国外 论文《3D打印原型与快速成型技术》 英文完整版

Modifi cation and Further Development of a Drop on Demand Printhead for Wax enabling Future 3D-Printing and Rapid Prototyping Thomas Ottnad*, Markus Kagerer, Franz Irlinger, and Tim C. Lueth, Member, IEEE AbstractDosing liquids in small quantities is present in a lot of different applications as inkjet printing, heating devices, and 3D-printing methods. This paper deals with a drop on demand (DOD) printhead based on a piezo-element bender actuator which can generate drops out of wax. The aim is to provide a testing system where changes of signifi cant design parameters can be done easily in order to proof the working principle in early stages of development. In this work a given drop on demand printhead is modifi ed making the manufacturing process more reliable. Putting single drops on each other can be regarded as both proof of functionality and an outlook on future applications for direct 3D-printing and rapid prototyping. Index Terms3D printer for wax, Formation of drops, Microdrop Generator, Piezo-element bender actuator, Rapid Prototyping. I. INTRODUCTION Drop on demand printheads come into action in numerous different technical applications. Inkjet printing was one of the fi rst applications and still inkjet printers are some of the best- selling Micro-Electro-Mechanical-Systems (MEMS). Today many other applications using drop on demand printheads have been established. The range of applications is as big as the range of used fl uids, working conditions, and working principles. 1 - 2 To show the big range of applications using drop on demand printheads dosing small quantities of fl uid for synthesis of radiopharmaceutical products, fuel-drop generation in heating devices, and providing binding material in 3D-printing techniques can be named. The main working principles are piezo-electrical, thermal, electrostatic, and acoustic methods to generate single droplets. In short: drop on demand printheads do fulfi ll different tasks dependent on the specifi c application and thus can be completely different from each other. 3 - 10 When sticking to piezo-electrical working principles there exist quite a few methods how to use this effect. Most often the idea is to apply the piezo-electrical effect in order to generate droplets by causing both change of volume in a fl uid chamber and pulse transmission to force a droplet out Manuscript received January 31, 2012. *T. Ottnad is with the Institute for Micro Technology and Medical Device Technology, Technische Universit at M unchen, 85748 Garching, Germany (e-mail: thomas.ottnadtum.de). M. Kagerer is with the Institute for Micro Technology and Medical Device Technology, Technische Universit at M unchen, 85748 Garching, Germany (e-mail: markus.kagerertum.de). F. Irlinger is with the Institute for Micro Technology and Medical Device Technology, Technische Universit at M unchen, 85748 Garching, Germany (e-mail: irlingertum.de; tim.luethtum.de). T. C. Lueth is with the Institute for Micro Technology and Medical Device Technology, Technische Universit at M unchen, 85748 Garching, Germany (e-mail: tim.luethtum.de). Fig. 1.Components of the drop on demand printhead for wax in exploded view. (1) Reservoir for wax and feeding of wax. (2) Sealing ring for reservoir. (3) Sealing ring for vent srew. (4) Vent screw. (5) Housing (illustrated transparent). (6) Electrical contacting. (7) Piezo-element bender actuator (8) Silicon plate with nozzle integrated (9) Layers of high- temperature glue. 10 of a nozzle. Examples how to use different piezo-electrical effects can be seen in using shear mode actuators, piezo- electrical pipes, or diaphragm design which additionally can make a printhead chemical resistant as the piezo-electrical material is not in contact with the fl uid. 9 Apart from specifi c working principles and how they are realized in practice certain parameters, as the geometry of nozzles, are very important. This comes into action not only when dispensing fl uids with low viscosities but also when The 2012 IEEE/ASME International Conference on Advanced Intelligent Mechatronics July 11-14, 2012, Kaohsiung, Taiwan 978-1-4673-2576-9/12/$31.00 2012 IEEE117 dealing with highly viscous fl uids as for example plastic melts. High temperatures and high working pressures make great demands on manufacturing such nozzles. 