Large die casting molds are the key to achieving the key technology for integrated die casting

Abstract Benefiting from the application of integrated die casting technology in new energy vehicles, it brings vitality to the manufacturing of large die casting molds. The manufacturing of large die casting molds has technical barriers and is the key technology to achieve integrated die casting. This article introduces the role and application effect of large die casting molds in integrated die casting, describes the difficulties in manufacturing large die casting molds, the need to pay attention to mold material selection and mold design simulation applications, and emphasizes the requirements of the temperature field thermal balance of large die casting molds. Measures used to assist thermal balance are proposed, and the application of in-mold sensors is forward-looking. With technological advancements, the technical barriers of large die casting molds are gradually being overcome and mastered, opening up new horizons for the manufacturing of large die casting molds and promoting the development of new energy vehicles towards energy saving and emission reduction.

Keywords: Large die casting mold, temperature thermal balance, design and simulation, material selection and use, mold temperature control and detection, in-mold sensor, 3D printing

1. Introduction

Integrated die casting technology was first proposed by Tesla in 2019. In 2020, the integrated die casting technology was applied to the production of the rear floor of the Model Y, integrating its original 80 stamping and welding parts into one part. The rear floor achieved a 10% weight reduction and a 40% cost reduction, attracting industry attention. Integrated die casting replaces several small assembled parts with a single large casting, offering three advantages: Firstly, it simplifies the production process. In traditional automotive production, dozens or hundreds of parts are assembled and welded. The types and quantities of parts are complex, requiring high production costs and time, and the assembly process is complex and time-consuming, requiring numerous robots to complete welding, assembly, and gluing. Integrated die casting only requires a die casting machine and mold to achieve one-step molding of the specified part, greatly simplifying the production process; secondly, the change in structure greatly improves hardware performance. Compared with assembly, integrated die casting has strong sealing and no gaps, giving the casting higher torsional stiffness and impact resistance, and reducing the trouble of part wear, aging, maintenance, and replacement during use; thirdly, lightweight. Integrated die casting uses aluminum alloy materials instead of steel. For example, using aluminum alloy materials in a 1.5-ton passenger car reduces weight by 30% compared to steel. Weight reduction is beneficial for automobiles; a 15% weight reduction in fuel vehicles can reduce fuel consumption by 5%, while in new energy vehicles, it reduces energy consumption by 10%. Aluminum alloy material is one of the most economical materials for lightweighting.

NIO and XPeng have launched integrated die casting production. NIO uses integrated die casting in the ET5 to produce the rear floor, reducing weight by 30% and increasing trunk space by 7L; XPeng has established a die casting factory, which is expected to be used in the production of new models to be launched in 2023; Volkswagen will also introduce integrated die casting to build a new factory for the production of the Trinity pure electric vehicle [1]. Die casting is a precision casting method that uses molten metal under pressure in a mold to cool and form. Die casting machines, alloy materials, and molds are known as the three elements of die casting [2]. Some institutions have investigated the technical barriers of integrated die casting as components > molds > equipment > materials, placing molds in a relatively high position [3]. Large die casting machines are produced by companies such as L&L, Haitian, and Yizumi, with 6000~12000T die casting machines already on the market [4-8]; there are many developers of heat-treatment-free aluminum alloy materials, including overseas companies such as Alcoa (USA), Rheinmetall (Germany), and Tesla's self-developed materials. Domestic companies have also developed heat-treatment-free aluminum alloy materials, with NIO, Litz Group, and Human Horizons collaborating with universities and enterprises such as Shanghai Jiao Tong University and Hubei Xinjinyang. These materials have been applied to products [9]. Currently, only a few units, such as Guangzhou Xingqiang Mold Manufacturing Co., Ltd., Ningbo Saiwida Machinery Co., Ltd., and Meilixin Technology Co., Ltd., can complete the manufacturing of key large die casting molds (≥6000T die casting machines) [10]. Recently, some companies are also entering the ranks of integrated die casting mold manufacturing, indicating that the manufacturing of ultra-large die casting molds is complex. In integrated die casting, the manufacturing of die casting molds is difficult and time-consuming, and mold wear is also significant. Relatively speaking, large die casting molds are the key technology for achieving integrated die casting.

2. Analysis of Large Die Casting Mold Manufacturing

2.1 Overview of Integrated Die Casting Molds
Under the "dual carbon" goal, new energy vehicles have become an important support for the automotive industry. With the acceleration of the automotive transformation process, where core lightweight structural parts are integrated into integrated die castings, replacing hundreds of stamping and welding parts with several parts, ultra-large, precision, and complex die casting molds have become a new favorite in automotive mold procurement, bringing many new challenges to the entire industry's technology and application, from mold materials to mold manufacturing [11].

