Micro-Piezoelectric Head Technology of Color Inkjet Printer

Transcription

1 DPP2: International Conferene on Digital Prodution Printing and Industrial Appliations Miro-Piezoeletri Head Tehnology of Color Inkjet Printer Takao Mimura & Shinri Sakai SEIKO EPSON Corporation Shiojiri-shi, Nagano-ken, Japan Abstrat In reent years, the inkjet tehnology has made great strides in the print quality. By introduing fine droplets, deep and light olor inks, optimized media, and advaned olor and image proessing, the latest inkjet printers an produe photo-quality prints. Of these many improvements, the most ontributive one is that the innovative piezoeletri print head has redued the ink droplet size by a fator of ten in the latest five years. This innovation has been aomplished by optimizing head dimensions and driving signals as well as improving the manufaturing and assembling proess. The piezoeletri atuator driven by the signal, optimized for the partiular print head model, an preisely ontrol the unrestrained ink menisus vibration at the nozzle openings to ahieve multi-sized droplets ejetion in a sequent manner as well as stabilized droplet ejetion up to the extremely high ejetion repetition frequeny. This Variable-Sized Droplet Tehnology (VSDT) has made it possible to introdue the digital photo-printers to the market without trading off the throughput. This paper shows how the piezoeletri inkjet tehnology has been improved and how muh flexibility it has for partiular use in home, offie and industry. Introdution Today, the inkjet tehnology has made it possible to produe high quality digital photo prints that ompete against the silver halide photographs by the ooperation between the improvements of the omponents that onstitute the inkjet tehnology and to extend its appliation to wide and pioneering area by its ompatibility and flexibility with digital soures. Two types of the inkjet head tehnologies, one is using piezoeletri atuators and the other is using heater elements, are well known in the market. Beause they have different physis and mehanism, they also have been taking different approahes to produing fine droplets. Figure shows the history of the ink droplet size redution ahieved by the EPSON s inkjet printers sine its first market model in 984. Throughout the evolution of the piezoeletri print heads, EPSON faed several diffiulties in the development. Two innovative MACH tehnologies broke through the diffiulties in 992 and 995 and made it possible to ontinue to make the suessful progress. As is shown in Figure, the minimum droplet volume has been redued every year and now reahed 3pl. This rapid and drasti evolution, that redued the ink droplet size by the fator of ten in the latest five years and still ontinues, have been making revolutions in the printing tehnology. This paper will desribe the struture of the inkjet heads, the dynamis of the heads, the basi approahes to generating fine droplets, and the multi-sized droplet tehnology aomplished by preisely ontrolling ink motion of the EPSON s piezo-type inkjet printers. The appliation area of the piezo-type head will also be mentioned. Figure. History of ink droplet volume of EPSON s piezo type printers. Struture of the Piezoeletri Print Head The piezoeletri inkjet head has a mehanism that transfers piezostrition to a volume hange of the pressure hamber and this volume hange indues osillating ink flow that generates droplets from a nozzle opening. There are several types of the piezoeletri inkjet heads in terms of the mehanism of the piezo transduer. 23

2 DPP2: International Conferene on Digital Prodution Printing and Industrial Appliations Conventional Piezo Head The early models sine 984 used piezo transduers similar to the head in the referene. 2 Eah pressure hamber, sealed by a thin elasti plate, has a piezo strip that is narrower than the hamber width. The unimorph atuator (or vibration plate), omposed of the laminate of the piezo strip and the elasti plate, transfers piezostrition to flexural deformation of the vibration plate. Figure 2 shows a plane view of the head of this type. The nozzles in a line are onneted to the pressure hambers plaed with large spae by the long and winding hannels. The manufaturing proess, that puts and glues the eah piezo strip on the hannel substrate of plasti or glass, restrits the use of thin piezo elements thinner than about µm, beause the piezo material is very brittle and easy to break in the proess. As desribed later, this dimensional restrition limits the redution of the droplets and the heads. together with the flow hannel substrate as a single layered eramis struture. As desribed later, it is strongly required to make both of the vibration plate and the piezo strip thinner to improve the inkjet head for jetting fine droplets. By taking the proess that unifies the ore parts of the head without mahining and gluing, the MLChips ould break the barrier, whih the onventional piezo heads had, to develop the high ost performane printers by reduing the head dimensions. Figure 3. Struture of the MLP type MACH Figure 2. Conventional Piezo Head. MLP (Multi-Layer Piezo) type MACH To break through this limitation, EPSON developed new inkjet head with multi-layer piezo atuators alled MLP type MACH (Multi-layer ACtuator Head), and introdued new printer with MACH in The MLP type MACH, shown in Figure 3, has multi-layer piezo elements slied to thin pillar shape, one end of them is fixed to the base and the other end is onneted to the vibration plates of the pressure hambers. This piezo element gets shorter to extend the pressure hamber by the transverse piezoeletri effet when voltage is supplied. Beside the unimorph atuator, the MLP atuator an transfer the piezostrition diretly to the displaement of the vibration plate regardless of the pressure hamber size. MLChips (Multi-Layer Cerami with Hyper Integrated Piezo Segments) type MACH While having high performane, the MLP type MACH osts a lot to be manufatured. To meet the market demand for low-ost and high performane inkjet printers, EPSON developed another MACH in Though the priniple of the transduer, shown in Figure 4, is similar to the onventional piezo head, the piezo strips are not independently manufatured but proessed and sintered Figure 4. Struture of MLChips type MACH. Dynamis of the Print Head Understanding the motion of the atuator and the ensuing ink flow, whih are atually oupled eah other, and preisely estimating the parameters that define and haraterize the motion, the aurate design an be done to meet the fine droplet target. In this setion, the basi analytial models for the estimation of the inkjet head s response will be explained and the responses to the simple basi inputs will be shown. Then, the relations between the analytial models and design parameters, and the strategies to the design for generating miro droplets will be desribed. 23

