Although we constantly remind ourselves to produce high-quality plastic molds that even a fool could debug, this does not mean we can overlook the technical expertise in injection molding and the vast data systems of injection molding machines. Third-tier injection molding technicians can only adjust the flow speed and injection pressure, which poses no issues for products requiring merely sufficient filling. Second-tier technicians will modify the filling method based on flow speed and injection pressure to resolve simple problems. First-tier injection molding technicians, in addition to mastering flow speed and injection pressure, possess a deeper understanding of the principles and functions of various machine parameters. They can effectively utilize these parameters to address issues in injection molding, such as the rarely adjusted back pressure. Below, we will briefly analyze the role of back pressure.

Firstly, we need to understand how back pressure is formed. During the process of plastic melting and plasticization, the molten material continuously moves towards the front end of the barrel, and gradually forms a pressure that pushes the screw back. In order to prevent the screw from retreating too quickly and ensure uniform compaction of the molten material, it is necessary to provide a reverse pressure to the screw, which is called back pressure. Back pressure, also known as plasticizing pressure, is controlled by adjusting the return throttle valve of the injection cylinder. The rear of the injection cylinder of the pre plasticizing screw injection molding machine is equipped with a back pressure valve to adjust the speed of oil leakage from the injection cylinder when the screw rotates and retreats, so as to maintain a certain pressure in the cylinder; The screw backward movement speed (resistance) of the full electric motor is controlled by an AC servo valve.

Before debugging the back pressure, we need to clarify what effect the debugging of back pressure will have on the plastic mold forming, or why we need to adjust the back pressure:

first,It can compact the molten material inside the barrel, increase density, and improve the stability of injection volume, product weight, and size.

second,The gas inside the melt can be 'extruded' to reduce surface gas bubbles, internal bubbles, and improve gloss uniformity of the product. Slow down the retreat speed of the screw to fully plasticize the molten material inside the barrel, increase the uniformity of color powder, masterbatch, and molten material mixing, and avoid color mixing in the product.

third,Slow down the retreat speed of the screw to fully plasticize the molten material inside the barrel, increase the uniformity of color powder, masterbatch, and molten material mixing, and avoid color mixing in the product.

fourth,Properly increasing the back pressure can improve the shrinkage of the product surface and the adhesive adhesion around the product.

fifth,It can increase the temperature of the melt, improve the plasticization quality of the melt, improve the flowability of the melt during mold filling, and eliminate cold glue lines on the surface of the product.

Both low and high back pressure in injection molding can affect the quality of the product. When the back pressure is too low, the screw retreats too quickly, resulting in a low density of molten material flowing into the front end of the barrel and a lot of trapped air. This can lead to poor plasticization quality, unstable injection volume, and significant changes in product weight and size. The surface of the product may exhibit defects such as shrinkage, air bubbles, cold texture, uneven gloss, etc. Moreover, bubbles are prone to occur inside the product, and the surrounding areas and bone positions are prone to insufficient adhesive; Excessive back pressure, high melt pressure, high material temperature, and decreased viscosity at the front end of the barrel can cause the melt to flow back in the screw groove and increase the leakage rate between the barrel and screw, which can reduce plasticization efficiency For plastics with poor thermal stability (such as PVC, POM, etc.) or coloring agents, thermal decomposition may occur due to the increase in temperature of the melt and the prolonged heating time in the barrel, or the discoloration of the coloring agent may increase, resulting in a decrease in the surface color/gloss of the product. Excessive back pressure, slow screw retreat, and long pre plasticization return time will increase cycle time and lead to a decrease in production efficiency. High back pressure and melt pressure make it easy for the nozzle to produce molten glue drooling after injection. During the next injection, the cold material in the nozzle channel may block the nozzle or cause cold spots in the product. During the injection molding process of plastic molds, excessive back pressure often leads to leakage of glue from the nozzle, wasting raw materials and causing the heating ring near the nozzle to burn out.

When there are slight air pockets, material streaks, color mixing, shrinkage, and significant changes in product size and weight on the surface of plastic molds during injection molding, back pressure can be appropriately increased. When there is glue leakage, salivation, molten material overheating decomposition, product discoloration, and slow material return in the nozzle, it is advisable to consider reducing the back pressure appropriately. The adjustment of back pressure is not fixed, but depends on the performance of the raw materials, drying conditions, mold structure, and product quality. The back pressure is generally adjusted between 3-15kg/cm3. The adjustment of each parameter in injection molding will have a certain impact on product quality. Injection molding technicians need to be familiar with the working principle and function of each parameter, and be able to perceive the interaction between parameters. Currently, no brand of injection molding machine can achieve perfect products with one parameter adjustment. Multiple parameters need to be combined and adjusted to the best state.