1. Introduction
In recent years, China's die casting industry has grown rapidly, with a significant increase in total output, making it a global leader in die casting. Molds, die casting machines, and die casting materials are the three key elements in die casting production, with molds being particularly crucial as they directly determine the quality of castings. However, die casting molds operate in extremely harsh conditions, where the mold cavity is exposed to high-temperature, high-pressure molten metal, leading to wear, oxidation, and corrosion. Additionally, the pursuit of high production efficiency causes periodic severe temperature fluctuations in molds, resulting in thermal fatigue cracks. The forced deformation of metals also wears down molds and reduces their hardness. Given the high cost, long manufacturing lead time, and difficulty in repairing die casting molds, it is essential to study the factors affecting mold performance and service life to improve casting quality and minimize economic losses from premature mold failure.
2. Die Casting Mold Maintenance Standards
Die casting molds are expensive and precision-intensive machinery, demanding high technical skills, meticulous work habits, and a strong sense of responsibility from mold maintenance personnel. These workers must be well-versed in the following eight essential technical standards for die casting molds:
(1) Mold Cleaning: Restoring the Mold's Original Appearance
Thoroughly clean all metal residues and accumulations from the mold to restore its original appearance. This step is crucial for ensuring the mold operates correctly and produces high-quality castings. Utilize specialized cleaning tools and agents to meticulously clean every part of the mold, including the cavity, core, and cooling channels. For stubborn accumulations, advanced processes like ultrasonic cleaning can be employed to ensure optimal cleaning results. Regular mold cleaning not only facilitates timely detection of potential issues, such as minor cracks and wear on the mold surface but also provides valuable information for subsequent mold repair and maintenance efforts.
(2) Problem Inspection: Comprehensive Mold Examination
Referencing the last die casting parts sent for repair, meticulously inspect the mold for issues such as scoring, sticking, deformation, metal loss, bending or breakage of small cores, misalignment of moving cores, broken or length-altered ejector pins, misaligned inserts, and loosened fasteners. Based on the extent of damage, decide whether repair or replacement is necessary. During the inspection, advanced detection equipment and tools, such as coordinate measuring machines and hardness testers, should be used to precisely measure and evaluate the mold's dimensional accuracy, geometric tolerances, and hardness. Additionally, a mold maintenance archive should be established to document each inspection and repair, enabling tracking and analysis of the mold's service life and performance changes over time.
(3) Local Welding Repair: Precision Repair to Extend Mold Life
For minor issues like cavity collapse, cracks, and metal loss that cause slight part scoring, local welding repair can be performed. However, strict adherence to welding repair procedures is essential to avoid significant reduction in mold life. Before welding, thoroughly clean the damaged area of the mold, including removing oil, rust, and cracks. Then, select appropriate welding materials and processes based on the mold material and damage severity, such as TIG welding or cold welding. During welding, carefully control parameters like current, voltage, and welding speed to minimize deformation and stress. After welding, conduct grinding, polishing, and other post-processing steps to ensure surface quality and dimensional accuracy of the repaired area. For smaller forming components with more severe problems or mold damage, greater caution is required, and replacing new mold components may be necessary when needed.
(4) Repair of Large Forming Components: Overcoming Challenges to Restore Mold Performance
If larger forming components exhibit severe issues such as surface collapse, cracks, and metal loss, local welding repair can also be performed, strictly following welding procedures. Compared to smaller forming components, damage to larger ones is often more severe and challenging to repair. Therefore, higher technical skills and greater attention are required. In addition to conventional welding methods, advanced repair technologies such as laser cladding and plasma spraying can be employed. These technologies form a high-performance alloy layer on the mold surface, not only repairing damaged areas but also enhancing surface hardness, wear resistance, and corrosion resistance. During the repair process, the mold's overall structure and mechanical properties should be fully considered to ensure the repaired mold meets production requirements.
(5) Maintenance of Sliding Components: Ensuring Smooth Mold Operation
Thoroughly clean, inspect, and repair sliding components such as core-pulling mechanisms and guide devices. Re-lubricate with high-temperature lubricating grease before reassembly to ensure smooth operation of these components, reduce friction and wear, and extend mold life. During maintenance, check parameters such as the fitting clearance and surface roughness of sliding components to ensure they remain within acceptable ranges. Replace severely worn sliding components promptly. Additionally, establish a regular lubrication and maintenance schedule for sliding components based on mold usage frequency and operating conditions, ensuring proper lubrication and optimal performance.
