Meaning
Flow behavior and deformation of molten plastics under thermal and mechanical stress govern the shaping of materials during manufacturing. Understanding polymer processing rheology is essential for designing extruders, dies and injection moulds that operate within safe processing windows. This discipline analyzes how polymers respond to high deformation rates.
Viscosity Variation
Thermoplastic resins exhibit non-Newtonian behavior where their flow resistance decreases as they are sheared. This shear-thinning behavior, which is a core concept in polymer processing rheology, allows thick melts to flow through thin gates and complex mould cavities at high speeds. It reduces the required injection pressure and avoids excessive thermal degradation caused by mechanical heating in localized narrow channels.
Shear Rate
Screw rotation and injection speeds determine the rate of deformation applied to the polymer. Measuring polymer processing rheology across a wide range of shear rates helps moulders predict how the material will behave during the filling stage. This measurement is necessary when switching from virgin to regrind resins.
Melt Fracture
Flow instabilities occur at high extrusion speeds when the shear stress at the die exit exceeds a critical threshold. These instabilities, studied under polymer processing rheology, result in surface defects such as sharkskin or melt fracture. Correcting these defects requires adjusting melt temperatures or die geometry.
