Family & Multi-Cavity Molding Application Case: Production Process of Disposable Medical Syringe Kit-Case Studies

A disposable medical syringe kit consists of three components: needle hub, plunger, and protective sleeve. It must meet requirements for precise dimensional matching, qualified biocompatibility, and mass high-efficiency production. Traditional single-cavity step-by-step production has drawbacks such as cumbersome processes, large part matching deviations, and low efficiency. This case adopts family & multi-cavity molding technology, designing a 24-cavity integrated mold (8 cavities for needle hub, 8 for plunger, 8 for protective sleeve) to simultaneously mold the three components using medical-grade PP/PE materials. The core control indicators are part dimensional tolerance ±0.02mm, assembly gap of the three components ≤0.03mm, and finished product qualification rate ≥99.5%. By optimizing cavity layout and parameter coordination, problems such as uneven melt distribution and cooling differences in simultaneous molding of multiple parts are solved. The following elaborates on the complete production process and technical key points in detail.

I. Product Specifications and Raw Material Preparation

The 10ml disposable medical syringe kit produced in this case is the core object of family molding, with closely interrelated structures of the three components: the needle hub (Φ8mm×12mm) requires an interference fit with the needle, the plunger (Φ7.8mm×80mm) has a sealing rubber ring groove at the front end, and the protective sleeve (Φ8.2mm×15mm) needs to wrap the tip of the needle hub. The three components must ensure no loosening or liquid leakage after assembly. The product complies with the GB 15810-2019 standard, and its biocompatibility meets ISO 10993 requirements, being non-cytotoxic and non-irritating.

Raw material selection adapts to the needs of simultaneous multi-cavity molding: medical-grade homopolypropylene (PP) resin (melt flow rate 10g/10min, 230℃/2.16kg) is selected for the needle hub and plunger, featuring excellent rigidity and fluidity; medical-grade LDPE resin (melt flow rate 8g/10min, 190℃/2.16kg) is selected for the protective sleeve, with good flexibility and easy demolding. Unified standards for raw material pretreatment: PP material is dried at 80℃ for 3 hours, and LDPE material is dried at 70℃ for 2 hours, with moisture content controlled below 0.02% to avoid bubbles and silver streaks after injection molding, while ensuring plasticization consistency of different materials in the same mold.

II. Design of Family & Multi-Cavity Mold and Equipment Selection

A Haitian MA3200/1100 horizontal injection molding machine is selected, equipped with a servo drive system and precision temperature control module, with a clamping force of 3200kN, an injection volume of 1100cm³, and a repeat positioning accuracy of ±0.005mm. It supports independent regulation of multi-material parameters, adapting to the needs of family & multi-cavity molding. The mold adopts an integrated hot runner structure and a 24-cavity symmetrical layout (8 cavities each for needle hub, plunger, and protective sleeve, evenly distributed in a ring shape) to ensure uniform melt distribution to each cavity.

Core design points of the mold: The hot runner system adopts a manifold layout, with a main sprue diameter of 8mm and a branch sprue diameter of 4mm. Each cavity is equipped with an independent hot runner nozzle, with a temperature control accuracy of ±1℃, avoiding molding defects caused by melt temperature differences between different parts. The cooling water channel adopts a zoned design: the water channel spacing of the needle hub and plunger cavities is 8mm, and that of the protective sleeve cavity is 6mm, adapting to the cooling shrinkage characteristics of different materials. A built-in automatic ejection and sorting mechanism is equipped to eject parts by type after molding, avoiding mixing. Meanwhile, the mold surface is mirror-polished (Ra≤0.2μm) to ensure the surface finish of parts.

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

(I) Mold Debugging and Cavity Positioning

After mold installation, cavity coaxiality calibration is first performed, and the position deviation between each cavity and the sprue is detected by a coordinate measuring machine to ensure ≤0.01mm. Then the hot runner temperature is debugged: the nozzle temperature of the cavity corresponding to PP is set to 210℃, and that corresponding to LDPE is set to 190℃, with constant temperature preheating for 30 minutes to eliminate mold temperature gradient. The ejection mechanism adopts a stepwise ejection strategy, with an ejection force of 15kN for the needle hub and protective sleeve, and 10kN for the plunger, avoiding part deformation or sticking.

