A mixing assembly for use in a semi-continuous process for producing liquid personal care compositions, such as shampoos, includes a main feed tube carrying a base of the composition to be produced, a plurality of injection tubes in selective fluid communication with the main feed tube, and an orifice provided in a wall at an end of the main feed tube downstream of the plurality of injection tubes. The wall in which the orifice is provided includes a curved (e.g., semispherical) entry surface on an upstream or inlet side of an orifice, and a curved (e.g., semi-elliptical) exit surface on a downstream or outlet side of the orifice. The orifice may have a rectangular or elliptical shape. By maintaining symmetry of the injection tubes with respect to the orifice, and leveraging delay between introduction of dosed modules and increased viscosity, effective mixing may be achieved with minimal energy.