11 - 13 Guaranteeing desired geometries of nozzles and analyzing the infl uence of geometry on fl ow behavior of a highly viscous fl uid is the fi rst step in order to assign principles of droplet generation later on. One certain working principle of a drop on demand printhead was presented at AIM 2011 10 which is based on a piezo-electrical bender actuator dosing liquid wax where droplet generation is caused by both change of volume and pulse transmission. II. COMPONENTS AND CONSTRUCTION OF THE DROP ON DEMAND PRINTHEAD As shown in Fig. 1 the drop on demand printhead consists of a reservoir for wax, sealing rings and a vent screw, a housing, and a piezo-element bender actuator which is mounted on a silicon plate with a nozzle integrated using thin layers of high-temperature glue. Design, construction and the manufacturing processes using rapid prototyping techniques are explained in detail in 10. In order to get a general idea of the design, the manufacturing, and working principle is summarized here. Whereas both housing and reservoir are made of aluminum and can be manufactured using milling technology the silicon plate is cut to size and structured using laser ablation. The drop on demand printhead fuzes wax from solid state to liquid state. For that purpose a simple heating element can be attached to one of the aluminum parts and due to the good heat transfer of aluminum it is easy to set up the temperature properly. Dependent on the melt temperature of a certain material, as for example different types of wax, the power of the heating element can be adapted. When screwing the reservoir on the housing a sealing ring is used in order to guarantee tightness. Although the drop on demand printhead is tolerant towards bubbles a vent hole can be used to exhaust the air before starting the printing process. The vent hole can be sealed using another screw and sealing ring. For the piezo-element bender actuator a ceramics material (PIC 151) is used. Due to its high Curie temperature of 250C it can be cut to size using laser ablation. Some data concerning the piezo-element bender actuator are listed in Table I. The laser used is an infrared laser, type LS2000, Nd:YAG with a wave length of 1064nm, maximum power of 25W, pulse duration greater than 100ns, pulse frequency of 4 kHz, and intensity of 1200MW/cm3. Laser ablation comes into action, too, when manufacturing the silicon plate in four steps which is shown in Fig. 2. After cutting a pocket for positioning the piezo-element bender actuator, next step is to cut a deeper pocket which allows the wax to fl ow around the actuator and additionally allows the bender actuator to overshoot without being damaged. Manufacturing the nozzle itself is the third step followed by cutting the silicon plate to size. TABLE I VARIABLES AND CHARACTERISTICS OF THE PIEZO-ELEMENT BENDER ACTUATOR WITH THE VALUE OF MAXIMUM BENDING10 VariableUnitValue d31 V/m 210 1012 UV40 am22 106 Eel V/m 1.8 106 lm7.7 103 hm0.55 103 wmaxm 30 106 When having manufactured all components of the drop on demand printhead it is ready to assemble. The piezo- element bender actuator is attached to the silicon plate using high temperature glue and is the fi rst pre-assembly. The high temperature glue will be used later on in order to seal the electrical conduit and additionally is used to achieve an electrical insulation. The same glue is used to fi x the silicon plate to the aluminum housing and after screwing the reservoir on the housing the printhead is ready to use. Fig. 2.Silicon plate machined using laser ablation. (1) Pocket for positioning the piezo-element bender actuator. Depth of the pocket of 30m. (2) Pocket to avoid damage of the piezo-element bender actuator when being defl ected with a depth of 60m. (3) Nozzle. (4) Outline of the silicon plate cut in the last step. 10 III. ADVANTAGES AND DISADVANTAGES OF THE DROP ON DEMAND PRINTHEAD The fact that this drop on demand printhead is tolerant towards bubbles and the fact that it can be heated and thus the range of dispensible fl uids is quite large are some of the advantages of this printhead. The working temperature can be set up to 80C and is only limited by the maximum temperature of the electrical contacting and of the glue but not in the fi rst instance by the piezo-element bender actuator as its Curie temperature is specifi ed with 250C. One of the most important advantages of this printhead is that it is easy adapting and manufacturing it in order to proof the principle in early stages. 