Mechanical polishing is a polishing method that obtains a smooth surface by cutting and plastic deformation of the surface of the material to remove the convex parts after polishing. Generally, you use whetstone strips, wool wheels, sandpaper, etc., mainly manual operation. Using auxiliary tools such as turntables, ultra-precise polishing methods can be used for high surface quality requirements. The ultra-precise grinding polishing adopts a special grinding tool. In the grinding polishing liquid containing abrasives, it is pressed against the surface of the workpiece to be processed for high-speed rotation. With this technology, a surface roughness of Ra0.008 μm can be achieved, which is the highest among various polishing methods. This method is often used in optical lens molds.
1.1 Basic procedures for mechanical polishing
In order to obtain high-quality polishing results, the most important thing is to have high-quality polishing tools and accessories such as whetstone, sandpaper and diamond abrasive paste. The choice of polishing program 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 (1) Rough polishing The surface after milling, electric spark, grinding and other processes can be polished by rotating surface polishing machine or ultrasonic grinding machine with a rotation speed of 35 000 ～ 40 000 rpm. A common method is to remove the white electric spark layer by using a wheel with a diameter of 3mm and a diameter of WA # 400. This is followed by manual whetstone grinding, with strips of whetstone plus kerosene as a lubricant or coolant. The general usage order is # 180 ～ # 240 ～ # 320 ～ # 400 ～ # 600 ～ # 800 ～ # 1000. Many mold makers choose to start with # 400 to save time.
(2) Semi-fine polishing Semi-fine polishing mainly uses sandpaper and kerosene. The order of the sandpaper is: # 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 you use a polishing cloth wheel to mix diamond abrasive powder or abrasive paste for grinding, the usual grinding sequence is 9 μm (# 1800) to 6 μm (# 3000) to 3 μm (# 8000). The 9 μm diamond abrasive paste and polishing cloth wheel can be used to remove hairline marks from # 1200 and # 1500 sandpaper. Followed by polishing with sticky felt and diamond abrasive paste, the order is 1 μm (# 14000) to 1/2 μm (# 60000) to 1/4 μm (# 100000).
The polishing process with accuracy above 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 required. Dust, smoke, dandruff and drool are all possible high-precision polished surfaces that can be scrapped for hours.
1.2 Problems to pay attention to in mechanical polishing
Pay attention to the following points when polishing with sandpaper:
(1) Polishing with sandpaper requires the use of a soft wood or bamboo stick. When polishing round or spherical surfaces, the use of cork rods can better match the arc of the round and spherical surfaces. Harder bars, like cherry wood, are more suitable for polishing flat surfaces. Trim the ends of the wooden strip to keep it in line with the shape of the surface of the steel. This can prevent the sharp angle of the wooden strip (or bamboo strip) from contacting the surface of the steel and causing deep scratches.
(2) When changing to different types of sandpaper, the polishing direction should be changed from 45 ° to 90 °, so that the stripes and shadows left by the sandpaper of the previous type can be distinguished. Before changing the different types of sandpaper, the polishing surface must be carefully wiped with a cleaning solution such as 100% cotton moistened with alcohol, because a small grit left on the surface will destroy the entire polishing process. 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 (3) To avoid scratching and burning the surface of the workpiece, special care must be taken when polishing with # 1200 and # 1500 sandpaper. It is necessary to load a light load and polish the surface using a two-step polishing method. Each type of sandpaper should be polished twice in two different directions, and each rotation between 45 ° and 90 °.
1.3 Diamond grinding and polishing should pay attention to the following points:
(1) This polishing must be performed under light pressure as much as possible, especially when polishing pre-hardened steel parts and polishing with a fine grinding 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 bar, such as adding a copper sheet; or cut out a part of the bamboo bar 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 work surface needs to be clean, but the workers’ hands must also be cleaned carefully.
(3) Each polishing time should not be too long, the shorter the time, the better the effect. If the polishing process takes too long, it will cause “orange peel” and “pitting”.
(4) To obtain high-quality polishing results, polishing methods and tools that are prone to heat should be avoided. For example: polishing wheel polishing, 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, and then spray a layer of anti-rust coating on the surface.
Because 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 good polishing quality. If the surface hardness of the steel is uneven or the characteristics are different, polishing will often be difficult. Various inclusions and pores in steel are not conducive to polishing.
Chemical polishing is to make the surface of the material microscopically convex 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, 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 liquid. The surface roughness obtained by chemical polishing is generally several 10 μm.
