Research status of warping deformation of the hott

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Research status of warpage deformation of thin-walled parts at home and abroad

warpage deformation is one of the common defects in injection molding of thin-walled plastic parts, because it involves the accurate prediction of warpage deformation, and the warpage deformation laws of injection molded parts of different materials and shapes are very different. The existence of warping deformation will affect the shape accuracy and surface quality of injection molded parts. When the warping deformation exceeds the allowable error, it will become a forming defect, and then affect the product assembly. Accurate prediction of the warping deformation of a large number of increasingly thin-walled parts (wall thickness less than 2mm) is the premise of effectively controlling warping defects. For a long time, people have done a lot of work in warping prediction, and the common research methods are experimental method and theoretical analysis method

research status of experimental methods

using experimental methods to study the warpage deformation of plastic products is mainly reflected in the study of the effects of material properties, product geometry and size, injection molding process conditions, etc. on the warpage deformation of products. As early as the 1960s, vleo and relliez designed a large number of experiments to obtain the effects of gate geometry, packing parameters (packing pressure and packing time) and mold elasticity on the final size of products. C and sin used Nylon6 and pet as polymer matrix to study the warpage characteristics of plates with different materials and wall thicknesses. Hiroyukikikiehi and kiyohitokoyama et al. Experimentally studied the relationship between the reinforcement ratio, anisotropy of linear thermal expansion coefficient, product thickness and warpage of 33% glass reinforced fiber PA66 injection molded disk, and proposed the concept of warpage index for the first time. They used warpage index to study the warpage characteristics of PA66 plastic products, and studied the relationship between warpage index, warpage and fiber orientation state, and the relationship between yield and warpage index

Figure 1: the shell is a typical thin-walled injection molded part

hy, etc. use the disk to test the mechanism of warping deformation of reinforced plastic products after mold out, and put forward the experimental formula of the disk being arched or saddle shaped deformation. Based on a large number of experimental data, ay and den established the experimental relationship between residual stress and warpage, so there is no need to consider ignoring the viscoelastic behavior of plastic and assuming that the material properties are independent of temperature during plastic curing, which will bring errors to warpage prediction. Radford and Diefendorf et al. Experimentally studied the deformation of composites during molding and use. They proposed a mathematical model based on the classical layered thin plate theory to predict the shape change of products

experimental methods to study warping deformation are often limited to a specific geometry, specific materials and process conditions, and can not fully consider the impact of many factors on warping deformation, nor can they predict the size of warping deformation that may occur in the product design stage. In practical use, the limitations of empirical formulas are also obvious, which are not only affected by experimental conditions, but also related to many factors such as the processing method of experimental data, the application conditions of empirical formulas, and an empirical formula is only applicable to production processes that are quite close to the experimental conditions

research status of theoretical analysis

before the 1970s, most of the warpage deformation analysis used qualitative analysis. Based on practical experience, measures were taken from the aspects of product design, mold design and injection molding process conditions to avoid large warpage deformation as far as possible. With the development of solid mechanics, computer science, numerical technology, material science and other disciplines, more and more scholars began to study the warpage of injection molded products theoretically

Research on shrinkage/warpage

because warpage deformation is related to uneven shrinkage, many scholars start with studying the shrinkage behavior of different plastics under different process conditions to analyze the relationship between shrinkage and product warpage. Based on the simulation of injection flow, pressure holding and cooling, Thomas, cffery proposed a model to predict the shrinkage of injection molded products through experiments and linear regression methods. On the basis of shrinkage prediction, the deformation of the products was calculated through the structural analysis simulation program

les et al. Conducted a detailed study on the shrinkage and warpage behavior of GMT materials. They believed that the fiber orientation of GMT materials would occur during the molding process. The joint effect of the different shrinkage characteristics between resin and fiber and the anisotropy formed after fiber orientation was the main reason for the warpage of products. They verified the influence of fiber orientation by changing the fabric method of raw materials with different fiber orientation, It is found that there is a certain correlation between shrinkage and warpage of products. When semmer establishes a mathematical model of shrinkage and warpage of plastic products, fiber orientation, process temperature, p-vt characteristics of materials and two special effects that can lead to shrinkage and warpage (i.e. heat island effect and rebound effect) are taken as the calculation basis

rich sail believes that it is difficult to obtain products with high dimensional accuracy with materials with high shrinkage. Strive for high accuracy, and try to use amorphous resins and resins with consistent shrinkage in all directions. Moldflow company measures the shrinkage of products for many materials under the conditions of changing parameters such as flow speed, pressure holding pressure, pressure holding time, mold temperature, mold filling time, product thickness and so on. According to the test results, the shrinkage of the product is divided into three parts: volume shrinkage, uneven shrinkage caused by molecular orientation, and uneven shrinkage caused by unbalanced cooling. On this basis, the wals electro-hydraulic servo hydraulic universal material experimental machine h, which is evaluated by nature as "a very promising material for printing 3D tissues and artificial organs", proposes a shrinkage prediction method that can consider more basic variables (volume shrinkage, crystal content, mold restrictions, plastic orientation, etc.), and uses the results of flow and cooling analysis to predict shrinkage strain. Walsh, Peter, etc. calculated the warpage of products on the basis of shrinkage prediction. Using the calculated shrinkage strain, the shrinkage strain is input into the result analysis program. After being equivalent to the node load, the warpage deformation of the product can be obtained through linear or nonlinear analysis. At the same time, the main and secondary factors that affect the warpage deformation of specific plastic products can be separated, which has great guiding significance for practice

