Mold polishing process, learn

Mold polishing process, learn

Polishing can not only increase the beauty of the workpiece, but also improve the corrosion resistance and wear resistance of the material surface. It can also make the mold have other advantages, such as easy demoulding of plastic products and reducing the production injection cycle. Therefore, polishing is a very important process in the process of plastic mold making.

1. Polishing method

At present, the common polishing methods are as follows:

1.1 mechanical polishing

Mechanical polishing is a polishing method to get a smooth surface by cutting and plastic deformation of the material surface to remove the convex part after polishing. It generally uses oilstone strip, wool wheel, sandpaper, etc. it is mainly manual operation. For special parts such as revolving body surface, auxiliary tools such as turntable can be used. For those with high surface quality requirements, ultra precision polishing method can be used. Ultra precision lapping and polishing is a kind of special abrasive tool. In the lapping and polishing fluid containing abrasive, it is pressed tightly on the machined surface of the workpiece to rotate at high speed. Ra0.008 can be achieved by using this technology μ The surface roughness is the highest among all kinds of polishing methods. This method is often used in optical lens mold.

1.2 chemical polishing

Chemical polishing is to make the micro protruding part of the material surface dissolve preferentially than 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 need complex equipment, can polish complex shape workpiece, can polish many workpieces at the same time, high efficiency. The key problem of chemical polishing is the preparation of polishing solution. The surface roughness obtained by chemical polishing is generally 10 μ m。

1.3 electropolishing

The basic principle of electropolishing 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, the effect of cathode reaction can be eliminated and the effect is better.

The electrochemical polishing process is divided into two steps

(1) The results show that the dissolution product diffuses into the electrolyte, the surface roughness decreases, and RA > 1 μ m。

(2) The surface brightness is improved and RA < 1 μ m。

1.4 ultrasonic polishing

The workpiece is put into the abrasive suspension and placed together in the ultrasonic field, and the abrasive is ground and polished on the workpiece surface by the ultrasonic oscillation. The macro force of ultrasonic machining is small, which will not cause the deformation of the workpiece, but it is difficult to make and install the 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 to separate the dissolved products from the surface of the workpiece, and the corrosion or electrolyte near the surface is uniform; The cavitation effect of ultrasonic in liquid can also inhibit the corrosion process, which is conducive to the surface lighting.

1.5 fluid polishing

Fluid polishing relies on the high-speed flow of liquid and abrasive particles to wash the workpiece surface to achieve the purpose of polishing. The common methods are abrasive jet machining, liquid jet machining, hydrodynamic grinding and so on. Hydrodynamic grinding is driven by hydraulic pressure, which makes the liquid medium with abrasive particles flow through the workpiece surface at high speed. The medium is mainly made of special compound (polymer like substance) with good flow property under low pressure and mixed with abrasive. The abrasive can be silicon carbide powder.

1.6 magnetic abrasive polishing

Magnetic abrasive polishing is to use magnetic abrasive to form abrasive brush under the action of magnetic field to grind workpiece. This method has the advantages of high efficiency, good quality, easy control of processing conditions and good working conditions. The surface roughness can reach ra0.1 with suitable abrasives μ m。


2. Basic methods of mechanical polishing

Polishing in plastic mold processing is very different from surface polishing required by other industries. Strictly speaking, mold polishing 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 requires only a bright surface.

The standard of mirror finishing is divided into four levels: Ao = ra0.008 μ m,A1=Ra0.016 μ m,A3=Ra0.032 μ m,A4=Ra0.063 μ m. Because it is difficult to accurately control the geometric accuracy of parts by electropolishing, fluid polishing and other methods, and the surface quality of chemical polishing, ultrasonic polishing, magnetic polishing and other methods can not meet the requirements, so the mirror machining of precision mold is mainly mechanical polishing.

2.1 basic procedure of mechanical polishing

In order to obtain high-quality polishing effect, the most important thing is to have high-quality polishing tools and accessories such as oilstone, sandpaper and diamond grinding paste. The choice of polishing procedure depends on the surface condition after the previous processing, such as machining, EDM, grinding and so on.

The general process of mechanical polishing is as follows:

(1) After milling, EDM, grinding and other processes, the surface of rough polishing can be polished by rotary surface polishing machine or ultrasonic grinding machine with rotating speed of 35000-40000 rpm. The common method is to use diameter Φ 3 mm, WA # 400 wheel to remove the white spark layer. Then it is hand-made whetstone grinding, strip-shaped whetstone with kerosene as lubricant or coolant. The general order of use is # 180 ~ # 240 ~ # 320 ~ # 400 ~ # 600 ~ # 800 ~ # 1000. In order to save time, many mold manufacturers choose to start from "400".

