In the injection molding process of medical plastic parts, precise control of process pressure is the core element to ensure product quality, performance, and safety. From injection pressure to holding pressure and then to back pressure, the parameter settings in each link must strictly adhere to the requirements of material properties and mold design. This article will systematically sort out the common process pressure issues in the production of medical plastic parts and propose targeted solutions based on practical cases.

I. Injection Pressure: The Art of balancing filling and flow

Problem manifestations: Insufficient injection pressure can lead to incomplete filling of the melt, resulting in defects such as short shots and sink marks. Excessive pressure, on the other hand, may cause flash, mold wear, and even cracking of the product due to internal stress. For example, when manufacturing central venous catheters, if the injection pressure is insufficient, uneven wall thickness of the catheter may occur, which could lead to medical accidents. Excessive pressure may cause burrs on the catheter surface, damaging patients' blood vessels.

Solutions:

1. Material adaptability adjustment: Set the pressure range according to the material properties. For instance, medical-grade PEI (polyetherimide) has a relatively high melt viscosity, and the injection pressure needs to be controlled between 100 - 180 MPa. Thin-walled products require higher pressure to ensure filling. PA9T (polyamide 9T), with a melting point of 306°C, has an injection pressure set between 100 - 150 MPa to avoid filling defects caused by insufficient pressure.
2. Mold structure optimization: For parts with complex structures, optimize the runner and gate designs through CAE simulation analysis. For example, when manufacturing ultrasonic instrument connectors, a multi-stage gate design can reduce local pressure losses and the risk of short shots.
3. Equipment precision upgrade: Select high-precision injection molding machines, such as the Yizumi FF-M 160 medical-specific machine, which has a pressure accuracy of ±0.1 MPa, effectively avoiding dimensional deviations caused by pressure fluctuations.

II. Holding Pressure: The key guarantee for dimensional stability

Problem manifestations: Insufficient holding pressure can lead to surface sink marks and depressions on the product, affecting its sealing and appearance. Excessive pressure may make it difficult to eject the product from the mold and even cause material degradation due to over-compaction. For example, when manufacturing PICC catheters, insufficient holding pressure may result in shrinkage holes at the catheter connector, causing liquid leakage. Excessive pressure may make the catheter wall thickness exceed the standard, affecting the smoothness of insertion.

Solutions:

1. Pressure gradient control: The holding pressure is usually 60% - 80% of the injection pressure, and the holding time is adjusted according to the thickness of the product. For example, when manufacturing vascular sheaths, the holding time for thick-walled parts should be extended to 15 - 30 seconds to ensure sufficient material replenishment, while that for thin-walled parts can be shortened to 10 seconds to avoid over-compaction.
2. Cooling system collaborative optimization: Balance the temperature field through the design of mold cooling water channels to reduce internal stress caused by uneven cooling. For example, when manufacturing tearable sheaths, using conformal cooling water channels can shorten the cooling time of the product to 20 - 30 seconds while reducing the risk of deformation.
3. Material pre-treatment enhancement: Fully dry materials with high moisture absorption (such as PEI) to avoid bubble defects caused by water vaporization. For example, PEI needs to be dried at 130 - 150°C for 4 - 6 hours to reduce the moisture content to below 0.02%, minimizing dimensional fluctuations caused by bubble expansion during the holding stage.

III. Back Pressure: The trade-off between plasticization quality and production efficiency

Problem manifestations: Insufficient back pressure can lead to uneven plasticization of the material, resulting in bubbles and silver streaking on the product. Excessive back pressure may cause excessive load on the screw, prolonged plasticization time, and even material decomposition. For example, when manufacturing syringe plungers, insufficient back pressure may cause flow marks on the plunger surface, affecting its sealing. Excessive back pressure may degrade the plunger material, reducing its hardness.

Solutions:

1. Segmented back pressure control: Set the back pressure range according to the screw speed and material properties. For example, when injection molding PA9T, the back pressure can be set between 3 - 8 MPa, and the screw speed controlled between 30 - 80 r/min to balance plasticization efficiency and quality.
2. Screw design optimization: Use high-compression-ratio screws (e.g., above 2.5:1) to enhance material conveying and plasticization capabilities. For example, when manufacturing infusion device valves, using a high-compression-ratio screw can reduce the back pressure requirement while improving plasticization uniformity.
3. Real-time monitoring and adjustment: Monitor the back pressure and screw position in real time through the injection molding machine's control system and adjust the parameters dynamically. For example, when manufacturing respirator masks, a closed-loop control system can automatically adjust the back pressure according to the melt temperature to ensure stable plasticization quality.

IV. Comprehensive Case: Insert injection molding of surgical instrument handles

When manufacturing surgical knife handles, metal knife tips need to be combined with plastic handles through insert injection molding. The following pressure issues need to be addressed in this process:

1. Insert positioning pressure: Use a vertical injection molding machine and an open-and-close mold method to fix the inserts, preventing displacement caused by pressure impact. For example, when manufacturing surgical clip handles, the insert positioning accuracy needs to be controlled within ±0.005 mm to ensure the bonding strength between the metal and the plastic.
2. Melt wrapping pressure: The injection pressure should be high enough to completely wrap the insert while avoiding insert deformation due to excessive pressure. For example, when manufacturing orthopedic surgical knife handles, the injection pressure can be set between 120 - 150 MPa, and the holding pressure between 90 - 120 MPa to ensure no gaps at the interface between the metal and the plastic.
3. Ejection pressure control: Reduce the ejection stress by increasing the number of ejector pins and the ejection draft angle (usually ≥3°). For example, when manufacturing minimally invasive surgical instrument handles, a four-pin ejection structure can reduce the ejection force by 40%, preventing product cracking due to concentrated pressure.

V. Industry Regulations and Quality Assurance

The production of medical plastic parts must strictly comply with the ISO 13485 medical device quality management system and FDA regulations, ensuring full control from raw material inspection to finished product testing. For example:

1. Raw material inspection: Each batch of materials needs to be tested for key indicators such as shrinkage and molecular weight to ensure compliance with medical-grade standards.
2. Process validation: Ensure the stability of process parameters through three-stage validation: IQ (Installation Qualification), OQ (Operational Qualification), and PQ (Performance Qualification). For example, when manufacturing pacemaker housings, three batches of small-scale validation need to be completed to form complete SOP operation documents.
3. Finished product testing: Use non-destructive testing (such as X-ray inspection) and biocompatibility testing (such as ISO 10993 standards) to ensure product safety. For example, when manufacturing artificial joints, CT scans are used to detect internal bubbles, and cytotoxicity tests are conducted to verify the biocompatibility of the materials.

Conclusion

The control of process pressure in medical plastic parts production is a deep integration of technology, experience, and regulations. From material selection to mold design, from parameter setting to quality inspection, every link requires a scientific attitude and strict standards. By precisely controlling injection, holding, and back pressures, combined with advanced equipment and testing technologies, medical injection molding enterprises can achieve "zero-defect" production and provide solid guarantees for patient safety and the development of the medical industry.