What are the injection process parameters of medical-grade rubber and plastic components?
The injection process parameters of medical-grade rubber-plastic components need to be accurately set around the three core requirements of material characteristics, product accuracy and biocompatibility. The parameters of different medical-grade materials (such as medical PP, PC, silicone, PEEK, etc.) are quite different. The following are the common core parameters and the key parameter ranges of sub-materials:
1. General core process parameters (basic control items of all medical-grade rubber and plastic injection molding)
Temperature parameter
Cylinder temperature: set in stages (feeding stage → compression stage → homogenization stage → nozzle), and match the melting temperature of materials, so as to ensure the complete melting of materials and avoid high-temperature degradation (degradation of medical-grade materials will produce harmful substances and destroy biocompatibility).
Mold temperature: it directly affects the dimensional stability, surface smoothness and internal stress of products. The mold temperature error of medical precision parts needs to be controlled within 1℃, and some materials need to be kept at constant temperature through a mold temperature machine.
Drying temperature: pretreatment parameters before injection molding. For materials with strong hygroscopicity (such as PC and PA), it is necessary to remove water to prevent bubbles and crazing in products.
Pressure parameter
Injection pressure: It can be divided into primary injection pressure (mold filling) and secondary injection pressure (feeding). The combination of high pressure and low speed or low pressure and high speed is required for precision medical components to prevent flash and under-injection, and at the same time reduce the shear degradation of materials.
Holding pressure: usually 30%~60% of the injection pressure, it is used to supplement the cooled and shrunk melt and eliminate shrinkage and warpage. Too high holding pressure will easily lead to excessive internal stress in the product and difficulty in demoulding.
Back pressure: control the screw back resistance to improve the melt plasticization uniformity. The back pressure of medical-grade materials is generally set at 0.5~3MPa, too high will increase the risk of material degradation.
Speed and time parameters
Injection speed: adjust by stages (slow → fast → slow), low speed at the gate to prevent jet flow, high speed at the cavity filling stage to ensure mold filling integrity, and low speed to reduce impact before pressure holding switching. The injection speed of precision micro-components should be accurate to mm/s level.
Holding time: according to the product wall thickness setting, it is necessary to ensure that the melt is completely fed back. If the time is too short, it is easy to produce shrinkage marks, and if it is too long, it will increase the internal stress.
Cooling time: the cooling time of medical-grade crystalline materials (such as PP) is longer based on the standard that the product is completely cured and does not deform after demoulding, and it needs to be combined with the design of mold waterway and the adjustment of material crystallization characteristics.
Injection cycle: including the total time of injection, pressure holding, cooling, mold opening and ejection. The cycle error of precision parts should be controlled within ±0.5s to ensure batch consistency.
Other key parameters
Screw speed: it affects the plasticizing efficiency and melt uniformity. The rotating speed of medical-grade materials is generally 50~150r/min to avoid the degradation of materials caused by shear heat generated by high rotating speed.
Dosage of release agent: medical-grade injection molding gives priority to reducing the use of release agent through mold polishing and process optimization. If necessary, medical-grade release agent must be selected, and the dosage should be strictly controlled.
Second, the reference range of injection parameters of common medical-grade rubber and plastic materials
Material type barrel temperature (℃) mold temperature (℃) drying conditions (temperature/time) injection pressure (MPa)
Medical PP 180 ~ 240 20 ~ 60 need not be dried (humidity < 0.02%) 60 ~ 120.
Medical PC 260 ~ 320 80 ~ 120 120℃/4 ~ 6 h 80 ~ 150
Medical silica gel (liquid LSR) 170 ~ 200 (mold temperature) 170 ~ 200 does not need to be dried for 30 ~ 80.
Medical peek 380 ~ 420 120 ~ 180 150℃/4 ~ 6h 100 ~ 180
Medical ABS 200 ~ 260 40 ~ 80 80℃/2 ~ 4h70 ~ 130
Iii. Special control requirements for medical-grade injection molding parameters
Parameter solidification and traceability
After determining the optimal parameters, it should be written into the standard operating instructions (SOP), and it is forbidden to adjust them at will; All parameters should be recorded in real time and incorporated into the production traceability system to meet the compliance requirements of ISO 13485 and FDA QSR 820.
process certification
Before mass production, it is necessary to verify the parameter window through DOE (experimental design) to ensure that the products still meet the quality standards within the range of parameter fluctuation; The upper and lower limits of key parameters need to be set, and the equipment will automatically stop when it exceeds the range.
Biocompatibility guarantee
To avoid material degradation caused by improper parameter setting, such as too high barrel temperature and too fast screw speed, small molecular volatiles will be produced, and the safety of process parameters needs to be verified by extraction test.