Die casting molds have different usage environments compared to other molds. Die casting involves injecting molten metal at high pressure (150~500MPa) and high temperature (300~1000℃, aluminum alloy 670~730℃) into the die casting mold cavity at a very high speed and in a very short time through the movement of a pressure injection plunger, and then crystallizing and solidifying under pressure to obtain a casting. During the forming process, the mold is repeatedly heated and cooled, and is subjected to wear and corrosion caused by the high-speed jetting of hot molten metal and high-speed scouring. The processing environment is harsh. The mold material requires high thermal fatigue resistance, thermal conductivity, and good wear resistance, corrosion resistance, and high-temperature mechanical properties [12-13]. The service life of aluminum alloy die casting molds is a comprehensive issue involving mold design, material selection, heat treatment, surface treatment, correct use, maintenance, and maintenance of the mold. Problems with the mold include wear, thermal fatigue, cracking, and erosion. Compared with foreign die casting molds, China's die casting molds have a significant gap, with the service life being only 1/5 to 1/3 of that of similar foreign molds [14]. The service life of aluminum alloy die casting molds is related to the wall thickness of the casting and the tonnage of the equipment. For castings with thicker walls, the mold life is relatively shorter, and for molds with larger tonnage, the mold life is shorter.

Large die casting molds are the key technology for achieving integrated die casting. Integrated die casting molds are characterized by their larger size and heavier weight than traditional die casting molds. Both forging and processing of mold materials and mechanical processing are severe tests. The first set of 6800T ultra-large integrated aluminum alloy die casting molds in China was independently developed by Guangzhou Xingqiang Mold Manufacturing Co., Ltd., with a mold weight exceeding 140T [15]. The integrated front engine compartment die casting (mold size 3300mm×2800mm×2000mm, weight approximately 120 tons) jointly designed and manufactured by Changan Automobile Manufacturing Center's casting process and its suppliers over six months was successfully trial-produced in Ningbo Beilun on January 15, 2023 [16]. Large die casting molds are the basic tooling equipment for achieving integrated die casting and are also a symbol of a country's industrial level and product development capabilities. The manufacturing of large die casting molds will have significant practical significance for reducing production costs and improving efficiency in integrated die casting.

2.2 Considerations Regarding Mold Material Selection
Integrated die casting has a greater impact on molds, with harsh working conditions and various stress influences leading to premature cracking and failure. Therefore, higher requirements are placed on mold design, materials, and heat treatment [17]. Die-casting molds can be divided into forming parts, gating systems, mold base parts, venting systems, temperature control systems, etc. The forming part is the core of the mold, including the die cavity and other structural components. The space formed between them is the cavity, forming the geometry of the die casting. Therefore, the mold design determines the shape and precision of the components. The die cavity (core) and the aluminum alloy contact parts generally use high-purity mold steel with low sulfur content. To improve high-temperature softening resistance, mold steel with high molybdenum content is selected, such as: H13, SKD61, 8407, 8417, 1.2344ESR, etc., of hot work tool steel. Improving the purity of mold steel and reducing or eliminating low-melting-point impurities is a fundamental and effective way to prevent premature cracking of die-casting molds. Integrated die-casting molds have complex structures, high manufacturing costs, and long preparation cycles, placing higher demands on the production of die-casting molds. Mold steel suitable for making die-casting molds must have good hot strength, hot fatigue resistance, oxidation resistance, and resistance to liquid metal corrosion. Under this premise, sufficient mold steel forging ratio, good heat treatment process, surface plating, and mold maintenance are used to extend the mold service life [18]. Commonly used hot work tool steel is refined by electroslag remelting (ESR), with a pure and fine microstructure that meets the needs of die-casting cores. Mold steel can be used rationally according to functional requirements to reduce mold costs. Module A is the part forming area, with a complex shape and high surface quality requirements, so imported high-quality hot work tool steel (W350, DIEVAR, etc.) is used. Module B is in contact with the gating system and can use ordinary mold steel (H13) [19]. The hardness of small die-casting molds is HRC 50～52. Integrated molds are particularly large and are expected to have good toughness and no cracking, so a hardness of HRC 45～48 is generally recommended.

