Extrusions, surface-layer orientation

The common feature of these processes is that two flat, or slightly convex, melt surfaces come together under pressure, with some outwards flow. The orientation produced is at right angles to the original flow direction. In the welding of extrusions, there is a layer of soft semisolid polymer behind the melt layer. When the melt flows outwards to form a bead at the free surfaces, this semisolid material undergoes shear deformation. 

The orientation affects flow profoundly, hence processability, as well as the product performance. It plays an important role in extrusion or injection molding where the anisometric particles may become oriented in a complex manner. Layered structures, weld lines, splice lines, swirls, and surface blemishes are well known. Mold geometry (e.g., inserts) and transient effects make predictions difficult. It has been theoretically and experimentally shown that, when designing... 

Many layered biological systems resemble cholesteric liquid crystals in the rotation of orientation from layer to layer. This has raised recent interest in whether they actually are liquid crystals, in that the rotation forms spontaneously as a result of interactions between fibers in successive layers. This would occur in a fluid state, which is subsequently embedded in a hard matrix. The core question is really whether the rotation pattern is directly controlled by some form of oriented extrusion during the deposition process or is controlled through the surface chemistry of fibrils deposited in successive layers. 

Microstructures that are typically observed in molded parts and extrudates include anisotropic textures. The higher orientation in extrusion can result in highly oriented rods or strands, at high draw ratios and/or small diameters, or in structures with an oriented skin and a less oriented central core in thicker strands. This skin-core texture is due to a combination of temperature variations between the surface and the bulk and the flow field in both extrusion and molding processes. For instance, the flow fields in a molded part are shown schematically in Fig. 1.7 [46]. Extensional flow along the melt front causes orientation. Solidification of the polymer on the cold mold surface freezes in this orientation. Flow between the solid layers is affected by the temperature gradient in the mold, and the resulting flow effects [57, 58] result in a rapidly cooled and well oriented skin... 

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