Pressure Dependent Polymer Viscosity Shifts within Sub-Micron Cavity Channels
Pressure-dependent viscosity increases melt resistance in sub-micron channels while wall slip and extreme shear thinning lower dynamic injection requirements.
Barus Equation
Meaning
Polymer melt swell exiting a die finds its analytical foundation in the barus equation, relating normal stress differences to recoverable elastic deformation. High elasticity causes extrudate swelling far beyond the geometrical dimensions of the orifice during profile extrusion. Polymer chains store energy under the severe shear rates applied inside the barrel and the die land.
Stored energy relaxes rapidly once the restriction vanishes at the exit plane. Excessive die swell disrupts dimensional control on medical tubing lines and film casting operations. Production engineers adjust melt temperature and throughput rates to counteract this phenomenon before scrap rates climb.
Stress Recovery
Molecular orientation relaxes immediately after the polymer clears the capillary exit. Normal stresses generated along the flow direction drive lateral expansion as the material recovers from shear strain. Shear viscosity alone fails to predict the magnitude of the diameter increase observed in amorphous thermoplastics and semicrystalline resins.
Elastic recovery depends directly on the ratio of the first normal stress difference to the shear stress at the wall. High molecular weight fractions increase relaxation times and amplify dimensional distortion.
Melt Fracture
Critical wall shear stress thresholds trigger flow instabilities known as melt fracture during high speed processing. Surface roughness appears on extruded strands once the recoverable shear strain exceeds a critical limit. Sharkskin defects ruin aesthetic appeal and compromise mechanical performance in finished profiles.
Internal tearing develops inside the die land when pressure oscillations exceed local tensile strength limits. Optimizing die geometry and applying fluoroelastomer processing aids suppress these defects without sacrificing line speed.
Viscoelastic Response
Characterizing polymer melts requires distinguishing steady shear viscosity from transient elastic properties. Oscillatory rheometry measures storage moduli that correlate directly with die swell performance under factory conditions. Virgin polymer batches exhibit predictable relaxation spectra during laboratory testing.
Regrind material undergoes chain scission that reduces molecular weight and lowers elastic recovery. Moulders compensate for batch variations by modifying thermal profiles on the extrusion line.