Polymer Melt Flow Behavior in High Shear Injection Molds
High shear molding relies on pseudoplastic shear thinning, requiring capillary rheometry over melt flow index to control viscous heating and pressure losses.
Extrudate Die Swell
Meaning
Post-extrusion dimensional expansion occurs when elastic polymer molecules relax upon exiting a restricted shaping orifice. This physical enlargement, recognized in polymer processing as extrudate die swell, reflects stored elastic strain accumulated during high-shear channel flow. Datasheet values for nominal resin viscosity do not predict die expansion ratios because elasticity depends on molecular weight distribution rather than average shear viscosity.
Process technicians adjust land length and melt temperature to control profile dimensions before the extrudate enters the quenching bath.
Elastic Recovery
Viscoelastic polymer chains undergo severe orientation and compression while flowing through narrow die lands. Upon emerging into the atmospheric exit zone, normal stresses force the polymer melt to expand radially while shortening along the flow direction. Operating parameters like die land geometry and shear rate govern the magnitude of this recovery response.
High molecular weight fractions produce higher normal stress differences, driving pronounced cross-sectional growth in extrudate profiles. Lowering melt temperature increases elastic energy storage, which elevates exit expansion ratios during continuous extrusion operations.
Profile Distortion
Uncontrolled cross-sectional expansion distorts wall thickness and alters final part geometry during extrusion processing. Applying extrudate die swell corrections during tooling design compensates for cross-sectional growth in complex profiles.
Die Calibration
Tooling designers modify die opening geometry to compensate for viscoelastic expansion during profile production. Incorporating land length extensions increases polymer residence time in the die land, allowing molecular relaxation before exit. Recycled resin lots introduce variations in elasticity that cause dimensional drift across production batches.
Correcting land geometry or adjusting draw-down ratios ensures extrudate dimensions remain within specified tolerances.