Insert Molding Application Case: Manufacturing Process of Medical Infusion Connector-Case Studies

As a core connecting component for clinical infusion therapy, medical infusion connectors must simultaneously meet requirements for sealing performance, biocompatibility, structural robustness, and resistance to repeated insertion and removal. Traditional assembly processes suffer from loose bonding between metal and plastic and high risks of seal failure. This case adopts insert molding technology to integrally form 304 stainless steel inserts with medical-grade PP materials, realizing the functional integration of "metal pressure-bearing + plastic sealing" for the connector. The core control indicators are bonding force between insert and plastic ≥500N, sealing pressure ≥0.6MPa, and no residual burrs on the surface. Through precise process design, problems such as insert positioning deviation and insufficient plastic wrapping are solved. The following elaborates on the complete manufacturing process and technical key points in detail.

I. Product Specifications and Preparation of Raw Materials and Inserts

The medical infusion connector processed in this case has an L-shaped structure with an overall size of Φ12mm×35mm. Its core is a Φ4mm 304 stainless steel insert (length 20mm, with annular anti-slip grooves at both ends), wrapped externally with medical-grade PP plastic to form a sealing surface and plug-in interface. It must withstand ≥1000 times of repeated insertion and removal without loosening or leakage. The product must comply with the GB 15810-2019 Medical Infusion Equipment Standard, and its biocompatibility must meet ISO 10993 requirements, being non-cytotoxic and non-skin-irritating.

Pretreatment of raw materials and inserts directly affects molding quality: medical-grade homopolypropylene (PP) resin is selected as the plastic material, with a melt flow rate of 12g/10min (230℃, 2.16kg), featuring excellent fluidity, chemical resistance, and biocompatibility. Before molding, it is dried at 80℃ for 4 hours to control the moisture content below 0.02%, avoiding bubbles and silver streaks after injection molding. The stainless steel insert is precision-ground on the outer circle by a centerless grinder, with a tolerance controlled within ±0.01mm, then subjected to electrolytic polishing to achieve a surface roughness Ra≤0.2μm. It is further ultrasonically cleaned with absolute ethanol for 30 minutes to remove oil stains and oxide films, and finally preheated at 120℃ for 30 minutes to reduce the temperature difference between the insert and plastic, preventing stress cracking caused by plastic shrinkage during cooling.

II. Insert Molding Process Planning and Equipment Selection

Combined with product structure and precision requirements, a Haitian MA1200/350 horizontal injection molding machine is selected, equipped with a servo control system, with a clamping force of 1200kN, an injection volume of 350cm³, and a repeat positioning accuracy of ±0.005mm, suitable for insert molding of small and medium-sized products. The mold adopts a two-cavity structure with a built-in automatic insert positioning mechanism (including elastic ejector pins and positioning pins) to ensure the insert coaxiality error ≤0.01mm; the mold cooling water channel is designed in an annular shape, fitting the plastic wrapping area to control mold temperature uniformity.

Tool and auxiliary material matching: a needle valve type injection nozzle is selected to avoid melt backflow; a TiAlN-coated screw is used to improve PP material plasticization efficiency; medical-grade anti-rust oil is selected as the cutting fluid (only for short-term protection after insert pretreatment) to avoid plastic contamination. The process route is planned as "insert pretreatment - automatic clamping and positioning - injection molding - cooling and demolding - deburring - cleaning and inspection", adopting a "low-temperature and low-speed" injection strategy to balance plastic fluidity and insert wrapping tightness. The entire process is carried out in a Class 10,000 clean room to avoid the impact of environmental pollutants.

III. Step-by-Step Insert Molding Process and Parameter Control

(I) Insert Clamping and Positioning

The clamping method of "robotic automatic feeding + mold positioning mechanism locking" is adopted. The robot has a pick-and-place accuracy of ±0.02mm, feeding preheated inserts into the mold cavity. It is matched with the grooves at both ends of the insert through 3 elastic positioning pins, with a positioning gap ≤0.005mm. At the same time, the negative pressure adsorption device in the mold is activated (suction force 0.4MPa) to fix the insert position and prevent insert deviation during injection molding. After clamping, the mold clamping force is set to 800kN, and the clamping speed is 5mm/s to ensure cavity sealing without leakage and avoid scratching the insert surface.

