With widespread application of plastic products, such as daily chemical products and beverage packaging containers, appearance requirements often require surface of plastic mold cavity to reach level of mirror polishing. Production of optical lenses, CDs and other molds have extremely high requirements for surface roughness, therefore extremely high requirements for polishability.
Polishing not only increases beauty of workpiece, but also improves corrosion resistance and wear resistance of surface of material. It can also make mold have other advantages, such as making plastic products easy to demold and reducing production injection cycle. Therefore, polishing is a very important process in production of plastic molds.

 

At present, commonly used polishing methods are as follows:

Mechanical polishing is a polishing method that relies on cutting and plastic deformation of material surface to remove polished convexities to obtain a smooth surface. Generally, oil stone sticks, wool wheels, sandpaper, etc. are used, manual operations are main ones. For special parts such as surface of rotating body, auxiliary tools such as turntables can be used, method of ultra-precision polishing can be used for high surface quality requirements. Ultra-precision polishing adopts a special abrasive tool, which is pressed tightly on processed surface of workpiece in a polishing liquid containing abrasives for high-speed rotation. Using this technology, surface roughness of Ra0.008μm can be achieved, which is the highest among various polishing methods. Optical lens molds often use this method.

Chemical polishing is to make surface microscopic convex part of material in chemical medium dissolve preferentially than concave part, so as to obtain a smooth surface. Main advantage of this method is that it does not require complex equipment, can polish workpieces with complex shapes, and can polish many workpieces at the same time, with high efficiency. Core problem of chemical polishing is preparation of polishing liquid. Surface roughness obtained by chemical polishing is generally several 10 μm.

Basic principle of electrolytic polishing is same as that of chemical polishing, that is, by selectively dissolving tiny protrusions on the surface of material to make surface smooth. Compared with chemical polishing, effect of cathode reaction can be eliminated, and effect is better.
Electrochemical polishing process is divided into two steps:
(1) Macroscopic leveling: Dissolved products diffuse into electrolyte, and geometric roughness of material surface decreases, Ra>1μm.
(2) Low-light leveling: Anode polarization, improved surface brightness, Ra<1μm.

Put workpiece in abrasive suspension and put it together in ultrasonic field, relying on oscillation effect of ultrasonic, so that abrasive is ground and polished on the surface of workpiece. Ultrasonic machining has a small macroscopic force and will not cause deformation of workpiece, but it is difficult to manufacture and install tooling. Ultrasonic processing can be combined with chemical or electrochemical methods. On the basis of solution corrosion and electrolysis, ultrasonic vibration is applied to stir solution, so that dissolved products on the surface of workpiece are separated, and corrosion or electrolyte near surface is uniform; cavitation effect of ultrasonic waves in liquid can also inhibit corrosion process, which is conducive to surface brightening.

Fluid polishing relies on high-speed flowing liquid and abrasive particles carried by it to wash surface of workpiece to achieve purpose of polishing. Commonly used methods are: abrasive jet processing, liquid jet processing, hydrodynamic grinding and so on. Hydrodynamic grinding is driven by hydraulic pressure to make liquid medium carrying abrasive particles flow back and forth across surface of workpiece at high speed. Medium is mainly made of special compounds (polymer-like substances) with good flowability under lower pressure and mixed with abrasives. Abrasives can be made of silicon carbide powder.

Magnetic abrasive polishing is to use magnetic abrasives to form abrasive brushes under action of a magnetic field to grind workpiece. This method has high processing efficiency, good quality, easy control of processing conditions and good working conditions. Using suitable abrasives, surface roughness can reach Ra0.1μm.

 

Polishing mentioned in plastic mold processing is very different from surface polishing required in other industries. Strictly speaking, polishing of 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 generally only requires a bright surface.
Standard of mirror surface processing is divided into four levels: AO=Ra0.008μm, A1=Ra0.016μm, A3=Ra0.032μm, A4=Ra0.063μm. Because it is difficult to accurately control geometric accuracy of parts by methods such as electrolytic polishing and fluid polishing, surface quality of methods such as chemical polishing, ultrasonic polishing, and magnetic abrasive polishing cannot meet requirements, so mirror surface processing of precision molds is still mainly mechanical polishing.

