Metal 3D printing is the most representative additive manufacturing technology, and its engineering application helps to improve the digital and intelligent design and manufacturing level of complex precision molds. Due to the common industrial characteristics with the mold manufacturing industry, metal 3D printing technology is becoming a new key technology for efficient and high-precision mold manufacturing. my country’s mold industry is taking the opportunity of promoting the innovative development of 3D printing mold making technology to enhance the advanced foundation of the mold industry and the modernization of the industrial chain, and promote the mold industry from large to strong.

1 Preface

Additive Manufacturing is a technology that uses a gradual accumulation of materials to manufacture solid parts or products. Since additive manufacturing technology does not require complex tools and molds and multiple processing processes to quickly manufacture workpieces and products of any shape, it has achieved rapid development in technology research and development and industrial application over the past 30 years, and has become one of the three basic technologies of material processing, along with subtractive manufacturing (mainly cutting) and equal material manufacturing (mainly mold forming). Additive manufacturing, especially the fully digitalized 3D printing technology, achieves the goal of “three reductions and one high” (reducing the number of parts, reducing the quality, reducing assembly, and producing highly complex parts) through 3D printing design of parts or whole machines and 3D printing manufacturing.

2 3D printing technology has become the main body of additive manufacturing technology

The widespread application of 3D printing technology has not only promoted the application of high-energy density processing technologies such as laser and electron beam in the manufacturing industry, but also is expected to revolutionize the entire design and manufacturing process of the manufacturing industry. It is an emerging intelligent manufacturing method, and has therefore received high attention and attention from governments and engineering and technical circles. The digital additive manufacturing technologies that have been developed are shown in Table 1.

Most of the key processes and equipment technologies of additive manufacturing belong to “3D printing technology”. Because they best reflect the technical characteristics of additive manufacturing and the direction of digitalization and intelligence, they have become the main body of additive manufacturing technology. Among them, metal 3D printing technology represented by metal powder bed fusion technology and sintering technology represents the technological frontier and industrial application level of additive manufacturing, and is the jewel in the crown of 3D printing technology.

3 Metal 3D printing has established a relatively complete industrial chain

In the 1980s, 3D printing had already developed commercial technology, but the initial printing materials were mainly non-metallic materials, so 3D printing was mainly used for the manufacture of non-metallic materials such as models, crafts, packaging and medical equipment. Entering the 21st century, metal 3D printing technology has developed rapidly, and the preparation technology of metal powder has become increasingly mature, making the engineering application scope of 3D printing technology from the initial product development prototype manufacturing, jewelry, dentistry and packaging manufacturing fields to quickly enter various fields of precision forming in the product manufacturing industry, becoming the most competitive manufacturing method for special complex parts and highly personalized products. Among them, the 3D printing equipment and corresponding layering software of companies such as EOS in Germany and 3D System in the United States once led the development of 3D printing technology. At the same time, the research and development, production and supply of various consumables used in 3D printing, such as non-metallic materials (nylon, ceramics, etc.) and metal materials (stainless steel, tool steel, heat-resistant alloys, aluminum alloys, copper, etc.) powders have gradually achieved industrialization and commercialization. However, there are few varieties of metal 3D printing consumables at present, and the price is relatively high. In addition, the investment in printing equipment is large and the printing efficiency is relatively low. At present, the scope of engineering application is mainly concentrated in non-market product fields such as aerospace, artificial joints (including teeth) and special tooling manufacturing industries.

In metal 3D printing technology, powder bed fusion technology is the most commonly used technology in engineering. One of its key technologies is the printing heat source, which generally uses a laser beam or a high-energy electron beam. The laser heat source can be used for laser cladding and selective laser sintering, while the high-energy electron beam is mainly used for electron beam melting. Due to the different propagation modes of laser and electron beam in the processing space, the working environment of the two is different – laser 3D printing is carried out in air or inert gas (such as nitrogen), and the environment of electron beam 3D printing requires a relatively high vacuum degree to reduce the energy loss caused by the collision of the electron beam with gas molecules before reaching the working surface. At present, both types of metal 3D printing equipment have established R&D design and production supply systems.

Another key technology of metal 3D printing is the preparation of printing materials. Powder bed fusion technology requires continuous and uniform spreading of metal powder. Therefore, the powder shape is required to be as spherical as possible, which is conducive to the smooth flow of the powder. At the same time, the uniformity of the powder particles and composition must meet the design requirements of the material. At present, the preparation technology of metal powders such as stainless steel, high-temperature alloys, titanium alloys, aluminum, copper, etc. with low carbon content is relatively mature and the varieties are also rich. Tool steel powders with high carbon content need to control carbon burning during preparation and use (melting), which affects the performance of printed products. Therefore, there are fewer varieties of tool steel powders at present, and the price is also high.

