Polishing is a very important process in the mold making process, and it is also a finishing work. With the widespread use of plastic products, the appearance quality requirements of plastic products are becoming higher and higher, so the surface of the plastic mold cavity is polished. The quality should also be improved accordingly, especially the molds with mirror surface and high gloss and high gloss surface have higher requirements on the mold surface roughness, so the requirements on polishing are also higher. Polishing not only increases the aesthetics of the workpiece, but also improves the corrosion resistance and wear resistance of the surface of the material. It can also facilitate the subsequent injection processing, such as making plastic products easy to demold and reducing the production injection cycle. At present, the commonly used polishing methods are as follows:
(1) Mechanical polishing
Mechanical polishing is a polishing method that obtains a smooth surface by cutting and plastic deformation of the material surface to remove the polished convex parts. Generally, whetstone strips, wool wheels, sandpaper, etc. are used, which are mainly manual operations. Special parts such as the surface of the rotating body can be Use auxiliary tools such as turntables, and those with high surface quality can use the method of super-finishing polishing. Super-precision grinding and polishing is the use of special grinding tools, in the grinding and polishing liquid containing abrasives, tightly pressed on the surface of the workpiece to be processed for high-speed rotation. The surface roughness of Ra0.008μm can be achieved by this technology, which is the highest among various polishing methods. Optical lens molds often use this method.
(1) Basic procedure 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 oilstone, sandpaper and diamond paste. The choice of polishing program depends on the surface conditions after the pre-processing, such as mechanical processing, EDM, grinding and so on. The general process of mechanical polishing is as follows:
① The surface after rough polishing, milling, electric spark, grinding and other processes can be selected for polishing with a rotating surface polishing machine or an ultrasonic grinding machine with a speed of 35 000-40 000 rpm. A common method is to remove the white electric spark layer by using wheels with a diameter of Φ3mm and WA #400. Then there is manual whetstone grinding, with strip whetstone plus kerosene as a lubricant or coolant. The general order of use is #180 ~ #240 ~ #320 ~ #400 ~ #600 ~ #800 ~ #1000. Many mold makers choose to start with #400 in order to save time.
② Semi-precision polishing Semi-precision polishing mainly uses sandpaper and kerosene. The numbers of sandpaper are: #400 ~ #600 ~ #800 ~ #1000 ~ #1200 ~ #1500. In fact, #1500 sandpaper only uses mold steel suitable for hardening (above 52HRC), but not for pre-hardened steel, because it may cause burns on the surface of pre-hardened steel parts.
③Precision polishing mainly uses diamond polishing paste. If the polishing cloth wheel is used to mix diamond grinding powder or grinding paste, the usual grinding sequence 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 to polish, in the order of 1μm(#14000)~1/2μm(#60000)~1/4μm(#100000).
The polishing process with an accuracy requirement of more than 1 μm (including 1 μm) can be performed 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 foam can all be scrapped from the highly precise polished surface obtained after several hours of work.
(2) Skills in mechanical polishing
Ⅰ The following points should be noted when polishing with sandpaper:
① Polishing with sandpaper requires the use of soft wooden or bamboo sticks. When polishing round or spherical surfaces, using 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. The end of the wooden strip is trimmed to keep it consistent with the shape of the surface of the steel piece, so that the acute angle of the wooden strip (or bamboo strip) can contact the surface of the steel piece and cause deep scratches.
②When changing to different types of sandpaper, the polishing direction should be changed from 45° to 90°, so that the streak shadow left by the previous model of sandpaper after polishing can be distinguished. Before changing to different types of sandpaper, the polishing surface must be carefully wiped with 100% cotton and a cleaning solution such as alcohol, because a small amount of sand left on the surface will destroy the entire polishing work. When changing from polishing with sandpaper to polishing with diamond paste, this cleaning process is equally important. Before polishing continues, 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, it should be polished twice in two different directions, each time between the two directions of 45 ° ~ 90 °.
Ⅱ Diamond grinding and polishing should pay attention to the following points:
① This kind of polishing must be carried out under light 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, 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 more soft. 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 also the hands of the workers must be carefully cleaned.
③ The polishing time should not be too long each time. 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 that are easy to generate heat should be avoided. For example: polishing wheel polishing, the heat generated by the polishing wheel can easily cause "orange peel".
⑤When 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.
Since mechanical polishing is mainly done manually, polishing technology is still the main reason for 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 hardness of the surface of the steel is uneven or the characteristics are different, it will often cause polishing difficulties. Various inclusions and pores in the steel are not conducive to polishing.
(3) The impact of different hardness on the polishing process
The increased 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.
(4) Influence of workpiece surface condition on polishing process
During the crushing process of steel machining, the surface layer will be damaged by 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 mechanical processing or heat treatment. Therefore, before the end of EDM, precision quasi-EDM should be used for trimming, otherwise the surface will form a hardened thin layer. If the electric spark finishing standards are not properly selected, the depth of the heat-affected layer can be up to 0.4 mm. The hardness of the hardened thin layer is higher than the hardness of the substrate and must be removed. Therefore, it is better to add a rough grinding process to completely remove the damaged surface layer to form an average rough metal surface, which provides a good basis for polishing.
(2) Chemical polishing
Chemical polishing is to make the microscopic convex part of the material in the chemical medium preferentially dissolve 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, 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 solution. 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 chemical polishing, that is, by selectively dissolving tiny protrusions on the surface of the material, the surface is smooth. 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 macro-leveling and dissolving products diffuse into the electrolyte, and the geometric roughness of the material surface decreases, Ra>1μm.
(2) Low-gloss flat anode polarization, surface brightness increases, Ra<1μm.
(4) Ultrasonic polishing
Put the workpiece in the abrasive suspension and put it in the ultrasonic field together, relying on the oscillation of the ultrasonic wave to make the abrasive grinding and polishing on the surface of the workpiece. Ultrasonic processing has a small macro force and will not cause deformation of the workpiece, but tooling is difficult to make and install. Ultrasonic processing can be combined with chemical or electrochemical methods. On the basis of solution corrosion and electrolysis, ultrasonic vibration is added to stir the solution to dissolve the dissolved products on the surface of the workpiece, 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 help the surface brighten.
(5) Fluid polishing
Fluid polishing relies on the high-speed flowing liquid and the abrasive particles it carries to wash the surface of the workpiece to achieve the purpose of polishing. Common 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) that flows through at a lower pressure and is mixed with an abrasive. The abrasive can be silicon carbide powder.
(6) Magnetic grinding and polishing
Magnetic grinding and polishing is the use of 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 appropriate abrasives, the surface roughness can reach Ra0.1μm.
The polishing mentioned in the processing of plastic molds 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 for polishing itself but also has high standards for surface smoothness, smoothness and geometric accuracy. Surface polishing generally only requires a bright surface. The standard of mirror 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, etc. However, the surface quality of chemical polishing, ultrasonic polishing, magnetic abrasive polishing and other methods cannot meet the requirements, so the mirror processing of precision molds is still mainly mechanical polishing.