In this work, a viscoelastic model was applied to describe the evolution of molecular orientation in injection molding. This model is an evolution of a previously applied nonlinear Maxwell model, in which the relaxation time was allowed to depend on a structural parameter rather than on shear rate as considered in our previous works. The parameter chosen was the difference between the two main eigenvalues of the molecular conformation tensor. The model was applied to some injection molding tests carried out on an amorphous PS. The results in terms of orientation were discussed, also in comparison with the results of the previous model in which relaxation time depended on shear rate. It was found that, apart from being much sounder on a physical basis, the model also introduces improvements on the description of experimental data.