The present invention provides an improved extruder assembly, that includes a wiper for wiping a screw portion in a low pressure extruder stage, whereby the wiping of the screw portion improves melt conveyance through the low pressure extruder stage. The present invention has particular utility when used in a low pressure venting and entrainment extruder stage of a single screw compounding extruder. The present invention also provides an extrusion/injection unit for use in-line compounding molding system that incorporates the improved extruder assembly.