Gregorio M. Vélez-García

Using transient shear rheology to determine material parameters in fiber suspension theory

Fiber suspension theory model parameters for use in the simulation of fiber orientation in complex flows are, in general, either calculated from theory or fit to experimentally determined fiber orientation generated in processing flows. Transient stress growth measurements in startup of shear flow and flow reversal in the shear rate range, γ=1–10 s−1, were performed on a commercially available short glass fiber-filled polybutylene terephthalate using a novel “donut-shaped” sample in a cone-and-plate geometry. Predictions using the Folgar–Tucker model for fiber orientation, with a “slip” factor, combined with the Lipscomb model for stress were fit to the transient stresses at the startup of shear flow. Model parameters determined by fitting at γ=6 s−1 allowed for reasonable predictions of the transient stresses in flow reversal experiments at all the shear rates tested. Furthermore, fiber orientation model parameters determined by fitting the transient stresses were compared to the experimentally determi...

Sample preparation and image acquisition using optical-reflective microscopy in the measurement of fiber orientation in thermoplastic composites

A complete sample preparation procedure used to determine three‐dimensional fiber orientation from optical micrographs of glass fiber‐reinforced thermoplastic composites is presented. Considerations for elimination of irregularities in the elliptical footprints, contrast enhancement between fibers and surrounding polymer matrix, controlled‐etching that allows the identification of small shadows where fiber recedes into the matrix, and topographical reconstruction of the elliptical footprint are described in the procedure. This procedure has produced high‐quality optical micrographs employed to obtain accurate fiber orientation data for thermoplastic composites using the method of ellipses. The optimal definition of the nonelliptical footprints’ borders allows an accurate measurement of orientation in small sampling areas.

Obtaining reliable transient rheological data on concentrated short fiber suspensions using a rotational rheometer

The conventional method for obtaining transient rheological data on short glass fiber-filled polymeric fluids is to use the parallel disk (PP) geometry in a rotational rheometer. Using the PP geometry large transient stress overshoot behavior was observed during the startup of flow measurements on a 30 wt % short glass fiber-filled polybutylene terephthalate. A contributing factor to this behavior is believed to be induced fiber collisions caused by the inhomogeneous velocity field (radial varying velocity gradient). A novel approach was taken in which a “donut” shaped sample was used in a cone-and-plate device (CP-D) to maintain a sufficient gap to fiber length ratio. The magnitude of the first normal stress difference was reduced by 70%, and the time to reach steady state was reduced by 100 strain units. The Lipscomb model coupled with the Folgar–Tucker model for the evolution of fiber orientation was fit to the stress growth behavior measured using both the PP geometry and CP-D resulting in different p...

Simulation of Injection Molding Using a Model with Delayed Fiber Orientation

Abstract Delay in fiber orientation evolution based on the Folgar-Tucker model with a slip parameter correction has been proposed as a simple alternative to improve predictions of fiber orientation in injection molded parts. Predictions based on this model and model parameters fitted to data from simple shear experiments were compared with experimentally determined fiber orientation in a center-gated disk. Three methods of fitting to simple shear data were assessed to obtain the isotropic diffusivity and the slip parameter. The model parameters and orientation data evaluated at the entry, lubrication and near-end-of fill regions in a center-gated disk for 30 wt.% short glass fiber-filled polybutylene terephtalate (PBT) were obtained from earlier efforts in our laboratory. Simulation results based on the Folgar-Tucker model with the slip correction using customarily assumed inlet orientation being random and experimentally measured at the gate disagreed with measured orientation values at certain positions along the disk. However, improvement in the prediction of orientation due the slip correction was found at the core and transition layers in the lubrication region. The use of inlet conditions washes out quickly in the absence of the slip correction and induced a general reduction of orientation towards the center of the sample causing underestimation of orientation at the entry and lubrication region. Model predictions combining the slip correction and experimentally determined orientation at the gate are in agreement with the experimental data for the core layers near the end-of-fill region.

Simulation of Orientation in Injection Molding of High Aspect Ratio Particle Thermoplastic Composites

A 2D coupled Hele‐Shaw flow approximation for predicting the flow‐induced orientation of high aspect ratio particles in injection molded composite parts is presented. For a highly concentrated short glass fiber PBT suspension, the impact of inter‐particle interactions and the orientation at the gate is investigated for a center‐gated disk using material parameters determined from rheometry. Experimental orientation is determined from confocal laser micrographs using the methods of ellipses. The constitutive equations are discretized using discontinuous Galerkin Finite Elements. Model predictions are significantly improved by using a localized orientation measured experimentally at the gate region instead of random or averaged gapwise measured orientation assumed in previous studies. The predicted profile in different radial positions can be related to the layered structure along the gapwise direction. Model modifications including interactions have lower impact than the initial conditions.