Polymer Engineering Faculty Research


Toward a viscoelastic modeling of anisotropic shrinkage in injection molding of amorphous polymers

Document Type


Publication Date

Fall 2005


A novel approach to predict anisotropic shrinkage of amorphous polymers in injection moldings was proposed using the PVT equation of state, frozen-in molecular orientation, and elastic recovery that was not frozen during the process. The anisotropic thermal expansion and compressibility affected by frozen-in molecular orientation were introduced to determine the anisotropy of the length and width shrinkages. Molecular orientation calculations were based on the frozen-in birefringence determined from frozen-in stresses by using the stress-optical rule. To model frozen-in stresses during the molding process, a nonlinear viscoelastic constitutive equation was used with the temperature- and pressure-dependent relaxation time and viscosity. Contribution of elastic recovery that was not frozen during the molding process and calculated from the constitutive equation was used to determine anisotropic shrinkage. Anisotropic shrinkages in moldings were measured at various packing pressures, packing times, melt temperatures, and injection speeds. The experimental results of frozen-in birefringence and anisotropic shrinkage were compared with the simulated data. Experimental and calculated results indicate that shrinkage is highest in the thickness direction, lowest in the width direction, and intermediate in the flow direction.



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