Process and skills of mold polishing

 Polishing is a very important process in the mold making process, and it is also the final work. With the increasing use of plastic products, the requirements for the appearance quality of plastic products are getting higher and higher. Therefore, the surface polishing of plastic mold cavities The quality should also be improved accordingly, especially the molds with mirror surface and high-gloss high-gloss surface have higher requirements for the surface roughness of the mold, so the requirements for polishing are also higher. Polishing not only increases the beauty of the workpiece, but also improves the corrosion resistance and wear resistance of the material surface, and can also facilitate subsequent injection molding processing, such as making plastic products easy to demould and reducing production injection molding cycles.

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

(1) Mechanical polishing   Mechanical polishing is a polishing method that removes the polished convex part by cutting and plastic deformation of the surface of the material to obtain a smooth surface. Generally, oil stone strips, wool wheels, sandpaper, etc. are used, and manual operations are mainly used. Auxiliary tools such as turntables can be used for the surface of parts such as rotary bodies, and ultra-fine grinding and polishing methods can be used for high surface quality requirements. Ultra-fine polishing uses a special abrasive tool, which is pressed against the surface of the workpiece to be processed in a polishing liquid containing abrasives 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. Optical lens molds often use this method.

⑴Basic procedures of mechanical polishing  To obtain high-quality polishing effects, the most important thing is to have high-quality polishing tools and accessories such as oilstone, sandpaper and diamond abrasive paste. The choice of polishing procedure depends on the surface condition after pre-processing, such as machining, EDM, grinding and so on.

The general process of mechanical polishing is as follows: ① After rough polishing, the surface after milling, EDM, grinding and other processes can be polished by a rotary surface polishing machine or an ultrasonic grinding machine with a rotation speed of 35 000-40 000 rpm. The commonly used method is to use a wheel with a diameter of Φ3mm and WA # 400 to remove the white spark layer. Then manual whetstone grinding, strip whetstone plus kerosene as lubricant or coolant. The general order of use is #180 ~ #240 ~ #320 ~ #400 ~ #600 ~ #800 ~ #1000. Many moldmakers choose to start with #400 to save time.

②Semi-fine polishing and semi-fine polishing mainly use sandpaper and kerosene. The numbers of sandpaper are: #400 ~ #600 ~ #800 ~ #1000 ~ #1200 ~ #1500. In fact, #1500 sandpaper is only suitable for hardened mold steel (above 52HRC), not for 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 grinding powder or grinding paste for grinding, the usual grinding sequence is 9μm (#1800) ~ 6μm (#3000) ~ 3μm (#8000). 9μm diamond 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 abrasive paste for polishing, the order is 1μm (#14000) ~ 1/2μm (#60000) ~ 1/4μm (#100000).  

The polishing process whose precision is required to be above 1 μm (including 1 μm) can be carried out in a clean polishing room in the mold processing workshop. For more precise polishing, an absolutely clean space is required. Dust, smoke, dandruff and drool can all ruin a highly polished finish after hours of work.

⑵Skills in mechanical polishing

Ⅰ The following points should be paid attention to when polishing with sandpaper:

① Polishing with sandpaper requires the use of soft wooden sticks or bamboo sticks. When polishing a round or spherical surface, using a cork stick can better match the curvature of the round and spherical surfaces. Harder woods, like cherry, are more suitable for polishing flat surfaces. Trim the end of the wooden strip so that it can keep in line with the surface shape of the steel part, so as to avoid deep scratches caused by the sharp angle of the wooden strip (or bamboo strip) touching the surface of the steel part.

② When using different types of sandpaper, the polishing direction should be changed by 45°~90°, so that the stripe shadow left by the former type of sandpaper after polishing can be distinguished. Before changing to a different type of sandpaper, the polishing surface must be carefully wiped with 100% pure cotton dipped in alcohol or other cleaning solution, because a small grain of sand left on the surface will destroy the entire subsequent polishing work. This cleaning process is equally important when switching from sandpaper polishing to diamond paste polishing. All particles and kerosene must be completely removed before polishing can proceed.

③ In order to avoid scratching and burning the surface of the workpiece, special care must be taken when polishing with #1200 and #1500 sandpaper. It is therefore necessary to apply a light load and polish the surface using a two-step polishing method. When polishing with each type of sandpaper, it should be polished twice in two different directions, with each rotation of 45°~90° between the two directions.

