Here are the detailed steps of polishing:
1. Excessive polishing
The biggest problem encountered in the daily polishing process is "excessive polishing", which means that the longer the polishing time, the worse the quality of the mold surface. There are two phenomena of excessive polishing: orange peel and pitting. Excessive polishing often occurs in mechanical polishing.
2. Causes of "orange peel" on workpiece
Irregular and rough surfaces are called "orange peel". There are many reasons for producing "orange peel". The most common reason is that the mold surface is overheated or carburized excessively. Excessive polishing pressure and polishing time are the main reasons for producing "orange peel". For example, when polishing with a polishing wheel, the heat generated by the polishing wheel will easily cause "orange peel". The harder steel can bear more polishing pressure, and the softer steel is prone to over polishing. Research has proved that the time of over polishing varies with the hardness of the steel.
3. Measures to eliminate "orange peel" of workpiece
When it is found that the surface quality is not polished well, many people will increase the polishing pressure and prolong the polishing time, which often makes the surface quality worse.
The following methods can be used to remedy:
(1) Remove the defective surface, grind the particle size slightly thicker than the previous sand number, and then grind it, and the polishing force is lower than the previous one.
(2) Stress relief shall be carried out at a temperature 25 ℃ lower than the tempering temperature, and the finest sand number shall be used for grinding before polishing until satisfactory results are achieved, and finally polishing shall be carried out with a light force.
4. Causes of "pitting corrosion" on workpiece surface
Because some non-metallic impurities in the steel, usually hard and brittle oxides, are pulled out from the steel surface during the polishing process to form pits or pitting corrosion, the main factors causing "pitting corrosion" are as follows:
(1) The polishing pressure is too high and the polishing time is too long.
(2) The purity of steel is not enough, and the content of hard impurities is high.
(3) The mold surface is rusty.
(4) The black leather is not removed.
5. Measures to eliminate pitting corrosion of workpiece
(1) Carefully regrind the surface, and the sand particle size is slightly larger than that used previously. Use soft and sharp oilstone for the final step of grinding before polishing.
(2) When the sand size is less than 1 mm, the softest polishing tool should be avoided.
(3) Try to use the shortest polishing time and the smallest polishing force.