Extrusion processes solidification/cooling

The stretching force is a direct consequence of the distance of polymer solidification (DPS) in both processes (i.e., FLH in film blowing processes or cooling length in ribbon extrusion). In this sense, controlling DPS offers the possibility to control the final properties of films and ribbons. Table 24.3 lists typical values of stretching force as function of FLH and DR for blown films of LDPE. 

If the break-up time of threads is longer than the time of cooling, threads will also remain present after solidification. These are, however, not permanent upon further processing by extrusion or injection moulding, the instabilities can grow further. 

Compared to extrusion, injection molding is presented as a cyclical process. A complete cycle consists of the following operations, starting with plastic granulate material dosage in the injection cylinder, followed by fusion of this material until it gets injection consistency. After the fusion, the molten plastic material is injected into the closed mold and this injected material is cooled until its solidification. Finally, the product is extracted with the opened mold. ... 

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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