Abstract: A high pressure mixing head for mixing reactive components in reaction injection molding includes a mixing chamber discharging into a quieting chamber and an injection valve for injecting reactive component into the mixing chamber. Recirculation passages for the reactive component are provided in the mixing chamber and internally of the injection valve. The recirculation passage internally of the injection valve is selectably opened and closed as is the outlet from the injection valve into the mixing chamber. Both the injection valve outlet and the recirculation passage internally of the injection valve are selectably throttleable.