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Abstract
In this article we demonstrate rapid manufacturing of an illumination quality double-sided centimeter-scale optics without any post-processing. Two methods are presented: additive manufacturing (AM) using an optically transparent material with a water-soluble support material (support method), and alternatively, using only the optically transparent material but flipping the optics mid-process (flipping method). The main advantage of the flipping method is that only one material is needed. However, the use of support material is more straightforward, allows better alignment between the two sides of the lens and enables more complex lenses manufactured in the future. Both methods result in sufficient surface quality, i.e. surface roughness Rq = 11.48 ±3.32 nm and form accuracy of ±10 μm, for the purposes of illumination optics.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Industries such as aerospace, medical/dental, automotive, and consumer products/electronics have been the front runners with the additive manufacturing (AM) technologies [1–4]. AM offers many benefits compared to conventional manufacturing where parts are done by removing material using different methods, e.g., milling, cutting, carving, shaping, turning or drilling [5]. The most important benefits of AM are accelerated product development speed and parts optimization time, complex geometries, mass customization and flexible production. Downsides can be the cost of high-tech AM printers, lack of material options and insufficient accuracy [6].
In optics, requirements for the surface topography accuracy are high. Low-frequency spatial wavelength errors, i.e., surface form irregularity or figure error, are affecting overall image quality as wavefront distortions. High-frequency, nanometer-scale spatial errors, i.e., surface roughness, are causing light scattering [7–11].
In millimeter and sub-millimeter-scale optics, AM based on multiphoton lithography has been demonstrated. Strengths of this method are high accuracy and therefore no need for post-processing [12–14]. Using this method, the volume for high-accuracy optics is restricted to mm$^3$-range. Another direct way to AM optics has been demonstrated using Projection Micro-Stereolithography (P$\mu$SL) [15]. This method works only for plano-convex, plano-concave or plano-freeform optics and is not applicable to double-sided or hollow structures.
Additive manufactured centimeter-scale optics using stereolithography (SLA), even double-sided, have been demonstrated, but the surface roughness has been an issue. Some post-processing methods have been applied and the most used methods for improving surface roughness are coating and mechanical polishing [16–18].
In inkjet printing based AM, the two most commonly used support materials are water-soluble photopolymer and hot melt wax based inks. Water-soluble support materials are in liquid form at room temperature, and the material is solidified using ultraviolet (UV) curing. These materials can be removed manually, using water pressure, by brushing, or by other mechanical means. Hot melt waxes are solid at room temperature, and the AM system has to support higher temperatures for these materials to be printed. A single droplet of liquid wax material solidifies instantly when hitting the substrate, so no additional curing is needed. Hot melt waxes can be removed by melting the wax and then using solvents. Currently, the use of support materials creates a matte surface on the final object after removal. The surface roughness is commonly in the range of $\mu$m, whereas in optics, preferable values are three orders of magnitude smaller [19,20].
In centimeter-scale, in our previous work, we have demonstrated inkjet printing based AM to achieve sub-micron form error and nanometer surface roughness for one-sided plano-convex lenses [21]. One of the main advantages of our inkjet printing based AM is that no post-processing is required. Post-processing increases manufacturing time, makes the process more complex and possibly creates additional shape errors.
In this paper, we demonstrate two inkjet printing based AM methods for manufacturing double-sided centimeter-scale optics. Using our AM printer, we realize positive meniscus lens using support material and, alternatively, printing the lens in two phases, where first a one-sided lens is printed and then flipped before printing the second side of the lens with a proper alignment procedure. Post-processing either removes (polishing) or adds (painting or coating) material, and is therefore another source for surface profile errors. For this reason, the aim in our printing process is to avoid all post-processing, that is often also a time-consuming process. After manufacturing we characterize surface profile and roughness. Both methods give more flexibility and potential for, e.g, complex optics designs to be manufactured for illumination purposes in centimeter-scale with all benefits of AM.
2. Printing methods and characterization
In our inkjet printing based AM process, three industrial quality printheads offer three thousand nozzles in total for use in parallel. Each nozzle can deposit a few picolitre sized droplet of optical quality UV curable resin on a substrate. Adjacent droplets merge into a smooth layer, which is cured afterwards. Assefa demonstrated earlier how to realize centimeter-scale imaging quality lens with iterative method [21].
