In the mold manufacturing process, the molding part of the mold often needs surface polishing. Mastering polishing technology can improve the quality and service life of molds, and then improve the quality of products.
1. Mould polishing method and working principle
Mould polishing usually uses oilstone strips, wool wheels, sandpaper, etc. , plastic deformation occurs on the surface of the material, and the convex part on the surface of the workpiece is removed to obtain a smooth surface, which is generally mainly manual operation. Ultra-precision grinding and polishing can be used for those with high requirements on surface quality. Ultra-precision grinding and polishing is that in the grinding polishing solution containing abrasives, a special abrasive tool is used to press tightly on the machined surface of the workpiece and rotate at high speed. The surface roughness of Ra0.008μm can be obtained by polishing.
2. Tools and specifications commonly used for mold polishing
Tools commonly used for mold polishing include: sandpaper, whetstone, felt wheel, grinding paste, alloy file, diamond grinding needle, bamboo chip, fiber whetstone, and circular rotary grinder.
Sandpaper: 150#, 180#, 320#, 400#, 600#, 800#, 1000#,1200 #;
Oilstone: 120#, 220#, 400#, 600 #;
Felt wheel: cylindrical, conical, square, with sharp mouth;
Grinding paste: 1# (white) 3# (yellow) 6# (orange) 9# (green) 15# (blue) 25# (brown) 35# (red) 60# (purple);
File: square, round, flat, triangular and other shapes;
Diamond grinding needle: generally 3/32 handle or18 handle, with circular waveform, cylinder, long straight cylinder and long cone;
Bamboo chips: various shapes are suitable for operators and mold shapes, and the function is to press sandpaper and polish it on the workpiece to achieve the required surface roughness;
Fiber millstone: 200# (black) 400# (blue) 600# (white) 800# (red)
3. Polishing process
(1) Rough throw
After finishing milling, EDM, grinding and other processes, the surface can be polished by a rotating surface polisher with a rotating speed of 35000 ~ 40000 r/min. Then grind the oilstone by hand, and add kerosene as lubricant or coolant to the strip oilstone. The order of use is180 # → 240 # → 320 # → 400 # → 600 # → 800 # →1000 #.
(2) Semi-fine polishing
Semi-finish polishing mainly uses sandpaper and kerosene. The quantity of sandpaper is: 400 #→ 600 #→ 800 #→1000 #→1200 #→1500 #. In fact, # 1500 sandpaper only uses die steel suitable for hardening (above 52HRC), and more NC knowledge pays attention to WeChat official account (NC programming teaching), which is not suitable for pre-quenched steel, because it may cause surface damage of pre-quenched steel and fail to achieve the expected polishing effect.
(3) Fine polishing
Diamond grinding paste is mainly used for fine polishing. If the polishing cloth wheel is mixed with diamond grinding powder or grinding paste, the usual grinding sequence is 9 microns (1800 #) → 6 microns (3000 #) → 3 microns (8000 #). 9μm diamond grinding paste and polishing cloth wheel can be used to remove the hairy wear marks left by 1 200# and 1 50 0# sandpaper. Then use felt and diamond grinding paste to grind in the order of 1 micron (14000 #) → 1/2 micron (60000 #) → 1/4 micron (100000 #).
(4) Working environment for polishing
The polishing process should be completed in two workplaces: the rough grinding place and the fine polishing place, and attention should be paid to cleaning the sand particles left on the surface of the workpiece in the previous process.
Generally, after rough polishing from oilstone to 1200# sandpaper, it is necessary to transfer the workpiece to a clean room for polishing to ensure that no dust particles in the air stick to the surface of the mold. Pay more attention to WeChat official account (CNC programming teaching) to learn more about CNC knowledge. Polishing processes with accuracy above 1μm (including 1μm) can be carried out in a clean polishing room. If more precise polishing is carried out, it must be carried out in an absolutely clean space, because dust, smoke, dandruff and saliva may scrap the high-precision polished surface.
