Process variations and the transient behavior of extruders

Restrictive or reverse-pumping elements are used in Twin-screw modular extruders to create filled-partially filled regions at strategic locations along the screw to manipulate the residence time distribution, dispersive and distributive mixing and applied stresses in the extruder. A predictive transient model of the extrusion process based on the mechanisms by which filled regions are formed and their behavior under non-steady state conditions was developed. A method for modifying existing pressure-throughput models for different screw geometries into inverse Q-P models for use in the transient model was also developed. Simulation studies of the effect of screw geometry and operating conditions on transient response of extruders to external disturbances, specific deliberate variations introduced into feed streams and other inputs to the process; were conducted on single screw and twin screw geometries and it was found that every extrusion screw design has an associated critical frequency. Disturbances of higher frequency are damped by the process and lower frequencies pass through to the output with little attenuation. This suggests a two-pronged strategy with closed-loop control to remove the lower frequency disturbances and robust screw design guided by the transient model to remove the higher frequency components by the inherent disturbance damping nature of the process. The critical frequency depends on the response speed of the extruder to the parameter of interest and is mainly affected by the conveying ability of the screws, fill level in the extruder, volume in the filled regions and the number of filled-partially filled regions in the extruder. Experiments conducted on a 28mm twin-screw extruder showed this to be true and good agreement was seen between experimentally observed response speeds and simulation results. When applied to the problem of production of energetic materials with designed variations in burn rate along the axial direction by varying the feed formula, it was seen that this response speed places a ceiling on the axial gradients of composition that can be achieved at the output. Steeper gradients can be achieved by making process changes such as reducing screw depth, reducing fill levels, etc. The model can be applied to design feed formula profiles to obtain specific, required composition variations at the output. An experimental and theoretical study of the behavior of filled regions showed that channel fill length at steady state is a linear function of the ratio of percent drag flow to percent empty fraction in the screw. Now, a minimum of only one test is required to characterize fill length for the entire range of screw speed and throughput for a given screw geometry.