4 Metal 3D printing technology has been successfully applied in the field of mold manufacturing

Molds are important basic process equipment in the product manufacturing industry. They have complex structures, high manufacturing precision, harsh working conditions, and high reliability and life requirements. At present, most mold materials are made of metal materials
(more than 90%), of which 90% are steel materials. In order to manufacture complex molds efficiently and accurately, the mold industry has been exploring various new processing methods and manufacturing technologies. However, even so, the processing technology level in the 20th century still cannot meet the demand for rapid improvement of mold design accuracy due to the advancement of digital design technology, resulting in many advanced design solutions that cannot be realized or forced to reduce manufacturing accuracy due to processing technology. For example, the cooling system of injection molds for precision and complex plastic products is difficult to process the conformal cooling water channel designed by CAE due to cutting processing technology, resulting in large local deformation of the parts.

At the beginning of the 21st century, with the breakthrough of key technologies such as laser power, spot focusing and mold steel powder, the engineering application of metal 3D printing technology in the production of conformal cooling water channels for injection molds has been realized, as shown in Figure 5. For large and complex injection molded parts, the cross-sectional shape and direction of the water channel are designed through simulation software analysis, and the conformal water channel insert is made by 3D printing technology and inserted into the mold base processed by cutting. The cooling water channels of the two parts are connected by hoses outside the mold. This additive and subtractive composite manufacturing technology not only avoids the weaknesses of 3D printing technology such as high mold steel powder price and low printing efficiency, but also solves the problem of unreasonable local cooling of traditional cooling water channels; for small precision structural parts injection molds, conformal cooling water channels are optimized and designed through CAD and CAE software, and the mold is directly printed out as a whole using metal 3D printing technology. The printed injection mold with accompanying water channels. In addition, some complex molds or mold parts, such as rubber tire mold tread blocks, sole pattern molds, etc., have also established metal 3D printing production lines. At present, metal 3D printing technology can control the comprehensive cost of such molds or mold parts within the acceptable range of the market.

In addition to its obvious advantages in terms of structural complexity, metal powder bed 3D printing technology also basically meets the requirements in terms of dimensional accuracy. The roughness of the free surface also basically meets the requirements, but the accuracy level of the working surface does not meet the design requirements and requires further machining or polishing.

5 Metal 3D printing technology is becoming an important part of intelligent mold design and manufacturing technology

5.1 The design and manufacturing of molds and metal 3D printing technology have common industrial characteristics

Mold forming is a precise and efficient processing method for parts (products), and the design and manufacturing of molds have typical “multi-variety, small batch” (single-piece production in most cases) characteristics. Metal 3D printing technology happens to have an indisputable advantage in meeting the needs of highly personalized high-end customers, and has a high degree of coordination with the industrial characteristics of the mold manufacturing industry, providing new technologies for efficient and high-precision mold processing.

5.2 Metal 3D printing technology accelerates innovation and development guided by market demand, and promotes technological breakthroughs in the design and manufacturing of mold cooling systems

Inspired by the feedback and encouragement of engineering applications, metal 3D printing technology is constantly moving forward with a positive and enterprising spirit to meet the requirements of metal 3D printing technology for high-quality development of user industries.

(1) Develop more metal 3D printing processes with the goal of improving efficiency. Powder bed fusion metal 3D printing technology has advantages in controlling the accuracy of printed products. The geometric structure of processed parts can be very complex. Selective laser melting (SLM) and direct metal laser sintering (DMLS) are typical representatives of powder bed fusion metal 3D printing processes. However, this process strongly relies on the heat source spot size and powder particle preparation quality, with low printing efficiency and high printing cost. The direct energy deposition technology (see Figure 8) that has been developed successively takes into account both dimensional accuracy and printing efficiency (the forming efficiency can reach 2 to 10 times that of powder bed technology). The preparation cost of printing materials (filaments, powders) is low, which is suitable for the manufacture of molds with larger cross-sections and slightly lower structural complexity and the repair of some locally failed molds.

Binder jet metal 3D printing technology provides new opportunities for the production of some molds that require uniform exhaust or vacuum forming.