Our AM printer can supply different material from each printhead, which is one thousand nozzles for a single material. This allows the printing of different materials simultaneously, but at the cost of lower print resolution when compared to using all printheads with a single material supply.
Thorlabs positive meniscus lens, model LE 1104, is selected for a design. It is one of the simplest cases which would demonstrate our goals and works as an easily accessible, industry standard reference in the form of a glass lens. A diagram of the lens is presented in Fig. 1. The lens has a focal length of 150 mm, radius of curvatures $r_1$ = 49.1 mm and $r_2$ = 131.6 mm, central thickness of 3.1 mm, and diameter of 25.4 mm.
The design STEP-file is modified and exported as stereolithrography (STL)-format using Autodesk Fusion 360. Autodesk Netfabbs is used as slicing software for creating individual 1-bit layer images.
Printing time for the lens is less than 40 minutes using the support material method and around 5-10 minutes more when using the flipping method due to alignment and setting up two print jobs instead of one. Dissolving support material took 1-2 hours. The flipping method needs 10-15 minutes extra time for laser-cutting the extra disc with alignment marks off from the actual lens part. There is no post-processing for the lens surfaces. In Fig. 2 there is a photograph of the lenses.
Fig. 2. Left lens: Thorlabs LE1104. Middle lens: the AM lens using the support material method. Right lens: the AM lens using the flipping method.
2.1 Support material method
Resins, in separate printheads, are LUX-Opticlear as optical material and 3Dresyns IJ WSS1-H as support material [23]. An optimal layer thickness for creating homogeneous layers using the support material is found to be 13 um and for LUX-Opticlear 9 um [24,25]. So, both materials have to be sliced separately. The total light transmission of a 2-mm-thick printed plate is around 91.1% for the visible wavelength range from 420 nm to 780 nm, and its internal transmission (when the Fresnel reflections are disregarded) is around 99.5% [26]. The yellowness of printed material can be decreased by keeping parts under blue light as seen in Appendix A, Fig. 9.
The negative lens with $r_2$ is firstly printed using the support material as seen in Fig. 3. Then the lens with $r_1$ is printed with LUX-Opticlear on top of the support. Printheads distance to the substrate stays fixed until the support material part of the lens is fully covered with LUX-Opticlear, after which normal stepping in $z$-direction for each layer is applied. Because the process continues uninterrupted from the support material to the lens material without transferring lens position, the alignment error between the lens sides remains minimal, on the order of single microns. Finally, support material is dissolved using sodium hydroxide (1%) aqueous solution.
Fig. 3. Basic concept of printing double-sided optical surfaces using the support material method: blue part denotes the glass substrate, green part is support material, and yellow marks the LUX-Opticlear.
Droplet position accuracy related to the printing distance and effect of air turbulence are presented in Appendix B. Because of big droplet size, slow printing speed (compared with typical inkjet printing speed) and the lens design used in this paper, the influence of the sources of error in Appendix B. for the final results is negligible.
2.2 Flipping method
In Fig. 4 the flipping method is presented where the lens is firstly divided horizontally at middle in two parts which are separated by a 1 mm thick, 50.8 mm diameter disc of LUX-Opticlear. The disc thickness is subtracted from the overall thickness of the lens. The disc is printed on top of the glass substrate. Then, four alignment marks are printed for later use. The positive lens with $r_1$ is printed above the disc, and the part is detached from the substrate. The part is flipped around 180 degrees and placed on a holder with correct sized recession. The part is aligned on the holder using the alignment marks and the printer’s internal alignment camera system. The alignment error is analyzed in Appendix C. The lens with $r_2$ is printed into correct position using the acquired alignment information. The disc is cut out using a carbon dioxide laser to match diameter of the lens.
Fig. 4. Basic concept of the flipping method: blue part denotes the glass substrate, black part is the holder, and yellow marks LUX-Opticlear.
2.3 Characterization methods for surface form and roughness
Panasonic UA3P-3100 is used for the surface form measurements. UA3P is a stylus based ultra-high precision measurement device which uses an atomic force probe for sensing the surface and an interferometric laser system is tracking $x$- and $y$-position of the sample while the probe is scanning. Accuracy of the device is 30 nm root mean square (RMS) and it is possible to measure freeform surfaces at up to 75 degree angle [27].
