Mold filling molecular orientation

Injection molded plaques or bars of PLC have a skin—core structure [33]. The molecular chains in the skin regions are largely aligned in the mold fill direction while the chain orientation in the core is more or less random. The high molecular alignment in the skin layer is induced by the elongational stress in the fountain flow and is immediately frozen upon contact with the mold surface. 

Quantitative predictions of the effects of fillers on the properties of the final product are difficult to make, considering that they also depend on the method of manufacture, which controls the dispersion and orientation of the filler and its distribution in the final part. Short-fiber- and flake-filled thermoplastics are usually anisotropic products with variable aspect ratio distribution and orientation varying across the thickness of a molded part. The situation becomes more complex if one considers anisotropy, not only in the macroscopic composite but also in the matrix (as a result of molecular orientation) and in the filler itself (e.g., graphite and aramid fibers and mica fiakes have directional properties). Thus, thermoplastic composites are not always amenable to rigorous analytical treatments, in contrast to continuous thermoset composites, which usually have controlled macrostructures and reinforcement orientation [8, 17]. 

An example of the multilayered structures common in polyacetals is shown in the polarized light micrographs (Fig. 5.35). They depict a uniformly nucleated crystalline structure formed due to mold filling and variations in the rate of cooling of the melt. The skin surface in the microtomed section (top in Fig. 5.35A) is birefringent, non-spherulitic and highly oriented. The molecular chains are oriented parallel to the injection direction. The central portion of the bar consists of a core (bottom Fig. 5.35A) with randomly oriented spherulites (Fig. 5.35C). It has no preferred molecular or lamellae orientation. There are usually one or more layers between the skin and core that are transitional shear zones with intermediate... 

---