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Basic Methods of Mechanical Polishing


Basic Methods of Mechanical Polishing (Part I)


The polishing said in the plastic mold processing and other industries required surface polishing is very different, strictly speaking, the polishing of the mold should be called mirror processing.It has high requirements not only for the polishing itself but also for the surface smoothness, smoothness and geometric accuracy.Surface polishing generally requires only a shiny surface.


The standard of mirror processing is divided into four levels:Ao = ra0.008μm, A1= ra0.016μm, A3= ra0.032μm, A4= ra0.063μm, because of the electropolishing, fluid polishing and other methods are difficult to accurately control the geometric accuracy of parts, and chemical polishing, ultrasonic polishing, magnetic polishing and other methods of surface quality and can not meet the requirements.Therefore, the mirror processing of precision molds is mainly based on mechanical polishing.


2.1 Basic procedures of mechanical polishing

In order to obtain high quality polishing effect, the most important thing is to have high quality polishing tools and accessories such as oilstone, sandpaper and diamond grinding paste.The choice of polishing program depends on the surface conditions after the previous processing, such as mechanical processing, EDM, grinding, and so on.


The general process of mechanical polishing is as follows:

(1) After rough milling, electric spark, grinding and other processes, the surface can be polished by rotating surface polishing machine or ultrasonic grinding machine with a rotating speed of 35,000-40,000 RPM.Commonly used methods include removing the white EDM layer with a wheel with a diameter of Φ3mm and WA # 400.This is followed by hand grinding, with strips of whetstone and kerosene as a lubricant or coolant.The general order of use is #180 ~ #240 ~ #320 ~ #400 ~ #600 ~ #800 ~ #1000.Many die makers choose to start with #400 to save time.


(2) half fine throwing half fine throwing mainly uses sandpaper and kerosene.The number of sandpaper is: #400~ #600 ~ #800 ~ #1000 ~ #1200 ~ #1500.In practice, #1500 sandpaper should only be used for hardened die steels (52HRC and above) and not for prehardened steels, as this may cause surface burns to the prehardened steel.


(3) fine polishing mainly uses diamond grinding paste.The usual grinding sequence is from 9μm (#1800) to 6μm (#3000) to 3μm (#8000) for grinding with a polishing cloth wheel mixed with diamond grinding powder or paste.9μm Diamond Grinding Paste and Polishing Cloth Wheel can be used to remove hair marks from #1200 and #1500 sandpaper.It was then polished with viscose felt and diamond grinding paste from 1μm (#14000) to 1/2μm (#60000) to 1/4μm (#100000).The polishing process with a precision of more than 1μm (including 1μm) can be carried out in a clean polishing room in the mold workshop.For more precise polishing, an absolutely clean space is necessary.Dust, smoke, dandruff, and drool can all ruin a highly polished surface after hours of work.


2.2 Problems to be noted in mechanical polishing The following points should be noted in sandpaper polishing:


(1) the use of sandpaper polishing needs to use soft wood or bamboo sticks.When polishing round surface or spherical surface, the use of cork rod can better match the radian of round surface and spherical surface.The harder wood, like cherry wood, is more suitable for polishing a flat surface.Finishing the end of the strip so that it matches the shape of the steel surface can avoid deep scratches caused by the sharp Angle of the strip (or bamboo strip) touching the steel surface.


(2) when changing to different types of sandpaper, the polishing direction should be changed to 45°~ 90°, so that the stripe shadow left by the polishing of the former type of sandpaper can be distinguished.Before changing to a different type of sandpaper, the polished surface must be carefully wiped with a cleaning solution such as alcohol and 100 percent pure cotton, because a small gravel left on the surface can ruin the entire finish.This cleaning process is also important when switching from sandpaper polishing to diamond paste polishing.Before polishing can proceed, all particles and kerosene must be completely cleaned.


(3) In order to avoid scratching and burning the workpiece surface, special care must be taken when polishing with #1200 and #1500 sandpaper.Therefore, it is necessary to apply a light load and a two-step polishing method to polish the surface.Each type of sandpaper should be polished twice in two different directions, 45°~ 90° for each rotation between the two directions.