A prediction method for the mold shrinkage rate of flexible plastic parts

Abstract

The present invention provides a method for predicting the mold shrinkage rate of flexible plastic parts, which includes: obtaining the 3D data, material information and molding process of the molded product to generate a finite element model; applying plastic injection molding simulation analysis software to calculate the result output of the selected analysis sequence; using the center measurement method to calculate the preliminary shrinkage rate m; obtaining the coordinate information of the main, secondary positioning and installation points of the molded 3D data; confirming the product installation point compensation amount: taking the shrinkage compensation amount of each installation point as the boundary constraint pre-input condition, processing the output result, and using the center measurement method to complete the confirmation of the final shrinkage rate M. The present invention can reduce the deviation determined by the empirical method, solve the problem that the evaluation of the warping deformation of the part in the free state in the simulation software cannot truly reflect the state after assembly, and provide a more comprehensive evaluation of the warping result.

Description

Technical Field

[0001] The present invention belongs to the technical field of injection molding, and particularly relates to a method for predicting the mold opening shrinkage rate of flexible injection molded parts. Background Art

[0002] Due to the characteristics of easy molding, light weight, excellent chemical stability, etc., plastics are widely used in chemical products. According to research, when the weight of a whole vehicle is reduced by 10%, the fuel efficiency can be increased by 6%-8%. With the application of the technology of replacing steel with plastic, the proportion of plastic products in automotive parts has been increasing year by year. Among them, injection molding is one of the most widely used molding methods, and the main applications on automobiles include covering parts such as bumpers, door interior panels, and instrument panels.

[0003] Injection molding is to inject completely molten plastic materials into the mold cavity at high pressure and high speed, and then obtain the molded product after cooling and solidification. The plastic shrinkage rate refers to the percentage of the difference between the size of a plastic part at the molding temperature and the size after being taken out of the mold and cooled to room temperature, which reflects the degree of size reduction of the plastic part after being taken out of the mold and cooled. Therefore, when designing the mold, it is necessary to consider the influence brought by the above material shrinkage. When opening the mold, the size on the mold is pre-scaled in advance to form a part that meets the size design requirements. The mold opening shrinkage rate of injection molded parts directly affects the size range of the molded product. How to accurately predict the mold opening shrinkage rate is a key indicator related to the success of part design.

[0004] The mainstream methods for confirming the mold opening shrinkage rate of traditional injection molded parts mainly include: 1. Based on past project experience, referring to the shrinkage rate data of parts in previous projects for confirmation, but there will be deviations from the actual shrinkage rate due to different part structure designs and processes; 2. Using the mold of parts with similar shapes and trial-producing with the actual production materials, and comparing the measurement results to confirm the shrinkage rate. The effect is better than the first method, but there are still deviations. 3. Adjust the shrinkage rate of the material to match the mold shrinkage rate after molding to achieve the design size, but this will lead to the coexistence of multiple small grades of the same material, resulting in chaotic logistics and storage management. Analyzing from the influencing factors of the mold opening shrinkage rate, the shrinkage rate of the product after injection molding is affected by multiple factors such as the structure design, material, wall thickness, injection molding machine model, and molding process of the product, and there is a certain interaction between various factors. Traditional methods cannot accurately confirm the mold shrinkage rate, and the mold shrinkage rate does not match the actual shrinkage rate of the product, resulting in the product not reaching the designed size requirements and causing size matching problems. Summary of the Invention

[0005] In order to solve the actual technical problems, the present invention proposes a method for predicting the mold opening shrinkage rate of flexible plastic parts, reducing the deviation determined by the empirical method, solving the problem that the evaluation of the warping deformation of parts in the free state in the simulation software cannot truly reflect the state after assembly, and providing a more comprehensive evaluation of the warping results.

[0006] The technical solution of the present invention is as follows:

[0007] The present invention proposes a method for predicting the mold opening shrinkage rate of plastic products, especially flexible injection molded parts. The method includes the following steps:

[0008] Step 1: Obtain the 3D data, material information, and molding process of the mold opening product; among them, the holding pressure and holding time in the molding process have a gradient range, and the 3D data of the product is preprocessed to generate a finite element model after eliminating the fillets below R2.

[0009] Step 2: Apply plastic injection molding simulation analysis software, import the finite element model generated in Step 1, and calculate the result output under the sequence; the selected analysis sequence is one of "filling + holding pressure + warping" or "filling + holding pressure + warping + cooling".

