ABSTRACT: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.

A novel approach to predict anisotropic shrinkage of slow crystallizing polymers in injection moldings was proposed, using the flow-induced crystallization, frozen-in molecular orientation, elastic recovery, and PVT equation of state. In the present study, three different polyesters, polyethylene terephthalate, polybutylene terephthalate, and polyethylene-2,6-naphthalate (PEN), are used. The anisotropic thermal expansion and compressibility affected by the frozen-in orientation function and the elastic recovery that was not frozen during moldings were introduced to obtain the in-plane anisotropic shrinkages. The frozen-in orientation function was calculated from the amorphous contribution based on the frozen-in and intrinsic amorphous birefringence and crystalline contribution based on the crystalline orientation function determined from the elastic recovery and intrinsic crystalline birefringence. To model the elastic recovery and frozen-in stresses related to birefringence during molding process, a nonlinear viscoelastic constitutive equation was used with the temperature-dependent viscosity and relaxation time. Occurrence of the flow-induced crystallization was introduced through the elevation of melting temperature affected by entropy production during flow of the viscoelastic melt. Kinetics of the crystallization was modeled using Nakamura and Hoffman-Lauritzen equations with the rate constant affected by the elevated melting temperature. Numerous injection molding runs were carried out by varying the packing time, packing pressure, flow rate, melt and mold temperature, and anisotropic shrinkage of moldings were measured. The experimental results were compared with the simulated data and found in a fair agreement.

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A novel approach for the numerical simulation of the development of crystallinity and birefringence on the spinline was proposed. The approach was based on the calculation of elastic recovery and crystalline and amorphous orientation functions without making any assumption about freezing. To model crystallization, the amorphous orientation function and the flow effect on the equilibrium melting temperature elevation due to the entropy reduction between the oriented and unoriented melts were incorporated. The crystalline orientation function was calculated from the frozen-in elastic recovery. The entropy change and frozen-in elastic recovery calculation were based on a nonlinear viscoelastic constitutive Eq. with the crystallinity and temperature dependent viscosity and relaxation time. The crystalline and amorphous contributions to the overall birefringence were obtained from the crystalline orientation function and the flow birefringence, respectively. The temperature, diameter, density, and birefringence profile in both lowand high-speed spun PET fibers were predicted and compared with the experimental data from literature. Theoretical predictions were found to be in agreement with the published experimental data.