Addressing Over-Shearing in TPE/TPR Processing: Preserving Properties via Low-Shear Screws

Thermoplastic Elastomers (TPE/TPR) are widely used in automotive parts, medical devices, and consumer electronics due to their combination of rubbery elasticity and plastic processability. However, TPE is highly sensitive to shear stress. During processing in a twin screw extruder, excessive shear can lead to polymer chain scission, a drop in melt viscosity, and hardness deviation. The key to solving this lies in a "low-shear, high-distribution" screw configuration that maximizes the retention of the material’s original mechanical properties.

In TPE/TPR compounding, over-shearing causes several negative outcomes:

• Degradation of Physical Properties: Localized "hot spots" from intense shear trigger thermal degradation, resulting in decreased tensile strength and increased compression set.

• Surface Defects: Over-pulverized fillers or degraded polymers can create gels on the pellet surface or cause a "sticky" feel in the final product.

• Hardness Inconsistency: Uncontrolled shear heat disrupts the micro-phase structure of base resins like SEBS/SBS, leading to hardness fluctuations across batches.

To achieve uniform mixing while protecting molecular chains, the screw and barrel configuration should adhere to these standards:

• Function: Utilizing large-pitch elements in the melting and conveying zones lowers the degree of screw filling, thereby reducing the average shear rate on the material.

• Technical Parameter: A pitch of 1.5 to 2 times the screw diameter is recommended for gentler material advancement.

• Selection Advice: Reduce the use of 90-degree neutral kneading blocks. Instead, use narrow kneading blocks with 30-degree or 45-degree forward staggering angles.

• Specialized Elements: Incorporate Screw Mixing Elements (SME) or Turbine Mixing Elements (TME). These components split and recombine the melt multiple times without generating intense frictional heat. (Reference: Distributive Mixing Efficiency Comparison - Ref: #QC-2024-EXP-08)

• Parameter Support: The unilateral clearance between the screw and the extruder barrel should be maintained at 0.05 mm - 0.10 mm.

• Logic: A slightly wider clearance compared to rigid plastics provides a broader flow path for the elastomer, preventing extreme shear forces in ultra-narrow spaces.

• Mirror Polishing: The surface roughness of the screw should reach Ra < 0.4 um. Superior smoothness reduces frictional resistance between the material and the metal surface, further lowering self-generated heat.

• Wear Resistance: While TPE is not highly abrasive, maintaining precision clearance requires a barrel hardness of 58-64 HRC.

In TPE/TPR extrusion, aggressive processing is often counterproductive. By selecting large-pitch elements, optimizing distributive mixing combinations, and ensuring temperature stability within +/- 1°C, manufacturers can perfectly preserve the material's elastic framework. For plants aiming for premium haptics and consistency, high-precision low-shear parts compatible with Coperion or Leistritz standards are essential for achieving technical superiority.