Preliminary study on volumetric 3D printing using visible light

The realm of 3D printing has been a valuable aspect of manufacturing and mechanical engineering to which complex geometries have been made that might be otherwise highly costly or not feasible by other manufacturing methods. This is where volumetric 3D printing has been advantageous by generating complex geometry with no defects on the surface. The problem with the research done so far is that the material used uses photoinitiators that photo-synthesize using ultraviolet (UV) light. The problem with this material is that it is attached to the destructive issues brought on by interacting with UV light, making some additives useless. To solve part of this problem, a solution to the material problem must be shown that a resin can be cured using visible light. This study has investigated the feasibility of a novel manufacturing process termed, the visible light–induced–volumetric 3D printing (VLI-V3DP) process that resulted in successfully finding resin that can be cured using visible light in the 470 and 530 nm range. By curing simple geometry using visible light, one can formulate a resin that can sustain just about any additives that can meet any goal, whether it is organic or not.


Introduction
A large portion of the machine capabilities that have dominated the manufacturing section has been subtractive manufacturing; this has been most notably in milling and turning parts to the desired dimensions from a larger block of material. An enormous appeal for subtractive manufacturing (SM) is the tolerance range achievable with modern machines maintaining a geometric error of 55-140 µm with a proper setup along with a wide range of material selection [1,2]. Additive manufacturing (AM) involves the process of increasing the material that is used to reach the final form of a part ultimately; currently, this has achieved the homes of many people by the use of thermoplastics in the form of filament used in such a way as to print a layer upon layer to finalize a part. In recent years AM has shown excellent accuracy with resin polymers as the media for generating components with good tolerance [3,4]. AM has grown to many different variations that are out in the market, including the 0D method (e.g., fused deposition modeling and stereolithography), 1D method (e.g., binder jetting), 2D (e.g., digital light processing) [5], and 3D (e.g., volumetric 3D printing) [6]. All these methods have unique characteristics that make the final part more usable in different situations based on additives and surface finish. A general process for AM is modeling a part using computer-aided design (CAD) software, which is imported into "sliced" sections with a uniform thickness stacked on a plate until the whole part is complete. A person needs no input by changing an operation sequence or indexing the origin; the only human information with the machine is preparing the surface at the start, then removing the part, and cleaning the cover for the next print. Geometry has little effect on getting the amount printed; the support material can be placed in any location to prop up overhangs. Still, this is negative because the surface finish/detail can be compromised [7]. More surface finish is susceptible to low quality from the effect of stacking the 2D layers on top of one another looking for a flight of stairs, and the processing time can be slow in the range of hours to complete apart.
Volumetric 3D printing (V3DP) has focused on the use of ultraviolet (UV) light that was near the 400 nm wavelength or less since it has a strong research background for free-radical polymerization with stereolithography-like platforms [6]. One issue of pressing concern is the findings of recent studies highlighting the potential toxicity of UVcurable photopolymers [8]. These resin materials contain potentially hazardous ingredients that have the relative ability to cause injury to biological tissues, affect biochemical functions, cause organ damage, and even lead to death [8,9]. Although the V3DP technology is touted as a potential candidate for 3D-bioprinting and tissue engineering applications [10], the overreliance on UV-light source and UVcurable resins has significantly hindered the progress in this direction [11]. Lastly, the UV radiation is harmful to the human tissues in the eyes and skin, leading to both acute and chronic effects, including eye inflammation, ocular cataracts, skin irritation, accelerated skin aging, and skin cancer [12].
Using wavelengths of light in the visible range would help understand better the opportunities of diverting from the UV spectrum of light Also, focusing on a single light source interaction will help to gain an understanding of the light-to-resin interaction [13]. Most common 3D printing technologies commercially available involve time-consuming completion times from the start of the printing process to the end of post-process cleaning and curing steps. This still leaves some visual imperfections from the layer-on-layer effect, most visible on contours and from the resolution of 100 nm in some processes [14]. To better suit, an optimal viscosity with parts suspended and final physical properties, other additives will need to be implemented by physical and hydrogel viscoelasticity [15].
This research aims to prove the possible nature of additively manufacturing a volumetric shape using only visible light. The process is termed the visible light-induced-volumetric 3D printing (VLI-V3DP) process. By isolating the light frequency to a specific wavelength, one can gauge the effect of time, intensity, and material selection. Light intensity has been shown to accelerate photopolymerization directly, achieving shorter exposure times by adjusting the polymerization rate. By modifying the power supplied to the various light sources, one can see the effect of the photopolymerization rate; this can help weed out any unfavorable parameters that hinder fast turnaround. It has been shown that a physical shape can be generated without the need for support material or a build plate. Keeping the light source in the visible light range (420 nm < λ < 600 nm) has significant benefits in promoting the appropriate longevity of the system by using natural light and expanding the manufacturing of a single object into the production of secondary materials. We currently dive into the effect of finding a sweet spot of reaction time and satisfactory material reaction; due to commercial use, these materials react with the help of UV light, but this paper reports that one can achieve a quick photopolymerization reaction to generate a solid structure. The forms of light delivered into the system will be coherent and in-coherent, allowing us to gauge the advantage of one versus the other with the reaction time required for a solid to form. This paper will be based on the ability to gather some components to properly cure 3D printing resin into a form that will retain its shape by employing visible light. By excluding UV light, one can extend the life of 3D printing devices by reducing the amount of high-level energy it experiences over their lifetime. The benefits of researching the possibilities of 3D printing without UV light bring the chances of producing a product that deteriorates or becomes unusable in UV light.