Figure 2: some medical products also need transparent thin-walled functions

because there are many factors affecting shrinkage, it is difficult to predict shrinkage in theory, so most of the work is based on experimental research, and the experimental shrinkage model is proposed according to the experimental results, and then the warpage deformation is calculated

Research on stress/warpage

during the molding process, the uneven orientation and shrinkage of plastic melt lead to uneven internal stress, so the warpage deformation occurs under the action of uneven internal stress after the product is molded. Therefore, many scholars analyze and calculate the internal stress and warpage of products from the perspective of mechanics. In some foreign literatures, warpage is considered to be caused by residual stress caused by uneven shrinkage

in the cooling stage of injection molding, when the temperature is higher than the glass transition temperature, the plastic is a viscoelastic fluid with stress relaxation: when the temperature is lower than the glass transition temperature, the plastic becomes solid. The liquid-solid transition and stress relaxation of plastics during cooling have great influence on the accurate prediction of residual stress and residual deformation of products. Osswald proposed a 3-D elastic phase transition model when predicting the warpage of compression mold parts. Matsuo Ka et al. Used a simple elastic model, combined with the integrated analysis of the injection molding process, to predict the warpage of reinforced fiber plastic products. Chiang et al. Used both LRW model and pure hydroelastic model to predict the shrinkage and warpage of injection molded products. Kabanami and crochet used a 3-D viscoelastic model to predict the residual stress and final shape of injection molded products. Shih jungliu considered the phase transition and stress relaxation behavior of plastic from liquid to solid during cooling. For the uncured area, the plastic presents viscous behavior, which is described by the viscous fluid model. For the cured area, the plastic presents viscoelastic behavior, which is described by the standard linear solid model. The viscoelastic phase transition model and two-dimensional finite element method are used to predict the thermal residual stress and the corresponding warpage deformation

the crystallinity of plastics also affects the residual stress and warpage deformation of products. Among them, the more mature method is adopted by Ang and AUR. They use the improved Tait equation to describe the pressure volume temperature relationship of crystalline plastics, use the Malkin crystallization dynamics to describe the crystallization behavior of plastics, use the linear thermal viscoelastic model to calculate the flow residual stress and thermal residual stress, use the calculated residual stress as the initial condition of solid mechanics analysis, and use the three-dimensional finite element method to calculate the warpage. The shrinkage and warpage of crystalline plastic polypropylene and amorphous ABS plate are predicted by the above method, and the results are compared

bushiro and Stokes used the method of nominal variables to study the influence of a wide range of parameters from material parameters to process parameters on the warpage of flat products. Assuming that the material is a simple thermoviscoelastic material in thermorheology, and ignoring the influence of flow, the mechanism of product warpage in injection molding is constructed by using amorphous thermoplastic melt layer in parallel to ensure the stability of the controller and solidification between cold plates

Jacques simulates the thermal warpage of amorphous plastic plates of automobiles caused by uneven cooling. After analyzing the heat transfer process of injection molding by one-dimensional finite difference method, the product is divided into multiple layers. According to the different temperatures of each layer, the stress after glass transition is calculated, and the warpage is calculated by using pure bending theory. This method can analyze the warpage deformation of simple products under the action of thermal stress. Tamma et al. Used the traditional finite element method and infinite element method to analyze the heat transfer of products, and used the bending theory of beams to calculate the thermal residual stress and warpage deformation caused by temperature changes. Takaakimatsuoka et al. Integrated mold cooling, plastic filling pressure maintaining cooling, fiber orientation, material properties and stress analysis to predict warpage by using the geometric model of three-dimensional thin-walled injection parts

Figure 3: thin-walled plastic shell of notebook

hiroyuki Kikuchi and Kiyohito Koyama use finite element method to calculate warpage. First, the flow field of melt and fiber orientation are calculated, then the thermal stress is calculated, and finally the warpage is calculated by using nonlinear structure analysis software Marc. Shen Changyu of Zhengzhou Institute of technology and other scholars put forward the concept of warpage deformation coefficient, mainly considering the warpage deformation caused by uneven temperature distribution, and calculated the warpage deformation coefficient by numerical calculation method. Then, the temperature stress is calculated by using the elastic non deformation theory and the finite element method, and the warpage is calculated by using the structural analysis program of Dalian University of technology. Wujianjun of Northwestern Polytechnic University and others solved the residual stress of plastic parts with one-dimensional viscoelastic constitutive equation, and solved the deformation of articles with cantilever beam theory. Dr. Lu Yiqiang from Huazhong University of technology analyzed the warping deformation of products with thin plate theory, regarded the in-plane deformation of products as a plane stress problem, and the deformation oriented as a thin plate bending problem. After the superposition of the two kinds of deformation, the finite element method of plane problem and thin plate bending problem was used to calculate the deformation of products in three-dimensional space coordinates

some scholars also considered the influence of the packing pressure on the orientation of plastic, the distribution of residual stress and the final deformation of products during the packing process. Kabanemi, crochet et al. Proposed a true three-dimensional method to calculate residual stress and final shape (shrinkage and warpage). They considered the influence of the packing stage, divided the product into three layers, and analyzed the residual stress and deformation by three-dimensional lattice.

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