Application of PSU in Injection Molding: Properties, Process, and Best Practices

Polysulfone (PSU), sometimes also referred to as PSF, is a high-performance thermoplastic engineering material. Known for its high heat resistance, mechanical strength, and excellent dimensional stability, PSU is widely used in injection molding for producing precision and functional parts. Its unique balance of strength, electrical insulation, and resistance to creep and aging makes it a preferred choice for many industries. In this article, we will explore the application of PSU in injection molding, from material properties to process requirements and design considerations.

Properties of PSU for Injection Molding

To begin with, PSU is an amorphous plastic with no clear melting point. Instead, it has a glass transition temperature (Tg) of about 190°C and requires molding temperatures above 280°C. Injection-molded PSU parts often display good transparency and mechanical strength.

One important characteristic of PSU is its melt behavior. It behaves similarly to PC (polycarbonate) with a high viscosity melt that shows sensitivity to temperature. While its viscosity decreases significantly when temperature rises above 330°C, the material still has relatively poor flowability. This makes mold design and processing parameters crucial to achieve defect-free products.

Additionally, PSU has low moisture absorption (around 0.6%), much lower than nylon. However, even small amounts of water can cause degradation during molding at high temperatures. Therefore, proper drying is always necessary before processing.

Common Applications of PSU Injection Molding

Due to its excellent thermal and electrical properties, PSU is used for a wide variety of functional injection-molded parts. Common applications include:

• Insulation components such as transformer parts, contactor housings, and thyristor caps.
• Coil bobbins, insulating sleeves, and terminal blocks.
• Precision electronic components such as printed circuit boards and bushings.
• Covers, housings, and other functional parts requiring high stability.

PSU’s performance in high-temperature and high-load environments makes it a valuable alternative to materials such as POM or PA66 when higher dimensional stability and heat resistance are required.

Internal Stress Issues in PSU Injection Molding

Like other engineering plastics, PSU injection-molded parts are prone to internal stresses, which can lead to cracking or premature failure. Four main types of stress are commonly observed:

• Thermal stress – caused by uneven cooling during molding. The outer layer solidifies faster than the inner layer, creating compression and tension stresses.
• Orientation stress – occurs when polymer chains align in the flow direction and cannot relax due to rapid cooling.
• Shrinkage stress – results from uneven shrinkage of different zones in the molded part.
• Demolding stress – arises when poor mold design or low draft angles cause deformation during ejection.

To minimize these stresses, it is essential to control mold temperature, part thickness, and cooling uniformity. For PSU, mold temperatures of around 130°C are generally recommended.

Processing Requirements for PSU

PSU requires specific processing conditions to achieve optimal results. Some key points include:

• Drying: PSU must be dried before molding, ideally at 120–140°C for 3–6 hours. For critical parts, a dehumidifying dryer is preferred to prevent degradation.
• Screw design: A low compression ratio screw with a length-to-diameter ratio of 14–20 is recommended to handle its high viscosity.
• Injection unit: Machines should have precise temperature control, capable of reaching above 300°C, with low screw speed (15–45 r/min).
• Nozzle design: Extended heated nozzles with diameters above 5 mm help reduce flow resistance and prevent drooling.

By following these processing requirements, manufacturers can ensure consistent part quality and prevent material breakdown.

Mold Design Considerations

PSU’s high viscosity and relatively low flow length-to-thickness ratio (around 80) require special attention in mold design. Some guidelines include:

• Maintain part thickness above 1.5 mm, preferably above 2 mm, to improve flow.
• Use rounded corners instead of sharp edges to reduce stress concentrations.
• Keep shrinkage allowance between 0.4% and 0.8%, with uniform shrinkage in all directions.
• Design short, wide runners and appropriately sized gates to ensure smooth filling.
• Provide adequate venting with slots less than 0.08 mm deep to allow air escape.
• Apply mold temperatures between 130°C and 150°C for optimal results.

With these measures, parts can achieve smooth surfaces, dimensional stability, and excellent performance.

Key Process Parameters

The success of PSU injection molding depends on careful parameter control:

• Barrel temperature: Typically set between 280°C and 315°C. Higher temperatures improve flow but risk degradation if above 320°C.
• Injection pressure: Around 100 MPa is common, though higher pressure may be needed for thin parts.
• Injection speed: A lower injection speed is usually better to avoid melt fracture, except for very thin walls requiring faster filling.
• Screw speed: Keep between 15 and 45 r/min to reduce shear heating.
• Mold temperature: Generally between 130°C and 150°C, never below 120°C.

Balancing these settings ensures consistent molding quality and avoids common defects like brittleness, voids, or stress cracking.

Conclusion

PSU is a high-performance engineering plastic well-suited for injection molding functional components that require high heat resistance, mechanical strength, and dimensional stability. Although its high viscosity and tendency to generate internal stress present processing challenges, proper drying, optimized mold design, and careful process control can ensure reliable results.

For manufacturers seeking a durable material for demanding injection-molded parts, PSU offers significant advantages over conventional plastics.

If you are looking for high-quality PSU materials for injection molding, Shengwen is a reliable polysulfone supplier providing consistent quality and technical support for your applications.