Particle Image Velocimetry (PIV) and Laser Doppler Anemometry (LDA) data is presented and used to characterize the dynamic behaviour of the flow field in a RIM machine, and the main flow patterns and mixing mechanisms are examined and established. A two-dimensional CFD model is used for the simulation of mass transfer and chemical reaction in the mixing chamber of a RIM machine. Some design and operational parameters are considered and its influence on the mixing behaviour of a RIM machine is analysed.