Medical plastic parts, as crucial components that directly come into contact with the human body or participate in medical procedures, have their surface quality directly impacting product performance and patient safety. Surface blurring not only impairs optical effects but may also trigger risks such as microbial growth and abnormal drug adsorption. Based on practices in the medical industry and principles of materials science, this article systematically analyzes the causes of this problem and proposes solutions.

I. Core Cause Analysis

1. Material Thermal Degradation and Impurity Precipitation

Medical-grade plastics like polypropylene (PP) and polycarbonate (PC) undergo molecular chain breakage when the processing temperature exceeds the critical value, generating carbonized particles. A case study by a medical device company showed that when the barrel temperature rose from 230°C to 260°C, the surface blurring rate of needle tubes surged from 3% to 22%. Additionally, an excessively high proportion of recycled materials (over 15%) introduces short-chain molecules, leading to uneven crystal grain sizes. An experiment by a syringe manufacturer revealed that adding 20% recycled materials reduced transparency by 40%.

2. Mold Design Defects

Inadequate mold venting is a common cause. For example, a stapler cartridge mold with a vent slot width of only 0.02 mm resulted in a 35°C temperature increase in the trapped gas area, triggering local material decomposition. Moreover, improper cooling water channel layout (e.g., runner spacing > 50 mm) creates a temperature gradient > 15°C, promoting surface stress crystallization.

3. Process Parameter Mismatch

Excessive injection speed (> 80 cm³/s) generates intense shear heat, causing the melt temperature to spike by over 50°C instantaneously. In the production of infusion set drip chambers, increasing the injection speed from 60 cm³/s to 90 cm³/s raised the surface blurring defect rate from 5% to 18%. Simultaneously, an excessively low mold temperature (< 60°C) inhibits molecular chain movement, increasing the surface roughness Ra value by 0.3 μm.

4. Environmental Control Failure

When the workshop humidity exceeds 70%, the moisture absorption rate of PC materials can reach 0.3%, and the vaporization of moisture during processing forms micropores. In the production of dialyzer housings, raising the humidity from 50% to 80% tripled the pore density on the surface. Additionally, volatile rust inhibitors condense on the mold surface, forming a 0.5 - 2 μm oil film.

II. Systematic Solutions

1. Material Optimization Strategies

• Modification Technology: Using nano-nucleating agents (e.g., sorbitol derivatives) reduces the PP spherulite size from 20 μm to 5 μm, improving transparency by 60%. A blood collection tube manufacturer achieved a product qualification rate increase from 82% to 97% after application.
• Recycled Material Treatment: Establishing a three-stage sorting system (optical sorting → density sorting → near-infrared detection), combined with a melt filtration device (filter mesh size ≥ 200) in a twin-screw extruder, can control the impurity content of recycled materials below 0.05%.

2. Mold Design Innovation

• Venting System Upgrade: Adopting vacuum venting technology, installing a 0.05 mm wide vent slot on the parting surface, and using a -0.08 MPa vacuum pump can reduce the trapped gas rate to below 0.5%. After mold reconstruction for a surgical instrument handle, the surface blurring defect rate dropped from 12% to 1.5%.
• Cooling Optimization: Applying conformal cooling channels manufactured via SLM metal 3D printing improves cooling efficiency by 40% and enhances temperature uniformity to ±2°C. In the production of endoscope lens seats, the cycle time was shortened by 18%, and surface quality significantly improved.

3. Precise Process Parameter Control

• Multi-stage Injection Control: Using a five-stage injection speed (30 - 50 - 70 - 50 - 30 cm³/s) with a back pressure of 15 MPa controls the melt temperature fluctuation within ±3°C. In the production of insulin pen needle seats, the surface blurring rate decreased from 9% to 1.2%.
• Dynamic Mold Temperature Control: Applying an oil-electric composite temperature control system to lower the mold temperature from 80°C to 60°C during the holding pressure stage promotes ordered molecular chain arrangement. In the production of artificial joint components, the surface roughness Ra value was reduced from 0.8 μm to 0.3 μm.

4. Intelligent Environmental Control

• Cleanroom Upgrade: Building an ISO Class 8 cleanroom equipped with a desiccant wheel dehumidification system (dew point -40°C) controls the humidity at 40% ± 5%. In the production of cardiac stent delivery systems, the micropore defect rate decreased from 0.3% to 0.02%.
• Volatile Substance Treatment: Installing an activated carbon adsorption device above the mold and using a centrifugal fan with an exhaust volume of 1500 m³/h reduces the volatile substance concentration to below 0.1 mg/m³. In the production of anesthesia masks, the surface oil stain rate dropped from 5% to 0.3%.

III. Quality Control System Construction

1. Online Inspection Technology: Applying a machine vision system (resolution 0.01 mm) with a ring LED light source enables real-time detection of surface defects larger than 0.05 mm. In the production of indwelling needles, the missed detection rate decreased from 2% to 0.05%.
2. Material Traceability System: Establishing an RFID material management system achieves full traceability from raw material batches to finished product serial numbers. After implementation by an orthopedic implant enterprise, the time for quality problem traceability was shortened from 72 hours to 2 hours.
3. Process Database Construction: Accumulating over 100,000 sets of process parameters and quality data and using AI algorithms to build a predictive model can shorten the process debugging time by 60%. In the production of IVD reagent bottles, the first-pass yield of new molds increased from 75% to 92%.

Conclusion

Resolving the surface blurring issue of medical plastic parts requires a collaborative approach from multiple dimensions, including materials science, mold engineering, process control, and environmental management. By applying innovative solutions such as nano-modification technology, intelligent temperature control systems, and machine vision inspection, combined with the ISO 13485 quality management system, a medical-grade surface quality with a surface roughness Ra ≤ 0.1 μm and transparency > 90% can be achieved, providing solid protection for the safety and effectiveness of medical devices.