0102030405
Problems you may encounter about “back pressure”
2025-07-11
Formation of back pressure
During the melting and plasticizing process of plastic, the molten material continuously moves to the front end of the barrel (inside the metering chamber), and more and more, gradually forming a pressure to push the screw backward. In order to prevent the screw from retreating too quickly and ensure that the molten material is evenly compacted, it is necessary to provide the screw with a reverse pressure. This reverse pressure that prevents the screw from retreating is called back pressure.
Back pressure is also called plasticizing pressure, and its control is achieved by adjusting the return oil throttle valve of the injection cylinder. The back pressure valve is installed at the rear of the injection cylinder of the pre-plasticizing screw injection molding machine to adjust the speed of the injection cylinder oil leakage when the screw rotates backward, so that the cylinder maintains a certain pressure (as shown in the figure below); the screw backward speed (resistance) of the full motor is controlled by the AC servo valve.
Benefits of properly adjusting the back pressure
1. It can compact the molten material in the barrel, increase the density, and improve the stability of the shot amount, product weight and size.
2. It can squeeze out the gas in the molten material, reduce the gas flowers and internal bubbles on the surface of the product, and improve the uniformity of the gloss. Slow down the screw retreat speed to fully plasticize the molten material in the barrel, increase the mixing uniformity of the toner, masterbatch and molten material, and avoid color mixing in the product.
3. Slow down the screw retreat speed to fully plasticize the molten material in the barrel, increase the mixing uniformity of the toner, masterbatch and molten material, and avoid color mixing in the product.
4. Properly increasing the back pressure can improve the shrinkage of the product surface and the glue flow around the product.
5. It can increase the temperature of the molten material, improve the plasticization quality of the molten material, improve the fluidity of the molten material during mold filling, and there is no cold glue pattern on the surface of the product.
When the back pressure is too low, the following problems are prone to occur
1. When the back pressure is too low, the screw retreats too fast, the density of the molten material flowing into the front end of the barrel is small (relatively loose), and more air is trapped.
2. It will lead to poor plasticization quality, unstable injection volume, and large changes in product weight and product size.
3. The surface of the product will have shrinkage, gas flowers, cold material patterns, uneven gloss and other undesirable phenomena.
4. Bubbles are prone to appear inside the product, and the periphery and bone position of the product are prone to insufficient glue.
Too high back pressure is prone to the following problems
1. The melt pressure at the front end of the barrel is too high, the material temperature is high, the viscosity decreases, the backflow of the melt in the screw groove and the leakage flow between the barrel and the screw increase, which will reduce the plasticization efficiency (the amount of plasticized material per unit time).
2. For plastics with poor thermal stability (such as PVC, POM, etc.) or colorants, the temperature of the melt increases and the heating time in the barrel increases, causing thermal decomposition, or the colorant discoloration increases, and the surface color/gloss of the product deteriorates.
3. The back pressure is too high, the screw retreats slowly, and the pre-plastic return time is long, which will increase the cycle time and reduce production efficiency.
4. The back pressure is high, the melt pressure is high, and the nozzle is prone to melt drooling after injection. The next time the glue is injected, the cold material in the sprue flow channel will block the sprue or cold material spots will appear in the product.
5. In the process of beer molding, the nozzle often leaks glue due to excessive back pressure, which wastes raw materials and causes the heating coil near the nozzle to burn out.
6. The mechanical wear of the pre-molding mechanism and the screw barrel increases.
Adjustment of back pressure
The adjustment of Injection Molding back pressure should depend on the performance, drying conditions, product structure and quality of the raw materials. The back pressure is generally adjusted at 3-15kg/cm3.
When there are a few gas flowers, mixed colors, shrinkage and large changes in product size and weight on the surface of the product, the back pressure can be appropriately increased. When the nozzle leaks glue, drools, the melt is overheated and decomposed, the product changes color and the return material is too slow, you can consider reducing the back pressure appropriately. Back pressure is one of the important parameters for controlling the quality of the melt and the quality of the product in the injection molding process. Appropriate back pressure plays an important role in improving product quality and can’t be ignored!
