With the widespread application of plastic products, such as daily necessities and beverage packaging containers, appearance needs often require the surface of the plastic mold cavity to reach the degree of mirror polishing. And the production of optical lenses, laser records and other molds have high requirements for surface roughness, so the requirements for polishing are also very high.
Polishing not only increases the beauty of the product, but also improves the corrosion resistance and wear resistance of the material surface, and can also make the mold have other advantages, such as making plastic products easy to release and reducing the production injection cycle. Therefore, polishing is a very important process in the process of making plastic molds.
At present, the commonly used polishing methods are as follows:
1.1 Mechanical polishing
Mechanical polishing is to remove the plastic deformation of the surface of the cutting material after the polishing of the convex part and get a smooth polishing method, the general use of stone strip, wool wheel, sandpaper and other tools, mainly manual operation, special parts such as the surface of the rotary body, can use auxiliary tools such as the turntable, surface quality requirements can be used super polishing method.
Super finishing polishing is the use of special grinding tools, in the polishing liquid containing abrasive, tightly pressed on the workpiece to be machined surface, for high-speed rotation. The surface roughness of Ra0.008μm can be achieved by using this technology, which is the highest among various polishing methods. This method is often used in optical lens molds.
1.2 Chemical Polishing
Chemical polishing is to dissolve the microscopic protruding part of the surface of the material in the chemical medium preferentially than the concave part, so as to obtain a smooth surface. The main advantage of this method is that it does not require complex equipment, can polish products with complex shapes, and can polish many plastic parts at the same time, with high efficiency. The core problem of chemical polishing is the preparation of polishing liquid. The surface roughness obtained by chemical polishing is generally 10μm.
1.3 Electrolytic polishing
The basic principle of electrolytic polishing is the same as that of chemical polishing, that is, the selective dissolution of small protruding parts on the surface of the material makes the surface smooth. Compared with chemical polishing, the effect of cathode reaction can be eliminated and the effect is better. The electrochemical polishing process is divided into two steps:
(1) The macro leveling dissolved product diffused into the electrolyte, the geometric roughness of the material surface decreased, Ra > 1μm.
(2) Low light level anodic polarization, surface brightness increased, Ra < 1μm.
1.4 Ultrasonic polishing
The plastic parts are put into the abrasive suspension and placed together in the ultrasonic field, and the abrasive is ground and polished on the workpiece surface through the oscillation of the ultrasonic wave. Ultrasonic processing macroscopic force is small, will not cause mold deformation, but tooling production and installation is more difficult.
Ultrasonic processing can be combined with chemical or electrochemical methods. On the basis of solution corrosion and electrolysis, ultrasonic vibration is applied to stir the solution, so that the dissolved products on the surface of the workpiece are separated, and the corrosion or electrolyte near the surface is uniform. Ultrasonic cavitation in the liquid can also inhibit the corrosion process, which is conducive to surface brightness
1.5 Fluid Polishing
Fluid polishing is the purpose of polishing the workpiece surface by washing the high-speed flowing liquid and the abrasive particles carried by it. Common methods are: abrasive jet processing, liquid jet processing, hydrodynamic grinding and so on. Hydrodynamic grinding is driven by hydraulic pressure to make the liquid medium carrying abrasive particles flow through the surface of the workpiece at high speed. The medium is mainly made of special compounds (polymer-like substances) with good flow under low pressure and mixed with abrasives, which can be silicon carbide powder.
1.6 Magnetic grinding and polishing
Magnetic grinding and polishing is the use of magnetic abrasive under the action of magnetic field to form an abrasive brush, grinding the workpiece. This method has the advantages of high processing efficiency, good quality, easy control of processing conditions and good working conditions. With suitable abrasive, the surface roughness can reach Ra0.1μm.
The polishing in plastic mold processing is very different from the surface polishing required in other industries, and strictly speaking, the polishing of the mold should be called mirror processing. It not only has high requirements for polishing itself but also has high standards for surface smoothness, smoothness and geometric accuracy. Surface polishing generally requires only a bright surface.
The standard of mirror processing is divided into four levels: AO=Ra0.008μm, A1=Ra0.016μm, A3=Ra0.032μm, A4=Ra0.063μm, because electrolytic polishing, fluid polishing and other methods are difficult to accurately control the geometric accuracy of the parts, and the surface quality of chemical polishing, ultrasonic polishing, magnetic polishing and other methods can not meet the requirements. Therefore, the precision mold mirror processing is still based on mechanical polishing.