Prediction of Fiber Orientation in a Rotating Compressing and Expanding Mold J. Wang, 1 C.A. Silva, 2 J.C. Viana, 2 F.W.J. van Hattum, 2 A.M. Cunha, 2 C.L. Tucker III 1 1 Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801 2 Institute for Polymers and Composites (IPC), Department of Polymer Engineering, University of Minho, Guimara ˜ es 4800-058, Portugal In the rotating/compressing/expanding mold (RCEM), one mold wall can expand, compress, and rotate dur- ing injection molding, thus offering opportunities to control the thermomechanical history of a polymer and its microstructure. A computer simulation of flow and fiber orientation in RCEM was developed. The predic- tive model extends the generalized Hele-Shaw formula- tion to account for compression/expansion and rota- tion of the mold wall, and uses the Folgar–Tucker model for fiber orientation predictions. A 20% GF poly- propylene was molded under various molding condi- tions. The predicted fiber orientation distributions were compared with experiments. The model compares favorably with experiments, provided that the fiber ori- entation equation is modified by a strain-reduction fac- tor that slows the transient development of fiber align- ment. The effect of fountain flow on orientation must also be included to correctly predict fiber orientation near the mold walls, mainly for the case of stationary and linear motions of the mold surface. Compression or expansion of the mold has only a small effect on fiber orientation, but rotation of the mold dramatically changes the orientation, causing fibers to align in the tangential direction across the entire thickness of the molding. This rotation action perturbs the fountain flow and becomes the dominant factor affecting fiber align- ment across the entire cavity thickness. POLYM. ENG. SCI., 48:1405–1413, 2008. ª 2008 Society of Plastics Engineers INTRODUCTION Injection-molded fiber-reinforced polymers, FRP, show a complex morphology featuring spatial variations of the mo- lecular and fiber orientation distributions, as a consequence of the thermomechanical history imposed during processing. In conventional injection molding, the composite morphology features two outer shell regions with fibers aligned in the flow direction (FD), and a central core layer where the fibers may be aligned transverse to FD. This morphological arrangement, generated during molding, determines the subsequent me- chanical behavior of the molded part [1–4]. Considerable progress has been made in predicting flow-induced fiber orientation in conventional injection molding. The standard model is based on Jeffery’s equa- tion for fiber orientation in a dilute suspension [5], as modified by Folgar and Tucker to account for fiber–fiber interactions [6]. The statistics of fiber orientation at any point in the mold cavity are represented by a second-order tensor [7]. This formulation is combined with mold filling simulations, which predict the velocity distribution during filling, to produce a comprehensive picture of the fiber orientation that results from filling (e.g. [3, 4, 8–10].). This capability is now available in commercial software. In contrast to the advances in predicting fiber orienta- tion, the possibilities for controlling the thermomechanical conditions applied to the polymer melt during conven- tional injection molding are limited. For a given molding geometry and material type, this limits our ability to con- trol the development of the composite morphology. To enlarge the possibilities for manipulating the thermome- chanical environment in injection molding, a novel special mold was designed and manufactured—the rotating, com- pression, and expansion mold (RCEM) in Fig. 1 [11]. The RCEM produces center-gated disk moldings under a wide range of thermomechanical conditions, including external mechanical loads. This is possible by mechani- cally controlling the motion of one of the cavity molding surfaces, which is able to rotate and to move axially (in either a steady or oscillating mode) during the filling and holding stages of the injection molding cycle. These cav- ity surface movements are driven mechanically by two servo-electric stepper motors, allowing for accurate con- trol of a previously defined motion sequence. The mold design allows for cooling channels near both mold walls, providing accurate control of the mold temperature. The molding process is monitored by two pressure–tempera- ture sensors placed along the flow path. Special dedicated Correspondence to: Julio Viana; e-mail: jvb@dep.uminho.pt DOI 10.1002/pen.20979 Published online in Wiley InterScience (www.interscience.wiley.com). V V C 2008 Society of Plastics Engineers POLYMER ENGINEERING AND SCIENCE—-2008