Since the success of a production process depends on its good planning and having a clear plan from the raw materials until the final product, the focus of this research is in modeling of the extrusion temperature profile of as-spun aliphatic-aromatic co-polyester fibers. The extrusion temperature profile affects the properties, productivity and product cost. In this work, as-spun aliphatic-aromatic co-polyester fibers were spun under a fractional factorial design as a function of the extrusion temperature profile using appropriate statistical methods. The influence of the extrusion temperature profile on the optical birefringence of the as-spun fibers was characterized. From the obtained results the overall orientation of the spun filaments has been modeled. For measuring the birefringence, an interferometric technique was employed and its microinterferograms were included for illustration. POLYM. ENG. SCI., 49:2492-2500, 2009. (C) 2009 Society of Plastics Engineers