Veeco Instruments WYKO NT9300 optical profilometer (currently Bruker) is used for surface roughness measurement. The measurement concept is based on white light optical interferometry (WLI) [28]. Phase shifting interferometry (PSI) is used to scan optically smooth surfaces with sub-nanometre vertical resolution. Usage of PSI is limited to surface roughness measurements or measuring flat samples where surface variations on the scale of tens of microns. Sudden height variations greater than 135 nanometres on optically smooth surfaces also might cause problems for this method. The light source is a white light-emitting diode (LED) which is filtered only for monochromatic 532 nm green light. During the measurement, a piezoelectric transducer (PZT) precisely alters the optical path length with a small step, which causes a lateral shift in the fringe pattern. The system periodically takes many relative steps and the fringes are recorded by the camera, which produces a set of interferograms. The surface height profile can be calculated by combining the interferograms [29,30].
3. Lens characterization results
The flipping and support material methods are producing similar LUX-Opticlear $r_1$-surface results because the processes are using same parameters and layer images. The same applies to $r_2$-surface in the flipping method. The error is in range of $\pm$10 um as seen in Fig. 5. This is well-aligned with our previous research, where we were able to use three times higher resolution, corresponding to ten times smaller droplet size [21].
Fig. 5. Panasonic UA3P error measurements of $r_1$-surface: (a) support material method (b) flipping method.
Surface roughness of three LUX-Opticlear surfaces are verified to be in range of a couple nm, just like in the previous research [21]. In the support material method the $r_2$-surface gets its quality from the support material, which acts as kind of a mold filled with liquid LUX-Opticlear. After removing the support material, the $r_{2}$-surface is characterized. 25 samples of 125 $\mu$m $\times$ 94 $\mu$m with interval of 150 $\mu$m around the center of the lens are measured. The RMS roughness calculated from these measurements is $R_{q}$ = 11.48 $\pm$3.32 nm. Figure 6 shows a typical optical profilometer measurements of surfaces manufactured using the support method. In Appendix E, Table 1. all single surface roughness measurements after removing the support material are presented.
Fig. 6. Wyko 9300NT PSI measurements of surfaces in the support material method: (a) $r_2$-surface roughness (b) $r_1$-surface roughness.
Surfaces form measurements are done using circular patterns with 0.5 mm interval. With this method the measurement time for a single lens is 10 minutes. Datapoint clouds are saved and imported to MATLAB, where data is compared to the CAD-model. The support material is removed before the form measurement. The $r_{2}$-surface accuracy for the lenses is $\pm$10 um as seen in Figs. 7. In summary, both materials can be printed with similar accuracy.
Fig. 7. Panasonic UA3P error measurements of $r_2$-surface: (a) support material method (b) flipping method.
The imaging performance of the lenses is evaluated with the USAF 1951 resolution target using collimated laser diode module (Thorlabs CPS635R) as light source. As shown in Fig. 8, both lenses were able to achieve the resolution of 57.0 lp/mm when using 12 mm aperture. The resolution of 45.3 lp/mm was archived when using 20 mm aperture.
Fig. 8. MTF measurements using the USAF 1951 resolution target (groups 4–5) with image resolution of 57.0 lp/mm : (a) flipping method (b) support material method.
4. Conclusions
We have demonstrated two methods using our inkjet printing based AM for manufacturing double-sided centimeter-scale optics. With the support material method, using a water-soluble support material it is possible to manufacture optically smooth surfaces if printing is done only one material at a time and the lens is UV-cured properly before continuing printing with another material. In this method there is no issues with alignment of parts because there is no need to remove the printed lens when different materials can be printed simultaneously from different printheads.
The flipping method offers good option if the lens can be printed in two parts and it is possible to do the alignment with good accuracy. Here we did not study how the thickness of the center part of the lens affects the final shape accuracy of the lens, but earlier experience shows that too thin printed parts are prone to deforming during the printing of the second surface.
In both methods the surface form accuracy is good enough for illumination optics.The worst case of 11 nm surface roughness in the support method induces about 3${\% }$ light scattering at the incident angle of 45 degrees, which is acceptable for illumination purposes [10,31].
