Back pressure is a key parameter that affects melt quality, appearance, dimensional stability, and efficiency. Reasonable back pressure eliminates bubbles, improves color mixing, and ensures consistent product weight. This article explains the definition, functions, calculation, setting steps, and troubleshooting of back pressure for on-site application.
I. What is Injection Molding Back Pressure
Back pressure, also called plasticizing pressure, is the resistance applied to the screw during rotation and retraction. It compacts the melt, removes gas, improves mixing, and stabilizes temperature and flow.
II. Functions of Back Pressure
Compact molten plastic for consistent weight and reduced shrinkage.
Remove air, moisture, and volatile components to eliminate splay marks and bubbles.
Improve color dispersion and prevent streaks or uneven coloring.
Stabilize melt temperature for better flow and filling.
III. Effects of Improper Back PressureLow Back Pressure
Insufficient plasticizing, loose melt, bubbles, poor color mixing, unstable weight, and dimensional variation.
High Back PressureExcessive shear heat, material degradation, yellowing, burning, longer cycle time, increased wear, and warping.
IV. Recommended Back Pressure for Common Materials
| Material | Back Pressure (MPa) | Description |
|---|
| PP, PE | 3–7 | Good flowability |
| ABS, PS | 4–8 | General purpose |
| PC, PMMA | 8–12 | Transparent parts |
| PA, PA66 | 5–10 | Must be dried |
| POM, PVC | 0.5–3 | Heat-sensitive |
| Glass-filled | 7–15 | Improve dispersion |
V. Practical Setting Steps
Start from the lower limit of the recommended range. Run 5–10 cycles and observe appearance. Increase by 0.5–1 MPa each time until defects disappear. Stop increasing if yellowing or burning occurs. Match screw speed: higher speed requires lower back pressure. Ensure the feeding time does not exceed cooling time.
VI. Fine-Tuning for Different ProductsTransparent and high-gloss parts require higher back pressure. Thin-walled and long-flow parts need medium-to-high back pressure. Thick-walled parts use medium-to-low back pressure. Heat-sensitive materials use low back pressure. Glass-filled materials use medium-to-high back pressure.
VII. Common Problems and Solutions
Bubbles and splay marks: increase back pressure and check drying.
Uneven color: increase back pressure and check color mixing.
Burning or black spots: reduce back pressure and barrel temperature.
Slow cycle: reduce back pressure and increase screw speed.
Unstable weight: stabilize back pressure and check check-ring.
VIII. Important NotesBack pressure should be sufficient but not excessive. Prioritize material drying before adjusting back pressure. Re-adjust after material or color change. Regularly maintain the check-ring. Small machines require lower back pressure.
SummaryBack pressure setting follows the principle of material adaptation, gradual adjustment, and balance between quality and efficiency. Proper back pressure improves molding stability, reduces defects, extends equipment life, and supports consistent mass production.
