Shrinkage anisotropy

Clearly shrinkage anisotropy is a eomplex issue. A number of faetors ean contribute and the relative importance of each will vary between timbers. In some cases a large microfibril angle might be significant, as in corewood and in compression wood. Ray tissue will be important in species such as beech and oak. Contrasting earlywood and latewood densities is a likely cause in Douglas fir, but would be irrelevant for a tropical hardwood. The effects of elastic anisotropy would be more apparent in low density softwoods. 

Shrinkage anisotropy is a common problem when using fiber-filled materials as the orientation of the fibers leads to quite extreme anisotropy in thermomechanical properties. 

The aspect ratio of nucleant particles induces preferential crystal orientation and contributes to the formation of superstractures (e.g., shish-kebab, etc.). This has negative effect on shrinkage of the parts during and after molding. Shrinkage anisotropy may also cause warpage of the parts. ... 

Shrinkage can influence product performances such as mechanical properties. Anisotropy directional property can be used when referring to the way a material shrinks during processing, such as in injection molding (Fig. 2-62) and extrusion. Shrinkage is an important consideration when fabricating... 

LCPs are handicapped by a high anisotropy of properties, shrinkage and thermal expansion low weld strength the cost, though justified by the performances and unusual design rules. 

Chain orientation effects anisotropy in the expansion in the orientation direction it is considerably lower than across. Moreover the tendency to (irreversible) shrinkage in the orientation direction upon temperature increase should be taken into account. 

Filling the Mold Cavity to Form the Product. The mold cavity is designed and machined to form the shape of the finished product. This is itself a complete art and science, based partly on experience, and increasingly on computerized engineering principles. Some major considerations are fast uniform flow, avoidance of degradation, minimization of orientation/anisotropy, fast cooling/ solidification, shrinkage and dimensional tolerances, and of course final properties of the product. 

The differences in thermal expansion coefficients of the individual phases and also their anisotropies result in non-uniform shrinkage on cooling. Tf this non-uniform shrinkage cannot be met by deformation, stresses arise restricted to short distances (microstresse.s). This phenomenon is characteristic for ceramics and influences their mechanical properties. High tensile stresses may even result in the formation of ciacks visible under the microscope, for example iji fireclay or porcelain. These cracks arc usually situated at phase boundaries. 

This analysis reveals that measurement of shrinkage or linear coefficients of thermal expansion (CTE s) in just flow and width directions, as is done for conventional polymers, is not sufficient for liquid crystal polymers (LCP s) and can lead to erroneous shrinkage predictions. This is a consequence of inherent LCP anisotropy, resulting in a relatively large linear CTE and shrinkage in the thickness direction of associated molded LCP parts. Linear and volumetric CTE data for neat and filled LCP molded parts of different geometries are presented. 

As expected, the anisotropy and volumetric CTE for the highly filled experimental compound (entry 15 in Table I) are much lower than the other XYDAR formulations. This material is particularly suited for applications where lower anisotropy and volumetric CTE match are needed and highlights the importance of adjusting filler loading in LCP s to meet shrinkage and CTE requirements for an end-use application. 

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