Description:
Traditionally, solids conveying has been modeled by assuming a separate friction factor for the screw and the barrel. Many models have been developed, notably the original Darnell and Mol model. However, such models are typically very unstable. That is, a small change (1%) in the value of either of the friction factors often drastically changes the result. Add to that the fact that there are two such parameters to consider, and practical use of such models is difficult. Friction factor data, in themselves, are difficult to accurately determine and repeat. The scatter in most friction factor data can cause models to produce an extreme range in results. However, actual extruder solids conveying typically functions more reliably than these models suggest. Therefore, one is lead to believe that there should be a different way to describe the performance solids conveying. An alternative approach presented here is based solely on the kinematics of the solids plug. A single solids conveying lead angle replaces the two friction factors as the governing empirical factor. The kinematics model describes solids conveying in cylindrical coordinates, and it is easily applied to operating extruders. The calculations are stable, simple, and mimic the actual performance of solids conveying very well. A detailed application to LDPE will be shown and discussed.