[0010] Step 3: Process the output result, and calculate the preliminary shrinkage rate m by the center measurement method;

[0011] Here, the present invention designs a method, abbreviated as the center measurement method. The center measurement method is to process the warping analysis deformation result, center it in the product length direction, select the position points at 1 / 2 and 4 / 5 positions on the left and right, and select the position points at 1 / 3 and 2 / 3 positions on the upper and lower. According to the formula: shrinkage rate = (design distance / shrinkage distance - 1), calculate the shrinkage rates at the four positions of ad, bc, eh, and fg respectively, and obtain the average value to get the shrinkage rate m.

[0012] Preferably, for the mesh preprocessing of the 3D data of the mold opening product, select the element types of neutral plane elements, double-sided elements, and 3D elements. For large flat parts with uniform wall thickness, neutral plane element meshes are recommended. For parts with uneven wall thickness, 3D element type meshes are recommended. The matching rate of the selected double-layer mesh elements should be greater than 90%;

[0013] The material data should at least include viscosity data, PVT data, mechanical property data, crystallization morphology data, filling data, stress-strain data, thermal property data, and shrinkage property data, and the shrinkage property should include the average shrinkage rates in the modified parallel and perpendicular directions, different shrinkage rate data under different processes, wall thickness, etc. The processing process parameters include melt temperature, mold temperature, filling time, filling volume, holding pressure, cooling time, and total molding cycle.

[0014] Step 4: Obtain the main, secondary positioning, and installation point coordinate information P0(x0, y0, z0), P1(x1, y1, z1)... Pi(x i , y i , zi )。

[0015] Step 5: Confirm the compensation amount of the product installation point: Assume that the coordinates of the main positioning installation point are P0(x0, y0, z0), and the coordinates of the Pi installation point are (x i , y i , z i ). Calculate the compensation amount of each installation point according to the following formula:

[0016] △X i =(x0 - x i )×m; △Y i =(y0 - y i )×m; △Z1=(z0 - z i )×m.

[0017] Step 6: Use the shrinkage compensation amount of each installation point as the boundary constraint pre-input condition. The selected analysis sequence is one of "Fill + Hold + Warp" or "Fill + Hold + Warp + Cool", and it is consistent with the previous analysis selection.

[0018] Step 7: Process the output result, and use the center measurement method to confirm the final shrinkage rate M; after the constraints of each installation point, the calculation result is that the free-state deformation is converted into the warpage deformation in the assembly design state, that is, input the overall compensation shrinkage rate to eliminate the normal shrinkage deformation part, so that the display result is converted from the free-state deformation to the warpage deformation in the assembly state. Measure the shrinkage rate in this state to guide the setting of the mold opening shrinkage rate.

[0019] Furthermore, the injection molding process includes melt temperature, mold temperature, filling time, filling volume, holding pressure, cooling time, and the total molding cycle.

[0020] Furthermore, Step 1 includes:

[0021] (1) Import the geometric model of the part to be analyzed into the mold flow analysis software for geometric processing. The processing process should take into account both calculation efficiency and geometric integrity. Fillets with a radius greater than R2 in the model simplification are not allowed to be removed, and the model should truly reflect the geometry and wall thickness information of the part.

[0022] (2) Model the hot and cold runners, gates, and waterway information. The model should truly reflect the dimensional information in the mold;

[0023] (3) Select the material parameters for the actual analysis. The analysis material should be the actual production material grade, and the material test data should include shrinkage correction-related parameters, the material shrinkage rates in the vertical and horizontal directions;

[0024] (4) Molding process confirmation. The injection, holding pressure, and cooling times are confirmed based on the big data statistics of the molding process parameters of previous product production. Among them, the holding pressure parameters (holding pressure and holding time) should have a gradient range to determine the adjustable range of the shrinkage rate. For example, the holding pressure parameters are analyzed with three gradients of ±2s above and below the determined value.

[0025] The beneficial effects of the present invention are as follows:

[0026] By adopting the above technical solutions, the present invention can reduce the deviation determined by the previous empirical method. At the same time, the problem that the evaluation of the warpage deformation of parts in the free state in the simulation software cannot truly reflect the state after assembly is solved in the solution, and the evaluation of the warpage result is more comprehensive. By accurately inputting boundary conditions such as materials and process conditions, the change of the shrinkage rate under the changes of product design, wall thickness, and process conditions is addressed. The simulation of the shrinkage rate of the product in the assembled state can be directly used for the mold opening of the product mold, avoiding the error problem caused by the previous experience-oriented method due to different product structures.

[0027] The technical solution of the present invention is more widely applicable, especially for accurately predicting the mold opening shrinkage rate of flexible injection molded parts with large deformations such as automotive bumpers and narrow decorative strips. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The above and / or additional aspects and advantages of the present invention will become apparent and be readily understood from the description of the embodiments in conjunction with the following drawings, wherein:

[0029] Figure 1 : The principle block diagram of the present invention;

[0030] Figure 2 : Schematic diagram of the shrinkage rate test by the centering method. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0031] The embodiments of the present invention are described in detail below. The main implementation process of the method can be seen in the attached Figure 1 As shown, the described embodiments are illustrative and are intended to explain the present invention and should not be construed as limiting the present invention.

