperature distribution, with the start point being the time when the injection-molded product is released after being cooled under pressure, and the end point being the time when a set time has elapsed from the start of injection.

5. 4. The method according to claim 3, wherein the gate seal is determined by setting molding conditions in a flow analysis of the analytical model such that a weight of the injection-molded product is maximized and a time until the gate seal is minimized.

6. 4. The method of claim 3, wherein the gate seal is at a point where the temperature of the gate center reaches a stop-flow temperature Ts.

7. 7. The method of claim 6, wherein the flow stop temperature Ts is calculated using data fitting coefficients b5 and b6 of a 2-domain Tait PVT model for the PVT data, where P is a pressure, Ts=b5+b6×P.

8. The method according to claim 6 , wherein the flow stop temperature Ts is an inflection point when cooling at a cooling rate in the range of 1° C. / min to 50° C. / min in specific heat measurement.

9. 2. The method according to claim 1, wherein in the step of determining the temperature distribution and pressure distribution of the analysis model in the process of molding the thermoplastic resin, a thermal conductivity obtained by a thermal conductivity measurement method called the AC steady-state method (ISO22007-6) is used for calculation.

10. A method for reducing voids that occur inside an injection-molded product obtained by injection molding a thermoplastic resin into a mold, comprising the steps of: predicting the amount of voids while changing one or more of the design, molding conditions, and molding material using the method described in claim 1; and repeating this process until the predicted amount of voids is reduced to a predetermined amount or less.

11. A program causing a computer to execute the method according to claim 1.

12. A program causing a computer to execute the method according to claim 10.

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