I. Introduction

Back pressure is a critical plasticizing parameter in injection molding, influencing melt uniformity, gas removal, and production efficiency. Incorrect settings can cause defects like material degradation, gas marks, or inconsistent plasticizing. This article provides a scientific approach to back pressure setting, tailored to material properties and production requirements.

II. Core Basic Principles

Back pressure should follow the principles of adaptability, margin, and coordination:

Adaptability: Match the material’s melt flow index (MFI)—higher MFI materials require lower back pressure to prevent drooling.

Margin: Set back pressure to 5-10% of injection pressure (maximum 15%) to avoid overloading the hydraulic system.

Coordination: Adjust back pressure in tandem with screw speed and barrel temperature to form a balanced plasticizing system. Aim for the lowest back pressure that meets plasticizing quality requirements.

III. Basic Setting Range by Raw Materials

General Non-Crystalline Plastics (PS, ABS): 1.0-2.0MPa (increase by 0.3-0.5MPa for high-gloss ABS).

General Crystalline Plastics (PE, PP): 0.5-1.5MPa (reduce by 0.2-0.3MPa for thin-wall parts).

Engineering Plastics (PC, PMMA): 1.5-3.0MPa (higher for transparent parts to remove gas); PBT/PA6: 1.0-2.0MPa.

Modified Plastics: Glass fiber-reinforced: 1.5-2.5MPa (avoid high pressure to prevent fiber breakage); mineral-filled: 1.0-2.0MPa.

Special Materials (PVC, TPE): 0.3-0.8MPa (PVC) and 0.5-1.0MPa (TPE/TPR) to prevent degradation and drooling.

IV. Fine-Tuning Methods

Appearance Defects: Increase back pressure by 0.3-0.5MPa for gas marks/sink marks; decrease by 0.5-1.0MPa for discoloration/burn marks.

Product Structure: Raise back pressure by 0.3-0.5MPa for thick-walled parts; lower it for thin-wall parts to maintain flowability.

Production Efficiency: Increase back pressure by 0.2-0.3MPa (with higher screw speed) to shorten pre-plasticizing time, if melt quality remains consistent.

V. Coordination with Other Parameters

Screw Speed: Higher screw speed increases shear, so reduce back pressure by 0.3-0.5MPa to prevent overheating.

Barrel Temperature: Lower barrel temperatures may require slightly higher back pressure to aid plasticizing; higher temperatures need lower back pressure to avoid degradation.

Feeding Amount: Adjust back pressure based on material volume—increase for overfeeding, decrease for underfeeding.

VI. Conclusion

Proper back pressure setting is essential for consistent melt quality and production efficiency. By aligning settings with material characteristics and production goals, manufacturers can minimize defects and optimize performance.