3. Mold Design Prevention Points

3.1 Demolding System Design

Medical molds strictly prohibit the use of external mold release agents; anti-adhesion demolding must rely entirely on the mold structure itself. Taking a disposable syringe plunger as an example, the root cause of difficult demolding is the strong gripping force exerted by the slender core after plastic cooling shrinkage, coupled with the deep-cavity structure that easily creates vacuum adsorption. Therefore, the core requires reasonable draft angles—$0.8^\circ \text{ to } 1.5^\circ$ for the main rod body, expanded to $2^\circ \text{ to } 3^\circ$ at structural transitions with smooth fillet blending. Cavities and cores must undergo high-precision single-direction mirror polishing along the demolding direction, controlling surface roughness within $Ra\ 0.025\ \mu\text{m}$. Addressing PP's strong adhesion characteristics, cavities can be treated with chromium plating, nickel plating, or diamond-like carbon (DLC) coatings to enhance surface hardness and hydrophobicity.

Ejection systems must prevent local stress concentrations that cause whitening. Practical designs frequently utilize a combination structure of an annular stripper sleeve matched with a central ejector pin, ensuring a wide and evenly distributed force-bearing area. For long-dimension components, auxiliary air ejection can be integrated, injecting compressed air through micro-holes to form an air film that breaks vacuum adsorption.

3.2 Gating and Cooling Systems

The gating system must eliminate dead corners to prevent melt from lingering and decomposing to form harmful substances. Cooling system design is equally critical—mold temperature during PP molding is typically controlled between $20\text{--}40^\circ\text{C}$, and cooling water circuits must achieve full coverage and uniform temperature control to prevent warping, ovality, or other deformations caused by uneven cooling. Clean, purified water should be used for cooling, with pipeline joints securely sealed to prevent water leakage from contaminating the plastic parts.

4. Precise Control of Injection Process Parameters

4.1 Temperature Control

The injection temperature of PP is typically controlled between $180\text{--}230^\circ\text{C}$, with a fluctuation range required to be $\le \pm 2^\circ\text{C}$. Excessively low mold temperatures directly cause defects—when the mold temperature drops below $60^\circ\text{C}$, the PP melt front solidifies too quickly, forming noticeable shear whitening in thin-walled regions. Research indicates that as mold temperature gradually increases, the crystallinity of PP plastic parts exhibits a non-linear upward trend, subsequently affecting the mechanical properties and appearance of the products.

4.2 Multi-Stage Injection Strategy

Multi-stage injection is an effective method for reducing internal stress and surface defects. Taking a three-stage strategy ("slow-fast-slow") as an example: the first stage uses a 20%–30% speed to smoothly fill the runner, the second stage accelerates to 80% passing through the gate to avoid cold material, and the third stage drops back to 40% to fill the cavity and minimize internal stress. Injection pressure for thin-walled parts can reach $100\text{--}150\text{ MPa}$, but excessive injection pressure (exceeding $150\text{ MPa}$) may cause melt fracture, generating silver streak-like whitening at the end of the runner.

4.3 Cooling and Post-Processing

Cooling time must be precisely calculated, generally taking 10–20 seconds for thin-walled parts and 20–40 seconds for thick-walled parts. The shrinkage rate of PP is typically controlled between $1.0\%$ and $1.5\%$. To prevent shrinkage deformation caused by post-crystallization, parts generally undergo hot water immersion treatment.

5. Production Environment and Contamination Control

5.1 Cleanliness Control

Medical injection molding production must be conducted in clean environments. Conventional consumables (such as syringes) require a Class 100,000 (ISO Class 8) cleanroom, while high-precision components must be upgraded to Class 10,000 (ISO Class 7). Workshop temperature should be stably controlled at $18\text{--}26^\circ\text{C}$, with humidity maintained between $45\%$ and $65\%$. When humidity exceeds $65\%$, PP materials easily develop sticky surfaces.

5.2 Static Electricity Control

Lacking polar groups internally and exhibiting poor hygroscopicity, PP features high surface and volume electrical resistivity, easily accumulating static electric charges during processing. Static electricity not only attracts dust to contaminate products but also affects precision demolding. Anti-static modified PP materials or electrostatic elimination devices like ionizing blowers can be deployed.

5.3 Operating Protocols

Operators entering the workshop must wear anti-static garments, masks, and gloves, undergoing purification through an air shower system. Wandering around or direct bodily contact with products during operation is strictly prohibited. Molds should avoid rusting, oil stains, and plastic carbon buildup during long-term production, as these attachments severely compromise demolding performance.

6. Conclusion

Defect prevention and contamination control in the precision injection molding of polypropylene medical supplies represent a systematic engineering task spanning materials, molds, processes, and environments. From strict material selection and drying pretreatment to anti-adhesion mold design and mirror polishing; from the precise implementation of multi-stage injection strategies to the comprehensive safeguarding of clean production environments—lapses in any single link can become the source of defects and contamination. In the medical injection molding field, prevention is not an added luxury, but a baseline quality requirement. Only by establishing a full-process, traceable control system can manufacturers ensure the safety and reliability of every polypropylene medical item.