ADDITIVE MANUFACTURING WITH UV LIGHT CURED RESIN. Vuorio, J.; Nikkilä, V.; Teivastenaho, V.; Peltola, J.; Partanen, J.; Kiviluoma, P. & Kuosmanen P.

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1 9th International DAAAM Baltic Conference "INDUSTRIAL ENGINEERING" April 2014, Tallinn, Estonia ADDITIVE MANUFACTURING WITH UV LIGHT CURED RESIN Vuorio, J.; Nikkilä, V.; Teivastenaho, V.; Peltola, J.; Partanen, J.; Kiviluoma, P. & Kuosmanen P. Abstract: This paper introduces a method of additive manufacturing by injection of light-cured material. A fabrication system to produce three-dimensional parts by extruding multiple layers of photosensitive resin was constructed. The polymerization of the resin is achieved in a curing process that utilizes ultraviolet light-emitting diodes. The device is able to manufacture basic three-dimensional shapes. Curing time of the photopolymer was discovered to be the dominating parameter with the used manufacturing method. Keywords: Additive manufacturing; photosensitive materials; UV-LEDs; Rapid prototyping 1. INTRODUCTION Additive manufacturing is a subject of intensive study. Additive manufacturing by injection of light-cured material is a manufacturing process for producing threedimensional objects by extruding multiple layers of photosensitive materials. The process utilizes ultraviolet light to cure materials upon completion. Recent studies in radiant power of ultraviolet lightemitting diodes (UV-LEDs) suggest that mercury-vapour lamps may be substituted for UV-LEDs [ 1 ]. However, the use of high power UV LEDs as a curing light source has not been thoroughly investigated in the application of additive manufacturing, especially with regard to the method of injection of the photopolymer. In this study a prototype printer was constructed and tested. Photopolymer is cured with UV light. The formula of the resin enables hardening process with UV light. When curing photopolymer by applying UV light, the cured layer absorbs light [ 2 ]. Curing process is halted when exposure subsides below threshold value. This restricts the thickness of one printed layer. The depth of cured layer can be expressed by Beer's law [ 2 ]: C= D pln(e/e c ), E > E c 0, E E c (1) Where C expresses the curing depth, E refers to UV light exposure and D p states the penetration depth of the resin, which is unique for each resin. Finally, E c indicates the critical exposure of the resin. Curing process will not happen if this threshold is not exceeded. Resin is extruded in liquid form. Initial curing is applied by focused UV-LEDs. These LEDs are attached to the extruder unit. However, to complete the process, additional UV light source is needed since the penetration depths of these LEDs are limited. 2. METHODS Advantages of LED lights are their high durability and longer lifespan. Also, LED lights can be controlled with a very short response time. However, led lights alone are insufficient to finalize the curing of the resin. The effect of the radiation area was analysed. Different amounts of radiation of light and its effects to the quality were analysed. Developed device was tested with several resins and the effects of material properties to manufacturing 317

2 quality were compared. Common shapes such as cubes and cones were used to compare the accuracy of the developed method against existing additive manufacturing systems. 2.1 Mechanical design of constructed printer The structural components of the printer are shown in Figure 1. The printer consists of the following subassemblies. Extruder unit syringes the resin by applying pressure with piston to the resin container. Stepper motor controls the piston by rotating gear which pushes a screw down and force syringe s piston to extrude material through nozzle. Unit is controlled by RAMPS (RepRap Arduino Mega Pololu Shield) [ 3 ]. Extruder moves along y- and z-axis. Also the LEDs responsible for the primary curing are located in the extruder unit. A detailed assembly of the extruder unit can be seen in Figure 2. Printbed functions as a platform for the printable object. This platform moves parallel to x-axis and thus has one degree of freedom. Movement is achieved with stepper motor controlling a belt drive. Print object can be moved in all three dimensions with printbed and extruder. Printer body is made of polycarbonate which provides mechanical strength while still being easy to manufacture. The polycarbonate sheets are connected to each other with fixit blocks and threaded rods to achieve easy and rigid assembly. Figure 2: Extruder unit. 2.2 Curing with UV-LEDs Four UV-LEDs were mounted to the extruder. Intensity of these LEDs stays constant but it can also be controlled by RAMPS unit. However, maximum intensity is generally desired at all times during the printing process. Properties of these LEDs are listed in Table 1. Manufacturer Model Peak wavelength Spectrum radiation bandwidth Tecled Lightning Co. XL5050UVC nm 28 nm DC forward current 60 ma Power dissipation 200 mw Luminous intensity 4.5 mw Life h Size 5 x 5 mm Table 1. Extruder LEDs for primary curing of the resin Figure 1: Structure of the printer 318

