Views: 0 Author: Site Editor Publish Time: 2026-02-01 Origin: Site
Polyetheretherketone (PEEK) is a high-performance semi-crystalline thermoplastic engineering plastic. Its crystallization behavior is the key factor determining the final performance of the products. During the injection molding process, the degree of crystallization, crystalline morphology, and crystalline perfection directly affect the mechanical properties, dimensional stability, heat resistance, and chemical stability of the products. However, the crystallization process of PEEK is influenced by multiple process parameters, making it difficult to control. In actual production, crystallization problems often lead to product defects. This article will start from the crystallization mechanism, systematically analyze the performance differences between the crystalline and non-crystalline states of PEEK, deeply explore the hazards of incomplete crystallization, and propose targeted improvement methods.
The crystallization process of PEEK is a phase transition process where the molecular chains change from an unordered state to an ordered arrangement. Under suitable temperature and pressure conditions, the mobility of the molecular chain segments increases, gradually forming nuclei and growing into spherical crystal structures. Completely crystallized PEEK (with a crystallinity of up to 30-40%) has the following performance characteristics:
Mechanical Performance Advantages: The crystalline regions act as physical cross-linking points, significantly enhancing the rigidity, strength, and hardness of the material. For every 10% increase in crystallinity, the tensile strength can increase by 15-20 MPa, and the bending modulus can increase by approximately 1 GPa. Products with high crystallinity exhibit excellent creep resistance, with minimal deformation under long-term loads.
Thermal Performance: The crystallinity directly affects the glass transition temperature (Tg) and melting point (Tm) of the material. For fully crystalline PEEK, the Tg is approximately 143°C, the Tm is approximately 343°C, and the heat distortion temperature can reach above 260°C, allowing for long-term use in high-temperature environments. The higher the crystallinity, the lower the thermal expansion coefficient and the better the dimensional stability.
Chemical Stability: The molecular chains in the crystalline region are closely arranged, making it difficult for solvent molecules to penetrate. Therefore, the chemical resistance and hydrolytic stability of PEEK in crystalline form are significantly superior to those in amorphous form. In high-temperature, high-pressure steam and acidic/alkaline environments, PEEK products with high crystallinity have a longer lifespan.
Dimensional Stability: Crystallization shrinkage is the main source of the forming shrinkage of PEEK. However, after complete crystallization, the product dimensions remain stable, the post-curing shrinkage is small, and the anisotropic shrinkage differences are relatively controllable.
When PEEK is rapidly cooled or when the process conditions are improper, the molecular chains fail to arrange in an orderly manner, resulting in an amorphous or low-crystallinity structure. The properties exhibit the following characteristics:
Mechanical performance degradation: The tensile strength and bending strength of amorphous PEEK are typically 20-30% lower than those of crystalline PEEK. Although the impact toughness has slightly improved, the overall load-bearing capacity is insufficient, and it is prone to creep deformation.
Thermal performance degradation: The heat distortion temperature has significantly decreased to below 150℃, limiting the long-term operating temperature. The thermal expansion coefficient has increased, resulting in significant dimensional fluctuations during temperature changes.
Poor chemical tolerance: The amorphous regions are prone to swelling by solvents, and are susceptible to stress cracking in organic solvents and in environments with strong acids and strong bases.
Inconsistent dimensions: Amorphous products exhibit a significant post-shrinkage phenomenon. During storage, they will continue to shrink and deform, making it difficult to ensure dimensional accuracy.
Insufficient strength leads to early damage: In actual use of partially amorphous PEEK products, when the load approaches the design limit, the amorphous regions become stress concentration points, which can easily cause crack propagation. Under dynamic loads or fatigue conditions, the service life is significantly shortened. For example, in the surrounding components of automotive engines, partially amorphous PEEK supports may fracture under the action of thermal cycles and vibrations.
Creep deformation impact on function: The molecular chain segments in amorphous state have strong movement ability and are prone to irreversible deformation under continuous stress. When used in precision transmission components, creep will cause changes in the mating gap, affecting the transmission accuracy and even leading to jamming and failure.
High Temperature Deformation and Dimensional Instability: The heat deformation temperature of non-fully crystalline products is low. When the operating temperature exceeds Tg, the material rapidly softens and deforms. For example, in electronic connectors, during high-temperature welding, the PEEK insulator may soften and deform, resulting in pin short circuits.
Accelerated Thermal Aging: The molecular chains in the amorphous regions have strong activity and are more prone to oxidative degradation at high temperatures. The chain breakage leads to performance deterioration. For uncrystallized products that have been used continuously at temperatures above 150℃, their service life may be shortened by more than 50%.
