Effect of hardness and Surface condition on polishing process

The key to the operation of the polishing machine is to try to get the maximum polishing rate in order to remove the damage layer caused by polishing as soon as possible. At the same time, it is also necessary to ensure that the polished damaged layer will not affect the final observed tissue, that is, it will not cause false tissue. The former requires the use of thicker abrasives to ensure a larger polishing rate to remove the polished damage layer, but the polishing damage layer is also deeper; the latter requires the use of the finest material to make the polishing damage layer shallow, but the polishing rate is low. .

The best way to solve this contradiction is to divide the polishing into two stages. The purpose of rough polishing is to remove the polishing damage layer, which should have the maximum polishing rate at this stage, and the surface damage formed by rough polishing is a secondary consideration, but it should also be as small as possible, followed by fine polishing (or final polishing). Its purpose is to remove the surface damage caused by rough polishing and minimize the polishing damage. When polishing the polishing machine, the grinding surface of the sample and the polishing disc should be absolutely parallel and evenly pressed on the polishing disc, and pay attention to prevent the sample from flying out and new wear marks caused by too much pressure. At the same time, the sample should be rotated and moved back and forth along the radius of the turntable to avoid local wear of the polished fabric too fast. micro-powder suspension should be continuously added in the polishing process to maintain a certain humidity of the polished fabric. Too much humidity weakens the wear mark effect of polishing, which makes the hard phase in the sample float and convex and the non-metallic inclusion in steel and graphite phase in cast iron produce "tail drag" phenomenon; when the humidity is too small, the heat generated by friction will heat up the sample and reduce the lubrication effect. The wear surface loses luster, and even black spots appear, and light alloy will damage the surface. In order to achieve the purpose of rough polishing, it is required that the rotating speed of the turntable should be low, preferably no more than 600 rpm; polishing time should be longer than the time required to remove scratches, because the deformation layer should also be removed. After rough polishing, the grinding surface is smooth, but dull, and there are uniform and meticulous wear marks under the microscope, which need to be eliminated by fine polishing.

As mechanical polishing is mainly done manually, polishing technology is still the main reason that affects the quality of polishing. In addition, it is also related to mold material, surface condition before polishing, heat treatment process and so on. High quality steel is a prerequisite for good polishing quality. if the surface hardness of steel is uneven or there are differences in characteristics, polishing difficulties often occur. All kinds of inclusions and pores in steel are not good for polishing.

The increase of hardness increases the difficulty of grinding, but the roughness decreases after polishing. Due to the increase in hardness, the polishing time required to achieve lower roughness increases accordingly. At the same time, the hardness increases and the possibility of excessive polishing decreases accordingly.

In the crushing process of cutting machining, the surface layer of steel will be damaged due to heat, internal stress or other factors, and improper cutting parameters will affect the polishing effect. The surface after EDM is more difficult to grind than the surface after ordinary machining or heat treatment, so precise EDM dressing should be used before the end of EDM, otherwise the surface will form a hardened thin layer. If the electric spark precision repair specification is not selected properly, the maximum depth of the heat affected layer can reach 0.4mm. The hardness of the hardened layer is higher than that of the matrix and must be removed. Therefore, it is best to add a rough grinding process to thoroughly remove the damaged surface layer to form an average rough metal surface, which provides a good foundation for polishing.

The polishing mentioned in plastic mold processing is very different from the surface polishing required by other industries. Strictly speaking, mold polishing should be called mirror processing. It has high requirements not only for polishing itself, but also for surface smoothness, smoothness and geometric accuracy. Surface polishing generally requires only a bright surface. The standard of mirror processing is divided into four levels: AO= Ra 0.008 μ m, A1 = Ra0.016 μ m, A3 = Ra0.032 μ m, A4 = Ra0.063 μ m. Because it is difficult to accurately control the geometric accuracy of parts, such as electrolytic polishing and fluid polishing, the surface quality of chemical polishing, ultrasonic polishing, magnetic polishing and other methods can not meet the requirements, so the mirror processing of precision moulds is mainly mechanical polishing.