Plastics are used more and more widely in automobiles, mainly including interior parts, exterior parts and functional structural parts. Automobile water chamber is a functional structural part of radiator. Its structure is complicated due to assembly of multiple parts, and product quality requirements are high. In addition to avoiding defects such as insufficient filling, flashing, air pockets, requirements for dimensional stability, sealing and flatness of assembly surface are relatively high. Automobile water chamber is a thin-walled long strip with an asymmetrical structure with bulges in the middle. After actual production, it is found that amount of warpage deformation is larger than that of conventional size products, there is a big difference between amount of warpage deformation and simulation. Therefore, warping deformation becomes the most important molding defect of water chamber. How to effectively control warpage and deformation of injection-molded long strip products is a technical problem that needs to be solved urgently, has attracted attention of experts and scholars in related fields. At present, control of warpage and deformation of plastic products focuses on following two aspects.
(1) Simulation optimization analysis, which is the most commonly used and studied method. There are many factors that affect warpage and deformation. Whether optimizing product structure and injection process, or optimizing casting system and cooling system of mold, although it can reduce warpage of molded product, for complex structure of automobile water chamber, due to complexity of model establishment, boundary condition determination, and process parameter selection, optimization results of this forward design are quite different from actual situation, and it is difficult to meet requirements. This method is suitable for situations where product structure is relatively simple and accuracy requirements are not high.
(2) Adopt pre-deformation measures and carry out reverse deformation design on products to achieve purpose of reducing warpage deformation. Domestic scholars have conducted research on pre-deformation compensation technology of some unequal thickness and thin-walled products under specific injection process conditions, and achieved good results. Zhang Bin carried out numerical simulation on box products to obtain an anti-deformation model, carried out retrograde modeling to reduce warping deformation of model. Under premise of optimizing injection process, Zeng Lei performed pre-deformation compensation for unequal thickness products through numerical simulation to reduce amount of warpage deformation. Huang Fengli proposed a robust and optimized pre-compensation method for product deformation. Huang Jiajia analyzed structural rigidity of carbon tank cover through comparison between physical object and CAE simulation, solved warpage deformation problem through pre-deformation method, and product met production requirements.
Under optimized injection process conditions of automobile water chamber, pre-deformation compensation technology is adopted to control warpage deformation by means of error offsetting, so as to meet molding quality requirements of product.

 

Figure 1 Model of car water chamber
Research object is automobile water chamber shown in Figure 1, with a mass of about 244g and an external dimension of 136mm*45mm*74mm. The whole is elongated, hollow wall is thin, and wall thickness is 2.7mm; there are 3 protruding features and 2 buckles on the left and right sides, ribs with a spacing of 16mm are distributed on the left and right sides of base. Bottom surface of water chamber is assembly surface, and its flatness is required to be within 1mm.

 

Table 1 Material parameters of automobile water chamber
Material of automobile water chamber is PA66+30% glass fiber, which has high strength, low permeability, heat resistance and aging resistance. Material parameters are shown in Table 1.

Setting of process parameters firstly takes into account factors such as production efficiency and production cost under premise of ensuring that product has no appearance defects. Through theoretical simulation and optimization, combined with actual working conditions and working conditions, injection process parameters are determined as follows: injection cycle is 61s, melt temperature is 290℃, injection time is 2.52s, cooling time is 25s, mold temperature is 60℃, holding time is 3s, and holding pressure is 50MPa.

 

Figure 2 Overall waterway layout

 

Figure 3 Movable waterway layout of right water chamber

 

Figure 4 Layout of fixed mold waterway in the right water chamber
Import processed model into MoldFlow for meshing, creating a gating system and a cooling system. Automobile water chamber is a functional part, and no weld marks are allowed to appear on assembly surface. Therefore, only one gate can be used, end pouring method that combines hot runner and ordinary runner is adopted. Diameter of cooling water path is φ10mm, and a water path with a diameter of φ8mm is added to raised features to avoid uneven cooling. The overall waterway layout is shown in Figure 2. Mold has a structure of 2 cavities. Now only right water chamber is analyzed. Specific layout of water circuit is shown in Figure 3 and Figure 4.

In addition to mold structure, cause of product warpage also includes factors such as uneven cooling, uneven shrinkage, uneven orientation, and corner effects. For long-shaped water chamber products, it is difficult to control warpage and deformation to meet accuracy requirements only through optimization of injection process. Therefore, for complex structural parts such as water chambers, it is planned to adopt pre-deformation compensation technology to control warpage deformation by means of error compensation to meet molding quality requirements of product.

 

Figure 5 Pre-deformation design steps
Pre-deformation compensation method is to preset amount of deformation in opposite direction on the basis of initial design of product to achieve purpose of offsetting or reducing deformation of product after mold is released, so that product reaches initial design state, and then mold is opened according to reverse model to form a product that meets quality requirements. Therefore, it is the key to accurately predict pre-deformation direction, trend and pre-deformation amount of water chamber. Now take actual deformation amount of water chamber as a reference, simulate actual situation through MoldFlow simulation, and consider accuracy of simulation and other factors to determine pre-deformation direction and pre-deformation of car's water chamber. According to above principles, pre-deformation design steps are shown in Figure 5.

 

Figure 6 Location of measuring point of car's water chamber
Mold is opened based on 3D model of automobile water chamber, considering influence of internal stress on product deformation and moisture absorption of nylon material during molding, product reaches a stable state after 24 hours. Under stable process conditions, molded product is placed in a room temperature environment for 24 hours, anchor plane is found by selecting 4 points, then different positions are selected, and distance between point and anchor plane is measured to obtain deformation of water chamber.
Position of measuring point is shown in Figure 6. Distance of first point in horizontal direction is 5.8mm, distance of second point is 9mm, and distance of middle point is 15mm. The first point in vertical direction is 6mm and middle distance is 3mm. This model has 76 measuring points and maximum deformation after measurement is 1.24mm.

