This paper proposes two important models useful for describing heat transfer and temperature profiles in single and modular self-Wiping co-rotating twin-screw extruders. One model predicts heat transfer coefficients. Calculations of the barrel and screw heat transfer coefficients are made for various modules in different diameter extruders. We compare the values of the heat transfer coefficients from the model with those from literature. The second model predicts the axial screw temperature profile when the barrel, but not the screw, is heated. Example calculations of the cup mixing and screw temperature profiles are also made for different diameter extruders.