Injection molding molecular orientation

Analysis of extruded and injection molded iPP shows that a skin-core structure is formed due to high shear and thermal gradients upon cooling. These frozen-in tensile stresses in the skin can act as the stress component in the environmental stress cracking process [4]. In injection moldings, molecular weight has a secondary effect. This effect is that residual orientation in the direction of flow increases with higher... 

Orientation. Most articles made of HDPE, including film, fiber, pipes, and injection-molded articles, exhibit some degree of molecular and crystal orientation (21). In some cases, orientation develops spontaneously for example, during melt flow into a mold and its subsequent crystallisation. When blown HDPE film and fiber are manufactured, orientation can be introduced dehberately by stretching. 

Injection-compression molding first injects the material into a partially opened mold, and then squeezes the material by closing the mold. Injection-compression molding is used for polymer products that require a high quality surface finish, such as compact discs and other optically demanding components because it practically eliminates tangential molecular orientation. 

Fig. 13.13 Shrinkage distributions of injection molded amorphous PS (a) at two injection rates longitudinal direction, (b) solid curves, longitudinal direction broken curve, transverse direction. [Reprinted with permission from G. Menges and G. Wiibken, Influence of Processing Conditions on Molecular Orientation in Injection Molds, Soc. Plastics Eng., 31st Annual Technical Conference, Montreal, Canada, 1973.]...

Fig. 13.15 Schematic representation of the flow pattern in the central portion of the advancing front between two parallel plates. The coordinate system moves in the x direction with the front velocity. Black rectangles denote the stretching deformation the fluid particles experience. [Reprinted by permission from Z. Tadmor, Molecular Orientation in Injection Molding, J. Appl. Polym. Sci., 18, 1753 (1974).]...

G. Menges and G. Wiibken, Influence of Processing Conditions on Molecular Orientation in Injection Molds, SPE ANTEC Tech. Papers, 19, 519 (1973). 

When polymer melts are accelerated during the manufacturing processes such as injection molding and afterwards cooled down quickly, the stretched molecular chains cannot relax, which leads to orientations in the direction of melt flow. These oriented polymer chains act differently toward ESC influences as compared to nonoriented molecules. Orientation perpendicular to the crack propagation direction may result in a higher ESCR, as will be shown in the following discussion. 

No microstructural differences between the mid and the core layers could be detected from comparison of the fracture surfaces (Figure 7c, d and 8c, d). The microstructures in these layers were better described as strips, and the average molecular orientations of the mid and the core layers were still parallel to the flow direction, in contrast with a relatively featureless core with little or no molecular orientation observed by other investigators Qj. The variations in the layered structures and the skin core phenomena observed in PECs could be correlated to the process history, the shapes of the specimens, the injection molding parameters, and the molecular structures of the polymers. 

Studies of the orientation phenomena in injection molding have shown that irrespective of the molding conditions and type of thermoplastic, molecular orientation is higher near the walls and decreases toward the interior. " " Low crystallinity and flow-induced structures have been observed near the walls for injection molded isotactic polypropylene parts. " High modulus materials have also been obtained with some injection molding techniques. " ... 

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