3 DPP2: International Conferene on Digital Prodution Printing and Industrial Appliations Analytial Models for the Piezoeletri Inkjet Heads The hannel flows that interat with the atuator as inkjet heads do are well explained by the aousti model that takes volume veloity and pressure as independent variables. Eah omponent of the inkjet head, suh as nozzle, pressure hamber, and vibration plate, is expressed in terms of aousti impedane, omposed of inertane, ompliane and aousti resistane, and is put together in eletri iruit as a equivalent model of the inkjet system. Figure 5 shows the simple equivalent iruit model for the MLChips type MACH, whih uses unimorph atuator, and two major vibration modes. The mode in Figure 5 is the vibration mode of the pressure hamber onneted to the ompliane of the hamber that results in ink droplet ejetion from the nozzle. The mode in Figure 5() is the vibration mode of the ink supply flow onneted to the apillary fore at the nozzle menisus that manages the frequeny harateristis of the droplet ejetion. Figure 6 shows the equivalent iruit model for the MLP type MACH. The longitudinal vibration atuator of MLP has high rigidity and an atuate the vibration plate foribly. Add to the vibration modes of the pressure hamber (Figure 6) and the ink supply flow (Figure 6), there is the vibration mode of the MLP (Figure 6()). Menisus Control When a drive signal is applied to the piezo element, indued piezoeletri strain generates a driving fore and exites the above mentioned vibration modes. Of these vibrations, the mode of the pressure hamber and its osillation period T determine the harateristis of ink ejetion. Figure 7 shows the osillating flow at the nozzle of MLChips type MACH onneted to the vibration mode of the pressure hamber in response to a drive pulse of whih width is the same as the osillation period T. Figure 8 shows the response to a negative drive pulse of whih width is half of the T..5 M n M s Driving pulse Atuator C a M a C =C a +C i Nozzle hannel Supply hannel C n M n + M s C n M n C Chamber i M s () Figure 5. Equivalent model of the MLChip type MACH. M n M s C =C v +C i C a C n M n + M s M a () Figure 6. Equivalent model of the MLP type MACH..5 Figure 7. Push-Pull pulse and response. Time in absissa is normalized by T, and also voltage and flow quantity in ordinates are normalized by the pulse height and flow amplitude respetively. The drive pulse shown in Figure 7 exites the ink osillation that flows outward first at the preeding positive edge of the pulse, and pushes the ink at nozzle to generate a droplet. The following edge of the pulse exites the ink osillation that flows inward first. As the following flow is in opposite phase to the preeding flow, i.e. out of phase, both flows anel eah other and no osillation flow remains. This driving method is alled Push-Pull operation. In the ase of the drive pulse shown in Figure 8, the pulse width of that is half of the T, an ink flow exited by the preeding pulse (negative) edge starts to draw ink menisus inward first, and at the time just when the ink flow begins to turn its flow diretion, the following edge of the pulse generates another ink flow. Beause these two ink osillation flows are in phase, they reinfore eah other, then onstrut a large osillation flow, and generate a droplet twie as fast as the Push-Pull operation. This driving method is alled Pull-Push operation. 232

