How to improve the mold level

The new surface treatment technology of die casting die is introduced in detail.

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Key points of design of molding die for precision plastic parts

In recent years, various new technologies for surface treatment of die casting dies have emerged, but they can be roughly divided into the following three categories: (1) improvement technologies of traditional heat treatment processes; (2) Surface modification technology, including surface thermal diffusion treatment, surface phase transformation strengthening and electric spark strengthening technology. (3) Coating technology, including electroless plating.

1, improvement technology of traditional heat treatment process

The traditional heat treatment process of die casting mold is quenching-tempering, and then the surface treatment technology was developed. Because there are many kinds of materials that can be used as die-casting dies, the same surface treatment technology and technology will produce different effects when applied to different materials. Kefu Shi recently put forward the substrate pretreatment technology and surface treatment technology of mold substrate. On the basis of traditional technology, suitable processing technology is put forward for different die materials, so as to improve die performance and prolong die life. Another development direction of heat treatment technology improvement is to combine traditional heat treatment technology with advanced surface treatment technology to improve the service life of die casting mold. For example, if the chemical heat treatment method of carbonitriding is combined with conventional quenching and tempering processes to strengthen NQN (namely carbonitriding-quenching-carbonitriding), not only higher surface hardness is obtained, but also the depth of effective hardened layer is increased, the hardness gradient distribution of carburized layer is reasonable, and the tempering stability and corrosion resistance are improved, so that the die-casting mold can obtain good core performance and improve surface quality and corrosion resistance.

2. Surface modification technology

2 1, surface thermal diffusion technology

This kind includes carburizing, nitriding, boronizing, carbonitriding, sulfur carbonitriding and so on.

2 1 1, carburizing and carbonitriding

Carburizing process can be used for cold working, hot working and surface strengthening of plastic mould, which can improve the service life of mould. For example, the die-casting die made of 3Cr2W8V steel is carburized first, then quenched at 1 40 ~150℃ and tempered twice at 550℃, and the surface hardness can reach HRC 56 ~ 6 1, which prolongs the die life of die-casting nonferrous metals and their alloys by/kloc-0. 8 ~ 3.0 times. When carburizing, the main technological methods are solid powder carburizing, gas carburizing, vacuum carburizing, ion carburizing and carbonitriding formed by adding nitrogen in carburizing atmosphere. Among them, vacuum carburizing and ion carburizing are technologies developed in recent 20 years, which have the characteristics of fast infiltration rate, uniform infiltration layer, gentle carbon concentration gradient and small workpiece deformation, and will play an increasingly important role in mold surface treatment, especially in precision mold surface treatment.

2 12, nitriding and related low-temperature thermal diffusion technology

This category includes nitriding, ion nitriding, carbonitriding, oxygen nitriding, sulfur nitriding, sulfur carbonitriding and oxygen, nitrogen and sulfur ternary nitriding. These methods have the advantages of simple processing technology, strong adaptability, low diffusion temperature (generally 480 ~ 600℃) and small workpiece deformation, and are especially suitable for surface strengthening of precision molds. The nitrided layer has high hardness, good wear resistance and good anti-sticking performance. After quenching and tempering nitriding at 520 ~ 540℃, the service life of the die casting die for 3Cr2W8V steel is 2 ~ 3 times longer than that without nitriding.

Many die-casting dies of H 13 steel in the United States need to be nitrided, and the surface hardness is as high as HRC65 ~ 70, while the die center has low hardness and good toughness, thus obtaining excellent comprehensive mechanical properties. Nitriding process is a common process for surface treatment of die casting dies. However, when a thin and brittle white layer appears in the nitride layer, it can not resist the action of alternating thermal stress, and it is easy to produce microcracks and reduce the thermal fatigue resistance. Therefore, in the process of nitriding, the process should be strictly controlled to avoid the generation of brittle layer. Recently, secondary and multiple nitriding processes have been proposed abroad. Repeated nitriding can decompose the white nitrided layer which is easy to produce microcracks in the service process, increase the thickness of nitrided layer, and at the same time produce a thicker residual stress layer on the die surface, so that the service life of the die is obviously improved. In addition, there are methods such as salt bath carbonitriding and salt bath sulfur carbonitriding. These processes are widely used abroad, but in China.

Rare. For example, TFI+ABI process is salt bath nitrocarburizing and then soaking in alkaline oxidation salt bath. The surface of the workpiece is oxidized and blackened, and the wear resistance, corrosion resistance and heat resistance are improved. The service life of aluminum alloy die casting die treated by this method is prolonged by hundreds of hours. Another example is the oxynit process developed in France, which is nitrided after sulpho-nitrocarburizing, which is more distinctive when applied to non-ferrous metal die-casting dies.

