Additive Manufacturing of star structured auxetic lattices with overhanging links

Additive manufacturing has been the driving force behind the growth of metamaterials as a field. Commonly taking the form of lattices, these structures can achieve a range of novel macroscale properties that stem from the cumulative effects of locally designed mechanisms. A wide array of mechanical metamaterials have already been designed using computational methods, but these rarely undergo physical testing, often as a result of manufacturing difficulties. This work approaches the problem of manufacturing complex metamaterial test samples though a case study of 3D petal-based auxetic star lattices. This structure was developed computationally to have a strict thickness minimum relative to cell scale. It has been manufactured and subjected to mechanical testing for validation. The unit cells used a 3D re-entrant mechanism with rotational symmetry so require a linkage structures with overhanging elements which is a common feature in metamaterial concepts but challenging to print. Trials of the test samples were manufactured using a thermoplastic polyurethane filament combined with polyvinyl acetate support at 20, 30 and 40 mm unit cell sizes. It was found that the main geometric challenges for successful printing were the link thickness and the reliability of the prints. To address unreliability, the geometry was cut into layers of cells with adhesive-connected feet and printed in parts for post-process assembly. The layered approach was tested successfully and was estimated to reduce the number of cells needed to be attempted to print the full lattice by over 80%. The use of dissolvable support material proved viable for printing overhanging links, but requires use of fused deposition modelling so a relatively low part resolution. The trial led to a five point design guide methodology for meta material test samples. Combined with cell mathematical definitions that strictly bound link thickness to take minimum print resolution into account, this methodology can be applied to other metamaterials and help bridge the gap between theoretical lattices and physical testing.