I. Introduction of electrostatic sanding
Since it has a special effect on improving the grinding efficiency of coated abrasives, especially abrasive belt grinding, it has been widely used by many large coated abrasives enterprises abroad since the 1950s and 1960s. In China, since the 1970s, a few enterprises have used electrostatic sanding for the production of water-resistant sandpaper, but due to a series of technical difficulties encountered in the use of DC high voltage at that time, after a short period of time, due to the technology and process has not been completely solved, it also ceased to be used. Individual enterprises use electrostatic - gravity sand planting hybrid method, barely retained the electrostatic sand planting this new process. Until the 1980s in the country's reform and opening up and the guidance of the government, the introduction of foreign advanced sets of coated abrasives equipment, in the new foreign technology, especially under the leadership of the belt manufacturing technology, the domestic electrostatic sand planting process has been a vigorous and healthy development. Such as white pigeon, Zibo Li research are producing electrostatic sanding resistant gauze.
There are many foreign manufacturers who are producing electrostatic sanding products. The main ones are 3M, Hermes, WANT, Mipox, Kovax, Fujistar, NIKEN, Kagami and so on. Utah has the most abundant products and the widest market coverage with 3M.
Electrostatic grit implantation is to implant fine abrasive grains on the high strength film with the help of high voltage electrostatic field force (as shown in Figure 1), so that the abrasive grains can be evenly distributed in a directional manner, which can provide higher grinding efficiency and bright and detailed grinding effect. At present, the abrasive grains mainly include aluminum oxide, silicon carbide, diamond, etc.
The advantage of electrostatic sanding is that the abrasive particles are arranged in a directional manner and the surface of the product is sharp, which can provide greater cutting power compared with conventional coated products.
Electrostatic sanding belt can be used to polish various metal rollers, rubber rollers, etc.; the other is used in the automobile manufacturing industry with great market potential, of which there are two major parts: one is the body processing, and the other is the final polishing of crankshaft, camshaft and universal shaft.
Introduction to abrasive belt grinding
Belt grinding is a new process that uses abrasive belts to gradually grind or polish the surface of the workpiece in accordance with the requirements of the workpiece to be processed by making the abrasive belt in contact with the surface of the workpiece at high speed in a certain mechanical device and under a certain pressure to produce friction. From a microscopic point of view, the abrasive grains on the surface of the abrasive belt resemble a cutting tool with a rounded tip and an obtuse or rounded edge. The radius of the arc ranges from a few microns to several tens of microns, depending on the material and size of the abrasive grain. Due to this geometric characteristic of the abrasive grain, the depth of cut is small (chip thickness is thin), generally around 0.005-0.05mm. Therefore, most of the abrasive cutting edges are cutting the workpiece under the condition of large negative front angle. In the relative motion of the abrasive grain and the workpiece surface, a certain interference between the abrasive grain and the workpiece surface is generated. According to the degree of interference, three different processes can be distinguished.
(1) Slip rubbing In fact, at this time began to contact the workpiece, interference is very little, the abrasive grains only rub the surface of the workpiece, "slip rubbing" role, when the abrasive grains on the workpiece slip rubbing, in fact, the elastic and plastic deformation of the excised material.
(2) plowing With the machine tool feed, cutting thickness increases, the interference increases, then the abrasive grains on the surface of the workpiece plowed out the "line", called "plowing". At this time the workpiece material plastic flow, the material produces a squeezing type of movement, and from the abrasive grain under the direction of the front and sides of the extrusion, while removing a small amount of material.
(3) Cutting Under the action of certain pressure, when there is enough interference and accompanied by a certain cutting temperature, the real "cutting" begins, at this time in front of the sliding abrasive grain to produce fracture and the formation of chips, there is a fairly fast removal rate. At the moment of contact with the workpiece, some of the abrasive grains on the sanding belt cut, others plow grooves, and others only play the role of sliding, and even different parts of the same abrasive grain and the same part play different roles at different processing times. In addition, the rotating motion of the abrasive belt plays a role in wiping away the chips, removing the chips in front of the advancing abrasive grains.
The proportion of the three stages of the abrasive cutting process in the whole grinding process varies depending on the material being processed.
