As the fineness requirements of materials become higher and higher, the use of sand mills is becoming more and more common, and there are many grinding media on the market. How to choose a grinding media that is more suitable for its own production process and conditions is a The pieces are more critical and troublesome. The following is a simple analysis of the following aspects. First, the choice of grinding beads: As the fineness requirements of materials become higher and higher, the use of sand mills is becoming more and more common, and there are many grinding media on the market. How to choose a grinding media that is more suitable for its own production process and conditions is a The pieces are more critical and troublesome. The following is a simple analysis of the following aspects. 1. Chemical composition Grinding media can be divided into different glass beads, ceramic beads materials (including zirconium silicate beads, zirconium oxide beads, aluminum oxide beads, the rare earth metal stabilized zirconia beads, etc.), steel and the like. Grinding medium Due to the difference in chemical composition and manufacturing process, the crystal structure of the beads is determined. The dense crystal structure ensures high strength of the beads, high wear resistance and low ink absorption. The difference in the percentage content of various components determines the specific gravity of the grinding beads, and the high specific gravity provides a guarantee for the high efficiency of grinding; the natural wear of the chemical composition of the grinding beads during the grinding process will have a certain influence on the performance of the slurry, so Considering the low wear rate, the chemical elements of scruples are also factors to consider. Such as grinding tape powder or other electronic component paste, metal Fe, Cu and other elements should be avoided, grinding beads containing Fe2O3 or CuSO4 and other components are not selected, so the choice of zirconium beads is often the common choice in this industry; such as grinding pesticides In medicine, biochemistry, heavy metals are scruples, and PbO is the most common ingredient. In short, some of the physical properties (hardness, density, wear resistance) determined by the chemical composition of the beads and the contamination of the slurry by the wear of the beads are factors to be considered in selecting the grinding media. 2, physical properties: 2.1 density of grinding beads Density is expressed in the general document as specific gravity (true specific gravity) and bulk density (false specific gravity). The molecular weight and percent composition of various oxides determine the density of the grinding media. The density of commonly used grinding media is shown in Table 1. Show. Table 1 Density of commonly used grinding media Under normal circumstances, the larger the specific gravity of the grinding beads, the larger the impulse, the higher the grinding efficiency, and the wear of the sand mill contact parts (inner cylinder, dispersion disc, etc.) is relatively large, so the viscosity and flow rate of the slurry Become a key. Low-density beads are suitable for low-viscosity slurries, and high-density beads are suitable for high-viscosity slurries. 2.2 The hardness of the beads Mohs hardness is a commonly used index. The grinding hardness of the beads is higher, and the wear rate of the beads is theoretically lower. The hardness of commonly used grinding beads and other materials is shown in Figure 1. Figure 1 Hardness of commonly used grinding media and materials For example, from the wear of the grinding beads to the contact parts of the sander (dispersion disc, rod pin and inner cylinder, etc.), the abrasive beads with high hardness are more abrasive to the contact parts, but can be adjusted by adjusting the filling amount of the beads. , the viscosity of the slurry, flow and other parameters to achieve the best optimization point. 2.3 Particle size of the beads The size of the grinding beads determines the contact point of the grinding beads and the material. The more contact points of the beads with smaller particle diameters under the same volume, the higher the theoretical grinding efficiency. On the other hand, the primary particles in the grinding are relatively large. In the case of materials, for example, for a 100 micron slurry, beads of D = 1 mm may not be used because the momentum of the beads does not reach the energy of sufficient grinding and dispersion, and beads having a larger particle size should be used. 3. Precautions for the first use of grinding media 1) Select zirconium beads or glass beads depending on the viscosity of the material 2) Select the appropriate size of beads according to the size of the raw material particles and the fineness required for the product. 3) Check that the separator or screen aperture of the grinder is properly selected and the gap should be one-third of the minimum bead diameter. For example: use a 1.2 to 1.4 mm bead with a gap of 0.4 mm. 4) Try to avoid turning on the grinder in the dry state, causing unnecessary loss of beads and accessories. 5) Different brands of beads cannot be mixed. 6) Try to avoid mixing the large and small beads. 