This paper sets out the theory and numerical methods used to simulate filling and fiber orientation is simple injection moldings (a film‐gated strip and a center‐gated disk). Our simulation applies to these simple geometry problems for the flow of a generalized Newtonian fluid where the velocities can be solved independently of fiber orientation. This simplification is valid when the orientation is so flat that the fibers do not contribute to the gapwise shear stresses. A finite difference solution calculates the temperature and velocity fields along the flow direction and through the thickness of the part, and fiber orientation is then integrated numerically along pathlines. Fiber orientation is three‐dimensional, using a second‐rank tensor representation of the orientation distribution function. The assumptions used to develop the simulation are not valid near the flow front, where the recirculating fountain flow complicates the problem. We present a numerrical scheme that includes the effect of the fountain flow on temperature and fiber orientation near the flow front. The simulation predicts that the orientation will vary through the thickness of the part, causing the molding to appear layered. The outer “skin” layer is predicted only if the effects of the fountain flow and heat transfer are included in the simulation.

A method is presented for measuring three‐dimensional fiber orientation in fiber‐reinforced polymers and placing confidence limits on the results. The orientations of individual fibers are determined from the elliptical intersections between the cylindrical fibers and a polished section. This can be done using either manual digitization or automated image analysis. Volume averages for the sample are computed using an orientation‐dependent weighting function that corrects for the bias of an area‐based sample. Equations are developed for nonuniform fiber lengths, using both number‐average and weight‐average measures of orientation. Sources of systematic, measurement, and sampling error are discussed and equations for sampling error and the propagation of measurement error are derived. The results use a second‐rank tensor to characterize fiber orientation, but the error analysis can be applied to any type of orientation parameter. We implement the technique using manual digitization of optical micrographs. Our implementation accurately measures samples with known orientation, and produces identical results from two perpendicular sections of a glass fiber/nylon injection‐molded sample.

Experimental measurements of fiber orientation are reported for two parts injection molded from nylon 6/6 reinforced with 43 wt% of glass fibers. The parts are a center‐gated disk and a film‐gated strip. Orientation is measured from polished cross sections and reported as a function of position, both across the thickness and in the flow direction. Both parts have a layered structure, with outer shell layers of flow‐aligned fibers surrounding a central core of either random‐in‐plane (strip) or transversely aligned fibers (disk). The disk also has surface skins with less alignment. The experiments are compared with predictions of the simulation presented in Part I. The simulation predicts the presence, nature, and location of the layers very well. However, it overpredicts the small out‐of‐plane fiber orientation and places the core‐shell transition too close to the midplane. A comparison with selected experimental results suggests that the major source of error is the closure approximation used by the fiber orientation equation. The simulation is exercised for a variety of cases to show the importance of material and process parameters. A polymer matrix with a small power‐law index or a large heat of fusion gives a thicker core and is less likely to have a skin. Injection time is an important parameter, but injection temperature and mold temperature have little effect on fiber orientation.