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 the steps of: 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; correcting pressure in a mold cavity; 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; 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, the effect of the crystallization temperature changing due to the application of pressure during flow is taken into consideration in the flow analysis of the injection molding process using flow analysis software.In Patent Document 2, data obtained by the 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 2009-233881 A [Patent Document 2] 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 during the injection molding process using pressurized resin material. 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 correcting the pressure in the mold cavity; 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; 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 a 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 a gate of the mold into a cavity and then releasing it from the mold.

[0011] [3] The method according to [1] or [2], wherein in the step (S3) of correcting the pressure in the mold cavity, the pressure Pc used in the calculation is calculated using a pressure correction coefficient α and a time t from the start of injection according to the following formula (1).

number

[0012] [4] The method according to [3], in the step (S3) of correcting the pressure in the mold cavity, the pressure correction coefficient α is set in the range of 0 to 30, time t is the time when the difference between the maximum pressure and the minimum pressure in the mold cavity is maximum, and the pressure Pc used in the calculation is calculated using the equation (1).

[0013] [5] The method according to any one of [1] to [4], wherein in the step (S4) of calculating the elastic constant distribution and the temperature load distribution of the analysis model, the temperature load distribution is calculated with the gate seal as the starting point and the elapse of a set time from the start of injection as the end point.

[0014] [6] The method according to [5], 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.

[0015] [7] The method according to any one of [1] to [6], wherein in the step (S5) of calculating the strain, the injection-molded product is cooled under pressure, and an elastic constant distribution is calculated taking into account the temperature dependency calculated from the temperature distribution, starting from a point in time between the gate seal and demolding, and ending from a point in time before demolding.

[0016] [8] The method according to [5], wherein the gate seal is at a point where the temperature at the center of the gate reaches the flow stop temperature Ts.

[0017] [9] The method according to [8], 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.

[0018]

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

[0019]

[11] The method according to any one of [1] to

[10] , wherein in the step (S2) of calculating the temperature distribution and pressure distribution of the analytical model, a thermal conductivity calculated by a thermal conductivity measurement method called the AC steady-state method (ISO22007-6) is used for the calculation.

[0020]

[12] 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

[11] 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.

[0021]

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

[11] .

[0022]

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

[12] . [Brief description of the drawings]

[0023] [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 data on the temperature dependence 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 shows a first shape example (flange type) according to the embodiment, where A is a 3D CAD shape of the bottom view and top view of the flange type, and B and C are X-ray CT scans of the actual molded product. [Figure 6] FIG. 6 shows an analytical model used in the embodiment for the shape example of FIG. [Figure 7] FIG. 7 is a diagram showing an example of pressure distribution at the time of VP switching for the shape example of FIG. [Figure 8] FIG. 8 is a diagram for explaining pressure distribution in the shape example of FIG. 5 according to a conventional method. [Figure 9]FIG. 9 shows a comparison of voids for the example shapes shown in FIG. 5, where A shows an X-ray CT scan of an actual molded product, B shows Comparative Example 1 using a conventional method that performs only flow analysis, C shows Comparative Example 2 using a coupled analysis of flow analysis and structural analysis (without pressure correction), and D shows an example using the improved method. [Figure 10] FIG. 10 shows a second shape example (window regulator type) according to the embodiment, in which A shows a three-dimensional CAD shape of the window regulator type, and B shows an X-ray CT scan of an actual molded product. [Figure 11] FIG. 11 shows an analytical model used in the embodiment for the shape example of FIG. [Figure 12] FIG. 12 is a diagram for explaining pressure distribution in the shape example of FIG. 10 according to a conventional method. [Figure 13] FIG. 13 shows a comparison of voids for the example shapes shown in FIG. 10, where A shows an X-ray CT scan of an actual molded product, B shows Comparative Example 1 using a conventional method that performs only flow analysis, C shows Comparative Example 2 using a coupled analysis of flow analysis and structural analysis (without pressure correction), and D shows an example using the improved method. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] Figure 1 is a diagram for explaining each step of injection molding. Figure 1 focuses on one mold and shows the timeline of molding. First, molding begins with injecting resin into the mold cavity through a gate. Immediately after injection, the resin delivery speed is controlled, 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 VP switching, pressure is held and injection continues. At the time of VP switching, it is considered that the difference between the maximum and minimum pressures in the mold cavity is at its maximum.