To obtain a high-quality polishing effect, the most important thing is to have high-quality polishing tools and auxiliary products such as whetstone, sandpaper and diamond lapping paste. Choice of polishing procedure depends on surface condition after pre-processing, such as machining, EDM, grinding and so on.
General process of mechanical polishing is as follows:
(1) Rough polishing. Surface after milling, electric spark, grinding and other processes can be polished by a rotating surface polishing machine or an ultrasonic grinding machine with a speed of 35000-40 000 rpm. Commonly used method is to remove white EDM layer with wheels with diameter Φ3mm and WA # 400. Then there is manual whetstone grinding, and strips of whetstone are added with kerosene as a lubricant or coolant. General order of use is #180 ~ #240 ~ #320 ~ #400 ~ #600 ~ #800 ~ #1000. Many mold manufacturers choose to start with #400 in order to save time.
(2) Semi-precision polishing. Semi-precision polishing mainly uses sandpaper and kerosene. Number of sandpaper is: #400~ #600 ~ #800 ~ #1000 ~ #1200 ~ #1500. In fact, #1500 sandpaper only uses die steel suitable for hardening (above 52HRC), not pre-hardened steel, because it may cause burns on the surface of pre-hardened steel.
(3) Fine polishing. Fine polishing mainly uses diamond abrasive paste. If polishing cloth wheel is mixed with diamond abrasive powder or abrasive paste for polishing, usual polishing order is 9μm (#1800) ~ 6μm (#3000) ~ 3μm (#8000). 9μm diamond abrasive paste and polishing cloth wheel can be used to remove hair-like wear marks left by #1200 and #1500 sandpaper. Then use sticky felt and diamond paste for polishing, order is 1μm (#14000) ~ 1/2μm (#60000) ~ 1/4μm (#100000). Polishing process with an accuracy requirement of 1μm or more (including 1μm) can be carried out in a clean polishing room in mold processing workshop. For more precise polishing, an absolutely clean space is necessary. Dust, smoke, dandruff and saliva can all scrap high-precision polished surface obtained after several hours of work.

Pay attention to following points when polishing with sandpaper:
(1) Polishing with sandpaper requires use of soft wood sticks or bamboo sticks. When polishing round or spherical surfaces, use of cork rods can better match curvature of round and spherical surfaces. Harder wood strips, like cherry wood, are more suitable for polishing flat surfaces. Trim ends of wooden strips so that they can be consistent with surface shape of steel parts, so as to prevent sharp angles of wooden strips (or bamboo strips) from contacting surface of steel parts and cause deep scratches.
(2) When changing to a different type of sandpaper, polishing direction should be changed from 45° to 90°, so that streaks and shadows left by previous type of sandpaper after polishing can be distinguished. Before changing to a different type of sandpaper, you must carefully wipe polishing surface with 100% pure cotton dipped in a cleaning solution such as alcohol, because a small grit on the surface will destroy entire subsequent polishing work. This cleaning process is equally important when changing from sandpaper polishing to diamond paste polishing. Before polishing can continue, all particles and kerosene must be completely cleaned.
(3) In order to avoid scratches and burns on the surface of workpiece, special care must be taken when polishing with #1200 and #1500 sandpaper. Therefore, it is necessary to apply a light load and use a two-step polishing method to polish surface. When polishing with each type of sandpaper, polishing should be done twice along two different directions, each rotation between two directions is 45°~90°.
Pay attention to the following points in diamond grinding and polishing:
(1) This kind of polishing must be carried out under lighter pressure as much as possible, especially when polishing pre-hardened steel parts and polishing with fine abrasive paste. When polishing with #8000 abrasive paste, usual load is 100~200g/cm2, but it is difficult to maintain accuracy of this load. To make this easier, you can make a thin and narrow handle on wooden strip, such as adding a copper sheet; or cut a part of bamboo strip to make it softer. This can help control polishing pressure to ensure that mold surface pressure is not too high.
(2) When using diamond grinding and polishing, not only working surface must be clean, but hands of worker must also be carefully cleaned.
(3) Each polishing time should not be too long, the shorter time, the better effect. If polishing process is too long, it will cause "orange peel" and "pitting".
(4) In order to obtain high-quality polishing results, polishing methods and tools that are prone to heat should be avoided. For example: polishing with a polishing wheel, heat generated by polishing wheel can easily cause "orange peel".
(5) When polishing process is stopped, it is very important to ensure that surface of workpiece is clean, carefully remove all abrasives and lubricants. Then a layer of mold anti-rust coating should be sprayed on the surface.

 

Since mechanical polishing is mainly done manually, polishing technology is still main reason that affects polishing quality. In addition, it is also related to mold material, surface condition before polishing, and heat treatment process. High-quality steel is a prerequisite for obtaining good polishing quality. If surface hardness of steel is uneven or there are differences in characteristics, polishing difficulties will often occur. Various inclusions and pores in steel are not conducive to polishing.
3.1 Impact of different hardness on polishing process. Increase in hardness increases difficulty of grinding, but roughness after polishing decreases. As hardness increases, polishing time required to achieve a lower roughness increases accordingly. At the same time, hardness increases, and possibility of over-polishing decreases accordingly.
3.2 Influence of surface condition of workpiece on polishing process. During crushing process of cutting machining of steel, surface layer will be damaged due to heat, internal stress or other factors, improper cutting parameters will affect polishing effect. Surface after EDM is more difficult to grind than surface after ordinary machining or heat treatment. Therefore, before end of EDM, it should be trimmed with precision EDM, otherwise surface will form a hardened thin layer. If EDM fine-tuning rules are not selected properly, depth of heat-affected layer can reach up to 0.4mm.
Hardness of hardened thin layer is higher than that of substrate and must be removed. Therefore, it is best to add a rough grinding process to completely remove damaged surface layer to form an evenly rough metal surface, which provides a good foundation for polishing process.