Twisted shells: unraveling under-extrusion instabilities in 3D printing*

In 3D printing, ensuring the precise extrusion of material is crucial for achieving the desired shape while minimizing filament usage. One strategy involves under-extrusion, where insufficient material is used to form a uniform layer. We discovered that this results in a periodic pattern of droplets and threads, highly dependent on the previous print layer and the printing properties. We developed a minimal model based on our experiments to explain the emergence and physics behind this instability. Extending our study to more complex printing paths, such as cylindrical ones, we demonstrated the creation of highly twisted shells, showcasing the potential of this method.