(2) Sandpaper and kerosene are mainly used for semi polishing and semi polishing. The numbers of sandpaper are: 400 ~ 600 ~ 800 ~ 1000 ~ 1200 ~ 1500. In fact, # 1500 sandpaper is only suitable for hardened die steel (above 52hrc), but not for pre hardened steel, because it may cause surface burn of pre hardened steel parts.

(3) Diamond grinding paste is mainly used in fine polishing. If the polishing cloth wheel is mixed with diamond grinding powder or grinding paste for grinding, the usual grinding sequence is 9 μ m(#1800)~ 6 μ m(#3000)~3 μ m(#8000)。 nine μ M diamond grinding paste and polishing cloth wheel can be used to remove hair like wear marks left by # 1200 and # 1500 sandpaper. Then, the polishing was carried out with felt and diamond grinding paste in the order of 1 μ m(#14000)~ 1/2 μ m(#60000)~1/4 μ m(#100000)。 The accuracy should be within 1 μ M (including 1 μ m) The polishing process can be carried out in a clean polishing room in the mold processing workshop. If more precise polishing is carried out, an absolutely clean space is necessary. Dust, smoke, dandruff and saliva are all likely to be discarded after several hours of work to obtain a high-precision polished surface.

2.2 problems needing attention in mechanical polishing

(1) Polishing with sandpaper requires the use of soft wood or bamboo sticks. When polishing round surface or spherical surface, using cork stick can better match the radian of round surface and spherical surface. The harder strips, like cherry, are more suitable for polishing flat surfaces. Trim the end of the wood strip to make it consistent with the surface shape of the steel part, so as to avoid the deep scratch caused by the sharp angle of the wood strip (or bamboo strip) contacting the surface of the steel part.

(2) When using different types of sandpaper, the polishing direction should be changed 45 °~ ninety °, In this way, the fringe shadow left by the previous model of sandpaper polishing can be distinguished. Before changing different types of sandpaper, the polished surface must be carefully wiped with a cleaning solution such as 100% pure cotton dipped in alcohol, because a small grit left on the surface will destroy the next polishing work. This cleaning process is also important when changing from sandpaper polishing to diamond polishing. All particles and kerosene must be completely cleaned before polishing continues.

(3) In order to avoid scratching and burning the workpiece surface, special care must be taken when polishing with # 1200 and # 1500 sandpaper. So it is necessary to load a light load and use two-step polishing method to polish the surface. When polishing with each type of sandpaper, polishing should be carried out twice in two different directions, with 45 turns each time between the two directions °~ ninety °。 Diamond grinding and polishing should pay attention to the following points:

(1) This kind of polishing must be carried out under light pressure as far as possible, especially when polishing pre hardened steel parts and polishing with fine grinding paste. When polishing with # 8000 grinding paste, the common load is 100 ~ 200g / cm2, but it is difficult to maintain the accuracy of this load. To make it easier to do this, you can make a thin and narrow handle on the wood bar, such as adding a piece of copper; Or cut part of the bamboo to make it softer. This helps to 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 must be clean, but also the hands of workers must be cleaned carefully.

(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 effect, polishing methods and tools which are easy to generate heat should be avoided. For example: polishing wheel polishing, polishing wheel heat will easily cause "orange peel".

(5) When the polishing process stops, it is very important to ensure that the surface of the workpiece is clean and all abrasives and lubricants are carefully removed, and then a layer of mold antirust coating should be sprayed on the surface.

3. Factors affecting the polishing quality of mould

As mechanical polishing is mainly done by hand, polishing technology is still the main reason affecting the polishing quality. In addition, it is also related to die material, surface condition before polishing and heat treatment process. High quality steel is the prerequisite to obtain good polishing quality. If the surface hardness of steel is uneven or there are differences in characteristics, polishing is often difficult. All kinds of inclusions and pores in steel are not conducive to polishing.

3.1 the influence of different hardness on the polishing process. The hardness increases the grinding difficulty, but the roughness after polishing decreases. Due to the increase of hardness, the polishing time required to achieve lower roughness increases correspondingly. At the same time, with the increase of hardness, the possibility of over polishing is reduced.

3.2 influence of workpiece surface condition on polishing process in the process of cutting and mechanical processing, the surface of steel 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 that after ordinary machining or heat treatment. Therefore, before the end of EDM, precision EDM should be used, otherwise the surface will form a hardened thin layer. The maximum depth of heat affected layer can reach 0.4mm if the EDM finishing gauge is not properly selected.

The hardness of the hardened layer is higher than that of the substrate and must be removed. Therefore, it is better to add a rough grinding process to thoroughly remove the damaged surface layer and form an average rough metal surface to provide a good foundation for polishing.

  报错 笔记

News : Prve


Ningbo Ruiyi Electronic Technology Co., Ltd. is located in Henghe Town, Cixi City, Zhejiang Province.

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