[0025] Here, gate seal is one of the indicators for obtaining stable injection molded products. Gate seal is a phenomenon in which the resin at the gate part solidifies and the flow stops. The time when the resin at the gate part solidifies and the flow stops is called the gate seal time. If the holding pressure is stopped before the gate seal time, the molten resin will flow back to the injection molding machine side through the gate, resulting in filling defects and weight loss. On the other hand, even if the holding pressure is stopped after the gate seal time, the gate is already solidified, so the resin will not flow back, and a stable injection molded product can be obtained. Therefore, it is an index that must be measured at the molding site.

[0026] The gate seal time may be determined by setting the molding conditions so that the weight of the molded product is maximum and the time until gate seal is minimum. The weight of the molded product and the time until gate seal may be obtained through experiments (weighing). Instead of experiments, the weight of the molded product and the time until gate seal 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 seal time. The time when the temperature at the center of the gate reaches the flow stop temperature Ts may be obtained through experiments or 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. Or, for the flow stop temperature Ts, in relation 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. Here, the 2-domain Tait PVT model assumes that the specific volume v(T,P) at temperature T and pressure P is represented as v(T,P)=v0(T)[1 - C×ln(1 + P / B(T))]+v t (T,P). In particular, v0(T) is a linear expression 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 expression 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 at b5, and a solid-liquid phase transition occurs.

[0027] 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 continue, for example, until the injection molded product reaches room temperature.

[0028] In actual injection molded products, the pressure is high near the gate, and the pressure is reliably transmitted from the gate, so it is believed that voids are unlikely to occur. On the other hand, the pressure is low in areas away from the gate, and the transmission of pressure from the gate is uncertain. Therefore, the volumetric shrinkage rate is large in areas away from the gate, so it is believed that voids are more likely to occur. In contrast, conventional analysis methods do not take the gate seal into account in their calculations, so the pressure can be the same at any point regardless of the gate position, which is thought to result in low accuracy in predicting the occurrence of voids.

[0029] In this disclosure, a flow analysis is performed that takes into account the pressure generated inside the mold. From the temperature and pressure data obtained from this, elastic constant distribution (particularly Young's modulus distribution, Poisson's ratio distribution, and shear modulus distribution), volumetric shrinkage distribution, and / or temperature load are obtained, and it has been found that by performing coupled analysis that applies this to structural analysis (strain analysis), it is possible to predict the location and amount of voids that will occur at a level comparable to the results in an actual product.

[0030] 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)

[0031] (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), correcting the pressure in the mold cavity (S3), calculating the elastic constant distribution and temperature load (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).

[0032] (Creating an analysis model (S1)) In the step (S1) of creating an analytical model in which an injection molded product is divided into a plurality of elements, the shape of the injection molded product is divided into minute elements to create a model necessary for performing a simulation. For example, the shape of the injection molded product (which may be the design shape of the injection molded product or the shape of the mold, etc., including 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 a plurality of 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.

[0033] (Calculation of temperature and pressure distribution in the injection molding process (S2)) In step (S2) of calculating the temperature distribution and pressure distribution 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 flow analysis calculates 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 cooling process). This calculates the temperature distribution and pressure distribution of the analytical model in the injection molding process. 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.

[0034] Next, input the analysis conditions for the thermoplastic resin flow analysis. The molding conditions include the resin (cylinder) temperature, mold temperature, injection speed, dwell pressure, and dwell time. 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 is when the molten thermoplastic resin is injected into the mold cavity, the cooling time ends, and the resin is released from the mold.

[0035] (Compensation for pressure in mold cavity (S3)) In the step (S3) of correcting the pressure in the mold cavity, the pressure distribution Pa(t) at a certain time t is corrected to calculate the pressure (hereinafter referred to as the pressure for volumetric shrinkage calculation) Pc to be used for calculating the volumetric shrinkage distribution. Note that the time t can be the time from the start of injection. By correcting the pressure inside the mold cavity, it is possible to predict the occurrence of voids taking into account the pressure generated inside the mold. In particular, the pressure Pc for calculating the volumetric shrinkage rate can be calculated by the following formula (1) using a pressure correction coefficient α.

number

[0036] Here, the pressure correction coefficient α can be set in the range of 0 to 30. Furthermore, the time t from the start of injection can be set to the time when the difference between the maximum pressure and the minimum pressure in the mold cavity becomes maximum. When the difference between the maximum and minimum pressures in the mold cavity is maximized, the pressure distribution is also expected to be the largest. Therefore, it is possible to predict the occurrence of voids that better reflects the pressure distribution occurring in the mold.

