Influence of Selected Process Parameters on Changes of the Fiber Orientation in Unidirectional Reinforced Thermoplastics During a Hot Pressing Process Rico Blei, 1 Teresa M € obius, 1 Camilo P  erez, 2 Tim A. Osswald, 2 Niels Modler 1 1 Institute of Lightweight Engineering and Polymer Technology, Technische Universit € at Dresden, Holbeinstr 3, 01307 Dresden, Germany 2 Polymer Engineering Center, University of Wisconsin-Madison, Madison, WI 53706 The research described in this paper is part of SFB 639 “Textile-reinforced composite components for function integrating multi-material design in complex lightweight applications.” In this study, changes in fiber orientation of unidirectional (UD) aligned glass polypropylene hybrid yarn were investigated. A new model for simulating changes in fiber orientation of unidirectional reinforced thermoplastics during a hot pressing process was developed. The material observed during this study is a hybrid yarn made out of homogenous aligned glass and polypropylene filaments. The mechanistic model, which was used to simulate the fiber behavior, allows the user to define different input data, such as velocity, vorticity, and viscosity of the molten polymer as well as fiber radi- us, initial fiber positions and fiber stiffness to character- ize material properties. To determine fiber orientations, a program was written in MATLAB. Hence, it is possible to calculate the angle of any desired segment at any point of time during the hot pressing process. Thereby, it was feasible to compare the numerical obtained changes in fiber orientation with those measured during the experiments conducted at the end of the research. The post-processing algorithm was used to determine the effects of selected processing parameters and mod- el parameters on the fiber behavior respectively. POLYM. COMPOS., 00:000–000, 2016. V C 2016 Society of Plastics Engineers INTRODUCTION Fiber reinforced composites are increasingly used to build highly stressed components and structures. Light- weight constructions are highly appreciated because of reduced material weight and lower moments of inertia, especially in such fields as transportation technology, sports equipment industry, wind power plant technology, and special purpose machines industry [1]. By using fiber reinforced composites, high specific stiffness and strength of structures can be realized. Not only the geometry of the design, but the material itself are variables that influence the performance of the light weight structure. Therefore, these materials offer the pos- sibility to design load-adapted constructions. In order to reach the composite’s outstanding properties during high- volume production, the usage of textile-reinforced compo- sites with thermoplastic matrices is advantageous. A fur- ther benefit is the fact that short processing times are feasible for such materials [2, 3]. UD reinforced fiber composites possess excellent light- weight properties when loaded with stresses in the fiber direction of the structure. Direct force flows ensure the effective usage of individual components. Hence, less material is required in comparison with structures where force diversions occur. Furthermore, it is possible to auto- mate composite manufacturing processes easily, thus allowing production of large structures with complex geometries in an economical way. The usage of glass polypropylene hybrid yarn offers additional benefits dur- ing the production process. Because of commingled glass and polypropylene filaments there is no matrix flow in through-thickness direction. Hence, impregnation of fibers can be achieved very quickly and production times are considerably lower [4, 5]. Changes in fiber orientation are observed during the hot pressing process of unidirectional reinforced thermoplas- tics. In order to design and manufacture parts effectively and reliably, the deformation process has to be investigated. The composite material examined during this study consists of a hybrid yarn, wherein glass and polypropylene fila- ments are aligned homogenously. Within the viscoelastic material complex flows are generated which are influenced by thermal and mechanical loads related to the process Correspondence to: R. Blei; e-mail: rico.blei@tu-dresden.de Prior oral presentation and original source of the material is the diploma thesis of Rico Blei. DOI 10.1002/pc.24201 Published online in Wiley Online Library (wileyonlinelibrary.com). V C 2016 Society of Plastics Engineers POLYMER COMPOSITES—2016