Method for predicting voids in resin molded products, method for reducing voids in resin molded products, program for predicting voids in resin molded products, and program for reducing voids in resin molded products.

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] This disclosure relates to the prediction of molding defects in resin molded products. [Background technology]

[0002] Injection molding is used in the manufacturing of complex-shaped parts using thermoplastic resins. Depending on the molding conditions or the shape of the product, molding defects called "sink marks" (indentations 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 injection molding of thermoplastic resins as the molten thermoplastic resin is cooled and solidified. In particular, with crystalline resins, the molecular chains, which were in a random state immediately after filling the mold, become oriented (folded and aligned) due to crystallization. As a result, the volume decreases (shrinks) below the volume immediately after filling the mold (mold dimensions), causing 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 parts used for airtight applications, indentations can create gaps in the sealing surface that contacts the mating component) or a decrease in strength (making the product more susceptible to fracture starting from voids). Therefore, there is a need for technological improvements in suppressing sink marks and voids.

[0005] To suppress sink marks and voids, one can check the molded product after it has been produced and modify the molding conditions, or redesign the gate or wall thickness of the molded product. However, because these methods are extremely time-consuming and costly, in recent years, researchers have been exploring whether it is possible to predict the occurrence of sink marks and / or voids using injection molding simulations with fluid analysis software, thereby optimizing the product shape and molding conditions. Patent Document 1 describes how data obtained from fluid analysis software is applied to strain analysis using structural analysis software to predict the occurrence of voids. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2009-233882 [Overview of the project] [Problems that the invention aims to solve]

[0007] Existing methods sometimes failed to predict with sufficient accuracy the occurrence of voids in molded products after cooling during injection molding processes using resin materials. The purpose of this disclosure is to provide a method for accurately predicting the void formation behavior of resin molded products produced by injection molding processes using resin materials. By solving this problem, it becomes possible to anticipate the product shape design, mold design, molding condition setting, and molding material in advance at the design stage in order to obtain molded products that do not generate voids, thereby enabling efficient product development. [Means for solving the problem]

[0008] This disclosure includes the following aspects:

[0009] [1] A method for predicting the void generation behavior inside an injection-molded product formed by injection molding a thermoplastic resin into a mold, Step (S1) is to create an analytical model by dividing the injection-molded product into multiple elements, Step (S2) of determining the temperature distribution and pressure distribution of the analytical model in the process of molding thermoplastic resin, Step (S3) of calculating the temperature distribution of the analytical model during the cooling process after demolding using the temperature distribution and pressure distribution, Step (S4) of calculating the elastic constant distribution and temperature load distribution of the analytical model using the temperature dependence data of the elastic constant, thermal expansion coefficient data and PVT data of the thermoplastic resin that have been measured in advance from the temperature distribution and the pressure distribution. Step (S5) is to calculate the strain generated in each element of the analysis model by performing a structural analysis using the elastic constant distribution and the temperature load distribution during the cooling process after demolding. A method comprising the step (S6) of predicting the location and / or amount of voids from the strain.

[0010] [2] The method according to [1], wherein the step (S2) of determining the temperature distribution and pressure distribution of the analytical model in the process of molding the thermoplastic resin is the step (S2) of determining the temperature distribution and pressure distribution of the analytical model in the process of injecting the thermoplastic resin from the gate of the mold into the cavity and then demolding the mold.

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

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

[0013] [5] The method according to [3], wherein the gate seal is determined by setting the molding conditions such that, in a flow analysis of the analytical model, 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 closed when 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 defined as Ts=b5+b6×P with respect to pressure P, using data fitting coefficients b5 and b6 of the 2-domain Tait PVT model for the PVT data.

[0016] [8] The method according to [6], wherein the flow stop temperature Ts is defined as the inflection point when cooled at a rate of 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 obtaining the temperature distribution and pressure distribution of the analysis model in the step of molding the thermoplastic resin, the thermal conductivity obtained by a thermal conductivity measurement method called the alternating steady-state method (ISO22007-6) is used for calculation.

