The process of mold design is generally as follows: from the user to the plastic product map (may be two-dimensional, or three-dimensional), the mold design engineer considers the appropriate structure according to the type, characteristics and shape of the product. That is, the plastic part can be formed by what kind of mold structure is used. In this process, the structural design engineer also needs to select the type of runner, size, gate type, quantity, size, position and determine the number and size of the runners according to some specific requirements of the product. In addition to being structurally sound, the designed mold is also feasible in terms of casting and cooling systems.
Obviously, mold design is very demanding for structural design engineers. He not only has the experience of structural design, but also has a lot of knowledge in molding materials and molding process, or the success or failure of design depends to a large extent on the experience of the mold designer. Even though a design engineer has a good mold design experience, he still faces many new problems, because the actual products are often ever-changing. Sometimes a subtle change or specific product requirement may make existing experience unavailable, or even if it is available, it cannot be validated before the actual test. This has led to a significant design-correction-redesign feature of the mold design process. How to reduce the number of cycles in the process, so that the initial design should achieve or approximate the results of the combination, which is of great significance in practice.
CAE analysis is the ideal tool to solve the above problems. The reason is that the tool is based on a relatively accurate mathematical model, so that the results of actual production practices can be approximated. In addition, the computational speed allows for multiple gating systems and cooling systems to be performed before the actual test. Evaluate until optimization, so as to shorten the design and manufacturing cycle and improve quality.
This article introduces some issues and practices that are noted in the CAE analysis, and gives some examples to illustrate.
(1) Clear analysis requirements and priorities. The purpose of CAE analysis is different for different products. For appearance parts, injection defects that affect the appearance, such as weld marks, are not allowed on the surface; for non-appearing parts, the position of the sintered marks is not important, but if the product has a strict assembly relationship, the allowable warpage and The amount of deformation has become the main goal of the pursuit. In addition, all articles are generally required to meet the requirements of balanced flow and uniform filling. In practice, it is difficult to meet multiple requirements at the same time. In most cases, it is mainly to meet a major requirement. Under this premise, other requirements are met. Therefore, on the basis of clear analysis requirements, it is also necessary to distinguish which are the main requirements of this analysis.
After determining the requirements and priorities of the analysis, it is necessary to initially determine which parameters are modified or changed. For example, the main factors affecting the position of the weld mark are the position of the gate and the shape characteristics of the product, and the shape of the product is generally not allowed to change; the factors affecting the amount of warpage and deformation are mainly the shape of the product, from the process From the point of view, too much injection pressure, too long holding time and uneven cooling can lead to excessive warpage. In addition to the characteristics of the product structure itself, the size of the gate and the runner can directly affect the injection pressure in the cavity. The size and arrangement of the cooling channel directly affect the uniformity of cooling. These factors indirectly affect the deformation and warpage of the product.
(2) Analysis of the characteristics of the product structure. Using professional CAE analysis software for injection flow simulation, geometric modeling is first required. In general, this process takes a long time. To this end, the geometric model should be simplified to an appropriate degree to shorten the modeling time and improve the calculation efficiency. At present, some CAE softwares have the function of directly inputting the solid model from the specialized CAD software, and then using the specific neutral extraction tool in the software to extract the neutral surface for calculation, but when the structure of the product is more complicated, it will be generated. Many problems. Therefore, it is still of great significance to simplify the details that do not significantly affect the calculation results.
The simplified approach can be based on different functions provided by specialized software. The principle of simplification is negligible fineness compared to the dimensions of the entire article, and details that are farther from the gate or parallel to the flow direction are also negligible. As for the division of the mesh, it is better to use different scale grids in different regions according to the different structures of the products to obtain higher calculation accuracy, especially when calculating warpage and deformation.
(3) Select the appropriate injection molding materials. The degree of credibility of the CAE analysis results depends on three aspects: first, the accuracy of the mathematical model of the molten state fluid used in the calculation; the second is the difference between the geometric model of the calculation time and the actual product; the third is the injection molding used. Whether the data source of the material is accurate and reliable. In general, errors from mathematical models are acceptable in practice, and many years of practical testing of commercial CAE analysis software have demonstrated this. Practice has proved that as long as the model is properly simplified according to the principle of model simplification, the resulting calculation error is also acceptable. On the contrary, if the material data used is inaccurate, even if the geometric model is more accurate, the calculation results are far from each other.
To this end, the name, grade, manufacturer or even production batch of the product material selected by the user must be clarified. In practice, the data error caused by different production batches of the same material is negligible. The material data of the same material produced by different manufacturers may vary greatly due to differences in formulation and process. The data of different grades of the same material should be larger.
At present, foreign commercialized CAE analysis software has established a certain number of material databases, but almost no domestic materials are involved. Obviously, calculations using foreign material data can only yield fairly rough calculations. Therefore, qualified users can select some commonly used materials for rheological parameter experiments according to their own product characteristics. On this basis, the input parameters of CAE analysis can be obtained, and a common material database suitable for the enterprise can be established.
(4) Analysis and evaluation of analysis results. The evaluation of the analysis results must be carried out in conjunction with the specific requirements of the analysis. That is to say whether the main goal of this analysis has been reached. If it is not met, it is necessary to seek new solutions based on the simulation results, including adjusting certain parameters, positions, etc., and then performing new simulations. Before seeking a new gating system, cooling system solution, it is necessary to carefully analyze the improvement plan or modify the parameters to make the calculation results gradually approach the analysis requirements, rather than deviate from the analysis objectives.
The evaluation of the calculation results must be considered comprehensively, because the final simulation results are often a compromise of multiple conditions, that is, the calculation results may be “only better, not the best”. This requires engineers and technicians to have a comprehensive trade-off of the analysis results. For example, under the premise of satisfying the flow balance, the injection pressure may be larger, which also brings some problems. However, as long as such injection pressure does not cause excessive shear rates or other injection defects, the solution can be considered to be useful, and the analysis can be ended. Of course, CAE analysis of as many scenarios as possible is best for finding a more appropriate solution. The analysis and evaluation work is best carried out by CAE analysts in conjunction with structural design engineers and experienced process personnel in the injection molding workshop.
(5) Correct understanding of the role of CAE analysis in mold design. Injection molding flow CAE analysis can help engineers evaluate multiple possible solutions in advance to achieve a relatively good gating system solution and cooling system solution. However, the calculation results cannot be expected to agree well with the actual situation, because the errors caused by the material data, the errors caused by the mathematical model of the calculation, and the errors caused by the simplification of the geometric model make the calculations The result is somewhat different from the actual injection molding process, and this difference increases significantly as the above error increases. Coupled with the complexity of the actual injection process, it is more difficult to obtain definitive analytical results.
If the material data is accurate, the model is simplified and it is entirely possible to obtain a more reliable analysis. It is also meaningful to use the obtained data as a process data to guide actual production.
Finally, let's talk about who is going to do the CAE analysis. CAE analysis has certain requirements for the comprehensive quality of analytical engineers due to the multiple sciences involved in materials, injection molding, mathematics, and computers. Experience has shown that CAE analysts are best served by engineers with a background in polymer materials. All in all, the more knowledge about the mold structure, injection molding process, and material knowledge, the richer the field experience, the more likely the CAE analysis results will be close to the real situation and the greater the guidance to production.