Additive manufacturing systems and apparatus include, in one aspect, a material deposition system including an extruder for deposition materials, the extruder including two or more material entry ports, a mixing chamber, and an exit orifice; and a controller coupled with the extruder to dynamically change delivery rates of the deposition materials to be mixed in the mixing chamber before flowing from the exit orifice; wherein the controller combines a desired volume flow rate of material to flow from the exit orifice with a mix ratio to specify the delivery rates of the deposition materials. The system can include filament drive systems to feed the thermoplastic materials in filament form into the entry ports, and the controller can dynamically change the mix ratio when operating the filament drive systems to control one or more properties of the material to flow from the exit orifice.