The grinding process is the process of cutting metal with the cutting edge of abrasive grains. Like the cutting of machine tools, the metal to be ground undergoes the processes of elastic deformation, plastic deformation and cutting formation, and a large amount of grinding force and grinding heat is generated. During the grinding process, due to the different shapes and distribution states of the abrasive grains, the actual effective abrasive grains on the surface of the abrasive belt are less than the total number of abrasive grains. As a result, the effect of the abrasive grains on the metal in the same time varies with the size of the sliding, plowing and cutting effects. Even different parts of the same abrasive grain and the same part at different processing times have different effects. It can be seen that the grinding of abrasive belts is very complicated. Especially in the negative front angle cutting process of the abrasive cutting edge, the cutting conditions are very poor, and the violent extrusion at each stage causes serious plastic deformation on the grinding surface, and a large amount of plastic deformed metal does not become chip out, but remains on the processed surface, so the hardening phenomenon on the processed surface is serious and the residual stress is large. Due to the high speed movement of the abrasive grains and the blunt cutting edge of the abrasive grains, large friction and elastic and plastic deformation are caused in the grinding area, and a large amount of heat is generated during the grinding process, resulting in a rise in the surface temperature of the workpiece in the grinding area, which will cause changes in the surface layer of the workpiece. Especially when the abrasive belt grit is severely worn, the grinding friction intensifies and generates a large amount of grinding heat, which causes the surface temperature of the workpiece to rise sharply, resulting in organizational changes (such as burns, cracks, thermal stresses, etc.) in the surface layer of the metal. This is exactly one of the reasons why grinding with abrasive belts sometimes still burns the surface of the workpiece.
So how can we extend the service life of abrasive belts during grinding so that they can sustain their abrasive power? We will continue to explore this in the next section.
III. How to extend the service life of abrasive belts
The production process of abrasive belts has extremely strict requirements for abrasive belt substrates, abrasives, binders and sanding density, etc. The premature end of the life of abrasive belts is often caused by improper use, and we analyze several typical problems below.
1. Sticky cover
When a layer of metal material compound cover in the abrasive cutting edge, that is, sticky cover, at this time the surface of the sanding belt becomes bright crystal, feel the feeling of slippery. The sticky cover mainly occurs in metal materials with high strength, especially in hard and tough materials.
--Insufficient grinding pressure is the main cause of sticky cover. For hard materials, insufficient pressure makes it difficult to cut the abrasive grain into the workpiece and to break the abrasive grain since it is sharp.
--Softer contact wheels or pressure plates, even with high enough grinding pressure, will only collapse badly and make it difficult for the abrasive grains to press into the workpiece.
--Higher belt speed, so that the abrasive grains do not spend enough time in the grinding area, cutting into the workpiece to a thin depth and generating severe heat.
The reasons for the occurrence of sticky cover are comprehensive, and its solution is also comprehensive, which means that a suitable contact wheel or pressure plate, sufficiently high grinding pressure, and lower abrasive belt speed are the fundamental solutions to the problem. Of course, it is also necessary to choose abrasives with good self-sharpening properties.
2. Dull grinding
In the grinding process, although all the abrasive grains are still present, the sharpness is already very poor. This is due to the dulling of the abrasive cutting edge due to wear, a phenomenon known as dulling. The normal dullness is the end of the belt's life, but obviously the "dullness" we are referring to here is caused by improper selection or use of the belt when the abrasive grains have not yet been consumed.
--Softer contact wheels or pressure plates make it difficult to cut the abrasive grains into the workpiece, resulting in a flat edge.
--Insufficient grinding pressure also tends to dull the abrasive belt, and it is difficult for the abrasive to sharpen itself.
-- When the workpiece is hard, the inappropriate choice of abrasive belt or the high speed of abrasive belt also makes it difficult to cut into the workpiece and dull it.
The abnormal wear of the sanding belt seriously affects the service life of the belt and greatly increases the processing cost, so it cannot be ignored.
Blockage is that the abrasive gap is quickly covered and filled with chips before the abrasive cutting edge is completely blunted, thus causing the abrasive belt to lose its cutting ability.
There are many reasons for clogging, mostly due to the use, processed materials and improper selection of abrasive belts caused by.
--Contact wheel or pressure plate is too soft, causing the abrasive grains to be difficult to cut into the workpiece, the abrasive belt to grind state, the processing area temperature due to friction and heat, so that the chips "welded" to the abrasive belt, resulting in blockage. The solution should be a hard contact wheel and pressure plate, or use the peak tooth back contact wheel and pressure plate, but also can use a small diameter contact wheel and so on.
-- High belt speed is also difficult to make the abrasive grain effectively cut into the workpiece, the same will appear blockage and workpiece burns, then the belt speed should be reduced.
-- processed materials are too soft (such as aluminum, copper and other non-ferrous metals) is very easy to cause clogging of the surface of the sanding belt, the solution should be: the use of sparse planting sanding belt; in order to meet the roughness requirements under the conditions of the use of coarse grit sanding belt. Adopt brittle silicon carbide abrasive abrasive belts; adopt the method of adding grinding aids, such as using lubricants, etc.
--Smooth processing surface of easily clogged materials, the abrasive belts that are easy to cause scratches such as grease and coarse grit cannot be used for such materials, and super-coated abrasive belts should be used, which have good chip removal and anti-clogging properties.
Of course, for the abrasive belt grinding life of the impact factors are many, can not lose one side, to generalize, should be all-round, multi-angle comprehensive consideration to truly understand the cause. This only in practice to continue to learn progress.