In many cases of composite processing by liquid matrix impregnation, the fiber reinforcement is compressed when it comes in contact with the liquid and then relaxes as the matrix flows within its pores. This phenomenon can be analyzed in terms of local fluid flow, mass conservation and mechanical equilibrium. A model is proposed to simulate the kinetics of impregnation, and the evolution of the fiber volume fraction profile as the resin front progresses, as well as after the front has reached the end of the mold. The analysis is then applied to the case of infiltration of needled glass fiber preforms by a polypropylene matrix, used in the production of Glass Mat Thermoplastic blanks. Aquantification of the effects of applied pressure and fluid viscosity on total process time is provided. It is shown that the time for preform relaxation after the fluid has filled the preform may be much larger than that for impregnation. As a result, an apparently well impregnated part may exhibit an inhomogeneous distribution of the reinforcement, in turn inducing a modification of the mechanical behavior and residual stress distribution.