According to actual production process of automobile water chamber, simulation is carried out in MoldFlow. After cooling + filling + pressure holding, deformation result of automobile water chamber is obtained, same measurement position as actual product is established to detect deformation of 76 measuring points on assembly surface of water chamber.

 

Figure 7 Comparison of actual deformation of water chamber and simulated CAE deformation
Simulation technology has an important guiding role in pre-deformation design. When pre-deformation is not used, comparison between simulated deformation C2 of automobile water chamber and actual deformation C1 is shown in Fig. 7, deformation deviation ΔC=C1-C2. It can be seen from Figure 7 that warpage trend obtained based on MoldFlow simulation is consistent with deformation trend measured by actual object, although values of the two in corresponding positions have a slight difference, they are mainly concentrated on both sides of water chamber, and difference is about 0.2mm, but peak position is basically consistent with maximum deformation. Point 15 is the first peak maximum deformation -1.09mm, actual deformation is -1.12mm, point 53 is second peak, maximum deformation is -1.29mm, and actual deformation is -1.25mm. Point 15 and point 53 are in corresponding positions on model, and are also positions where model has the thinnest wall thickness, where deformation is the largest in terms of geometric structure.
Above analysis shows that adopted CAE simulation analysis results are more consistent with actual warpage deformation results, trends are consistent, indicating correctness and effectiveness of  simulation analysis; without pre-deformation, maximum deformation exceeds design accuracy, indicating that molded product cannot meet quality requirements. Therefore, pre-deformation measures are needed to solve problem of excessive deformation.

 

Figure 8 Pre-deformation compensation
Pre-deformation compensation is shown in Figure 8. Choose a line on assembly surface of water chamber model, f0 represents ideal position, f1 represents position after injection molding without pre-deformation, and f2 represents position before pre-deformation injection molding. Position, f0’ represents position after pre-deformed injection molding. Select any point d on line, and vector displacement passing through position of point d can be expressed by following formula.

 

From equations (1) to (3), it can be seen that value of pre-deformation coefficient λ determines shape of f2. Practice has shown that actual deformation of product is caused by interaction of multiple factors. Shape of f2 also needs to refer to actual deformation of car's water chamber and simulated deformation. At the same time, it also needs to refer to main deformation trend of water chamber, accuracy of simulated deformation and its structural characteristics to make some adjustments.

 

Figure 9 Pre-deformation compensation design
In actual design process, center position of anchor plane is taken as design center to control deformation of assembly surface of water chamber. When λ is too large, it is easy to overcompensate and lead to overcorrection; when λ is too small, pre-deformation compensation will not have desired effect. According to scholars' research on compensation of pre-deformation of long strip or thin-walled products, when λ ∈ [0.6,1], deviation of f0' and f0 is small. There are errors between CAE simulation and actual conditions, pre-deformation will also change product structure and melt flow, resulting in different deformations. Due to complexity of product end pouring method and product shape characteristics, considering that product wall thickness is 2.7 mm and maximum deformation is 1.29 mm, value of λ is temporarily set to 0.8. Protruding feature on the left side of water chamber has a greater impact on deformation, error between measured value of measuring point and simulated deformation is large. Combining actual measurement value and simulated deformation amount to carry out pre-deformation design of water chamber model, while ensuring that pre-deformation model and theoretical design size are same to obtain pre-compensated automobile water chamber pre-deformation model. Actual deformation of pre-deformed model at measurement position is shown in Figure 9.

 

Figure 10 Comparison of simulation deformation before and after pre-deformation
Pre-deformed automobile water chamber model is simulated and analyzed in MoldFlow according to above process conditions, simulated deformation amount C 3 of assembly surface of automobile water chamber after pre-deformation is obtained, compared with simulated simulated deformation amount C 2 obtained without pre-deformation, as shown in Figure 10. Results show that peak value of simulated deformation C 3 after pre-deformation is greatly reduced, maximum deformation is 0.46 mm, and deformation of other measuring points is within 1 mm.

 

Figure 11 Actual car water chamber

 

Figure 12 Comparison of actual deformation
Mold design is completed according to pre-deformation design, and actual car water chamber produced is shown in Figure 11. Same detection method is used to obtain actual deformation C4 of car water chamber after pre-deformation, compare it with deformation C1 without pre-deformation. Result is shown in Figure 12. It can be seen from Figure 12 that warpage deformation C4 of automobile water chamber after pre-deformation is improved significantly, and maximum deformation is 0.47mm, which meets accuracy requirements of automobile water chamber assembly surface.
Comparing CAE simulation and actual measurement data of automobile water chamber under actual injection process conditions, it is shown that warpage deformation trend of the two is same before pre-deformation, deformation amount difference is about 0.2mm, indicating that simulation analysis result is in line with actual warpage deformation result, and maximum deformation reaches 1.24mm, which cannot meet quality requirements of product. Through analysis of warpage deformation simulation of automobile water chamber after pre-deformation, maximum deformation is about 0.46mm. Finally, mold was opened to verify that actual warpage deformation was within 0.47mm, indicating that pre-deformation compensation measures can effectively control warpage deformation. Pre-deformation design method provides a reference for control of warpage and deformation of similar products, has a good reference value for ensuring quality of products, shortening development cycle, and reducing production costs.