Polishing is a crucial step in the plastic mold making process. It generally involves using oilstones, sandpaper, and polishing compounds to grind the mold cavity surface, making the working surface of the mold mirror-like.

Mold polishing makes the surface of the produced products smooth and beautiful, and also makes it easier for the products to be demolded from the mold.

The standards for mirror polishing of molds are divided into four levels:

A0=Ra0.008μm

A1=Ra0.016μm

A3=Ra0.032μm

A4=Ra0.063μm

1. Methods of Mold Polishing

When polishing molds, do not start with the finest oilstones, sandpaper, or polishing compounds. This will not remove coarse textures. The surface may appear bright, but when viewed from the side, coarse textures will be visible. Therefore, start with coarser oilstones, sandpaper, or polishing compounds, then switch to finer ones, and finally use the finest polishing compound.

This may seem troublesome and involve many steps. In fact, it is not slow. Each step removes the coarse processing textures from the previous step, preventing rework, and allowing the mold to achieve the required smoothness in one go.

Generally, coarse oilstones are first used to roughly grind the surface of the mechanically processed mold cavity to remove machine tool marks. Then, fine oilstones are used to remove the marks left by the coarse oilstones. Fine sandpaper is then used to further grind the surface polished by the fine oilstones. Finally, polishing or grinding paste is used to perform the final fine polishing of the mold cavity surface, achieving a mirror-like effect. This is the general process of mold polishing.

Of course, if possible, an ultrasonic polishing machine can be used to polish the mold, which is more efficient and less strenuous.

2. Points to Note When Polishing with Sandpaper

(1) Sandpaper polishing requires the use of soft wooden or bamboo sticks. When polishing curved or spherical surfaces, soft wooden sticks better match the curvature of the surface. Harder wood, such as cherry wood, is more suitable for polishing flat surfaces. The end of the stick should be shaped to match the shape of the steel surface to avoid deep scratches caused by sharp corners of the stick contacting the steel surface.

(2) When changing to different grades of sandpaper, the polishing direction should be changed by 45°-90°. This allows the streaks left by the previous grade of sandpaper to be identified. Before changing to a different grade of sandpaper, the polished surface must be carefully wiped with 100% pure cotton soaked in a cleaning solution such as alcohol, as even a small grain of sand left on the surface will ruin the entire polishing process. This cleaning process is equally important when switching from sandpaper polishing to diamond polishing paste. All particles and kerosene must be completely cleaned before continuing polishing.

(3) To avoid scratching and burning the workpiece surface, extra care must be taken when polishing with #1200 and #1500 sandpaper. Therefore, it is necessary to apply a light load and use a two-step polishing method. When polishing with each grade of sandpaper, polish twice in two different directions, each time rotating 45°-90° between the two directions.