The remaining geometrical parameters are easily derived from the measured parameters presented above. Several of the screw parameters are functions of the screw radius. They include the perpendicular distance from flight to flight, W(r); the width of the flights in the axial direction, b(r); and the helix angle, θ(r), the angle produced by the flight and a plane normal to the screw axis. These parame- ters will be discussed later. At the barrel wall these parameters are subscripted with a b. The helix angle at the barrel wall is θb and is calculated using Eq. 1.1. The helix angle at the barrel wall for a square-pitched screw is 17.7°.