Direct Ink Writing in Multi-Material AM

Combining DIW and vat printing to create a hybrid structure. 5

Although capable of generating parts from photopolymers in high resolution, a challenge of vat-based approaches is their limitations in multi-material applications. 1 In the preceding column, I reviewed multi-vat and multi-wavelength approaches to this challenge. The current column reviews technical approaches and applications of additively manufactured multi-material parts using direct ink writing in combination with vat printing as an additional method to address these limitations.

What is Direct Ink Writing?

General comparison of droplet and continuous printing technologies. 2
Figure 1. General comparison of droplet and continuous printing technologies. 2

Direct ink writing (DIW) involves a droplet or continuous deposition of ink to generate three-dimensional structures, as shown in Figure 1. 2 A comparison of typical droplet and continuous formulations is shown in Table 1 (a). Some material flow occurs for both cases upon deposition, but for the case of the continuous deposition process, shear thinning is necessary so that the extruded material retains its shape long enough for UV curing to occur. Inkjet printheads typically are operated significantly above ambient to reduce viscosity and facilitate jetting. For this column, DIW will refer only to the continuous deposition shown in (b).

General comparison of droplet and continuous printing material requirements.
Table 1. General comparison of droplet and continuous printing material requirements.

Applications of Vat Photopolymerization Combined with DIW

The viscosity and particle sizes of inkjet printheads are limited by the nozzle size, which delivers tens of picoliters per drop. Continuous printing technology utilizes pressure to force material through a dispensing needle, and so the resolution, layer thickness and print speed depend on the viscosity of the formulation at temperature, including particle size and loading.

The additive manufacturing of ceramics incorporates a two-step procedure in which the first step utilizes a printing process to generate the desired geometry. The subsequent step exposes the part to elevated temperatures in which residual organics are volatilized and the ceramic particles fuse into a cohesive object through a process known as sintering. Increasing the weight percentage of ceramic in the UV formulation increases the viscosity, but decreases shrinkage and deformation during the sintering process.

Combining DIW and vat printing to create a hybrid structure. 5
Figure 2. Combining DIW and vat printing to create a hybrid structure. 5

As discussed by Williams and coauthors, the layer height typically is limited to 50%-100% of the nozzle diameter. 5 A feature size requiring a nozzle diameter of 0.5-5.0 microns also will require limits in particle size and formulation viscosity to avoid needle clogging or pressures in excess of 400 psi. The Williams group was able to load Alumina with a 10 micron particle size into a UV ceramic formulation at 85 weight percent (61 volume percent) and sinter it at 1650° C for 2,000 minutes. This was combined with vat printing in the process shown in Figure 2.

(a) Structure before (left) and after (right) sintering (b) SEM prior to sintering shows no evidence of layers or extrusion beads (c) SEM of sintered part shows porous structure (d) Three-point bend test shows consistent performance at 25° C and 400° C.
Figure 3. (a) Structure before (left) and after (right) sintering (b) SEM prior to sintering shows no evidence of layers or extrusion beads (c) SEM of sintered part shows porous structure (d) Three-point bend test shows consistent performance at 25° C and 400° C. 5

As implemented by Williams and coauthors, the DIW print cycle does not introduce stresses into the previously printed layers, and minimal flow occurs upon deposit due to the shear-thinning rheological properties of the resin. By using the resolution of the vat printing process to create cavities into which the DIW ink is extruded, the mechanical properties are determined by the DIW ink; however, the dimensional resolution is defined by the vat print, as shown in Figure 3.

Martinez and coauthors recently have used vat photopolymerization to generate gel polymer electrolytes for sodium ion batteries in combination with positive electrodes produced with DIW. Sodium ion batteries are of interest as a lithium ion alternative for space programs, such as Artemis, due to the abundance of the critical raw materials contained in the regolith present on the surface of the Moon and Mars. They used a combination of tape casting (film coating using a doctor blade), vat photopolymerization and DIW to generate coin cells and carry out electrochemical testing. The PEG diacrylate-based Gel Polymer Electrolyte (GPE) was formulated with a range of ratios of resin to electrolyte and both tape casted and 3D printed. A terpineol solvent-based formulation was used for both tape casting and DIW of the working electrode (WE). The combination of techniques facilitated a range of designs and compositions. The approach is illustrated in Figure 4.

Coin cell prototype structures generated using a combination of the techniques discussed.
Figure 4. Coin cell prototype structures generated using a combination of the techniques discussed. 6

For the GPE optimization, Martinez and coauthors varied the ratio of resin to NaClO4 electrolyte over a range of 1:0 to 1:5, and selected 1:4 as the highest level of electrolyte with dimensional accuracy but without undesirable thermal effects. Direct ink writing was used to print the working electrode from 56.1 weight percent Na0.44MnO2, 6.9% carbon black, 3.7% cellulose binder and 33.3 weight percent terpineol. A range of both working electrode and GPE geometries were evaluated. Discharge cycling of coin cells assembled from DIW working electrodes in four different configurations combined with a vat printed GPE is shown in Figure 5, with an evident decrease in capacity with increasing discharge rate.

Half-cell battery performance of DIW electrodes combined with vat polymerized GPE. Designs 1, 2 and 3 correspond to a DIW path spacing of 0.6 mm, 0.9 mm and 1.2 mm, respectively, with a full electrode as reference. C/20 refers to a 20-hour discharge of a 1 Ah battery and 1C refers to a corresponding 1-hour discharge.
Figure 5. Half-cell battery performance of DIW electrodes combined with vat polymerized GPE. Designs 1, 2 and 3 correspond to a DIW path spacing of 0.6 mm, 0.9 mm and 1.2 mm, respectively, with a full electrode as reference. C/20 refers to a 20-hour discharge of a 1 Ah battery and 1C refers to a corresponding 1-hour discharge. 6

In future work, Martinez and coauthors plan to implement two-photon polymerization to enhance the spatial resolution of the GPE geometry and further enhance cell performance. 6

The examples discussed are only an indication of the potential for the uses of DIW to enhance the potential applications of vat-based 3D printing in applications as disparate as ceramics and batteries. Future columns will include a focus on medical applications. 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 or DLP or a liquid crystal display (LCD) screen. There are also many permutations of these approaches which may utilize multiple light sources and optics to achieve specific performance advantages.
  2. Lewis, J. Direct-write Assembly of Ceramics from Colloidal Inks Current Opinion in Solid State and Materials Science 2002, 6, 245–250
  3. https://www.quantica.io/jetpack (accessed Sept. 21, 2025)
  4. https://industry.ricoh.com/en/industrialinkjet/mh/5421f_5421mf (accessed Sept. 21, 2025)
  5. Rau, D.; Forgiarini, F.; Williams, C. Hybridizing Direct Ink Write and Mask-Projection Vat Photopolymerization to Enable Additive Manufacturing of High Viscosity Photopolymer Resins Additive Manufacturing, 2021, 42, 101966-101980
  6. Martinez, A.; Schiaffino, A.; Aranzola, A.; Fernandez, C. Spatially Controlling the Mechanical Properties of 3D Printed Objects by Dual-Wavelength Vat Photopolymerization Additive Manufacturing 2022, 57, 102977-102985

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