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Fabricating Photopolymer Objects by Mold 3D Printing and UV Curing

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Fabricating Photopolymer Objects by Mold 3D Printing and UV Curing Computational Design and Fabrication FabSquare: Fabricating Photopolymer Objects by Mold 3D Printing and UV Curing Vahid Babaei, Javier Ramos, Yongquan Lu, Guillermo Webster, and Wojciech Matusik ■ Massachusetts Institute of Technology apid prototyping tools provide practical of the mold content is carried out by ultraviolet methods for personal fabrication, al- (UV) energy. We replace the expensive and time- lowing designers and engineers to refine consuming mold-making stage in injection molding Rand iterate their ideas quickly. Because of their with 3D printing and substitute the high-pressure, low cost and versatility, 3D printers in particu- high-temperature process involving large and costly lar are becoming increasingly popular. Although hardware with simple injection and UV curing. 3D printers are incredibly powerful devices for Recently, 3D printing has been used as a tool to creating complex geometries, they are inherently fabricate molds for traditional thermoplastic injec- slow and only print materials tion molding. The molds are temporary and used that are within a small range to test mold geometries and for short production The FabSquare system of properties. This puts them in runs. Typically, the molds only last for several dozen fabricates objects by casting clear contrast with classic fabri- injections because the mold polymer degrades un- photopolymers inside a 3D cation processes such as injection der high pressure and heat. Moreover, such molds printed mold. The molds are molding, where speed and high still require the heavy machinery used for injection printed with UV-transparent throughput come at the expense molding. UV injection molding has also been used materials to allow for UV of cost and flexibility. at the µm and mm scales. In biology, UV microin- photopolymerization. Because Injection molding is the pro- jection molding allows for fabricating biocompat- it uses simple and inexpensive cess of injecting molten ther- ible hydrogel structures1 and biosensors.2 Micro UV hardware, FabSquare is moplastic materials into a mold molding has been used to fabricate micro-optical 3 suitable for ordinary users in cavity, inside which they harden devices. While, to the best of our knowledge, there nonspecialized labs. as they cool. The conventional is no literature on the fabrication of large-scale ob- mold-making stage is painstak- jects via 3D printing combined with UV injection ing, expensive, and slow. Apart from the mold molding, we believe that the main contribution development, the required machinery is also pro- of our work is to introduce a flexible fabrication hibitively expensive and best suited for producing method that is suitable for researchers without ac- thousands to millions of parts. cess to complicated hardware, such as computer To address these challenges, we developed Fab- graphics researchers. (See the “Related Work in Square, a system that brings injection molding into Casting and Personal Fabrication” sidebar for more the realm of personal fabrication by leveraging 3D details about previous work.) printing and interactive design. The FabSquare sys- Compared with injection molding, FabSquare is tem is similar to injection molding, except that it significantly less expensive and less complicated, works with photopolymers and the solidification with much simpler hardware requirements, and 34 May/June 2017 Published by the IEEE Computer Society 0272-1716/17/$33.00 © 2017 IEEE g3bab.indd 34 4/3/17 5:25 PM Related Work in Casting and Personal Fabrication njection molding is one of the most popular methods Hopper Ifor manufacturing plastic parts1 because it is capable of fabricating relatively complex parts with high accuracy. In Parting line this method, raw materials melted under high tempera- Mold ture and high pressure are fed into a mold and then hard- ened via cooling. Figure A shows a simplified diagram that illustrates the injection molding process. Injection molding Screw plunger Part requires high startup costs and long lead times before the Barrel Heater Ejection pins first part is produced.2 The metal mold is expensive—on the order of tens of thousands of dollars—and takes weeks Figure A. Simple demonstration of the injection-molding process. to draft, design, and manufacture. This approach is best The material is fed from the hopper into the barrel and forced by the suited for the mass production of end-use products. rotating screw plunger into the mold cavity. The heaters melt the Personal fabrication devices, such as 3D printers and material and keep them molten. After cooling and hardening inside laser cutters, are versatile tools for producing single-item the mold cavity, the part is released with the help of ejection pins. parts.3 Numerous recent works have been focused on speeding up various personal fabrication processes. For example, parallelizing