Post-shrinkage causes assembly difficulties: After injection molding, the partially crystallized products will continue to crystallize during storage, resulting in post-shrinkage. For precision components, post-shrinkage may cause interference fits to become clearance fits, or clearance fits to become interference fits, affecting assembly and performance. For instance, a 0.3% post-shrinkage of the bearing cage may cause the bearing to become stuck.
Anisotropic shrinkage differences are significant: The crystallization behavior of PEEK varies in the flow direction and the perpendicular direction. This difference becomes more pronounced when it is not fully crystallized. Thin-walled products are prone to warping and distortion, while thick-walled products may develop shrinkage cavities internally.
Stress Cracking Risk: In chemical media or high-temperature and high-humidity environments, the amorphous regions of uncrystallized products are prone to swelling due to solvents, and under the action of residual stress, silver streaks will form and eventually develop into cracks. For example, in the sealing components of chemical pumps, stress cracking leakage may occur when in contact with organic solvents.
Hydrolysis Acceleration: In a high-temperature water or steam environment, the ester bonds in amorphous PEEK are more prone to hydrolyze and break, resulting in a decrease in molecular weight and deterioration of performance.
The core significance of mold temperature: Mold temperature is the most direct parameter for controlling the crystallinity of PEEK. When the mold temperature is below 140℃, the molecular chain movement ability is insufficient, and the crystallization rate is extremely slow, resulting in products being basically amorphous; when the mold temperature is within the range of 160-200℃, the crystallization rate reaches its optimum, and a crystallinity of 30-40% can be achieved; when it exceeds 200℃, the crystallization rate is too fast, which may lead to the formation of coarse spherical crystals and a decrease in impact toughness.
Influence of barrel temperature: The barrel temperature (340 - 400℃) mainly affects the melt and thermal history. If the temperature is too high or the residence time is too long, it will lead to degradation of molecular chains and a decrease in crystallization ability; if the temperature is too low, the melt viscosity will be high, making filling difficult.
The drawbacks of rapid cooling: In injection molding, once the molten material enters the mold cavity, it cools rapidly. If the cooling rate is too fast (such as when the mold temperature is too low or the cooling time is too short), the molecular chains cannot have time to arrange in an orderly manner before being "frozen", resulting in a non-crystalline or low-crystallinity structure.
The Challenge of Slow Cooling: To achieve high crystallinity, slow cooling is required, but this will prolong the molding cycle and reduce production efficiency. Additionally, the difference in cooling rates between the inner and outer surfaces of thick-walled products is significant, which can lead to internal stress.
The influence of holding pressure: The pressure applied during the holding stage can promote the orientation and crystallization of molecular chains, but excessive pressure will lead to an increase in internal stress, making the product prone to warping. Insufficient holding time will result in insufficient secondary pouring, and internal shrinkage cavities are likely to form in the product.
Shear-induced crystallization: Near the gate and at the flow front, the melt is subjected to high shear forces, which may trigger crystallization prematurely, forming a shear-induced crystallization layer. This results in uneven crystallinity in different regions of the product.
Molecular Weight and Crystallization Ability: The high-molecular-weight PEEK has a high melt viscosity, with the molecular chains having difficulty moving and a slow crystallization rate; the low-molecular-weight PEEK has a fast crystallization rate, but its mechanical properties are poorer. The crystallization behaviors of different grades of PEEK vary.
The influence of nucleating agents: The addition of nucleating agents can increase the crystallization rate and lower the crystallization temperature, which is beneficial for achieving higher crystallinity at a higher cooling rate. However, it may affect the transparency and toughness of the product.
5. Systematic Approach to Improving PEEK Crystallization Issues
5.1 Process Parameter Optimization Strategy
Precise Control of Mold Temperature: Utilize a high-precision mold temperature controller to maintain the mold temperature at a stable 160-200°C (select the specific value according to product requirements). For thick-walled products, the mold temperature can be appropriately increased to 180-200°C; for thin-walled products, it can be slightly lower (160-180°C). The mold temperature fluctuation should be controlled within ±2°C.
Cooling Time Optimization: The cooling time should ensure that the product is fully crystallized, but it should not be too long as it may affect efficiency. It is recommended to determine the crystallization completion time through DSC testing. The actual cooling time should be 1.5 to 2 times the crystallization time. For products with a wall thickness of 3mm, the cooling time usually needs to be 60 to 120 seconds.
