Hybridizing Direct Ink Write and mask-projection Vat Photopolymerization to enable additive manufacturing of high viscosity photopolymer resins

Abstract

Vat Photopolymerization (VP) is an additive manufacturing (AM) process that enables the fabrication of parts with high-resolution features and sharp corners through its selective patterning of UV irradiation. Currently, VP is capable of only processing low viscosity photoresins due to the constraints imposed by the recoating process, which limits both the available materials and the mechanical properties of the final parts. Conversely, Direct Ink Write (DIW), a material extrusion process, is capable of processing high viscosity photoresins but patterns features directly via a nozzle, which limits both achievable feature resolution and shape. To enable the layered processing of high viscosity photoresins (and thus high-performance materials) while retaining high-resolution features, the authors have created a novel AM process that integrates DIW and VP printing modalities. The resulting hybrid process uses the DIW system to extrude an unpatterned layer of photoresin that is then selectively photocured into the desired layer shape by a UV DLP projector. The pressurized extrusion mechanism of DIW and shear-thinning behavior of the photoresins enable the processing of photoresins with viscosities that are multiple orders of magnitude larger than which can be currently processed via traditional VP. The DLP’s selective irradiation of the dispensed layer enable the realization of high-resolution features that are orders of magnitude smaller than that which can be currently processed via DIW. The hybrid process’s relaxation of viscosity constraints allows for the printing of new photoresins with novel chemistries, high molecular weight monomers, and/or high solids loading that ultimately result in greatly improved material properties compared to traditional photoresins. To demonstrate the capabilities of the hybrid system three materials were printed from novel, high-viscosity photoresins: (1) an all-aromatic polyimide, (2) an urethane acrylate elastomer, (3) a highly loaded alumina photopolymer suspension. The all-aromatic polyimide photoresin had a viscosity of 770 Pa s and the final parts had a degradation temperature of 534 °C. The urethane acrylate elastomer photoresin had a viscosity of 21.7 Pa s and the parts exhibited an average elongation of 599% and an average stress of 16.3 MPa at failure. A stiffer reinforced urethane acrylate photoresin with 5 wt% silica had a viscosity of 38.6 Pa s, and the parts exhibited an average elongation of 430% and an average stress of 14.8 MPa at failure. The 85 wt% alumina photopolymer suspension had a viscosity of 1350 Pa s, exhibiting a final sintered flexural strength at 25 and 400 °C of 154.9 and 156.6 MPa respectively. These three case studies demonstrate the hybrid AM process’s ability to print high-performance materials from high viscosity photoresins while retaining the high-resolution and sharp features expected of VP parts.