The main functions of sand blasting are surface descaling, descaling, etc., such as heat-treated parts, and the effect and function of shot blasting are more: not only rust removal, but also surface roughness, surface roughness, part removal machining The burr eliminates the internal stress of the part, reduces the deformation of the part after the heat treatment, and improves the wear resistance and pressure capacity of the part.
Common polishing method
1.1 Mechanical polishing
Mechanical polishing is a polishing method that removes the polished convex portion by plastic deformation on the surface of the material to obtain a smooth surface. Generally, oil stone strips, wool wheels, sandpaper, etc. are used, which are mainly manual operations, and special parts such as the surface of the rotating body can be used. Using an auxiliary tool such as a turntable, the method of super-fine polishing can be used with high surface quality requirements. Ultra-fine grinding and polishing is a special-purpose grinding tool. In the polishing liquid containing abrasive, it is pressed against the machined surface to perform high-speed rotary motion. 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.
1.2 Chemical polishing
Chemical polishing is a process in which a portion of a material that is microscopically convex in a chemical medium is preferentially dissolved in a concave portion to obtain a smooth surface. The main advantage of this method is that it can polish a workpiece with complex shapes without complicated equipment, and can polish many workpieces at the same time with high efficiency. The core issue of chemical polishing is the formulation of the polishing fluid. The surface roughness obtained by chemical polishing is generally several 10 μm.
1.3 Electropolishing
The basic principle of electropolishing is the same as chemical polishing, which is to selectively dissolve the surface of the material to make the surface smooth. 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) The macro-leveling dissolved product diffuses into the electrolyte, and the surface roughness of the material decreases, Ra > 1 μ m.
(2) The low light leveling anodic polarization increases the surface brightness, Ra < 1 μ m.
1.4 Ultrasonic polishing
The workpiece is placed in an abrasive suspension and placed together in an ultrasonic field, and the abrasive is ground and polished on the surface of the workpiece by the action of ultrasonic waves. Ultrasonic machining has a small macroscopic force and does not cause deformation of the workpiece, but it is difficult to manufacture and install the 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 to dissociate the dissolved products on the surface of the workpiece, and the corrosion or electrolyte near the surface is uniform. The cavitation of the ultrasonic wave in the liquid can also inhibit the corrosion process and facilitate surface illuminating.
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 for polishing purposes. Common methods include: abrasive jet processing, liquid jet processing, hydrodynamic grinding, and the like. Hydrodynamic grinding is driven hydraulically to allow the liquid medium carrying the abrasive particles to flow back and forth across the surface of the workpiece at high speed. The medium is mainly made of a special compound (polymeric substance) which flows at a relatively low pressure and is doped with an abrasive, and the abrasive can be made of silicon carbide powder.
1.6 Magnetic Abrasive Polishing
Magnetic grinding and polishing is the use of magnetic abrasive to form an abrasive brush 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 a suitable abrasive, the surface roughness can reach Ra 0.1 μ m.
The polishing described 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. Not only does it have high requirements for polishing itself, but it also has a high standard 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 μ m , A1=Ra0.016 μ m , A3=Ra0.032 μ m , A4=Ra0.063 μ m, due to electropolishing, fluid polishing, etc. It is difficult to accurately 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 processing of precision molds is mainly based on mechanical polishing.
2.1 Basic procedures for mechanical polishing
For high-quality polishing, the most important thing is to have high-quality polishing tools and accessories such as oilstone, sandpaper and diamond paste. The choice of polishing procedure depends on the surface condition of the previous processing, such as machining, EDM, grinding and so on. The general process of mechanical polishing is as follows:
(1) The surface after rough polishing, electric sparking, grinding, etc. can be polished by a rotary surface polishing machine or an ultrasonic grinder with a rotational speed of 35 000 to 40 000 rpm. A common method is to remove the white spark layer by using a wheel with a diameter of Φ 3 mm and WA # 400. Then there is a manual oil stone grinding, strip of oil stone plus kerosene as a lubricant or coolant. The general order of use is #180 to #240 to #320 to #400 to #600 to #800 to #1000. Many moldmakers choose to start with #400 in order to save time.
(2) Semi-finishing semi-finishing mainly uses sandpaper and kerosene. The number of sandpaper is: #400 ～#600 ～#800 ～#1000 ～#1200 ～#1500 。. In fact, #1500 sandpaper is only used for hardened die steel (above 52HRC), not for pre-hardened steel, as this may cause burns on the surface of pre-hardened steel.
