Key Considerations for PEEK Injection Molds

PEEK (Polyether Ether Ketone) is a high-performance specialty engineering plastic known for its exceptional thermal resistance, mechanical strength, chemical stability, and dimensional stability. It is widely used in demanding industries such as aerospace, medical implants, electronics, and automotive. However, due to its challenging processing characteristics, PEEK imposes stringent requirements on injection mold design, material selection, manufacturing, and process control.

Below is a hierarchical detailed analysis of the key points for PEEK injection molds.

1. Mold Material Selection

The high processing temperatures (melting point 360–400°C) and the common inclusion of abrasive fillers like glass or carbon fibers in PEEK necessitate mold materials with exceptional high-temperature resistance, wear resistance, and corrosion resistance.

High-Temperature Tool Steels: It is essential to select premium steels capable of withstanding high temperatures and pressures, such as H13, P20 (for general structures), NAK80 pre-hardened steel (for high-precision molds), or high-chromium steels like the SK, SKD, SKH series134. For precision molds (e.g., medical implants), steels must exhibit high hardness (HRC 55 or above) and excellent polishability14.

Corrosion Resistance and Wear Treatment: The potential for chemical reactions at high temperatures requires the mold cavity to undergo bulk heat treatment (e.g., quenching) and surface treatments (like nitriding, chrome plating, or hardening) to enhance wear resistance and service life37.

Other Mold Plate Materials: To balance costs, non-cavity plates can be made from materials like Cr12 or CrWMn.

2. Critical Mold Design Aspects

Mold design directly impacts product quality and production efficiency, requiring careful attention to the gating system, temperature control system, venting, and ejection mechanism.

Gating System Design:

Runners: Circular or trapezoidal runners are preferred to ensure smooth flow and reduce resistance and shear stress. Runners should be smooth without sharp turns34. A cold slug well at the runner's end is recommended.

Gates: Gates should be positioned at the thickest section of the part to minimize weld lines and stress concentration. Gates should be short, with a diameter typically one-third to one-half of the wall thickness (≥1mm for unfilled, ≥2mm for reinforced grades), and not less than 0.6mm48. Valve-gated hot runner systems or direct sprue gates are recommended to reduce flow marks.

Temperature Control System:

Mold Temperature: PEEK is sensitive to mold temperature, usually requiring 150–220°C. Thin-walled molds (150–180°C) facilitate rapid cooling, while thick-walled molds (180–220°C) help minimize sink marks16. Uniform temperature distribution is crucial, achieved through well-designed cooling channels (e.g., conformal cooling) or oil circuits to prevent warpage.

Insulation Design: Insulating plates (e.g., made from asbestos or old PCB, >8mm thick) should be installed between the mold and the injection machine. Thermal insulation around the mold perimeter can reduce heat loss.

Venting System: PEEK's poor flowability can trap air, leading to bubbles, burn marks, or surface defects. Venting slots should be 0.02–0.05mm deep and about 2mm wide. Venting can also utilize ejector pin clearances or slider gaps.

Cavity and Ejection Design:

Draft Angles: The relatively high shrinkage rate of PEEK (1.1–2.5%) necessitates draft angles of 1–3° for easy part release.

Surface Finish: The cavity surface should be highly polished (Ra <0.8μm or below 10S) to prevent sticking and surface defects.

Ejection Mechanism: For standard molds, ejector pin layout must be even. For high-precision molds (e.g., optical lenses), gas-assisted ejection or valve-gated hot runner ejection can be used to avoid ejection marks.

3. Injection Process Control

The mold must work in precise harmony with the injection process to ensure material properties and production efficiency.

Injection Parameters:

Injection Pressure and Speed: Molds with simple structures require lower pressure (80–120MPa) and can use faster speeds. Complex molds (e.g., with slender cores) need higher pressure (120–180MPa) and multi-stage injection speeds (slow first then fast) to avoid short shots, trapped air, or weld lines.

Holding Pressure and Cooling Time: Appropriate holding pressure (e.g., 7.5MPa) and cooling time help reduce shrinkage and deformation.

Barrel and Nozzle Temperature: Barrel temperature is typically 360–400°C (higher for reinforced grades). The nozzle requires independent heating to prevent freezing610. The feed zone of the barrel should be cooled to 70–100°C for smooth feeding.

Material Handling and Downtime Control: PEEK material must be dried at 150°C for 3 hours or 160°C for 2 hours, with strict avoidance of contamination610. If downtime exceeds 1 hour, the barrel temperature should be reduced to 340°C; if exceeding 3 hours, the barrel should be purged.

4. Mold Manufacturing and Maintenance

Machining Precision: Precision manufacturing techniques like CNC and EDM are essential to ensure dimensional stability. Shrinkage compensation (typically 1.1–1.4%) must be applied, noting the difference in shrinkage between perpendicular and parallel flow directions.

Strength and Wear Resistance: Molds require reinforced cavities to withstand high pressures (100–200MPa) and wear-resistant treatments.

Application Case: For instance, a medical device manufacturer successfully produced high-precision PEEK surgical instrument components by optimizing mold design and process (including mold flow analysis and annealing), enhancing product consistency.

5. Future Development Trends

With the advancement of Industry 4.0, PEEK injection molds are moving towards intelligence (real-time monitoring, automated production), application of new materials (improved temperature and wear resistance), and green manufacturing (energy saving and environmental protection).

Conclusion

PEEK injection molding is a systematic project requiring meticulous coordination of materials, design, process, and manufacturing. Only by comprehensively controlling these aspects can high-performance and high-precision PEEK products be manufactured to meet the demands of advanced industries.