2.3 Mold Design and Simulation
One of the difficulties in manufacturing die-casting molds lies in the design. Die-casting molds are crucial to the success of die casting and have always been considered key process equipment in die-casting production. Oversized molds represent a significant leap in size, and there is no precedent for the design of such large molds domestically or internationally [17]. Integrated molds are generally very large, with significant investment, making mold design a crucial step. Integrated die castings have large dimensions, thin and uneven wall thicknesses, complex shapes, long injection processes, and short injection times. Coupled with the characteristics of die casting itself, shrinkage and deformation of castings are unavoidable. Of course, there are also some unexpected incidents in actual operation. The structural design, manufacturing, and reliability verification of large die-casting molds are all challenging. Structural design relies heavily on experience and computational experiments. Integrated molds are even larger, with more complex runner designs, larger wall thickness variations, and greater processing difficulty. This places higher demands on the design of the gating, overflow, venting, and cooling systems, relying on prior experience and extensive experimentation. Higher requirements are placed on mold strength and toughness. Because integrated die casting is performed in an ultra-high vacuum environment with high-speed filling and high-pressure solidification, higher requirements are placed on the strength, toughness, precision, and sealing of the mold. Molds are a key barrier link in the industrial chain, and the reliability of die-casting molds is crucial to the yield rate of integrated die casting. The mold determines the shape and dimensional tolerances of the casting; its gating system determines the filling of the molten metal; the mold strength limits the maximum injection pressure; and the mold can control and adjust the heat balance of the die-casting process. Currently, the yield rate of integrated die casting varies across the industry, with leading companies expected to achieve higher yield rates [20].

In integrated die casting, CAE simulation plays an irreplaceable role in the component and mold design stages. Accurate simulation results serve as an evaluation criterion or standard for design quality. Generally, mesh quality is an important guarantee for reliable simulation results. Integrated castings weigh 70-100 kg, with wall thicknesses of 3-4 mm, and contain numerous reinforcing ribs, rounded corners, and other special structures. Accurate structural mesh is needed for description, requiring at least 100 million cavity mesh elements. Including the mold mesh, the total number of mesh elements is at least 300-500 million. Traditional single-machine software is limited by algorithms and hardware, requiring a network scale of tens of millions to support [21].

ZhiZhu CAE is a locally deployed high-performance die-casting simulation software with simple operation and rich functions. Its powerful post-processing capabilities provide users with in-depth and diverse professional analysis capabilities. SARES, relying on supercomputing clouds, provides users with low-cost, intelligent, and efficient die-casting CAE simulation services in a SaaS format. Its core functions include: die-casting process analysis (minimum casting wall thickness, draft angle, casting fillet radius analysis), mold gating and venting design (parting surface, inlet, runner, venting, overflow system simulation and auxiliary design), mold cooling design (circulating water channels, point cooling, heating oil channels simulation and auxiliary design), mold thermal balance analysis (mold temperature analysis, casting solidification time, mold opening time, spraying time, etc., simulation and optimization of die-casting cycle processes), and injection process analysis (optimization of slow injection process formulation and prediction of air entrainment in the pressure chamber simulation) [22-23].

2.4 Thermal Balance of Integrated Die-Casting Molds

In die-casting production, temperature is one of the core process elements, and proper control of the die-casting mold temperature directly affects product quality and production efficiency. Temperature is an important parameter in the entire die-casting process. Melting temperature, mold temperature, die-casting temperature, water temperature, oil temperature, etc., are all importantly related to the entire die-casting process. In integrated die-casting production, the temperature field changes are more complex: first, the casting weight is close to or exceeds 100 kg, releasing a large amount of heat; second, the wall thickness is thin and the process is long, resulting in uneven temperatures in various parts of the mold. Generally, the mold temperature near the gate and runner is quite high, requiring urgent cooling and heat dissipation; while the mold temperature at the end is too low, resulting in reduced melt fluidity and easily causing casting cold shuts and incomplete injection defects, requiring urgent heating. Past designs mainly focused on the gating system, including the runner, slag well, and vent design, without paying much attention to the internal water and oil channels in the mold. At that time, smaller castings could manage.

Mold thermal management is very important in large-scale integrated die casting. Due to the large size of the mold, the difficulty of mold thermal control increases. The reasonable design of the cooling/heating system is a prerequisite for effectively controlling the mold temperature in production. Mold temperature control has a significant impact on the quality of integrated die castings. Temperatures that are too high or too low will cause defects in the castings [17]. For large integrated castings, the problem of mold temperature thermal balance must be considered. On the one hand, die-casting simulation software is used to analyze the temperature distribution, and on the other hand, practical measures are needed to solve the mold thermal balance [21]. Currently, the main methods include mold temperature control, infrared imaging detection, in-mold sensors, and 3D printing.

