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This paper presents a study on the flow of molten plastic in a single screw plastic extrusion device. The flow characteristic of molten plastic in a single screw plastic extruder is similar to that of a viscous liquid between two infinitely parallel plates, one moving and the other fixed. Assuming that the molten plastic was a Newtonian fluid, a differential equation describing the relationship between the extrusion speed, the pressure at the head of the extruder, and the screw geometry, the properties of the molten plastic was established. Hence, the integrated flow equation is derived in the case of isothermal extrusion and the fluid viscosity was constant inside the extruder. These flow behavior theories were then utilized to established an equation for screw diameter calculation and verified through the design and construction of a filament extrusion device for a 3D printer.