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With the widespread application of plastic products, such as daily chemical products and beverage packaging containers, the appearance needs often require the surface of the Metal Parts Machining cavity to reach the level of mirror polishing. The production of optical lenses, CDs and other molds have extremely high requirements for surface roughness, and therefore extremely high requirements for polishability.
Polishing not only increases the beauty of cnc machinined workpieces, but also improves the corrosion resistance and wear resistance of the material surface. It can also provide molds with other advantages, such as making plastic products easy to demold and reducing production injection molding cycles. Therefore, polishing is a very important process in the production of plastic molds.

1. Polishing method

At present, the commonly used polishing methods are as follows:

1.1 Mechanical polishing
Mechanical polishing is a polishing method that relies on cutting and plastic deformation of the material surface to remove the polished convexities to obtain a smooth surface. Generally, oil stone strips, wool wheels, sandpaper, etc. are used. Manual operation is the main method. CNC turning special parts such as the surface of the rotating body , Auxiliary tools such as turntable can be used, and ultra-precision polishing method can be used for high surface quality requirements. Ultra-precision polishing is a special abrasive tool, which is pressed tightly on the surface of beryllium copper in a polishing liquid containing abrasives for high-speed rotation. Using this technology, the surface roughness of Ra0.008μm can be achieved, which is the highest among various polishing methods. Optical lens molds often use this method.

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

1.3 Electrolytic polishing
The basic principle of sand casting electrolytic polishing is the same as that of chemical polishing, that is, the surface is made smooth by selectively dissolving tiny protrusions on the surface of the material. 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) Macroscopic leveling The dissolved products diffuse into the electrolyte, and the geometric roughness of the material surface decreases, Ra>1μm.

(2) Low-light leveling: Anode polarization, surface brightness is improved, Ra<1μm.

1.4 Ultrasonic polishing
Put the workpiece in the abrasive suspension and put it together in the ultrasonic field, relying on the oscillation effect of the ultrasonic to make the abrasive grinding and polishing on the surface of the workpiece. Ultrasonic machining has a small macroscopic force and will not cause deformation of the 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 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; the cavitation effect of ultrasonic in the liquid can also inhibit the corrosion process and facilitate surface brightening.

1.5 Fluid polishing
Fluid polishing relies on high-speed flowing liquid and abrasive particles carried by it to wash the surface of the workpiece to achieve the 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 the liquid medium carrying abrasive particles flow back and forth across the surface of the workpiece at high speed. The medium is mainly made of special compounds (polymer-like substances) with good flowability under lower pressure and mixed with abrasives. The abrasives can be made of silicon carbide powder.

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

2. Basic method of mechanical polishing

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 generally only requires a bright surface.

The standard of mirror surface processing is divided into four levels: AO=Ra0.008μm, A1=Ra0.016μm, A3=Ra0.032μm, A4=Ra0.063μm. It is difficult to precisely control the geometric accuracy of parts due to methods such as electrolytic polishing and fluid polishing. However, the surface quality of chemical polishing, ultrasonic polishing, magnetic abrasive polishing and other methods are not up to the requirements, so the mirror processing of precision molds is still mainly mechanical polishing.

2.1 Basic procedures of 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. The choice of polishing procedure depends on the surface condition after pre-processing, such as machining, edm machining, grinding and so on.

The general process of mechanical polishing is as follows:
(1) Rough polishing The surface after milling, electric spark, grinding and other processes can be polished by a rotating surface polishing machine or an ultrasonic grinder with a speed of 35000-40 000 rpm. The commonly used method is to remove the white EDM layer with wheels with diameter Φ3mm and WA # 400. Then there is manual whetstone grinding, and the strip whetstone is added with kerosene as a lubricant or coolant. The 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. The 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 the polishing cloth wheel is mixed with diamond abrasive powder or abrasive paste for polishing, the usual polishing order is 9μm (#1800) ~ 6μm (#3000) ~ 3μm (#8000). 9μm diamond abrasive paste and polishing cloth wheel can be used to remove the hair-like wear marks left by #1200 and #1500 sandpaper. Then use sticky felt and diamond paste for polishing, the order is 1μm (#14000) ~ 1/2μm (#60000) ~ 1/4μm (#100000). The polishing process with an accuracy requirement of 1μm or more (including 1μm) can be carried out in a clean polishing room in the mold processing workshop. For more precise polishing, an absolutely clean space is necessary. Dust, smoke, dandruff and saliva can all scrap the high-precision polished surface obtained after hours of work.

