Core Principles
1. High-Temperature Rheology Principle
Polyether Ether Ketone (PEEK) is a semi-crystalline high-performance engineering plastic with a high melting point, reaching 343°C. Its melt exhibits typical pseudoplastic non-Newtonian fluid characteristics, conforming to the power-law equation τ = Kγ˙ⁿ. During the injection molding process, thorough plasticization must be achieved within the range of 360-400°C. Temperatures that are too low result in excessively high melt viscosity, preventing mold filling, while temperatures that are too high can cause thermo-oxidative degradation, subsequently affecting the material's mechanical properties and biocompatibility.
2. Crystallization Control Principle
The crystallinity of PEEK directly impacts its strength, hardness, and dimensional stability. Its crystallization behavior follows the Avrami equation. When the mold temperature is ≥140°C, the crystallinity can reach 30%-35%, achieving optimal modulus and wear resistance. Conversely, if the mold temperature falls below 120°C, a low-crystallinity transparent layer tends to form, leading to reduced dimensional stability.
3. Internal Stress Control Principle
The cooling shrinkage rate of PEEK melt ranges from 1.0% to 2.0% and exhibits significant anisotropic characteristics. Measures such as employing high holding pressure, slow cooling, and uniform mold temperature can effectively reduce molecular orientation stress, prevent warpage and cracking in implant-grade components, and ensure the product's suitability for long-term in-vivo service.
Process Flow
Raw Material Pre-treatment
The process begins with vacuum drying: drying at 150°C for 4-6 hours ensures the moisture content is strictly controlled to ≤0.01%. A dust-free, closed-loop feeding system is utilized to prevent contamination from dust and metal particles.
Equipment Preparation
Select high-temperature resistant screws (nitrided with a wear-resistant coating) and barrels with a maximum temperature control capability of ≥420°C. The mold utilizes a high-temperature oil heating system to maintain a stable mold temperature within the 140-160°C range.
Melting and Plasticization
The barrel employs segmented temperature control: 360°C, 380°C, and 390°C, with the nozzle temperature set to 385°C. Screw speed is controlled at 30-60 r/min, with back pressure maintained at 10-15 MPa to ensure a homogeneous melt free of bubbles.
Mold Filling
Injection pressure is set between 180-250 MPa, with an injection speed of 20-40 mm/s. This facilitates rapid filling of thin-walled, complex cavities and prevents premature cooling and solidification of the melt.
Packing & Holding
Holding pressure is set at 70%-80% of the injection pressure. The holding time is controlled between 20-60 seconds, effectively compensating for volumetric shrinkage in thick-walled sections and preventing voids and sink marks.
Cooling and Setting
The mold maintains high-temperature circulation, with a cooling time of 60-180 seconds. Controlled slow cooling regulates the crystallization rate and reduces internal stresses.
Demolding & Annealing
The ejection temperature is maintained at ≥100°C to prevent cracking from cold ejection. Annealing is performed: holding at 200°C for 2-4 hours to completely eliminate internal stresses and stabilize product dimensions.
Post-Processing & Inspection
Deburring and precision grinding are carried out. Dimensional verification via CMM, mechanical property testing, and biocompatibility validation (ISO 10993) are performed.
Key Parameters & Material Properties
Material Properties
Melting Point: 343°C, requiring ultra-high-temperature molding, which presents a high processing threshold.
Tensile Strength: ≥95 MPa, close to bone density, enabling it to replace metal bone implants.
Crystallinity: 30%-35%, determining rigidity, wear resistance, and dimensional stability.
Biocompatibility: Compliant with ISO 10993-5/-10 standards, non-toxic, suitable for long-term human implantation.
Radiopacity: Can be modified for radiopacity, facilitating image localization in orthopedic and spinal surgeries.
Shrinkage Rate: 1.0%-2.0%, anisotropic, requiring pre-compensation in mold design.
Process Parameters
Temperature Control: Barrel temperature 360-390°C (too low results in incomplete filling; too high causes thermal degradation). Mold temperature 140-160°C (determines crystallinity and dimensional accuracy).
Pressure Control: Injection pressure 180-250 MPa (to overcome high-viscosity melt flow resistance). Holding pressure 130-200 MPa (to eliminate sink marks and voids).
