Breakthroughs in projection-enabled additive manufacturing: From novel strategies to cutting-edge applications

Abstract

As one of the crucial technologies of additive manufacturing (AM), projection technology, which features fast printing speed, high molding accuracy, and low cost, exhibits great potential in cutting-edge applications. The basic principle of projection-enabled AM (PAM) is the stacking of layer-wise structures, in which the dynamic mask pattern is projected onto the surface of the liquid photosensitive resin, triggering polymerization in a specific area to accomplish single-layer printing. This facilitates the layer-wise manufacture of complex three-dimensional (3D) structures. Currently, the projection technologies of PAM can be mainly classified into liquid crystal displays, digital light processing (DLP), and silicon-based light crystal spatial light modulators. Among these, DLP is distinguished by its programmable two-dimensional (2D) projection ability. Initially, DLP was used as an imaging technique in which image signals were digitally processed and then projected. In recent years, DLP technology has been widely employed in optical AM. Figure 1 shows the development of PAM technologies, including typical processing schemes, processable materials, and concrete applications. Compared with conventional voxel-by-voxel AM strategies, DLP enables layer-wise 3D structuring, revealing distinct advantages including high efficiency, reasonable resolution, low cost, and great scalability. To date, DLP-enabled AM (DLP-AM) has opened a new avenue for advanced optical AM, attracting tremendous research interest. To achieve high processing speeds, tremendous breakthroughs have been made in DLP-AM technologies during the past few years. As a successful example, continuous liquid interface production (CLIP) technology has been developed to achieve fast and large-scale 3D printing through a dead zone formed by UV light and oxygen. However, further increase of the printing speed is challenging. When the printing parts rise too fast or the resin is too viscous, the resin does not fill in time, which determines the maximum printing speed. A new injection continuous liquid interface (iCLIP) method was proposed to overcome this limitation.4Lipkowitz G. Samuelsen T. Hsiao K. et al.Injection continuous liquid interface production of 3D objects.Sci. Adv. 2022; 8: eabq3917Crossref PubMed Scopus (7) Google Scholar The core component of this method is the microfluidic channel created and integrated into the growing parts. The resin can be injected into the printed area through the viaduct, overcoming the passive resin flow process of the original CLIP technology and thereby achieving actively controlled mass transfer (Figure 1). Therefore, the printing speed has been increased by 5–10 times, and resins that are an order of magnitude more viscous can be processed. In addition, the iCLIP method facilitates printing using various types of resins by injecting them individually through different channels. To further improve fabrication efficiency and resolution, Regehly et al. introduced a two-color linear volumetric AM (VAM) technology called Xolography. Using photoswitchable photoinitiators, this technology initiates local polymerization in a confined monomer volume by the linear excitation of intersectant light beams of different wavelengths. This yields a characteristic resolution of up to 25 μm and a curing rate of 55 cm3/s.3Regehly M. Garmshausen Y. Reuter M. et al.Xolography for linear volumetric 3D printing.Nature. 2020; 588: 620-624Crossref PubMed Scopus (143) Google Scholar In addition, Taylor and co-workers utilized the computed axial lithography (CAL) technique to develop a microscale CAL (m-CAL) VAM technology, which demonstrates microscale processing capability and possesses several distinct advantages such as high printing speed, layer-free manufacturing, and high-viscosity resin printing.1Toombs J.T. Luitz M. Cook C.C. et al.Volumetric additive manufacturing of silica glass with microscale computed axial lithography.Science. 2022; 376: 308