The basic principle of electrolytic polishing is the same as chemical polishing, that is, the surface is smooth by selectively dissolving the tiny protruding parts on the surface of the material. Compared with chemical polishing, the effect of the cathode reaction can be eliminated, and the effect is better. The electrochemical polishing process is divided into two steps:
(1) Macro leveling The dissolved product diffuses into the electrolyte, the surface roughness of the material decreases, and Ra> 1 μm.
(2) Low light leveling Anodic polarization, surface brightness improved, Ra <1 μm.
The workpiece is placed in an abrasive suspension and placed in an ultrasonic field together, and the abrasive is ground and polished on the surface of the workpiece by the ultrasonic vibration. Ultrasonic machining has a small macro force and does not cause workpiece deformation, but tooling production and installation are difficult. 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 to dissolve the dissolved product on the surface of the workpiece, and the corrosion or electrolyte near the surface is uniform; the cavitation of ultrasonic waves in the liquid can also suppress the corrosion process and facilitate the surface brightening.
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. 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 workpiece surface at high speed. The medium is mainly made of special compounds (polymer-like substances) that flow through under low pressure and mixed with abrasives. The abrasives can be silicon carbide powder.
Magnetic grinding and polishing is the use of magnetic abrasives to form abrasive brushes under the action of a magnetic field to grind workpieces. This method has high processing efficiency, good quality, easy control of processing conditions, and good working conditions. With a suitable abrasive, the surface roughness can reach Ra0.1 μm.
所说 The polishing in 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 requires only a shiny surface. The standard for mirror processing is divided into four levels: AO = Ra0.008 μm, A1 = Ra0.016 μm, A3 = Ra0.032 μm, A4 = Ra0.063 μm, due to the methods of electrolytic polishing and fluid polishing, etc. It is difficult to precisely control the geometric accuracy of parts, and the surface quality of chemical polishing, ultrasonic polishing, magnetic polishing and other methods can not meet the requirements, so the mirror surface processing of precision molds is mainly mechanical polishing.
2.1 Influence of different hardness on polishing process
The higher the hardness, the more difficult it is to grind, but the roughness after polishing is reduced. As the hardness increases, the polishing time required to achieve a lower roughness increases accordingly. At the same time, the hardness is increased, and the possibility of excessive polishing is correspondingly reduced.
2.2 Influence of workpiece surface condition on polishing process
During the crushing process of steel cutting machinery, 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, precision EDM trimming should be used before the end of EDM, otherwise the surface will form a hardened thin layer. If the EDM finishing standard is not selected properly, the depth of the heat affected layer can be up to 0.4mm. The hardness of the hardened layer is higher than that of the matrix 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, which provides a good basis for polishing.
Shot blasting is the use of a high-speed rotating impeller to throw small steel shots or small iron shots to hit the surface of the part at high speed, so the oxide layer on the part surface can be removed. At the same time, the steel shot or iron shot hit the surface of the part at high speed, causing the lattice distortion of the part surface and increasing the surface hardness. It is a method for cleaning the surface of the part. Shot blasting is often used to clean the surface of the casting or strengthen the surface deal with.
General shot blasting is used for regular shapes, etc. Several shot heads are up and down and left and right together, high efficiency and low pollution.
In the repair and shipbuilding industry, shot blasting and sand blasting are commonly used. However, whether it is shot blasting or sand blasting, compressed air is used. Of course, it is not necessary to use a high-speed rotating impeller for shot blasting. Generally speaking, in the repair and shipbuilding industry, shot blasting (small steel shot) is mostly used for pretreatment of steel plates (rust removal before painting); sandblasting (ore sand used in repair and shipbuilding) is mostly used in forming ships or sections, The role is to remove old paint and rust from the steel plate and repaint. In the repair and shipbuilding industry, the main role of shot blasting and sand blasting is to increase the adhesion of steel plate coating paint.
In fact, the cleaning of castings is not just by shot blasting. For large pieces, sandblasting is generally performed first. That is, the riser of the casting is cut off and placed in the drum. The parts collide with each other in the drum, and most of the sand on the surface is first removed. Remove it before blasting or blasting.
The size of the shot peening balls is 1.5mm.
Studies have shown that, in terms of failure, tensile stress is much easier than compressive stress on the surface of metallic materials. When the surface is under compressive stress, the fatigue life of the material is greatly improved. Therefore, spraying parts that are prone to fatigue fracture such as shafts are usually sprayed. Pills form surface compressive stress and increase product life. In addition, metallic materials are very sensitive to tensile. This is why the tensile strength of the material is much lower than the compressive strength. This is also the general tensile strength (yield, resistance Pull) indicates the cause of material properties.
