Medical injection-molded parts, due to their stringent requirements for precision and quality, are significantly impacted by surface streak defects, which directly affect product performance and safety. This article systematically analyzes the causes of streak defects from four dimensions: material selection, mold design, injection molding process parameter optimization, and production environment control. Targeted solutions are proposed. Through practical case verification, the defect rate of streak defects can be reduced to below 0.5% after process optimization, providing a reference for the high-quality production of medical injection-molded parts.

1. Introduction

Medical injection-molded parts are widely used in surgical instruments, diagnostic equipment, implants, and other fields. Their surface quality directly influences product sealing, biocompatibility, and service life. Streak defects (such as flow marks, weld lines, and silver streaks) not only affect appearance but may also become stress concentration points, leading to cracking or functional failure during use. Therefore, resolving streak issues is crucial for enhancing the quality of medical injection-molded parts.

2. Causes of Streak Defects

2.1 Material Factors

• Insufficient material fluidity: High-viscosity or low-melt-flow-rate materials are prone to flow marks when filling molds, especially in thin-walled structures or parts with complex geometries.
• Uneven distribution of additives: Additives such as lubricants and color masterbatches that are not fully mixed can lead to local density differences, resulting in streaks.
• Material degradation: High temperatures or prolonged heating can cause material decomposition, generating gases or carbonized substances that form silver streaks.

2.2 Mold Design Defects

• Improper gate location: An unreasonable number, location, or size of gates can lead to uneven melt flow, resulting in weld lines.
• Unreasonable runner design: Long runners, abrupt cross-sectional changes, or uneven cooling in the runner system can cause significant temperature gradients in the melt, leading to flow marks.
• Insufficient venting system: Poor mold venting can trap gases in the cavity, forming bubbles or silver streaks.

2.3 Injection Molding Process Parameters

• Low injection speed: Slow melt filling speed allows the front end to cool and solidify, while the rear end pushes, creating flow marks.
• Improper melt temperature: Low temperatures reduce fluidity, while high temperatures can trigger material degradation.
• Insufficient packing pressure: Inadequate pressure during the packing stage can result in uneven melt shrinkage, causing depressions or streaks.
• Insufficient cooling time: Premature ejection of parts that have not fully cooled can lead to shrinkage deformation or surface streaks.

2.4 Production Environment Factors

• High ambient humidity: Hygroscopic materials (such as PA and PBT) can absorb moisture in humid environments, decomposing at high temperatures to produce gases that form silver streaks.
• Insufficient equipment cleanliness: Residual impurities in the screw or barrel can mix into new materials, causing streaks.

3. Solutions and Implementation Steps

3.1 Material Optimization

• Select high-fluidity materials: Choose materials with a higher melt flow rate (MFR) based on product requirements, or add flow modifiers.
• Optimize additive ratios: Ensure uniform dispersion of lubricants, color masterbatches, and other additives to avoid local high concentrations.
• Strict drying treatment: Pre-dry hygroscopic materials (e.g., PA should be dried at 80-100°C for 4-6 hours) to reduce moisture content to below 0.02%.

3.2 Mold Design Improvements

• Optimize gate design: Use multi-gate or hot runner systems to balance melt flow; match gate size to part wall thickness to avoid excessive shear stress.
• Improve runner structure: Shorten runner length, adopt gradual cross-sectional designs to reduce pressure loss; place cold slug wells at runner ends to capture front-end cold material.
• Enhance venting system: Add venting slots (depth 0.02-0.05mm) at parting lines and between cores and cavities, or use vacuum venting technology.

3.3 Process Parameter Adjustments

• Segmented injection control: Use multi-stage injection speeds, filling quickly at the front end to reduce cooling and slowing down at the rear end to avoid vortices.
• Temperature gradient management: Maintain melt temperatures 5-10°C above the lower limit of the recommended range and ensure uniform mold temperature (error ≤2°C) to avoid local overheating or undercooling.
• Packing pressure optimization: Set packing pressure at 50%-80% of the injection pressure, with duration adjusted based on part wall thickness (1-2 seconds for thin-walled parts, 3-5 seconds for thick-walled parts).
• Cooling time control: Ensure cooling time allows the part center temperature to drop below the material's heat distortion temperature, monitored in real-time with an infrared thermometer.

3.4 Production Environment Control

• Control workshop humidity: Use dehumidifiers to maintain ambient humidity below 40%, especially in areas producing hygroscopic materials.
• Regular equipment cleaning: Clean the screw, barrel, and mold surfaces after each shift to avoid impurity residue; replace filters regularly (suggested every 5,000 mold cycles).

4. Case Study

A medical enterprise producing disposable syringe pistons encountered periodic streak defects with a defect rate of 5%. Analysis revealed:

1. Material issue: PA66 material was not fully dried, with a moisture content of 0.15%.
2. Mold defect: The gate location was off-center, causing uneven melt flow.
3. Process parameter issue: Low injection speed (30 mm/s) and insufficient packing pressure (40 MPa).

Improvement measures:

1. Dry PA66 at 100°C for 6 hours, reducing moisture content to 0.02%.
2. Redesign the gate location to a central position.
3. Adjust injection speed to 80 mm/s and packing pressure to 60 MPa.
4. Increase mold venting slot depth to 0.03 mm.

Effect verification: After improvements, the streak defect rate dropped to 0.3%, and product qualification rate rose to 99.5%, meeting medical-grade standards.

5. Conclusion

Resolving streak defects in medical injection-molded parts requires a comprehensive approach addressing material, mold, process, and environmental factors. By optimizing material fluidity, improving mold design, precisely controlling process parameters, and strictly managing the production environment, the streak defect rate can be significantly reduced, enhancing product quality stability. In practice, it is recommended to combine the Design of Experiments (DOE) method to quickly identify key factors and validate improvement effects, achieving efficient and cost-effective quality enhancement.