The conformal cooling mold was manufactured by 3D printing technology, and the substrate of 3D printing was studied. The compressive strength and compressive yield strength of the 3D printed composite material with 30% (mass fraction) tungsten carbide (WC) added reached 1757MPa and 1677MPa respectively, which is about 20% higher than the compression performance of the mold steel matrix. In view of the problems of long cooling time, product warping and dimensional instability in mold design, a conformal cooling water channel mold based on metal 3D printing was designed, and the simulation analysis was carried out with the help of Moldflow software. The results show that the conformal water channel solution can significantly improve the cooling efficiency and reduce product deformation. Metal 3D printing technology has a wide range of application value in the field of conformal cooling molds. Plastic products are mainly formed in injection molding machines using molds. In the entire production process, the time and cost of manufacturing injection molds are the highest. Nowadays, the increasing demand for personalized small-batch products has led to the rapid development of the mold industry and increasingly fierce competition. Therefore, in addition to continuously improving the quality and performance of injection molds, it is also necessary to minimize its production costs and shorten the product development cycle in order to quickly seize the market.
In the process of plastic part molding, the cooling stage accounts for 2/3 of the entire molding cycle. Insufficient cooling time may cause defects such as shrinkage marks and warping of plastic parts. Too long cooling time will also affect the production efficiency and cost of plastic parts. Therefore, improving the heat dissipation efficiency of the mold and shortening the cooling time have become the key research directions of the mold industry. Among them, the design of special-shaped water channels is one of the effective solutions to improve the heat dissipation efficiency of the mold and shorten the cooling time. The traditional mold cooling water channel is limited by the processing technology and can only be achieved by drilling holes in the mold. However, a mold often has complex mechanisms, such as push rods, inclined guide columns and sliders. When designing the water channel, it is necessary to avoid certain existing mechanisms of the mold. Therefore, it has become a very difficult task to make an effective cooling water channel in the mold to cool the plastic parts quickly and evenly.
Taking injection molds as an example, existing injection molds have the following main problems due to cooling factors: (1) The existing mold processing technology limits the water channel processing technology, resulting in the inability to fully meet the cooling requirements of plastic parts, resulting in problems such as substandard quality of plastic parts or inability to directly mold; (2) The existing mold processing technology limits the cooling efficiency of the water channel, prolongs the cooling time, further affects the ejection time of the injection molded product, thereby affecting production efficiency and delaying product market promotion; (3) The cooling problem during the product injection molding process often restricts the structural and functional design of the product. The spraying process during high-pressure injection has two main purposes: one is lubrication to improve demolding; the other is to act as a spray medium to cool. The industrial development trend is to minimize or eliminate the spraying step. The use of conformal cooling design can reduce the demand for spraying, extend the life of the mold and ensure the quality of the injection molded parts1.
Metal laser 3D printing technology is based on the principle of layered manufacturing and layer-by-layer superposition molding. According to the three-dimensional structural model of the required parts, the slice model file is designed. The metal powder material is melted according to the established scanning trajectory by laser 3D printing equipment, solidified and formed, and superimposed layer by layer, which can form parts with any complex geometric shape7. Compared with traditional processing methods, laser selective melting (SLM) can produce mold cores and cavities with high-precision complex conformal water channels, which greatly reduces the cooling time of the mold and shortens the manufacturing cycle of the mold.
Based on metal 3D printing technology, this paper studies the performance of the printing substrate, designs a precise conformal cooling system for injection molds, uses mold flow analysis software to analyze the performance of conformal cooling molds, optimizes the key process parameters of conformal cooling molds, and uses 3D printing technology to print the mold and conduct injection molding tests.
1 Material research
According to the needs of mold parts, select a low-carbon high-strength steel with good corrosion resistance and high hardness. Composite material printing is the current research direction of metal additive manufacturing. By adding tungsten carbide (WC) ceramic phase, the strength and hardness of the matrix, as well as the wear resistance and heat resistance, can be improved, but at the same time, it will also bring about problems such as decreased plasticity. Analyzing and exploring the strengthening mechanism of the ceramic phase and the bonding between the ceramic phase and the matrix interface is an effective way to control the overall performance of the composite material and improve toughness.