[0032] In this embodiment, some existing commercial software in the prior art can be used to assist in predicting the mold opening shrinkage rate, such as commercial mold flow analysis software (CAE analysis of injection molding). In the following embodiments, the prediction method of the present invention is described by taking the commercial mold flow analysis software as an example. However, obviously, the concept of the present invention is the selection and combination of parameters in the prediction method and the optimized calculation of the prediction model, which is a general and universal method and is not limited to the above specific software.

[0033] See Figure 1, a method for predicting the mold shrinkage rate of plastic products, especially flexible injection-molded parts, and the specific process is as follows:

[0034] Step 1.1: Import the geometric model of the part to be analyzed into the mold flow analysis software for geometric processing. The processing process should consider both computational efficiency and geometric integrity. Fillets with a radius greater than R2 during model simplification are not allowed to be removed, and the model should truly reflect the geometry and wall thickness information of the part.

[0035] Step 1.2: Model the hot and cold runners, gates, and waterway information, and the model should truly reflect the dimensional information in the mold.

[0036] Step 1.3: Select the material parameters for actual analysis. The analyzed material should be the brand of the actual production material, and the material information should include viscosity data, PVT data, mechanical property data, crystallization morphology data, filling data, stress-strain data, thermal property data, and shrinkage property data. The shrinkage property should include different shrinkage rate data under different processes, wall thicknesses, etc. (Table 1) and the average shrinkage rates in the parallel and perpendicular directions after correction (Table 2).

[0037] Table 1: Test data of average shrinkage rate under different wall thicknesses and processes;

[0038]

[0039] Table 2: Material shrinkage rate correction

[0040] Whether the residual stress is corrected Yes Parallel shrinkage rate 0.688 Vertical shrinkage rate 0.8724

[0041] Step 1.4: Confirm the molding process. The injection, holding pressure, and cooling times are confirmed based on the big data statistics of the molding process parameters of previous products. Among them, the holding pressure parameters (holding pressure and holding time) should have a gradient range to determine the adjustable range of the shrinkage rate. For example, the holding pressure parameters are analyzed with three gradients of ±2s above and below the determined value.

[0042] Step 2: After importing the analysis model into the simulation analysis software, select the analysis sequence and complete the analysis;

[0043] Step 3: Analyze the warpage deformation results using the center measurement method. Centered in the product length direction, select the 1 / 2 and 4 / 5 position points on the left and right, and the 1 / 3 and 2 / 3 position points on the top and bottom. Calculate the shrinkage rates of the four distances ad, bc, eh, and fg respectively and obtain the average value m. If the product shape is restricted and points cannot be taken at the above positions, points can be appropriately taken on the plane based on the product shape ratio, such as Figure 2 .

[0044] Step 4: Confirmation of product installation point coordinates. Based on the geometric information (3D data) of the molded product and the surrounding environment, the main positioning F0, secondary positioning F1, and other installation points Fn are respectively confirmed, and the coordinate information P0(x0, y0, z0), P1(x1, y1, z1)……Pi(x i , y i , z i ) etc. are respectively confirmed.

[0045] Step 5: Confirmation of product installation point compensation amount. Assume that the coordinate of the main positioning installation point P0 is (x0, y0, z0), the coordinate of the installation point P1 is (x1, y1, z1), … the coordinate of the installation point PN is (x n , y n , z n ), and the compensation amounts of the installation point PN in the three coordinate directions are respectively: △X n = (x0 - x n ) × m; △Y1 = (y0 - y n ) × m; △Z1 = (z0 - Z n ) × m,

[0046] Calculate the compensation amount of each installation point in this way.

[0047] Step 6: Constraint of product installation points. Enter the boundary constraint interface of the simulation analysis software, select, and perform constraints on the installation points of the product. The direction in which the degrees of freedom of the installation points are released is not input.

[0048] Step 7: Post-processing of the warpage deformation results of three pressure-holding process gradients. The deformation results include the deformation under normal shrinkage. After inputting the value m in the shrinkage compensation, refer to Step 3 to take points and calculate the average shrinkage rate M.

[0049] Recommend the shrinkage rate M confirmed by the simulation to the mold supplier for mold design.

[0050] The preferred embodiments of the present invention have been described in detail above in conjunction with the accompanying drawings. However, the present invention is not limited to the specific details in the above embodiments. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solutions of the present invention, and these simple modifications all fall within the protection scope of the present invention.