[0018]

[10] A method for reducing voids generated inside an injection-molded article formed by injection-molding a thermoplastic resin into a mold, the method comprising: comparing prediction results when one or more of design, molding conditions, and molding materials are changed with respect to the void amount predicted by the method according to any one of [1] to [9], and repeating this until the void amount 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 for causing a computer to execute the method according to

[10] . [Brief Description of the Drawings]

[0021] [Figure 1] Figure 1 is a diagram for explaining each step of injection molding. [Figure 2] Figure 2 is a flowchart illustrating an example of a method for predicting void generation behavior according to the embodiment. [Figure 3] Figure 3 is a diagram illustrating an example of temperature dependence data of the elastic constant of a thermoplastic resin. [Figure 4]Figure 4 shows an example of PVT data for a thermoplastic resin. [Figure 5] Figure 5 shows a 3D CAD model of the shape according to the embodiment. [Figure 6] Figure 6 is a cross-sectional view of the analytical model for the shape used in the embodiment. [Figure 7] Figure 7 shows a comparison of void formation, where A is an X-ray CT scan of an actual molded product, B is a comparative example using a conventional method involving only fluid analysis, C is a reference example of coupled analysis of fluid and structural analysis, and D is an example of an improved method. [Modes for carrying out the invention]

[0022] Figure 1 is a diagram illustrating each step of injection molding. Figure 1 focuses on a single mold and shows the molding process in chronological order. Injection 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 a predetermined amount of resin has been filled, the control switches to resin pressure control (holding pressure) (VP switching). After the switch, injection continues under holding pressure.

[0023] One indicator for obtaining stable injection-molded products is gate seal. Gate seal is the phenomenon where the resin at the gate solidifies and stops flowing. The time during which the resin at the gate solidifies and stops flowing is called the gate seal time. If holding pressure is stopped before the gate seal time, molten resin will flow back through the gate to the injection molding machine, resulting in poor filling and weight loss. On the other hand, even if holding pressure is stopped after the gate seal time, the gate is solidified, so the resin will not flow back, and stable injection-molded products can be obtained. Therefore, it is an indicator that is always measured in the molding process.

[0024] The gate seal time may be determined by setting molding conditions such that the weight of the molded article is maximized and the time to gate seal is minimized. The weight of the molded article and the time to gate seal may be obtained by experiment (measurement). The weight of the molded article and the time to gate seal may be determined by simulation through flow analysis instead of experiment. Alternatively, the time point at which the temperature at the gate center reaches the flow stop temperature Ts may be defined as the gate seal time. The time point at which the temperature at the gate center reaches the flow stop temperature Ts may be obtained by experiment or determined by simulation. The flow stop temperature Ts is 1°C / min to 50°C / min in specific heat measurement Cooling speed within the range of [this range] and may be the inflection point when cooled. Alternatively, for the flow stop temperature Ts, Ts = b5 + b6 × P may be set for the pressure P using the data fitting coefficients b5 and b6 of the 2-domain Tait PVT model with respect to the aforementioned PVT data. Here, in the 2-domain Tait PVT model, the specific volume v(T,P) at temperature T and pressure P is expressed as v(T,P) = v0(T)[1 - C × ln(1 + P / B(T))] + v by a constant C and an exponential function B(T). t It is expressed as (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). In this model, fitting is performed with a linear expression of P such that Tt = b5 + b6 × P from the PVT data. 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.

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

[0026] In actual injection-molded products, especially when the product is thick, voids may occur after the product has been removed from the mold (after demolding). Conventional flow analysis software can only calculate temperature and pressure inside the mold, and therefore cannot calculate the temperature changes outside the mold and take into account the actual phenomenon. Therefore, it is thought that conventional methods using fluid analysis software may not be able to accurately predict the occurrence of voids. Furthermore, there is no known method for calculating temperature changes outside the mold and predicting void formation from that data.

