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How to solute overpolishing in polishing process
May 26, 2018

The biggest problem encountered in polishing is "over-polishing". Excessive polishing means that the longer the polishing time is, the worse the surface quality of the mold is. There are two phenomena when excessive polishing occurs: the "orange peel" (orange peel) and the "dimple" (pinhole). Excessive polishing occurs in mechanical polishing.


Orange peel (orange peel)

An irregular, rough surface is called "orange peel" (orange peel) and there are many different reasons for producing "orange peel" (orange peel). The most common cause is overheating of the mold surface or excessive carburization, excessive polishing pressure, and excessive polishing time. Harder steels can tolerate greater polishing stresses, softer steels are prone to over-polishing, and studies have shown that excessive polishing time varies with the hardness of the material.

It was found that the surface quality is not good, many people will increase the polishing pressure and prolong the polishing time. This practice tends to make the surface quality worse. Use one of the following methods to remedy:

method 1:

The defective surface was removed and the grind was slightly coarser than the previously used sand number. Then finish grinding, the polishing pressure is lower than the previous pressure.

Method 2:

The stress was removed at a temperature lower than the tempering temperature of 25°C. Use the finest sand number before grinding to achieve a satisfactory result.


Finally, it is polished with slightly lower pressure. If the effect is still not satisfactory, the hardness of the mold must be increased. The following tips for several methods:

Use nitrided or carburized methods to increase the surface hardness of steel.

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Heat treatment techniques are used to increase the overall hardness of the steel.

"Dimple" (pinhole)

The formation of "dimples" or "pinholes" is due to non-metallic inclusions (impurities) in the steel, which are usually hard and brittle oxides; they are pulled from the surface of the steel during the polishing process to form "dim pits" or "Pinhole." The main influencing factors are as follows:

1, polishing pressure and polishing room.

2, the purity of steel, especially hard inclusions (impurities) content.

3, polishing tools.

4, grinding materials.


The reason for the generation of dimples (pinholes) is due to the different hardness of the steel substrate and the inclusions (impurities). When polishing, the substrate is removed by grinding faster than the inclusions. If the polishing continues, the inclusions will protrude and eventually will be pulled out leaving tiny holes (pinholes).


If the size of the diamond paste is less than 10 μm and a softer polishing tool is used (eg, hair loss), micro-pits (pinholes) are most likely to occur. One of the ways to reduce the appearance of micro-pits (pinholes) is to use high-purity die steels that have been vacuum degassed or ESR refined.


If dimples (pinholes) still appear, take the following actions.

1. Re-grind the surface carefully. The grain size of the sand is slightly coarser than the one used earlier.

2. The polishing procedure is carried out using the soft and sharpened oil stone for the final step of grinding.

3. When the grain size is less than Цm, the softest polishing tool should be avoided.

4. Use the shortest polishing time and the smallest polishing pressure possible.


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