In the medical injection molding industry, product cleanliness and safety directly impact patient health and treatment outcomes. However, black spots and impurities—common defects in injection-molded parts—not only compromise appearance but also risk bacterial growth, posing medical hazards. This article analyzes the root causes of black spots and impurities, offering systematic solutions through process optimization, material management, equipment maintenance, and more, to help enterprises enhance product quality and production efficiency.

1. Causes of Black Spots and Impurities

Black spots typically manifest as black, brown, or gray particles on or inside injection-molded parts, originating from:

• Material Issues: Contaminated recycled materials, foreign objects (e.g., metal shavings, fibers), or additive decomposition.
• Process Parameter Errors: Excessive temperature causing material degradation, high injection speeds generating shear heat, or insufficient backpressure allowing gas entrapment.
• Mold/Equipment Problems: Dirty mold cavities, poor exhaust design, worn screw/barrel components, or defective cold slug wells.
• Environmental/Operational Factors: Dust contamination in workshops or improper material handling during production changes.

2. Systematic Solutions

Material Management:

• Use medical-grade raw materials and avoid recycled materials.
• Dry materials thoroughly (e.g., PA at 120°C for 4 hours) to reduce moisture content.
• Add antioxidants to delay thermal degradation.

Process Optimization:

• Lower melt temperature (5–10°C below recommended values) and adopt multi-stage injection to minimize shear heat.
• Adjust backpressure (5–15 MPa) and shorten packing time.

Mold/Equipment Maintenance:

• Clean molds with ultrasonic systems and optimize exhaust slots (0.02–0.05mm width).
• Replace worn screw/barrel parts and improve cold slug well design.

Environmental Control:

• Maintain ISO 8-class cleanrooms and standardize operator protocols (e.g., dust-free clothing).

3. Case Study

A medical manufacturer reduced black spot defects from 3% to 0.1% by:

• Switching to virgin PP material and extending drying time.
• Lowering melt temperature to 260°C and cleaning exhaust channels.
• Enhancing workshop cleanliness and operator training.

4. Conclusion

Eliminating black spots requires holistic control across materials, processes, equipment, and environments. Future advancements in AI-driven quality monitoring and predictive analytics will further enable lean manufacturing in the medical sector.