Time Control: Cooling time 60-180 seconds (to ensure adequate crystallization and reduce internal stress). Annealing time 2-4 hours (to stabilize dimensions and prevent post-deformation).
Industrial Applications
Orthopedic Implants
Products include spinal fusion cages, acetabular liners, bone plates, and bone screws. The advantage lies in its elastic modulus being close to that of human bone, resulting in low stress shielding, along with X-ray transparency.
Dental Implant Systems
Encompasses products like dental abutments, temporary crowns, and superstructures for implants. Features include excellent biocompatibility, wear resistance, aesthetic appeal, and no metal allergies.
Surgical Instruments
Includes minimally invasive instrument handles, grasping forceps, and endoscopic accessories. Characterized by resistance to high-temperature steam sterilization, chemical corrosion, and lightweight properties.
Interventional Device Components
Products include catheter support structures and housings for interventional devices. Offer high strength, thin-wall capabilities, and high dimensional precision.
Technological Advancements
Carbon Fiber Reinforced PEEK
Utilizes 10%-30% milled carbon fibers for reinforcement, increasing the modulus to 15-20 GPa, closer to the properties of cortical bone, making it suitable for load-bearing orthopedic implants.
Micro-Injection Molding Technology
Enables precision molding of micro-components at the 0.1mm scale, applied in neuro-interventional and ophthalmic minimally invasive instrument fields.
Intelligent Optimization Technology
Utilizes in-mold sensing and AI-based parameter optimization for real-time monitoring of cavity pressure and temperature, automatically adjusting packing and cooling parameters to increase product yield to over 99%.
Localization Progress
Domestically produced high-purity PEEK achieves import substitution with lower impurity content and a 30%-50% reduction in cost.
Sterilization-Tolerant Modification
Adapted for repeated sterilization using gamma radiation, electron beam, and high-temperature steam, retaining ≥95% of mechanical properties.
Process Comparison & Optimization
Comparison with Similar Processes
PEEK Injection Molding vs. Metal MIM: PEEK offers light weight, X-ray transparency, and corrosion resistance; metal provides higher strength, suitable for load-bearing applications.
PEEK Injection Molding vs. Machining: Injection molding is efficient for high-volume, complex geometries; machining is suited for custom single pieces with higher precision.
PEEK vs. PLA/TPU: PEEK is for permanent implantable applications, high-temperature resistant, and high-strength; PLA is biodegradable; TPU offers high flexibility.
Defect Analysis & Optimization
Short Shots / Incomplete Filling: Caused by insufficient temperature, inadequate pressure, or undersized runners. Solutions: increase barrel and mold temperature, raise injection pressure, optimize gate location.
Voids / Sink Marks: Caused by insufficient holding pressure, excessively fast cooling, or non-uniform wall thickness. Solutions: extend holding time, design uniform wall thickness, increase mold temperature.
Cracking / Warpage: Caused by excessive internal stress, uneven mold temperature, or inadequate annealing. Solutions: implement high-temperature slow cooling, add annealing process steps, optimize mold cooling channels.
Engineering Practice Solutions
Spinal Fusion Cage Process Parameters
Barrel Temperature: Segmented control – rear zone 360°C, middle zone 380°C, front zone 390°C; nozzle at 385°C.
Mold Temperature: 150-160°C, ensuring crystallinity reaches 30%-35%.
Packing Pressure: Injection pressure 200-220 MPa, holding pressure 150-165 MPa (75% of injection pressure), holding time 40-60 seconds.
Annealing Process: Hold at 200°C for 3 hours, followed by slow cooling to room temperature within the furnace.
Quality Issue Rectification Measures
Addressing issues like edge brittleness and cracking after high-temperature sterilization:
Material Drying: Strictly implement vacuum drying process – dry at 150°C for 4-6 hours, ensuring moisture content ≤0.01%. Utilize a closed-loop feeding system to prevent moisture reabsorption.
Process Temperature: Lower barrel and nozzle temperatures (not exceeding 400°C), increase mold temperature to above 140°C, and verify the accuracy of the temperature control system.
Post-Processing: Perform standard annealing immediately after demolding (hold at 200°C for 2-4 hours), controlling heating and cooling rates. Inspect the mold ejection system to prevent demolding damage.
Through systematic process control and rigorous quality management, medical-grade PEEK injection-molded products can meet the high standards required for implantable medical devices, providing patients with safe and reliable medical solutions.
