Injection molding screw back pressure
Injection molding screw back pressure refers to the pressure applied to the rear end of the screw during the plasticizing process. Its function is to increase the density of the melt, improve the plasticizing quality, and expel air and volatiles from the melt. Properly setting screw back pressure is crucial for ensuring product quality and stabilizing the production process. Too little back pressure can lead to insufficient plasticization, low melt density, and high air content, resulting in defects such as bubbles and sink marks in the product. Excessive back pressure increases screw torque and energy consumption, prolongs plasticizing time, and can even cause overheating and decomposition of the plastic. Therefore, in injection molding production, screw back pressure must be precisely controlled based on factors such as the type of plastic and product requirements.
Screw backpressure is typically expressed as hydraulic system pressure in megapascals (MPa). The setting range is generally 0.5 to 20 MPa, with the specific value determined based on the characteristics of the plastic. For plastics with lower viscosity and better fluidity, such as PE and PP, the backpressure can be set lower, typically 0.5 to 3 MPa, to prevent melt flashing during plasticization and reduce energy consumption. Plastics with higher viscosity and lower fluidity, such as PC and POM, require higher backpressure to promote plasticization and mixing, typically set between 3 and 10 MPa. For modified plastics containing additives (such as glass fiber and flame retardants), backpressure may need to be increased, typically between 5 and 15 MPa, to ensure uniform dispersion of the additives in the melt. Furthermore, for plastics prone to generating volatiles, such as PVC and ABS, higher backpressure helps dissipate these volatiles and reduce bubbles and streaks in the finished product. Backpressure is typically set between 2 and 8 MPa.
Screw back pressure has a significant impact on the plasticization quality of the melt. Properly increasing the back pressure can increase the shearing effect of the screw on the melt, allowing the plastic to be more fully mixed and plasticized, and improving the uniformity and stability of the melt. At the same time, the increase in back pressure makes it easier to expel air and volatiles in the melt, thereby reducing defects such as bubbles and pinholes in the product. For example, when producing transparent plastic parts, in order to ensure the transparency of the product, a higher back pressure is usually required to expel tiny bubbles and impurities in the melt. The back pressure is generally set at 5-8MPa. However, excessive back pressure will cause the melt temperature to increase due to the increased frictional heat generated by the enhanced shear effect. For plastics with poor thermal stability, such as PVC and POM, this may cause plastic decomposition, so the upper limit of the back pressure needs to be strictly controlled.
Backpressure setting is closely related to parameters such as screw speed and plasticizing capacity, and these two parameters must be coordinated to achieve optimal plasticizing results. When the screw speed is high, the melt’s residence time in the barrel is short, potentially leading to inadequate plasticization. In this case, appropriately increasing the backpressure can increase shearing and plasticizing time, compensating for the insufficient speed. Conversely, when the screw speed is low, the melt’s residence time in the barrel is long. Excessive backpressure can easily lead to melt overheating, so the backpressure needs to be reduced. When plasticizing at high rates, backpressure should also be appropriately increased to ensure melt uniformity and adequate melting and mixing of the plastic. For example, when the injection molding machine’s plasticizing capacity exceeds 80% of its rated value, the backpressure typically needs to be increased by 20% to 30% compared to normal conditions.
In actual operation, the adjustment of the screw back pressure needs to be gradually optimized based on the quality feedback of the product. During the initial setting, an intermediate value can be selected according to the type of plastic and the product requirements, and then adjusted by observing the appearance and internal quality of the product. If ripples, cold spots, etc. appear on the surface of the product, it may be caused by too low back pressure and insufficient plasticization. The back pressure should be appropriately increased; if the product shows discoloration, scorch marks, or the screw has difficulty turning or the noise increases, it may be that the back pressure is too high and the back pressure needs to be reduced. At the same time, operators also need to pay attention to the impact of back pressure on the production cycle. Excessive back pressure will prolong the plasticization time and reduce production efficiency. Therefore, under the premise of ensuring product quality, a lower back pressure should be selected as much as possible to improve production efficiency. By reasonably setting and adjusting the screw back pressure, efficient and energy-saving production can be achieved while ensuring product quality.
