Multi-color dual wavelength vat photopolymerization 3D printing via spatially controlled acidity

Additive manufacturing (AM), or 3D printing (3DP), offers precise control over object shape and function. While most AM focuses on single materials, there's increasing demand for multimaterial printing to expand design freedom and functionality by spatially controlling properties like stiffness, thermomechanical properties, conductivity, color, and solubility. Common multimaterial AM methods are deposition-based (e.g., melt extrusion, material jetting, direct ink write), which involve delivering different materials to print heads. Vat photopolymerization (VP) is attractive due to its high resolution, smooth finish, and speed, but multimaterial VP is challenging due to the inherent homogeneity of the resin vat. Current methods often involve switching entire resin vats or using fluidic devices, resulting in longer print times, increased complexity, resin contamination, and limitations in material diversity and placement (often restricted to z-axis variation). A more efficient approach involves manipulating the chemistry of a single homogeneous resin vat during printing by altering light properties (wavelength and intensity), controlling the degree of monomer conversion. However, these approaches often require post-processing or result in minimal property disparity. Dual wavelength VP offers better control using photochemical orthogonality, allowing distinct chemistries to be controlled by different wavelengths. Previous work using dual wavelength VP focused primarily on stiffness control, motivating this research to explore color control. Multi-color objects are valuable for applications like data storage, camouflage, and education. While previous approaches coupled the printing process and color modulation or involved a two-step process, this research aimed to decouple color modulation from object formation using multiple wavelengths simultaneously, with one wavelength for color modulation independent of the wavelength dictating object structure.

The literature review extensively covers existing multi-material 3D printing techniques, emphasizing the limitations of current vat photopolymerization approaches. The authors highlight the challenges associated with resin switching and fluidic systems, emphasizing the need for a more efficient, single-vat method. Previous work using dual-wavelength VP to control stiffness is discussed, establishing the foundation for this research's focus on color control as a novel application of this technique. Existing methods for multi-color 3D printing, such as those using gradient light intensity or post-processing color modulation, are critically analyzed to showcase the innovation of the proposed approach, which aims to achieve independent control of object formation and color using simultaneous dual-wavelength projection.

The researchers developed multi-color resins incorporating photoacid generators (PAGs), specifically onium salt PAGs (TAS), and pH-responsive dyes (bromocresol green (BG) and methyl red (MR)). Upon UV exposure, the PAG generates acid, changing the pH and thus the color of the dye. The color change was initially demonstrated in solution and then in cured resin samples, confirming the efficacy of the acid-mediated color modulation. The researchers then explored 3D printing of these resins using an Elegoo Mars 3 LCD printer with a 405-nm light source, achieving good resolution and complex object formation. Post-printing UV exposure was used to modulate the color of the printed objects, with the color change controllable by UV light dosage and spatial patterning using masks or projections. The light penetration depth was investigated, revealing limitations in color modulation for thicker objects due to the Beer-Lambert law. Wavelength effects were also studied, showing that 365-nm light produced the most drastic color change, while 405-nm light showed a slower response, and 455-nm light had no effect. Environmental stability was assessed, noting color changes due to ambient light and mitigating this with the addition of a UV absorber (avobenzone). Color leaching into water was minimal due to the crosslinked nature of the printed parts. To expand the color palette, methyl red (MR) was introduced, and mixtures of BG and MR produced further color variations. A grayscale projection platform was developed to create customized patterns by controlling local light dosage. This involved creating a color palette calibration, converting a target color image into a grayscale projection via minimum variance quantization, and then using this projection to pattern the resin during printing. Finally, a custom dual-wavelength printer was constructed with a UV projector (365 nm) and visible light projector, allowing for simultaneous curing and color modulation during 3D printing. A type II photoinitiation system (camphorquinone (CQ) and ethyl 4-(dimethylamino)benzoate (EDMAB)) was incorporated to improve print quality and reduce outgrowth with higher concentrations of TAS and the addition of hydroquinone. The final properties of the printed objects (tensile strength, modulus, and elongation) were characterized, showing no significant difference between different colors printed from the same resin. Various multi-color structures were printed to demonstrate the versatility of the approach.

The study successfully demonstrated multi-color 3D printing from a single resin vat using a dual-wavelength approach. The key findings include: (1) successful color modulation through spatially controlled acidity using a photoacid generator (PAG) and pH-responsive dyes; (2) development of two distinct approaches for achieving multi-color prints: a two-step process (3D printing followed by UV exposure) and a single-process dual-wavelength approach; (3) successful printing of complex, high-resolution objects with various colors and patterns; (4) exploration of multiple dyes (BG and MR) and their mixtures to achieve a wider range of colors; (5) creation of a grayscale projection platform for precise color patterning; (6) optimization of the dual-wavelength printing process using a type II photoinitiation system to mitigate outgrowth and improve color change kinetics; (7) demonstration of multi-color structures that are difficult or impossible to fabricate using subtractive manufacturing; and (8) characterization of the mechanical properties of the printed parts showing no significant difference in tensile strength, modulus, or elongation across different colors printed from the same vat.

The findings address the research question by demonstrating a novel, efficient method for multi-color 3D printing using dual-wavelength photopolymerization and spatially controlled acidity. The significance lies in the decoupling of object formation and color modulation, enabling greater design freedom and flexibility. The results are highly relevant to the field of additive manufacturing, offering a significant advancement in multimaterial 3D printing capabilities. The approach avoids the limitations of existing methods and provides a pathway for creating complex, multi-colored objects with improved efficiency and control.

This research successfully demonstrated a versatile, two-step and single-process dual-wavelength 3D printing method for creating multi-color objects from a single resin vat. The approach leverages spatially controlled acidity using a photoacid generator and pH-responsive dyes. Future research could explore additional resin systems to control other properties, and further optimize the process through grayscale control of both visible and UV light to control multiple properties simultaneously.

The study notes limitations in color modulation for objects thicker than 1 mm due to light penetration limitations governed by the Beer-Lambert law. The color stability of printed parts in ambient light was addressed but requires further investigation or protective coatings. While the dual-wavelength printing method was demonstrated, optimization of layer times for faster printing could be explored through the use of more efficient PAGs or higher light intensities.