Notes on PI Plastic Injection Molding Processing
2026-04-27 11:07:22
Plastic Injection Molding
Polyimide (PI) is a high-performance special engineering plastic with ultra-high temperature resistance, high rigidity, excellent thermal stability and insulation properties, widely used in precision electronics, new energy, aerospace and high-end insulating parts manufacturing. Compared with ordinary engineering plastics, PI has extremely strict injection molding requirements, featuring strong hygroscopicity, high molding temperature, sensitive internal stress and easy thermal decomposition during processing. It is prone to defects such as hydrolytic bubbles, silver streaks, filling deficiency, brittle cracking and mold sticking. Based on on-site precision injection molding experience, this paper systematically sorts out full-process processing specifications and key precautions from raw material pretreatment, equipment selection, mold design, process parameter setting, defect improvement and post-processing standards to ensure stable molding, accurate size and reliable structural strength of PI products.
I. Raw Material Pretreatment and Hygroscopicity ControlPI resin has extremely strong moisture absorption capacity, and trace moisture will cause hydrolysis reaction under ultra-high temperature plasticization conditions, resulting in material degradation, surface bubbles, silver streaks and product embrittlement. Dehumidification and drying is the core pre-processing procedure. Only low dew point dehumidifying dryers can be used, and ordinary hot air drying is prohibited. The drying temperature is strictly controlled within the standard range, and the drying time is maintained sufficiently. Glass fiber reinforced PI materials need extended drying cycles to ensure the moisture content of raw materials is less than 0.02%.
Completely dried materials shall be stored in a closed and heat-preserved hopper to prevent secondary moisture absorption in the workshop environment. Unused raw materials that have been opened must be re-dried before reuse. Strictly avoid mixing PI with other high-temperature plastics such as PEEK and PEI. Thorough barrel cleaning is required during material replacement to eliminate hybrid impurities and prevent black spots, decomposition and abnormal molding.
II. Injection Molding Machine Selection and Adaptation Requirements
PI molding belongs to ultra-high temperature processing, and ordinary injection molding machines cannot meet the production conditions. The equipment must be equipped with a high-precision temperature control system to ensure stable and accurate temperature control of each barrel section. Adopt high-temperature special bimetallic screws with reasonable length-diameter ratio and compression ratio to reduce shear heat and avoid thermal decomposition of PI materials. The nozzle adopts an independent heating structure to prevent low-temperature cold material blockage and local temperature imbalance.
Strictly control the residence time of materials in the barrel to avoid thermal degradation caused by long-term high-temperature accumulation. For short-term shutdown, reduce barrel temperature or empty residual materials in time. Regularly clean carbon deposits inside the screw and barrel to ensure uniform plasticization. Equip high-temperature insulation protection for equipment heating parts and strengthen workshop ventilation and exhaust to discharge harmful gas generated by high-temperature processing and ensure operation safety.
III. Key Points of Mold Design and ManufacturingPI melt has poor fluidity, fast curing speed and large molding internal stress, which requires targeted mold optimization. Select high-hardness, high-temperature resistant and wear-resistant mold steel to adapt to long-term mass production of special plastics. The cavity surface adopts mirror polishing and DLC wear-resistant coating treatment to reduce demolding friction, effectively solve mold sticking, product pulling and whitening defects, and improve mold surface durability.
The mold must be equipped with a high-temperature oil-type mold temperature machine to maintain a stable high mold temperature environment. Conformal cooling water channels are arranged evenly to eliminate cooling dead angles, ensure uniform mold temperature distribution, and avoid product warping, deformation and dimensional deviation caused by temperature difference. Reasonably design exhaust structures on parting surfaces, weld line positions and deep cavity dead corners to smoothly discharge trapped gas and prevent burning, material shortage and obvious weld lines.
The ejection mechanism adopts balanced layout with sufficient ejection area to avoid top cracking and deformation of thin-walled PI products during demolding. The matching clearance of sliding blocks and inserts is precisely controlled to prevent flash and material overflow while ensuring flexible movement.
IV. Standardization of Core Injection Process ParametersTemperature control is the primary key of PI injection molding. The barrel temperature is set in a gradient manner, with a higher nozzle temperature to ensure smooth melt flow. Glass fiber modified PI materials adopt the upper limit of the temperature range. The mold temperature must be maintained at a high level; too low mold temperature will cause sharp increase of internal stress, product cracking and decreased welding strength.
Medium and low injection speed is adopted to prevent material decomposition caused by high-speed shear overheating. For thin-walled and complex structural parts, segmented speed regulation is used to balance filling efficiency and molding quality. The injection pressure is set reasonably to avoid excessive high-pressure filling and accumulation of internal stress. Low-pressure and short-time pressure holding is implemented to prevent structural deformation caused by excessive feeding.
Screw speed and back pressure are kept at a low and stable level to ensure uniform plasticization and avoid fracture of glass fiber and material performance damage caused by excessive shear. The cooling time is reasonably prolonged according to product thickness; sudden forced cooling is strictly prohibited to prevent stress cracking of PI products.
V. Improvement of Common Molding DefectsBubbles and silver streaks are mainly caused by incomplete drying, excessive material temperature and poor exhaust. The problems can be solved by strengthening dehumidification treatment, appropriately reducing molding temperature and optimizing mold exhaust structure. Product cracking and embrittlement stem from insufficient mold temperature, excessive internal stress and unreasonable pressure holding. Increasing mold temperature, reducing holding pressure and optimizing cooling rhythm can effectively improve such defects.
Mold sticking is mainly related to rough mold surface, large demolding resistance and mismatched temperature. Mirror polishing, mold surface coating and reasonable adjustment of mold temperature can stabilize demolding effect. Flash burrs are caused by insufficient clamping force, high material temperature and worn parting surfaces, which can be eliminated by matching clamping parameters, optimizing temperature and repairing mold sealing surfaces. Black spots and yellowing are induced by material retention and overheating decomposition; shortening material residence time and regular machine cleaning can stabilize product appearance quality.
VI. Post-Processing and Production Management SpecificationsPI products retain high internal stress after demolding. Annealing treatment is essential to eliminate residual stress. The products are kept warm in an oven at a constant temperature and then cooled slowly to improve product toughness, dimensional stability and prevent later cracking. Newly demolded high-temperature products are prohibited from contacting low-temperature environment directly, and slow cooling placement is adopted to reduce deformation caused by cold and hot temperature difference.
Establish standardized workshop operation procedures, unify parameter standards, and prohibit arbitrary modification of key data such as temperature, pressure and cycle. Regularly maintain mold water channels, exhaust grooves and ejection parts, clean carbon deposits and residual glue, maintain optimal equipment and mold working conditions, and realize long-term stable mass production of high-precision PI products.
ConclusionPI special plastic injection molding focuses on three core points: thorough dehumidification drying, high-temperature adaptive molding and low-stress processing. Strict implementation of raw material pretreatment, high-temperature equipment matching, mold temperature and exhaust optimization, and standardized process parameters, combined with professional post-annealing treatment, can effectively solve various difficult molding defects. Standardized operation and daily maintenance can significantly improve product yield, reduce mold repair and production loss, and meet the mass production requirements of high-end precision PI structural parts.