(II) Multi-Parameter Coordinated Injection Molding

A "segmented plasticization and stepwise injection" strategy is adopted to meet the molding needs of two materials. In the plasticization stage, the barrel temperature is set in segments (PP side: feeding section 170℃, melting section 200℃; LDPE side: feeding section 160℃, melting section 185℃), with a screw speed of 70r/min and a back pressure of 2.0MPa, ensuring full plasticization of the two materials without degradation. The injection stage is controlled in three levels: the first-level injection pressure is 70MPa and speed is 40mm/s, pushing the melt to fill the sprue and cavity edge, prioritizing uniform feeding; the second-level injection pressure is 90MPa and speed is 60mm/s, filling the core area of the cavity, and strengthening filling for precision structures such as the plunger sealing groove and needle hub mating surface; the third-level holding pressure is 60MPa and holding time is 4s, compensating for cooling shrinkage differences of different parts and improving dimensional consistency.

The cooling stage adopts zoned temperature control: the mold temperature of the PP cavity is controlled at 45℃, and that of the LDPE cavity is 35℃, with a cooling time of 18s. Real-time monitoring is performed by built-in mold temperature sensors to ensure the temperature fluctuation of each cavity ≤±2℃, avoiding part warpage caused by uneven cooling. During injection molding, a continuous release agent spray (medical-grade residue-free type) is activated, spraying once every 50 molds to reduce the risk of sticking.

(III) Automatic Sorting and Preliminary Finishing

After molding, the mold opens, and the automatic ejection mechanism ejects parts in three groups by type, synchronously linking with the sorting robot arm. The visual recognition system distinguishes the needle hub, plunger, and protective sleeve with a sorting accuracy of ±0.5mm, and sends the parts to three bins respectively. Subsequently, preliminary impurity removal is performed on each group of parts, and residual melt flash on the surface is blown off by high-pressure air flow (0.4MPa) before being sent to the clean area for inspection.

IV. Quality Inspection and Finished Product Acceptance

The inspection adopts the mode of "100% inspection of key dimensions + sampling inspection of assembly compatibility + mandatory inspection of biocompatibility". Key dimensions are inspected by a fully automatic coordinate measuring machine, with 80 pieces randomly sampled from each batch (20 pieces of each part) to test 10 indicators including needle hub inner diameter, plunger diameter, and protective sleeve inner hole. The dimensional tolerance must be controlled within ±0.02mm, and the assembly gap of the three components ≤0.03mm. Assembly compatibility sampling uses simulated assembly tests, with 50 sets of finished products sampled to test plunger push-pull smoothness and needle hub sealing performance. The push-pull resistance must be between 5-10N, and the sealing pressure ≥0.5MPa (no leakage for 30s holding pressure).

Biocompatibility and safety tests are carried out in accordance with medical standards: 10 sets of finished products are sampled from each batch for cytotoxicity and skin irritation tests, both of which must meet ISO 10993 standards. Surface cleanliness is inspected by a particle counter, requiring the number of particles ≥0.5μm per square centimeter ≤8. Qualified finished products are packaged as kits, marked with batch number and production date to realize full-process quality traceability.

V. Case Summary

In this case, the syringe kit is produced by family & multi-cavity molding technology. A single mold can simultaneously produce 24 parts per cycle (8 sets of kits), with a production efficiency of 800 sets per hour, which is more than 70% higher than traditional single-cavity step-by-step production, and the mold utilization rate is increased to 85%, significantly reducing unit production cost. By optimizing cavity layout, hot runner zoned temperature control, and multi-parameter coordination strategies, problems such as dimensional deviation and uneven cooling in simultaneous molding of different materials are effectively solved, and the finished product qualification rate is stably above 99.6%. This case verifies the core advantages of family & multi-cavity molding in the production of medical kit parts. Its process plan can be replicated for the production of similar medical kits such as infusion sets and blood collection needles, providing technical reference for mass, efficient, and precision production in the medical industry.