I. Core Causes of Discoloration Issues in Medical Injection-Molded Products

1. Material Contamination and Degradation

• Raw Material Pollution: Medical-grade plastics (such as PPSU, PC, PEEK) can develop local color abnormalities if contaminated with dust, metal debris, or cross-contaminated with other materials during storage or transportation. For example, a medical device company once experienced mottled discoloration in injection-molded parts due to black rubber particles mixed into the raw material warehouse.
• Additive Decomposition: Additives commonly used in medical products, such as color masterbatches, antistatic agents, or lubricants, can decompose if they lack sufficient temperature resistance in the high-temperature barrel. For instance, a company using a color masterbatch rated for 180°C but operating the barrel at 220°C caused yellowing on the product surface.
• Moisture and Volatiles: If medical-grade plastics are not adequately dried (e.g., PC requires 120°C drying for 4 hours), moisture vaporizes under high temperatures, forming bubbles, while residual volatiles accelerate material degradation, leading to a darker color.

2. Uncontrolled Process Parameters

• Excessive Temperature: Medical products are highly sensitive to temperature. For example, a company producing syringes experienced edge burning and discoloration when the nozzle temperature rose from 230°C to 250°C.
• Shear Heat Effects: High screw speeds or injection rates generate excessive shear heat. For instance, a company manufacturing catheter connectors observed flow mark discoloration after increasing the screw speed from 80 rpm to 120 rpm.
• Prolonged Residence Time: Complex medical product structures may extend injection cycles. If materials remain in the barrel for over 10 minutes, thermal degradation can occur. For example, a company producing surgical instrument handles encountered uniform yellowing after a 20-minute mold stoppage.

3. Mold Design Defects

• Poor Venting: Inadequate venting in medical product molds can cause intense plastic-oxygen reactions under high pressure, leading to burn marks. For example, a company producing stethoscope earplugs observed black burn spots due to poor cavity venting.
• Undersized Gates: Medical products often feature small gates for precision, but overly narrow gates increase plastic flow resistance and local temperature rise. For instance, a company manufacturing microscope slide clips reduced the gate diameter from 0.8 mm to 0.5 mm, causing yellowing near the gate.
• Mold Release Agent Residue: Medical products frequently use silicone-based release agents for surface finish, but excessive application can cause whitening. For example, a company producing artificial joints encountered cloudy discoloration after over-spraying release agent by 30%.

4. Inadequate Equipment Maintenance

• Temperature Control System Failures: Aging thermocouples or inaccurate temperature controllers in medical injection molding machines can cause actual temperatures to deviate by over ±5°C from setpoints. For example, a company producing infusion pump housings experienced batch yellowing due to an 8°C controller error.
• Barrel Contamination: Medical products demand high cleanliness, but metal debris or residual materials from previous runs in the barrel can scratch plastic surfaces and cause discoloration. For instance, a company producing endoscope lens mounts observed silver scratches due to a 0.5 mm iron chip in the barrel.

II. Solutions to Discoloration Issues in Medical Injection-Molded Products

1. Optimized Material Control

• Dedicated Material Management: Medical companies should establish separate raw material warehouses with sealed packaging and dust prevention measures to avoid cross-contamination. For example, a company set up a dedicated drying room for PPSU materials, maintaining humidity ≤30%.
• Additive Screening: Prioritize medical-grade color masterbatches and additives, verifying their temperature resistance. For instance, a company confirmed via DSC testing that a color masterbatch decomposes at 280°C, higher than the actual barrel temperature of 250°C.
• Standardized Drying Processes: Develop drying parameter tables for medical materials, such as 120°C for 4 hours for PC and 150°C for 6 hours for PEEK.

2. Precise Process Parameter Control

• Temperature Gradient Management: Divide the barrel into 3–5 zones with temperature differentials ≤10°C. For example, a company producing cardiac stents set barrel temperatures at 220°C-230°C-240°C-235°C-230°C.
• Shear Heat Suppression: Optimize runner designs via Moldflow simulations to reduce injection speeds and screw speeds. For instance, a company lowered injection speed from 150 mm/s to 100 mm/s and screw speed from 100 rpm to 80 rpm, reducing discoloration rates by 70%.
• Residence Time Monitoring: Install barrel material level sensors to trigger alarms if materials exceed 8 minutes of residence time. For example, a company reduced material degradation rates from 5% to 0.3% through this measure.

3. Improved Mold Design

• Venting System Optimization: Add 0.02–0.05 mm deep venting slots at cavity ends and clean them regularly. For example, a company reduced burn marks in surgical knife handle molds from 15% to 0.5% after adding venting slots.
• Gate Structure Adjustment: Replace straight gates with fan gates or pin gates to reduce flow resistance. For instance, a company eliminated gate-area discoloration in syringe molds by switching to fan gates.
• Mold Release Agent Standardization: Establish spraying standards, such as 0.1–0.2 ml per mold. For example, a company reduced surface whitening rates from 8% to 0.2% using quantitative spraying devices.

4. Enhanced Equipment Maintenance

• Temperature Control System Calibration: Calibrate barrel temperatures quarterly using infrared thermometers, replacing thermocouples if errors exceed ±3°C. For example, a company improved temperature control precision to ±1.5°C through this practice.
• Barrel Cleaning Cycles: Clean barrel interiors with copper brushes every 5,000 mold cycles and inspect screw wear. For example, a company reduced discoloration caused by barrel debris by 90% through regular cleaning.

III. Preventive System for Discoloration Issues in Medical Injection-Molded Products

1. Discoloration Issue Database: Record discoloration types, locations, and causes for each batch, using big data analytics to identify high-frequency problem points. For example, a company discovered that 70% of discoloration issues occurred near gates, prompting gate design optimizations.
2. SPC Process Control: Implement control charts for key process parameters (e.g., temperature, pressure, speed) to trigger automatic shutdowns if parameters exceed control limits. For example, a company reduced discoloration rates from 3% to 0.1% through SPC control.
3. Discoloration Simulation Experiments: Use DSC, TGA, and other instruments to simulate material degradation under different temperatures and durations, establishing process parameter boundaries in advance. For example, a company determined that PC materials should not exceed 5 minutes of residence time at 260°C.

Conclusion

Discoloration issues in medical injection-molded products involve multidisciplinary challenges across material science, fluid mechanics, and thermodynamics. A holistic approach spanning raw material selection, process design, mold optimization, and equipment maintenance is essential. By establishing data-driven preventive systems, medical companies can reduce discoloration-related scrap rates from the industry average of 2%–5% to below 0.5%, significantly enhancing product competitiveness and patient safety.