1 Volumetric Helical Additive Manufacturing Antoine Boniface 1 Florian Maître 1 Jorge Madrid -Wolff 1 Chris tophe Moser 1

2025-04-30 1 0 1.68MB 18 页 10玖币

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Volumetric Helical Additive Manufacturing

Antoine Boniface 1, Florian Maître 1, Jorge Madrid-Wolff 1, Christophe Moser 1, *

1 Laboratory of Applied Photonics Devices, School of Engineering, Ecole Polytechnique Fédérale de Lausanne, CH-

1015 Lausanne, Switzerland

Corresponding author: christophe.moser@epfl.ch

Abstract

3D printing has revolutionized the manufacturing of volumetric components and structures for various

fields. Thanks to the advent of photocurable resins, several fully volumetric light-based techniques have

been recently developed to overcome the current limitations of 3D printing. Although fast, this new

generation of printers cannot fabricate objects whose size typically exceeds the centimeter without

severely affecting the final resolution. Based on tomographic volumetric additive manufacturing, we

propose a method for volumetric helical additive manufacturing (VHAM) multi-cm scale structures

without magnifying the projected patterns. It consists of illuminating the photoresist while the latter

follows a helical motion. This movement allows for increasing the height of the printable object.

Additionally, we off-center the modulator used for projecting the light patterns to double the object's

lateral size. We demonstrate experimentally the interest in using these two tricks for printing larger

objects (up to 3 cm × 3 cm × 5 cm) while maintaining high resolution (< 200 μm) and short print time (<

10 min).

1. Introduction

Over the last decade, 3D printing technologies have experienced unprecedented developments and changes.

They now enable fabricating complex volumetric objects rapidly and inexpensively. This makes 3D printers

especially attractive and pertinent for various fields including the aerospace industry or medical applications 1,2.

Until recently, the paradigm in light-based 3D printing or additive manufacturing (AM) mainly relied on using

a vat of liquid photopolymer resin, out of which the object is constructed sequentially, layer by layer or voxel

by voxel3. An ultraviolet (UV) light beam cures the resin one layer at a time whilst a platform moves the object

being made downwards after each new layer is hardened. The UV light is either raster scanned, which solidifies

the resin point by point as in stereolithography (SLA)4, or flashed onto the resin curing the whole layer at once

as in digital light processing (DLP) technologies5,6. Due to the layer-by-layer nature of the printing process, these

light-based additive manufacturing techniques are subject to major geometric constraints and throughput

limitations.

The past few years have seen the emergence of several fully volumetric additive manufacturing (VAM)

technologies that move away from the layer-by-layer approach. Two-photon photopolymerization represents the

state-of-the-art of volumetric printing with light7. It enables the fabrication of microscale objects with a lateral

resolution of 100 nm and axial resolution of 300 nm but is slow, with a printing speed of just 1–20 mm3/h, and

requires expensive femtosecond laser sources. Recently, teams have demonstrated that two-photon lithography

can be sped up through spatiotemporally focusing of ultrafast lasers8,9; while two-step absorption, instead of

two-photon absorption, has enabled microfabrication without the need for pulsed lasers10,11. For high-speed

solidification of centimeter-scale objects, two volumetric approaches have been recently developed. In one,

called xolography, polymerization is locally induced at the intersection of two light beams of different

wavelengths using photoswitchable photoinitiators12. It is a fast technique that can print building blocks of only

tens of microns but which requires special photoinitiators and is strictly limited to optically clear and low-

absorptive resins. In the second approach, coined computed axial lithography (CAL) or tomographic volumetric

additive manufacturing, an entire three-dimensional object is simultaneously solidified by irradiating a liquid

photo-sensitive resin volume from multiple angles with dynamic 2D light patterns13–15. These 2D light patterns

are calculated from the target 3D object using 3D-to-2D transforms, like the Radon transform, similarly to X-

ray Computer Tomography Scans16. In a tomographic printer, a Digital Micromirror Device (DMD), which

