Pressure die casting (referred as diecast) has characteristics of high production efficiency, short production process, high smoothness and strength of castings, low machining allowance and metal material saving. In recent years, China's diecast industry has developed rapidly and its total output has grown significantly. It become already a veritable diecast country. Molds, diecast machines and diecast materials are three major elements of diecast production. Only high-quality molds can produce high-quality castings with stability and efficiency. If working environment of die casting mold is very bad: cavity is directly in contact with high-temperature, pressure, speed metal liquid phase during diecast production process, and is directly washed by molten metal, which is easy to wear, high-temperature oxidation and various corrosion. High-efficiency production periodically increased and decreased sharply mold temperature, working surface is prone to thermal fatigue cracking; when metal is forced to deform, it rubs against surface of cavity, easily wears mold and reduces its hardness. Cost of molds is high, production cycle is long, and repairs are difficult. So it is very important in terms of service life. Therefore, research on factors affecting mold performance and service life is beneficial to improve casting quality and reduce economic losses due to premature scrapping of molds. In general, factors that affect performance and service life of die casting mold include mold materials, molding design and manufacturing, surface treatment techniques, and mold specific use.
Die casting mold is a very expensive special precision machine. It requires mold repairer not only to have superb technology, fine style, but also to have a serious and responsible spirit. Mold repairers should be familiar with technical standards of die casting mold as follows:

 

1, After metal pin and fouling of mould is cleaned, showing true color of mold.
2. Refer to the last diecast product that was sent for repair with mold and carefully check mold for problems. Whether there is no strain, sticking, crushing, whether there is a small core bent or broken, whether movable core is inserted or not positioned, whether there is a broken push rod or length of push rod changes, whether insert is positioned or not precise, with or without fastening bolts. Make repairs or replacements based on damage.
3, Cavity collapse, cracks, falling blocks which cause slight strain on casting, can be repaired locally. Repair should be strictly in accordance with welding repair process, otherwise many die life will be lost. Smaller mold parts have more serious problems or mold damage.
4. Surface of larger mold parts is seriously collapsed, cracked, dropped and can be repaired by local welding. Repair and repair should be strictly in accordance with welding repair process, otherwise many die life will be lost. Smaller mold parts have more serious problems or mold damage.
5. For sliding parts such as core pulling mechanism and guiding device, it should be thoroughly cleaned, carefully inspected and repaired. Then relubricate with high temperature grease and assemble.
6. If there is hydraulic core pulling, hydraulic part and mold are repaired at the same time. It should pays special attention to cleaning to prevent pollution for hydraulic part maintenance, otherwise it will pollute entire die casting machine hydraulic system.
7. When mold fails or is damaged in production process, repair plan should be determined according to specific conditions. If necessary, follow above comprehensive repairs.
8. Complete maintenance and repair of mold, and make molding surface, parting surface and mounting surface to be rust-proofed, close and fixed, and place it on the backing plate according to installation direction of mold on the machine. Mold attachment is 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, high hardness, high temperature oxidation resistance, high 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 widely used in China, and life of die-casting die was about 50,000. In 1990s, H13 hot work die steel was introduced, and die-casting die produced can have a service life of 150 to 200,000 molds and become a widely used die casting mold material. 3Cr2W8V hot work die steel has high strength and hardness, good resistance to cold and heat fatigue, and good hardenability, but poor toughness and plasticity, long service life, high alloying degree and high cost. H13 has good comprehensive performance at medium temperature (~600 °C), high hardenability (hardenable in air), low heat treatment deformation rate, and its performance and service life is higher than 3Cr2W8V.
Selection of die casting mold, in addition to temperature and type of cast metal, should also consider impact and wear of components of die casting mold. The higher the temperature, the higher the thermal fatigue properties and high temperature properties of material. Parts with more severe wear should have higher hardness. With working conditions of die casting mold are becoming more and more demanding, requirements for metallurgical quality, performance and life of mold materials are continuously improved, especially requirements for material purity and isotropicity are high. Some high-alloy, high-quality and optimized mold materials are constantly appearing. This in turn promotes development of die casting industry.

Reasonable molding design is an important prerequisite for extending service life of die casting mold. Reasonable wall thickness and cooling channel design ensure mold strength and heat balance. In molding design, special attention should be paid to parts of work where stress is concentrated and there is a large abrasion. Accuracy of each part selected should be reasonable: if gap is too large, heat conduction is not good, resulting in thermal fatigue damage; if gap is too small, extrusion pressure and tensile stress will be generated. Internal stress is easily generated during mold manufacturing process, and internal stress has a great influence on the life of mold. Therefore, internal stress should be avoided and eliminated as much as possible during manufacturing process. For example, after rough machining, stress should be tempered in time, and electric pulse is used instead of electric spark to reduce tension on the surface 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 major categories: traditional heat treatment process improvement technology; surface modification technology, such as surface laser processing technology; coating technology.

Traditional die casting mold heat treatment process is quenching-tempering. The so-called conventional heat treatment process is an improved technique that combines quenching-tempering with advanced surface treatment processes. Such as NQN (carbonitriding-quenching-carbonitriding composite strengthening), higher mold surface hardness, increased internal strength, reasonable hardness gradient of layer, improved tempering stability and corrosion resistance, and substantially increase comprehensive performance and service life.

Surface modification technology refers to use of physical or chemical methods to change surface properties of mold. Generally, there are two kinds of surface heat, expansion, infiltration techniques and surface laser treatment.
Surface heat, expansion, infiltration techniques include carburizing, nitriding, boronizing, and carbonitriding, and sulfur-carbonitriding. Carburizing helps to strengthen surface hardness of mold. Carburizing processes include carburizing of solid powder, gas, vacuum, ion. Vacuum carburizing and ion carburizing have a fast osmosis rate, uniform infiltration layer, gentle carbon concentration gradient and small deformation of workpiece. Nitriding process is simple, die nitride layer has high hardness, good wear resistance and good anti-sticking performance. Boronizing enhances surface performance most obviously, hardness, wear resistance, corrosion resistance and blocking resistance of mold are significantly improved, but process conditions are harsh.
Laser processing of mold surfaces is a technology that has emerged over past three decades to improve mold surface performance in two ways. One is surface of laser melting mold, which is then combined with processes of carburizing, nitriding, and plating. Another method is to combine laser surface technology with some metal materials with good physical properties to integrate into surface of die casting mold.

Coating technology is to apply a coating on surface, such as PTFE composite plating, the main purpose is to enhance wear resistance, corrosion resistance and resistance to cold and heat.

Choosing a reasonable die casting process and maintenance is critical to life of mold. Most of mold damage is caused by improper use and lack of scientific maintenance. First, special attention should be paid to temperature control of mold. Mold should be preheated before production, and proper temperature range should be maintained during production to prevent surface cracks or even cracks caused by excessive temperature gradient inside and outside the cavity. Secondly, use of high-quality die-casting release agent, its thickness is moderate, thin and evenly coated with 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 periodically use tempering and other techniques to eliminate thermal stress.

 

Die casting mold materials, molding design and manufacturing, mold surface treatment technology and mold use affect performance and service life of mold. By combining these factors and taking effective measures, performance of die casting mold can be effectively improved and service life of die casting mold can be extended.