[0037] (Calculation of elastic constant distribution and temperature load distribution (S4)) In the step (S4) of calculating the elastic constant distribution and the temperature load distribution, the calculated temperature distribution and pressure distribution are used to obtain 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.

[0038] (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 (Young's modulus, etc., 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 dependency of the elastic constant (Young's modulus) 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 elastic constant distribution 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.

[0039] (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.

[0040] (shear modulus distribution) The shear modulus distribution of an injection molded product can be determined by applying values ​​calculated from temperature dependency data of the elastic constant (Young's modulus) and temperature dependency data of the Poisson's ratio to the temperature distribution obtained by flow analysis.

[0041] (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 specific volume is approximately 0.70 cm3 when the temperature of the thermoplastic resin is 40°C. 3 / g.

[0042] The volumetric shrinkage distribution of an injection molded product is calculated as follows. First, the temperature (T1) and pressure (P1) at a certain point before shrinkage are obtained from the temperature distribution data and pressure distribution data obtained by flow analysis. These are then applied to the PVT data of the thermoplastic resin obtained in advance to convert them into the volume (V1) at the point before shrinkage. In the example of the gate seal time in FIG. 4, T1 = 200°C, P1 = 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 V1 = approx. 0.82 cm 3 It is. Similarly, the temperature (T2) and pressure (P2) at a certain point after contraction are obtained from the temperature distribution data and pressure distribution data obtained by flow analysis. These are then applied to the PVT data to convert them into the volume (V2) at the time after contraction. 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 comparing with the time it takes for the injection-molded product to reach the mold temperature in the actual process). In the example shown in FIG. 4, when the mold temperature is cooled to the mold temperature, T2 = 40°C, P2 = 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 V2 = approx. 0.70 cm 3 It is. Then, from the volumes (V1, V2) of each element before and after shrinkage, the distribution of the volumetric shrinkage rate of each element ((volume before shrinkage - volume after shrinkage) ÷ (volume before shrinkage), i.e. (V1-V2) / V1) is calculated. In other words, when the pressure is constant at 50 MPa (P1=P2=50 MPa) and cooling is performed from T1=200°C to T2=40°C, the volumetric shrinkage rate can be calculated as (V1-V2) / V1=(0.82-0.70) / 0.82=0.146, or 14.6%. Even if the pressure is not constant before and after shrinkage, the temperature (T1) and pressure (P1) of each element before shrinkage and the temperature (T2) and pressure (P2) of each element after shrinkage are calculated by flow analysis. If the relationship (PVT data) between the temperature (T1, T2) and the specific volume at the pressures (P1, P2) before and after shrinkage is obtained in advance, the volumetric shrinkage rate (V1-V2) / V1 of each element can be calculated. In this way, the volumetric shrinkage distribution can be calculated from the PVT data, temperature distribution, and pressure distribution.

[0043] (temperature load) The temperature load (distribution of temperature difference) is calculated by dividing the volumetric shrinkage rate of each element by the previously obtained volumetric expansion rate of the thermoplastic resin and converting it into 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 calculated from PVT data, but in the case of isotropic resin, the volumetric expansion rate is three times the linear expansion rate, so it may be calculated from a previously measured linear expansion rate instead of the volumetric expansion rate calculated 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.

[0044] (Calculation of strain occurring in each element of the analytical model (S5)) In the 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, shear modulus distribution, and volumetric shrinkage distribution), and the volumetric shrinkage distribution and / or temperature load distribution (at least one of the volumetric shrinkage distribution and the temperature load distribution), and the strain generated in each element of the analytical model (distribution of the amount of strain in the analytical model) is calculated. The time start point for calculating the strain is the time when the injection molded product is cooled under pressure, and can be a certain time point between gate sealing and demolding. The time end point for calculating the strain is a certain time point (e.g., the time of demolding) after the time start point for calculating the strain and up to (before) demolding.