[0027] This disclosure describes an analysis that takes into account cooling after demolding from the mold. From the temperature data obtained, the elastic constant distribution and volume shrinkage rate distribution are obtained and applied to structural analysis (strain analysis) through coupled analysis, and it was found that the location and amount of voids can be predicted at a level comparable to that of actual products. In this disclosure, cooling by air cooling inside and outside the mold is described in detail, but the cooling means is not limited to air cooling.

[0028] The following describes in detail a method for predicting void generation behavior according to embodiments of this disclosure. However, this disclosure is not limited to the embodiments described below. (One embodiment)

[0029] (Method for predicting void formation behavior) An example of a method for predicting the void formation behavior in this embodiment will be described in detail with reference to Figure 2. This method involves a coupled analysis that applies the results of the fluid analysis to the structural analysis. As shown in the flowchart of Figure 2, an example of a method for predicting void generation behavior in 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 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 and / or amount of voids (S6).

[0030] (Creation of analysis model (S1)) Step (S1) involves creating an analysis model by dividing the injection-molded part into multiple elements. This involves dividing the shape of the injection-molded part into minute elements to create a model necessary for running the simulation (see Figure 6 as an example). For example, the shape of the injection-molded part (which can be the design shape of the injection-molded part or the shape of the mold, etc., and for the mold shape, conditions such as the position, number, and size of the runners and gates) is input into the computer using 3D shape measurement or a CAD system. Then, the shape input into the computer is divided into multiple 3D elements using an element division preprocessor or the like to create an analysis model. In coupled fluid analysis and structural analysis, it is possible to prepare a separate model for fluid analysis and a separate model for structural analysis and perform the analysis. However, in this embodiment, we will explain in detail an example in which the same analysis model is used from the fluid analysis to the structural analysis.

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

[0032] Next, input the analysis conditions for performing the flow analysis of the thermoplastic resin. Molding conditions include resin (cylinder) temperature, mold temperature, injection speed, holding pressure, and holding pressure time. Note that the mold temperature is the same as the cooling water temperature or heater setting temperature. Furthermore, the molding conditions include specifying the start and end points of the flow analysis. The start point of the flow analysis can be, for example, the point at which the thermoplastic resin begins to be injected from the mold gate into the cavity. The endpoint of the flow analysis can be the period from the injection of molten thermoplastic resin into the mold cavity, through the cooling period, and before demolding. In particular, it can be defined as the point of demolding. In the flow analysis, the point of demolding can be specified as the time elapsed from the start of injection (which may be determined by comparing it with the time until demolding in the actual process).

[0033] (External mold cooling analysis (S3)) In step (S3) of the mold external 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 performed using structural analysis software. The starting point for external cooling analysis is demolding. In the case of analysis within the mold, in addition to the injection molded product, the mold itself must be modeled, and a finite element model of the mold must be used to consider the interaction between the molded product and the mold. On the other hand, in the case of analysis outside the mold, a model of the mold itself is not necessary, so the model of the injection molded product itself and the boundary conditions must be changed significantly, and even as an extension of analysis within the mold, setting the analysis conditions becomes extremely difficult. The endpoint of the out-of-mold cooling analysis can be, for example, the point at which the injection-molded part reaches room temperature outside the mold. The "point at which the injection-molded part reaches room temperature outside the mold" can be specified, for example, by the time measured during the actual process from the start of injection until the injection-molded part reaches room temperature outside the mold. Alternatively, the point at which the temperature of the injection-molded part reaches room temperature can be determined within the out-of-mold cooling analysis itself. Generally, the endpoint of the out-of-mold cooling analysis can be set to the point after a certain set time has elapsed from the start of injection.

[0034] (Calculation of elastic parameter distribution and temperature load distribution (S4)) In step (S4), which calculates the elastic constant distribution and temperature load distribution, the calculated temperature distribution (temperature distribution during the molding process of the thermoplastic resin and the temperature distribution during the cooling process after demolding) and pressure distribution (pressure distribution during the molding process of the thermoplastic resin) are used to determine the elastic constant distribution and temperature load distribution (temperature difference distribution) of the injection-molded product. In particular, the elastic constant distribution and temperature load distribution of the injection-molded product can be calculated from the temperature distribution and pressure distribution using a conversion program for computers. Furthermore, the volume shrinkage rate distribution can also be calculated along with the temperature load distribution.

