In the field of medical injection molding, the surface gloss of products is not only an important indicator of their appearance quality but also directly affects their biocompatibility, cleanliness, and clinical user experience. Taking precision medical components such as orthopedic implants and surgical instrument handles as examples, enhancing surface gloss requires systematic optimization through four core aspects: material selection, mold design, process parameter control, and post-processing techniques. The following analyzes the key implementation steps based on industry practices and cutting-edge technologies.

1. Material Selection: Ensuring a Solid Foundation for Gloss from the Source

The surface gloss of medical injection-molded materials is influenced by three factors: resin type, additives, and pre-processing techniques.

1.1 Resin Type Selection

High-gloss resins should be prioritized, such as polylactic acid (PLA), polyether ether ketone (PEEK), and polyamide (PA). For instance, PLA can improve surface cell adhesion and gloss uniformity by modifying its molecular structure (e.g., introducing hydrophilic hydroxyl or carboxyl groups). PEEK, known for its excellent mechanical properties and biocompatibility, is commonly used in orthopedic implants. Its surface gloss can be further enhanced by adding nano-silica fillers.

1.2 Additive Optimization

Avoid using materials with a high content of fillers (e.g., calcium carbonate and talc), as they reduce surface gloss. If performance enhancement is required, nano-scale additives or brightening agents can be used. For example, adding 0.5%-1% silicone powder to PA can significantly improve melt flowability, reduce surface flow marks, and enhance gloss.

1.3 Pre-processing Techniques

For reinforced materials containing glass fibers (e.g., GF-PA66), drying treatment should be carried out to reduce the moisture content to below 0.02%, preventing hydrolysis-induced surface roughness. For high-temperature materials like PEEK, pre-crystallization treatment in a vacuum drying oven is necessary to avoid uneven crystallization and gloss differences during injection molding.

2. Mold Design: Precisely Controlling Surface Replication Capability

The surface quality of the mold directly affects the gloss of the product, and optimization should be carried out from the following dimensions.

2.1 Surface Polishing and Coating Treatment

• Polishing Grade: The mold cavity surface should reach a mirror polishing standard (Ra ≤ 0.05 μm). For transparent medical components (e.g., endoscope lenses), ultra-precision polishing with diamond abrasive paste is required.
• Coating Technology: Physical Vapor Deposition (PVD) coatings (e.g., TiN and TiAlN) or electroless nickel-phosphorus alloy layers can be used to increase the mold surface hardness to above HV1200, reducing wear caused by melt erosion and lowering the friction coefficient to improve demolding effects. For example, after TiN coating treatment, the surface gloss of a cardiac stent mold increased by 30%, and the mold life extended to over 500,000 cycles.

2.2 Cooling System Design

Conformal cooling channels should be adopted to ensure a temperature difference of ≤ 1°C across different parts of the mold, preventing gloss differences caused by uneven cooling. For example, by optimizing the cooling channel layout of an acetabular cup mold, the surface temperature uniformity of the product improved by 40%, and the standard deviation of gloss decreased from 0.8 to 0.3.

2.3 Venting System Optimization

Venting slots with a width of 0.02-0.05 mm should be set at the parting line and the mating surfaces of the core and cavity to prevent gas entrapment and dark spots. For micro-structured medical components (e.g., microfluidic chips), a vacuum venting system can be used to reduce the cavity pressure to below -90 kPa, completely eliminating bubble defects.

3. Process Parameter Control: Dynamically Balancing Filling and Cooling

Injection molding process parameters should be precisely adjusted according to material characteristics and product structure.

3.1 Injection Speed and Pressure

• Thin-walled Components (e.g., surgical knife handles): High-speed and low-pressure injection (speed: 80-120 mm/s, pressure: 60-80 MPa) should be used to avoid melt degradation due to excessive shear heating and reduce surface flow marks.
• Thick-walled Components (e.g., orthopedic implant shells): Low-speed and high-pressure injection (speed: 10-20 mm/s, pressure: 100-120 MPa) should be used to ensure complete mold filling and reduce sink marks.

3.2 Mold Temperature Control

• Crystalline Materials (e.g., PLA and PEEK): The mold temperature should be set 5-10°C higher than the material's crystallization temperature. For example, when the mold temperature of PLA is set at 65-70°C, the crystallinity of the product increases by 15%, and the surface gloss improves by 20%.
• Amorphous Materials (e.g., PC and PMMA): The mold temperature should be controlled at 80-90°C to reduce internal stress and avoid surface whitening.

3.3 Packing Pressure and Cooling Time

• The packing time should be set according to the product wall thickness, generally 1-2 seconds per millimeter of wall thickness. For example, a 3 mm-thick cardiac stent requires 4-6 seconds of packing to ensure dimensional stability.
• The cooling time should be optimized through CAE simulation to avoid premature demolding and product deformation. For PEEK materials, the cooling time should be extended to over 30 seconds to ensure complete solidification.

4. Post-processing Techniques: Refining Surface Modification

For high-precision medical components, post-processing is a key step in enhancing surface gloss.

4.1 Polishing Treatment

Mechanical polishing or electrolytic polishing can be used to treat the product surface. For example, after electrolytic polishing, the surface roughness of an artificial joint prosthesis decreased from Ra 0.8 μm to Ra 0.1 μm, and the surface gloss increased by 50%, while also reducing bacterial adhesion.

4.2 Coating Technology

• Bioactive Coatings: Hydroxyapatite (HA) or collagen coatings can be sprayed on the product surface to improve biocompatibility and gloss uniformity. For example, after HA coating treatment, the standard deviation of surface gloss of a dental implant decreased from 0.5 to 0.2.
• Antibacterial Coatings: Silver ion or photocatalytic coatings (e.g., TiO₂) can be used to inhibit bacterial growth while maintaining stable surface gloss.

4.3 Laser Micro-machining

Femtosecond lasers can be used to create nano-scale textures on the product surface, achieving a "superhydrophobic + high-gloss" dual effect. For example, after laser micro-machining, the contact angle of an endoscope lens increased to 150°, while the surface gloss remained above 90%.

5. Case Study: Enhancing Surface Gloss of a PEEK Interbody Fusion Cage

A company addressed the issue of insufficient surface gloss in PEEK interbody fusion cages through the following measures:

1. Material Upgrade: Using a PEEK composite material containing 0.5% nano-silica, the surface gloss increased from 75 GU to 85 GU.
2. Mold Optimization: Applying PVD coating treatment to the mold cavity increased the surface hardness to HV1500 and extended the mold life to 800,000 cycles.
3. Process Adjustment: Increasing the mold temperature from 80°C to 90°C and reducing the injection speed from 50 mm/s to 30 mm/s decreased the standard deviation of surface gloss from 1.2 to 0.5.
4. Post-processing: Adding an electrolytic polishing step, the final product surface gloss reached 92 GU, meeting the stringent requirements for high-precision medical components in clinical applications.

Conclusion

Enhancing the surface gloss of medical injection-molded products requires optimization across the entire chain of material selection, mold design, process control, and post-processing. By implementing scientific material selection, precision mold design, dynamic process control, and refined post-processing, both the gloss and functionality of medical products can be simultaneously optimized, providing higher-quality solutions for the medical industry. In the future, with the integration of cutting-edge technologies such as nanotechnology and intelligent injection molding, the control of surface gloss in medical injection-molded products will achieve higher precision and reliability.