Structure skin-core

The basic steps of the IM process produce unique structures in all molded products, whether they are miniature (micro) electronic components, compact discs, or large automotive bumpers. These structures have frequently been compared to plywood with several distinct layers, each with a different set of properties. In all IM products, a macroscopic skin-core structure results from the flow of melt into an empty cavity. Identifiable zones or regions within the skin are directly... 

The basics observed in molded products are always the same only the extent of the features varies depending on the process variables, material properties, and cavity contour. That is the inherent hydrodynamic skin-core structure characteristic of all IM products. However, the ratio of skin thickness to core thickness will vary basically with process conditions and material characteristics, flow rate, and melt-mold temperature difference. These inherent features have given rise to an increase in novel commercial products and applications via coinjection, gas-assisted, low pressure, fusible-core, in-mold decorating, etc. 

Frische, S., Maunsbach, A. B., and Vollrath, F. (1998). Elongate cavities and skin-core structure in Nephila spider silk observed by electron microscopy. Journal of Microscopy 189, 64-70. 

The substrate also has an important influence on diffusion of the dye. The diffusion rate increases with caustic soda treatment or mercerization of cotton. However, with regenerated cellulose fibers, which have a marked skin-core structure, the outer parts of the fiber can act as a diffusion barrier. 

Table 3. The toughness improvement induced by the -modification, rationalized in terms of the ratio Gt0t(/ -nuclcalcd)/Gtot (non-nucleated), is markedly stronger for the series tested under biaxial conditions. As both sets of investigated samples were injection molded, the skin-core structure (discussed in Sect. 3.2.3) could be inferred to be at least partly responsible for this feature. This was, however, confirmed in a recent work, where impacted /3-PP samples (tested at 5 ms-1, T = 23 °C and - 20 °C) with or without skin layers showed the same fracture performance within the limits of the experimental error [116].

Figure 2.12 Schematic of a skin core structure in a high speed spun fiber showing a greater degree of crystallization in the skin than in the core.

Figure 3.6 Two possible structures of spider frame-silk (a) microcomposite structure consisting of randomly distributed protein crystallites in anamorphous matrix (after Gosline et al 1986) and (b) skin-core structure. The two core regions are marked 1 and 2 and the thin skin is marked 3. Both the core regions contain pleated fibril-like strands (after Li et al., 1994).

Molded samples of LCP s often form a "skin-core" structure. The phenomenon is depicted by a significant dependence of the anisotropic properties of molded parts on part thickness ( 5.). Scanning electron photomicrographs of the cross-section of a molded part reveal a highly oriented "skin" layer surrounding a less ordered inner "core." Apparently, the fraction of disordered core material diminishes as the sample thickness decreases. 

Kevlar 149 has a substantially higher degree of crystallinity and larger crystal size than for Kevlar 49. The unit cell dimensions for the two also differ significantly. In addition, while there are present skin-core structure in both Kevlar 29 and Kevlar 49 fibers, there is no definable difference between the skin and core for Kevlar 149. 

It causes migration of the dispersed phase, thus imposing global changes of morphology in the formed parts, viz., skin-core structures, weld lines, blush lines, etc. 

Figure 9.39 Skin-core structure in the injection-molded products for the iPP and blends. (From Reference 65 with permission from John Wiley Sons, Inc.)...

One of the most important features of extruded and moulded articles is the presence of a skin-core structure caused by high shear gradients in the flowing polymer close to the die or mould surface, which induce high orientation. In rods or other extrudates the skin-core effect can be removed by subsequent drawing to increase the overall orientation of the sample this cannot, of course, be done for mouldings. 

Morphological level (nm to um). This level describes the organization of the microfibrils and macrofibrils into layers and walls the existence of distinct cell wall layers in native cellulose libers or in skin core structures in man-made cel-lulosic fibers are discussed. 

A detailed analysis of the Raman spectrum of syndiotactic PS in the region 600-850/cm was undertaken. As syndiotactic PS exhibits considerable polymorphism, spectra of various preparations including melt-crystallised syndiotactic PS, solvent-crystallised syndiotactic PS and quenched glassy materials, were studied. Peaks are assigned to conformational changes and sequences. Comparison with atactic PS is made. Study of the cross section of a compression moulded plaque exhibiting a skin/core structure revealed the continuous way the structure varied with changing crystallinity. The various physical forms of syndiotactic... 

The morphology and properties of injection molded thermotropic longitudinal PLC products are similar to those of extrudates, i.e., macroscopic skin/core structure, microscopic hierarchically layered and... 

Morphology (skin/core structure, orientation degree and profile)... 

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