Pressure Holding Curve Design: A multi-stage pressure holding strategy is adopted: the first stage is high-pressure rapid retraction (80% of the injection pressure), the second stage is medium-pressure maintenance (50-60%), and the third stage is low-pressure to eliminate internal stress (20-30%). The holding time is determined based on the freezing time of the gate, and is generally 2-3 times the injection time.
Injection Speed Adjustment: Use medium-high speed injection to prevent the temperature at the melt front from dropping too rapidly. However, too fast a speed can lead to shear heating, resulting in degradation, and this needs to be optimized through mold flow analysis.
5.2 Mold Design Improvement
Mould Temperature System Optimization: Design uniform cooling channels to ensure consistent temperature across all areas of the mould cavity, avoiding uneven crystallinity due to temperature differences. For complex products, zone-controlled temperature can be adopted.
Gate and Runner Design: Use large-sized hot runners or hot nozzles to minimize the loss of melt temperature. The position of the gate should avoid creating weld marks in the stressed areas, and the size of the gate must ensure adequate secondary cooling.
Complete Exhaust System: Adequate exhaust to prevent trapped gases from burning. The exhaust groove has a depth of 0.02 - 0.05 mm and is positioned in the final filling area of the melt.
5.3 Material Pre-treatment and Post-treatment
Complete drying: Bake the material at 120-150℃ for 3-6 hours, and keep the material hopper at a temperature of 80-120℃ continuously to ensure the moisture content is below 0.02%. Moisture can hinder crystallization and cause the product to become foggy.
Annealing treatment: For products with insufficient crystallinity or high internal stress, annealing at 150-200℃ for 2-4 hours can be performed. Annealing can promote secondary crystallization, eliminate internal stress, and enhance crystallinity and dimensional stability. However, it is necessary to pay attention to the annealing temperature and time to avoid excessive crystallization leading to brittleness.
Application of nucleating agents: For thin-walled products that require rapid crystallization, it is advisable to add 0.1-0.5% of nucleating agents (such as talc powder, boron nitride, etc.). However, the impact on mechanical properties and appearance should be evaluated.
5.4 Process Monitoring and Quality Assurance
Online Monitoring: Utilizing cavity pressure sensors and temperature sensors, the process status is monitored in real time, enabling timely detection of any abnormalities.
Crystallinity Testing: Regularly take samples for DSC testing to monitor the changes in crystallinity. Density method and infrared spectroscopy can also be used for rapid detection.
"Establishing the process window": Through DOE experimental design, determine the safe ranges of each process parameter and establish standardized operation instructions.
VI. Case Analysis
Case 1: Warping and Deformation of the Automobile Engine Mounting Bracket
Problem Description: The PEEK engine bracket was stable in size after injection molding, but during assembly, it was found to have warping deformation, making it impossible to install.
Cause Analysis: Through DSC testing, the crystallinity of the product was only 25%, and the crystallinity distribution in the thickness direction was uneven. The reason was that the mold temperature was set at 150°C which was too low, and the cooling time was insufficient (40 seconds), resulting in incomplete crystallization and high internal stress. During storage, further crystallization and shrinkage occurred, causing warping.
Solution: Increase the mold temperature to 180℃, extend the cooling time to 90 seconds, and add the annealing process (180℃ × 3 hours). After the improvement, the crystallinity reaches 35%, the size is stable, and the warping deformation is eliminated.
Case 2: High Temperature Failure of Electronic Connectors
Problem Description: The PEEK connector deformed during the 260℃ reflow soldering process, resulting in a short circuit between the pins.
Reason Analysis: The heat distortion temperature test was only 180℃, which is far below the standard requirements. The DSC analysis showed that the crystallinity was only 20%, and there was a distinct cold crystallization peak, indicating that the cooling process during molding was too rapid.
Solution: Increase the mold temperature to 190℃, adopt a slow cooling process (controlled by the mold temperature controller), and the crystallinity of the product is raised to 38%, with the heat distortion temperature reaching 270℃, meeting the usage requirements.
VII. Summary and Outlook
The control of crystallization in PEEK injection molding is a complex system engineering that involves multiple factors such as materials, processes, molds, and equipment. Incomplete crystallization not only fails to meet the design requirements of the product's performance, but also may lead to early failure during use, posing safety risks. By precisely controlling the mold temperature, optimizing the cooling rate, improving the holding pressure process, and combining necessary post-treatment, the crystallinity and crystallization quality can be effectively enhanced. In the future, with the development of online monitoring technology, intelligent temperature control systems, and new nucleating agents, the control of PEEK injection molding crystallization will become more precise and efficient, providing more reliable solutions for high-end manufacturing.