Warpage deformation is one of the common defects in the injection molding of thin-shell plastic parts. The warpage deformation rules of injection molded parts with different materials, shapes and different forming processes are very different. The existence of warping deformation problems will affect the shape accuracy and surface quality of the injection molded parts, and even become molding defects, which will affect the assembly and appearance of the products. Warpage is one of the important features of plastic deformation, and its research has important application value. Using numerical simulation technology to study the injection molding of parts and reduce the amount of warpage of plastic parts is of great significance for improving the precision of injection molding products, shortening the development cycle of new products, reducing costs and increasing productivity.
1. Warpage deformation and its influencing factors
(1) Warpage deformation and its research
Warpage deformation is a common defect in injection molded articles. Warpage deformation can be divided into two forms, namely stable warping and unsteady warpage. Stable warping means that warpage deformation and shrinkage should be proportional. Unsteady warpage means that the product is bent due to excessive plane strain, and generally this warpage is large.
Before the 1970s, most of the analysis of warpage deformation used qualitative analysis. With the development of numerical technology, materials science and other disciplines, the study of warpage theory of injection molded products is more in-depth. Because warping deformation is related to uneven shrinkage, many scholars have studied the shrinkage behavior of different plastics under different process conditions to analyze the relationship between shrinkage and product warpage. During the molding process, due to the uneven orientation and shrinkage, the internal stress is not uniform. Therefore, after the product is released from the mold, warping deformation occurs under the action of uneven internal stress. Therefore, many scholars analyze and calculate the internal stress and warpage of the product from the perspective of mechanics. In some foreign literature, warpage is considered to be caused by residual stress caused by uneven shrinkage.
(2) Factors affecting warpage deformation
For the warpage analysis, there are three main reasons for the warpage deformation: 1 uneven cooling. Uneven cooling results in a large temperature difference between the articles, which causes the secondary shrinkage values ​​of the articles to differ greatly after ejection. This difference in shrinkage results in the generation of bending moments that cause the article to warp. 2 area contraction. Area shrinkage refers to the difference in shrinkage between different areas leading to warpage of the product. In general, regional shrinkage can be used to measure the difference in shrinkage of different regions of a product, while molecular orientation is used to measure shrinkage in different directions of the article. 3 molecular orientation. Molecular orientation causes inconsistencies in the contraction of the parallel flow direction and the vertical flow direction, which causes warpage. For fiber reinforced plastic products, the orientation effect is one of the main causes of warpage of the product.
There are many factors that affect the warpage deformation, which are related to the entire injection molding process and the mold structure, and these effects are complicated. The main process parameters involved in the whole process of injection molding are: melt temperature, mold temperature, filling time (injection speed), injection pressure, holding pressure, holding time and cooling time. These parameters all affect the quality of the product. The effect of each process parameter on warpage is different. It is of great significance to analyze the influence degree of each factor on the warpage to improve product quality and warp optimization design. Through this analysis, the less influential factors can be removed from the optimized design variables, which reduces the amount of computation, saves time, and improves the effectiveness of the optimized design.
The main measures to reduce warpage are: modify the geometry of the part; modify the mold structure (including the design of the gating system, cooling system and ejector system); adjust the process parameters and so on. In actual production, the geometry of the part and the mold structure have been fixed, so it is more practical to adjust the process parameters to reduce the warpage.
2. Numerical simulation using orthogonal test method
(1) Orthogonal test method
It is a scientific and effective method for researching and processing multi-factor experiments. It uses orthogonal tables to conduct overall design, comprehensive comparison and statistical analysis of the test, and achieves better production conditions through a few trials to achieve the highest production. Process effect. The orthogonal table can equalize the sampling within the range of factors, so that each test has a strong representativeness. Because the orthogonal table has the characteristics of balanced dispersion, it guarantees some requirements of the comprehensive test. These tests are often better or Better achieve the purpose of the test. China has made great progress in the theoretical research of orthogonal test methods, so that this scientific method can serve economic development.
(2) Injection molding CAE
Injection molding CAE establishes an effective numerical calculation method by establishing mathematical and physical models of the plastic molding process, realizing dynamic simulation analysis of the molding process, and making the understanding of the plastic molding process from macroscopic to microscopic, qualitative to quantitative, and static to dynamic. Provides scientific basis and design analysis tools for optimizing mold design and controlling the product forming process to achieve the desired end product. The use of CAE technology is an important milestone in the development of plastic molding technology. Injection molding CAE technology generally has flow analysis, pressure holding analysis, cooling analysis, shrinkage/warpage deformation analysis, etc. Warpage deformation analysis can complete the identification of shrinkage and warpage, and can improve the dimensional accuracy of products and reduce The influence of residual stress on the final use performance of the product provides engineering technicians with reference data for application design, mold design and molding process.
3. Warpage simulation analysis and process optimization of plastic parts
(1) Basic situation of plastic parts
The plastic part is a kind of cover-type part, the material is PA-709AABS plastic, the average wall thickness is 2mm, it is the appearance part, the plastic part has the assembly structure, the plastic part extension and the plane requirement are straight. As shown in Figure 1.
Figure 1 Plastic mesh model
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