Stamping parts strain is a common quality defect in the production process, which is common in major automobile manufacturers. On the one hand, it reduces the stability and production efficiency of the production process, and the scrap rate of parts increases. On the other hand, it will cause more serious wear of the mold, reduce the life of the mold and the precision of stamping parts, and increase the number of mold repairs and production downtime.
The essence of napping is due to local adhesion (occlusion) on the surface of the workpiece and the mold. There are many ways to improve the napping problem. The basic principle is to change the nature of the friction pair between the mold and the processed part, so that the friction pair is made of materials that are not easy to adhere to. replace. After the mold enters the debugging stage of the production site, there are generally the following methods to improve the picking problem: 1. Change the mold material and increase the hardness of the mold; 2. Treat the surface of the mold, such as hard chrome plating, PVD and TD; Coating with nano-coating, such as RNT technology, etc.; 4. Add a layer of other substances between the mold and the processed parts to separate the processed parts from the mold (such as applying lubrication or special lubricants or adding a layer of PVC and other materials ); 5. Use self-lubricating coated steel plate.
In terms of mold materials, mold steel SKD11, CR12MOV, etc. are recognized as wear-resistant and anti-occlusion materials. After heat treatment, the hardness can reach about chromium hardness HRC58-63 degrees. Such materials can be used when the mold is small and the shape of the part is relatively simple. However, this material is difficult to process after heat treatment, very brittle, easy to crack, high cost, and limited in size, and this kind of material has large deformation after heat treatment, and the research and development work after heat treatment is huge.
The shape of the inner panel of the car is relatively complex and more and more high-strength steel plates are used. This kind of parts has higher requirements on the overall performance of the mold. It usually adopts an inlaid structure. The surface treatment process of the inlay currently includes TD, plating Hard chrome, nitriding, PVD, etc.
TD treatment is the abbreviation of Thermal Diffusion Carbide Coating Process (Thermal Diffusion Carbide Coating Process). This technology was first developed and patented by Toyota Central Research Institute in Japan in the 1970s. It is also called Toyota Diffusion Process, or TD for short. Process, that is, TD processing. It is also called molten salt infiltration metal in our country. Regardless of its name, its principle is to place the workpiece in a molten borax mixture, and form a metal carbide coating on the surface of the workpiece through high-temperature diffusion.
The main characteristics of TD coating treatment are: high coating hardness, HV can reach about 3000, high wear resistance, tensile resistance, corrosion resistance and other properties, and the service life of TD coating is about 100,000 units; but TD coating Layer treatment has high requirements on mold materials, and the thermal stress, phase transition stress, and specific volume changes generated during high-temperature treatment will easily cause deformation or even cracking of the mold during heat treatment. There will also be cracking. TD coating treatment has high requirements on the processing quality and shape of the mold; in addition, it is difficult to process after TD coating treatment, which cannot meet the needs of design changes and mold adjustment and repair. For molds with other surface treatments, the original surface treatment needs to be completely removed, otherwise it will affect the surface quality of the TD cladding. In addition, the TD cladding treatment technology will generally reduce the service life after 3-4 treatments.
PVD (Physical Vapor Deposition) is the physical vapor deposition method, and the PVD coating is the surface coating manufactured by the physical vapor deposition method. It has good anti-stretch performance, and the hardness of the coating can be as high as HV2000-3000, or even higher, so it has excellent wear resistance, and its processing temperature is relatively low, the deformation of the processed workpiece is small, and it can be processed many times without affecting the service life. and other advantages, but the bonding force between the coating and the substrate is poor, and it is easy to cause the coating to fall off when used on deep-drawing molds and molds with high forming pressure, and cannot exert its anti-strain and wear-resistant effects.
PVD coating
The size of the outer plate mold is generally large. If the mosaic structure is used, there will be strain at the seam, so most of them adopt the overall structure, and the material is generally made of cast iron such as ductile iron. The hardness of the forming feeding part can reach about HRC50-55 degrees after flame quenching.
Most of the surface treatment of the outer plate mold of the overall structure adopts hard chromium plating process, but its surface hardening effect is limited, and the surface hardness is about 1000HV. In addition, the hard chromium plating layer is mechanically combined with the mold base material, which is easy Once the coating falls off, the anti-scratch performance will be lost. When the surface hardening layer is worn, the roughness will appear again, and the service life of the surface hardening layer is generally about 50,000 to 100,000 units.
chrome
RNT is an emerging technology in recent years. Its working principle is that after coating the mold cavity with the RNT coating liquid, the coating nano-molecules are diffused by pressure and act on the surface of the mold to form a nano-metal carbide coating. The process expands from the inside to the outside, and the thickness and hardness vary with the The working time of the mold increases, the thickness of the coating is 0.1-1μm, and the hardness of the coating is HV1100-1600. Even when the mold bears a large load, the coating layer on the surface will not fall off and fail due to the plastic deformation of the substrate. Its thickness and hardness increase with the working time of the mold and the number of coatings from the inside to the outside. Applying RNT coating once can generally guarantee 100-500 pieces without napping. However, the application of this technology to parts with severe napping, parts that generate heat during production and ultra-high-strength plates is still immature, and the cost of use is relatively high.
The use of reasonable lubricants in the production process can effectively improve the friction conditions and reduce fuzzing. Its main function is to separate the contact pairs with a lubricating oil film. Oiling is generally done manually or with automatic equipment on the line. In addition, the use of lubricants can also effectively reduce dark spots and cracking problems. However, the use of lubricants will make the environment dirty and slippery. In order to improve the impact of oil coating on the working environment, steel companies such as Baosteel, Wuhan Iron and Steel, and Maanshan Iron and Steel have developed self-lubricating steel plates in recent years. The use of self-lubricating coated steel plates has excellent self-lubricating properties. Properties such as corrosion resistance, fingerprint resistance, processing formability and paintability, etc. It is mainly to roll-coat a layer of organic coating on the steel plate, and there is no need to apply lubricating oil during the stamping process. However, the cost of use is slightly higher, and it has not been widely used.
Due to the wide variety of molding loads and molding materials, which kind or several measures are used to solve the problem of workpiece strain, in addition to considering the effectiveness of the effect, the batch size of the product, the difficulty of realization and its economy must also be considered. and other issues, and finally choose the most suitable method.
