Development of functionally graded metamaterial using selective polymerization via digital light processing additive manufacturing

Abstract

Functionally graded materials (FGMs) are characterized by gradually changing material compositions or properties designed to provide desired functional requirements. Recently, FGMs have been manufactured by multi-material additive manufacturing (AM) processes such as material jetting or directed energy deposition, which require high manufacturing costs. This study aims to develop FGMs through vat photo-polymerization AM, specifically the single-vat digital light processing (DLP) process. To differentiate material properties in the single-vat DLP process, two approaches were proposed in the AM and post-curing stages. In the AM stage, we differentiated the brightness of printing images to manipulate the degree of polymerization, so-called graded AM, instead of utilizing the conventional black-and-white images. In the post-curing stage, the additively manufactured parts were selectively cured in accordance with functional requirements. The effects of selective post-curing and graded AM were investigated in terms of the hardness and flexural deformation behavior of the printed specimens. These approaches were then combined to selectively manipulate the degree of polymerization, and thus be used to develop hard yet flexible FGMs. Considering that hardness and flexibility are contradicting properties, the developed FGMs serve as structural metamaterials with high surface hardness and flexural flexibility. The developed hard yet flexible metamaterials can be utilized in the preparation of novel functional structures, without requiring additional coating or assembly processes.