Beijing – Melt extrusion 3D printing is increasingly important in industrial applications, but mechanical anisotropy can occur, and the mechanism by which this occurs is unclear. A team in the polymer additive manufacturing lab at the Beijing University of Science and Technology have been studying the reasons behind the phenomenon.
The mechanical properties of the printed parts are largely down to the interface bonding strength between the adjacent melt lines formed during the printing process. The group looked at how temperature attenuated as layers of thermoplastic polyurethane melt were deposited, and how this affected the development of the interfaces. In an open environment, it tied in with the power law function.
An electric heating bed was used to look at the effect of envelope temperature on tensile properties, bringing the printed sheets close to the bed’s temperature. Tensile testing was then carried out both longitudinally and transversely to look at how the mechanical anisotropy developed.
Both ultimate tensile strength and elongation at break measured transversely increased as the temperature rose. They put this down to an improvement in interface bonding. However, the longitudinal ultimate tensile strength remained fairly stable – it went up slightly to start with, and then dropped back a little.
They found that the printed parts’ mechanical properties were affected by both the wall-shear induced orientation effect of molecular chains at the melt line surface, and the morphology of crystallisation within the deposited molten filament. Raising the envelope temperature increased bonding at the interface and overcame mechanical anisotropy, while also promoting crystallisation.
This printed TPU had better longitudinal yield stress once the temperature was above a critical point. The team suggests that this insight could help in achieving the goal of printing high-strength parts using melt extrusion 3D printing.
The work has been published in the journal Additive Manufacturing.