2 13, boronizing

The boronizing layer has high hardness (Feb: HV 1800 ~ 2300, Fe2B: HV 1300 ~ 1500), wear resistance and red hardness, and certain corrosion resistance and anti-adhesion, so the boronizing technology has achieved good application effect in the mold industry. However, due to the bad working conditions of die-casting dies, boronizing technology is rarely used for surface treatment of die-casting dies. However, in recent years, an improved boronizing method has appeared to solve the above problems, and it has been applied to the surface treatment of die casting dies, such as multi-element and coating powder infiltration. Coating agent powder boronizing method is to mix boron compounds with other boronizing agents and coat them on the surface of die casting die. After the liquid volatilizes, it is packaged and sealed according to the general method of powder boronizing, heated at 920℃ for 8 hours, and then cooled with air. This method can obtain a dense and uniform infiltrated layer, improve the hardness, wear resistance and bending strength of the infiltrated layer on the die surface, and increase the average life of the die by more than 2 times.

2 14, rare earth surface strengthening

In recent years, the method of adding rare earth elements to strengthen the mold surface has been widely respected. This is because rare earth elements have many functions [13], such as improving infiltration rate, strengthening surface and purifying surface. It has a great influence on improving the surface structure, physical, chemical and mechanical properties of the mold, and can increase the infiltration rate, strengthen the surface and generate rare earth compounds. At the same time, it can eliminate the harmful effect of trace impurities distributed on the grain boundary, and play a role in strengthening and stabilizing the grain boundary on the surface of the cavity. In addition, rare earth elements react with harmful elements in steel to produce high melting point compounds, which can also inhibit the segregation of these harmful elements on grain boundaries, thus reducing deep brittleness and so on. Adding rare earth elements in the surface strengthening treatment of die-casting die can obviously increase the thickness of the infiltrated layer and the surface hardness of various infiltration methods, at the same time, make the infiltrated layer structure fine and dispersed, and reduce the hardness gradient, thus significantly improving the wear resistance, cold resistance and thermal fatigue performance of the die, thus greatly improving the life of the die. At present, the treatment methods applied to the cavity surface of die casting die include rare earth carbonitriding, rare earth carbonitriding, rare earth boron carbonitriding, rare earth boron aluminum carbonitriding, rare earth soft nitriding, rare earth sulfur nitrogen carbonitriding and so on.

22, laser surface treatment

Laser surface treatment is to heat the surface of the workpiece with a laser beam to make it quickly melt into a thin layer with a certain depth. At the same time, alloy elements are coated on the surface of the workpiece by vacuum evaporation, electroplating and ion implantation. And fully mixed with the base metal under laser irradiation. After condensation, an alloy layer with special properties with a thickness of 10 ~ 1000 micron is obtained on the surface of the die, and the cooling rate is equivalent to quenching. If the surface of H 13 steel is rapidly melted by laser, the melted zone has high hardness, good thermal stability and strong plastic deformation resistance, which obviously inhibits the initiation and propagation of fatigue cracks. Recently, if Saha and Dahot adopt the method of laser cladding VC layer on H 13 substrate, the research shows that the obtained mold surface is essentially a continuous, dense and nonporous VC steel composite coating, which not only has strong oxidation resistance at 600℃, but also has strong resistance to molten metal reduction [19]. 23 EDM cermet deposition process In the continuous development of surface modification technology, EDM deposition process appeared. In this process, under the action of electric field, an instantaneous high temperature and high pressure zone is generated on the surface of the base metal, and ionic cermet materials are infiltrated at the same time, forming metallurgical bonding on the surface, and the surface of the base metal also undergoes instantaneous phase transformation, forming martensite and fine austenite structure [20]. This process is different from welding, spraying or element infiltration, and should be somewhere in between. It makes full use of the characteristics of high wear resistance, high temperature resistance and corrosion resistance of cermet materials, and has simple process and low cost. This opens up a new way for surface treatment of die casting die.

3. Coating technology

As a kind of mold strengthening technology, coating technology is mainly applied to mold surface treatment with relatively simple working environment, such as plastic mold, glass mold, rubber mold and stamping mold. Die-casting dies need to withstand the harsh environment of alternating cold and hot stress, so coating technology is generally not used to strengthen the surface of die-casting dies. However, in recent years, it has been reported that the surface of die casting die is strengthened by chemical composite plating to improve the anti-sticking and demoulding performance of the die surface. In this method, PTFE particles are soaked in an aluminum-based die-casting die, and then (NIP)- PTFE composite plating is carried out. Experiments show that this method is feasible.

4. It is feasible in art and performance, which greatly reduces the friction coefficient of the die surface.

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Die pressure machining is an important part of mechanical manufacturing, and the level, quality and life of die are closely related to die surface strengthening technology. With the progress of science and technology, various die surface treatment technologies have made great progress in recent years. The performance is as follows: ① the improvement of traditional heat treatment process and its combination with other new processes; (2) Surface modification technologies, including carburization and low-temperature thermal diffusion (various nitriding, carbonitriding, ion nitriding, ternary diffusion, etc.). ), salt bath thermal diffusion, boronizing, rare earth surface strengthening, laser surface treatment and electro-spark deposition of cermets. ③ coating technology, etc. However, for die-casting dies with extremely bad working conditions, the existing new surface treatment technology can no longer meet the growing requirements, and more advanced technology can be expected, and it is also expected to be applied to the surface treatment of die-casting dies. In view of the fact that surface treatment is one of the important means to improve the service life of die-casting dies, surface treatment technology will play an important role in improving the overall production level of die-casting dies in China.

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