118 Although manufacturing the printhead is quite easy it demands some skills concerning glueing and electrical con- tacting. Putting the piezo-element bender actuator on the silicon plate is the fi rst important step which is quite easy to control. Glueing the silicon plate to the aluminum housing is much more diffi cult. Using the proper amount of glue is necessary in order to avoid plugging of the nozzle integrated in the silicon plate. Whether the nozzle is plugged or not cannot be checked easily. Thus you can only assume that this might be the reason why the drop on demand printhead doesnt work. To disassemble the printhead is only possible when heating the assembly above the melt temperature of the glue but removal of any glue located under the piezo-element bender actuator in close location to the nozzle is only possible when removing the piezo-element bender actuator, too. When trying this, the silicon plate often breaks as it is very brittle material (see Fig. 3). Fig. 3.Breakage of the silicon plate when trying to disassembe the print- head due to its brittle property. Some glue covering the nozzle integrated in the silicon plate was the reason for disfunction of the drop on demand printhead. The glue still can be seen on the surface of the silicon plate. Using laser ablation can be seen as an advantage as well as a disadvantage of the printhead. As illustrated in Fig. 2 laser ablation is used to manufacture the silicon plate with several pockets resulting in quite large surfaces. When using laser ablation the specifi c laser source and laser settings effect the quality of surfaces. In this case a high quality is aimed for as surface roughness infl uences the fl ow of the fl uid. The higher the surface roughness the higher the resistance breaking down the fl uid fl ow. Additionally when using a laser unsteady performance of the laser infl uence manufacturing duration enormously as abration has to be checked continuously. 14 Aside from the problem of avoiding any glue to plug the nozzle and the quite complex control of geometry when man- ufacturing the silicon plate, electrical contacting represents another problem. As there is only little space for the electrical contacting soldering points and cables have to be quite small. This in turn means that any loads as buckling or pulling-out can damage the electrical contacting. IV. REDESIGN OF THE DROP ON DEMAND PRINTHEAD In order to meet the challenges of rapid, accurate, and reproducible manufacturing the design of the drop on de- mand has been changed. Using standard tooling machinery to manufacture the complete printhead is aimed for. That target can be achieved by adapting the design of the former printhead using an aluminum housing with a nozzle integrated (see Fig. 4). Sticking to the basic idea of the printheads design but doing it without the use of silicon and therefore no more need of laser ablation production time even can be reduced while functionality still keeps the same. The piezo-electrical bender actuator will be attached to the aluminum housing still using high temperature glue as this allows easy electrical insulation. Best idea to avoid any glue plugging the nozzle when doing the fi nal assembly is not to use any glue any more at all. For that reason a cover plate made of aluminum can be screwed on top of the aluminum housing. To guarantee tightness between both of the aluminum parts a sealing ring comes into action. Fig. 4.Components of the redesigned drop on demand printhead for wax in exploded view. (1) Reservoir for wax and feeding of wax. (2) Vent screw (3) Sealing rings for reservoir and vent srew. (4) Cover plate. (5) Sealing ring for cover plate. (6) Housing with nozzle integrated. (7) Piezo-element bender actuator (8) Layer of high-temperature glue. The other components as sealing rings for the vent screw and the reservoir which still is screwed to the aluminum 119 housing have not been changed. The redesign with the new aluminum housing provides much more space for the electrical contacting and thus allows to use conductors with larger diameter. Additionally solder- ing points are not limited in their size, too. Combination of both makes the electrical contacting much more robust. Using standard tooling machinery for manufacturing the aluminum housing with the nozzle integrated even allows to control geometry much better. First step is to mill an aluminum block to a specifi c size out of which several printheads can be manufactured. This is kind of half-fi nished product and allows