Application of the hottest CAE Technology in the d

Application of CAE Technology in injection mold design

Abstract: through several typical examples, this paper explains how to use the analysis results of CAE of injection mold to solve the problems in the design of gating system and cooling system of injection mold

key words: injection mold; CAE； Flow analysis; Cooling system

1 introduction

for a long time, the design of injection mold in China mainly depends on the experience and intuition of designers. Through repeated mold trial and repair, the design scheme is modified, which is lack of scientific basis and has great blindness, which not only makes the production cycle of mold long, the cost high, but also the quality difficult to guarantee. For large-scale precision and new structure products, the problem is more prominent. With the increasingly extensive application of plastic products, the traditional production mode of injection mold can no longer meet the needs of the development of modern society for the output, quality and replacement speed of plastic products. For many years, people have been expecting to predict the flow of plastic melt in the mold cavity during injection molding and the cooling and curing process of plastic products in the mold cavity, so that the problems existing in the design can be found before the mold is manufactured, and the drawings can be modified instead of the molds. The emergence of CAE Technology in injection mold makes people's wish come true

injection mold CAE technology is to establish the physical and mathematical model of the flow and heat transfer of plastic melt in the mold cavity according to the basic theories of plastic processing rheology and heat transfer, construct its solution method by using numerical calculation theory, and visually simulate the dynamic filling and cooling process of the melt in the actual molding on the computer screen by using computer graphics technology, The state parameters of the forming process (such as pressure, temperature, speed, etc.) are given quantitatively. Using the CAE technology of injection mold, the mold design scheme can be analyzed and simulated on the computer to replace the actual mold test before the manufacture of camphor ware, predict the potential defects in the design, break through the traditional bondage of repeated mold test and repair on the injection molding machine, and provide a scientific basis for designers to modify the design. Many CAE enterprises have changed their systems and are still developing the application of technology to varying degrees. The direct benefits brought by the application of technology are to save time and effort, reduce the number of mold trials, mold repairs and mold scrap rate, shorten the mold design and manufacturing cycle, reduce costs and improve product quality

Since the 1980s, some commercialized CAE software for injection mold have appeared in the international market, such as C-MOLD of AC Teclnology company in the United States and mold flow of MF company in Australia, which have been used to guide actual production and achieved remarkable economic benefits. In China, the research and development of injection mold CAE technology began during the Eighth Five Year Plan period. In recent years, some practical injection mold CAE software with independent copyright have emerged, such as z-mold developed by the national rubber and plastic mold engineering research center of Zhengzhou University of technology. CAE software is only a tool to assist analysis. Therefore, like other tools, whether it can make the best use of everything depends on the user's level of use. Although there are many literatures about the design of injection mold and the introduction of CAE technology of injection mold, there are few literatures about how to use CAE analysis results to guide mold design. Here are some typical CAE analysis examples to illustrate how to use CAE technology to solve problems in mold design

2 flow analysis and its application in mold design the purpose of flow analysis is to predict the process of melt flowing through the runner gate to fill the cavity. Flow simulation can help optimize product and cavity design, determine reasonable gate and runner, predict the required injection pressure and clamping force, and find possible defects. Due to the non Newtonian characteristics of plastic melt and the non isothermal and non steady flow process, the simulation of melt filling flow process is quite difficult, which must be realized by numerical methods

There are two main methods of flow analysis: one is the branch flow method, which is based on one-dimensional flow analysis. It geometrically decomposes three-dimensional plastic parts into a series of flow paths composed of one-dimensional flow units in series. In the calculation process, iterative calculation is used to make the pressure drop of each flow path equal under the condition that the sum of the flow of each flow path is equal to the total injection volume. This method has short calculation time, but it is difficult to analyze plastic parts with complex shapes. The other is the flow network method. Its basic idea is to divide the whole cavity into grids and form volume units corresponding to each node, establish the relationship between the node pressure and the flow into the node volume unit, obtain a set of control equations with each node pressure as a variable, and update the flow front according to the filling condition of the node volume unit. At present, generalized hel is widely used in flow analysis. A Shaw flow model, using the mixed finite element/finite difference method to solve the control equation, basically follows the basic idea of the flow network method, uses the control volume method to establish the finite element equation for solving the pressure field, and for the time and the difference along the thickness direction, establishes the energy equation for solving the temperature field, so as to realize the dynamic simulation of the filling process of the injection mold

2.1 application of flow analysis in mold cavity design

for injection mold cavity with complex shape, the change of product shape and thickness will affect its filling mode. The filling information of different areas, as well as the information about material shortage, welding line, cavitation position and so on, are very important to the cavity design. In order to obtain this information, the traditional method is to use the experimental mold or the real mold to get it through the "lack of material" injection again and again, and use the flow analysis to get some key information in the cavity design in the conceptual design stage of the product, such as the welding line/fusion line and cavitation position, the degree of flow balance, the runway effect, the stagnation and accelerated flow of the melt, the filling condition at any time or under any filling volume, etc, Using these analysis results, we can judge how to modify the product to obtain a better filling mode

