Hinges for Origami-Inspired Structures by Multi-Material Additive Manufacturing

Additive manufacturing of structures capable of changing their properties or shape in a programmed controlled manner is a booming research field. A large number of studies show that the unique capabilities of 4D printing (freedom of geometrical design space and locally controllable material in combination with active materials) enable the development of structures with completely new properties, e.g. programmable mechanical or thermal response and shape transformations. This new way of fabrication is particularly of interest in the field of origami. Origami-inspired structures can be used in deployable structures, where a minimum amount of storage volume is desirable. Further, studies show that two dimensional origami patterns can be assembled into three-dimensional metamaterials, capable of changing their properties. Conventional fabrication of these structures often requires assembly and joining limiting the achievable designs. This can be overcome by an additive manufacturing approach, which allows the fabrication highly complex origami structures in a one-shot process. Seemingly, the only limitation for these newly presented structures is the imagination of the engineer. In reality, structures finding their way into real life applications are an exception. The reason for this gap between laboratory scale and industry are the to-date insufficient mechanical properties. Usually loading capacity and fatigue resistance of the hinges are insufficient for the requirements imposed by possible applications. This holds particularly true for living hinges. 3D printed pin joint designs usually require much larger dimensions compared to living hinges. For the majority of the origami structures this is not feasible. This study addresses these limitations of 3D printed origami-inspired structures. We focus on the characterization and optimization of the mechanical properties of 3D printed living hinges, including strength, bending stiffness and fatigue. This often-disregarded aspect is essential for pushing the existing origami-inspired structures further towards real life applications. We introduce a new type of 3D printed hinge, fabricated by multi-material FDM printing using continuous fibers. These composite hinges show large potential to significantly improve the loading capacity of the existing structures. Other hinge designs investigated are multi-material hinges fabricated by ink jet printing of photo-curable polymers and single material hinges printed by FDM from common 3D printing materials. The influence of design parameters such as cross section area and length of the hinge, as well as number of loading cycles, on the mechanical properties are investigated. Mechanical characterization of the hinges show tremendous difference in the loadbearing capacity. The maximum tensile load of composite hinges are up to two orders of magnitude larger than the one of inkjet printed multi-material hinges. Finally, maps correlating mechanical properties with design parameters are developed as a tool for selection of manufacturing technology, material and design of hinges for desired combinations of bending stiffness and loading capacity tailored to given applications. This work represents a first step towards bringing the advances in the field 3D printed origami-inspired structures closer towards application.