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Swell and distortions of high‐density polyethylene extruded through capillary dies
Author(s) -
Eggen Svein,
Hinrichsen Einar L.
Publication year - 1996
Publication title -
polymer engineering and science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.503
H-Index - 111
eISSN - 1548-2634
pISSN - 0032-3888
DOI - 10.1002/pen.10428
Subject(s) - die swell , materials science , extrusion , swell , capillary action , die (integrated circuit) , mechanics , composite material , slip (aerodynamics) , pressure drop , instability , thermodynamics , physics , nanotechnology
The effect of varying the die entrance angle and the die length on extrudate swell and on the onset of extrudate distortion in capillary extrusion has been studied. Using theory from the literature, we have analyzed the contribution to the total pressure drop from the elongational and shear deformation in the entrance region, and from the capillary pressure drop in the land region of the die. From the contribution of the elongational deformation, we obtained an estimate for the elongational viscosity of the polymer. The same analysis was used to study the influence of the die geometry on the stick‐slip instability. It is found that the elongational component at the inlet region mainly influences the extrudate distortions. The onset of the stick‐slip instability occurs within 10% at a wall stress τ w of 0.3MPa, where τ w is calculated from expressions assuming fully developed flow. The variation around this average value is systematic with changes in die geometry, and the observed variations are probably due to the non‐homogeneous pressure field in the die. We also propose a model for predicting extrudate swell. Input to the model are material parameters obtainable from oscillatoric measurements of the loss and storage modulus and residence times calculated from the geometry of the die. The swell model includes a fitting parameter that sets the overall scale of the swell.