No 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 nearend-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.

Fiber orientation in the frontal region of a center-gated disk was measured for the first time and compared with orientation predictions using standard Folgar-Tucker and the reduced strain closure (RSC) model in coupled flow simulations. Fiber orientation was experimentally measured along three different heights representative of shell, transition and core layers, in order to understand the evolution of orientation along the radial direction in the frontal region. Orientation predictions of the Folgar-Tucker model and its two modified versions, the delayed Folgar-Tucker model and the RSC model were assessed against the measured experimental data. Orientation predictions with all three models showed a drop in orientation near the front, which was in qualitative agreement with the experimental data. Modified versions of the Folgar-Tucker model showed a relatively larger drop in orientation in the shell layer with predictions being relatively closer to experimental values. However, no significant slowdown was observed with the modified versions in the transition and core layers. With coupling of flow and orientation, the frontal flow region was slightly larger and orientation predictions showed only slight improvement. A significant improvement in the frontal region was obtained when a lower value of the interaction coefficient was used.

The effect of sparse long chain branching, LCB, on the shear step‐strain relaxation modulus is analyzed using a series of eight high‐density polyethylene (HDPE) resins. Strains of 1 to 1250% are imposed on materials with LCB content ranging from zero to 3.33 LCB per 10,000 carbon atoms. All materials are observed to obey time–strain separation beyond some characteristic time, τk. The presence of LCB is observed to increase the value of τk relative to the linear resin. The behavior of the relaxation modulus at times shorter than τk is investigated by an analysis of the enhancement seen in the linear relaxation modulus, G0(t), as a function of strain and LCB content. This enhancement is seen to (1) increase with increasing strain in all resins, (2) be significantly larger in the sparsely branched HDPE resins relative to the linear HDPE resin, and (3) increase in magnitude with increasing LCB content. The shape and smoothness of the damping function is also investigated. The finite rise time to impose the desired strain is compared to the Rouse relaxation time of linear HDPE resins studied. Sparse LCB is found to increase the magnitude of the relaxation modulus at short times relative to the linear resin. POLYM. ENG. SCI., 2010. © 2010 Society of Plastics Engineers