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The mold determines the contour of the die casting and affects its performance. This article explains the basic principles of how to shape the melt.
Die casting is a forming process for mass production of aluminum, magnesium and their alloy parts. The casting process is carried out in a die-casting machine, which is divided into a hot-chamber die-casting machine and a cold-chamber die-casting machine. The main difference is that in the hot chamber casting machine, the container containing the molten metal is located inside the machine, while in another case, the container is located outside the machine. In both types of machines, the molten metal is pressed from the casting chamber through one or more casting channels into the cavity of the permanent steel mold, where it adopts the shape determined by the mold and solidifies. These die casting molds consist of two halves so that the casting can be removed from the mold. The feed-side mold halves are mounted on a fixed plate on the rigid side of the die-casting machine, while the ejection-side mold halves are mounted on a movable plate on the other side. Before closing the mold, the two halves will be sprayed with mold release agent so that the casting can be easily demolded from the mold later and the plate will not overheat. Depending on the size of the casting, up to 300 casting cycles per hour can be carried out.
When the mold is closed, the melt is pressed into the mold at a pressure of up to 1,200 bar, reaching a maximum filling speed of 150 m/s (540 km/h) [1]. High closing force and clamping force are required to compress the mold halves and keep the mold closed: up to 8,000 kN (800 tons) in hot chamber die casting machines and up to 45,000 kN (4,500 tons) in cold chamber die casting machines. By using such high forces , Can manufacture large castings. Regarding materials and design, the mold used for this purpose must be designed to permanently withstand the loads associated with large amounts of melt. When the metal is solidified, the mold is opened in half, and the casting is ejected by bolts or taken out by a robot and transported for further processing.
One of the core issues regarding the die-casting process is the mold. It determines the contour that must be transferred to the casting, and it should also allow the casting to solidify as quickly as possible. In this way, the formation of fine-grained structure is promoted, which is conducive to the quality of castings. In order to achieve the best cooling effect, the mold will be cooled in some parts. Another effect is to shorten the production time, which provides economic advantages. The design of die casting tools is described in the standard DIN 16760-1 [2]. The tools used in the die-casting process will inevitably endure high thermal and mechanical loads, so they must be able to withstand these loads permanently. For example, the service life of zinc die casting molds can reach 500,000 to 2 million cycles. In order to achieve this performance, in addition to the above-mentioned molds, die-casting tools also include mold inserts, cores, sliders and ejector devices, all of which are made of high-strength hot work steel such as X40CrMoV5-1 (1.2344) or special materials, such as hard Metal. The characteristics that play a very important role in these tools are high abrasion resistance, high ductility, high heat resistance, high thermal tear resistance and thermal wear resistance, and good thermal conductivity. When selecting a material, one must consider its technical characteristics, tool design, heat treatment, and last but not least the complex interaction between the tool and the metal to be cast. To this end, manufacturers and suppliers of suitable steel provide information brochures and consulting services [3].
In the past, tools for die-casting technology were manufactured according to drawings, but now designers use 3D CAD data and use the most advanced IT technology. When designing a mold, one must consider the casting process—and therefore melt flow and cooling—as well as the geometry and dimensions of the die casting to be manufactured. The casting should have the characteristics of uniform, fine-grained microstructure, high dimensional accuracy and dimensional stability, and high surface quality. Computer-aided simulation calculations help to make the tool most suitable for die-casting parts. Tool makers and mold makers use CAM systems for manufacturing. CNC-controlled milling machines as well as punching dies and cutting-erosion machines are used to integrate forming contours into the forming material with high precision. The manufacture of the mold is very complicated and therefore expensive.
Prototypes made from sand molds for thin-walled and fine die-cast parts
The mold cost is as high as 20% of the total cost of aluminum die casting [3]. However, for high-volume production of components, starting from a specific batch is more cost-effective than manufacturing parts in other ways, such as through a machining process. In addition, the manufacturing time for each part is shorter. The Mechanical Design Institute of Magdeburg University in Germany has developed a standardized and time-saving die-casting tool design program [4].
The design diversity of die-casting parts and the requirements for them continue to grow. Therefore, the requirements for the performance of tool steels used for die casting and the structural design of tools and molds made of these steels are also increasing. These steels, the software programs used for design and simulation, and the capabilities of the machining systems are constantly evolving. The topics "digitalization" (industry 4.0) and "3D printing" are becoming more and more important. Trade fairs are responding to this trend. EUROGUSS pays special attention to this topic through the special program "Additive Manufacturing". By using digital technology, processes can be controlled more effectively and optimization potentials can be better identified.
Through the 3D printing process, it is possible to manufacture parts that cannot be manufactured by traditional processes, such as die-casting mold inserts with complex shapes and integrated cooling channels with close contours and curves. According to Dr.-Ing. Ioannis Ioannidis, president and CEO of die casting machine manufacturer Oskar Frech, chairman of the Foundry Machinery Association, and member of the board of directors of the German Machinery and Equipment Manufacturers Association VDMA Additive Manufacturing Association, believes that there is still a lot of mold manufacturing potential in this area: Complete thermal management may be affected, for example, molds can be better protected from wear, and the quality of the parts to be cast may be affected."[5]
How can foundries benefit from the many advantages provided by the additive 3D printing process? Today, innovative foundries also use a combination of 3D printed cores and conventionally produced molds, as long as there is a return. In addition to printing sand molds, more and more foundries are also using 3D printer models for investment casting. For more information, check out the Voxeljet case study.
3D printing in tool and mold making
[1] K. Herfurth, N. Ketscher, M. Köhler: Giessereitechnik kompakt. Werkstoffe, Verfahren, Anwendungen. (Compact casting technology. Material, technology, application. German) Hrsg. Verein Deutscher Giessereifachleute. Dusseldorf 2013. S. 109 f.
[2] DIN 16760-1: Forming tools-Part 1: Machined undrilled plates. Berlin 2008.
[3] Uddeholm tool steel for die casting. Hagfors, Sweden 2016, S.16. https://www.uddeholm.com/files/AB_die_casting_eng.pdf
[4] A. Berkau, C. Birke, S.-J. Brockop: Vorgehen zur rechnergestützten Konstruktion von Rapid-Tooling-Werkzeugen am Beispiel eines Druckgießwerkzeuges. (Take the German die-casting tool as an example, a computer-controlled rapid tool design program). In: 5. Magdeburger Maschinenbautage. Magdeburg, September 2001, S. 195-203. http://www.pemos.de/artikelsammlung/beitraege01/bebibr.pdf
[5] Additive manufacturing bietet Druckgießereien große Chancen. Interview with mit Ioannis Ioannidis. (Additive manufacturing provides huge opportunities for die casting foundries. Interview Ioannis Ioannidis in German.). At: Giesserei 11/2017. https://www.giesserei.eu/magazin/interview/2016/interviewioannidis/?L=0zudem
This article was originally published by EUROGUSS.
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