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How to control the surface of the stamping part?
Wen / Fengchao
Stamping is a common quality defect in the production process and is common in large steel drum production plants.
On the one hand, the pulling process reduces the stability and production efficiency of the production process, the scrap rate of the parts rises, on the other hand, the mold is more severely worn, the die life and the precision of the stamping parts are reduced, and the number of mold repairs and production downtime are increased.
Pulling hair
The essence of the napping is due to local sticking (biting) of the workpiece and the surface of the mold. There are many ways to improve the problem of pulling the hair. The basic principle is to change the nature of the friction pair between the mold and the part to be machined, so that the friction pair is replaced by a material that is not easily adhered.
After the mold enters the debugging stage of the production site, the following methods are generally used to improve the pulling problem:
1. Change the mold material and increase the hardness of the mold;
2. Process the surface of the mold, such as hard chrome plating, PVD and TD;
3. Applying a nano-coating to the mold cavity, such as RNT technology;
4. Add a layer of other substance between the mold and the part to be machined to separate the machined part from the mold (such as lubrication or special lubricant or a layer of PVC);
5. Use self-lubricating coated steel plate.
In terms of mold materials, mold steel SKD11, CR12MOV, etc. are recognized as wear-resistant anti-seize materials. After heat treatment, the hardness can reach about 255C-63 degrees Chrom hardness. When the mold is small and the shape of the parts is simple, such materials can be used. However, the material is difficult to process after heat treatment, has high brittleness, is easy to crack, has high cost, and has limited size, and the deformation of such materials after heat treatment is large, and the research and distribution work after heat treatment is huge.
The shape of the inner plate of the automobile is relatively complicated and more and more high-strength steel plates are used. These parts have higher requirements on the overall performance of the mold. They are usually inlaid with a mosaic structure. The surface treatment process of the insert is currently TD, plated. Hard chrome, nitriding, PVD, etc.
TD treatment is the abbreviation of the thermal diffusion method carbide coating treatment. This technology was first developed and patented by the Toyota Central Research Institute in Japan in the 1970s. It is also called the Toyota Diffusion Process, or TD Process. China is also known as molten salt metal infiltration. Regardless of its name, the principle is to place the workpiece in a molten borax mixture and form a metal carbide coating on the surface of the workpiece by high temperature diffusion.
The main features of TD coating treatment are: high hardness of cladding, HV up to 3000, high wear resistance, tensile strength, corrosion resistance, etc. The service life of TD coating is about 100,000 units.
However, the TD coating treatment has high requirements on the mold material, and the thermal stress, phase transformation stress and specific volume change generated during the high temperature treatment tend to cause deformation or even cracking of the mold during the heat treatment process. Generally, the repaired mold is in the mold. Cracking also occurs at the weld, and TD coating has high requirements on the quality and shape of the mold.
In addition, it is difficult to process the TD coating after processing, which can not meet the needs of design change and mold adjustment. For the molds that have been surface treated, the original surface treatment should be completely removed, otherwise the surface quality of the TD coating will be affected. In addition, the TD coating treatment technology generally has a service life reduction after 3-4 treatments.
TD processing
PVD (Physical Vapor Deposition) is a physical vapor deposition method. The PVD coating is a surface coating made by physical vapor deposition.
It has good tensile resistance, coating hardness 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 treated workpiece is small, and it can be processed multiple times without affecting the life. The advantages are the same, but the plating layer and the substrate have poor adhesion. When used on the drawing mold and the mold with high forming pressure, the coating layer is easily peeled off, and the tensile and abrasion resistance effects are not exerted.
PVD coating
The size of the outer plate mold is generally large. For example, the structure of the insert block is used, and there is a strain at the joint. Therefore, the whole structure is mostly adopted, and the material of the outer plate is generally cast iron such as ductile iron. After the flame-hardening of the forming and feeding portion, the hardness can reach a chromium hardness of HRC of 50-55 degrees.
The surface treatment of the outer structure of the overall structure is mostly hard chrome plating, but the surface hardening effect is limited, the surface hardness is about 1000 HV. In addition, the hard chrome plating is mechanically combined with the mold base material, which is easy when the forming pressure is large. When it falls off, the coating will lose its tensile and tensile properties. When the surface hardened layer is worn, the pulling will occur again, and the life of the surface hardened layer is generally about 50,000-100,000 units.
chrome
RNT is an emerging technology in recent years. The working principle is that after the RNT coating liquid is coated on the mold cavity, the nano-metal carbide coating is formed by the pressure diffusion of the coating nano-molecule and acts on the surface of the mold, and the process expands from the inside to the outside, and the thickness and hardness accompany The working time of the mold increases, the thickness of the coating is 0.1-1 μm, and the hardness of the coating is HV1100-1600. Even when the mold is subjected to a large load, the coating of the surface will not fall off due to plastic deformation of the substrate. Its thickness and hardness increase from the inside to the outside with the increase of the working time of the mold and the number of coatings.
Once coated with RNT coating, it is generally guaranteed that 100-500 pieces are not pulled. However, the technology is not mature enough for the parts with severe hair pulling, the parts that are heated during the production process, and the ultra-high strength boards, and the use cost is high.
Pulling before using RNT
Pulling after using RNT
The use of a reasonable lubricant in the production process can effectively improve the friction conditions and reduce the pulling of the hair. Its main function is to separate the contact pair with a lubricating oil film. The oil is generally applied by hand or by using an automatic device on the line.
In addition, the use of lubricants can also effectively reduce the problem of dark spots and cracks. However, the use of lubricants will make the environment dirty and slippery. In order to improve the impact of oiling on the working environment, in recent years, Baosteel, Wuhan Iron and Steel, Maanshan Iron and Steel and other steel companies have developed self-lubricating steel sheets, which use self-lubricating coated steel sheets with excellent self-lubricating. Properties such as properties, corrosion resistance, fingerprint resistance, processability and paintability are mainly applied to a roll of an organic coating on the steel sheet, and no need to apply lubricating oil during the press forming process. However, the cost of use is slightly higher and has not been widely used.
Since the forming load and the material to be formed vary widely, and what kind of measures are taken to solve the problem of the strain of the workpiece, in addition to the effectiveness of the effect, the batch size of the product must be considered, the difficulty of realization and its economy. In the end, I will choose the most appropriate method.
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