Apparatus (20) for injection of fluid into extrusion system components such as a preconditioner (24) or extruder (100) is provided, preferably as a composite assembly including a fluid injection valve (52) and an interconnected static mixer section (54). Alternately, use may be made of the fluid injection valve (52) or static mixer section (54) alone. The invention greatly simplifies the fluid injection apparatus used in extrusion systems, while giving more efficient absorption of thermal energy with a minimum of environmental contamination, and the ability to inject multiple streams into the extrusion systems.