offers millions of degrees of freedom to spatially modulate the light intensity of the input beam, is used to

produce the 2D patterns. By projecting patterned light into the liquid resin from multiple angles, a 3D energy

dose is accumulated. In the regions where such energy dose exceeds a polymerization threshold, solidification

occurs. The object is finally printed when all target voxels inside the liquid precursor receive an irradiation dose

above this said threshold17. Thanks to high-power laser diodes, the tomographic photopolymerization of cm-

scale objects can be performed as fast as within 30 to 120 seconds with resolutions of around 100 μm14. After

printing, the surrounding liquid unpolymerized resin can be washed away to reveal the desired solid printed part.

Using this process, it was first possible to obtain cm-scale objects with a resolution of 300 μm (ref. 13). When

the polymerization starts, the refractive index of the resin changes, locally perturbing the propagation of light.

This intricate phenomenon was not taken into account initially for computing the 2D light patterns, limiting print

fidelity. It has been shown that re-calculation of the patterns based on measured feedback from a first sacrificial

print combined with an optimized low-étendue illumination can bring the final print resolution to 80 μm (for

positive features) and 500 μm (for negative features)15. However, print resolution and fidelity are still limited,

mainly by three primary sources of error: (1) the projected light patterns; (2) the chemical and optical properties

of the resin (e.g., polymerization diffusion, absorbance or scattering, and change in refractive index upon

polymerization), and (3) the optical projection setup. The goodness of the algorithm is essential to produce

patterns whose back projections form a volumetric dose of light that best represents the target object inside the

resin. Several recent works have been proposed to optimize the set of patterns with respect to (i) the target dose

by including physics priors18,19 or (ii) more appropriate loss functions that enhance contrast20. Regarding the

resin's opacity, it is clear that optically transparent materials allow for propagating sharp patterns of high fidelity,

but the same patterns would be inevitably blurred and thus lose their finest features in scattering media. The

detrimental effect of scattering can be reduced through refinements to the calculated tomographic patterns18 or

through refractive-index matching of the resin21. The chemical diffusion of free radicals may also cause

unwanted polymerization that enlarges the print resolution by a few microns22,23, but this effect can be mitigated

by doping the resin with free-radical quenchers24,25. Ultimately, the optical resolution of the printer dictates the

achievable printed voxel size. In DLP and tomographic VAM, optical resolution is determined at best by the

features of the modulator used to pattern light, namely the DMD. Here we use a DLP7000 chip from Texas

Instrument that has on its surface Nx × Ny = 768 × 1024 micro-mirrors (pitch = 13.6 μm) arranged in a rectangular

array capable of displaying 8-bit images.

In our optical system, the DMD image is magnified by a factor of 1.66; the resulting pattern onto the vial is 1.74

cm × 2.33 cm with a resolution of 23 μm (see a detailed sketch of the experimental setup in Supplementary S.1).

The only way to increase the size of the printed objects without compromising the resolution is to move the

DMD with respect to the vial or vice versa. Inspired by spiral CT (ref. 26), we propose to move the sample around

the light beam with a helical trajectory. Additionally, we show that lateral printable size can be doubled without

compromising resolution by off-centering the optical axis with respect to the rotation axis of the photoresist vat

(Fig. 1.a). Together, these two tricks increase the number of building blocks inside the vial by a factor of up to

12. Here these available printed voxels are used to print larger objects up to 3 cm × 3 cm × 5 cm in a few minutes

without compromising the printing resolution (< 200 μm).

Figure 1: Principle of tomographic volumetric helical additive manufacturing (VHAM). a. Simplified schematic of the helical

printer. A laser beam (λ = 405 nm) is modulated in intensity with a DMD before propagating through the photoresist. The vial

containing the latter is off-centered, such that one lateral edge of the rectangular beam intersects the center of the cylindrical

container. The vial is placed in rotation and move continuously up and down defining a helical trajectory. b. Schematic

representation of the helical printing procedure. A series of patterns of light projected over multiple angles trigger at different time

the polymerization. At a given time only a subpart of the resin is exposed to light. In fact, it is the helical movement that ensures

the solidification of the whole object. c. Time lapse. Two cycles up/down and 12 turns were necessary to solidify the resin in the

desired geometry (see 3D model in b.). This is achieved in less than 10 min. The rotation stage holds the vial by the bottom. A white

cap prevents resin's leakage to the outside. d. Final part obtained after washing out the unpolymerized resin. Scalebar: 10 mm.