4. How to determine the particle size of zirconium beads: 4.1. Requirements for grinding equipment: 1) Screen separation: the minimum diameter of the beads = the gap of the screen * 1.5; 2) Ring separation: the minimum diameter of the beads = the gap of the ring * 3; 4.2. Process requirements: 1) The initial diameter of the material: the minimum diameter of the beads = the initial diameter of the material * 30 ~ 50; 2) Final diameter of the material = diameter of the beads. 4.3. Bead diameter 1.5mm is the preferred choice. Second, how to avoid the crushing of the beads After we choose the beads, how to use it properly, avoid the crushing of the beads, and prolong the life of the grinding media and the sander contacts. 1, the cause of broken beads The glass beads, zirconium silicate beads and pure zirconium beads which are popular in the market are basically two types of electrofusion method and sintering method in terms of production process. The beads are formed in hot air, cold air or electrolyte. If the technical parameters are not controlled, the following fragile beads will be produced: bubble beads, snowman beads, tail beads, flat (elliptical) beads. Since the above beads have stress-concentrated areas and are easily broken during grinding, such beads should be avoided as much as possible when selecting beads. 2. Judging the normal wear and tear of the beads Normally, the grinding beads that work for a period of time become smaller, the surface is smooth and not angular, which should be the wear of normal beads; on the contrary, if there are angular and flaky shaped beads in the beads, this should produce broken beads. 3, the cause of broken beads As far as the beads themselves are concerned, on the one hand, the quality of the beads, on the other hand, or the strength of the beads of such materials cannot compete with a certain type of sander. For example, the Dyno-mill ECM horizontal sand mill in Switzerland and the DCP vertical sand mill in Drais, Germany, are usually recommended to use pure zirconium beads due to the high energy density of the input. In addition, the grinding beads are subjected to the normal operation of the sander to receive a pressure of about 1 kg, and the glass beads can withstand about 5000 kg of force and 9000 kg of zirconium silicate beads. The force of the grinding beads in the sand mill is Insignificant, so the reason for the broken beads should be concentrated on the equipment and the process, and the corresponding solutions are taken. 4, broken beads solution A. Equipment aspects 1). Dispersion disc: Disperse discs are reversed, loose or cracked. 2). Separation device: The dynamic sieve ring has a gap and the screen is broken. 3). Feed pump: After the gear pump is suddenly turned off, the pressure inside the sander will press the beads back into the pump. It can be avoided by adding a check valve while keeping the feed pump cleaned regularly. 4). Inner cylinder: The inner cylinder has defective points. B. Production operations: 1). Mixed use of large and small beads: In this case, there is an indication that the grinding efficiency is improved at first, but as the grinding time is lengthened, a large bead grinding bead is produced, and finally the deformation of the beads is accelerated to break. The solution is to use beads of uniform particle size. 2). Mixed use of different brands of beads: Due to the inconsistency of the hardness and density of various beads, it is easy to produce hard beads to eat soft beads, so this form should be eliminated. 3). The viscosity of the slurry is too thin or excessive: compared to a certain density of beads, it is easy to cause the accumulation of beads and direct contact with the wear parts of the sander to accelerate the wear and breakage of the beads. 4). The flow rate of the material is too fast: this will cause the beads to accumulate at the material outlet, which will accelerate the wear of the beads and the sander accessories. The solution is to loosen the backlogged beads by intermittent start-up and re-distribute evenly, and then adjust the flow of the material. 5, how to solve the crushing of the grinding beads First, identify the "broken" beads. If the particles are smoother, rather than sharp-edged like broken glass, this is actually a natural wear-resistant bead. These beads may be ground into various shapes such as discs, ovals, or smaller spheres. This situation can be called wear, but the degree of wear is different. From the perspective of force, when the dispersing disc transfers energy to the beads, if the viscosity of the liquid is very low, for example, when the sander is cleaned with solvent or water, the thrust of the material is not large enough, and the beads may come into contact with the dispersing disc. broken. This is an important reason why we recommend minimizing cleaning time. If the machine is cleaned with a low viscosity liquid, the beads will also wear contact parts of other beads or sanders. In order to obtain the longest service life of the grinding beads and sand mill, it is best to use resin for cleaning and maintain sufficient material viscosity during the production process. Many people came across the old beads and put them into the new beads to test the broken beads. Let's analyze this problem and explore solutions. 