Short fibers controlling

The aspect ratio, i.e., the length/diameter ratio (L/D) of the fibers is a major parameter that controls the fiber dispersion, fiber-matrix adhesion and optimum performance of short fiber-mbber... 

Figure 5. Reaction mixtures demonstrating short-fiber-forming activities of endo-l,4-/3-D-glucanases. The reaction mixtures are shown after two weeks incubation at 40°C with 20 mg filter paper (3). Represented from left to right are a control without enzyme, Endoglucanase TV (235 fig/ mL), Endoglucanase III (87 fig/mL), and Endoglucanase II (75 fig/mL).

For short fiber systems, the fiber orientations obtained are less controllable, but techniques are being developed to control fiber orientation during injection molding to some degree. 

An outline of the viscose process is shown in Figure 10.52. By this process, short-fibered cellulose (wood pulp) is converted in a series of controlled and coordinated steps to a spinnable solution and then into longer filaments, which may be precisely controlled as to length, denier, cross-sectional shape [118], and other physical properties. 

Short-fiber thermoplastic composites are typically injection molded, which allows rapid, high-volume, and economical production but requires expensive tooling. The fiber length and volume fraction are limited by this method, and fiber orientation and distribution are difficult to control. 

Quantitative predictions of the effects of fillers on the properties of the final product are difficult to make, considering that they also depend on the method of manufacture, which controls the dispersion and orientation of the filler and its distribution in the final part. Short-fiber- and flake-filled thermoplastics are usually anisotropic products with variable aspect ratio distribution and orientation varying across the thickness of a molded part. The situation becomes more complex if one considers anisotropy, not only in the macroscopic composite but also in the matrix (as a result of molecular orientation) and in the filler itself (e.g., graphite and aramid fibers and mica fiakes have directional properties). Thus, thermoplastic composites are not always amenable to rigorous analytical treatments, in contrast to continuous thermoset composites, which usually have controlled macrostructures and reinforcement orientation [8, 17]. 

Two important modifications have to be mentioned, which permit to control stiffness and tonghness of a pol3rmer material separately polymer orientation and reinforcement through a second phase such as core-shell particles, mineral fillers, or short fibers (see Reinforcement). 

Cullity, D., and Boiler, T. Development of Ballistic-Damage-Tolerant Flight Control Components Molded of a Short-Fiber Reinforced Composite Material. Goodyear Aerospace Corp., Sept. 1973. 

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