Injection back pressure problem
Formation of back pressure in injection moldingFirst, the formation of back pressureIn the process of melting and plasticizing of plastics, the melt moves to the front of the barrel and more and more, gradually forming a pressure to push the screw back. In order to prevent the screw from backing too fast and to ensure uniform compaction of the melt, it is necessary to provide an opposite direction of pressure to the screw, which is the reverse pressure that prevents the screw from retreating. Back pressure is also known as plasticizing pressure, and its control is achieved by adjusting the oil return valve of the injection cylinder. Pre injection plasticizing screw injection molding machine is equipped with the rear end of the oil cylinder back pressure valve, regulating screw back when the injection cylinder oil cylinder speed, to maintain a certain pressure; shift speed screw motor after (resistance) is composed of AC servo valve control.
Five, the adjustment of back pressureThe adjustment of the injection back pressure shall be determined according to the raw material performance, drying conditions, product structure and quality, and the back pressure should be adjusted at 3-15kg/cm 3. When the surface of the product has little Qihua, mixed color, shrink and product size and weight changes, may be appropriate to increase the back pressure. When the nozzle leakage glue, salivation, Melt Overheating product decomposition, discoloration and feeding back too slow may be considered appropriate to reduce the back pressure.Back pressure is one of the important parameters to control melt quality and product quality in injection molding process. Proper back pressure plays an important role in improving product quality, and can not be ignored!
Three, back pressure is too low, prone to the following problems1, the back pressure is too low, screw back into the barrel too fast, the front end of the melt density (Jiao Songsan), into the air. 2, will lead to poor quality of plastic, the amount of plastic spray instability, product weight, product size changes. 3, the product surface will appear shrinkage, gas flower, cold material lines, uneven luster and other undesirable phenomena. 4, the product prone to bubbles, the product surrounding and bone position, easy to walk dissatisfied glue.Four, high back pressure, prone to the following problemsMelt pressure is 1, the front end of the barrel is too high, high temperature material, decrease viscosity, melt in the screw slot of the upstream and the barrel and screw clearance leakage flow rate increases, will reduce the plasticizing efficiency (feed plasticizing unit time).2, for the poor thermal stability of plastic (such as PVC, POM etc.) or coloring, thermal decomposition caused by melt temperature and heating time of growth in the barrel, increases the degree of discoloration or coloring, product surface color / gloss variation.3, back pressure is too high, screw back slow, pre plastic back material time is long, will increase cycle time, lead to production efficiency decline.4, high pressure, high melt pressure, injection nozzle after prone to melt salivation phenomenon, next injection, cold flow feeding nozzle will plug the outlet or product in cold spot.5, in the process of beer molding, often due to excessive back pressure, nozzle leakage phenomenon, waste of raw materials, and lead to near the nozzle heat ring burned.6. The mechanical wear of the preform and screw barrel increases.
Two, proper adjustment of the benefits of back pressure1, can melt inside the barrel compaction, increase density, improve the stability of injection quantity, weight and size of the products. 2. The gas in the melt can be extruded to reduce the gas, the inner bubbles and the gloss uniformity of the products. Slow speed screw back, so that melt inside the barrel full plasticizing, mixing and melting toner, color uniformity, avoid product color mixing phenomenon. 3, slow speed screw back, so that melt inside the barrel full plasticizing, mixing and melting toner, color uniformity, avoid product color mixing phenomenon. 4, appropriately increase the back pressure, can improve the product surface shrinkage and product periphery walk glue situation. 5. It can raise the temperature of molten material, improve the plasticizing quality of melt material, improve the fluidity of melt filling, and no cold glue on the surface of the melt.