varying the distance between the nozzle and the bearing area of the piezo-electrical bender actuator. Achieved accuracy is no longer dependent on any unsteady performance of the laser but only dependent on the accuracy of the milling machine. As shown in Fig. 5 six exemplars of printheads have been manufactured and you can see the different components and stages of assembly. Finally the drop on demand printhead was attached above a 3D-table. The size of the drop on demand printhead (without the reservoir screwed on top) just changed a little. Whereas the fi rst design has dimensions of 16mm x 9mm x 5mm (length x width x height) the new one has the size of 20mm x 10mm x 4.5mm (length x width x height) and the drop on demand printhead is still very handy. Fig. 5.Components of the redesigned drop on demand printhead for wax. (1) Aluminum housing with nozzle integrated (2) Piezo-element bender actuator with electrical contacting. (3) Pre-assembly of the piezo-element bender actuator and the housing. (4) Several manufactured printheads (5) Final assembly with cover plate. (6) Printhead attached above a 3D table for 3D-printing applications. All of the six exemplars of the manufactured printheads differ from each other only concerning the aluminum hous- ing. Whereas position of both the nozzle and the surface where the bender actuator is glued to stay the same, length of the nozzle and the distance between the nozzles inlet and the surface where the bender actuator is glued to are different. Table II lists the characteristics and in Fig. 6 they are illustrated. TABLE II CHARACTERISTICS OF THE NOZZLE GEOMETRY AND DISTANCE TO THE PIEZO-ELEMENT BENDER ACTUATOR DiameternozzleLengthnozzleDistancenozzleactuator 180m470m0m 280m460m10m 380m450m20m 480m440m30m 580m430m40m 680m420m50m Fig. 6. Simplifi ed sketch of he aluminum housing illustrating characteristics of the nozzle geometry and distance between the nozzles inlet and the piezo-element bender actuator integratend in the aluminum housing. The distance between the nozzles inlet and the piezo- element bender actuator actually is addditionally affected by the layer of glue. As the fi lling operation of the glue still is a manual process fi nal control of the distance is not possible yet. V. RESULTS Screwing the aluminum reservoir on the aluminum hous- ing and thus sealing the drop on demand printhead and attaching a heating element to melt wax makes the drop on demand printhead ready to use. A proof of functionality can be seen in Fig. 7 and Fig. 8 where a lot of small droplets have been printed on the same spot resulting in a much bigger drop. When applying a voltage of 40V all of the manufactured printheads where able to produce droplets. The results shown in Fig. 7 and Fig. 8 have been achieved applying an rect- angular signal at a frequency of 50Hz using the printhead no. 3 listed in Table II with nozzle diameter of 80m, nozzle lenghth of 450m, and a distance of 20m between the nozzles inlet and the piezo-element bender actuator. Sporadic meausrements of the size of droplets resulted in 120 diameters of 50m to 110m. The smaller droplets can be regarded as satellite droplets. Fig. 7. Photo taken with a microscope in sixfold optical magnifi cation showing multiple droplets of wax which form one single big drop. Sur- rounding the drop some much smaller satellite droplets can be seen. For fi rst tests to show the functionality a standard candle wax was used. The temperature was set to 80C and the distance of the nozzle to the printing table was set to 30mm. The big distance between nozzle and printing table explains the big area where drops fi nally striked on the printing table as small puffs of air lead to big defl ections. Fig. 8. Photo taken with a microscope in sixfold optical magnifi cation showing the same drop as in Fig. 6. The satellite droplets surrounding the big drop can be seen. Characterizing the generated drops concerning repro- ducibility, size, velocity, and other parameters dependent on nozzle geometry, applied voltage, and frequency was not object of this work. Main target was to proof the functionality and the working principle of the drop on demand printhead. VI. CONCLUSIONS This work is the continuation of Design, construction, and verifi cation of a printhead - tolerant towards bubbles - dosing liquid wax using rapid prototyping techniques 10 which was presented last year at AIM conference. The design and construction of this drop on