Bubble formation at fixed positions in storage-type blow molding machines can be addressed by optimally adjusting backpressure parameters. Backpressure (plasticizing pressure) directly impacts melt density and gas expulsion efficiency. Improper settings may cause air or volatiles to remain in the melt, ultimately forming bubbles at specific product locations. Given your current production scenario, scientifically setting backpressure values combined with multi-stage control strategies is essential to enhance melt density and fundamentally reduce bubble formation.

I. Backpressure Setting Principles: Moderate Increase with Segmented Control
1. Initial Recommended Settings
For most general-purpose plastics (e.g., PE, PP), initially set backpressure between 6–10 MPa.
If noticeable bubbles appear near the gate, gradually increase to 8–12 MPa to enhance melt compression and force trapped gas out through the barrel front;
For thermosensitive materials like PVC and POM, maintain lower ranges (5–8 MPa) to prevent thermal decomposition from elevated temperatures.

Note: Higher back pressure is not always better; excessively high back pressure can cause adverse effects (see below).

2. Implement Multi-Stage Back Pressure Control

Modern injection molding machines support differential back pressure settings at different screw positions, a key technique for eliminating fixed-position bubbles:

Front-end material storage (within 70% of stroke): Use moderate back pressure (e.g., 6–8 MPa) to ensure efficient plasticization.
Late-stage material storage (final 30% stroke): Gradually increase back pressure to 10–12MPa while reducing screw speed to achieve “high pressure, low speed” melt compaction.
This method effectively minimizes screw overtravel, ensures consistent material storage volume per cycle, and prevents localized cavitation caused by loose melt.
II. Complementary Process Optimization: Collaborative Elimination of Bubble Root Causes

Adjusting backpressure alone is insufficient to resolve the issue completely. A closed-loop control system must incorporate the following measures:

Supporting Measures    Operational Key Points    Mechanism of Action
Reduce screw speed‌    Recommended control below 50 rpm (low-to-medium speed)    Minimizes shear heat, extends plasticization time, and facilitates degassing
Check raw material drying status‌    PE/PP materials: 80°C baking for 2–4 hours    Prevents moisture vaporization and bubble formation
Clean barrel and check ring    Regular disassembly inspection to remove carbon deposits and wear residues    Prevents density inconsistencies from melt backflow
Monitor melt temperature    Calibrate using temperature guns or built-in sensors    Prevents localized overheating that causes degradation and gas generation
III. Risks of Excessive vs. Insufficient Back Pressure
Back Pressure Level    Primary Issues    Impact on Products
Too Low (<6MPa)    Low melt density, trapped air, uneven plasticization    Prone to flash, cold spots, shrinkage
Too High (>15MPa)    Elevated melt temperature, intense shear, increased energy consumption    Causes material degradation, color deterioration, cold spots

Therefore, the optimal back pressure requires fine-tuning within the 8–12 MPa range through trial and error, observing changes in product appearance to find the balance between stability and efficiency.

IV. Practical Recommendations: Three-Step Adjustment Method

Step 1: Baseline Testing
Set back pressure to 8 MPa and screw speed to 40–50 rpm. Produce 10 molds continuously, recording bubble occurrence frequency and location.

Step 2: Gradient Increase
Increase backpressure by 1 MPa at a time while maintaining all other parameters constant, until bubbles significantly decrease or disappear.

Step 3: Stability Verification
Run continuously for 1 hour at the optimal parameters, monitoring product weight variation (should be ≤±2g) to confirm consistent plasticization.