The difference between the twin screw extruder and the single screw extruder is mainly reflected in the following two aspects.
Transfer mode of material
In the single screw extruder, the solid conveying section is frictional drag and drag, and the melt conveyer section is sticky and drag. The friction properties of solid materials and the viscosity of the molten materials determine the transport behavior. If some materials have bad friction performance, if the feeding problem is not solved, it is difficult to feed the material to a single screw extruder. In a twin screw extruder, especially for a meshing twin screw extruder, the transmission of material is in some way a positive displacement. The degree of forward displacement depends on the degree of proximity between a screw's screw rod and the relative screw groove of another screw. The screw geometry of the compact intermeshing revolving extruder can get a high positive displacement transmission characteristic. 
Flow velocity field of material
At present, the flow velocity distribution of materials in a single screw extruder has been described quite clearly, while in a twin screw extruder, the flow velocity distribution of materials is rather complex and difficult to describe. Many researchers only analyze the material flow velocity field without considering the material flow in the mesh area, but these results are quite different from the actual situation. Because the mixing characteristics and the overall behavior of the Twin Screw Extruder mainly depend on the leakage in the meshing area, the flow situation in the meshing area is quite complex. In the twin screw extruder, the complex flow spectrum of the material shows the advantages that the single screw extruder can't match at macroscopic level, for example, the mixing is sufficient, the heat transfer is good, the melting capacity is large, the exhaust capacity is strong, and the material temperature control is good.
For more on these themes, see below.