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Home / Injection Molding of Flexible Materials: Precautions for TPU/TPE Processes

Injection Molding of Flexible Materials: Precautions for TPU/TPE Processes

2025/11/07 By le zhan

TPU and TPE injection molding

Two common mistakes are easily made when injection molding TPU/TPE: using injection pressures below 1400 kgf/cm² and using standard (non-nitrided) screws and barrels. TPU and TPE offer unparalleled flexibility and durability, but their unique flowability and abrasion resistance require specialized process control to achieve optimal results. Therefore, several key considerations must be taken into account when injection molding TPU/TPE materials, focusing on mandatory high pressure, nitriding of parts, and other critical steps designed to help you obtain consistent, high-quality parts.

Why is an Injection Pressure of 1400 kgf/cm² or Higher Required for TPU/TPE Injection Molding?

TPU and TPE have higher viscosity than rigid plastics like PP or ABS, meaning they are difficult to flow into the mold cavity. Therefore, injection molding processes for these materials require a minimum injection pressure of 1400 kgf/cm² on the injection molding machine, and often higher pressures for complex parts.

Low pressure (below 1400 kgf/cm²) can cause gaps in the mold because material cannot reach all cavities or thin-walled areas. One medical tubing manufacturer solved 80% of its TPU under-injection problems by increasing the pressure from 1200 kgf/cm² to 1500 kgf/cm².

Additionally, high-pressure compacts TPU/TPE, eliminating air bubbles, and ensures consistent overall flexibility of the part. Under-pressurized parts often have soft spots or brittle edges. For parts with complex details (e.g., textured grips) or long flow paths, increase the pressure to 1600–1800 kgf/cm². Use pressure profiling to avoid shear damage—TPU/TPE may degrade if the pressure peaks are too high.

Injection Pressure

Use nitrided screws and barrels in injection molding machines.

TPU and TP are abrasive and can cause excessive wear on the screws and barrels of standard injection molding machines. This wear leads to uneven plasticization, material degradation, and frequent part replacements. To ensure reliable TPU/TPE injection molding, always use nitrided screws and barrels in your injection molding machine.

What is nitriding? Nitriding is a heat treatment process that creates a hard, wear-resistant surface on steel parts. Nitrided surfaces resist the wear of TPU/TPE, extending screw/barrel life by 3-5 times compared to standard chrome-plated parts. Nitrided parts maintain consistent melt quality by preventing material buildup and ensuring uniform flow. One automotive parts factory reduced the discoloration rate of its TPU parts by 90% after upgrading to nitrided screws. Never use bimetallic screws for ordinary TPU/TPE—it is both wasteful and expensive. Nitrided screws strike a perfect balance between durability and cost, making them suitable for flexible materials. Topstar’s service team frequently observes this: “Factories using ordinary screws to secure TPE materials need to replace them every 6 months. Factories using nitrided screws, however, experience no problems for 2-3 years.”

Injection molding machine barrel

TPU/TPE Material Drying Precautions

TPU is highly hygroscopic; even small amounts of moisture can disrupt the injection molding process, leading to bubbles, cracks, and reduced part strength. TPE varies depending on grade; some grades are non-hygroscopic, but most perform better after drying. Please follow these steps:

TPU Drying: Dry in a dryer at 80–100°C for 2–4 hours. Use a hopper dryer with a dew point of -40°C to ensure complete removal of moisture. Please do not skip the drying step, as we have found that up to 20% of TPU parts are defective due to undried material.

TPE Drying: Thermoplastic olefin TPE is non-hygroscopic, but styrene-based TP and TPE blends stored in humid environments may require drying at 60–80°C for 1–2 hours.

The best storage method is to store TPU/TPE products in a sealed bag containing desiccant. If opened, air dry immediately, as moisture will be absorbed within hours in humid environments.

TPU and TPE Material Drying Precautions

Temperature Control Techniques for TPU/TPE Processes in Injection Molding

Temperature directly affects the flowability, flexibility, and part quality of TPU/TPE. Unlike rigid plastics, even small temperature fluctuations (±5°C) can cause serious problems. Temperature can be controlled as follows:

Melt Temperature:

TPU: 160–220°C. Temperatures above 230°C can lead to thermal degradation (yellowing, brittle parts).

TPE: 130–190°C (varies depending on the matrix polymer; styrene-based TPEs require lower temperatures, while olefin-based TPEs require higher temperatures).

At the injection molding machine, maintain the mold temperature at TPU at 30–60°C and TPE at 20–40°C. Higher mold temperatures improve flowability but increase production cycles; lower temperatures accelerate cooling but may cause shrinkage marks. Utilize water cooling to maintain a consistent temperature in the mold cavity. Also, maintain the nozzle temperature 5–10°C higher than the melt temperature to prevent material solidification.

Post-molding treatment to maintain TPU/TPE properties

TPU and TPE parts are flexible but prone to deformation after molding. Proper post-molding treatment ensures that they retain their shape and properties. Trim or pack only after the parts have completely cooled. Premature cooling can lead to deformation. Trim the gate with sharp, clean tools. Dull blades can tear TPU/TPE, resulting in rough edges and reduced part strength. For high-volume production, use a die-cutting machine for clean, consistent cuts. Lay parts flat in a cool, dry place. Avoid stacking heavy objects on TPU/TPE parts to prevent deformation under pressure. Use breathable packaging to prevent moisture buildup.

TPU/TPE Process and Core Material Considerations

67% of common problems in TPU/TPE injection molding can be solved by prioritizing two key factors: using an injection pressure of 1400 kgf/cm² or higher, and equipping the injection molding machine with a nitriding screw and barrel. When combined with proper drying, temperature control, mold design, and post-processing, stable, high-quality flexible parts can be achieved.

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