3 2.3 Beam steering The alignment of the LEDs and lenses relative to the plane of the printbed is crucial. By overlapping the focused beams of UV light that are coming through the lenses will be concentrating the effective radiation of the UV light into a fixed area [ 6 ]. The optical properties of the lenses that were used for are shown in Table 2. The resin will be extruded in such way that it will be completely exposed to the fixed area. This will have the most beneficial effect to the overall irradiance efficiency of the LEDs and thus in this case accelerate the polymerization process of the resin. The LED and lens alignment is shown in the Figure 3. Type FCA10911 NIS33U-SS Optic Material Cyclic Olefin Polymer (COP) Holder Material Polycarbonate Fastening Tape Viewing Angle(FWHM) 10 degrees Table 2. Lenses for concentrating the beam of light. Figure 3: LED alignment and steering of the beam. 2.4 Evaluation of resins We compared two different resins: Somos ProtoGen O-XT and Somos WaterShed XC Both of these resins are used in additive manufacturing processes, e.g., stereolithography. These both photopolymers resemble traditional engineering plastics, such as acrylonitrile butadiene styrene (ABS), and these materials were selected to this study due their mechanical properties, which makes it possible to create durable and functional parts. Properties of both resins are listed in Table 3. Colour ProtoGen O-XT White WaterShed XC Optically clear, near colorless Viscosity at 30 C Density (g/ cm 2 ) Critical exposure (mj/cm 2) D p, Slope of cure-depth vs. ln(e) curve (mm) Table 3: Properties of the resins. [ 7,8 ] 3. RESULTS This study demonstrated that photopolymer can be solidified with LED lights. Critical exposure was exceeded and with proper calibration of printing parameters, it is possible to manufacture solid objects. In the photopolymerisation process the radiation of the UV light will be partially absorbed by the cured layer. This follows the Beer s law of absorbance which states that the exposure will decrease exponentially with the curing depth. Calculated values of cure depth for both used resins are less than 1 mm. This and practical experiments with LEDs and resins indicate that limiting factor with minimum layer thickness is the accuracy of the extruder. While the maximum layer thickness is limited by the curing depth. The ability of the light sources to cure sample lines of resin was also tested in practice. This test provided how long it takes for the LEDs to cure the resin. From the curing time it is possible to predict the 319

speed that the extruder can be moved while extruding material. These curing tests were executed with and without lenses and with variable number of LEDs.

4 speed that the extruder can be moved while extruding material. These curing tests were executed with and without lenses and with variable number of LEDs. It was discovered that with one LED, curing process takes too long to be used in AM even with a lens focusing the beam. Although it also proved that the lens provided a remarkable improvement to the curing speed of the resin. In final construction, four LEDs with lenses were found to be good compromise between curing speed and keeping the size of the LED module small enough to fit in to the extruder. enough to create a concentrated hotspot near the head of the syringe. At the same time combined beam needs to cover as large area as possible, so that the extruded resin is exposed to the UV-light long time enough for it to cure completely. However even with optimized light source the curing time achieved with the developed prototype is 5 10 s, under the UV-light exposure, depending on the layer thickness. This means that curing time of the resin is the limiting factor of the printing speed the device is able to achieve. The aforementioned also means that the shape and size of the printed object have huge impact on final printing speed reached. Figure 4 illustrates the light pattern and how long is the exposure distance in a case when straight line is printed. From the figure it can be seen that if the outlines of the manufactured object can be fitted in the 40x40 mm area, the extruded resin is constantly exposed to the UV-light and therefore faster printing speed can be used. However the optimized printing speed versus the dimensions of the printed object was not in the scope of this study and the overall printing speed achieved with the built prototype is poor compared to modern thermoplastic printers. 4. DISCUSSION Figure 4: Topside view of the light pattern and printing process. The reason the light sources were placed approximately 30 mm from the printbed was to get the most beneficial effect of overall irradiance efficiency of LEDs and thus get the most effective polymerization process. This distance was evaluated empirically by changing the distance from the LED lenses relative to the printbed and by changing the alignment of the angle of the UV-light beam from LEDs. The alignment of the LEDs needs to be done so that the beams are overlapping each other s The constructed prototype is able to provide an alternative method to additive manufacturing that is able to compete with more common layer manufacturing (LM) methods (e.g., fused deposition modelling). It has many advantages: increased accuracy, efficiency, controllability, response and safety. The developed method does not require heating of the printbed and extruder whereas it is required in normal thermoplastic printing. Therefore, the device consumes less power and does not require initialization time for the components to reach the desired temperature. In addition, cold printing process increases the safety and efficiency 320

