Volumetric Additive Manufacturing

The dual-wavelength photoinitiation and printing geometry of the light sheet-based process (UV is blue and visible is red). 5

The UV additive manufacturing techniques discussed in this column have focused on approaches in which the part is printed layer by layer in series. 1 The current column reviews technical approaches to volumetric additive manufacturing (VAM, also referred to as Deep Vat Printing, or DVP), a general class of additive techniques in which the part is freely suspended within the matrix of the liquid resin during the build process. The motivation for the development and application of VAM is to address shortcomings of conventional printing techniques in which the part generally is constructed one layer at a time while attached to a build plate that moves in a vertical direction. Parts printed conventionally can suffer from surface ridges, anisotropic mechanical properties and failures due to adhesion to a build plate or resin tray film, neither of which is a component of VAM processes. Additionally, VAM offers the potential for higher print speeds with a simplified mechanism.

What is Volumetric Additive Manufacturing?

General comparison of layer-based and continuous printing technologies. 2
Figure 1. General comparison of layer-based and continuous printing technologies. 2

There are different technical approaches to VAM, but they all share some common features, as shown in Figure 1. 2 Because the part is not constructed on a build platform, there are no support structures. Because there are no layers, there is no directional asymmetry resulting from interlayer adhesion.

Tomographic Printing

Tomographic printing (Computed Axial Lithography, or CAL), shown in Figure 1, utilizes the same mathematical model as the Computed Axial Tomography (CAT) scan technique used for medical imaging. Rather than using the detected signal variation from a moving source to calculate the underlying tissue structure of the patient, tomographic printing spatially varies the UV energy using a DLP projector and rotates the sample to create the structure being printed, as shown in Figure 2. The projector’s spatial control of the curing process results from the balance between absorption by the photoinitiator and resulting radical formation and the quenching of radicals by oxygen. Because there is no build platform, the viscosity of the formulation must be high enough to minimize part motion during the print process. According to Kelly and coauthors, their tomographic approach can print formulations with viscosities up to 90,000 cP, beyond the range of conventional DLP printers. 3

Computed axial lithography used to generate a model of The Thinker by Auguste Rodin. 3
Figure 2. Computed axial lithography used to generate a model of The Thinker by Auguste Rodin. 3

Kelly et al. also performed a detailed comparison between the print times of CAL, conventional DLP printing and CLIP. 3 (Continuous Liquid Interface Production, or CLIP, is a DLP technique in which the printed part and resin tray film are separated by a zone of oxygen inhibition. 4) Because the CAL technique does not rely on the motion of a build plate through a fluid, it does not have the same viscosity limitations as the other two methods. Without the material flow requirements of CLIP and conventional DLP, the print speeds of the CAL approach are correspondingly faster. 3 Since all three methods involve the generation of a part immersed in uncured UV resin, postprocess removal of excess resin in combination with potential thermal and UV postcuring may affect the relative production speeds, particularly for dual-cure resins that require thermal post-processing. All three techniques also encounter limitations in print cross-sectional area due to available pixel resolution.

Light Sheet-Based Photochemistry

The dual-wavelength photoinitiation and printing geometry of the light sheet-based process (UV is blue and visible is red). 5
Figure 3. The dual-wavelength photoinitiation and printing geometry of the light sheet-based process (UV is blue and visible is red). 5

Regehly and coauthors have reported a dual wavelength light sheet approach to volumetric printing, which involves the use of a specialized photoinitiator that first is converted under UV irradiation to a UV-transparent Type II photoinitiator and then irradiated in the visible to generate radicals in the presence of a coinitiator. The UV light is focused into a thin sheet and projected perpendicular to the visible image slice projection. This thin sheet of UV light is swept through the reaction volume as the image slices successively are projected to enable the part to be generated volumetrically as contiguous slices, as shown in Figure 3 and Figure 4. 5 As reported by Regehly et al., the light sheet design requires a very specific photoinitiator but is capable of printing at speeds 10 times faster than conventional DLP printers. The print resolution in the z-direction is determined by the width of the UV beam, which is 34 microns, 59 microns and 108 microns for a cuvette depth of 1, 3 and 10 cm, respectively, limiting this technique to smaller parts when higher resolution is required.

Figure 4. Calibration data for the light sheet scanning process. UV curing indicates the undesired region in which the sample is solidified by UV alone, and no solidification indicate the undesired region in which no parts are formed. 5
Figure 4. Calibration data for the light sheet scanning process. UV curing indicates the undesired region in which the sample is solidified by UV alone, and no solidification indicate the undesired region in which no parts are formed. 5

In addition to VAM applications in which resins are polymerized, light sheet technology also has been utilized recently in a depolymerization process for biological imaging. 6 Rather than using a microtome to slice tissue sections that can result in sample damage, Wang and coauthors used a UV light sheet to irradiate a tissue sample embedded in a hydrogel containing a photodegradable crosslinker. The degraded material is cleared, exposing a fresh layer of tissue that undergoes nanoscale fluorescence imaging. Automation of this process permits the high-resolution analysis of biological samples of virtually unlimited size, as shown in Figure 5.

Left: Configuration of the optical system (Photochemical Sectioning, or PS) Right: UV photochemical sectioning of the fluorescent hydrogel. 6
Figure 5. Left: Configuration of the optical system (Photochemical Sectioning, or PS) Right: UV photochemical sectioning of the fluorescent hydrogel. 6

The examples discussed are only an indication of the potential design and applications of volumetric additive manufacturing processes. Their commercial potential has yet to be fully realized, even though medical research already is benefiting from advances in the technology. Reference 2 is recommended for a more thorough summary of the field.

For feedback, or if there are specific topics readers would like to see discussed, contact me at pshare@admatdesign.com.

References

  1. This column is focused on photopolymer applications in additive manufacturing, in which a reservoir or vat of liquid resin is photopolymerized 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. This column is focused on techniques in which the resin is cured volumetrically.
  2. Chansoria, P.; Rizzo, R.; Ruetsche, D.; Liu, H.; Delrot, P.; Zenobi-Wong, M. Light from Afield: Fast, High-Resolution, and Layer-Free Deep Vat 3D Printing, Chemical Reviews 2024, 124, 8787–8822
  3. Kelly, B.; Bhattacharya, I.; Heidari, H.; Shusteff, M.; Spadaccini, C.; Taylor, H. Volumetric Additive Manufacturing via Tomographic Reconstruction, Science 2019, 363, 1075-1079
  4. Tumbleston, J.; Shirvanyants, D.; Ermoshkin, N.; Janusziewicz, R.; Johnson, A.; Kelly, D.; Chen, K.; Pinschmidt, R.; Rolland, J.; Ermoshkin, E,; DeSimone, J. Continuous Liquid Interface Production of 3D Objects, Science 2015, 347, 1349-1351
  5. Regehly, M.; Garmshausen, Y.; Reuter, M.; Koenig, N.; Israel, E.; Kelly, D.; Chou, C.; Koch, K.; Asfari, B.; Hecht, S. Xolography for Linear Volumetric 3D Printing, Nature 2020, 588, 620-627
  6. Wang, W.; Ruan, R.; Liu, G.; Milkie, D.; Li, W.; Betzig, E.; Upadhyayula, S.; Gao, R. Mesoscale Volumetric Fluorescence Imaging at Nanoscale Resolution by Photochemical Sectioning, Science 2025, 390, 6770

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

RELATED ARTICLESMORE FROM AUTHOR