What are the precautions in the polishing stage of optical glass lenses?

Polishing materials and tools

Material selection of polishing die

For optical glass lenses with different shapes and accuracy requirements, appropriate polishing die materials should be selected. For example, polyurethane polishing die is suitable for some lenses with complex shapes and relatively low precision requirements. It has good elasticity and adaptability, can better fit the lens surface, and reduce the excessive local pressure during polishing. The asphalt polishing die is often used for high-precision lens polishing, which can provide a very high surface finish. Because asphalt can gradually deform according to the shape of lens surface during polishing, polishing is more uniform.

Pay attention to the hardness and toughness of the polishing die. Improper hardness may lead to scratches on the lens surface or can not effectively remove grinding marks. If the polishing die is too hard, it may scratch the glass when contacting with the lens; Too soft may not provide enough polishing pressure, affecting polishing efficiency. At the same time, the polishing die with poor toughness is easy to be damaged, which shortens the service life.

Shape making of polishing die

The shape of the polishing die should be made accurately according to the shape of the lens. For spherical lenses, the curvature of the polishing die should closely match the curvature of the lens. If the curvature radius of the polishing die is different from that of the lens, it will lead to uneven polishing and local over-polishing or under-polishing. For example, when polishing a convex spherical lens, the radius of curvature of the concave surface of the polishing die should be slightly larger than the radius of curvature of the convex surface of the lens to ensure good contact and polishing effect.

For aspheric lens, the shape design of polishing die is more complicated. It is necessary to use high-precision processing equipment to make a polishing mold suitable for aspheric lens, and to take into account the complex curve and changing curvature of aspheric surface to ensure that the polishing mold can achieve uniform polishing on the entire lens surface.

Preparation and selection of polishing solution

The components of polishing solution mainly include polishing powder and chemical additives. There are many kinds of polishing powders, such as cerium oxide (CeO₂), which has good chemical activity and grinding ability for optical glass. When choosing polishing powder, we should consider its particle size, purity and other factors. Particle size directly affects the polishing accuracy and surface quality. Finer particles can obtain a smoother surface, but the polishing speed may be slow. The coarser particles have a faster polishing speed, but may leave deeper scratches.

Chemical additives play a role in regulating the polishing process in the polishing solution. For example, some additives can adjust the pH value of the polishing solution, so that the polishing solution can chemically react with the glass surface at a suitable pH value, soften the glass surface and improve the polishing efficiency. At the same time, additives can also prevent the agglomeration of polishing powder and ensure the stability of polishing solution. When preparing polishing solution, it should be carried out in strict accordance with the formula proportion to ensure the accurate concentration of each component, because improper concentration will affect the polishing effect.

Polishing process control

Polishing speed control

Polishing speed has an important influence on lens surface quality and polishing efficiency. If the polishing speed is too fast, it will produce a large friction force, which may lead to too high temperature of the lens surface, thermal deformation of the glass surface, and wear of the polishing die and lens surface. For example, when polishing small-sized and high-precision lenses, the polishing speed should generally be controlled in a lower range to ensure the surface quality.

Too slow polishing speed will affect production efficiency. The appropriate polishing speed should be determined according to the size, shape and material of the lens and the characteristics of the polishing die and polishing solution. Generally speaking, the polishing speed of large-size lenses can be faster, but we should also pay attention to avoid quality problems caused by too fast speed.

Polishing pressure control

Proper polishing pressure is one of the key factors to ensure the surface quality of lens. When the pressure is too high, the contact between the lens and the polishing die will be too close, which will easily lead to scratches, pits and other defects on the lens surface. Moreover, excessive pressure will make the polishing die deform excessively, which will affect its service life. For example, when polishing an ultra-thin lens, a smaller polishing pressure should be used to prevent the lens from breaking.

If the pressure is too small, the grinding marks can't be effectively removed and the expected polishing effect can't be achieved. The polishing pressure should be determined comprehensively according to the hardness of lens material, the characteristics of polishing die and the specific requirements of polishing stage. The best polishing pressure range can be found through experiments and experience accumulation.

Temperature control

During the polishing process, the temperature of the lens and the surface of the polishing die will increase due to the friction. Too high temperature will bring many problems, such as the thermal deformation of glass mentioned above, and it will also dry up the polishing solution, leading to the agglomeration of polishing powder and affecting the polishing effect. Therefore, it is necessary to take effective temperature control measures.

A cooling liquid circulation system can be used to spray the cooling liquid evenly on the contact surface between the lens and the polishing mold to take away the heat. At the same time, attention should be paid to the selection of cooling liquid to avoid chemical reaction between cooling liquid and polishing liquid or lens material. In addition, in the design and operation of polishing equipment, heat dissipation should also be taken into account, for example, reasonable arrangement of equipment vents to improve heat dissipation conditions.