The parameters affecting the packing stress of fiber mats for melt impregnation, resin transfer molding and compression molding are systematically investigated. An analytical equation based on local bending of fibers, which was previously derived for non-impregnated networks, is applied to composite mats of dispersed planar bundles impregnated with molten polypropylene. It is shown that many simultaneous mechanisms interact during packing of impregnated bundle mats, in particular when the mats are needled. These include in-plane bending of the bundles, compaction of the fibers within a bundle, and buckling, slippage or breakage of the out-of plane fibers. In order to identify and decouple these features, aspect ratio of the bundles, lubrication, needling intensity and packing history are varied. A microstructural experiment is also developed to evaluate the extent of bundle spreading. It is found that dispersed fibers or bundles roughly follow the equation based on local bending, but that needled bundle networks deviate from the power law behavior. Three regions were identified. The first is attributed to self-loading of the mats and to buckling of the out-of plane fibers. The second region is due to slippage and breakage of the out-of-plane fibers and depends on the loading history and on the needling intensity. The third region is due to packing of the in-plane bundles, which do not really bend, or spread under load, but are locally compressible, owing to misalignment and waviness of the individual fibers forming the bundles. In compression molding, the influence of the in-plane reorientation of the initially out-of-plane bundles on the packing stress is observed.