Multi-Material Additive Manufacturing

In the preceding column, I reviewed recent applications of UV vat printing technologies in manufacturing for electronics applications. 1 The current column reviews technical approaches and end-use applications of vat-based additively manufactured multi-material parts.

Why Multi-material Additive Manufacturing is Important

A wide range of applications exist in which it is desirable to have multiple properties within a single object. In graphic arts applications, techniques such as inkjet, lithography or flexography are used to differentiate spatial regions in two dimensions according to properties such as color, reflectance or tactility. In additive manufacturing, it is of interest to generate parts in three dimensions in which special regions are differentiated not only by color but also by mechanical or electrical properties. For example, the contact area of a robotic gripper may be more elastic than a stiffer substructure so that the gripper can survive many duty cycles without damaging the surface of the object that is gripped. In conventional manufacturing, this might involve a metal gripper with a rubber insert requiring two-part suppliers and their associated inventories. As described in the previous column on electronics applications, conductive metals often need to be combined with ceramic insulators. Additive manufacturing has the potential to reduce inventory to a single part while increasing design flexibility.

Multi-Vat Approaches

Figure 1. Multi-material printing using vats containing different UV formulations. 2

Conventional vat-based printing consists of a single formulated resin with a single set of properties cured and a printer operating at a single wavelength. Spatial differences in mechanical properties can be achieved through designs such as lattices or other structures, but the resulting part is produced from a single formulation. Differences in properties are obtained using multiple vats containing multiple resin formulations, as shown in Figure 1. 2

There are several challenges to this approach to multimaterial printing. The region of the part on which the next layer is cured is immersed in the formulation in the vat during the printing step, as described by the Ware group and illustrated in Figure 1. 2 Immersion of the part in different vats containing different formulations is likely to lead to cross contamination. Various cleaning methods, including rinsing, ultrasound and wiping, 3 as well as combinations of these methods, have been demonstrated but may provide challenges in a production environment. As outlined by Ware, the multi-vat approach also has the adverse effect of slowing the printing process in comparison to a single vat and potentially decreasing interlayer adhesion due to factors such as oxygen inhibition. Using a selective materials approach, Lithoz has commercialized a dual-vat LED printer for hybrid ceramics in which a ceramic can be combined with another ceramic, a metal or a polymer, in either a single layer or a multilayer or gradient at a maximum build speed of 100 layers per hour and a build volume of 76 x 43 x 170 mm. 4 Applications of this technology include the electronics industry as discussed in the previous column.

Multi-Wavelength Approaches

Figure 2. Mechanical properties of the dual wavelength printing samples following exposure at 405 nm for 1 second and 365 nm for 4 seconds, followed by postcuring at 120° C. The blue and purple reference samples were printed at the single wavelength indicated, and each half of the two-color sample was printed at 405 nm and 365 nm, respectively. 5

Another approach to the localized control of material properties within a single part utilizes a single vat containing a single formulation but multiple wavelengths of light to control the photoinitiation process. The Schloegl group used a dual projector system at 405 nm to cure an acrylate modified polyester with a radical initiator and at 365 nm to cure an epoxy resin with a photoacid generator, followed by thermal postcuring, as shown in Figure 2. 5 Schloegl, et al. reported that the cohesive failure observed during the elongation testing of the heterogeneous samples exclusively occurred in the soft radical cured domain, suggesting that diffusion of the acid photocatalyst species leads to a gradient across the interface.

Figure 3. Selective solubility vat polymerization, where the soluble portion of the print functions as a recyclable support structure. 6

Properties other than mechanical also have been explored using the single-vat, dual-wavelength approach. The Hart group recently has reported results using the same wavelengths as the Schloegl group and a combination of acrylate and cationic chemistries in combination with a controlled thermal postprocess to obtain a high level of differentiation in solubility properties, as shown in Figure 3. 6 An advantage of this approach is a reduction in material waste, as the dissolved support material, which typically constitutes a significant portion of total resin usage, can be reclaimed and reused rather than discarded. Hart, et al. described challenges that remain for this approach, which include dimensional accuracy during the print process and material shrinkage and distortion during the post-process heating cycle.

Figure 4. Dual-wavelength vat polymerized multicolor parts. The image of each part is accompanied by its respective CAD model .7

The Boydston group has reported the use of photoacid catalysts in combination with pH-responsive dyes to spatially control color in a single-vat, dual-wavelength process, utilizing the same wavelengths as the Schloegl and Hart groups. 7 Careful adjustment of the concentrations of photoacid and dye, as well as an inhibitor and amine synergist, were necessary to balance absorption profiles and radical propagation. The results are shown in Figure 4. Additional work was reported using a single wavelength during the printing process and a second wavelength during post-process, varying the post-process UV dose to adjust the shade of the part.

The range of applications of multi-wavelength single-vat photopolymerization continues to expand, and the examples described in this column are only a portion of the ongoing development work in this area. The forthcoming column will address UV additive processes that are not strictly vat-based. For feedback, or if there are specific topics readers would like to see discussed, contact me at pshare@admatdesign.com.

Paul Share, Ph.D.
Principal Consultant
Advanced Materials Design LLC

 

References:

  1. This column is focused on photopolymer applications in additive manufacturing, in which a reservoir or vat of liquid resin is photopolymerized layer by layer to generate a part. When a layer is cured using a narrow laser beam in a series manner, one point at a time, it is referred to as stereolithography or SLA. Simultaneous illumination of a large area to cure one full layer at a time can be accomplished with a digital light projector (DLP) or a liquid crystal display (LCD) screen. There also are many permutations of these approaches that may utilize multiple light sources and optics to achieve specific performance advantages.
  2. Subedi, S.; Liu, S.; Wang, W.; Shovon, S.; Chen, X.; Ware, H. Multi-material Vat Photopolymerization 3D Printing: A Review of Mechanisms and Applications Advanced Manufacturing 2024, 1:9, 1-17
  3. Shaukat, U.; Thalhamer, A.; Rossegger, E.; Schloegl, S.; Dual-vat Photopolymerization 3D Printing of Vitrimers Additive Manufacturing 2024, 79, 103930-103938
  4. https://www.lithoz.com/en/3d-printer/cerafab-multi/ (accessed June 23, 2025)
  5. Cazin, I.; Gleirscher, M.; Fleisch, M.; Berer, M.; Sangermano, C.; Schloegl, S. Spatially Controlling the Mechanical Properties of 3D Printed Objects by Dual-Wavelength Vat Photopolymerization Additive Manufacturing 2022, 57, 102977-10298
  6. Diaco, N.; Thrasher, C.; Hughes, M.; Zhou, K.; Durso, M.;Yap, S.; Macfarlane, R.; Hart, J. Dual-Wavelength Vat Photopolymerization with Dissolvable, Recyclable, Support Structures Advanced Materials Technologies, 2025, e00650
  7. Chin, K.; Ovsepyan, G.; Boydston, A. Multi-Color Dual Wavelength Vat Photopolymerization 3D Prionting via Spatially Controlled Acidity Nature Communications, 2024, 15, 3867-3874

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