Easy snap-folding of hexagonal ring origami by geometric modifications

Abstract

Hexagonal ring origami is a type of foldable structure that has impressive packing abilities and can be tessellated into two-dimensional or three-dimensional surfaces without any gap or overlap. It can be folded under bending or twisting loads into a peach core-shaped configuration with only 10.6% of its initial area. However, in applications of large-scale foldable structures, folding by bending or twisting is usually technically difficult. Here, we propose strategies to facilitate easy snap-folding of the hexagonal ring by a simple point load or localized twist or squeeze. This is enabled by two geometric modifications made to the hexagonal ring: introducing residual strain and creating pre-twisted edges. By combining theoretical modeling, finite element simulations, and experiments, we systematically investigate the snap-folding behaviors of modified hexagonal rings with residual strain and pre-twisted edges. It is found that the geometric modifications promote easy snap-folding of the hexagonal ring by different mechanisms: introducing residual strain can significantly decrease the energy barrier and thus reduce the required moment to snap-fold the ring, while creating pre-twisted edges allows for easy out-of-plane deformation which is a necessary condition for a ring to fold. Combining the two methods further enables the snap-folding of the hexagonal ring by a point load or localized twist or squeeze. To demonstrate the easy folding of large assemblies of the modified rings, we construct various structures that can be snap-folded from their initial three-dimensional states to significantly lower-volume final states by a simple compression at the corners of the rings. We envision that the proposed geometric modification strategies can provide a new perspective on the rational design of easy-to-fold ring origami-based foldable functional structures with extremely high packing ratios.