Materials and methods
The specific wavelength light source is essential to keep a consistent energy level of photons hitting the resin to control the cure rate over time. This was accomplished using various light sources and wavelengths to study its effect. The remainder of the setup was a rotating table, glass vials, a mirror, and a power supply, as shown in Fig. 1.
The light sources evaluated are from 2 different vendors; the first one was from Mightex, and it consisted of a Hipower LED collimated source with a wavelength of 470 nm. The second light source is Lights88 532 nm; more detail on each light source is given in Table 1. After the initial findings of the commercial photocentric resin, a new formulation had to be made. The custom resin was made using poly(ethylene glycol) diacrylate as the polymer, which is supplied in liquid form with a minimum purity of 92% (Manufacturer: Sigma-Aldrich, USA) and Bis(2,6-difluoro-3-(1-hydropyrrol-1-Yl)phenyl)titanocene functioning as the photoinitiator in solid form with a minimum purity of 95%, the molecular weight of 534.39 g/ mol (manufacturer: Gelest, a group company of Mitsubishi Chemical) shown in Fig. 2.
The polymer is mixed with the radicals that are produced from the photoinitiator by photo cleaving, which is induced by absorption when introducing the suitable wavelength. The radical will combine with the photopolymer and increase the length of the molecule until a radical-to-radical interaction drives the termination of the polymerization at the respective end. The mixture is comprised of 0.11% photoinitiator and 99.89% polymer.

