Challenges and Solutions for Injection Molding and High-Temperature Engineering Plastics

In the pursuit of efficiency and high output in modern manufacturing, injection molding has become an indispensable method due to its cost-effectiveness. However, for applications requiring high strength and corrosion resistance, conventional injection molding materials often fall short. At this point, high-temperature engineering plastics, such as PEEK and Ultem, with their outstanding mechanical, thermal, and corrosion-resistant properties, become ideal alternatives. Nevertheless, the high melting points of these materials pose unprecedented challenges for injection molding. This article will delve into the issues encountered in the injection molding of these high-performance plastics and explore corresponding mold design strategies.

1. Features of High-Temperature Engineering Plastics

High-temperature plastics, with melting points ranging from 410°F (216°C) to 720°F (382°C), include both amorphous and semi-crystalline types. These materials are extremely sensitive in their liquid state, and prolonged residence times may lead to a decline in their mechanical properties.

2、Material Features and Injection Molding Challenges

1). Amorphous Thermoplastics: Materials like Ultem (polyetherimide or PEI) have a random molecular structure and a broad softening range. Although they are easy to thermoform and can be bonded with adhesives, they exhibit varying performance in terms of stress cracking, fatigue resistance, corrosion resistance, and wear resistance. Products made from these materials tend to stick to molds and may develop cracks during demolding. To mitigate these issues, high-temperature molds should be used to reduce internal stress, and a high-pressure injection with a gradually decreasing holding pressure strategy is recommended.

2). Semi-Crystalline Thermoplastics: Materials like PEEK have an orderly crystalline structure and a narrow melting range. They offer excellent corrosion resistance and wear resistance, but their impact resistance is relatively poor, and maintaining dimensional stability can be challenging. During injection molding, these materials are prone to sink marks and voids, requiring consistent holding pressure, medium-pressure injection, and maintaining a high mold temperature to ensure proper material crystallization.

3. Mold Design Strategies

  Cooling and heating are particularly critical in mold design for high-temperature engineering plastics.

  Cooling Channel Design: 

          Ensure that the coolant flow creates high turbulence to improve heat transfer efficiency. Channels are typically designed to achieve a Reynolds number exceeding 10,000.

  Coolant Selection: 

         Since the melting temperatures of high-performance plastics may be significantly higher than the boiling point of water, consider using oil or other alternative coolants.

  Use of Cooling Pins: 

         In areas where cooling channels are not feasible, cooling pins made from materials such as copper-beryllium alloy are commonly used to conduct heat.

  Mold Heating: 

        After demolding, the mold needs to be quickly reheated, which can be achieved using hot steam or oil.

  Mold Maintenance: 

       Regular maintenance and cleaning are essential to ensure efficient heat transfer.

  Mold Material Selection: 

       Balance wear resistance, cost, heat transfer capability, and machinability. Mold materials with high hardness and good thermal conductivity are typically chosen.

4. Conclusion

Designing injection molds for high-temperature engineering plastics is a complex and precise task that requires a comprehensive consideration of material characteristics, molding requirements, and mold performance. Although parts made from these high-performance materials can replace metals in many high-end applications, the design and manufacture of their molds are far more complex than those for standard plastics. Therefore, for manufacturers aiming for high quality and efficiency, a deep understanding of material properties and mastery of mold design techniques are crucial for achieving cost-effectiveness and efficient production.