12.03.2020 •

Stereolithography and Softlithography

Stereolithography‐based additive manufacturing is rapidly gaining interest for manufacturing ceramics due to its ability to form complex‐shaped architectures without molds, and to offer good alternative. Stereolithography is an effective UV light‐cured technology based on the photopolymerization of a photosensitive ceramic slurry, exhibiting a great potential in the fabrication of complex‐shaped ceramic parts with high accuracy. During the process, 3D model is firstly imported with the x‐y resolution, layer thickness, and exposure parameters into a printer. Then each individual layer is cured by a UV light. After the first layer is cured, the supporting platform is moved up, and the ceramic slurry is recoated with a blade.

Then, the second layer is cured analogously. These steps are repeated until the whole green part is eventually produced. In the past years, many kinds of oxide ceramics, including Al2O3,19-21 ZrO2,22-24 ZTA,25 and other oxide ceramics have been widely reported using this method. However, it is noted that all these reported oxide ceramics are in white color. For SiC ceramic, it is usually in gray or dark color. As known, the color of particle has an obvious impact on the light transferring behavior and it's curing ability. Unfortunately, the stereolithography‐based additive manufacturing of gray‐colored or dark‐colored SiC ceramic is therefore very difficult and challenged, as well as has not been reported before. Moreover, the intrinsic relationships between the particle color and curing ability has not been understood. Moreover, the particle size, solid loading, as well as the stereolithography parameters, have great effects on the curing ability of ceramic slurries. However, till now, there is still no report about these aspects and no study finding these intrinsic relationships.

Among all 3D printing methods, stereolithography apparatus (SLA) and digital light processing (DLP) offer great advantages, making them ideal candidates for microfluidics and biomedical applications. However, one of the limitations of 3D printed SLA/DLP master molds for softlithography is the requirement for tedious pretreatments prior to PDMS casting. The pretreatment of the resin is necessary to ensure the complete curing of the PDMS in contact with the resin. Otherwise, the surface of the PDMS replica in contact with the resin cannot be polymerized due to the presence of residual catalyst and monomers, and its transparency would be also compromised. It has been observed that the effects of pretreating the master mold are more significant in channels with smaller feature sizes and, in the case of relatively larger 3D printed parts, this challenge is not significant. To address this issue, many researchers have proposed various pretreatment protocols to treat the 3D printed master mold before PDMS casting. As one of the first attempts, Comina et al. proposed to cover the 3D printed template with a specialized ink via airbrushing. However, the effectiveness of that method depended largely on the thickness of the ink. Four procedures are commonly used among other proposed postprinted protocols: 1) UV curing; 2) surface cleaning (e.g., ethanol sonification and soaking); 3) preheating; 4) surface silanization. Waheed et al. introduced an efficient yet time‐consuming pretreatment protocol for PDMS softlithography. The postprocessing included a 5 min UV treatment followed by 6 h of soaking in an ethanol bath. Following the air plasma treatment for 1 min, the surface of the 3D printed template was silanized by perfluorooctyl triethoxysilane for 3 h.