After the polishing process is completed, the surface of the workpiece should be dustproof. After the polishing process stops, all abrasives and lubricants should be carefully removed to ensure the surface of the workpiece is clean, and then a layer of mold antirust paint should be sprayed on the surface of the workpiece.
5. Factors affecting the polishing performance of mold surface
(1) Surface condition of workpiece
In the process of machining, the material surface will be damaged by heat, internal stress or other factors, and improper cutting parameters will affect the polishing effect. The surface after EDM is more difficult to grind than the surface after machining or heat treatment, so finish machining by EDM should be adopted before the end of EDM, otherwise a hardened thin layer will be formed on the surface. If the EDM finishing quantity is not properly selected, the maximum depth of heat affected layer can reach 0.4 mm The hardness of the hardened thin layer is higher than that of the matrix and must be removed. So it is best to add a rough grinding process to provide a good foundation for polishing.
(2) the quality of steel
High-quality steel is the premise of obtaining good polishing quality, and various inclusions and pores in steel will affect the polishing effect. In order to achieve a good polishing effect, more CNC knowledge should pay attention to WeChat official account (CNC programming teaching). When machining a workpiece, the polished surface roughness must be marked. When it is determined that the workpiece needs mirror polishing, steel with good polishing performance must be selected and all of them must be heat treated, otherwise the expected effect will not be achieved.
(3) heat treatment process
If the heat treatment is improper, the surface hardness of steel is uneven or the characteristics are different, which will make polishing difficult.
(4) Polishing technology
Because polishing is mainly done by hand, human skills are still the main reason that affects polishing quality.
It is generally believed that polishing technology affects surface roughness. In fact, a good polishing process should be combined with high-quality steel and correct heat treatment process to get satisfactory polishing effect. On the other hand, the polishing technology is not good, even if the steel is no matter how good, it can not achieve the mirror effect.
6. Precautions for different types of polishing
(1) Precautions for Sandpaper Grinding and Oilstone Grinding of Mould
1) Only clean and soft oilstone tools can be used to polish the high-hardness mold surface.
2) When changing the sand grade during grinding, the workpiece and the operator's hands must be cleaned to avoid bringing coarse sand to the next fine grinding operation.
3) During each grinding process, more NC knowledge should be paid attention to WeChat official account (NC programming teaching). Sandpaper should be sanded in different 45 directions until the sand particles at the next higher level are eliminated. After the sand grains of the previous level are removed, the grinding time must be extended by 25% to convert the finer sand marks of the next level.
4) Changing different directions during grinding can avoid uneven phenomena such as waves.
(2) Matters needing attention in diamond grinding and polishing
Diamond grinding and 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 8 000# grinding paste, the common load is 100 ~ 200 g/cm2, but it is difficult to keep the accuracy of this load. In order to do this conveniently, you can make a thin and narrow handle on the wooden strip, or cut off a part of the bamboo strip to make it softer. This can help control the polishing pressure to ensure that the surface pressure of the mold is not too high. When using diamond grinding and polishing, not only the working surface is required to be clean, but also the workers' hands must be very clean.
(3) Precautions for polishing plastic molds
The polishing of plastic molds is quite different from the surface polishing required by other industries. Strictly speaking, the polishing of plastic molds should be called mirror processing. Not only the polishing itself requires high standards, but also the flatness, smoothness and geometric accuracy of the surface. Pay more attention to WeChat official account (CNC programming teaching) to acquire CNC knowledge. The standard of mirror polishing is divided into four grades: A0=Ra0.008 μ m, A 1=Ra0.0 16 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 electrolytic polishing and fluid polishing, chemical polishing and fluid polishing, etc.
Matters needing attention during grinding are as follows:
1) When the new mold cavity starts machining, the surface of the workpiece should be inspected first, and the surface should be cleaned with kerosene, so that the surface of the oilstone will not stick dirty and lose its cutting function.