1) The internal contact parts of the sander are not installed correctly: the dispersing disc is loose or the edge of the dispersing disc has sharp corners, the inside of the sanding machine has foreign matter, the inside of the valve is loose, etc., which may cause the grinding beads to break. 2) The sander contact is worn or not properly installed: the dynamic separator or screen may be worn, the screen may be broken or reversed, and if the grinding medium passes through the separator and enters the feed pump, the pump is The beads are crushed before being blocked or shut down. 3) Back pressure: When the feed pump is turned off, the residual pressure in the sander presses the beads into the pump so that when the pump is started again, it will crush the beads. One-way valves are not necessarily reliable and sometimes allow the beads to pass before the valve closes. 4) Grinding ball backlog: This can be caused if the grinding beads accumulate at the bottom of the sander or if the working pump is too fast and the grinding beads are concentrated at the exit of the horizontal sander. The solution is to loosen the backed grinding beads by the on-off-on-off jog mode when starting the sander. 5) Dispersion disc wear: This situation may be difficult to determine. The edges of the worn dispersing disc are ground and rugged, and the edge of the dispersing disc is where the rotational speed of the dispersing disc is greatest and a strong eddy current may form. In addition, for a vertical mill, the beads are concentrated by their own weight to the vicinity of the edge of the lower dispersion disk. When all these factors are combined, it is possible to cause the grinding beads to break. The wear of the dispersing disc is too large due to the slow rotation of the feed pump and the excessive density of the grinding beads. This problem can be solved by replacing worn parts with the same batch of beads. 6) Mix the grinding beads produced by different manufacturers. The products of different manufacturers may be different in terms of density. Doing so will exacerbate the mutual wear between the beads. The solution is to never mix grinding beads from different manufacturers. If you suspect that there is already a mix, you can only dump them all. 7) Because the used old beads are accidentally added to the sander. 8) Garbage or some small worn beads: There are often rubbish or some small worn beads attached to the screen, on the shaft, on the bracket, or in other machine corners. When adding new beads, these small things may loosen from the attachment position into the newly added beads. This will cause the new beads to have the illusion of breaking, but what you see is just some of the natural wear beads. If the problem is not too big, let these small things be naturally discharged after a certain period of time. Otherwise, you must completely remove the old beads and replace them with new ones. 9) The cleaning beads can be cleaned by selecting water, solution or resin according to the formula; the cleaning should be kept at a low speed, and the “Start-Close†button of the grinding machine should be used for intermittent cleaning. 6, the principle of adding grinding media If the grinding efficiency of the grinder is found to be reduced, it is a possible signal for adding beads. Users can master the actual loss rate of the beads according to their own process conditions, and do regular screening and adding. The fine beads are sieved out with a sieve size of two-thirds of the diameter of the beads (for example, a bead screen of 0.8 to 1.4 mm should be 0.8 mm), and the difference beads are added. 7, how to add grinding media Due to the natural loss of the beads, the particle size of the beads will become smaller and smaller. In order to maintain a uniform filling amount and avoid clogging of the beads or entering the separation device, the beads should be sieved and supplemented according to the life of the grinding medium and the conditions of the user's own process. Amount of grinding media. It is recommended to sift the beads after 100-200 hours of operation and add the appropriate amount of new beads.
The fundamental reason why leather is soft is that when leather is made, oil is introduced into the interior of animal skin to form an oil film that completely covers the surface of the fibers inside the skin, that is, a layer of oil film with appropriate thickness is used to separate the surface of the fibers inside the skin. In this way, the friction between the fibers inside the leather is equivalent to the friction of the oil molecules, so the leather will be very soft. Its principle is similar to that of sponge, but the difference is that the inner surface fibers of dermis are separated by oil film, which makes the inner surface of dermis have sliding pores, but its pores are smaller than that of sponge, and the distribution of the inner surface fibers of dermis is not as regular as that of sponge.
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Types of glass bead Zirconium silicate beads Pure zirconium beads Alumina beads Steel ball Rare earth zirconium beads proportion 2.5 4.3 5.5 5.7 7.8 6.1 Scattered weight 1.5 2.5 3.2 3.4 4. 3.