Meaning
Stretching of polymer chains along the direction of flow when the melt passes through converging or diverging channel geometries defines a critical mode of rheological behavior. Polymer melts in extrusion and injection moulding experience both shear and tensile forces as they move through die lips or gates. This phenomenon of extensional deformation occurs when the fluid accelerates, pulling the polymer molecules into a highly aligned state.
The resistance of the melt to this stretching action determines how well it can maintain its shape during free-flow processes like blow moulding or film blowing.
Melt Strength
High molecular weight resins with long-chain branching resist rapid thinning when subjected to tensile forces during processing. When extensional deformation occurs in a melt with low melt strength, the polymer web or parison will sag and tear. This behavior is critical in film extrusion where a stable bubble requires a material that exhibits strain hardening under tension.
Flow Instability
Excessive stretching at high throughput speeds can trigger severe surface defects and structural failures in the extruded product. When the rate of extensional deformation exceeds the relaxation time of the polymer, the melt can fracture as it exits the die. This fracture results in sharkskin or melt fracture defects, which ruin the surface finish and reduce the strength of the final part.
To avoid these issues, processors must limit line speeds or modify the flow channel angles to allow a gentler acceleration of the melt.
Processing Performance
Monitoring how a resin behaves under tensile forces helps moulders select the correct grade for complex hollow parts. While standard datasheets focus on shear viscosity, measuring the response of the polymer to extensional deformation provides a more accurate prediction of its performance in the mould. In extrusion blow moulding, a stable parison requires a balance of elasticity and tensile resistance to prevent uneven wall thickness.
Ensuring that the melt resists excessive stretching allows the production of parts with consistent barrier properties and structural integrity.