Ⅱ Diamond grinding and polishing should pay attention to the following points:

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

②When using diamond grinding and polishing, not only the working surface is required to be clean, but also the workers' hands must 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 carried out for too long it will cause "orange peel" and "pitting".

④ In order to obtain a high-quality polishing effect, polishing methods and tools that are prone to heat should be avoided. For example: buffing wheel polishing, the heat generated by the buffing wheel can easily cause "orange peel".

When the polishing process stops, it is very important to keep the workpiece surface clean and carefully remove all abrasives and lubricants, followed by spraying a mold anti-rust coating on the surface.   Since mechanical polishing is mainly done manually, the polishing technology is still the main reason affecting the 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 good polishing quality. If the surface hardness of the steel is uneven or there are differences in properties, polishing difficulties will often occur. Various inclusions and pores in steel are not conducive to polishing.

(3) The influence of different hardness on the polishing process The increase of 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 excessive polishing decreases accordingly.

⑷The effect of the surface condition of the workpiece on the polishing process. During the crushing process of steel in cutting machining, the surface layer will be damaged due to heat, internal stress or other factors. Improper cutting parameters will affect the polishing effect. The surface after EDM is more difficult to grind than the surface after ordinary machining or heat treatment. Therefore, fine gauge EDM should be used before the end of EDM, otherwise the surface will form a hardened thin layer. If the EDM specification is not properly selected, the depth of the heat-affected layer can reach a maximum of 0.4mm. The hardened thin layer is harder than 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 average rough metal surface and (2) Chemical polishing   Chemical polishing is to make the microscopic convex part of the surface of the material dissolve preferentially compared with the concave part in the chemical medium, so as to obtain a smooth surface. The 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. The core problem of chemical polishing is the preparation of polishing fluid. The surface roughness obtained by chemical polishing is generally several 10 μm.

(3) Electrolytic polishing   The basic principle of electrolytic polishing is the same as that of chemical polishing, that is, to make the surface smooth by selectively dissolving the tiny protrusions on the surface of the material. Compared with chemical polishing, it can eliminate the influence of cathode reaction, and the effect is better.

The electrochemical polishing process is divided into two steps:

⑴ The macroscopically leveled dissolved product diffuses into the electrolyte, and the geometric roughness of the surface of the material decreases, Ra>1μm.

⑵ Twilight flat anodic polarization, surface brightness is improved, Ra<1μm.

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(iv) Ultrasonic polishing &#160; Put the workpiece into the abrasive suspension and put them together in the ultrasonic field, relying on the oscillation of the ultrasonic wave, the abrasive is ground and polished 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 machining 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 waves in the liquid can also inhibit the corrosion process and facilitate surface brightening.

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(5) Fluid polishing &#160; Fluid polishing relies on the high-speed flowing liquid and the abrasive particles carried by it to scour the surface of the workpiece to achieve the purpose of polishing. Commonly used methods are: abrasive jet processing, liquid jet processing, hydrodynamic grinding, etc. Hydrodynamic grinding is driven by hydraulic pressure, so that the liquid medium carrying abrasive particles flows back and forth across the surface of the workpiece at high speed. The medium is mainly made of a special compound (polymer-like substance) with good flowability under relatively low pressure and mixed with abrasives. The abrasives can be silicon carbide powder.

(vi) Magnetic grinding and polishing &#160; Magnetic grinding and polishing uses 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. With suitable abrasives, the surface roughness can reach Ra0.1μm. &#160;

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The polishing mentioned in plastic mold processing is very different from the surface polishing required in other industries. Strictly speaking, the polishing of molds should be called mirror processing. It not only has high requirements on the polishing itself but also has high standards on surface flatness, smoothness and geometric accuracy. Surface polishing is generally only required to obtain 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 accurately control the geometric accuracy of parts due to electrolytic polishing, fluid polishing and other methods , and the surface quality of chemical polishing, ultrasonic polishing, magnetic grinding and polishing methods can not meet the requirements, so the mirror processing of precision molds is still dominated by mechanical polishing. provide a good foundation for polishing.