Digital Materials – Photopolymers with increased toughness

Abstract

Common Lithography based Additive Manufacturing Technologies (L-AMTs) are well known for their excellent Surface quality and precision as well as for their nearly unlimited freedom of design. However, in some cases L-AMTs are limited in thermomechanical properties of the final part material. This is of specific concern for photopolymers which typically exhibit rather low fracture toughness values. Therefore, a lot of effort is put into the development of new concepts for toughening of 3D printed materials and expand the field of industrial applications. The opportunity of individually tailoring each segment or layer within the part allows the production of so-called digital materials, where each volume element (voxel) can carry different structural or functional material properties. This allows for mimicking toughening concepts which are used in biological materials like nacre or glass sponges where alternating soft and hard layers help to significantly increase the toughness of the final compound. [1, 2] Fratzl et al. figured out that an effective shielding effect can be observed if the difference in elastic moduli of soft and hard phase is a factor 5 or higher.[3] By combining different L-AMTs, e.g. inkjet printing with digital light processing, such toughening concepts relying on hybrid materials can be realized. A digital light process unit (used wavelength λ = 375 nm, pixel size 50 μm) and an inkjet unit (λ = 455 nm, resolution 600 dpi) are utilized to produce bioinspired heterogenous materials. By printing the primary hard matrix material by a lithography-based AMT-printer and adding a soft ink as secondary material by an inkjet-printer, toughening of the digital material could be achieved. Tensile testing, stress relaxation and impact testing were performed to characterise thermomechanical properties. Furthermore, light, and scanning electron microscopy were utilized to assess properties of fractured samples and inkjet droplet quality. Assessed values agree with high performance polymers and printed digital materials reach an increase in tensile toughness of more than 60 %.