Capillary Rheometry Corrections and Mooney Wall Slip Analysis for Polyolefin Blends
Capillary rheometry requires Bagley and Rabinowitsch-Weissenberg corrections alongside Mooney wall slip analysis to prevent polyolefin blend die miscalculations.
Capillary Die Geometry
Meaning
Design parameters of a rheological flow channel determine the shear rates and pressure drops achieved during melt characterisation. In capillary rheometry, the term capillary die geometry refers to the length, diameter, and entry angle of the die.
Orifice Influence
Melt processing depends on the aspect ratio of the flow channel to stabilize flow and minimize surface defects. When capillary die geometry has a low length-to-diameter ratio, the entrance effects dominate the total pressure drop and complicate calculations. Increasing the length helps establish a fully developed velocity profile.
Extrusion Boundary
Surface distortion often occurs when elastic memory from the reservoir remains unrelaxed as the melt exits the die. Optimizing capillary die geometry reduces the severity of sharkskin and melt fracture in extruded parts. A longer die provides more residence time for the polymer chains to untangle and relax, which lowers the stored elastic energy and improves the surface finish of the extruded profile.
Viscosity Calibration
Process conditions must be adjusted to match the flow behavior established by the specific die dimensions. Moulders rely on capillary die geometry to model the high-shear environments of injection gates. Correct dimensions ensure that the viscosity measurements represent the true shear conditions of production.