[0045] (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 a structural analysis of the shape in which the sink marks have occurred is performed, 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 flow analysis software does not consider the temperature dependency of elastic constant distribution (Young's modulus, Poisson's ratio, and shear elastic modulus) and linear expansion coefficient, so it does not take into account the change in elastic constant of each part when the temperature changes, and it is not possible 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 due to temperature changes of each element (especially Young's modulus distribution, Poisson's ratio distribution, and shear elastic modulus distribution) based on the temperature distribution and pressure distribution obtained from the flow analysis, thereby improving the prediction accuracy.

[0046] Furthermore, in the method disclosed herein, by correcting the pressure within the mold cavity (S4), it is possible to predict the occurrence of voids taking into account the pressure generated within the mold, thereby further improving prediction accuracy.

[0047] Next, for the flange type (first shape example) and the window regulator type (second shape example), 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 are shown, and the prediction of voids according to the embodiment of the present disclosure is specifically described. Note that the present disclosure is not limited to these examples.

[0048] For the flange type and window regulator type, the molding material (thermoplastic resin) of the actual molded products is Polyplastics' Duracon (registered trademark) POM M90-44 (unfilled material). The specific heat of the molding material was measured using a differential scanning calorimeter (DSC), and the thermal conductivity was obtained using a thermal conductivity measurement method called the AC steady-state method (ISO22007-6).

[0049] In addition, 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 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).

[0050] In the example, after flow analysis (S2 in Fig. 2) for injection, pressure holding, and cooling, the pressure in the mold cavity was corrected (S3 in Fig. 2), and the elastic constant distribution and temperature load of the injection molded product (inside and surface) were obtained by a conversion program (S4 in Fig. 2). Furthermore, a structural analysis was performed to calculate the strain (S5 in Fig. 2), and the location of void generation and the amount of voids were predicted based on the magnitude of the strain (S6 in Fig. 2).

[0051] (Flange type) Fig. 5 shows a first shape example (flange type) according to the embodiment. Fig. 5A to Fig. 5C (hereinafter referred to as Fig. 5A to Fig. 5C) show a 3D CAD shape of the flange type (Fig. 5A) and X-ray CT of an actual molded product (Fig. 5B and Fig. 5C). Figure 5A shows the 3D CAD shapes of the bottom and top views of the flange mold, respectively. In the top view, four protrusion-like features, features 1 to 4, can be seen. Feature 1 is close to "gate1" in the figure. Feature 2 is close to "gate2" in the figure. Figure 5B is an X-ray CT scan of an actual molded product that was injection molded from the gate labeled "gate 1" in the figure. An enlarged image of the area around feature 1 (area surrounded by a square) is attached. Feature 1 is located near gate 1. No voids are observed near feature 1. Figure 5C is an X-ray CT scan of an actual molded product that was injection molded from the gate "gate 2" in the figure. An enlarged image of the area around feature 1 (area surrounded by a square) is attached. Feature 1 is located far from gate 2. Voids can be seen near feature 1.

[0052] (Example of flange type) The injection molding conditions for 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 in the actual molded products. [Table 1]

[0053] The analytical model used in the examples is shown in Figure 6. The 3D CAD shape of the injection molded product was divided into minute elements to create the analytical model (Figure 6) required to perform the simulation.

[0054] In step (S4) of correcting the pressure in the mold cavity, the pressure distribution Pa(t) at the VP switching time t is corrected by equation (1) to calculate the volumetric shrinkage calculation pressure Pc to be used in calculating the volumetric shrinkage distribution. In equation (1), α=10. In the analysis, t was set to approximately the same value as the VP switching time t in the actual molding (experiment), and the pressure distribution Pa(t) was corrected. FIG. 7 is a diagram showing an example of pressure distribution at VP switching for the shape of FIG. 5A. FIG. 7 shows the distribution of Pc when the gate is at gate1. Note that at the time of VP switching, the pressure at the gate1 position is 7.578 MPa. If α=10 in formula (1), the pressure Pc for calculating the volumetric shrinkage rate at the gate1 position is c is 75.78MPa. If a holding pressure of 50MPa is applied, a total pressure of 125.78MPa is applied at the gate 1 position. The pressure of 125.78MPa is an apparent pressure for calculating the amount of shrinkage.

