Using a polishing machine and polishing powder or polishing liquid together for polishing requires setting polishing time, pressure and other parameters. After polishing, it must be cleaned or soaked immediately, otherwise the polishing powder will solidify on the glass and leave traces.
1. Material of polishing powder
Polishing powder is usually composed of cerium oxide, aluminum oxide, silicon oxide, iron oxide, zirconium oxide, chromium oxide and other components. Different materials have different hardness and different chemical properties in water, so the use occasions are different. The Mohs hardness of aluminum oxide and chromium oxide is 9, cerium oxide and zirconium oxide are 7, and iron oxide is lower. Cerium oxide has a high chemical activity and comparable hardness to silicate glass, so it is widely used in glass polishing. In order to increase the polishing speed of cerium oxide, fluorine is usually added to cerium oxide polishing powder to increase the grinding rate. Mixed rare earth polishing powder with low cerium content is usually doped with 3-8 fluorine; pure cerium oxide polishing powder is usually not doped with fluorine. For ZF or F series glass, because of its low hardness and high fluorine content, it is better to use fluorine-free polishing powder.
2. Particle size of cerium oxide
The larger the particle size of cerium oxide, the greater the grinding force, and the more suitable it is for harder materials. ZF glass should use finer polishing powder. It should be noted that all cerium oxide particle sizes have a distribution problem. The size of the average particle size or median diameter D50 only determines the speed of polishing, while the maximum particle size Dmax determines the polishing accuracy
The level of optical cold processing production operation 3. Therefore, to obtain high-precision requirements, the maximum particle size of polishing powder must be controlled.
3. Hardness of polishing powder
The actual hardness of polishing powder is related to the material. For example, the hardness of cerium oxide is about Mohs hardness 7, and all kinds of cerium oxide are similar. But different cerium oxide bodies give people different hardness because cerium oxide polishing powder is usually agglomerates. Of course, some polishing powders add harder materials such as aluminum oxide, which will increase the grinding rate and wear resistance.
4. Concentration of polishing slurry
The concentration of the slurry during polishing determines the polishing speed. The higher the concentration, the higher the polishing speed. When using small-particle polishing powder, the slurry concentration should be appropriately adjusted to a lower level.
Lens polishing
After the optical lens is finely ground with abrasive liquid, there is still a crack layer about 2-3 m thick on its surface. The method to eliminate this crack layer is polishing. The mechanism of polishing is the same as that of grinding, but the tool material and polishing liquid (slurry) used are different. The materials used for polishing are flannel (cloth), polishing leather (polyurethane) and asphalt (pitch). Usually, to achieve a high-precision polishing surface, the most commonly used material is high-grade polishing asphalt. Using asphalt for polishing is to use the fine surface of asphalt to drive the polishing liquid to grind the surface of the lens to generate heat, so that the glass melts and flows, melts the rough tops and fills the bottom of the cracks, and gradually removes the crack layer. At present, the polishing powder used for polishing glass lenses is mainly cerium oxide (CeO2). The proportion of polishing liquid varies according to the polishing period of the lens. Generally, a high-concentration polishing liquid is used in the initial polishing and when the polishing mold is combined. After the lens surface is bright, a dilute polishing liquid is used to avoid the orange peel phenomenon (fogging of the lens surface) on the mirror surface. The motion mechanism used for polishing and grinding is the same. In addition to the different polishing tools and working fluids, the environmental conditions required for polishing are also more stringent than those for grinding. The following are the general matters to be paid attention to during polishing: There must be no impurities on the surface of the polishing asphalt and in the polishing liquid, otherwise it will cause scratches on the mirror surface. The surface of the polishing asphalt must match the surface of the lens, otherwise it will jump during polishing, thereby biting the polishing powder and scratching the lens surface. Before polishing, it is necessary to determine whether there are scratches or punctures on the lens surface left after grinding. Whether the size and material of the polishing tool are appropriate. Whether the hardness and thickness of the asphalt are appropriate. During the polishing process, attention must be paid to the surface condition and precision inspection of the lens at all times. Inspection of lens surface defects, because the inspection process is based on personal vision and methods, so the inspector should have a deep understanding of the specifications of scratches and sand holes, and should often compare the standard samples of scratches and sand holes to ensure the correctness of the inspection.
Detailed description of optical cold processing process data (process aging)
1. Polishing powder
1.1 Requirements for polishing powder
a. The particle size should be uniform, and the hardness should generally be slightly harder than the polished material;
b. The polishing powder should be pure and contain no impurities that may cause scratches;
c. It should have a certain lattice morphology and defects, and have appropriate self-sharpening;
d. It should have good dispersibility and adsorption;
e. It has good chemical stability and will not corrode the workpiece.
1.2 Types and properties of polishing powder Commonly used polishing powders are cerium oxide (CeO2) and iron oxide (FeO3).
a. The particles of cerium oxide polishing powder are polygonal, with obvious edges and corners, an average diameter of about 2 microns, Mohs hardness of 7 to 8, and a specific gravity of about 7.3. Due to the different manufacturing processes and cerium oxide content, cerium oxide polishing powder is white (content reaches more than 98%), light yellow, brown yellow, etc.
b. Iron oxide polishing powder is commonly known as red powder. The particles are spherical, the particle size is about 0.5 to 1 micron, the Mohs hardness is 4 to 7, and the specific gravity is about 5.2. The color ranges from yellow-red to dark red.
In summary, cerium oxide has a higher polishing efficiency than red powder, but for parts with high surface finish requirements, red powder is still better for polishing.
2. Polishing mold layer (underlay) material) Commonly used polishing mold layer materials are polishing glue and fiber materials.