Bubbles appearing at the fixed discharge position of a storage-type blow molding machine are typically closely related to gas entrapment during raw material plasticization, poor mold venting, or improper process parameter settings. Given your plastic product manufacturing context, this issue directly impacts product yield and surface quality, necessitating systematic troubleshooting across four dimensions: materials, equipment, process, and molds.

I. Core Cause Analysis and Targeted Solutions
1. Gas Entrapment During Screw Plasticization (Primary Cause)

During material storage, improper plasticization parameters can cause raw material to incorporate air into the melt, ultimately forming fixed-position bubbles near the discharge point.

Common Contributing Factors:

- Excessively fast storage speed, allowing material to enter the metering section without sufficient compaction;
Insufficient backpressure setting, failing to effectively expel trapped gases from the melt;
Excessive barrel temperature causing localized thermal decomposition of raw material and gas generation.

Solutions:

Appropriately increase storage backpressure (recommended gradual adjustment to 8–12 MPa) to enhance melt density and reduce gas retention;
Reduce storage speed to extend plasticization time, ensuring uniform melting of raw material;
Verify barrel temperature settings to prevent excessive feed section temperatures causing reflux and material return; maintain reasonable temperature gradients across sections.
2. Poor Mold Venting (Secondary but Critical Factor)

Even with normal melt gas content, inadequate mold venting channels allow gas accumulation at the filling end (near the gate) to form trapped gas bubbles.

Key Inspection Points:

Verify venting grooves are present at parting lines, slides, ejector pins, etc.;
Confirm vent groove depth aligns with material properties (typically 0.02–0.04mm for PE);
Observe if bubbles concentrate at corners or junctions to determine if flow front entrained air.

Optimization Measures:

Add or deepen vent grooves in bubble-prone areas; install venting pins if necessary;
Clean existing venting channels to prevent carbon buildup or dust blockages;
Relocate gate position to thick-walled areas to improve gas escape pathways.
3. Mismatched Injection Process Parameters

High injection speeds or insufficient holding pressure prevent timely gas escape, causing residual gas to form bubbles in the melt.

Adjustment Recommendations:
Implement multi-stage injection control: Initial medium-speed filling, switching to low speed near the end to facilitate venting;
Extend holding pressure time to ensure complete melt shrinkage, reducing false vacuum bubble detection;
Moderately reduce injection pressure to prevent material degradation and gas generation from high-speed shear.
4. ‌Excessive moisture or volatile content in raw material‌

If raw material is insufficiently dried, moisture vaporization at high temperatures can form bubbles in relatively fixed locations.

Countermeasures:
For hygroscopic materials like PE and PP, pre-dry thoroughly (recommended: 80°C baking for 2–4 hours);
Inspect raw material for moisture-induced clumping or excessive recycled content;
Regularly clean hoppers to prevent bridging and ensure uniform feeding.
II. Rapid Diagnosis Flowchart (Auxiliary Judgment)
Symptom Characteristics    Possible Type    Preliminary Judgment
Bubbles visible immediately after mold opening, often at the end of filling    Trapped air bubbles    Process or venting issues
Bubbles appear after cooling, concentrated in thick-walled ribs    Vacuum bubbles (shrinkage cavities)    Insufficient holding pressure or uneven cooling
Bubbles accompanied by black spots or scorch marks    Thermal decomposition bubbles    Excessive temperature or prolonged dwell time
Bubbles appear in fixed locations and recur across batches    Structural issues with raw material or equipment    Inspect cleanliness of sprue and runner

Tip: Identify bubbles by sound when crushing samples—bubbles burst with a “pop,” while vacuum bubbles are silent.