Jet impingement mixing in an L-type mixhead: comparison of mixing criteria

Abstract

High velocity jet impingement mixing is a unique operation of the reaction injection molding process. The quality of mixing depends on processing parameters for a given system, and can have significant effect on the properties of the product. We have compared three methods used for characterization of mixing quality reported in the literature for a cross-linking polyurethane system processed on an industrial scale high pressure RIM machine, with an L-type mixhead. The methods are based on, (i) visual appearance of a molded sample, (ii) adiabatic temperature rise during reaction and, (iii) flexural modulus of elasticity of a molded sample. The visual appearance test was made quantitative by using a color matching optical system. The adiabatic temperature rise increases rapidly with nozzle Reynolds number (Re_nzl) for polyol, up to a critical value (Re_nzl = 380) beyond which there is a slow increase. The latter is attributed to the geometry of the L-type mixhead. A simple analysis is presented to explain the limiting extent of mixing at high Re_nzl. The flexural modulus and the visual appearance of molded samples are found to improve until Re_nzl=500, implying a greater sensitivity to mixing as compared to the adiabatic temperature rise.