The mass fraction of WC in the mold steel is 30%, and the SLM molding process parameters are shown in Table 1. The density of the molded sample measured by the drainage method is as high as 9.11g·cm’-3. After forming a 30% (mass fraction) WC mold steel composite material sample, after high-temperature solution at 900℃ for 1h and aging heat treatment at 500℃ for 4h, the hardness is detected to be as high as 52.4HRC. Through experiments, it can be seen that its hardness is positively correlated with its density. The higher the density, the fewer the number of internal pores and the higher the hardness. The hardness of the mold steel material without additives after solution-aging heat treatment is 48HRC. In comparison, the overall hardness of the mold steel with WC added is improved to a certain extent, indicating that a small amount of WC particles are dissolved in the matrix during the solution treatment process.
As can be seen from Figure 1, except for some WC particles that maintain a complete and smooth spherical interface, other WC particles are dissolved during the solution treatment process, becoming elliptical or defective spheres, and small particles of WC disappear. This shows that the WC particles are partially dissolved in the matrix, and a strong metallurgical bond is formed between the particles and the substrate, which significantly improves the overall strength and hardness of the material, which can also be reflected in the hardness change of the composite material.
The compression performance test results of composite materials with different mass fractions (WC) show that as the mass fraction of WC increases, the compressive strength and compressive yield strength of the composite material gradually increase. The compressive strength and compressive yield strength of the composite material with 30% WC added are as high as 1757MPa and 1677MPa, respectively, which is about 20% higher than the compression performance of the mold steel matrix, and the compressed parts are not fractured, but compression deformation occurs. Therefore, WC composites used as substrates are beneficial to improving the performance of the mold.
2 Design and analysis of mold conformal water channel
In view of the structural characteristics of conformal water channel mold design products, we first used Moldflow analysis software to compare the mold flow analysis of several typical cases with traditional water channels, explored the hot spots, temperature gradients, thermal deformation and other issues in the injection molding process, designed conformal water channels with different curvatures, distances from the cavity, and cross-sectional shapes, and continuously iterated and optimized them, with the goal of increasing production efficiency by more than 30%. We analyzed and summarized the relationship between the cooling effect and the structural characteristics and dimensions of the conformal water channel, focusing on the minimum distance between the water channel and the mold surface under different working conditions, as well as the life performance and cooling performance at different distances, the requirements of different structural characteristics and injection molding material systems for water flow, and the consistency between the simulation analysis results of the software and the actual results.
2.1 Conformal water channel design
Through the study of the typical structure and material characteristics of the product, the temperature gradient of the injection molding process is analyzed using finite element analysis software, and conformal water channels with different curvature, distance from the cavity, and cross-sectional shapes are designed. The cooling effect of the conformal water channel is analyzed and optimized iteratively, and the analysis combining theory and practice is carried out through experiments to establish the relationship between the temperature gradient and the conformal water channel structure, and preliminarily determine the conformal water channel design standard.
Take the upper cover mold of a certain product as an example: according to the principle that the distance between the cooling water channel and the glue surface must be greater than 2/3 of the water channel diameter, the deep bone layout and the innovative waist-shaped cross-section smooth conformal water channel are first designed, as shown in Figure 2.
2.2 Mold hot spot and deformation analysis
The hot spot position is analyzed by Moldflow software, as shown in Figure 3. The hot spot position is 12 protruding small columns, which are the parts that need to be cooled. The temperature gradient affects the cooling deformation of the product. Generally speaking, the more uniform the temperature, the better, and the difference between the highest and lowest temperatures should not exceed 20℃.
As can be seen from Figure 4, the highest temperature of the mold surface of the traditional machined water channel is 102℃, the lowest is 30℃, the extreme temperature difference is 72℃, and the protrusion is seriously insufficiently cooled. In actual production, it is easy to cause uneven cooling of injection molded products, large deformation, and substandard product quality; the mold of the conformal water channel is more uniform than the mold of the traditional machined water channel, the temperature gradient is gentler, the highest temperature is about 40℃, and the cooling effect is ideal, which is conducive to controlling product cooling and deformation and improving product quality.
The main factors affecting thermal deformation are trend effect, uneven cooling and uneven shrinkage. Through Moldflow analysis, the factors of each deformation are decomposed, the main factors affecting deformation are found, and then corresponding improvements are made to reduce deformation. Because the material does not contain glass fiber, there is no oriented deformation. As can be seen from Figure 5, the main deformation factor affecting this product is the deformation caused by uneven shrinkage, so the product wall thickness can be changed to make it more uniform.