(3) Fine polishing fine polishing mainly uses diamond abrasive paste. If the polishing powder is mixed with a diamond abrasive powder or a polishing paste, the usual polishing order is 9 μm (#1800) to 6 μm (#3000) to 3 μm (#8000). The 9 μm diamond paste and polishing cloth wheel can be used to remove the hair marks from the #1200 and #1500 sandpaper. The polishing is then carried out using a felt and a diamond paste in the order of 1 μm (#14000) to 1/2 μm (#60000) to 1/4 μm (#100000).
Accuracy requires a polishing process of more than 1 μm (including 1 μm) in a clean polishing chamber in the mold shop. For more precise polishing, an absolutely clean space is required. Dust, smoke, dandruff and saliva can all leave high-precision polished surfaces that are obtained after hours of work.
2.2 Problems to be paid attention to in mechanical polishing
Polishing with sandpaper should pay attention to the following points:
(1) Polishing with sandpaper requires the use of soft wood sticks or bamboo sticks. When polishing round or spherical surfaces, use cork sticks to better match the curvature of the round and spherical surfaces. The harder wood strips are like cherry wood, which is more suitable for the polishing of flat surfaces. The end of the trim strip is adjusted to match the shape of the steel surface so that the acute angle of the strip (or bamboo strip) is prevented 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 stripe shadow left by the polishing of the former type of sandpaper can be distinguished. Before changing the different types of sandpaper, the polished surface must be carefully wiped with 100% pure cotton and a cleaning solution such as alcohol, because a small gravel left on the surface will destroy the entire polishing work. This cleaning process is equally important when polishing from sandpaper to diamond polishing. All pellets and kerosene must be completely cleaned before polishing continues.
(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 load 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 between 45 ° and 90 °.
Diamond Grinding and Polishing Precautions (1) This 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 strip, such as adding a piece of copper; or cut a part of the strip to make it softer. This helps 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 worker must be carefully cleaned.
(3) The polishing time should not be too long, the shorter the time, the better the effect. If the polishing process is carried out 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: 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. 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 the quality of polishing. In addition, it is related to the mold material, the surface condition before polishing, the heat treatment process, and the like. High-quality steel is a prerequisite for good polishing quality. If the surface hardness 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.
3.1 Influence of different hardness on polishing process
The increase in hardness increases the difficulty of grinding, but the roughness after polishing decreases. Due to the increased hardness, the polishing time required to achieve lower roughness is correspondingly increased. At the same time, the hardness is increased and the possibility of over-polishing is correspondingly reduced.
3.2 Influence of surface condition of the workpiece on the polishing process
During the crushing process of steel machining, the surface layer may be damaged by heat, internal stress or other factors, and improper cutting parameters may affect the polishing effect. The surface after EDM is more difficult to grind than the surface after ordinary machining or heat treatment. Therefore, the fine gauge quasi-electric spark should be used before the end of EDM, otherwise the surface will form a hardened thin layer. If the EDM is not properly selected, the depth of the heat affected layer can be up to 0.4mm. The hardness of the hardened 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.
The difference between shot blasting and sand blasting Shot blasting is the use of a high-speed rotating impeller to throw a small steel shot or a small iron shot to the surface of the part at high speed, so the oxide layer on the surface of the part can be removed. At the same time, the steel shot or the iron shot hits the surface of the part at high speed, causing the lattice distortion of the surface of the part to make the surface hardness increase. It is a method for cleaning the surface of the part. The shot blasting is often used to clean the surface of the casting or to strengthen the surface of the part. deal with.
Generally, shot blasting is used for regular shapes, etc. Several heads are placed up and down together, with high efficiency and low pollution.
Repair, shipbuilding, shot blasting, sand blasting are commonly used. But whether it is shot blasting or sand blasting, it is in the form of compressed air. Of course, it is not the impeller that does not use high-speed rotation. In the repair and shipbuilding industry, shot blasting (small steel shot) is often used in steel plate pretreatment (pre-painting rust removal); sand blasting (repairing and shipbuilding is used in ore) is often used in forming ships or segments. The role is to remove the old paint and rust on 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 coating paint.
In fact, the cleaning of casting parts is not only the use of shot blasting. For large parts, the drum is first cleaned. The riser of the casting is cut off and placed in the drum. The parts collide with each other in the drum, and the sand of the surface is mostly Remove it and then perform shot blasting or shot peening.
The size of the shot blasting ball is 1.5mm.
Studies have shown that in terms of damage, the tensile stress on the surface of the metal material is much easier than the compressive stress. When the surface is under compressive stress, the fatigue life of the material is greatly improved. Therefore, it is usually sprayed on components such as shafts that are susceptible to fatigue fracture. The pellet forms surface compressive stress and improves product life. In addition, the metal metal material is very sensitive to stretching, which is why the tensile strength of the material is much lower than the compressive strength. This is also the tensile strength of the metal material (yield, anti-wear). Pull) indicates the reason for the material properties.
