General Steps of Mechanical Polishing
- Coarse Polishing
Surfaces processed by milling, EDM, grinding, and other techniques can be polished using a rotating polishing machine or an ultrasonic grinding machine at speeds between 35,000 to 40,000 rpm. A common method involves using a wheel of diameter Φ3mm and WA #400 to remove the white EDM layer. This is followed by manual oil stone grinding, using strip-shaped oil stones with kerosene as a lubricant or coolant. The typical sequence of grits used is #180, #240, #320, #400, #600, #800, and #1,000. To save time, many mold manufacturers start with #400 grit. - Semi-Fine Polishing
Pre-polishing mainly utilizes sandpaper and kerosene. The sequence of sandpaper grits is: #400, #600, #800, #1,000, #1,200, and #1,500, and more. In reality, #1,500, and above, sandpaper is only suitable for hardened mold steel (above HRC 52) and not for pre-hardened steel, as it might cause surface burnishing on pre-hardened steel parts. - Fine Polishing
Fine polishing primarily employs diamond polishing paste. When mixed with a polishing cloth wheel, the usual sequence of diamond paste grits is 9 μm (#1,500), 6 μm (#2,000), and 3 μm (#4,000). The 9 μm diamond paste and polishing cloth wheel are used to remove the scratches left by #1,200 and #1,500 sandpaper. Then, polishing is performed with felt and diamond paste in the order of 1 μm (#10,000), 1/2 μm (#20,000), and 1/4 μm (#50,000).
*All mesh numbers use the JIS standard.
What is Chemical Mechanical Polishing
Chemical Mechanical Polishing (CMP) is a technique that employs both chemical and mechanical actions to polish the surface of a workpiece. During the CMP process, the workpiece surface contacts a polishing pad filled with a chemical solution. Under externally applied pressure and relative motion, the material on the surface of the workpiece is softened through a chemical reaction, and the softened material is mechanically worn away, achieving the polishing effect. CMP technology can effectively remove the damage layer and cracks on the workpiece surface, enhancing the surface finish. It's capable of achieving a roughness of Sa.0.2nm.
It's important to differentiate this from the Chemical Mechanical Planarization technology used in semiconductors. Although polishing and planarization use the same processing principle, planarization demands higher equipment performance and polishing pad hardness, as this process aims to improve both the roughness and planarity of the workpiece.
As industrial technology advances and evolves, CMP has found wide applications across various materials used in modern industries, including metals, ceramics, plastics, glass, and more. However, some special materials may undergo chemical alteration after CMP. Thus, the polishing liquid needs to be specially formulated, and the processed products require proper preservation, such as cover with antioxidants.
Ceramics
Ceramics possess unique superior properties, making them increasingly used in various fields from smartphone cases to aerospace equipment. In the ultra-precise processing and polishing of ceramic materials, grinding and polishing processes hold an irreplaceable position. The polishing of ceramic materials mainly includes abrasive polishing, laser processing, and ultrasonic polishing.
Application Example
Some smartphones use a microcrystalline zirconia ceramic back plate. Due to the high hardness of the ceramic body and the strong light absorption of the ceramic surface, ordinary grinding and polishing cannot achieve a mirror polish effect. Therefore, it requires plane and arc lapping and polishing. The process flow is:
- Arc Buffing Lapping
- Arc Buffing Polishing
- Plane Lapping
- Plane Pre-Polishing
- Plane CMP polishing
The plates processed for 313 minutes become smooth and even, with a gentle color.
Plastics
Often, the appearance of plastic cover directly affects consumers' perception of the product, so these types of products usually require a mirror polish. Polishing plastic products can improve the surface water resistance and translucency of the material, making polishing an important process in plastic manufacturing. CMP is commonly used for polishing plastic products.
Application Example
Past semi-smartphones and modern digital displays mostly use resin materials. Before polishing, the material appears similar to frosted glass. After CMP polishing, the screen becomes transparent with a glassy texture, and the surface feels smoother and more delicate. This type of screen process involves flattening the product through flat lapping and then achieving the ideal effect directly through soft CMP polishing. The products produced through these processes are as glossy and transparent as glass.
Glass
The main economic and technical indicators of glass polishing processing are characterized by the finish polishing efficiency, dimensional accuracy of the processed glass edge, roughness, and the consumption condition or service life of the polishing tools. There are several types of glass processing equipment on the market, but the intrinsic quality of the glass itself and the operator's skill level directly affect the efficiency and outcome of the glass. Therefore, choosing an efficient and simple polishing process is the best solution.
Application Example
Smartphone touch screen glass lapping and polishing processing works on uneven glass surfaces to make them smooth and relatively polished. The common steps are rough lapping, fine lapping, and finish polishing. Rough lapping uses larger-grained abrasives to remove most of the excess material from the blank, and maintaining the approximate geometric shape and roughness. Fine lapping aims to maintain the most accurate geometric shape and fine crack depth. CMP is the final process step and the last part to achieve an optical surface layer. Before CMP polishing, try to remove the damage layer left by fine and rough grinding to achieve the ideal optical surface effect.
Conclusion
The common thread across all materials is the strategic use of CMP polishing, which stands out for its ability to deliver ultra-smooth and precise surfaces, enhancing both the functionality and aesthetics of the final products. This process underscores the importance of choosing the right polishing method, tailored to the specific requirements of the material and its end-use, ensuring that each piece meets the highest standards of quality and performance.
