First: the formation of back pressure In the process of plastic melting and plasticizing, the molten material keeps moving to the front of the cylinder (the metering chamber), and more and more, gradually forming a pressure, pushing the screw back. In order to prevent the screw from receding too quickly and to ensure that the melt is evenly compacted, the screw needs to be provided with a pressure in the opposite direction. The pressure that prevents the screw from receding is called back pressure. Back pressure, also known as plasticizing pressure, is controlled by adjusting the return throttle valve of the injection cylinder. Back pressure pre-plasticized screw injection molding machine is equipped with a back pressure valve at the back of the injection cylinder, which can adjust the oil discharge speed of the injection cylinder when the screw rotates and receding, so that the cylinder can maintain a certain pressure; The full motor screw speed (drag) is controlled by the AC servo valve. Benefits of proper back pressure regulation: 1, can be the gun barrel of molten material compaction, increase the density, improve the injection volume, product weight and size of stability. 2. It can "extrude" the gas in the molten material to reduce the gas flowers and internal bubbles on the surface of the product and improve the uniformity of luster. Slow down the screw receding speed, so that the gun barrel melt fully plasticized, increase the toner, color master and melt mixing uniformity, to avoid the phenomenon of mixing color products. 3, slow down the screw receding speed, so that the gun barrel melt fully plasticized, increase the toner, color master and melt mixing uniformity, to avoid the phenomenon of mixing color products. 4. Proper lifting of the back pressure can improve the shrinkage of the product surface and the gluing around the product. 5, can raise the temperature of the melt, so that the melt plasticizing quality, improve the flow of the melt filling mold, the surface of the product without gellan. When the back pressure is too low, the following problems are easy to occur: 1. When the back pressure is too low, the screw recedes too fast, and the molten material at the front end of the gun barrel has a small density (relatively loose), and is trapped in more air. 2, will lead to poor plasticizing quality, injection volume is not stable, product weight, product size changes. 3, the product surface will appear shrinkage, gas flower, cold material grain, luster uneven and other undesirable phenomena. 4. Bubbles are easy to appear inside the product, and glue is easy to leave around the product and bone position. High back pressure, prone to the following problems: 1. The melting pressure at the front of the gun barrel is too high, the melting temperature is too high, and the viscosity drops. The reverse flow of the melting material in the screw groove and the leakage flow between the barrel and the screw increases, which will reduce the plasticizing efficiency (plasticized material amount per unit time). 2. For plastics (such as PVC, POM, etc. 3. The back pressure is too high, the screw recede slowly and the premolding material return time is long, which will increase the cycle time and lead to the decline of production efficiency. 4, the back pressure is high, the molten material pressure is high, after spraying nozzle is easy to occur molten glue saliva phenomenon, the next time when spraying, the nozzle runner in the cold material will block the nozzle or cold material spot in the products. 5, in the process of beer molding, often due to the back pressure is too large, nozzle leakage phenomenon, waste of raw materials and lead to the heating ring near ** burned out. 6. Mechanical wear of premolding mechanism and screw barrel increases. Second: back pressure adjustment: The back pressure of injection molding should be adjusted according to the performance of raw materials, drying conditions, product structure and quality conditions. The back pressure is generally adjusted at 3-15kg/cm3. When the surface of the product has a little air flower, mixed color, shrink and product size, weight changes, can increase the back pressure. When ** leakage of glue, saliva, melting material overheating decomposition, product discoloration and return material too slow can consider appropriate reduction of back pressure. Back pressure is one of the important parameters to control the melt quality and product quality in injection molding process. Appropriate back pressure plays an important role in improving product quality.