5 of the system. Moreover the manufactured objects does not suffer from distortion caused by the shrinkage of material cooling down which is a common problem in most AM methods. Because the extruded material has rather low viscosity and the material flow is controlled with syringe, an increased controllability can be reached compared to AM methods using more viscous materials. On the other hand, the low viscosity of extruded resins limits the overhanging features that can be manufactured. It was originally thought that the manufacturing process with photopolymer would be faster than what was achieved in this study. It is crucial that built layer is solid enough to support the layers that will be extruded on top of it. Therefore, LEDs needs to be focused to the extruded resin long enough to be cured almost completely thus liming the speed of the printer. Compared to other radiation sources, LEDs advantage is the emission of radiation of required wavelength. Therefore, LED light will cure the resin with high efficiency. The current problem of LED lights is their small power output, which will result in slow print speed. Lack of irradiation will also limit the layer thickness. Higher wavelength would allow deeper penetration depth. [ 1 ] Future technology will provide us more efficient, higher power LEDs with lower price. As a result, we will likely see faster, cheaper and more power efficient photopolymer printers that can be used for rapid prototyping. 5. CONCLUSION This study demonstrates that UV LED technology can be successfully applied to rapid prototyping and LM applications. The study demonstrated LED s capability to cure photopolymer. A prototype printer was built and simple objects were successfully created with it and numerous advantages compared to more common LM methods were discovered. However the achieved printing speed of the prototype printer was low due to the slow curing speed of the resin. 6. CORRESPONDING ADDRESS Panu Kiviluoma, D.Sc. (Tech.) Aalto University School of Engineering, Department of Engineering Design and Production P.O. Box Aalto, Finland Phone: panu.kiviluoma@aalto.fi 7. REFERENCES 1. S.L. McDermott; J.E. Walsh and R.G. Howard. A comparison of the emission characteristics of uv-leds and fluorescent lamps for polymerisation applications. Optics and Laser Technology, 40(3), Kim, J.S.; Kim, D-S and Lee, M-C. 3d printing method of multi piezo head using a photopolymer resin. International Conference on Control, Automation and Systems, RepRapWiki. Ramps Tecled Lighting Co nm smd uv led reference. product-detail/ nm-smd-uv- LED_ html, LEDiL. Fca10911 nis033u-ss reference PhD Richard Sahara. Uv led 3 lens technology. rray_paper_ pdf, DSM Somos Materials. Somos ProtoGen O-XT reference. 321

6 SLA_Somos_18420.pdf, DSM Somos Materials. Somos Water- Shed XC content/uploads/2013/07/somos- WaterShed_XC_11122_Datasheet.pdf, ADDITIONAL DATA ABOUT AUTHORS Vuorio, Jaakko, B.Sc. (Tech.) Phone: Teivastenaho, Ville Phone: Peltola, Jukka Phone: Nikkilä, Vesa Phone: Kuosmanen, Petri, D.Sc. (Tech.), Professor Phone: Partanen, Jouni, Ph. D. (Tech.), Professor Phone:

THE EFFECTS OF MINITUARISATION OF PROJECTION STEREOLITHOGRAPHY EQUIPMENT ON PRINTING QUALITY

9th International DAAAM Baltic Conference "INDUSTRIAL ENGINEERING 24-26 April 2014, Tallinn, Estonia THE EFFECTS OF MINITUARISATION OF PROJECTION STEREOLITHOGRAPHY EQUIPMENT ON PRINTING QUALITY Rayat,