[0055] (Comparative Example 1 of Flange Type) In Comparative Example 1 (see FIG. 9 ) relating to the conventional method, voids were predicted from the volumetric shrinkage distribution value calculated by flow analysis using the same analytical model as in the example, the same physical properties of the thermoplastic resin (excluding temperature dependence of elastic constants, etc.) and the same analytical conditions as in the example. That is, in Comparative Example 1, a coupled analysis was not performed, and voids were predicted from the volumetric shrinkage distribution obtained from the flow analysis. Furthermore, in Comparative Example 1, no correction was made for the pressure inside the mold cavity. Fig. 8 is a diagram for explaining the pressure distribution by the conventional method for the shape of Fig. 5 A. The pressure during gate sealing is zero over the entire analytical model, so there is no pressure gradient.

[0056] (Flange type comparison example 2) In Comparative Example 2 (see FIG. 9), a coupled analysis was performed on the same analytical model as in the Example, using the same thermoplastic resin property values ​​(including temperature dependence of elastic constants, etc.) and the same analysis conditions as in the Example, and voids were predicted from the strain obtained by the structural analysis. In Comparative Example 2, the pressure inside the mold cavity was not corrected.

[0057] (Comparison of flange type void predictions) Figure 9 is a comparison of voids for the example shapes in Figure 5. Figures 9A to 9D (hereafter referred to as Figures 9A to 9D) are diagrams showing a comparison of voids in an actual molded product corresponding to Figure 5, voids predicted by a conventional method, and voids predicted by the present method, respectively. The upper part of Fig. 9A shows an X-ray CT scan of an actual molded product when the gate is gate 1. The upper part of Fig. 9A is the same as Fig. 5B. The lower part of Fig. 9A shows an X-ray CT scan of an actual molded product when the gate is gate 2. The lower part of Fig. 9A is the same as Fig. 5C. The upper part of FIG. 9B shows Comparative Example 1 based on the conventional method using only flow analysis, in the case where the gate is gate1. The lower part of FIG. 9B shows Comparative Example 1 using the conventional method with only flow analysis, in the case where the gate is gate2. The upper part of FIG. 9C shows Comparative Example 2 in which a coupled analysis of flow analysis and structural analysis is performed when the gate is set to gate1 (however, the pressure gradient in the mold cavity is not corrected). The lower part of FIG. 9C shows Comparative Example 2 in which a coupled analysis of flow analysis and structural analysis is performed when the gate is gate2 (however, the pressure gradient in the mold cavity is not corrected). The upper part of FIG. 9D shows an example of the improved method when the gate is gate1. The lower part of FIG. 9D shows an example of the improved method in which the gate is gate2.

[0058] (Comparison when gate is gate1) As mentioned above, the upper part of Figure 9A (actual molded product) includes an enlarged view of the vicinity of Feature 1 in Figure 5A (the area enclosed in a square). Feature 1 is located near gate 1. No voids are observed near Feature 1. In the upper row of FIG. 9B (Comparative Example 1), voids are predicted near all four protrusion-like features, Features 1 to 4. In the upper part of FIG. 9C (Comparative Example 2), a void is predicted near feature 1. In the top row of FIG. 9D (example), no voids are predicted near feature 1. Therefore, in Comparative Example 1, voids occurred near all of Features 1 to 4, and there was a large deviation from the locations where voids occurred in the actual molded product. In Comparative Example 2, voids also occurred near Feature 1, which differed from the location of void occurrence in the actual molded product. In the embodiment, no voids are generated near feature 1, making it possible to accurately represent the actual void generation phenomenon.

