Pressure casting (die casting for short) has characteristics of high production efficiency, short production process, high casting finish and strength, small machining allowance, and saving of metal materials. China's die-casting industry has developed rapidly in recent years, with total output increasing significantly, and it has become a veritable die-casting power. Molds, die-casting machines and die-casting materials are the three major elements of die-casting production. Only high-quality molds can produce high-quality castings stably and efficiently. Working environment of die-casting molds is very harsh: During die-casting production process, mold cavity is in direct contact with high-temperature, high-pressure, and high-speed molten metal, and is directly washed by molten metal. It is prone to wear, high-temperature oxidation, and various corrosions: high-efficiency production causes mold temperature to rise and fall dramatically periodically, and working surface is prone to thermal fatigue cracks: when metal is forced to deform, it rubs against surface of cavity, easily wearing out mold and reducing its hardness. Mold cost is high, production cycle is long, and repair is difficult. Service life is also particularly important. Therefore, research on factors that affect mold performance and service life is beneficial to improving casting quality and reducing economic losses caused by early scrapping of molds. Generally speaking, factors that affect performance and service life of die-casting molds include mold materials, mold design and manufacturing, surface treatment technology and specific usage of mold.

 

Die-casting molds are very expensive special precision machinery, which require mold maintenance workers not only to have superb skills and meticulous work style, but also to be serious and responsible. Mold maintenance workers should be familiar with technical standards of die-casting molds as follows:
1. It is all about cleaning metal cracks and scales everywhere in mold to reveal true color of mold.
2. Refer to the last die-cast product sent for repair together with mold to carefully check problems with mold. Is there any strain, sticking to mold, pressure or loss of flesh? Is there any small core that is bent or broken? Is there any movable core that is inserted incorrectly in position? Is there a broken push rod or a change in length of push rod? Is there any insert that is not positioned correctly? Is there fastening bolts are loose, etc. Depending on damage, repair or replacement is determined.
3. For cavity collapse, cracks, falling pieces, etc. that cause slight strain on casting, partial welding repairs can be performed. Welding repairs should be performed strictly according to welding repair process, otherwise a lot of mold life will be lost. Above failures of smaller molded parts are more serious or mold is damaged.
4. If molding surface of larger molded parts is seriously collapsed, cracked, or dropped, it can be repaired locally by welding. Welding repair should be performed strictly according to welding repair process, otherwise a lot of mold life will be lost. Above failures of smaller molded parts are more serious or mold is damaged.
5. Sliding parts such as core pulling mechanisms, guide devices, etc. should be thoroughly cleaned, carefully inspected, and repaired. Re-lubricate with high-temperature grease before assembly.
6. If there is hydraulic core pulling, hydraulic part and mold should be repaired at the same time. When repairing hydraulic part, pay special attention to cleanliness to prevent contamination, otherwise the entire hydraulic system of die-casting machine will be polluted.
7. When mold fails or is damaged during production process, repair plan should be determined according to specific situation. Perform comprehensive repairs as above if necessary.
8. After maintenance of mold has been completed, rust-proof molding surface, parting surface, and installation surface, close and set mold, and place it on backing plate according to installation direction of mold on machine. Mold accessories are placed with mold.

Performance and service life of die-casting mold are closely related to material of mold. Good die-casting mold manufacturing materials generally have following characteristics: good machinability and forgeability; high wear resistance and corrosion resistance: high strength at high temperatures, high red hardness, high temperature oxidation resistance, impact toughness and tempering stability at high temperatures: good thermal conductivity and fatigue resistance; small thermal expansion coefficient: small heat treatment deformation rate and good hardenability.
In the past, 3Cr2W8V hot work die steel was commonly used in China, and life of die-casting mold was about 50,000 mold times. H13 hot work die steel was introduced in the 1990s, and die-casting molds produced have a service life of 150,000 to 200,000 molds. It is currently a widely used die-casting mold material. 3Cr2W8V hot work die steel has high strength and hardness, good cold and heat fatigue resistance, and good hardenability, but has poor toughness and plasticity, short service life, high alloying degree, and high cost. H13 has good comprehensive properties at medium temperature (~600℃), high hardenability (can be hardened in air), low heat treatment deformation rate, and its performance and service life are higher than 3Cr2W8V.
Material selection of die-casting mold should not only be based on temperature of casting metal and type of casting metal, but also impact and wear of various parts of die-casting mold by casting metal. The higher temperature, the higher thermal fatigue performance and high temperature performance material should have. Parts that are more severely worn should have higher hardness. Working conditions of die-casting molds are becoming increasingly demanding, and requirements for metallurgical quality, performance, and lifespan of mold materials are constantly increasing. In particular, requirements for material purity and isotropy are higher. Some high-alloy, high-quality, and optimized mold materials are constantly emerging. In turn, it also promotes development of die-casting industry.

