Shrinkage differential

Other elastomeric-type fibers iaclude the biconstituents, which usually combine a polyamide or polyester with a segmented polyurethane-based fiber. These two constituents ate melt-extmded simultaneously through the same spinneret hole and may be arranged either side by side or ia an eccentric sheath—cote configuration. As these fibers ate drawn, a differential shrinkage of the two components develops to produce a hehcal fiber configuration with elastic properties. An appHed tensile force pulls out the helix and is resisted by the elastomeric component. Kanebo Ltd. has iatroduced a nylon—spandex sheath—cote biconstituent fiber for hosiery with the trade name Sidetia (6). 

Bicomponent fibers have also provided a route to self-texturing (self-crimping) fibers. The crimp results from the length differential developed during processing caused by differential shrinkage in the two polymers in side-by-side or eccentric core—sheath configurations (50). 

Abrupt changes of section cause poor flow and differential shrinkage, giving sink marks (Fig. 28.11 - you can find them on the surface of many small polymer parts), distortion, and internal stress which can lead to cracks or voids. The way out is to design in the way illustrated in Fig. 28.12. Ribs, which are often needed to stiffen polymer parts, should have a thickness of no more than two-thirds of the wall thickness, and a height no more than three times the wall thickness. Corners are profiled to give a uniform section round the corner. 

FIGURE 13.25 Differential shrinkage during sintering of porcelain hollow pipes of various sizes made by extrusion. In the ceramic powder mixture used for porcdain are kaolin platelet particles which orient in the shear of extrusion. From Funk [64], Reprinted by permission of the American Ceramic Society. 

Stresses in solvent based coatings arise from the differential shrinkage between the thin film coatings and the corresponding substrates. These stresses are due to volume changes associated with solvent evaporation, chemical reaction (i.e. cyclization in polyimide formation) and differences in thermal expansion coefficients of the coating and substrate (4>5). The level of residual stress depends on the material properties such as modulus, residual solvent content and crosslinking (5) and its thermal-mechanical history. 

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