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The material flow in the weld zone of friction stir welded bi-modal high density polyethylene (HDPE) was investigated under different in-process temperatures. The in-process temperatures were controlled by changing the tool rotation speed, welding speed and the material initial temperature. Boundary temperature measurements were recorded on the top and bottom surfaces of the welded plates and the temperature distribution in the workpiece was determined numerically using an iterative inverse heat conduction method. Material flow patterns were observed by welding two different colors of the polymer blanks, white on the advancing side and black on the retreating side. Joint quality was assessed using optical microscopy, and joint strength and elongation were measured by tensile testing. It was found that process temperatures greatly affect the extent of material flow in the weld zone, which in turn affects the tendency to form defects and the overall joint mechanical properties. It was also found that proper fusion across the interfaces occurs when the material in the welding zone was maintained at temperatures in excess of 100 °C across the thickness. The high temperature conditions instigated proper fusion of the material, which in turn resulted in large weld efficiencies and elongations in excess of 100%.