Studies on the Effect of Pellet Size on Positive Conveying in Helically Grooved Single Screw Extruders

The helical grooved barrel feed section permits, when the barrel channel aspect ratio and the barrel channel helical angle are properly designed, an operating mode where the positive conveying is prevailing. While the pellet size of materials is varied, different dimension relationships of the barrel channel depth and pellet size could induce different equivalent barrel channel aspect ratios with the transformation of solids conveying mechanisms from the positive conveying to the friction-drag conveying. Considering the effect of dimension relationships on the solids conveying mechanism, a novel pellet-size physical model was established to guide the available design of the barrel channel geometry for positive conveying. In the pellet-size model, one or two shear interfaces were identified inside the solid-plug resulting in different equivalent barrel channel aspect ratios, based on which the solids conveying mechanism on the shear interfaces was detailed by the boundary condition equations for positive conveying. Besides, the theoretical simulations are also compared with the on-line measuring experimental data. The results show that the equivalent barrel channel aspect ratio on the shear interfaces exists indeed and determines the solids conveying mechanism, which is well confirmed by the excellent consistence between the predicted and measured data.