Medical injection-molded products, due to their direct contact with the human body or use in precision medical treatments, demand extremely high standards of safety and reliability. However, cracks—a common defect in injection molding—not only compromise product aesthetics but also pose significant risks to functionality, potentially leading to medical accidents. This article systematically explores the root causes of cracks in medical injection-molded products from four key dimensions: material properties, process parameters, mold design, and post-processing.

1. Material Factors: The Invisible Driver of Internal Defects

• Improper Material Selection: Medical products often use high-transparency, high-strength materials (e.g., PC, PPSU). If the material lacks toughness or impact resistance, cracks may form under stress. For example, PC, while highly transparent, is prone to photodegradation under UV exposure, increasing brittleness.
• Incompatible Additives: Additives like plasticizers or stabilizers may phase-separate if incompatible with the base material, creating stress concentration points. Some plasticizers, for instance, can migrate during high-temperature sterilization, causing localized embrittlement.
• Excessive Recycled Material: Medical-grade products require strict material purity. High recycled content (e.g., >30%) may introduce impurities or degradation products, reducing mechanical performance. Tests show a 40% drop in impact strength under such conditions.

2. Process Parameters: The Direct Consequence of Thermal-Mechanical Imbalance

• Uncontrolled Injection Pressure/Speed: Excessive pressure induces turbulence, creating weld lines, while overly fast speeds generate shear stress, breaking molecular chains. A medical device housing cracked near the gate due to rapid injection.
• Imbalanced Mold Temperature/Cooling Time: Low mold temperatures cause rapid solidification, trapping internal stress. Insufficient cooling leads to post-molding shrinkage, increasing residual stress. Studies show a 20% rise in crack risk for every 10°C drop in mold temperature.
• Inadequate Packing Pressure: Low packing pressure fails to compensate for material shrinkage, forming voids or sink marks that act as crack initiation sites. A syringe piston leaked due to insufficient packing.

3. Mold Design: The Root of Structural Defects

• Uneven Wall Thickness: Thickness variations (>3mm) cause differential cooling, concentrating stress in thick sections. A bottle cap cracked at the edge due to a 3mm wall-thickness difference.
• Insufficient Draft Angle: Draft angles <0.5° increase friction during ejection, causing surface scratches that propagate into cracks. A medical connector failed after repeated ejection with inadequate draft.
• Poor Gate Placement: Gates located at weak points (e.g., corners) align melt flow with stress directions, exacerbating crack risk. Relocating the gate reduced crack rates from 15% to 2%.

4. Post-Processing and Environmental Factors: The Compounding Effect of External Stressors

• Improper Sterilization: Ethylene oxide (EO) sterilization may leave acidic residues, accelerating hydrolysis, while steam sterilization induces thermal aging. A silicone catheter developed surface cracks after repeated steam sterilization.
• Harsh Storage Conditions: Prolonged UV or humidity exposure triggers photodegradation or hydrolysis. PC stored at 85°C/85%RH for 6 months lost 50% of its impact strength.
• Mechanical Fatigue: Repeated bending/stretching (e.g., catheter insertion) exceeds material fatigue limits, initiating and propagating cracks. An endoscope tip cracked after frequent flexing.

5. Solutions and Preventive Measures

• Material Optimization: Use medical-grade alloys (e.g., PC/ABS) for balanced properties. Limit recycled content to ≤15% and add UV/hydrolysis stabilizers.
• Process Adjustments: Employ multi-stage injection to control speed/pressure. Optimize mold temperature (60–80% of melt temperature) and cooling time. Increase packing pressure/duration.
• Mold Improvements: Uniform wall thickness (Δ≤0.3mm), increased draft angles (1–2°), and optimized gate placement.
• Post-Processing Reinforcement: Replace EO with low-temperature plasma sterilization. Store at 25°C/50%RH and conduct fatigue testing.