2.4.1 Application of Mold Temperature Control

Temperature control units offer intelligent temperature control for molds, heating or cooling them to maintain the set operating temperature. In integrated die casting, more than 30 temperature control units are equipped. A series of high-temperature, medium-temperature, and low-temperature control units (200℃ water temperature unit, 320℃ oil temperature unit, cold and hot temperature control station, multi-channel mold chiller, multi-channel spot chiller, integrated control system) are matched according to the temperature control needs of the casting. A wide temperature range of 20℃~320℃ is designed to provide a stable and controllable temperature source output for mold temperature balance, and the large temperature output range can not only control the mold temperature balance but also extend to the control of the die chamber, punch, flow divider, and sprue bushing, ensuring the needs of die casting [24]. Now, integrated temperature control units have been developed, such as the CTM large-scale high-temperature water-based integrated control station (referred to as "CTM"). A single CTM can replace more than ten temperature control units in the temperature control group unit, making the ultra-large die-casting island temperature control group unit more space-saving, cost-effective, energy-efficient, and precise in temperature control. Usually, more than 30 temperature control units are required, while using the CTM large-scale central hot water control system to replace ordinary temperature control units only requires 2-3 units, which not only reduces the floor space but also lowers the procurement cost, saving more than 30% on equipment costs and more than 70% on installed power. At the same time, CTM has high transmission pressure and faster heating speed; mold temperature data acquisition and centralized data control [25].

2.4.2 Infrared Imaging Detection Application

All objects emit electromagnetic radiation depending on their temperature. The portion of the spectrum with wavelengths between 2.0 and 1000 micrometers is called thermal infrared radiation. Infrared imaging detection uses photoelectric technology to detect the specific infrared band signals of the thermal radiation of objects, converts these signals into images or graphics that can be visually distinguished by humans, and can calculate the temperature value. Infrared thermal imaging technology allows people to "see" the surface temperature distribution of objects. Thermal infrared imaging uses a thermal infrared-sensitive CCD (Charge Coupled Device) to image objects, reflecting the temperature field on the object's surface. Thermal infrared has wide applications in industry and medicine [26-27].

The infrared thermal imaging temperature measurement system uses a thermal imaging thermometer to perform real-time and precise temperature measurement of the mold, providing automatic tracking and alarming of temperature anomalies. Through infrared thermal imaging, it promptly interacts with the PLC (Programmable Logic Controller) for real-time alarming. After alarming, the PLC controls the increase in the spraying time, thus achieving process automation.

The main applications are: meeting the needs of all-day temperature monitoring, setting alarm thresholds for each area to facilitate engineers to adjust die-casting machine parameters in time; real-time display of the full radiation heat map, setting display points, lines, and regional temperature data, directly displaying in temperature anomaly areas; wide temperature monitoring coverage, allowing for large-area viewing of temperature uniformity; automatically locating high-temperature points in the entire image, intuitively seeing problem points, clearly showing high-temperature points on the mold surface, and early detection of potential problem areas; when temperature anomalies occur, the monitoring background can promptly detect them, triggering alarms, and the audio-visual alarm module will issue alarms to remind on-site personnel to adjust the die-casting machine parameters in time; supporting timed storage of images for secondary analysis, facilitating review of the die-casting machine status; the system can define three different alarm thresholds and levels to assist personnel in evaluating the urgency and development trend of potential problems [28]. Because it is an application of infrared technology, various problems in the casting process can be effectively prevented and eliminated in their infancy without interrupting the production process. Because it is unnecessary to use temperature regulation, compressed air, water-based lubricants, mold release agents, etc., the excessively high or low mold temperature during processing has a negative impact on the quality of parts, mold service life, production cycle, energy consumption, and maintenance costs, and it plays a positive role in optimizing the die-casting process [29].

2.4.3 In-Mold Sensor Application

In-mold sensors were used in the 1980s to detect melt pressure or temperature in plastic molds for injection molding process control. In recent years, driven by 5G application scenarios, it has brought more applications and innovations to the development of in-mold sensors. The in-mold state sensor is an injection molding visualization tool developed by a domestic research institute, which has functions such as intelligent sorting of injection molding quality, rapid setting of injection molding parameters, reduction of molding cycle, mold flow analysis verification, and mold temperature monitoring. It has a unique effect on quality variations caused by material changes [30].

The sensor is installed in the mold core and needs to be in direct contact with the material, leaving a mark on the surface (similar to a ejector pin mark), with a diameter of 8 mm. The installation position requires a flat surface (generally choose the gate, temperature fluctuation point, and multiple flow ends). Installing the in-mold sensor in the mold cavity, the data and curves after technical processing of the output melt state information are used to analyze the in-mold sensor curve and obtain the melt state change during the entire injection process. Its main purpose is to improve product quality, reduce waste, shorten processing cycles, and increase efficiency.