[0051] In addition, it should be noted that, in the above specific embodiments, the various specific technical features described can be combined in any suitable manner without conflict. To avoid unnecessary repetition, the present invention does not separately describe various possible combination methods. In addition, any combination can be made between various different embodiments of the present invention, as long as it does not violate the idea of the present disclosure, it should also be regarded as the content disclosed by the present invention.

Claims

1. A prediction method for the mold shrinkage rate of a flexible plastic part, characterized in that, Including: Step 1: Obtain the 3D data, material information, and molding process of the molded product; Among them, the holding pressure and holding time in the molding process have a gradient range. The 3D data of the product is preprocessed to generate a finite element model after removing fillets below R2; Step 2: Apply plastic injection molding simulation analysis software, import the generated finite element model, and calculate and output the results under the analysis sequence; The selected analysis sequence is one of "Fill + Hold + Warp" or "Fill + Hold + Warp + Cooling"; Step 3: Process the output results and calculate the preliminary shrinkage rate m using the center measurement method; The center measurement method is to process the warping analysis deformation results. Centered in the length direction of the product, select the 1 / 2 and 4 / 5 position points on the left and right, and select the 1 / 3 and 2 / 3 position points on the top and bottom. According to the formula: shrinkage rate = design distance / post-shrinkage distance - 1, calculate the shrinkage rates at the four positions of ad, bc, eh, and fg respectively, and obtain the average value to get the shrinkage rate m; Step 4: Obtain the primary, secondary positioning and installation point coordinate information P0 (x0, y0, z0), P1 (x1, y1, z1) …… Pi (x i , y i , z i ); Step 5: Confirm the compensation amount of the product installation point: Assume that the coordinates of the main positioning installation point are P0(x0, y0, z0), and the coordinates of the Pi installation point are (x i , y i , z i ). Calculate the compensation amount of each installation point according to the following formula: △X i = (x0 - x i ) × m; △Y i = (y0 - y i ) × m; △Z1 = (z0 - z i ) × m Step 6: Use the shrinkage compensation amount at each installation point as the boundary constraint pre-input condition. The selected analysis sequence is one of "Fill + Hold + Warp" or "Fill + Hold + Warp + Cooling", and it is consistent with the previous analysis selection; Step 7: Process the output results and confirm the final shrinkage rate M using the center measurement method; After constraining each installation point, the calculation result is the warping deformation converted from the free state deformation to the assembly design state. The shrinkage rate is measured in this state to guide the setting of the mold opening shrinkage rate.

2. The prediction method for the mold opening shrinkage rate of the flexible plastic part according to claim 1, characterized in that, Preprocess the 3D data of the product to generate a finite element model. The selectable model types include midplane elements, double-sided elements, and 3D elements; For large flat parts with uniform wall thickness, midplane element meshes are recommended. For parts with uneven wall thickness, 3D element type meshes are recommended. For the selected double-layer mesh elements, the matching rate should be greater than 90%.

3. The prediction method for the mold opening shrinkage rate of the flexible plastic part according to claim 1, characterized in that, The data included in the material information should at least include viscosity data, PVT data, mechanical property data, crystallization morphology data, filling data, stress-strain data, thermal property data, and shrinkage property data. The shrinkage property should include the average shrinkage rates in the corrected parallel and perpendicular directions, and different shrinkage rate data under different processes and wall thickness conditions.

4. The prediction method of the mold opening shrinkage rate of the flexible plastic part according to claim 1, characterized in that, The molding process includes melt temperature, mold temperature, filling time, filling volume, holding pressure, cooling time, and total molding cycle.

5. The prediction method for the mold opening shrinkage rate of the flexible plastic part according to claim 1, characterized in that, The said Step 1 includes: (1) Import the geometric model of the part to be analyzed into the mold flow analysis software for geometric processing. The processing process should consider both calculation efficiency and geometric integrity. Fillets with a radius greater than R2 in the model simplification are not allowed to be removed, and the model should truly reflect the geometry and wall thickness information of the part; (2) Model the hot and cold runners, gates, and waterway information. The model should truly reflect the dimensional information in the mold; (3) Select the material parameters used in the actual analysis. The analysis material should be the actual production material grade, and the material test data should include shrinkage correction related parameters and the material shrinkage rates in the vertical and horizontal directions; (4)Molding process confirmation. The injection, holding pressure, and cooling times are confirmed based on the big data statistics of the molding process parameters of previous product production. Among them, the holding pressure parameters have a gradient range, which is used to determine the adjustable range of the shrinkage rate. The holding pressure parameters are analyzed with three gradients of ±2 s taken above and below the determined value. The holding pressure parameters are the holding pressure and the holding time.

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