[0035] (Elasticity parameter distribution) The distribution of elastic constants of injection-molded products can be determined by applying temperature dependence data of the elastic constants (generally the proportionality coefficient between stress and strain) of the molding material (thermoplastic resin), which has been acquired in advance, to the temperature distribution obtained by flow analysis. Figure 3 shows the temperature dependence of the elastic constant of a certain thermoplastic resin. The horizontal axis represents temperature, and the vertical axis represents the elastic constant (Young's modulus). The temperature Tc on the horizontal axis represents the transition temperature of the thermoplastic resin. The transition temperature is the temperature at which the solidification region and the molten region are separated, and is also called the no-flow temperature, solidification temperature, solidification temperature, or solid-liquid transition temperature. As shown in Figure 3, if temperature dependence data for the elastic constant of a thermoplastic resin is available, the distribution of the elastic constant can be obtained from the temperature distribution. Furthermore, elastic constants can include three types of values: the modulus of elasticity (Young's modulus) for uniaxial stress, Poisson's ratio, and shear modulus. Using these three types of elastic constants enables more accurate analysis.

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

[0037] (Shear modulus distribution) The shear modulus distribution of injection-molded products can be determined by applying values ​​calculated from temperature dependence data of the elastic constant (Young's modulus) and the temperature dependence data of Poisson's ratio to the temperature distribution obtained through fluid analysis.

[0038] (Volume contraction rate distribution) The volume shrinkage rate distribution of injection-molded products can be determined by applying pre-acquired (measured) data representing the relationship between pressure, volume, and temperature of the molding material (thermoplastic resin) ("PVT data") to temperature and pressure distribution data obtained through fluid analysis. Figure 4 shows PVT data for a thermoplastic resin. The horizontal axis represents temperature (in °C), and the vertical axis represents the reciprocal of density, i.e., specific volume (in cm³). 3The formula is ( / g). Figure 4 shows the relationship between temperature and specific volume when the pressure (P) is 50 MPa. It is assumed that the relationship between temperature and specific volume for several pressures other than 50 MPa has been measured in advance. The relationship between temperature and specific volume for pressures for which no measured values ​​exist can be obtained, for example, by interpolation from the relationship between temperature and specific volume for pressures for which measured values ​​exist. In Figure 4, at a pressure (P) of 50 MPa, the time it takes for the thermoplastic resin at the gate to solidify and stop flowing (referred to as the "gate seal time") is such that the temperature of the thermoplastic resin is 200°C and the specific volume is approximately 0.82 cm³. 3 This indicates that the value is / g. Similarly, at the point when the temperature of an injection-molded product made of thermoplastic resin reaches the mold temperature, the temperature of the thermoplastic resin is 40°C and the specific volume is approximately 0.70 cm³. 3 This indicates that it is / g.

[0039] The volume shrinkage rate distribution of injection-molded products is calculated as follows: First, the temperature (T1) and pressure (P1) at a specific point in time before shrinkage are determined from the temperature and pressure distribution data obtained through fluid analysis. These are then applied to the previously acquired PVT data of the thermoplastic resin to convert them to the volume (V1) at the time before shrinkage. In the example of gate sealing time shown in Figure 4, T1 = 200°C, P1 = 50 MPa, and the specific volume is approximately 0.82 cm³. 3 It is / g. In other words, the volume per unit weight (1g) at this point is V1 = approximately 0.82cm³. 3 That is the case. Similarly, the temperature (T2) and pressure (P2) at a certain point after contraction are determined from the temperature and pressure distribution data obtained from the fluid analysis. By applying these to the PVT data, the volume (V2) at the point of contraction is converted. Furthermore, the point in time after shrinkage can be specified as the time after a set period of time has elapsed since the start of injection (for example, this may be determined by comparing it 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, where the mold has been cooled to its temperature, T2 = 40°C, P2 = 50 MPa, and the specific volume is approximately 0.70 cm³. 3 / g. That is, the volume per unit weight (1 g) at this point is V2 = approximately 0.70 cm 3 . Then, from the volumes (V1, V2) of each element before and after shrinkage, the distribution of the volume shrinkage rate of each element ((volume before shrinkage - volume after shrinkage) ÷ (volume before shrinkage), that is, (V1-V2) / V1) is determined. That is, when the pressure is constant at 50 MPa (P1=P2=50MPa) and cooling is performed from T1=200°C to T2=40°C, the volume shrinkage rate can be calculated as (V1-V2) / V1=(0.82-0.70) / 0.82=0.146, that is, 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 temperature (T1, T2) and specific volume at the pressures (P1, P2) before and after shrinkage is acquired in advance, the volume shrinkage rate (V1-V2) / V1 of each element can be calculated. In this way, the volume shrinkage rate distribution can be calculated from the PVT data, the temperature distribution, and the pressure distribution.

