Back Pressure in Injection Molding: Typical Ranges & Guidelines

Back pressure is a critical process parameter in injection molding, referring to the resistance applied to the screw during its backward rotation (plasticizing stage). Proper back pressure helps improve melt homogeneity, color dispersion, and degassing, thereby reducing defects such as silver streaks, bubbles, and flow marks. However, setting back pressure too high or too low can negatively impact both part quality and production efficiency.

1 Typical Back Pressure Range

In general, the industry-recognized range for back pressure is 5–30 bar. The optimal value depends on the material’s viscosity, flowability, thermal stability, and the specific requirements of the molded part.

2 Material-Specific Back Pressure Guidelines

General-purpose plastics (PP, PE, ABS)

For materials with good flowability and moderate viscosity, a back pressure of 10–20 bar is commonly used. This range ensures sufficient melting and mixing without causing excessive shear heating or prolonging the plasticizing cycle.

Engineering plastics with low flowability (PC, PMMA)

Materials like PC and PMMA require better melt uniformity to fill complex cavities and maintain dimensional stability. A higher back pressure of 15–30 bar is often recommended to enhance mixing and packing of the melt.

Heat-sensitive materials (PVC, POM)

PVC, POM, and similar materials are prone to degradation when subjected to high shear stress and temperature. To minimize the risk of decomposition, back pressure should be kept low, typically between 5–10 bar.

3 Effects of Back Pressure on Molding

Positive effects

Increasing back pressure can:

Eliminate air bubbles and volatiles by improving degassing.

Enhance the dispersion of colorants and additives, resulting in a more uniform appearance.

Improve melt density and consistency, leading to better dimensional stability.

Negative effects

Excessively high back pressure can:

Extend the plasticizing time, increasing the overall cycle time.

Increase the motor load and energy consumption.

Cause overheating of the melt, leading to material degradation, discoloration, or even nozzle drooling.

4 Practical Setup and Adjustment Principles

When setting back pressure, it is advisable to start at the midpoint of the recommended range for the specific material. During trial runs, observe the part surface for streaks or bubbles. If such defects appear, gradually increase the back pressure by 5–8 bar and re-evaluate. Conversely, if the part shows signs of degradation or the cycle time becomes too long, reduce the back pressure accordingly. The goal is to use the minimum necessary back pressure that achieves the desired part quality while maintaining a stable and efficient production cycle.