Compensation of Under- and Over-Extrusion caused by sudden Feed Rate Changes using a Single-Screw Extruder

Abstract

Additive extrusion processes using single-screw extruders usually face various challenges. One of those is the inconsistent extrusion caused by sudden feed rate changes as they occur in sharp changes in direction. In these cases, the resulting strand shows a widening, followed by a narrowing, before resettling on a constant cross-section.

The reason for this behaviour is that even if the change in direction may seem abrupt, it is governed by the dynamic limits of the machine, inducing a finite deceleration and acceleration on all axes. In parallel, the fluidic system within the extrusion nozzle is highly inertial. The pressure only slowly decreases, causing the molten polymer to ooze out of the nozzle. The following delayed pressure generation leads to under-extrusion during and after the acceleration phase.

This poster provides an approach to compensate said effect by manipulating the extruder speed upon a feed rate gradient, aiming at congruent trajectories of pressure and feed rate.

The experimental characterisation presented in this poster includes the physical printing of a one-dimensional change in direction: A conveyor belt is used as a support plate, being driven at a predetermined velocity, decelerated and accelerated as abruptly as feasible within the dynamic machine limits. The compensation is based on a polynomial, offsetting the extruder screw’s rotational speed whenever the nominal extruder speed changes, ultimately aiming to shift the pressure history to be congruent with the feed rate. With this approach, a strong improvement of the strand’s uniformity has been visually confirmed.