[0059] (Comparison when gate is gate2) As mentioned above, the lower part of Figure 9A (actual molded product) includes an enlarged view of the vicinity of features 1 and 2 in Figure 5A (both enclosed in squares). Feature 1 is close to "gate 1". Feature 2 is close to "gate 2". Voids can be seen near feature 1. No voids can be seen near feature 2. In the lower part of FIG. 9B (Comparative Example 1), no voids are predicted near feature 1. 9C (Comparative Example 2), a void is predicted near feature 1. A void is predicted near feature 2, which is in the vicinity of gate 2. In the lower part of Figure 9D (Example), voids are predicted near feature 1. No voids are predicted near feature 2, which is in the vicinity of gate 2. Therefore, since no voids occurred near Feature 1 in Comparative Example 1, there was a large discrepancy with the location where voids occurred in the actual molded product. In Comparative Example 2, voids also occurred near Feature 2, which differed from the location of void occurrence in the actual molded product. In the embodiment, a void occurs near feature 1, but no void occurs near feature 2, so that the actual void occurrence phenomenon can be accurately represented.

[0060] (Evaluation of void prediction for flange type) From the prediction of void generation when the gate is gate1 or gate2, Comparative Example 2, which performs coupled analysis, better represents the void generation phenomenon than Comparative Example 1. The embodiment shows the void generation phenomenon better than the comparative examples 1 and 2.

[0061] (Window regulator type) Fig. 10 shows a second shape example (window regulator type) according to the embodiment. Fig. 10A and Fig. 10B (hereinafter referred to as Fig. 10A and Fig. 10B) show a 3D CAD shape of the window regulator type (Fig. 10A) and an X-ray CT scan of an actual molded product (Fig. 10B). Figure 10A shows the 3D CAD geometry of the window regulator mold. Protrusion-like features (features 5 to 7) are visible. Figure 10B is an X-ray CT image of an actual molded product that was injection molded from the gate marked "gate" in the figure. Feature 5 is located near the gate. No voids are observed near feature 5.

[0062] (Example of window regulator type) The injection molding conditions for the actual molded products and the analysis conditions by this method are shown in Table 2. 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.

[0063] [Table 2]

[0064] The analytical model used in the example is shown in Figure 11. The 3D CAD shape of the injection molded product was divided into minute elements to create the analytical model (Figure 11) required to perform the simulation.

[0065] In step S4 of correcting the pressure gradient in the mold cavity, the pressure distribution Pa(t) at the VP switching time t is corrected by equation (1) to calculate the volumetric shrinkage calculation pressure Pc to be used for calculating the volumetric shrinkage distribution. In equation (1), α=10.

[0066] (Window regulator type comparison example 1) In Comparative Example 1 (see FIG. 13B) relating to the conventional method, the same analysis model as in the example was used, and the same physical properties of the thermoplastic resin (excluding temperature dependence of elastic constants, etc.) and the same analysis conditions as in the example were used, but no coupled analysis was performed. Instead, voids were predicted from the volumetric shrinkage distribution value calculated by flow analysis. Fig. 12 is a diagram for explaining the pressure distribution by the conventional method for the shape of Fig. 10A. During the pressure holding process from when the mold cavity internal pressure reaches its maximum, the pressure is almost uniform over the entire analysis model, and there is almost no pressure gradient.

[0067] (Window regulator type comparison example 2) In Comparative Example 2 (see FIG. 13C), a coupled analysis was performed on the same analytical model as in the Example, using the same physical properties (including temperature dependence of elastic constants, etc.) of the thermoplastic resin as in the Example, and the same analysis conditions as in the Example, and voids were predicted from the strain obtained by the structural analysis. In Comparative Example 2, the pressure gradient in the mold cavity was not corrected.

[0068] (Comparison of void predictions for wind regulator types) Figure 13 is a comparison of voids for the example shape of Figure 10. Figures 13A to 13D (hereinafter referred to as Figures 13A to 13D) are diagrams showing a comparison of voids in an actual molded product corresponding to Figure 10, voids predicted by a conventional method, and voids predicted by the present method, respectively. Figure 13A shows an X-ray CT scan of the actual molded product. Note that Figure 13A is the same as Figure 10B. FIG. 13B shows Comparative Example 1 based on the conventional method using only flow analysis. FIG. 13C shows Comparative Example 2 in which a coupled analysis of flow analysis and structural analysis is performed (however, the pressure inside the mold cavity is not corrected). FIG. 13D shows an embodiment of the improved method.

