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Mould polish(1)


As plastic products become more and more widely used, such as daily cosmetics and beverage packaging containers, the need for appearance often requires the surface of the plastic mold cavity to reach the degree of mirror polishing. The production of optical lenses, laser records and other molds require extremely high surface roughness, and thus the polishability requirements are also extremely high. Polishing not only increases the beauty of the workpiece, but also improves the corrosion resistance and wear resistance of the material surface, and can make the mold have other advantages, such as making the plastic products easy to release, reducing the production injection cycle. Therefore, polishing is a very important process in the process of plastic mold making.
At present, the following polishing methods are commonly used.
 
1.1 mechanical polishing
Mechanical polishing is by cutting, material surface plastic deformation to remove the convex part of the polished and smooth surface polishing method, generally using oil stone, wool wheel, sandpaper, etc., mainly by hand, special parts such as the surface of the rotary body, can use the rotary table and other auxiliary tools, the surface quality requirements can be used to ultra-precise polishing method. Ultra-precision grinding and polishing is the use of special abrasives, containing abrasives in the grinding and polishing fluid, pressed on the surface of the workpiece being processed, for high-speed rotary motion. Using this technology can achieve Ra0.008μm surface roughness, is the highest among various polishing methods. Optical lens molds often use this method.
 
1.2 Chemical polishing
Chemical polishing is to let the material in the chemical medium surface microscopic convex part than the concave part of the preferential dissolution +, so as to get a smooth surface. The main advantage of this method is that it does not need complicated equipment, can polish complex shape workpiece, can polish many workpieces at the same time, high efficiency. The core problem of chemical polishing is the preparation of polishing solution. The surface roughness obtained by chemical polishing is generally several 10μm.
 
1.3 electrolytic polishing
The basic principle of electrolytic polishing is the same as chemical polishing, that is, by selectively dissolving the tiny projections on the surface of the material to make the surface smooth. Compared with chemical polishing, the effect of cathodic reaction can be eliminated, and the effect is better. Electrochemical polishing process is divided into two steps.
(1) Macro-leveling Dissolution products diffuse into the electrolyte, and the geometric roughness of the material surface decreases, Ra>1μm.
(2) Micro-leveling Anodic polarization, surface brightness increases, Ra <1μm.
 
1.4 Ultrasonic polishing
The workpiece is put into the abrasive suspension and placed in the ultrasonic field together, relying on the oscillating effect of ultrasonic waves to make the abrasive grinding and polishing on the surface of the workpiece. Ultrasonic processing has small macro force and will not cause deformation of the workpiece, but it is difficult to make and install the tooling. Ultrasonic processing can be combined with chemical or electrochemical methods. On the basis of solution corrosion, electrolysis, then apply ultrasonic vibration to stir the solution, so that the workpiece surface dissolution products from the surface, corrosion or electrolyte near the surface evenly; ultrasonic cavitation in the liquid can also inhibit the corrosion process, facilitate the surface brightening
 
1.5 Fluid polishing
Fluid polishing is to rely on the high-speed flow of liquid and the abrasive particles carried by the workpiece surface to achieve the purpose of polishing. Commonly used methods are: abrasive jet processing, liquid jet processing, hydrodynamic grinding, etc. Fluid power grinding is driven by hydraulic pressure, so that the liquid medium carrying abrasive particles flowing over the surface of the workpiece at high speed reciprocally. The medium is mainly made of a special compound (polymer-like substance) with good flow through under low pressure and mixed with abrasive, and the abrasive can be made of silicon carbide powder.
 
1.6 Magnetic grinding and polishing
Magnetic grinding and polishing is to use magnetic abrasives to form abrasive brushes under the action of magnetic field to grind and process the workpiece. This method has high processing efficiency, good quality, easy control of processing conditions and good working conditions. With suitable abrasives, the surface roughness can reach Ra0.1μm.
The polishing mentioned in the plastic mold processing is very different from the surface polishing required in other industries, 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 flatness, smoothness and geometric accuracy. Surface polishing is generally only required to obtain a shiny surface. Mirror processing standards are divided into four levels: AO = Ra0.008μm, A1 = Ra0.016μm, A3 = Ra0.032μm, A4 = Ra0.063μm, due to electrolytic polishing, fluid polishing and other methods are difficult to accurately control the geometric accuracy of the parts, while chemical polishing, ultrasonic polishing, magnetic polishing and other methods of surface quality can not meet the requirements, so precision The mirror surface processing of the mold or mechanical polishing is the main focus.
 
2.1 Mechanical polishing basic procedures
To obtain high quality polishing effect, the most important thing is to have high quality oil stone, sandpaper and diamond polishing paste and other polishing tools and auxiliary products. And the choice of polishing procedure depends on the surface condition after pre-processing, such as mechanical processing, EDM processing, grinding processing, etc. The general process of mechanical polishing is as follows.
(1) rough polishing
After milling, EDM, grinding and other processes, the surface can be polished by choosing a rotary surface polishing machine with a speed of 35 000-40 000 rpm or an ultrasonic lapping machine. Commonly used methods include removing the white EDM layer using a wheel of Φ3mm diameter, WA # 400. This is followed by manual oilstone polishing with strips of oilstone with kerosene as lubricant or coolant. The general order of use is #180 ~ #240 ~ #320 ~ #400 ~ #600 ~ #800 ~ #1000. many mold makers choose to start with #400 to save time.
(2) Semi-finishing polishing
Semi-finishing is mainly done with sandpaper and kerosene. The number of sandpaper is #400 ~ #600 ~ #800 ~ #1000 ~ #1200 ~ #1500. In fact, #1500 sandpaper is only suitable for hardened die steel (52HRC or more), but not for pre-hardened steel, as it may cause surface burns on pre-hardened steel parts.
(3) Fine polishing
Fine polishing mainly uses diamond polishing paste. If the polishing cloth wheel is mixed with diamond grinding powder or paste, the usual grinding sequence is 9μm (#1800) to 6μm (#3000) to 3μm (#8000). 9μm diamond grinding paste and polishing cloth wheel can be used to remove hairline abrasion marks left by #1200 and #1500 sandpaper. Next, polishing is done with sticky felt and diamond polishing paste in the order of 1μm (#14000) ~ 1/2μm (#60000) ~ 1/4μm (#100000).
Polishing processes requiring a precision of 1μm or more (including 1μm) can be performed in a clean polishing room in the mold shop. For more precise polishing, an absolutely clean space is necessary. Dust, fumes, dandruff and saliva can ruin the high precision polished surface obtained after hours of work.