The principle of tomographic volumetric helical additive manufacturing (VHAM) is given in figure 1.a. We

combine a rotation and a translation stage to set the glass vial (diameter = 32 mm) containing the photoresist in

a helical motion. We must emphasize here that all the resin is not illuminated at once as in conventional

tomographic VAM. Here, in VHAM, the whole resin is entirely excited only after one complete cycle comprising

a bottom-up and a top-down pass. Half a cycle (only up or down) includes α rotations of the vial. As the vial

follows a helical trajectory, α also represents the number of patterns stacked along the vertical axis. There are

some overlapping regions between the patterns so that after a turn its lower and upper parts coincide. The size

of the overlap is fine-tuned by adjusting the vial's rotation speed to the vertical movement of the translation

stage; which is essential to ensure continuity of the printed objects. A detailed workflow regarding the

computation of the VHAM patterns is provided in Methods and fig. S.2. In this work, the rotation speed is

between 8 and 10 °.s-1 which respectively gives a vertical linear speed of 458 μm.s-1 and 366 μm.s-1. After a few

up and down cycles, the light dose accumulated inside the resin at different heights and over multiple angles is

sufficient to solidify it as shown on the schematic figure 1.b. This usually happens after 2 or 3 vertical cycles

and is in general completed in less than ten minutes (fig. 1.c). Note that because of light absorption, patterns

projected at θ and θ+180° do not irradiate the volume of resin in the same way. We take this into account by

performing a blank half turn (simply put, no projection and no vertical translation over 180°) between two

vertical cycles. It results that for 3D structures with no central symmetry, the number of projected patterns

doubles. To give an idea, one may need to send around 10,000 patterns for printing with α = 3 and an angular

resolution of 0.18°.

The final 3D printed structure is obtained after some post-processing including a washing and post-curing steps

as described in Methods. For this particular helical tower structure α = 3, which means 4α = 12 times more

printable voxels inside the resin compared to conventional tomographic VAM.

2. Results

We report on the capabilities of helical tomographic VAM through a series of different printed structures in

transparent acrylics. The photo-curable resin used in this work is prepared by combining a commercial

polyacrylate (PRO21905 from Sartomer), with 0.6 mM phenylbis (2,4,6-trimethylbenzoyl) phosphine oxide as

a photoinitiator (TPO from Sigma Aldrich) in a planetary mixer. The optical transparency together with its

stiffness and ease of use makes it a candidate of choice to present our printer's competences in terms of object

size and resolution. In figure 3, we show the prints of five different 3D models, relatively large (2 cm wide at

least), with different heights. For all of them, the DMD is off-centered with respect to the vial's rotation axis, but

the parameter α is adjusted to fit the height of each object. As in conventional tomographic VAM, these complex

and hollow geometries are printed in a short time (3-10 min) without the need for support structures.

Figure 2: Examples of 3D printed objects using tomographic VHAM. a. 3D model. b. Photographs of the obtained prints.

c. Microscopic images to better appreciate the printed fine details. Scalebars:b. = 10 mm, c.= 1 mm.

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1VolumetricHelicalAdditiveManufacturingAntoineBoniface1,FlorianMaître1,JorgeMadrid-Wolff1,ChristopheMoser1,*1LaboratoryofAppliedPhotonicsDevices,SchoolofEngineering,EcolePolytechniqueFédéraledeLausanne,CH-1015Lausanne,SwitzerlandCorrespondingauthor:christophe.moser@epfl.chAbstract3Dprintinghasrevolu...

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