1. The Unique Risks of Cracking in Medical Products

The hazards of cracking in medical injection-molded products far exceed those in ordinary industrial items:

• Biocompatibility risks: Cracks may become breeding grounds for bacteria, increasing infection risks.
• Functional failure risks: For example, cracking in syringe connectors can lead to liquid leakage.
• Regulatory compliance risks: Certification systems like FDA and CE impose strict requirements on product integrity.

Case Study: A brand of disposable syringes experienced piston detachment due to cracking at the ejector pin area, triggering a mass recall. This underscores the critical importance of cracking control in medical products.

2. Material Selection and Pre-Treatment

2.1 Medical-Grade Material Adaptability

• Priority choices: Polycarbonate (PC), Polyphenylsulfone (PPSU), and other chemically resistant materials.
• Special requirements:
  • DEHP-free PVC requires special stabilizers.
  • Transparent components demand colorless, UV-resistant PC.
• Recycled material restrictions: Medical products must use virgin medical-grade materials; recycled materials are prohibited.

2.2 Pre-Drying Process Optimization

• Drying parameters:
  • PC: 120°C/4–6 hours (dew point ≤ -40°C).
  • PPSU: 150°C/8 hours (nitrogen protection required).
• Equipment requirements: Use dehumidifying dryers with humidity monitoring to ensure moisture content ≤ 0.02%.

3. Key Control Points in Mold Design

3.1 Ejection System Optimization

• Balanced ejection: Multi-point ejection with ejector pins ≥3mm in diameter and spacing ≤50mm.
• Draft angles: Sidewall draft angles ≥3°, rib areas ≥5°.
• Surface treatment: Cavity surfaces coated with hard chromium (HV ≥ 800) and polished to Ra ≤ 0.2μm.

3.2 Gating System Design

• Gate selection:
  • Thin-walled parts (≤2mm): Submarine gates, 0.8–1.2mm in size.
  • Thick-walled parts (>2mm): Fan gates, width 1.5× wall thickness.
• Runner optimization: Use hot runner systems with temperature control accuracy ±1°C.

3.3 Structural Stress Design

• Fillet transitions: All internal corners R ≥ 0.5mm, external corners R ≥ 0.3mm.
• Rib design: Thickness ≤60% of main wall thickness, height ≤3× wall thickness.
• Metal insert treatment: Preheat inserts to 120–150°C and apply nickel plating.

4. Precision Control of Molding Processes

4.1 Temperature Management

• Barrel temperatures:
  • PC: 280–320°C (five-zone control).
  • PPSU: 340–380°C (requires dedicated high-temperature screw).
• Mold temperatures:
  • Transparent parts: 80–100°C (oil temperature control).
  • Non-transparent parts: 60–80°C (water temperature control).

4.2 Pressure Curve Optimization

• Injection pressure: Multi-stage control (example):
  • First stage: 80% max pressure (filling).
  • Second stage: 60% max pressure (packing).
  • Third stage: 40% max pressure (compensation).
• Packing time: Set based on wall thickness (2–3 seconds per 1mm).

4.3 Speed Control Strategy

• Injection speed: Slow-fast-slow three-stage control:
  • Initial stage: 10–20% max speed (prevents vortexing).
  • Middle stage: 80–90% max speed (filling).
  • Final stage: 30–40% max speed (prevents overfilling).

5. Environmental and Post-Processing Controls

5.1 Cleanroom Requirements

• Temperature/humidity control: 23 ± 2°C, 45 ± 5% RH.
• Cleanliness: ISO Class 7 (10,000) or higher.
• Static control: Floor resistance 10⁶–10⁹Ω, personnel wear ESD garments.

5.2 Post-Processing Techniques

• Annealing:
  • PC products: 110°C/2 hours (air circulation oven).
  • PPSU products: 180°C/4 hours (nitrogen protection required).
• Surface treatment: Plasma treatment for critical contact surfaces to enhance surface energy.

6. Quality Inspection and Traceability

6.1 Online Inspection Systems

• Visual inspection: High-resolution CCD cameras (resolution ≥ 0.02mm).
• Stress detection: Polariscopes for residual stress distribution.
• Dimensional inspection: Coordinate measuring machines (accuracy ±0.005mm).

6.2 Batch Traceability System

• Material traceability: Each batch includes COA (Certificate of Analysis) and MSDS.
• Process traceability: Record key parameters (temperature, pressure, time).
• Product traceability: Laser marking or RFID chips for full lifecycle tracking.

7. Typical Case Studies

Case 1: Cracking in Syringe Connectors

• Problem: Ring-shaped cracks after 6 months of storage.
• Root causes:
  1. Large CTE mismatch between aluminum insert (23×10⁻⁶/°C) and PC (65×10⁻⁶/°C).
  2. Unpreheated inserts causing interface stress concentration.
• Solutions:
  1. Switch to PPSU (CTE 50×10⁻⁶/°C).
  2. Preheat inserts to 150°C.
  3. Add a 0.2mm silicone buffer layer.

Case 2: Cracking in Syringe Piston Ejector Areas

• Problem: Whitening cracks at the ejector pin.
• Root causes:
  1. Insufficient draft angle (actual 2° vs. designed 3°).
  2. Rough ejector pin surface (Ra 0.8μm, exceeding standard).
• Solutions:
  1. Increase draft angle to 3.5°.
  2. Hard chrome-plate and polish ejector pins to Ra 0.2μm.
  3. Reduce ejection speed from 150mm/s to 80mm/s.

8. Future Technological Directions

1. Smart injection systems: Integrate pressure, temperature, and stress sensors for real-time closed-loop control.
2. Nanocomposite materials: Develop self-healing nano-coatings.
3. Simulation optimization: Use CAE software to predict cracking risks.
4. 3D-printed molds: Accelerate mold verification and shorten development cycles.

Conclusion: Cracking control in medical injection-molded products requires a holistic quality management system from material selection to post-processing. By implementing the proposed solutions, a medical enterprise reduced cracking rates from 3.2% to 0.15%, saving over $500,000 annually in quality losses. As reliability demands rise, refined and intelligent cracking control technologies will become core competitive advantages in the industry.