Results
Based on the preliminary results, the VLI-V3DP process is feasible. The commercial resin was able to react to a generic LED projector. The curable resin undergoes photopolymerization with light, not in the UV realm, as shown in Fig. 3.
The challenge of curing a volume brought into account the size of the container and the difficulties of curing resin as the depth of material increases. To overcome some of the challenge's investment, a higher output light source was used instead of the self-LED projector-the introduction of the Mightex collimated light source. The setup shown in Fig. 4 was isolated from outside light to better gauge the viability of the higher energy; the distance we set at 25 inches and cured for 25 min. The distance and the curing time were optimized through multiple trials. The next step in the experiment was to cure resin selectively. At the same time, having a container with uncured resin will allow for future selective geometry. The same Mightex light source as above at 9.5″ was used on a vial of 0.87″ diameter. The power supply output was set at 15 V and 1 A. The results are shown in Fig. 5. It is seen that some of the material on the wall had been cured.
To gain a better understanding of selective curing, we introduced the coherent light source. With a 9-min exposure time, and a power setting of 2.38 V with 0.25 A, the resin was cured only at a specific section of the vial. Figure 6 shows more accurately a solid layer separating two liquid bodies, but also, the figure on the right can be seen as a small volume not attached to the wall that has been solidified.
To build on the findings so far, the coherent light source is used to cure a complete selective layer of photosensitive resin while evaluating the effect of distance and power. The results give a good indication of the value of power vs. the   In the next iteration, the power of the light source is increased to 0.595 W while keeping the same distance. It significantly improves the result by more than doubling the capacity by curing a resin layer in 14 min. Finally, in the third iteration, the power is kept at 0.595 W, and 4 in of separation of the vile from the light source is finally cured in 9 min. To compare the coherent light source results, a comparison of the in-coherent is made to gauge the importance of selecting the spread of the photons. In the first iteration, the power supply was set up with a power of 10 W, and the container was located 26 in away from the light source; this was finally cured in 60 min. The second iteration increased the power to a maximum of 15 W; this ultimately achieved a curing time of 25 min. Finally, on the third iteration, the Mightex collimated light source was brought closer to the container to achieve a shorter curing time of 6 min. The distance and curing time were adjusted to achieve the optimal results. Figure 7 shows the tensile strength vs. time graph of the resins cured during the initial tests. It is seen that increasing the curing time results in increased strength of the cured sample.
Testing the curing material with a change in power and time will be explored to gauge how well the end-of-cure properties compare. This information set will be used to understand the relation of time and power investment to know what parameter to change when results are not satisfactory. Allowing the turntable to rotate multiple times, full coverage of the container is reached, and the layer cure will attribute to the strength of the bonds created between the radicals produced and the polymer. One full rotation was completed in a time of 20 s, making 15 rotations. The Mightex results show a similar effect at 10 watts for 5 min and 15 W for 2 min; it would be more beneficial to keep the power high for a shorter amount of time than to keep power low for an increase of time. This helps in structuring the exemplary scenario to print a volumetric layer without having any support structure. To understand primary data on the feasibility of repetitive part creation, this will be at a given power and a fixed time to complete a set number of rotations: 2 min will produce six revolutions, 3 min with nine processes, and finally 5 min 15 revolutions.   Developing the findings further, a mixture of resin is mixed to produce a better and more quickly generate a solid before a minute is elapsed. A polymer and photoinitiator combination are mixed to create a custom mixture; the successful reaction will be resin to cure without being attached to the perimeter of the vile. The mixture is exposed to 10 s of 532 nm light produced from the Lights88 laser; the mixture has a higher polymer ratio than the photoinitiator. Light exposure will be emitted to 0.75 W for the duration of the rotation, and the power is supplied by the BK Precision power supply. Figure 9 shows the stress variation with respect to time for the newly created resin combination. From the figure, it is seen that the stress values are steadily increasing with the curing time. The increase in stress value can be attributed to the hardening of the material as a result of cross-linking when exposed to focused light. The significant increase of strength at 5 min of curing time would be due to the specific behavior of the resin during curing, which is further attributed to exothermic and thermally activated mechanisms [16]. The printing results are given in Fig. 10.
Curing material depends on controlling the reaction rate, allowing all the necessary information to be channeled into the resin, causing the correct solidification to occur, as shown in Fig. 11. By limiting the amount of photoinitiator to be mixed into the solution, one can maintain some control of the radicals being produced and cause a photopolymerization between the monomers and radicals.

Conclusion
This study reports the preliminary investigations on the VLI-V3DP process and found that the resin can be cured in the visible light region with a suitable photoinitiator. The range of curing beyond the green state will depend on the final application. Some applications will require a form  that can deform to some degree to move with its surroundings. On the other hand, some applications require a rigid body that will not bend easily. The preliminary study on the VLI-V3D process showed that it is feasible to generate a model without curing the container's walls. It produced smooth contours with no signs of a stair-casing effect. The model was developed in a short 10-s time frame at a power of 0.75 W. It was achieved with a ratio of 0.11% photoinitiator and 99.89% polymer. A beam was optimal for generating a cylinder model since the collimated light cannot disperse in unwanted areas.
Publisher's note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Springer Nature or its licensor (e.g. a society or other partner) holds exclusive rights to this article under a publishing agreement with the author(s) or other rightsholder(s); author self-archiving of the accepted manuscript version of this article is solely governed by the terms of such publishing agreement and applicable law.