In-situ observation of the extrusion processes of Acrylonitrile Butadiene Styrene and Polylactic Acid for material extrusion additive manufacturing

In material extrusion (MatEx) additive manufacturing, solid polymer filaments are fed into extruders to generate extrudates that are needed to build 3D plastic parts. There is a finite gap between the inner surface of an extruder and the edge of a filament, which facilitates the insertion and translation of the filament inside the extruder. It is still not clear how this gap is filled during the extrusion. On the other hand, it is important to know this information, for example, to model the distributions of temperature and pressure during the extrusion. In this work, the gap-filling processes of Acrylonitrile Butadiene Styrene (ABS) and Polylactic Acid (PLA) filaments were directly observed through a heated glass tube. The glass tube functioned as the extruder in our case. A criterion was first developed to determine whether the gap was filled at a specific location of the tube. The gap-filling processes were then observed for different combinations of extrusion temperatures and feed rates. ABS and PLA filaments, which consisted of different colors, were custom-made to aid in the observation. Although ABS and PLA were, respectively, amorphous and semi-crystalline polymers, for both materials, we found: i) the gap inside an extruder was filled through three different steps during the initial stage of extrusion; ii) the gap-filling level depended on extrusion temperature and feed rate; and iii) there was no noticeable difference in the gap filling level when the extrusion started with an empty tube or resumed after the printing had been paused.