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Research on the application of water-resistant PVA Abrasives

water resistant PVA abrasives are original products in Japan. In the 1980s, they were first used for fine grinding magnetic heads in China. There are magnetic head factories in Shenzhen, Suzhou, Chengdu, Ningbo and other places. At first, in order to meet the localization needs of these manufacturers, the second grinding wheel factory began to carry out this research. At that time, the best use was the Nissan FBB grinding wheel, with a density of about 0.8g/cm3. After the diffusion of this technology, a number of grinding tool factories have been established in China, specializing in the production of magnetic head grinding wheels. However, the overall level is low, and the adjustment range of abrasive performance is limited. It only partially replaces the Nissan grinding wheel in the production of domestic magnetic heads. In recent years, with the rapid development of electronic technology and the increasing improvement of manufacturing level at home and abroad, various new water-resistant PVA abrasives, especially low-density products, continue to emerge, and the domestic demand is very strong. According to the original manufacturing technology, low-density products can only be obtained by adjusting the sand knot ratio, which will inevitably lead to low grinding ability and grinding efficiency of the abrasive tools, "rubbing is greater than grinding", and increase the grinding heat. Obviously, in order to change this backward situation, it is necessary to further study the manufacturing process of water-resistant PVA abrasive tools

1. Performance characteristics of water-resistant PVA Abrasives

ideal water-resistant PVA Abrasives have a large number of rich micro pores similar to sponge structure, and have good elasticity and water absorption. As an "elastic terrazzo", water-resistant PVA Abrasives have high strength, less heat and fast heat dissipation, which can avoid the high temperature generated by grinding, are not easy to burn the workpiece, and can prevent spiral burns, spots and vibration lines; It has strong wear resistance, good sharpness, the ability to change the rake angle of the grinding edge, excellent self sharpening, and uniform grinding resistance. There is no need to repair the abrasive tools in the grinding process, and there will be no dust pollution. It is especially suitable for precision grinding, ultra precision grinding and mirror grinding of heat sensitive materials such as stainless steel, non-ferrous metals and cemented carbide

2. Analysis of technical elements

the key to the manufacturing technology of water-resistant PVA abrasive tools is to accurately control the porosity and sand binding ratio of abrasive tools and adjust the acetalization speed

the difficulty of sand binding ratio control is to determine the addition proportion of PVA and thermosetting resin. The proportion is too large, the water resistance is not good, and the combination is not firm; If the proportion is too small, the slurry is difficult to mix evenly, and even condensation and sedimentation may occur. It is an effective way to solve this technical problem to choose thermosetting resin with good mixing performance and large range of miscible proportion with PVA solution. According to the information, the thermosetting resins that have been successfully used abroad at present include phenolic resin, epoxy resin and amino resin

the porosity of abrasive tools is guaranteed by two processes: mechanical stirring and mixing with air and adding foaming agent to produce pores. Abrasive tools with high density are generally directly controlled only by adjusting the mixing speed and mixing time. The faster the mixing speed is, the longer the mixing time is, the more foam in the slurry is, the smaller the pore diameter is, and the more uniform the distribution is, the higher the porosity of the product is. For those products with low density, chemical foaming agents should be added. Introduce sufficient gas through foaming, adjust the bubble size and uniformity of foam by mechanical foaming method before acetalization molding, and then select the appropriate acetalization reaction conditions, so as to control the density and cell structure of finished products. The addition amount of foaming agent should be accurately controlled. If it is too small, the product will form a closed cell structure, which will affect the grinding effect; If there are too many bubbles, the size of bubbles will be different, the strength of products will be reduced, and the abrasive particles will fall off unevenly. Generally controlled within 5% of the abrasive quality. In addition, to produce products with sufficient density, it is also necessary to add a certain amount of water in the mixing process of the slurry, and add thickener, thixotropic agent, foam stabilizer and other mixed additives to improve the stability of the slurry system. Because after increasing the amount of water, the slurry becomes thinner, the abrasive is easy to settle and layer, and the foam is unstable. Foam is originally a thermodynamic unstable system. Both bubble film drainage and gas diffusion in the bubble will lead to bubble film rupture, bubble coalescence, and even bubble collapse in serious cases. Increasing the mixing temperature and prolonging the mixing time will make the slurry mixed more evenly, and the more evenly the slurry is stirred, the more stable the abrasive particles can be suspended in the slurry

pva acetalization is a relatively slow process. Under the condition of lower than 50 ℃, it takes nearly 40 hours for the acetalization degree to reach 60-70%. Increasing the temperature can shorten the acetalization time. Increasing the degree of acetalization can increase the brittleness of the product and reduce the elasticity. In order to ensure that abrasive products have moderate elasticity, the acetalization degree is generally controlled at about 70%. In addition, the initial acetalization temperature is too high, and the gas in the bubble expands, which makes the bubble wall thinner, reduces the elasticity, accelerates the drainage speed, and breaks bubbles, small bubbles and form large bubbles, resulting in changes in pore structure and distribution, and it is not easy to get products with uniform organization. The initial acetalization temperature is too low, and the acetalization speed is too slow, which is prone to the sedimentation and stratification of abrasive particles, and the floating of low-density bubbles. Similarly, products with uniform pores cannot be obtained. And the low temperature time should not be too long, otherwise the bubble is easy to be fixed into a closed cell structure. For products with closed hole structure, the coolant is not easy to soak, the lubrication and cooling effect is not good, and the grinding is not easy to be uniform. In short, it is necessary to control the appropriate initial reaction temperature, not only to make the bubbles break and merge prematurely, but also to make the bubbles relatively fixed in a uniform state, and then raise the temperature in a timely manner, so that the relatively fixed bubbles break, form an open structure, and get products with uniform density up and down

