A double-acting compounder comprises: an outer housing; an inner mandrel disposed telescopically within the outer housing to define a fluid chamber; an equalizer, such as an equalizer piston, between the fluid chamber and an exterior of the outer housing for equalizing pressure; a movable seal assembly within the fluid chamber with a first end defining an end of the fluid chamber; the inner mandrel having an outer shoulder for moving the movable seal assembly with the inner mandrel when the inner mandrel moves in a first direction relative to the outer housing to compresses fluid within the fluid chamber; and the outer housing having an inner shoulder for preventing the movable seal assembly from moving with the inner mandrel when the inner mandrel moves in a second direction relative to the outer housing to compresses fluid within the fluid chamber.