multiple heads can significantly in- 3. N. Gershenfeld, Fab: The Coming Revolution on Your Desktop– crease the printing speed.4 WirePrint fabricates low-fidelity From Personal Computers to Personal Fabrication, Basic Books, previews of a prototype by printing a wireframe mesh.5 2008. faBrickation limits 3D printing to the parts of the object 4. C.J. Hansen et al., “High-Throughput Printing via Microvascular where a higher resolution is needed, and the user puts Multinozzle Arrays,” Advanced Materials, vol. 25, no. 1, 2013, Lego-like bricks everywhere else.6 CofiFab is a computa- pp. 96–102. tional method that fabricates highly detailed, large-scale 5. S. Mueller et al., “WirePrint: 3D Printed Previews for Fast objects rapidly.7 It fabricates the interior shell with a laser Prototyping,” Proc. 27th Ann. ACM Symp. User Interface Software cutter and 3D prints the outer surface. and Technology, 2014, pp. 273–280. There have also been recent efforts to create custom 6. S. Mueller et al., “faBrickation: Fast 3D Printing of Functional thermoforming setups for generating one-off, textured ob- Objects by Integrating Construction Kit Building Blocks,” jects.8,9 Like FabSquare, one of these approaches uses molds Proc. 32nd Ann. ACM Conf. Human Factors in Computing Systems, to cast the object made of gypsum.8 However, in that ap- 2014, pp. 3827–3834. proach, the mold is destroyed when the object is removed. 7. P. Song et al., “CofiFab: Coarse-to-Fine Fabrication of Large 3D Objects,” ACM Trans. Graphics, vol. 35, no. 4, 2016, article no. 45. References 8. C. Schüller et al., “Computational Thermoforming,” ACM 1. D.V. Rosato and M.G. Rosato, Injection Molding Handbook, Trans. Graphics, vol. 35, no. 4, 2016, article no. 43. Springer, 2012. 9. Y. Zhang, Y. Tong, and K. Zhou, “Coloring 3D Printed Surfaces 2. G. Menges et al., How to Make Injection Molds, Carl Hanser by Thermoforming,” IEEE Trans. Visualization and Computer Verlag, 2001. Graphics, preprint, 2016; doi:10.1109/TVCG.2016.2598570. is therefore within the reach of ordinary users. FabSquare Workflow and Compared with 3D printing, the printed mold Technical Details in FabSquare can be reused to rapidly create ad- To fabricate a CAD model, the FabSquare workflow ditional copies of the intended object. Hence, it is follows these steps: well suited to situations where more than a hand- ful of parts are needed. Moreover, many differ- 1. Design a mold geometry. ent additive components can be mixed with the 2. Fabricate the mold with a 3D printer. photopolymer to improve its physical properties. 3. Fill the mold with a specified polymer material. This is an important advantage over 3D printing, 4. Solidify the fluid inside the mold with UV- which only works with a limited range of mate- triggered polymerization. rial properties. FabSquare does have some limita- tions: Similar to injection molding, the maximum Figure 1 and an online web extra video (see achievable thickness with UV curing is limited. https://youtu.be/a-s13U5uLAE) illustrate each of Moreover, the FabSquare editor does not support these four steps. First, the user specifies the geom- molds with more than two parts. etry in the form of a digital model. The mold can IEEE Computer Graphics and Applications 35 g3bab.indd 35 4/3/17 5:25 PM Computational Design and Fabrication then be designed from the model in a 3D CAD software. The output is two STL files that can be directly printed with UV-transparent materials. Next, the photopolymer fluid is injected into the tightly clamped mold. Then, the mold contain- ing the material is exposed to UV light, and the polymerization takes place. After the liquid inside the mold solidifies, the object can be removed from the mold. (a) (b) Figure 2 shows a set of example objects created with FabSquare. Mold Design The mold’s digital design can take place in a CAD environment, such as SolidWorks. We have also developed a stand-alone application that performs the same design steps with a high degree of auto- (c) (d) mation. (We describe our mold-design application in detail later in this article.) The intended 3D geometry for fabrication is used to generate two halves of a mold: A side and B side. The design includes a material inlet and an air outlet in order to let materials flow through (e) the mold. We should be able to tightly clamp the Figure 1. The FabSquare workflow. To fabricate a CAD model, the two halves of the mold together in order to prevent FabSquare workflow follows these steps: (a) design a mold geometry, the material from leaking out of the model region. (b) fabricate the mold with a 3D printer, (c) fill the mold with a specified For this, we distribute screw holes along the mold’s polymer material, (d) solidify the fluid inside the mold with UV- periphery evenly and densely. Figure 3a shows the triggered polymerization, and (e) remove the object from the mold. outline of a designed mold after printing. The maximum mold outline surface area is lim- ited by the area of the 3D printer’s tray and the size of the UV-curing module.
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