Adjusting back pressure is one of the most critical yet misunderstood aspects of the injection molding process. While it might seem like a minor setting, it directly impacts the homogenization of the melt, the density of the part, and the overall stability of your production cycle.
Here is a comprehensive guide on how to understand and adjust back pressure effectively.
1. Understanding the Role of Back Pressure
Back pressure is the amount of pressure the injection cylinder exerts against the screw as it moves backward during the “plasticizing” (dosing) phase. Instead of the screw simply retreating as it melts plastic, back pressure forces the screw to work harder to move back.
Why do we use it?
- Homogenization: It ensures the color and temperature of the melt are uniform.
- Air Removal: It squeezes out trapped air or volatiles from the resin.
- Shot Consistency: It ensures each “shot” of plastic has the same density, preventing weight fluctuations in parts.
2. Step-by-Step Adjustment Procedure
Step 1: Start with a Baseline
Always begin with the material manufacturer’s recommendations. Most resins (like PP or ABS) require a back pressure between 50 to 150 psi (plastic pressure). If you are using a hydraulic machine, remember to distinguish between hydraulic gauge pressure and actual plastic pressure.
Step 2: Observe the Melt Quality
Purge a small amount of material and inspect it.
- If the melt looks “marbled” or has color streaks: Your back pressure is too low. Increase it in increments of 10–20 psi.
- If you see bubbles or “splay” (silver streaks) in the molded part: This often indicates trapped air. Increasing back pressure will help “squeeze” that air back through the hopper.
Step 3: Monitor Screw Recovery Time
As you increase back pressure, the screw will take longer to reach its set position.
- The Rule of Thumb: The screw should finish rotating (dosing) about 1 to 2 seconds before the mold opens. If back pressure is so high that the screw is still turning when the mold is ready to open, you are wasting cycle time.
3. The “Sweet Spot” and Potential Risks
Finding the balance is key. Adjusting back pressure is a game of trade-offs.
| Pressure Level | Benefits | Risks |
| Too Low | Faster cycle times; low material degradation. | Poor color mixing; air bubbles; inconsistent part weight. |
| Too High | Excellent color/heat uniformity; high part density. | Excessive screw wear; material degradation (burning); increased cycle time. |
Signs of Excessive Back Pressure
If you go too high, the mechanical shearing of the plastic generates frictional heat. This can lead to “degradation,” where the plastic chemically breaks down, resulting in brittle parts or dark “burn marks.” Furthermore, for glass-filled materials, high back pressure can actually break the glass fibers, reducing the structural integrity of the final product.
4. Final Verification
Once you have made adjustments:
- Check Part Weight: Run 10 consecutive cycles and weigh the parts. If the weight varies significantly, increase the back pressure slightly to stabilize the melt density.
- Visual Inspection: Look for “splay” or “silvering” on the surface.
- Nozzle Drool: If plastic is oozing out of the nozzle excessively (drooling) after dosing, you may have too much back pressure or need to adjust your decompression (suck-back) settings.
Conclusion
Adjusting back pressure is not a “set it and forget it” task. It requires a balance between melt quality and cycle efficiency. Always adjust in small increments, allow the machine to stabilize for 5–10 cycles after each change, and document your results. For more information about injection molding, pls visit our website.
