In the injection molding process, back pressure is a crucial parameter that not only affects the quality of the molten material but also directly impacts the quality of the final product. This article will delve into the reasons for the formation of back pressure, its effects, and how to make improvements, to help readers better understand this key process aspect.

The formation reasons for back pressure.

Back pressure, also known as plasticizing pressure, is gradually formed during the melting and plasticizing process of plastics. When the screw rotates, plastic pellets are transported to the front end of the screw in the melting zone, where they gradually melt under heating and shearing action. The molten material continuously moves towards the front end of the barrel (in the metering chamber), gradually forming a pressure that pushes the screw backward. To prevent the screw from retracting too quickly and to ensure uniform compaction of the molten material, a counteracting pressure needs to be applied to the screw, which is known as back pressure.

The formation of backpressure is influenced by various factors, including:

The principle of screw rotation: During rotation, the screw pushes the plastic particles forward through the screw flights, forming melt and generating pressure.

The resistance during the melting process: The plastic melt has a certain viscosity, which creates resistance to the rotation of the screw, and this is an important factor in the formation of back pressure.

The structure of the injection system: The check valve and nozzle structure in the injection system also affect the formation of back pressure. The check valve prevents melt backflow, increasing the resistance to forward flow of the melt; the diameter and internal structure of the nozzle influence the resistance as the melt passes through.

II. Effects of Backpressure

The size of the back pressure has a significant impact on the injection molding process and product quality.

Positive impact:

Negative impact:

III. Improvement Methods for Back Pressure

To address the impact of back pressure, the following methods can be adopted for improvement:

**Adjusting Backpressure Properly:** Set the backpressure appropriately based on the material properties, drying conditions, product structure, and quality requirements. Typically, backpressure should be adjusted within the range of 3–15 kg/cm³. For higher-viscosity plastics, the backpressure can be slightly increased, while for heat-sensitive materials, it should be set lower.

Optimize the injection system structure: Improve the nozzle structure to reduce the resistance when the melt passes through; optimize the design of the check valve to improve the flow efficiency of the melt.

Strengthen mold management: Inspect the gate size and type of the mold, adjust the back pressure according to the melt flow conditions; Regularly maintain the mold to keep it clean and in good condition.

Control plastic quality: Use plastic raw materials with stable quality, avoid using deteriorated or impure plastics; regularly clean the storage machine and injection molding machine to ensure the normal supply of materials.

In summary, back pressure plays an important role in the injection molding process. By reasonably adjusting back pressure, optimizing the structure of the injection system, strengthening mold management, and controlling plastic quality, the positive effects of back pressure can be fully utilized to improve product quality and production efficiency.