(II) Core Injection Molding Process

The injection molding process adopts three-stage parameter control to balance plastic filling and insert wrapping effects: in the plasticization stage, the barrel temperature is set in sections as 160℃ for the feeding section, 200℃ for the melting section, and 210℃ for the nozzle section, with a screw speed of 80r/min and a back pressure of 1.5MPa to ensure full plasticization of PP material without degradation. In the injection stage, a stepwise injection strategy is adopted: the first-stage injection pressure is 60MPa and speed is 30mm/s, pushing the melt to fill the cavity edge and wrap the insert bottom; the second-stage injection pressure is 80MPa and speed is 50mm/s, filling the core area to ensure the plastic fully wraps the insert anti-slip grooves; the third-stage holding pressure is 50MPa and holding time is 3s, compensating for plastic cooling shrinkage and improving bonding tightness.

In the cooling stage, the mold temperature is controlled at 40℃, and the cooling time is 15s. Uniform heat dissipation is achieved through the annular water channel to avoid plastic cracking caused by local excessive cooling speed. During demolding, an ejector pin demolding method is adopted, with an ejection force of 20kN and a demolding speed of 2mm/s. Combined with the anti-stick coating in the mold, plastic adhesion to the cavity is prevented, and the bonding surface between the insert and plastic is protected from damage.

(III) Subsequent Processing Procedures

After demolding, the products are first deburred manually combined with ultrasonic cleaning. A special nylon brush is used to clean burrs at the injection gate and parting surface (burr height ≤0.002mm) to avoid sharp edges scratching medical staff or damaging infusion tubes. They are then placed in a medical pure water ultrasonic cleaning tank for 20 minutes (water temperature 50℃) to remove residual injection auxiliaries and impurities on the surface. Finally, they are dried with hot air (60℃, 15 minutes) to ensure no moisture remains on the product surface before entering the inspection link.

IV. Quality Inspection and Finished Product Acceptance

Finished product inspection adopts the mode of "100% inspection of key indicators + sampling inspection of performance parameters" to ensure compliance with medical standards: dimensional accuracy is inspected by a coordinate measuring machine, with 50 pieces randomly sampled from each batch to test 8 indicators including insert coaxiality and plastic sealing surface size, requiring a qualification rate of over 99.8%. Bonding force inspection uses a tensile testing machine to apply axial tension to the insert, requiring a breaking force ≥500N without separation between plastic and insert. Sealing performance inspection uses an air tightness tester, applying 0.6MPa pressure for 30s, with no air leakage as qualified.

Biocompatibility and safety sampling inspection: 10 finished products are sampled from each batch for cytotoxicity test and skin irritation test, both of which must meet the ISO 10993-5 and ISO 10993-10 standard levels. Surface cleanliness is inspected by a particle counter, requiring the number of particles ≥0.5μm per square centimeter ≤10. Qualified finished products are packaged aseptically, marked separately with batch number, production date, and inspection results to realize full-process quality traceability.

V. Case Summary

In this case, medical infusion connectors are manufactured by insert molding technology. Through insert pretreatment, precise positioning, and optimization of stepwise injection parameters, traditional process pain points such as loose bonding between metal and plastic and insufficient sealing performance are effectively solved. The processing efficiency reaches 300 pieces per hour, which is more than 60% higher than traditional assembly processes, and the scrap rate is controlled below 0.02%. This case verifies the core advantages of insert molding in the production of medical connectors—integral molding not only simplifies processes and reduces assembly pollution risks but also significantly improves product structural robustness and sealing reliability. Its process plan can be replicated for similar medical insert molding products (such as syringe push rods and catheter connectors), providing technical reference for the production of high-precision and high-safety insert products in the medical industry.