Reaction Injection Moulding, RIM, is a technology for the production of plastic parts using reactive in situ polymerization from the mixing of two or more liquid monomers. Although RIM is a widely used process, nowadays there are no means to effectively control mixing. This work assesses a new control variable of mixing in RIM: the differential static pressure between the liquid monomers injectors. This control variable is related to the mechanical properties of injected RIM parts. The dynamics of the control variable is also assessed. A model is introduced to relate the control variable with the mixing conditions in RIM. (C) 2015 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.