After a plastic injection mold has been machined using milling, electric discharge machining, or wire cut machining, the surface of its cavity has to be finished mainly by human hands or by polishing using a machine so that the surface is made extremely smooth.
If the condition of polishing is not proper, the glossiness decreases of the transferred surface of the molded item, and the quality of the surface becomes poor. In addition, at the time that the molded item is separated from the cavity, the undulations in the surface become undercuts which may cause defects in separating the molded item from the mold.

The polishing of the cavity surface is done using a hard grinding stone or abrasive powder. Grinding stones or abrasive powders are used successively starting from those with a large grain size and ending with those a small grain size. The polishing work is done while applying cutting oil so that there is no clogging or biting.
The direction of polishing is not only one fixed direction, but the polishing direction is changed frequently such as in the left to right direction, top to bottom direction, or circular direction. Considerations are given so that the polished surface is uniform.

The following are the main polishing materials used.

The granularity of polishing materials can be in the range of #10 to #3000. As the number becomes smaller, the grain becomes rougher.
(Example: The finish will be rough when polished with a grain size of #10 to #30, and the finish will be fine and smooth when polished with a grain size of #1000 to #2000.)

When using a fine grinding stone, the polishing is done by mixing olive oil or a vegetable oil to a wooden material such as maple, pine, willow, balsa, etc.