Impregnation of compressible fiber mats with a thermoplastic resin. Part I: Theory

In many cases of composite processing by liquid matrix impregnation, the fi ber reinforcement is compressed when it comes in contact with the liquid an d then relaxes as the matrix f lows within i ts pores. This phenomenon can be analyzed in terms o f 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 fibe r preforms by a polypropylene matrix, used in the production of Glass Mat T hermoplastic blanks. A quantification of the effects of applied pressure an d fluid viscosity on total process time is provided. It is shown that the t ime for preform relaxation after the f luid has filled the preform may b e much larger than that for impregnation. As a result, an apparently well imp regnated part may exhibit an inhomogeneous distribution of the reinforcemen t, in turn inducing a modification of the mechanical behavior and residual stress distribution.