Extrusion of TPU (Thermoplastic Polyurethane)

1. Material Preparation

• TPU Pellets Selection: Choose TPU pellets with appropriate hardness (shore hardness, typically ranging from 50A – 90D), melt flow index (MFI), and performance characteristics (e.g., high abrasion resistance, elasticity, and chemical resistance) according to the final product requirements.
• Drying: TPU is hygroscopic, so it must be pre – dried before extrusion to remove moisture. Moisture can cause bubbles, surface defects, and reduced mechanical properties in the extruded products. Drying is usually carried out at a temperature between 80 – 100°C for 3 – 6 hours.

2. Extrusion Process

• Extruder Components
  • Barrel: The barrel of the extruder is heated in multiple zones to gradually melt the TPU pellets. The temperature profile is carefully set to ensure proper melting without over – heating the material, which could lead to degradation. For example, the feed zone temperature might be around 160 – 180°C, the compression zone around 180 – 200°C, and the metering zone around 200 – 220°C, but these values can vary depending on the TPU grade.
  • Screw: The screw rotates within the barrel, conveying, compressing, and melting the TPU pellets. Different screw designs (e.g., single – screw or twin – screw extruders) can affect the mixing, melting efficiency, and output rate of the extrusion process. Twin – screw extruders generally offer better mixing and more uniform melting, especially for complex formulations.
• Melting and Mixing: As the TPU pellets move through the barrel, they are gradually melted by the combination of heat from the barrel and shear generated by the screw rotation. The molten TPU is thoroughly mixed to ensure a homogeneous melt.
• Die Design and Shaping: The molten TPU is then forced through a die, which determines the cross – sectional shape of the extruded product. Dies can be customized to produce different shapes, such as round for tubes, rectangular for profiles, or flat for sheets and films. After passing through the die, the extruded TPU is cooled and solidified, usually by passing through a water bath or using air cooling.

3. Post – Processing

• Calibration and Sizing: For some extruded products, calibration and sizing operations are required to ensure accurate dimensions. This may involve using calibration sleeves, vacuum sizing tanks, or other devices to control the outer diameter, thickness, or other critical dimensions of the product.
• Cutting or Winding: Depending on the application, the extruded TPU product is either cut into specific lengths (for profiles, tubes, etc.) or wound onto rolls (for sheets and films).