[0040] (Temperature load) The temperature load (temperature difference distribution) is obtained by dividing the volume shrinkage rate of each element by the pre-acquired volume expansion coefficient of the thermoplastic resin to convert it into a temperature difference. The volume expansion coefficient is the coefficient β in the relational expression ΔV / V=βΔT when the original volume V changes by ΔV due to a temperature rise ΔT. In general, the volume expansion coefficient is obtained from PVT data, but when the thermoplastic resin is isotropic, the volume expansion coefficient is three times the linear expansion coefficient. Therefore, instead of the volume expansion coefficient obtained from PVT data, it may be obtained from a pre-measured linear expansion coefficient. The volume expansion coefficient and the linear expansion coefficient are collectively referred to as the thermal expansion coefficient. The thermal expansion coefficient (volume expansion coefficient and linear expansion coefficient) may have temperature dependence. It should be noted that temperature load refers to the "temperature difference before and after shrinkage," and is distinct 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 determining the shrinkage rate distribution. The latter temperature difference, calculated from the temperature data obtained from the flow analysis, simply calculates "temperature only." In contrast, the former temperature load is a value obtained by first calculating the volume from "temperature and pressure" using PVT data and then converting it back to temperature using the volume expansion coefficient. Therefore, it is calculated using "temperature and pressure" rather than "temperature only." Thus, temperature load is a "temperature difference" that takes into account the effect of the pressure that the resin experiences during actual molding. Unlike the difference between the temperature at a certain point before shrinkage and a certain point after shrinkage, using temperature load yields more accurate analysis results.

[0041] (Calculation of strain generated in each element of the analysis model (S5)) In step (S5), which calculates the strain generated in each element of the analytical model, a structural analysis is performed using the distribution of elastic constants (particularly the Young's modulus distribution, Poisson's ratio distribution, and shear modulus distribution) during the cooling process after demolding, and the distribution of volume shrinkage rate and / or temperature load distribution (at least one of the volume shrinkage rate distribution and temperature load distribution) to calculate the strain generated in each element of the analytical model (the distribution of strain amounts in the analytical model). Furthermore, in order to determine the temperature distribution after demolding, it is necessary to determine the initial temperature immediately after demolding, and the initial temperature is determined by the temperature distribution inside the mold. The temporal starting point for strain calculation can be, for example, the time at which the molded product is demolded after holding pressure cooling. The temporal ending point for strain calculation can be, for example, the time at which the injection-molded product reaches room temperature. Alternatively, the temporal ending point for strain calculation can be the time at which a set period of time (which may be set by comparison with the actual process) has elapsed since the start of injection.

