Abstract: An L-shaped mixing head for reaction injection molding apparatus comprises a first chamber wherein a plurality of reactive molding components are impingement mixed under high pressure. A second chamber intersecting with the first chamber quiets the resulting stream of mix product prior to its exiting the mixing head. The outlet end of the second chamber is defined by the inner surface of a sleeve, the material of which has a yield strength less than that of the materials of the mixing head body and the second plunger. Most preferably, the sleeve material is also chemically nonreactive with and nonadherent to the reactive molding components to be used therewith, both individually and as mixed.