2.2 Problems to be paid attention to in mechanical polishing: The following points should be paid attention to when polishing with sandpaper:

(1) Polishing with sandpaper requires the use of soft wood sticks or bamboo sticks. When polishing round or spherical surfaces, the use of cork rods can better match the curvature of the round and spherical surfaces. The harder wood strips, like cherry wood, are more suitable for polishing flat surfaces. Trim the ends of the wooden strips so that they can be consistent with the surface shape of the steel parts, so as to avoid the sharp angles of the wooden strips (or bamboo strips) from contacting the surface of the wondercopper and causing deep scratches.

(2) When changing to a different type of sandpaper, the polishing direction should be changed from 45° to 90°, so that the streaks and shadows left by the previous type of sandpaper after polishing can be distinguished. Before changing to a different type of sandpaper, you must carefully wipe the polishing surface with a 100% pure cotton moistened with a cleaning solution such as alcohol, because a small grit left on the surface will destroy the 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 the 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 the surface. When polishing with each type of sandpaper, polishing should be done twice along two different directions, and each rotation between the two directions is 45°~90°.

What are the common methods of mold polishing?

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, the usual load is 100~200g/cm2, but it is difficult to maintain the accuracy of this load. To make this easier, you can make a thin and narrow handle on the wooden strip, such as adding a copper sheet; or cut a part of the bamboo strip to make it softer. This can help control the polishing pressure to ensure that the mold surface pressure is not too high.

(2) When using diamond grinding and polishing, not only the working surface is required to be clean, but the hands of the worker must also be carefully cleaned.

(3) Each polishing time should not be too long, the shorter the time, the better the effect. If the 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, the heat generated by the polishing wheel can easily cause “orange peel”.

(5) When the polishing process is stopped, it is very important to ensure that the surface of the workpiece is clean and carefully remove all abrasives and lubricants. Then a layer of mold anti-rust coating should be sprayed on the surface.

3. Factors affecting the quality of mold polishing

Since mechanical polishing is mainly done manually, polishing technology is still the main reason that affects polishing quality. In addition, it is also related to the mold material, the surface condition before polishing, and the heat treatment process. High-quality steel is a prerequisite for obtaining good polishing quality. If the surface hardness of the steel is uneven or there are differences in characteristics, polishing difficulties will often occur. Various inclusions and pores in the steel are not conducive to polishing.

3.1 The impact of different hardness on the polishing process The increase in hardness increases the difficulty of grinding, but the roughness after polishing decreases. As the hardness increases, the polishing time required to achieve a lower roughness increases accordingly. At the same time, the hardness increases, and the possibility of over-polishing decreases accordingly.

3.2 The influence of the surface condition of the workpiece on the polishing process During the crushing process of the cutting machining of the steel, the surface layer will be damaged due to heat, internal stress or other factors, and improper cutting parameters will affect the polishing effect. The surface after EDM is more difficult to grind than the surface after ordinary CNC Machining or heat treatment. Therefore, before the end of EDM, it should be trimmed with precision EDM, otherwise the surface will form a hardened thin layer. If the EDM fine-tuning rules are not selected properly, the depth of the heat-affected layer can reach up to 0.4mm.

The hardness of the hardened thin layer is higher than that of the substrate and must be removed. Therefore, it is best to add a rough grinding process to completely remove the damaged surface layer to form an evenly rough metal surface, which provides a good foundation for the polishing process.

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