Bubble Defects in Plastic Injection Parts: Causes and Solutions

1. Core Causes of Bubble Defects

1.1 Raw Material and Temperature-Related Factors

• Excessive Temperature: Prolonged exposure to high temperatures causes plastic degradation, releasing gas that forms bubbles. Overheating can also lead to black decomposition lines; mild cases can be resolved by cooling, while severe barrel decomposition requires disassembly and cleaning.

• Moisture and Volatiles in Raw Materials: Unprocessed or improperly stored raw materials contain excess moisture or volatile substances, which vaporize during molding and form bubbles.

• Poor Raw Material Quality: Impure raw materials or incompatible additives may react at high temperatures, generating gas.

1.2 Process Parameter Issues

• Improper Injection Pressure/Speed: Excessively high injection pressure (often caused by overfeeding) or speed traps air in the melt. In high-pressure environments, trapped air is scorched by friction and encapsulated in the product, forming internal cavities—especially in solid plastic injection parts.

• Insufficient Cooling Time: Incomplete cooling of thick-walled parts leads to internal shrinkage, creating voids that appear as bubbles. External cold pressing molds can be used to supplement cooling if needed.

1.3 Custom Plastic Molds Design and Maintenance Problems

• Poor Ventilation: Accumulated debris on the mold cavity or inadequate venting channels prevent air from escaping, leading to bubble formation—particularly in thick-walled sections.

• Undersized Gates or Imbalanced Gating: Mismatched gate sizes and product weights in multi-cavity custom plastic molds cause uneven filling, with thick parts more prone to bubbles. Undersized gates increase flow resistance, trapping gas.

• Lack of Regular Maintenance: Neglected mold cleaning leads to residue buildup, blocking vents and disrupting normal filling.

2. Targeted Solutions for Common Bubble Scenarios

2.1 Surface Swelling and Bubbles in Thick-Walled Parts

• Phenomenon: Surface swelling of thick-walled products, with visible bubbles in cross-sections after cutting.

• Solutions:

• Adjust temperature parameters to avoid overheating and plastic degradation.

• Strictly control raw material quality: use dry, pure materials and store them in moisture-proof conditions.

• Optimize cooling: extend cooling time or use external cold pressing molds to ensure full solidification.

• Check and adjust feeding quantity to prevent overfeeding-induced high pressure.

2.2 Internal Cavities with Blackening in Solid Parts

• Phenomenon: Internal voids in solid plastic injection parts, often accompanied by blackening (caused by scorched trapped air).

• Solutions:

• Reduce injection pressure and slow down injection speed to minimize air entrapment.

• Regularly verify feeding quantity to avoid overfilling the barrel.

• Enhance custom plastic molds ventilation: clean vents regularly and add vent grooves at key gas-trapping points.

2.3 Bubbles in Thick Sections of Products

• Phenomenon: Bubbles concentrated in thick areas of plastic injection parts, especially in multi-cavity molding.

• Solutions:

• Optimize custom plastic molds gating system: balance gate sizes with product weights to ensure uniform filling.

• Avoid overly thick part designs; use gradual thickness transitions instead of abrupt changes.

• Improve mold ventilation: add vent channels near thick sections and clean mold residues regularly to maintain smooth air escape.

3. Comprehensive Prevention and Control Measures

3.1 Raw Material Management

• Pre-dry raw materials (especially moisture-sensitive plastics) to remove moisture and volatiles.

• Select high-purity raw materials and compatible additives to avoid thermal reactions.

3.2 Process Parameter Optimization

• Fine-tune temperature, injection pressure, and speed based on material properties and part structure—avoid overheating, overpressure, or excessive speed.

• Extend cooling time appropriately for thick-walled parts to ensure complete solidification.

3.3 Custom Plastic Molds Optimization and Maintenance

• Design adequate venting channels (recommended depth: 0.02–0.05mm) at flow ends, weld lines, and thick sections.

• Balance gate sizes in multi-cavity molds and avoid undersized gates.

• Establish a regular mold maintenance system: clean residues, inspect vents, and polish cavities to ensure smooth filling.

4. Conclusion