the addition amount of acid and aldehyde should not be less than the formula requirements, and too much is a waste. The amount of catalyst is small, the reaction speed is slow, the acetalization time is long, and the acetalization degree is low. With the increase of catalyst dosage, the acetalization reaction rate increases rapidly in the initial stage, and gradually tends to be flat in the middle and late stages, and the acetalization degree of the product increases. The effect of formaldehyde dosage on the acetalization reaction speed and the acetalization degree of the product is similar to that of acid catalyst, that is, the higher the concentration of formaldehyde in the system, the faster the initial reaction speed, and basically unchanged in the middle and late stages. The acetalization degree also increases with the increase of formaldehyde concentration. The amount of acid catalyst and formaldehyde is too small and too large, which will destroy the balance of the slurry system. If it is too small, the reaction speed is too slow, and the slurry will have a concentration gradient due to too long solidification time, and even sedimentation and stratification in serious cases; When it is too large, because the reaction speed is too fast, the curing reaction speed of thermosetting resin is faster than the acetalization reaction of PVA, and settlement delamination will also occur due to the polymerization of resin. In addition, the greater the amount of acid catalyst and formaldehyde, the greater the difficulty of water washing, which not only increases the cost, is not easy to recover, but also pollutes the environment. Moreover, if the water washing is not clean, it will also corrode the surface of the product. In the light case, the color of both sides of the product is inconsistent, affecting the appearance, and in the serious case, the surface of the product will become black due to corrosion

the drying process is the process of water evaporation from the wet body of abrasive tools. Among them, moisture moves from the inside of the wet body to the surface of the body, which is called internal expansion, and different processes need to repeatedly put and take out the aluminum profile; The moisture on the surface of the green body absorbs heat, vaporizes and evaporates into the atmosphere, which is called external diffusion. The initial drying temperature is too high, and the external diffusion speed is greater than the internal diffusion speed, which will cause the blank electronic universal testing machine to adopt the design body of microcomputer controlled electro-hydraulic servo valve loading and manual hydraulic loading, and the separation of the main body and the control frame to deform due to uneven shrinkage, which will cause crack waste products in serious cases. According to the change of the moisture content of the green body in different stages and the internal and external diffusion rate, the drying humidity should be controlled, and the measures of heating up in sections should be taken to shorten the drying cycle on the premise of ensuring the drying quality. After drying, continue to raise the temperature to an appropriate temperature to completely harden the resin bond in the green body. If the temperature rises too fast in the hardening stage and the final temperature is too high, the quality of abrasive tools will be affected

3. Research results

this study has made a technical breakthrough in the density and hardness control of abrasive tools, and mastered the core technology of manufacturing water-resistant PVA abrasive tools. The stable production process and density are determined to be 0 8g/cm3 low density water-resistant PVA abrasive product formula. It can produce water-resistant PVA products with different hardness and density by using different abrasive materials and abrasive particle sizes according to the needs of users. The grinding performance of the product is significantly higher than that of domestic current products. After being used by many users, it can completely replace the abrasive tools imported from Japan

4. Application prospect

water resistant PVA abrasive tools replace traditional polishing and grinding materials, and are widely used in the precision grinding of non-ferrous metals, cemented carbide and various non-metallic materials, especially for grinding processes and grinding objects with strict requirements on grinding heat. The commonly used grinding processes and grinding objects are: mirror grinding of photographic printing zinc plate and magnesium plate; Stainless steel surface polishing; Remove oxide scale, grind and polish thin copper surface; The price of copper strip fell by 0.1 US dollars/ton compared with the previous trading day, and the surface of steel strip was polished; Grinding and mirror polishing of stainless steel tableware, watch cases, stainless steel pipes and other stainless steel workpieces; Fine grinding and polishing of permalloy, copper chromium alloy, magnesium aluminum alloy materials; After long-term clinical verification, PMMA is an ideal intraocular lens material with stable, light weight, good transparency and refractive index of 1.49, which is hydrophobic; It has good anti-aging and anti environmental change characteristics, good acid, alkali and organic solvent resistance characteristics, and good biocompatibility head, fine grinding of card swiping head; Fine grinding and polishing of aluminum discs in pcb/ccl laminates and computer hard disks in IT industry; Fine grinding and polishing of non-metallic materials such as kinescope glass shell and LCD plane, hard synthetic resin, optical glass, spectacle lens, rubber roller for printing, high-grade leather, jade, ivory, craft ceramics, glass ceramics, etc; Calibration correction of measuring instruments, Superfinishing of beauty knives, Superfinishing of sliding surfaces of micro precision bearings, Superfinishing of automotive shock absorber rods, processing of various bearing pins, transmission shafts in compressors and various molds, etc