Impingement mixing of liquids in reaction injection molding

Reaction injection molding (RIM) requires the mixing of two prepolymers that will react to form the polymer part molded in situ. An important operation in the sequence of steps that make up RIM is the homogeneous mixing of the two (or more) viscous liquids. In many RIM applications, the preferred mixing route is direct jet-to-jet impingement of the reactant streams. Fluid velocity measurements have been obtained under a variety of conditions to determine the effect of geometrical parameters on the RIM mixhead on the observed flowfield. No 'coning' or 'fanning' of the jets was observed and the flow is laminar and unidirectional after 2 mixhead diameters downstream of the impingement point. Retracting the clean-out piston (closed end of the mix chamber) increases the amount of backflow observed in the chamber. The cases that involved a smaller nozzle opening diameter to chamber diameter seemed more sensitive to the geometrical parameters investigated.