(6) Hydraulic System Maintenance: Safeguarding the Mold's Critical System
If the mold is equipped with a hydraulic core-pulling system, the hydraulic components should be repaired simultaneously with the mold. During maintenance, pay special attention to cleanliness to prevent contaminants from entering the hydraulic system, which could damage the entire die casting machine's hydraulic system. Begin by conducting a comprehensive inspection of the hydraulic system, including components such as hydraulic pumps, cylinders, solenoid valves, and oil lines, to identify issues like leaks, blockages, and wear. Then, disassemble and clean the hydraulic system, using specialized cleaning agents and filtration equipment to thoroughly remove contaminants and impurities. When reassembling the hydraulic system, ensure correct installation of all components, proper alignment, and secure connections. Finally, perform debugging and testing of the hydraulic system to verify that parameters such as working pressure, flow rate, and speed meet design requirements, ensuring proper operation of the hydraulic system.
(7) Fault Repair Strategy: Tailored Solutions for Optimal Results
When mold failures or damage occur during production, develop a repair strategy based on the specific situation. If necessary, carry out comprehensive repairs in accordance with the above standards to restore mold performance and ensure production efficiency and casting quality. When formulating a repair strategy, consider factors such as the extent of mold damage, repair complexity, cost, and timeline. For minor faults, simple repairs like replacing damaged components or adjusting mold fitting clearances may suffice. For more severe damage, comprehensive mold repair, including structural optimization and improvement, may be required. Strictly follow relevant process standards and operating procedures during the repair process to ensure repair quality. After repair, conduct rigorous inspection and testing of the mold, including visual examination, dimensional accuracy measurement, and performance testing, to confirm that the mold meets production requirements.
(8) Anti-Rust Treatment: Protecting the Mold with a "Coating"
After completing mold maintenance, apply anti-rust treatment to the forming surfaces, parting lines, and mounting surfaces. Close the mold, fix it on a pad according to its installation orientation on the machine, and store it with all accessories to prevent rust and corrosion during idle periods, which could affect mold life. Various methods can be used for anti-rust treatment, such as applying anti-rust oil, spraying anti-rust paint, or using anti-rust paper. Select appropriate anti-rust methods and materials based on the mold material, storage environment, and usage requirements. Ensure uniform and complete coverage of the mold surface with the anti-rust agent. Store the mold in a dry, well-ventilated environment free from corrosive gases to prevent moisture and corrosive substance exposure. Regularly inspect and maintain stored molds; if the anti-rust layer is damaged or signs of rust appear, promptly repair and treat the affected areas.
3. Mold Materials: The Foundation of Mold Performance
The performance and service life of die casting molds are closely related to the mold material. High-quality die casting mold materials typically exhibit the following characteristics: excellent machinability and forgeability; high wear and corrosion resistance; high strength and red hardness at elevated temperatures, along with resistance to high-temperature oxidation, impact toughness, and temper stability; good thermal conductivity and fatigue resistance; low thermal expansion coefficient; minimal heat treatment distortion; and good hardenability.
In the past, 3Cr2W8V hot work mold steel was widely used in China, with a die casting mold life of approximately 50,000 cycles. Since the introduction of H13 hot work mold steel in the 1990s, the service life of die casting molds has increased to 150,000–200,000 cycles, and H13 is now extensively applied. While 3Cr2W8V offers high strength and hardness, good resistance to cold and thermal fatigue, and excellent hardenability, its inferior toughness and ductility limit its service life and increase its cost due to high alloy content. In contrast, H13 demonstrates superior comprehensive performance at moderate temperatures (~600°C), with high hardenability, low heat treatment distortion, and longer service life compared to 3Cr2W8V.
When selecting materials for die casting molds, factors such as the temperature and type of molten metal being cast, as well as the impact and wear on mold components, must be considered. Materials should possess higher thermal fatigue and high-temperature resistance at elevated temperatures and greater hardness in high-wear applications. As die casting mold operating conditions become increasingly demanding, requirements for mold material metallurgical quality, performance, and service life continue to rise. Purity and isotropy are of particular concern, with the emergence of high-alloy, high-quality, and optimized mold materials driving advancements in the die casting industry.
4. Mold Design and Manufacturing: Precision Design and Careful Production
Reasonable mold design is a crucial prerequisite for extending the service life of die casting molds. Proper wall thickness and cooling channel design ensure mold strength and thermal balance. During mold design, special attention should be given to areas prone to stress concentration and significant wear. The precision of mating parts must also be carefully controlled: excessive clearances can impair heat transfer, leading to thermal fatigue damage, while insufficient clearances can generate compressive and tensile stresses. Mold manufacturing can induce internal stresses, which significantly impact mold service life. Therefore, during mold processing, measures should be taken to minimize and promptly relieve internal stresses. For instance, stress-relieving annealing should be performed promptly after rough machining, and electrical discharge machining (EDM) can be replaced with electro pulse machining to reduce surface tensile stresses on molds.