Figure 1 shows a shallow box product with ribs. The thickness of side wall and ribs is 3mm, and the thickness of bottom plate is 1 5 mm, feed from the bottom center, and the filling mode is shown in Figure la. This design forms an air pocket at the bottom of the product, which is mainly caused by the convergence of non-uniform flow (runway effect) caused by the change of wall thickness. If the air at this place cannot be discharged, it will form a coke mark at the bottom of the product. A ejector pin must be designed at the air pocket to make the air escape from the ejector pin hole. At the same time, by changing the gate position or product thickness, we can try to avoid cavitation in the mold, and make cavitation appear at the edge of the product or at the parting line. The air can be discharged from the mold gap or the external exhaust hole. Figure 1b shows the analysis results when the bottom thickness increases to 3mm. It can be seen from this that the melt front is pushed forward evenly, and finally all the gas is squeezed to the edge of the product, so that the gas is discharged from the parting surface, simplifying the mold structure

2.2 application of flow analysis in gate design

there are many types of gates, generally including side gate, point gate, latent gate, fan gate, film gate, etc. according to their different characteristics, the gates are generally small in different occasions. Because of this great flow resistance, subtle changes will have a great impact on the filling of plastic melt. Gate design mainly includes the number, position, shape and size of gates. The number and location of gates mainly affect the filling mode, while the shape and size of gates mainly affect the melt flow properties. On the one hand, the gate design should ensure to provide a fast, uniform, balanced and single direction filling mode, on the other hand, it should avoid the occurrence of jet, stagnation, depression and other phenomena

stagnant flow or stagnant flow spots are surface defects caused by the stagnation of polymer melt. When there are areas with large thickness differences in the product, the plastic melt will flow in the direction of thicker and easier filling, and the plastic melt at the thinner part will stagnate. The plastic melt will not return to fill the thinner part until the thicker area is full. If the stagnation time of plastic melt is too long, it will be cooled and solidified at the stagnation point, resulting in a sharp rise in short shot or flow shear stress. When the solidified melt is pushed to the surface of the product, stagnation spots will be formed on the surface. Using flow analysis, the location of stagnation phenomenon can be found, and this phenomenon can be improved by modifying the gate position. Figure 2 is a simple example

when the gate is set at a (Fig. 2a), the melt will stagnate at the thin wall. If the gate position is set at B (Fig. 213), the melt will first fill the thick wall, and then accelerate the filling of the thin wall without stagnation. Therefore, in order to avoid stagnation, on the one hand, the sudden change of product thickness should be avoided, on the other hand, the gate should be set in the thick wall area that is easy to fill and as far away from the sudden change of thickness as possible

flow equilibrium requires that all flow paths be filled at the same time, otherwise residual stress will be generated due to uneven orientation caused by undervoltage or overpressure; The position and number of gates have a great influence on the flow balance. For complex parts, it is often impossible to determine the appropriate position and number of gates to ensure the flow balance in the mold cavity. Using flow analysis, the influence of different position and number of gates on the flow balance can be quickly predicted. Figure 3 shows the washing machine cover plate product with one mold and two cavities. In the initial design (Fig. 3a), considering the large filling volume of the large cover plate, two gates are set on the large cover plate, and one gate is set on the small cover plate. During the mold test, it is found that the large cover plate has been expanded and the small cover plate is not full. This is also fully verified by flow analysis. The reason is that the structure of the small cover plate is complex, the flow resistance is large, and the filling is difficult, resulting in the flow imbalance in the two cavities. According to the flow analysis results, two fan-shaped gates are set on the small cover plate, while only one gate is set on the large cover plate, so that the flow is balanced and the injection pressure is greatly reduced, as shown in Figure 3B

in the multi gate cavity mold, there are often problems such as flow imbalance and difficult to change the position of the fusion line. Using the valve gate, the opening time of each gate can be controlled, so the filling mode and the position of the fusion line can be changed. Using flow analysis can help set the opening time of different valve gates to obtain better filling mode and weld line position. Figure 4 shows an application example of using the valve gate design to eliminate the welding line in the multi gate cavity mold. Figure 4A shows that only the central gate is opened at the beginning. When the front edge of the melt reaches the gates on both sides (Figure 4A state), then open the gates on both sides. At this time, the central gate can be closed or continuously filled, and figure 4B shows the final full state

2.3 application of flow analysis in runner design

runner is mainly used to transport plastic melt to each gate. The commonly used runner shapes are round, trapezoidal, U-shaped, etc. It can be determined according to different occasions and processing convenience. It is still a new topic. If the flow resistance is compared with the same cross-sectional area, the circular section is the best choice. However, due to the need for double-sided processing, the processing is difficult and costly. Generally, the hydraulic diameter of the section is used to compare the flow resistance. The cross-sectional size and length of the runner will affect the flow resistance. If the flow resistance is too large, most of the injection pressure will be wasted in the runner, and the proportion of pressure drop in the mold cavity will be reduced; However, if the runner resistance is reduced and the runner size is arbitrarily increased, the cooling time will be prolonged and the material consumption will be increased. Using flow analysis can