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Gear pumps have been applied to thermoplastic extrusion lines since 1978, the most typical of which are plastic sheets, plastic profiles, cast films or blown films. Gear pumps are now an indispensable device in these physical processes, so that instrument manufacturers typically ship gear pumps and single-screw extrusion equipment for sale. Gear pumps are widely used for many reasons, including: increased throughput; better quality; higher throughput; effective raw material savings and reduced energy consumption, allowing investment payback periods to be reduced to within 6 months. When gear pumps are used to mix and extrude rubber products, these advantages are naturally brought into the industry. This article focuses on the basic knowledge of gear pumps used in rubber extrusion and the resulting advantages and challenges. The discussion focused on two areas of the rubber market: profile extrusion and custom compounding / compounding. In the field of profile extrusion, we cite a realistic example of a series of manufacturing positives experienced by an end user using gear pumps to retrofit existing lines (Figure 1). Figure 1, gear pump practical application of the basic structure of the gear pump Extruder gear pump is a positive displacement device, usually installed between the extruder and the die. The gear pump consists of two symmetrical shafts, each shaft also contains intermeshing gears and radial bearings, and the whole pump is covered with an external casing, as shown in Figure 2. The gears provide the pumping action directly, and the bearings determine the position of the gear pump shaft and support the entire pump shaft. The gear and shaft are of an integrated design because the torques and shear loads acting on the gear pump can be large when the product viscosity is high or under special pressure conditions and this design allows the unit to withstand even greater forces . One of the gears is a drive wheel, which is connected to a long shaft that can be passed out of the gear pump housing through the seal ring and connected to the gearbox outside the pump, while the gearbox is driven by a motor. The bearings in gear pumps are lubricated with rubber. The Gear Pump rack houses a reverse-threaded labyrinth seal called Visco Seals to prevent the rubber in the extruder from leaking out. However, in a typical rubber production process, these seals are designed to allow a small amount of rubber product to be pumped from the pump in order to avoid scorching or premature curing of the rubber in the pump (if there is a curing agent in the rubber compound) Exudation. Figure 2, the working principle of the extrusion gear pump structure shown in Figure 3, when the gear pump shaft rotation, the gear teeth at the pump inlet end are separated from each other, which makes the entry of rubber products can enter the gap between the gear teeth . Then, as the gear rotates, rubber is brought to the other side of the gear. When the teeth of the two gears are engaged on that side, the rubber between the original gears is released and a pumping operation is completed. A typical gear pump system should contain a pump driven by an alternating current. The drive for the pump includes a gear reducer, a drive shaft, and a dedicated control system dominated by the pressure circuit. The drive is connected to the output of the extruder (typically 0-10 volts) and the extruder becomes the source of power for the pump. Pressure sensors are installed in the gear pump inlet and outlet, and its role is not only a control system, but also to prevent gear pump insufficiency or system over-pressure. Figure 4 shows the gear pump and its drive used in a pipe unit. Figure 4. Silicone Rubber Tubing and Its Drives Special Requirements for Gear Pumps in Rubber Production Typical rubber compounds for gear pumps used in the rubber industry include polybutadiene rubber (BR), neoprene (CR), and B Propylene terpolymer rubber (EPDM), nitrile rubber (NBR), styrene butadiene rubber (SBR), and natural rubber. These rubber compounds pose a number of challenging problems for gear pumps. Although the processing temperatures of these rubbers are already very low compared to thermoplastics, they are still subject to combustion or coking and these problems present a number of potential problems for rubber processors. These rubbers also have problems of high viscosity, low lubricity and high abrasion resistance. In addition, many dies are designed to work under high pressures (above 5000 psi). Therefore, to keep the performance of the gear pump high for a long time, all of these issues must be taken into account when designing the gear pump. Based on experience, we have concluded that a well-designed gear pump in rubber processing must include the following elements: complete penetration of the rubber material at the bearings and journals; the design of the gear should not deviate more than 5,000 psi; wear out Components need to be coated with special hard material; pump and pump shaft cooling device to control the system temperature. The design of a good gear pump with a gear pump installed in the terminal line determines that the dimensional error of the product is well controlled, especially since it is important to control the size of the profile during the profile extrusion process. By reducing pressure fluctuations, giving a smoother die pressure, the gear pump can control the dimensional error of the extruded product very well (typically ± 1.5%). This is the first major positive, which can significantly improve product yield and material savings. In general, the pumping capacity of the extruder is poor, and its working efficiency is generally 40 ~ 50%. Adding a gear pump to the extruder to increase the pumping efficiency to 98% greatly reduces the pumping pressure on the extruder and maximizes the mixing and grinding of the extruder. At the same time, the introduction of the gear pump also greatly reduces the shear force inside the rubber, thus greatly reducing the rubber temperature (Figure 5). This also makes the line speed in the extruder is improved, making the output of the product has been 25 to 40% increase. This is the second major benefit to gear pumps. & nb
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