5. Mold Surface Treatment Technology: A Key to Enhancing Mold Performance
Rigorous and appropriate surface treatment of die casting molds can significantly improve mold performance and longevity. Die casting mold surface treatment technologies primarily fall into the following three categories:
(1) Improved Traditional Heat Treatment Processes: Optimizing Processes to Boost Performance
The traditional heat treatment process for die casting molds involves quenching and tempering. The improved process combines quenching and tempering with advanced surface treatment techniques, such as NQN (i.e., carbonitriding - quenching - carbonitriding composite strengthening). This composite strengthening process enhances surface hardness, increases internal strength, ensures a reasonable hardness gradient in the case, and improves temper stability and corrosion resistance, thereby significantly elevating the mold's overall performance and service life. In practical applications, optimizing heat treatment parameters such as temperature, time, and cooling rate can further enhance mold performance. Additionally, selecting appropriate combinations of heat treatment and surface treatment processes based on mold operating conditions and requirements can achieve optimal performance improvements.
(2) Surface Modification Technologies: Innovative Methods to Enhance Surface Performance
Surface modification technologies alter the surface properties of molds using physical or chemical methods and are mainly categorized into surface thermo-diffusion treatments and laser surface treatment technologies. Surface thermo-diffusion treatments include processes such as carburizing, nitriding, boriding, and carbonitriding. Carburizing enhances mold surface hardness and can be performed using methods like solid powder carburizing, gas carburizing, vacuum carburizing, and ion carburizing. Among these, vacuum and ion carburizing offer fast penetration, uniform case depth, gradual carbon concentration gradients, and minimal workpiece distortion. Nitriding is a simple process that produces high hardness and wear-resistant nitride layers on molds, providing excellent anti-adhesion properties. Boriding offers the most significant improvements in mold surface properties, significantly enhancing hardness, wear resistance, corrosion resistance, and anti-galling performance. However, this process requires stringent conditions. Laser surface treatment technology, an emerging technique of the past three decades, enhances mold surface performance in two primary ways: laser melting of the mold surface followed by combination with other processes such as carburizing, nitriding, or coating; and integration of laser surface treatment with metallic additives boasting superior physical properties to fortify the mold surface. Through surface modification technologies, a high-performance modified layer can be formed on the mold surface, thereby extending mold service life and boosting production efficiency.
(3) Coating Technologies: Protection and Decoration
Coating technology involves applying a protective layer on the mold surface, such as PTFE composite coatings. This technology primarily aims to enhance the mold's wear resistance, corrosion resistance, and ability to withstand thermal fluctuations. The protective layer effectively shields the mold surface from wear and corrosion, prolonging mold service life. In addition to its protective function, coating technology can also enhance the mold's appearance, improving its aesthetic and decorative qualities. When selecting coating materials and processes, it is essential to consider the mold's operational requirements and working environment, taking into account factors such as coating performance, cost, and environmental impact to achieve optimal coating results.
6. Mold Operation and Maintenance: Scientific Management to Extend Mold Life
Selecting appropriate die casting processes and implementing scientific maintenance practices are crucial for prolonging mold service life. Many mold damages result from improper use and lack of systematic maintenance. Firstly, mold temperature control is of paramount importance. Preheat the mold before production and maintain an appropriate temperature range during production to prevent surface cracks or even cracking caused by excessive temperature gradients between the cavity and exterior layers. Induction heating devices or hot oil circulation systems can be employed for uniform mold preheating. During production, real-time monitoring and temperature regulation of the mold can be achieved through cooling water systems and temperature control systems to ensure stable mold temperature. Secondly, use high-quality die casting release agents with moderate thickness and uniform application on the mold surface to protect mold materials. The release agent should possess excellent lubricity, easy release properties, and stability, forming a uniform film on the mold surface to prevent adhesion between castings and the mold, reducing ejection resistance. It should also be compatible with mold materials and die casting alloys, without adversely affecting the mold or castings. Finally, to minimize heat stress accumulation and prevent mold cracking, regularly use tempering techniques to eliminate heat stress. Based on mold usage and heat treatment requirements, establish a reasonable tempering schedule and process to promptly relieve internal stresses in the mold, restoring its microstructure and properties. These measures can effectively extend mold service life, enhance production efficiency, and improve casting quality.
7. Conclusion
The performance and service life of die casting molds are influenced by a combination of factors, including mold materials, mold design and manufacturing, mold surface treatment technologies, and mold operation and maintenance. By comprehensively considering these factors and implementing effective optimization and improvement measures, the performance of die casting molds can be significantly enhanced, and their service life prolonged. This not only improves the overall quality and efficiency of die casting production but also reduces economic losses from premature mold failure, providing robust support for the sustainable development of the die casting industry.