2) When grinding coarse particles, it should be carried out in the order of easy first and then difficult, especially for some dead corners that are difficult to grind, the deeper bottom should be ground first, and then the side and large plane should be ground.
3) Some workpieces may be polished together. First, grind the coarse particles or fire particles of a single workpiece separately, and then grind all the workpieces together until smooth.
4) For the workpiece with large plane or side, grind rough lines with oilstone, and then use flat steel sheet for light transmission detection to check whether there is unevenness or inverted thread. If there is back thread, it will lead to difficulty in demoulding or strain.
5) In order to prevent the die workpiece from being inverted or some joint surfaces need to be protected, you can stick it with a saw blade or stick it on the edge with sandpaper to obtain ideal protection effect.
6) Pull the grinding die plane back and forth, and the handle of the whetstone shall be leveled as far as possible, not exceeding 25. Because the inclination is too large, the force will rush from top to bottom, which will easily lead to many rough lines on the workpiece.
7) If you use copper or bamboo sandpaper to grind the workpiece plane, the sandpaper should not be larger than the tool area, otherwise it will grind to the wrong place.
8) Try not to use a grinder to repair the parting surface, because the parting surface repaired by the grinding wheel head is rough and wavy. If necessary, the grinding wheel head must be glued to the concentricity balance.
9) The shape of the grinding tool should be close to the surface shape of the mold, so as to ensure that the workpiece is not deformed by grinding.
7. How to solve common polishing problems?
(1) Overpolished
The biggest problem encountered in the daily polishing process is "over-polishing", that is, the longer the polishing time, the worse the surface quality of the mold. There are two phenomena of over-polishing: orange peel and pitting. Overpolishing mostly occurs in mechanical polishing.
(2) The reason of "orange peel" on the workpiece
The irregular and rough surface is called "orange peel". There are many reasons for the "orange peel". The most common reason is overheating or carburization of the die surface. More CNC knowledge pays attention to WeChat official account (CNC programming teaching). The main reasons for "orange peel" are too high polishing pressure and too long polishing time. For example, when the polishing wheel is polished, the heat generated by the polishing wheel is easy to cause "orange peel". Hard steel can withstand greater polishing pressure, while relatively soft steel is easy to be over-polished. It is proved that the time of over-polishing will change with the hardness of steel.
(3) Measures to eliminate "orange peel" of workpieces
When it is found that the surface quality is not polished well, many people will increase the polishing pressure and extend the polishing time, which will often make the surface quality worse.
The following methods can be used for remediation:
1) to remove the defect surface, and the grinding granularity is slightly coarser than that of the sand used before, and then the polishing strength is lower than that before.
2) Stress relief is carried out at a temperature lower than the tempering temperature of 25℃, before polishing, it is polished with the finest sand number until the satisfactory effect is achieved, and finally it is polished with light force.
(4) Causes of "pitting" on the surface of workpiece.
Because some nonmetallic impurities in steel, usually hard and brittle oxides, are pulled out from the surface of steel during polishing to form pits or pitting, the main factors causing "pitting" are as follows:
1) The polishing pressure is too high and the polishing time is too long.
2) The purity of steel is not enough, and the content of hard impurities is high.
3) The mold surface is rusted.
4) Black leather is not removed.
(5) Measures to eliminate pitting corrosion of workpieces
1) Carefully re-grind the surface, and the grain size of the sand is slightly coarser than before. Before the polishing process, the final grinding step is carried out with a soft and sharp oilstone.
2) When the sand size is less than 1 mm, the softest polishing tool should be avoided.
3) Try to use the shortest polishing time and minimum polishing force.
Cavity polishing is a very important process in the mold manufacturing process, which is related to the quality and life of the mold and also determines the quality of the product. Mastering the working principle and technological process of polishing and choosing a reasonable polishing method can improve the quality and life of the mold, and then improve the quality of the product.
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