An additive injection system for use in injection molding machines to inject additive materials, such as fluids or powders, downstream of a nozzle of the injection molding machine. The additive injection unit has an additive tank to store or contain additive material and additive injectors that inject an additive material into an injection molding machine. A pump pumps the additive material from the tank through a common manifold connected to the additive injectors to be injected into the injection molding machine. A controller controls operation of the additive injection device and one or more sensors coupled to the additive injection device can provide feedback to the controller.