In future, printing quality can be further improved by, e.g., new generation printheads with higher resolution that used today (even pico-litre scale droplets for over 1000 dpi are possible). Accurate measurement of the surface profile while printing could be also possible. This could lead for correcting any potential shape errors while printing. With these improvements quality of complex optical components can be further increased.
In conclusion, it is possible to manufacture complex, double-sided optics in centimeter-scale with short production times using inkjet printing based AM without post-processing. However, surface roughness stays a little bit behind in the support material than in LUX-Opticlear.
A. Optical transmission of printed material
Fig. 9. Optical transmission of 2-mm-thick printed LUX-Opticlear material, measured with a spectrophotometer. Fresh sample has been measured after printing and the other has kept under blue light.
B. Single droplet accuracy and atmospheric drag
In cases where picolitre droplets are used, droplet accuracy degradation due to air flows inside the printing system will rise even with under millimeter flight distances. Duineveld represented the relationship between the dimensionless velocity of the droplet and the dimensionless distance travelled from printhead which describes how a single droplet is losing it’s initial velocity after ejecting from a nozzle [25,32]. Figures 10 and 11 show how droplet position accuracy decay with higher distances between the printhead and the substrate.
Fig. 10. Microscope image of a substrate where single droplets are printed with different distances between the printhead and the substrate. Upper horizontal line is printed for the reference line and at the lower line the leftmost droplet is from distance of 1.0 mm and next ones with distance increased by 0.5 mm for each drop. (a) 20 picolitre droplets and (b) 2 pl droplets.
Fig. 11. Droplet position error with different printing distances between the printhead and the substrate. (a) 20 picolitre droplets and (b) 2 pl droplets.
C. Alignment error
The flipping method relies to a manual alignment after the disc and the first part of the lens has been place on the holder. The alignment error is measured by printing 25 separated single droplets to the substrate as seen in Fig. 12. The center droplet is printed as normal printing process. After this the substrate is shifted randomly a small distance and the center droplet position is measured using a camera of the printer. Based on this measurement, the required offset for the next droplet is calculated and the droplet is printed accordingly. This is repeated until all 25 droplets are printed. Then, the droplet positions in relation to the center droplet are measured. RMS errors for the droplet position are 6 um for $x$-axis direction and 8 um for $y$-axis.
Fig. 12. Microscope image of alignment error test where 25 single droplets are printed separately on a substrate.
D. Position accuracy of individual droplets of a single printhead
One reason for droplet positioning errors is variation in the printhead nozzles which is causing non-uniformity in the droplet velocity and ejection direction as seen in Fig. 13. A new printhead produces better than few micron accuracy for droplet position when distance between the printhead and the substrate is 1 mm. Over time, the accuracy of the position decreases due to degeneration of the nozzles, which is most often caused by dried or hardened material around the nozzle orifices. Proper upkeep and cleaning of the nozzle plate is important.
Fig. 13. Single droplet position accuracy compared to theoretical values of a single printhead. Each one of the 1000 nozzles is used to create a group of calibration droplets on to the substrate, which positions are measured afterwards. Distance between the printhead and the substrate is 1.0 mm. Std error for $y$-direction is 2.815 um and $x$-direction is 2.743 um.
E. Surface roughness measurements of the $r_{1}$ and $r_{2}$-surfaces manufactured using the support method
Table 1. Wyko NT9300 optical profilometer RMS measurements of surfaces after removing support material. $r_{2}$:$R_{q}$ = 11.48 $\pm$3.32 nm. $r_{1}$:$R_{q}$ = 2.78 $\pm$1.1 nm.
Funding
Jane ja Aatos Erkon Säätiö; Teknologiateollisuuden 100-Vuotisjuhlasäätiö; Suomen Akatemia, Academy of Finland.
Acknowledgments
This document is the result of the research project funded by Jane and Aatos Erkko Foundation, and the Technology Industries of Finland Centennial Foundation. The work is part of the Research Council of Finland Flagship Programme, Photonics Research and Innovation (PREIN), decision 346518, and partially funded by Research Council of Finland 3D-GRINO-PPCI project, decision 355542.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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