Investigation on the Forming Process of Polylactic Acid in Material Extrusion Additive Manufacturing Technique

The internal hydrodynamic parameters of extrusion liquefier and bonding neck have an important influence on the forming quality of material extrusion additive manufacturing (MEAM) products. To investigate the relationship, theoretical research and experimental analysis are carried out on both the melt flow behavior (MFB) of the molten polylactic acid (PLA) inside the extrusion liquefier and the bonding neck. They are theoretically modelled based on Newton's power law equation and the viscous sintering phenomenon of the extrudate, respectively. The measurement on the melt pressure drop is then performed with a self-made equipment, and the bonding neck of the sample is observed and measured by scanning electron microscope (SEM). Through the comparison between the predicted and measured results, the proposed theoretical models are validated, and they can give reliable predictions in terms of MFB and bonding neck. The results also show that increasing the extrusion temperature and width will reduce the hydrodynamic parameters (pressure drop, shear stress and apparent viscosity), and increase the bonding neck size of the sample, and thereby improve the forming quality of MEAM products. While for the printing speed, the situation is to the contrary.