Reasonable mold design is an important prerequisite for extending service life of die-casting molds. Reasonable wall thickness and cooling water channel design can ensure strength and thermal balance of mold. When designing mold, special attention should be paid to the areas where stress concentration and greater abrasion occur during work. Accuracy of each selected part must be reasonable: if gap is too large, heat conduction will be poor, leading to thermal fatigue damage; if gap is too small, extrusion force and tensile stress will occur. Internal stress is easily generated during mold manufacturing process, and internal stress has a great impact on service life of mold. Therefore, in process of manufacturing and processing molds, internal stress should be avoided and eliminated in time. For example, after rough machining, timely stress relief and tempering can be performed, and electric pulses can be used instead of electric sparks to reduce surface tension of mold.

Through rigorous and reasonable technical treatment of surface of die-casting mold, its performance and life can be greatly improved. Die-casting mold surface treatment technology can be roughly divided into three categories: traditional heat treatment process improvement technology; surface modification technology, such as surface laser treatment technology: coating technology.
(1) Improvement technology of traditional heat treatment process. Traditional die-casting mold heat treatment process is quenching-tempering. So-called improvement technology of traditional heat treatment process is to combine quenching-tempering with advanced surface treatment technology. Such as NQN (i.e. carbonitriding-quenching-carbonitriding composite strengthening), surface hardness of mold is higher, internal strength is increased, hardness gradient of casing layer is reasonable, tempering stability and corrosion resistance are improved, the overall performance and service life are greatly improved.
(2) Surface modification technology. Surface modification technology refers to use of physical or chemical methods to change properties of mold surface. Generally speaking, there are two types: surface heat, expansion and penetration technology and surface laser treatment technology. Surface heat, expansion and carburizing technologies include carburizing, nitriding, boronizing, carbonitriding, sulfur carbonitriding, etc. Carburizing helps strengthen surface hardness of mold. Carburizing process methods include solid powder carburizing, gas carburizing, vacuum carburizing, and ion carburizing. Vacuum carburizing and ion carburizing have fast carburizing speed, uniform carburizing layer, gentle carbon concentration gradient and small deformation of workpiece. Nitriding process is simple, and nitrided layer of mold has high hardness, good wear resistance, and good mold sticking resistance. Boriding improves surface properties most obviously, and mold hardness, wear resistance, corrosion resistance and adhesion resistance are significantly improved, but process conditions are harsh.
Laser treatment of mold surfaces is a technology that has emerged in the past thirty years. It improves surface performance of molds in two ways. One is to melt mold surface with laser and then combine it with carburizing, nitriding, plating and other processes. Another method is to combine laser treatment surface technology with some metal auxiliary materials with better physical properties to integrate them into surface of die-casting mold.
(3) Coating technology. Coating technology is to put a layer of protective clothing on mold by coating the surface, such as polytetrafluoroethylene composite plating. Main purpose is to enhance wear resistance, corrosion resistance and cold and heat resistance of mold.

Choosing a reasonable die-casting process and maintenance are crucial to service life of mold. Most mold damage is caused by improper use and lack of scientific maintenance. First of all, special attention should be paid to temperature control of mold. Mold should be preheated before production and an appropriate temperature range should be maintained during production to prevent surface cracks or even cracking caused by excessive temperature gradients between inner and outer layers of cavity. Secondly, choose a high-quality die-casting release agent with a moderate thickness and evenly coat mold surface, which plays an important role in protecting mold material. Finally, in order to reduce accumulation of thermal stress and avoid cracking of die-casting mold, it is necessary to regularly use techniques such as tempering to eliminate thermal stress.

Die-casting mold materials, mold design and manufacturing, mold surface treatment technology and mold usage comprehensively affect performance and service life of mold. Combining these factors and taking effective measures can effectively improve performance of die-casting molds and extend service life of die-casting molds.