The present invention relates to a process and device RIM involving a mixing chamber (1) with opposing injectors (2, 3). The efficiency is maximized through the operation in flow regimes with very strong mixing dynamics, which is assured by means of the introduction of schemes to: detect the flow regime within the mixing chamber, through the dynamic pressure measurement upstream of the injectors (2, 3), using means (5); and/or impose a chaotic flow regime, through the pulsation of the opposed jets, using means (6). Alterations in design, comparatively to the traditional geometries, are also proposed, concerning: the mixing chamber (1), being foreseen a prismatic rectangular chamber; the injectors (2, 3), being foreseen elongated injectors; and even the scheme of injection of the materials, using a third injector (4).