A novel building strategy to reduce warpage in droplet-based additive manufacturing of semi-crystalline polymers

Warpage is induced by a shrinkage difference during the manufacturing process. A main source for a shrinkage difference is a temperature gradient within the manufactured part. In the field of additive manufacturing (AM) this effect is even stronger compared to conventional fabrication techniques like injection molding, due to the layer-wise and time-staggered fabrication principle. A novel droplet-based AM process has been developed, based on squeezing a polymer melt through a nozzle. The process is currently limited to amorphous plastics because of warpage problems with the fabrication of semi-crystalline material. Several compensation techniques for warpage effects are discussed for the proposed process. Optimizing the tool path planning process is identified as the best approach. The building strategy has a significant influence on the temperature distribution during the building process. This affects the resulting shrinking rates and arising residual internal stresses. Most building strategies for extrusion based processes use parallel hatching (zigzag) lines to fill the contours and minimize the building time. In this paper, a novel building strategy is introduced for droplet-based AM, which tries to reduce thermal gradients during the building process. The so-called 'island printing' uses an infill strategy based on the division of the layered data into fields. With the printing order of the fields, the heat impact can be controlled and used to generate a homogeneous temperature distribution. First parts have been fabricated with semi-crystalline plastics showing the occurring warpage problem. A significant warpage reduction of 22% could be reached with the novel island printing strategy in first tests.