[0042] (Prediction of void location and / or void quantity (S6)) In step (S6), which predicts the location and / or amount of voids, the deformation is calculated based on the strain distribution in the analytical model to predict the occurrence of voids. By first predicting the occurrence of sink marks from the distribution of strain, and then analyzing the shape of the resulting sink marks, it is possible to predict the occurrence of voids. Alternatively, without predicting sink marks, it is possible to predict that voids will occur in an element if, for example, the strain in each element of the analytical model exceeds a void occurrence threshold determined in advance by actual measurements for each resin material. Conventional analysis software does not consider the temperature dependence of elastic constants (Young's modulus, Poisson's ratio, and shear modulus) and the coefficient of linear expansion. Therefore, it can only input the elastic constant at room temperature (a single value), and thus fails to adequately consider the changes in the elastic constants of each part when the temperature changes, making it impossible to accurately predict void formation. In contrast, the improved method according to this disclosure can calculate the amount of strain in each part of the molded product based on the temperature distribution and pressure distribution obtained from the flow analysis, using the distribution of elastic constants due to temperature changes of each element (especially the Young's modulus distribution, Poisson's ratio distribution, and shear modulus distribution), as well as the temperature load calculated from the volume shrinkage rate distribution. This improves the prediction accuracy.

[0043] Furthermore, the method of this disclosure allows for the prediction of void formation considering cooling outside the mold by calculating the temperature and pressure distribution of each element of the analytical model during cooling outside the mold (S3), thereby further improving the prediction accuracy.

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

[0045] (Product shape and molding material) Figure 5 shows a 3D CAD model of a bolt-shaped, thick-walled molded product with ribs, according to an example. The molding material (thermoplastic resin) of the actual molded product is Polyplastics' Duracon® 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 determined using 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 fluid analysis in the examples and comparative examples, Autodesk Moldflow® Insight 2019.0.5 (3D solid model) build 20180921.0959_C70L71 (hereinafter referred to as Moldflow) was used. In the examples, the Adventure Cluster2021 (hereinafter referred to as ADVC) manufactured by Allied Engineering Co., Ltd. was used for the mold cooling analysis and structural analysis. In the example, a custom conversion program using Visual Basic® was used to calculate the elastic constant distribution and volume shrinkage rate distribution (S5).

[0047] (Examples) Table 1 shows the molding conditions for the actual molded product and the analysis conditions using this method. In the examples, the physical properties of the thermoplastic resin were set to be the same as those of the thermoplastic resin in the actual molded product. [Table 1]

[0048] Figure 6 also shows a cross-sectional view of the analytical model used in the example.

[0049] In the examples, injection, holding pressure, and in-mold cooling analyses were performed (S2 in Figure 2). From the temperature distribution obtained from the out-mold cooling analysis (S3 in Figure 2), a conversion program was used to obtain the temperature load calculated from the elastic parameter distribution (especially the Young's modulus distribution, Poisson's ratio distribution, and shear modulus distribution) and volume shrinkage rate distribution of the injection-molded product (internal and surface) (S4 in Figure 2). A structural analysis was then performed to calculate the strain and predict the location and amount of voids (S5 and S6 in Figure 2).

[0050] (Comparative example) In the comparative example using the conventional method, voids were predicted from the volume shrinkage rate distribution values ​​calculated by flow analysis, using the same analytical model as the example, the same thermoplastic resin physical properties (excluding temperature dependence of elastic constants, etc.) and the same analytical conditions as the example. As mentioned above, the conventional method does not consider the temperature dependence of elastic constants (Young's modulus, Poisson's ratio, and shear modulus) and linear expansion coefficient, and therefore does not consider the changes in the elastic constants of each part when the temperature changes. In other words, the comparative example shows inferior analytical accuracy for cooling within the mold. In the comparative example, coupled analysis was not performed to predict voids; instead, voids were predicted from the volume shrinkage rate distribution obtained from the flow analysis.

[0051] (Reference example) Furthermore, in the reference example, coupled analysis was performed using the same analytical model as in the example, the same thermoplastic resin properties (including temperature dependence of elastic constants, etc.) and the same analytical conditions as in the example, and voids were predicted from the strain obtained from the structural analysis. The analytical conditions in the reference example are the same as the molding conditions of the actual molded product shown in Table 1. Also, in the reference example, the thermoplastic resin properties are the same as those of the thermoplastic resin in the actual molded product. Note that the reference example does not include analysis of cooling outside the mold; only analysis of cooling inside the mold is performed.