[0069] (Comparison of void occurrence) As mentioned above, in Fig. 13A, feature 5 in Fig. 10A is near the gate. No voids are observed near feature 5 (area surrounded by a square in the upper center), but voids are generated in the lower left area (area surrounded by a square in the lower left), which is away from the gate. In FIG. 13B, the amount of voids generated in the lower left part, which is far from the gate, is small, so there is a discrepancy between the amount of voids generated in the actual molded product. In FIG. 13C, voids are predicted throughout most of the region, including near feature 5. In FIG. 13D, no voids are predicted near feature 5, but a void is predicted in the lower left portion, away from the gate. Therefore, in Comparative Example 1, the predicted voids are generally smaller than those in the actual molded product, and the deviation from the amount of voids generated in the actual molded product is large. In Comparative Example 2, voids also occurred near feature 5, which differed from the location of void occurrence in the actual molded product. In the embodiment, no voids are generated near feature 5, and voids are predicted throughout the entire part, similar to the actual molded part. Therefore, the actual void generation phenomenon can be accurately represented.

[0070] (Evaluation of void prediction) From the void prediction when the gate is gate1 or gate2 in the flange type and the void prediction for the window regulator type, the embodiment better represents the void generation phenomenon than the comparative examples 1 and 2.

[0071] (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.

[0072] 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.

[0073] 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 generated inside an injection molded product formed by injection molding a thermoplastic resin into a mold, comprising: creating an analysis model obtained by dividing the injection molded product into a plurality of elements; obtaining the temperature distribution and pressure distribution of the analysis model in the process of molding the thermoplastic resin; correcting the pressure in the mold cavity; calculating the elastic constant distribution and temperature load distribution of the analysis model from the temperature distribution and the pressure distribution using temperature-dependent data of elastic constants, data of thermal expansion coefficients, and PVT data of the thermoplastic resin measured in advance; calculating the strain generated in each element of the analysis model by structural analysis using the elastic constant distribution and the temperature load distribution; predicting the void generation location and / or void amount from the strain.

2. The step of obtaining the temperature distribution and pressure distribution of the analysis model in the process of molding the thermoplastic resin is the step of obtaining 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 demolding, according to the method of Claim 1.

3. In the step of correcting the pressure in the mold cavity, the pressure Pc used in the calculation is calculated by the following formula (1) using the pressure correction coefficient α and the time t from the start of injection. 【Number 1】 However, in the formula (1), Pc is the pressure for calculating the volume shrinkage rate, Pa(t) is the pressure obtained by flow analysis, α is the pressure correction coefficient, t is the time from the start of injection.

4. In the step of correcting the pressure in the mold cavity, the pressure correction coefficient α is set in the range of 0 to 30, the time t is set as the time when the difference between the maximum pressure and the minimum pressure in the mold cavity is the largest, and the pressure Pc used in the calculation is calculated by the formula (1), according to the method of Claim 3.

5. In the step of calculating the elastic constant distribution and temperature load distribution of the analysis model, the temperature load distribution is calculated starting from the gate seal and ending when the set time has elapsed from the start of injection, according to the method of Claim 1.

6. The method according to claim 5, wherein the gate seal is determined by setting molding conditions such that, in the flow analysis of the analysis model, the weight of the injection molded product is maximum and the time to the gate seal is minimum.

7. The method according to claim 1, wherein, in the step of calculating the strain, the injection molded product is pressure-held and cooled, and an elastic constant distribution is calculated in consideration of the temperature dependence calculated from the temperature distribution with a certain point between the gate seal and demolding as a starting point and a certain point before demolding as an end point.

8. The method according to claim 5, wherein the gate seal is the point in time when the temperature at the center of the gate reaches the flow stop temperature Ts.

9. The method according to claim 8, wherein the flow stop temperature Ts is set to Ts = b5 + b6 × P for the pressure P using the data fitting coefficients b5 and b6 of the 2-domain Tait PVT model for the PVT data.

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

11. The method according to claim 1, wherein, in the step of obtaining the temperature distribution and pressure distribution of the analysis model, the thermal conductivity obtained by a thermal conductivity measurement method called the alternating current steady state method (ISO22007-6) is used in the calculation.

12. A method for reducing voids generated inside an injection molded product formed by injecting a thermoplastic resin into a mold, wherein the void amount is predicted while changing one or more of the design, molding conditions, and molding material by the method according to claim 1, and the prediction is repeated until the predicted void amount is reduced to a predetermined amount or less.

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

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

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