Die swell and recoverable shear strain in polyethylene extrusion

The present paper treats die swell as unretarded recovery of elastic strain imparted during flow into and through the die. Three mechanistic models are developed. All three models treat the material as an elastic solid for the purpose of elucidating die swell, and two models utilize rubber‐like elasticity theory. Equations relating swelling ratio, recoverable shear strain (S R ), axial normal stress, shear stress, and shear modulus are derived for the three models. While the forms of the results differ somewhat from model to model, the numerical results are quite similar. Experimental die swell data were obtained for four high density polyethylene resins in several large L/D capillaries using an Instron Capillary Rheometer. A procedure for obtaining equilibrium, or strain‐free, values of the swelling ratio by annealing the extrudates above the melting point is discussed. Values of S R and normal stress were calculated and compared to corresponding quantities obtained directly from measurements using a cone‐and‐plate Weissenberg Rheogoniometer. Excellent agreement between results from the widely divergent methods of measurement was obtained.