Theories of fiber packing, of use in manufacturing composite materials, are developed. The maximum packing fraction of force free fibers is estimated based on a statistical analysis of the distribution of fiber‐fiber contact points. The new expressions are more general than previous ones by allowing for a distribution in fiber length and orientation. The forced packing beyond this limit is governed by the bending of fiber segments between contact points. A micromechanical theory is developed for this, again based on the contact point statistics, and equations relating the force response per unit area of a fiber bed to the fiber volume fraction are derived for three basic types of assembly: a general 3D wad, a planar mat of dispersed fibers, and a bundle of almost parallel fibers. Other types of reinforcement structure, such as woven fabrics, and the effect of lubrication are also treated briefly.

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The variation of fiber orientation and content within injection molded rectangular plates is investigated. Fiber orientation and volume fraction are measured using metallographic polishing and a newly developed image‐analysis system; fiber weight‐fractions are measured using an ashing method. Three different moldings are studied; two of the long‐fiber compound Verton, Molded at different injection speeds, and one of the short‐fiber compound Maranyl. Microstructural regions are identified and characterized, and the flow mechanisms by which they are formed are discussed. The results show that the long fibers segregate extensively, whereas the short ones do not. The major differences between the two compounds are found in the core: the core of the long‐fiber compound has a higher content and orientation of fibers than that of the short‐fiber compound.