The working surface of the steel plate of our daily car is strengthened by shot peening, which can significantly improve the fatigue resistance of the material.
Shot blasting is to use an electric motor to drive the impeller body to rotate. By the action of centrifugal force, throw balls with a diameter of 0.2 to 3.0 (including cast shot, cut stainless steel shot, etc.) onto the surface of the workpiece, so that the surface of the workpiece reaches a certain degree of roughness. Make the workpiece beautiful, or change the welding tensile stress of the workpiece to compressive stress, and improve the service life of the workpiece. Almost used in most fields of machinery, shipbuilding, auto parts, aircraft parts, guns, tanks, surface bridges, steel structures, glass steel pipes, etc. Sand blasting (pellet) is to use compressed air as power to spray sand with a diameter of 40 to 120 mesh or about 0.1 to 2.0 on the surface of the workpiece, so that the workpiece achieves the same effect. Different pellets have different treatment effects. The key point is that shot peening can also play a strengthening role. Now domestic equipment has entered a misunderstanding, thinking that only shot blasting can achieve the purpose of strengthening. US and Japanese companies use shot peening for strengthening! Each has its own advantages. For workpieces such as gears, the shot blasting angle cannot be changed, and the initial speed can only be changed by frequency conversion. However, it has a large amount of processing and fast speed, and shot peening is just the opposite. The effect of shot blasting is not as good as shot peening.
Sandblasting is a method of cleaning the surface of parts by blowing out quartz sand at high speed using compressed air. The factory is also called sand blowing, which not only removes rust, but also can remove oil, which is very useful for painting. It is often used for rust removal on the surface of parts; it is used for the surface modification of small parts (the small wet sand blasting machine sold on the market is used for this purpose, the sand is usually corundum and the medium is water); in steel structures, the use of high-strength bolts for connection is a relatively advanced Method, because the high-strength connection uses the friction between the bonding surfaces to transmit force, so the quality of the bonding surface is very high. At this time, the bonding surface must be treated by sandblasting.
Sandblasting is used for complex shapes, easy to remove rust by hand, low efficiency, poor site environment and uneven rust removal.
General sand blasting machines have sand blasting guns of various specifications. As long as they are not particularly small, you can put the guns in and clean them.
Ancillary products for pressure vessels—The head uses sandblasting to remove oxide scale on the surface of the workpiece. The diameter of quartz sand is 1.5mm ～ 3.5mm.
One kind of processing is to use water as a carrier to drive silicon carbide to process parts. It is a type of sandblasting.
Both shot blasting and sand blasting can clean and decontaminate the workpiece. The purpose is to prepare for the next sequence, that is, to ensure the roughness requirements of the next process, and some to ensure the consistency of the surface. Strengthening effect, sand blasting is not obvious. Generally, shot blasting is small steel balls, and sand blasting is quartz sand. According to different requirements, the number of subheads.
1. Both shot blasting and sand blasting are surface treatments, but it is not to say that only shot castings are shot blasted.
2. The main function of sand blasting is to remove surface rust and scale, such as parts after heat treatment, and the functions and functions of shot blasting are more: not only rust removal, surface oxide scale removal, but also surface roughness improvement and component removal Machining burrs, eliminating internal stress of the part, reducing deformation of the part after heat treatment, improving the wear resistance and pressure capability of the part surface.
3. There are many processes for shot peening, such as: castings, forgings, surface parts after machining, surface parts after heat treatment, etc.
4. Sand blasting is mainly manual operation, while shot blasting is more automated and semi-automatic.
5. The steel shots and iron shots used for shot blasting are not actually shots in the true sense. To be precise, they are small steel wires or small steel rods, which only look like balls after a period of use. To put it bluntly, it is just river sand. It is no different from that used in construction, except that the sand blasting is sieved with less mud and particle size specifications. Of course, some industries are also different. For example, the shot blasting industry uses real steel shot blasting and metal blasting sand blasting (not river sand-quartz sand). Pills are generally spherical particles without angular edges. Such as wire cut pellets, etc. Sand refers to angular grains such as brown corundum, white corundum, river sand, etc.
6. Spray and throw
The blasting sprays the sand or shot on the surface of the material with compressed air as the power to achieve the removal and certain roughness.
Blasting is a method of centrifugal force generated when the pellet is rotated at high speed, which impacts the surface of the material to achieve removal and a certain degree of roughness.