I. Causes of Gas Marks

Gas marks (also known as flow marks or silver streaks) are common yet challenging defects in medical injection-molded parts. These defects not only affect appearance but may also degrade mechanical properties, leading to issues such as compromised sealing in medical devices or drug compatibility problems. The causes of gas marks in medical injection molding are particularly unique:

1. Material Factors
   • The molecular weight distribution, viscosity, and additives (e.g., lubricants, flame retardants) of medical-grade plastics (such as PP, PE, PC, PPSU) directly influence melt flowability. For example, high-viscosity materials are prone to turbulence during filling, trapping gases and forming marks.
   • Material hygroscopicity: Materials like PA and PBT absorb moisture; if not fully dried, water vaporizes during melting, generating gas.
2. Mold Design Flaws
   • Improper runner system design: Excessively long runners, abrupt cross-sectional changes, or insufficient cold slug wells can cause uneven cooling of the melt front, leading to turbulence and gas entrapment.
   • Ineffective venting: Medical products require high cleanliness, so clogged or inadequately designed venting slots prevent gas escape.
   • Wall thickness variations: Thick-to-thin transitions increase the risk of weld lines and associated gas marks.
3. Process Parameter Miscontrol
   • Excessive injection speed: High-speed filling generates shear heat, causing localized overheating and material degradation, which produces gas.
   • Insufficient holding pressure: Fails to compact the melt adequately, leaving trapped gas.
   • Abnormal mold temperature: Low temperatures cause premature solidification, trapping gas; high temperatures lead to material decomposition and gas generation.
4. Environmental & Equipment Issues
   • Residual contaminants in the barrel: Degraded material from previous runs decomposes at high temperatures, producing gas.
   • Hydraulic system contamination: Oil pollution can infiltrate the melt, causing localized overheating.
   • Workshop humidity: High humidity accelerates moisture absorption in hygroscopic materials.

II. Detection & Evaluation of Gas Marks

Gas mark detection in medical parts must combine visual, performance, and regulatory assessments:

1. Visual Inspection
   • Use a standard light box (D65 light source) to observe surface defects at a 45° angle. Gas marks typically appear as silver-white or light-yellow streaks.
   • Focus on weld lines, gate areas, and thickness transitions.
2. Microscopic Analysis
   • Examine cross-sections via metallurgical microscopy or SEM to confirm gas entrapment morphology and material degradation.
3. Performance Testing
   • Sealing tests: For products like infusion sets or syringes, conduct water bath pressure tests.
   • Tensile strength tests: Gas mark regions may act as stress concentrators, reducing mechanical performance.
4. Regulatory Compliance
   • Ensure gas marks do not compromise functionality or biocompatibility per ISO 13485, FDA 21 CFR Part 820, etc.

III. Systematic Solutions

1. Material Optimization

• Drying Treatment: Dry hygroscopic materials (e.g., PA, PBT) at ≥120°C for 4–6 hours to reduce moisture content to <0.02%.
• Material Modification: Add 0.1%–0.5% nano-SiO₂ or talc to improve flowability and reduce turbulence.
• Alternative Materials: For gas-sensitive products, consider low-viscosity medical-grade materials (e.g., TPE, LCP).

2. Mold Improvements

• Runner Optimization: Use hot runner systems to minimize cold material; design circular or trapezoidal runner cross-sections with smooth transitions.
• Enhanced Venting: Add 0.02–0.05 mm venting slots at parting lines or use porous steel for venting.
• Gate Design: Employ submarine or pin gates to reduce melt impact; position gates away from thin-wall areas.

3. Process Adjustments

• Multi-Stage Injection: Use a “slow-fast-slow” injection profile to vent gas during initial filling, accelerate mid-filling, and slow down for holding pressure.
• Temperature Control: Set barrel temperatures as “higher front, lower rear”; adjust mold temperature per material (e.g., 80–100°C for PC, 40–60°C for PP).
• Holding Pressure Optimization: Apply 60%–80% of injection pressure for holding, with duration adjusted by wall thickness (1–5 seconds).

4. Equipment & Environment Management

• Barrel Cleaning: Thoroughly purge the barrel with cleaning material before production to remove contaminants.
• Hydraulic System Maintenance: Regularly replace hydraulic oil to prevent oil infiltration into the melt.
• Workshop Climate Control: Maintain 20–25°C and ≤50% humidity; install dehumidifiers in hygroscopic material production zones.

5. Process Monitoring & Traceability

• Inline Inspection: Deploy machine vision systems for real-time defect detection and SPC analysis of process variations.
• Batch Management: Document material drying records, mold maintenance logs, and process parameters for traceability.

IV. Case Study: Gas Mark Reduction in Infusion Set Components

A manufacturer faced gas marks on infusion set drip chambers, resulting in an 85% sealing test pass rate. Improvements included:

1. Material: Replaced PA66 with PA66+30%GF for better flowability; increased drying temperature from 80°C to 120°C.
2. Mold: Added venting slots at parting lines and enlarged gate diameter from 1.2 mm to 1.5 mm.
3. Process: Reduced injection speed from 80 mm/s to 60 mm/s; increased holding pressure from 120 MPa to 150 MPa.

Post-implementation, gas mark defects dropped to 2%, and sealing test pass rates rose to 99.5%.

V. Conclusion

Controlling gas marks in medical injection-molded parts requires a multi-dimensional approach integrating material, mold, process, equipment, and management optimizations. Enterprises should establish a “prevention-detection-improvement” closed-loop quality system, leveraging DFMEA and PFMEA tools to systematically reduce gas mark risks. As medical industry safety standards tighten, refined, data-driven injection molding process management will become a core competitive advantage.