The working surface of the steel plate of the car we ride every day is reinforced with shot peening, which can significantly improve the fatigue strength of the material.
Shot blasting is to use the motor to drive the rotation of the impeller body. By the action of centrifugal force, the ball with a diameter of 0.2-3.0 (with casting pill\cutting pill\stainless steel] target=_blank>stainless steel pill, etc.) is thrown to the surface of the workpiece to make the workpiece The surface reaches a certain degree of roughness, which makes the workpiece beautiful, or changes the welding tensile stress of the workpiece to compressive stress, and improves the service life of the workpiece. It is used in most fields of machinery, repairing ships\auto parts\aircraft parts \ Guns tank surface \ bridge \ steel structure \ glass \ steel \ pipe \ etc. Sandblasting (pill) is the use of compressed air as the power to spray the diameter of 40 ~ 120 mesh or 0.1 ~ 2.0 pill to the workpiece The surface makes the workpiece achieve the same effect. The size of the pellets is different, and the treatment effect is different. The key point is that the shot peening can also play a strengthening role. Now the domestic equipment has entered a misunderstanding, and it is considered that only shot blasting In order to achieve the purpose of reinforcement, the US and Japanese companies are used to strengthen the blasting and use! Each has its own advantages. For example, a workpiece such as a gear, the angle of the shot peening can not be changed, can only be used Frequency change the initial velocity, but it handles large, fast, and shot peening is just to the contrary, the effect of blasting on the shot no good effect.
Sand blasting is a method of using compressed air to blow out quartz sand at high speed to clean the surface of the part. The factory is also called sand blowing, not only to rust, but also to remove oil, which is very useful for painting. Commonly used for surface derusting of parts; surface modification of parts (small wet sand blasting machine sold in the market is this use, sand is usually corundum, medium is water); in steel structure, the application of high-strength bolts is a relatively advanced The method is because the high-strength joint uses the friction between the joint surfaces to transmit the force, so the quality of the joint surface is very high, and the joint surface must be treated by sand blasting.
Sandblasting is used for complex shapes, easy to remove by hand, low efficiency, poor on-site environment, and uneven rust removal.
The general sand blasting machine has various types of sand blasting guns. As long as it is not a particularly small box, the gun can be put in and cleaned.
The supporting product of the pressure vessel—the head is sandblasted to remove the scale on the surface of the workpiece. The diameter of the quartz sand is 1.5mm~3.5mm.
One kind of processing is to use water as a carrier to drive the diamond to machine parts, which is a kind of sand blasting.
Both shot blasting and sandblasting can clean and decontaminate the workpiece. The purpose is to prepare for the next step, that is, to ensure the roughness requirements of the next process, and also for the consistency of the surface, the shot peening has on the workpiece. For reinforcement, sand blasting is not obvious. Generally, shot peening is a small steel ball, and sand blasting is quartz sand. According to different requirements, the number of sub-items.
Precision casting is used almost every day with sand blasting and shot blasting.
Supplement 1. Shot blasting and sand blasting are surface treatments, but it is not that only castings are shot.
2. The main functions of sand blasting are surface descaling, descaling, etc., such as heat-treated parts, and the effect and function of shot blasting are more: not only rust removal, but also surface roughness, surface roughness, removal of parts Machining burrs, eliminating internal stress in parts, reducing deformation of parts after heat treatment, improving wear resistance and pressure capacity of parts.
3. There are many processes for shot peening, such as castings, forgings, parts of parts after machining, and parts after heat treatment of parts.
4. Sandblasting is mainly manual operation, while shot blasting is automated and semi-automatic.
5. Steel shots and iron shots used in shot blasting are not really pills in the true sense. It is precisely a small steel wire or a small steel stick. It only looks like a meatball after a period of use. The so-called sandblasted sand To put it bluntly, it is just river sand. It is no different from the construction. It is only sieved with sandblasting, with less mud and particle size specifications. Of course, some industries are also different. For example, the shipbuilding industry uses real steel shots for shot blasting and metal ore (not river sand-quartz sand) for sandblasting. Pills are generally spherical and have no angular particles. Such as wire cutting pills, etc.; sand refers to angular sand, such as brown corundum, white corundum, river sand and so on.
6. Spray and throw
The spray is sprayed with compressed air as the power to spray the sand or pellet onto the surface of the material to achieve a clear and a certain roughness.
The throwing is a method of centrifugal force generated when the pellet is rotated at a high speed, impacting the surface of the material to achieve removal and a certain roughness.
Http://news.chinawj.com.cn Editor: (Hardware Business Network Information Center) http://news.chinawj.com.cn
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