In the field of medical injection molding, the phenomenon of top-white on products has become a core pain point restricting the yield rate. Take precision components such as surgical instrument casings and implant stents as examples. Top-white not only leads to appearance defects but may also cause structural failures due to stress concentration, directly affecting product safety and service life. This article, considering the characteristics of the medical industry, systematically expounds on the root causes and solutions of top-white issues from three dimensions: mold design, process control, and material selection.

I. The Essence of Top-White Phenomenon: Microscopic Manifestation of Uncontrolled Stress

The essence of top-white is the molecular chain breakage caused by the ejection force exceeding the plastic's bearing limit during the demolding process, resulting in local stress concentration. Medical-grade materials (such as PC, POM, PA66) are more susceptible to stress due to the need to meet biocompatibility requirements. For example, when glass-fiber-reinforced PA66 is ejected, the breakage or misalignment of glass fibers can form white streaks similar to "sweater snags," while transparent PC materials may experience a decrease in light transmittance due to stress whitening, directly affecting the performance of optical medical devices.

II. Mold Design: Eliminating Stress Concentration from the Source

1. Optimization of the Ejection System
   • Multi-stage Ejection Technology: Adopt a graded ejection strategy of "light pushing (50% pressure) first, then full pressure," allowing the plastic part to loosen gradually. For example, a mold for cardiac stents reduced the top-white rate from 12% to 2% by increasing the pre-ejection stroke.
   • Scientific Layout of Ejector Pins: Place ejector pins under thick walls, ribs, or bosses to avoid single-point force on thin ribs. For fine ribs with a diameter of less than 2mm, replace single ejector pins with a combination of "ejector blocks + angled ejectors" to expand the contact area by three times.
   • Precise Control of Draft Angles: Medical-grade transparent parts require a draft angle of ≥3°, and even 5° for ribbed areas. A mold for syringe pistons reduced the ejection force by 40% by increasing the draft angle from 1° to 2.5°.
2. Surface Treatment and Auxiliary Structures
   • Mirror Polishing: The surface roughness of the cavity and core should reach Ra ≤ 0.1μm (medical-grade standard), equivalent to the level of optical mirrors. A mold for artificial joints reduced the friction coefficient from 0.3 to 0.05 through electrolytic polishing.
   • Demolding Auxiliary Design: Create 0.1mm-deep pits or textures at the ejector pin positions to make the pins "embed" into the plastic part, distributing the pushing force more evenly. For example, a mold for infusion set needle hubs reduced top-white defects by 65% by adding a honeycomb texture.

III. Process Control: Precisely Regulating Stress Release

1. Temperature Management
   • Material Temperature Optimization: Medical-grade PC material should be controlled at 290-300°C, 10-20°C higher than conventional materials, to reduce melt viscosity. A mold for dialyzer casings reduced the ejection resistance by 28% by increasing the material temperature from 280°C to 295°C.
   • Zoned Mold Temperature Control: For thick-walled parts (such as orthopedic implants), the mold temperature should be maintained at 80-100°C, while for thin-walled parts (such as blood collection tubes), it should be controlled at 40-60°C. A mold for minimally invasive surgical instrument components used a mold temperature machine bypass system to keep the temperature fluctuation of local ejector pin areas within ±2°C.
2. Pressure and Speed Control
   • Gradient Packing Pressure Design: Adopt a three-stage packing pressure curve of "high pressure - medium pressure - low pressure" to avoid residual stress accumulation. For example, a mold for insulin pen needles reduced the top-white rate from 15% to 3% by gradually reducing the packing pressure from 80MPa to 50MPa.
   • Graded Ejection Speed Adjustment: Control the first-stage ejection speed at 10-20mm/s and increase it to 30-50mm/s in the second stage. A mold for endoscope lens components increased the ejection stroke to 15mm (conventional is 8mm) for a smoother ejection process.
3. Lubrication and Toughening
   • Addition of Internal Lubricants: Add 0.5%-1% silicone oil or stearic acid to the raw material to reduce melt viscosity. A mold for catheter connectors reduced the ejection friction force by 35% by adding 0.8% silicone oil.
   • Selection of High-Toughness Materials: For areas prone to top-white, use "toughened-grade" PA66 or PC/ABS alloys. For example, a mold for surgical knife handles reduced the ejection fracture rate from 5% to 0.2% by switching to toughened PA66.

IV. Material Selection: Special Solutions Matching Medical Needs

1. Development of Low-Stress Materials: For medical implants, develop special materials with low shrinkage and high toughness. For example, a special material for orthopedic screws reduced the shrinkage rate from 0.6% to 0.3% and the ejection stress by 40% by adding nano-calcium carbonate.
2. Control of Glass Fiber Orientation: For glass-fiber-reinforced materials, adjust the screw speed and back pressure to make the glass fibers distribute uniformly along the flow direction. A mold for internal fixation plates increased the glass fiber orientation by 25% and reduced top-white phenomena by 70% by reducing the screw speed from 150rpm to 100rpm.
3. Biocompatibility Verification: All modified materials must pass ISO 10993 biocompatibility tests to ensure that the top-white repair solutions do not introduce harmful substances. For example, a mold for infusion pump casings required retesting for cytotoxicity and skin irritation after adding lubricants.

V. Case Practice: Overcoming Top-White Issues in a Medical Catheter Connector

A company's production of catheter connectors had a yield rate of only 68% due to top-white defects. The following measures led to a breakthrough:

1. Mold Modification: Replace the original four Φ2mm ejector pins with eight Φ1.5mm ejector pins, doubling the distribution density; increase the draft angle from 1.5° to 3°.
2. Process Optimization: Increase the material temperature from 275°C to 290°C and the mold temperature from 50°C to 70°C; reduce the packing pressure from 60MPa to 45MPa and the packing time from 15s to 10s.
3. Material Upgrade: Switch to a toughened-grade PC/ABS alloy and add 0.6% silicone oil.
   Finally, the top-white defect rate dropped from 32% to 1.5%, and the yield rate increased to 98.2%, saving more than 2 million yuan in annual costs.

VI. Future Trends: Intelligent and Digital Solutions

As medical injection molding moves toward higher precision and complexity, solutions to top-white issues are integrating Industry 4.0 technologies:

1. Mold Flow Analysis Simulation: Use Moldflow software to simulate the stress distribution during the ejection process and identify high-risk areas in advance.
2. Real-Time Sensor Monitoring: Install pressure sensors at the ejector pin positions to dynamically adjust ejection parameters.
3. AI Process Optimization: Train AI models based on historical data to automatically generate optimal process parameter combinations.

Conclusion
The top-white issue in medical injection-molded parts needs to be systematically solved from the aspects of mold design, process control, and material selection. By scientifically designing the ejection system, precisely controlling process parameters, selecting low-stress materials, and incorporating intelligent technologies, the fundamental elimination of top-white defects can be achieved, providing a solid guarantee for the safety and reliability of medical products.