Figure 6 is a comparison of the thermal deformation of mold inserts between traditional water channels and conformal water channels. It can be seen that the deformation of conformal water channels and traditional machined water channels is close and both are within the controllable range. This is mainly because the main factor affecting deformation is shrinkage deformation caused by uneven wall thickness.
2.3 Analysis of conformal water channel layout
Conformal water channels can be closer to the cavity than traditional machined water channels, and the distance between the water channel and the cavity is greater than or equal to 2/3 of the water channel diameter. It depends on the structure of the mold. If there are ejector holes, inclined ejector holes, insert holes, etc. next to it, it is recommended that the distance between the water channel and the cavity is greater than or equal to the water channel diameter. Figures 7 and 8 are respectively a comparison of the cooling effects of water channels 2.5mm and 5.0mm away from the cavity. By comparison, it can be seen that the cooling effect of the water channel 2.5mm away from the cavity is better than that of the water channel 5.0mm away from the cavity. The shorter the distance, the more uniform the insert temperature, and the shorter the cooling cycle. It can be seen that under reasonable principles, the closer the water channel is to the cavity, the better the cooling effect, but it should be noted that the distance between the water channel and the cavity is always uniform.
2.4 Analysis of conformal water channel cross-section design
The cross-sectional area of conformal water channels is the same as that of traditional drilled water channels, and both should be kept consistent as much as possible during design. This experiment proposes elliptical and circular cross-sectional design schemes, as shown in Figure 9(a) and Figure 10(a). From the above comparison, it can be seen that the water channel with an elliptical triangular cross-section has a larger water flow rate, better cooling effect, shorter cycle, and more uniform insert temperature than the water channel with a circular cross-section. Therefore, the larger the water transport cross-sectional area, the better the cooling effect. The cooling cycle is an important parameter for injection molding product trial molds, and conformal water channels can improve the cooling cycle well.
The traditional machined water channel and the conformal water channel are introduced into Moldflow, and the comparative effect of the two is analyzed by loading the same injection molding material (PETG, material information see Table 2) and the same water temperature process parameters (the inlet water temperature is set to 20℃).
From the Moldflow simulation analysis results in Figures 11 and 12, it can be seen that the cooling cycle of the conformal water channel is 19s (including 5s of mold opening and closing time), while the cooling cycle of the traditional machined water channel is 27s (including 5s of mold opening and closing time). The conformal water channel is superior to the traditional machined water channel, and its cycle is shortened by 30%, achieving the optimization goal.
3 Mold Verification
After mold flow analysis, the mold structure design is determined, and the SLM metal 3D printing equipment (HBDG350) independently developed by Hanbang Technology is used for printing. The printing process and the actual product after printing are shown in Figure 13. After printing, wire cutting, heat treatment and machining are performed, and then the mold is installed for product trial mold injection molding; the trial mold mainly verifies the cooling cycle of the mold and the temperature of the insert, because the cooling cycle will affect the production efficiency, and the insert temperature will affect the production efficiency and product quality. The shorter the cooling cycle, the higher the production efficiency and the higher the economic benefits; the more uniform the insert temperature, the better the product quality and the higher the production efficiency. After the trial mold verification, the products formed by the 3D printing conformal water channel cooling solution have a production efficiency improvement of more than 30% compared with the traditional water channel mold, and the defective rate is almost zero, which fully meets the production and use requirements.
4 Conclusion
1) The WC particle reinforced mold steel composite material is used as the substrate, and its performance is improved after 3D printing. The compressive strength and compressive yield strength of the composite material with 30% (mass fraction) WC added reached 1757MPa and 1677MPa respectively, which is about 20% higher than the compression performance of the mold steel matrix. The composite material can be used in 3D printing molds to reduce mold deformation and improve product quality.
2) The self-supporting elliptical water channel conformal cooling mold design breaks through the conventional water channel diameter limit and improves the mold cooling efficiency. The conformal design can reduce the maximum temperature of the mold surface by 47.4%, the average temperature of the mold surface by 40.9%, and the uniformity of the mold surface temperature by 1.8%. The cooling effect is significantly improved, and the conformal water channel mold can effectively reduce the deformation of the product, improve the dimensional stability of the product, and significantly improve the quality of the product.
3) Compared with traditional water channel molds, the production efficiency of products formed by 3D printing conformal water channel cooling scheme is increased by more than 30%, which fully meets the production and use requirements.