(2) Optimize the mold steel powder preparation process to reduce the production cost of 3D printing. Powder performance and quality are key factors in the shape and controllability of metal 3D printing. With the in-depth understanding of the physical mechanism of how the metal 3D printing process affects the quality of printed products, new results have been achieved in the research on powder composition, particle shape, particle size, particle distribution state and ratio of different printing processes (see Table 2), and different powder making technologies have been developed based on the research results.

In order to prepare metal powders that meet the above reference standards, European and American companies have developed gas atomization, ion atomization and electrolytic powder making methods. For the powder bed melting process, high-quality and expensive metal powders are usually used. These powders are usually prepared by gas atomization or plasma atomization processes, which melt the metal by induction heating or plasma torch respectively. The molten metal liquid is injected into the atomization chamber and broken into small droplets by high-speed airflow, which gradually solidifies during the falling process. Gas atomization powder making is highly efficient, and more than 80% of metal 3D printing powders (including mold steel powders) are made by this method; the sphericity of the powder prepared by the plasma atomization process is higher than that of the gas atomization process; the particle size of the powder prepared by the two processes is Gaussian distribution.

Around 2010, European and American countries began to produce 3D printing mold steel powder (S316 stainless steel, P20 plastic mold steel, etc.), with a particle size of 20-40μm and a price of 200-250 US dollars/kg. They also used steel powder to print and manufacture injection mold inserts such as conformal water channels. Around 2015, China successfully developed mold steel powder for laser cladding.

(3) Increased freedom in mold cooling system design and improved mold forming production efficiency. The engineering application of metal 3D printing technology has greatly increased the freedom in mold cooling system design, without worrying about the impact of the processability of the cooling system on the accuracy of the results.

Take the conformal cooling water channel design of TPE cable casing mold insert (see Table 3) as an example for analysis. Design 6 in Figure 1 is a water channel structure designed based on the traditional “drilling” process, and the other five designs are water channel structures designed based on the metal 3D printing process. These six mold inserts were printed using 316L stainless steel and heat treated to a hardness of 54HRC. After actual testing, design 5 (fountain type), design 1 (thin U-type) and design 4 (thick spiral type) have the best cooling performance. The actual objects of these three mold inserts are shown in Figure 1. The injection mold uses the above-mentioned conformal cooling inserts to shorten the injection cooling time from about 30s in design 6 to about 6s, thereby reducing the injection cycle from 60.5s to 14.7s, and shortening the production cycle by 75%.

In addition to using metal 3D printing technology to directly print molds with conformal water channels, a method has also been developed to complete the manufacturing of molds with conformal water channels together with mechanical processing technology (see Figure 2). This additive and subtractive composite manufacturing technology is also called “3D printing grafting”. This method can not only meet the design cooling conditions, but also improve the efficiency of mold manufacturing.

5.3 Metal 3D printing technology promotes intelligent mold design and manufacturing

1) The innovative development of metal 3D printing technology has promoted the improvement of the freedom of collaborative design of parts (products) from product development, forming process design, and mold design. It can also compare the shape control and controllability effects of various forming schemes in virtual space to complete the design of high-precision and high-efficiency mold forming schemes, and innovate the entire process of mold design and manufacturing.

2) The engineering application of metal 3D printing technology helps to improve the digitalization, networking and intelligent design and manufacturing level of complex precision molds or mold parts, and is becoming a new key technology for efficient and high-precision mold manufacturing.

3) Since the 13th Five-Year Plan, my country has maintained its position as the world’s largest mold consumer, manufacturer and exporter for many years, but in terms of original innovation capabilities of technology and competitiveness of the industrial chain, my country’s mold industry is still “big but not strong”. From the perspective of improving the advanced foundation of the mold industry and the modernization of the industrial chain, my country’s mold industry should seize the opportunity of metal 3D printing technology to have a revolutionary breakthrough in the intelligentization of mold design and manufacturing, and strive to promote the mold industry from big to strong.

6 Conclusion

Based on the above analysis, the following suggestions are put forward: ① Actively carry out basic theoretical research on metal 3D printing molding technology, especially the research on the organizational evolution mechanism of 3D printing process of commonly used mold steel powder, and strive to keep pace with or lead the world. ② Innovate the research and development of metal 3D printing process and equipment technology, and establish a standard system for metal 3D printing molding technology, including process, equipment, printing materials, post-printing processing and evaluation methods. ③ According to the characteristics of the processed mold or mold parts and the 3D printing technology used, optimize the mold design and manufacturing process, and accelerate the construction of 3D printing molding technology application platform and supply chain. It is estimated that by 2035, (metal) 3D printing molding will account for 15% to 20% of the national mold production(45 billion to 60 billion yuan).