Theoretical and experimental studies of anisotropic shrinkage in injection moldings of various polyesters

Abstract

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.

Analysis of Selected Properties of Injection Moulded Sustainable Biocomposites from Poly(butylene succinate) and Wheat Bran

“…The processing shrinkage of injection-moulded parts made of partially crystalline polymers depends on many factors, which include heat transfer during cooling, volume shrinkage due to thermal expansion, flow-induced residual stresses, orientation of macromolecules, and crystallization. The above factors are in turn dependent on the processing parameters and properties of the injected material [88]. All of the measurement series studied are characterized by partial crystallinity [66], by which the effect of shrinkage anisotropy was observed.…”

The Elastic Mold Deformation During the Filling and Packing Stage of the Injection Molding Process

“…[12] Due to the complexity of the phase change especially when considering semi crystalline polymers, this simple approach is still state of the art in commercial software. [21][22][23] Lucyshyn et al [24] investigated the influence of the cooling rate on the transition temperature. He found, that a higher cooling rate causes a decrease in transition temperature for semi crystalline materials while no significant change was observed for amorphous materials.…”

Determination of non-isothermal crystallization rate constant for pseudo-experimental calorimetric data

“…Using the calculated K(T) and assuming n equal to 3, the differential form of the Nakamura equation 10 , given by Equation 7, can be used to simulate the dθ/dt vs. T curves, which are integrated to obtain almost exclusively to the Nakamura equation, mostly in Isayev's various scientific works [6][7][8][9] . In this respect, nonlinear regression is more flexible; moreover, it can be applied to other types of equation, not only to those that fit into the form of Equation 1.…”