2.2 Problems to be noted in mechanical polishing
Polishing with sandpaper should pay attention to the following points.
(1) Polishing with sandpaper requires the use of soft wooden or bamboo sticks. When polishing round or spherical surfaces, the use of soft wood sticks can better match the curvature of the round and spherical surfaces. Harder sticks like cherry wood, on the other hand, are more suitable for polishing flat surfaces. Trim the end of the wood bar so that it can match the shape of the steel surface, which can avoid the sharp angle of the wood bar (or bamboo bar) from touching the steel surface and causing deeper scratches.
(2) when changing to a different model of sandpaper, polishing direction should be changed 45 ° ~ 90 °, so that the previous model of sandpaper polishing after leaving the stripes shadow can be distinguished. Before changing to a different type of sandpaper, you must use 100% pure cotton with alcohol and other cleaning solutions to polish the surface for careful wiping, because a very small grit left on the surface can ruin the entire polishing work that follows. This cleaning process is equally important when switching from sandpaper polishing to diamond polishing paste polishing. All particles and kerosene must be completely cleaned before polishing continues.
(3) Special care must be taken when polishing with #1200 and #1500 sandpaper in order to avoid abrasions and burns on the surface of the workpiece. It is therefore necessary to load a light load and to polish the surface using a two-step polishing method. Polishing with each type of sandpaper should be done twice in two different directions, with a 45° to 90° rotation between each direction.
Diamond grinding and polishing should pay attention to the following points.
(1) This polishing must be done under as light a pressure as possible, especially when polishing pre-hardened steel parts and polishing with fine abrasive paste. In polishing with #8000 polishing paste, the common load is 100~200g/cm2, but it is difficult to maintain the accuracy of this load. To make it easier to do this, you can make a thin and narrow handle on the wooden strip, such as adding a copper piece; or cut off a part of the bamboo strip to make it softer. This can help control the polishing pressure to ensure that the pressure on the surface of the mold is not too high.
(2) When using diamond grinding and polishing, not only the work surface requires cleanliness, but also the worker's hands must be carefully cleaned.
(3) Each polishing time should not be too long, the shorter the time, the better the result. If the polishing process is too long will cause "orange peel" and "pitting".
(4) in order to obtain high-quality polishing effect, easy to heat polishing methods and tools should be avoided. For example: polishing wheel polishing, the heat generated by the polishing wheel will easily cause "orange peel".
(5) when the polishing process stops, to ensure that the surface of the workpiece clean and carefully remove all abrasives and lubricants is very important, followed by a layer of mold rust-proof coating should be sprayed on the surface.
Since mechanical polishing is still mainly done by hand, so polishing technology is still the main reason for the impact of polishing quality. Besides, it is also related to the mold material, surface condition before polishing, heat treatment process, etc. High-quality steel is a prerequisite for good polishing quality, if the steel surface hardness is uneven or there are differences in characteristics, it will often produce polishing difficulties. Various inclusions and pores in the steel are not conducive to polishing.
3.1 The effect of different hardness on the polishing process
Increasing hardness makes it more difficult to grind, but the roughness after polishing is reduced. Due to the increase in hardness, the polishing time required to achieve a lower roughness grows accordingly. At the same time, the possibility of over-polishing is reduced by the increase in hardness.
3.2 Surface condition of the workpiece on the polishing process
Steel in the cutting mechanical processing of the crushing process, the surface layer will be damaged by heat, internal stress or other factors, 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 EDM finishing should be used before the end of EDM, otherwise the surface will form a hardened thin layer. If the EDM finishing gauge is not selected properly, the depth of the heat affected layer can be up to 0.4mm. the hardened thin layer is harder than the substrate hardness and must be removed. Therefore, it is best to add a rough grinding process to completely remove the damaged surface layer, forming an evenly rough metal surface to provide a good basis for polishing processing.