[0052] (Comparison of void predictions) Figure 7 shows a comparison of void generation in an actual molded product, a comparative example using a conventional method, a reference example without out-of-mold analysis, and an example using an improved method. Figures 7A to 7D (hereinafter referred to as Figures 7A to 7D) show, respectively, a comparative example using conventional methods involving only X-ray CT and flow analysis of actual molded products, a reference example in which coupled flow and structural analysis is performed but external cooling analysis is not performed, and an example using the improved method.

[0053] In the comparative example, voids were generated throughout the entire interior of the molded product, resulting in a large discrepancy with the location of voids in the actual molded product. In this embodiment, the void generation locations in the actual molded product and those in the analysis are close together, allowing the actual void generation phenomenon to be accurately represented in the coupled analysis.

[0054] (Methods to reduce voids) This disclosure also includes a method for reducing voids that occur inside injection-molded articles obtained by injection molding thermoplastic resin into a mold. In other words, by the method described above for predicting the void generation behavior , established By changing one or more of the following: total, molding conditions, and molding material Predicting the amount of voids , Predicted Until the amount of voids is reduced to a predetermined amount or less and Try to retaliate.

[0055] This disclosure is not limited to the embodiments described above, but also includes various modifications in which components are added, removed, or replaced from the above-described configuration. Furthermore, each embodiment can be combined in various ways. In particular, this disclosure should not be construed as being limited to the above-described embodiments or examples with respect to shape, material, or conditions.

[0056] Furthermore, the Disclosure also includes a program for causing one or more processors to execute a method for predicting the void generation behavior relating to the Disclosure, or a method for reducing the void relating to the Disclosure. Such program may be provided on a computer-readable, non-transitory storage medium.

Claims

1. A method for predicting the void formation behavior inside an injection-molded product formed by injection molding a thermoplastic resin into a mold, The steps include creating an analytical model by dividing the injection-molded product into multiple elements, A step of determining the temperature distribution and pressure distribution of the analytical model in the process of molding thermoplastic resin, The steps include: using the aforementioned temperature distribution to calculate the temperature distribution of the analytical model during the cooling process after demolding; From both the temperature distribution and the pressure distribution, the step of calculating the elastic constant distribution and temperature load distribution of the analytical model using previously measured temperature dependence data of the elastic constant, thermal expansion coefficient data and PVT data of the thermoplastic resin, A step of calculating the strain generated in each element of the analytical model by performing a structural analysis using the elastic constant distribution and the temperature load distribution during the cooling process after demolding, A step of predicting the location and / or amount of voids from the strain, A method that includes this.

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

3. The method according to claim 1, wherein, in the step of calculating the elastic constant distribution and temperature load distribution of the analytical model, the elastic constant distribution and temperature load distribution are calculated with the gate seal as the starting point and the elapsed time from the start of injection as the ending point.

4. The method according to claim 1, wherein in the step of calculating the strain, the elastic constant distribution and the temperature load distribution are used, taking into account the temperature dependence calculated from the temperature distribution, with the start point being demolding when the injection molded product has cooled under holding pressure, and the end point being the time set from the start of injection.

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

6. The method according to claim 3, wherein the gate seal is at the point when the temperature of the center of the gate reaches the flow stop temperature Ts.

7. The method according to claim 6, wherein the flow stop temperature Ts is determined by using the data fitting coefficients b5 and b6 of the 2-dominant Tait PVT model with respect to the PVT data, and the value of pressure P is given as Ts = b5 + b6 × P.

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

9. The method according to claim 1, wherein in the step of determining the temperature distribution and pressure distribution of the analytical model in the process of molding the thermoplastic resin, the thermal conductivity obtained by a thermal conductivity measurement method called the AC steady-state method (ISO 22007-6) is used in the calculation.

10. A method for reducing voids generated inside an injection-molded product obtained by injection molding a thermoplastic resin into a mold, comprising 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 until the predicted amount of voids is reduced to a predetermined amount or less.

11. A program that causes a computer to perform the method described in claim 1.

12. A program that causes a computer to perform the method described in claim 10.

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