(526d) Stretching Properties of the Flow and Impinging Jets Stability Studies in RIM

Reaction Injection Molding (RIM) is an industrial process for the production of plastic parts where two or more viscous liquid monomers are injected and mixed in a cylindrical mixing chamber by opposed impinging jets. The mixture that leaves the mixing chamber is discharged into a mold where the polymerization occurs and the plastic part is formed. The mixing chamber is a cylinder with 1 cm diameter and 5 cm length, the injectors’ diameters are in the range of 1 – 2 mm and the mean residence time in the mixing chamber is in the range of 10 – 100 ms. An efficient mixing of the two monomers in the mixing chamber before entering the mold must be achieved in order to avoid zones of unreacted monomers that may lead to structural defects and the eventual rejection of the plastic parts [1]. The jets' Reynolds number range of industrial practice goes up to 600. Previous computational and experimental works [2, 3] have shown that mixing in RIM mainly occurs above a critical jets' Reynolds number that marks the transition from a steady segregated laminar flow regime to a self-sustained laminar chaotic regime with strong mixing dynamics. It has also been shown that in equal injector diameter chambers the iso-momentum condition between the opposed jets is fundamental to ensure the continuous presence of the self-sustainable chaotic flow regime [4].

In this work the stretching properties of the flow in the RIM machine mixing chamber are studied from 3D Computational Fluid Dynamics (CFD) simulations and Lagrangian tracking of the position and deformation of material spherical elements. The flow regimes and the jets' impingement point position under unequal flow rates and different injectors' diameters are studied from PIV experiments. An elastic analogue model of the jets is proposed to predict its impingement point.

Results show that the flow under certain conditions develops the ability to reorient trajectories and produce a nearly exponential average stretching of fluid elements with time, a feature of chaotic flows. Both CFD simulations and experimental results show that Re > 110 is the value that sets the transition from a steady laminar flow regime to a self-sustained laminar chaotic flow regime. The good agreement between the PIV experiments and the elastic analogue model indicates that the jets' kinetic energy feeding rate ratio is the key parameter to control mixing in RIM for formulations with stoichiometric and density ratios different from 1.

[1] P. Kolodziej, W.P. Yang, C.W. Macosko, S.T. Wellinghoff, Impingement Mixing and its Effect on the Microstuture of RIM Polyurethanes, Journal of Polymer Science, Part B: Polymer Physics, 24 (1986) 2359-2377.

[2] R.J. Santos, A.M. Teixeira, J.C.B. Lopes, Study of mixing and chemical reaction in RIM, Chemical Engineering Science, 60 (2005) 2381-2398.

[3] R.J. Santos, E. Erkoç, M.M. Dias, A.M. Teixeira, J.C.B. Lopes, Hydrodynamics of the mixing chamber in RIM: PIV flow-field characterization, AIChE Journal, 54 (2008) 1153-1163.

[4] M.I. Nunes, R.J. Santos, M.M. Dias, J.C.B. Lopes, Micromixing assessment of confined impinging jet mixers used in RIM, Chemical Engineering Science, 74 (2012) 276-286.