4 DPP2: International Conferene on Digital Prodution Printing and Industrial Appliations Figure 8. Pull-Push pulse and response Figure 9. Ramp step input and response. As well as the pulse width desribed above, rise-time or fall-time, in that time the pulse ramps up or down between two voltages, is also very important parameter. Compared with the sharp edge ase, if the rise-time or fall-time is equal to the T, no overshoot is observed in Figure 9. This pulse edge draws ink menisus to some amount without any residual osillations and is very useful for preise menisus ontrol. Approah to Fine Droplets Generation Generally, reduing the droplet volume auses slowing down the droplet speed. In the shuttle type inkjet printers, the inkjet head ejets droplets while moving fast, so the slower droplets degrade dots positioning auray and resultant image quality. Aordingly, the way to the faster droplets is exatly the way to the smaller droplets. The droplet veloity is estimated by integrating ink momentum going out of the nozzle and then dividing the figure of the momentum by the droplet mass. In the ase of the Push-Pull operation, integrating the flow, thik line in Figure 7, gets the droplet veloity V m as V m 2 π Q 4 TA = () where Q is the droplet volume and A is the area of the nozzle aperture. And the osillation period T is T 2π MC = (2) where C is the ompliane of the pressure hamber and M is total inertane of parallel onnetion of the nozzle and supply ink hannels. From Equation (), the smaller the osillation period T and nozzle beome, the faster droplets ejet. Although it helps the droplets to speed up, reduing the nozzle size auses logging up of the nozzles, so it is diffiult to make use of that. On the other hand, from Equation (2), the osillation period T is proportional to the square root of C, and as the C is strongly influened by the dimension of the vibration plate, reduing the hamber ompliane is very effetive to shorten the osillation period T. In priniple, the ompliane of the vibration plate is proportional to the 5 th power of the vibration plate width and is inversely proportional to the 3 rd power of the plate thikness. Add to this, in the ase of the MLChips type MACH, theoretially, the volume displaement of the vibration plate driven by the same voltage is proportional to the 3 rd power of the width and is inversely proportional to the 2 nd power of the thikness. By optimizing both width and thikness of the vibration plate, i.e. reduing both width and thikness, for the speifi droplet size, the onsistent design with the evolution of the droplet size an be made. Variable-Sized Droplet Tehnology It is evident that small droplets size needs the large number of droplets to over the whole media with ink. But the piezoeletri inkjet head is diffiult to redue the nozzle spaing and inrease the number of nozzle in terms of ost. So if only the redution of the droplet size is made, the print head would have to shuttle lots of times and this would result in a large drop in the throughput. The VSDT (Variable-Sized Droplet Tehnology) an overome this diffiulty, ejeting differently sized ink droplets in one sanning. The number of sanning is defined 233

5 DPP2: International Conferene on Digital Prodution Printing and Industrial Appliations by the largest droplet size the head an ejet, not by the smallest. Figure shows the VSDT applied to MLP type MACH. 5 The pulse pattern from the ommon pulse generator, applied to eah piezo transduer, has two driving pulses in one yle, orresponding to the pixel grid point. When the swith of the piezo transduer selets the pulse part2, small droplet of 3pl would ejet and form a small dot (Figure [C]). When the pulse part3 is seleted, large droplet of pl would ejet and form a middle dot (Figure [D]). In the ase that both pulses are seleted, the large droplet would be larger than the ase D by the effet of the preeding small dot ejetion, two droplets would ejet and total ink volume of 9pl would form a large dot (Figure [E]). veloity. While the simple Pull-Push operation would ejet droplets twie as fast as the Push-Pull operation, by taking large Pull and small Push, the smaller droplet ejetion was made possible. Also in the VSDT, new idea of the Pull-Push-Pull operation, whih is the ombination of the Pull-Push and the Push-Pull, is implemented. The last Pull is set to anel the previously generated flow like the Push-Pull operation. The response to this omplex waveform an be easily understood by the superposition of the osillations generated by eah pulse edge. Evolution of the Piezo Inkjet Tehnology As above desribed, the piezoeletri inkjet head an preisely ontrol the ink menisus motion and hange the droplet volume by means of the driving signal. This ontrollability made possible to adapt the inkjet heads to plenty of ink and media ombinations and apply them to from the photo quality home print to the professional large format and arts. From the viewpoint of the droplet size, if only a small droplet is pursued, it is easy to establish a new reord. On the other hand, piezoeletri inkjet head an aept a variety of inks other than water base inks. These potentials an lead the piezo inkjet head to the new appliation area of the marking tehnology. Conlusion The piezoeletri inkjet heads has been evolving at marvelous speed, introduing new atuators. The high linearity of the atuators has given the high ontrollability, and the design flexibility and ultimate optimizing tehnique established appliability of the piezo inkjet tehnology to the all sort of digital printings. Referenes. M. Fujino, Pro. SPIE, Vol. 3963, pp (2). 2. E. L. Kyser, S. B. Sears, US patent 3,946,398, (976). 3. T. Kitahara, Pro. IS&T s th International Congress on Advanes in Non-Impat Printing Tehnologies, pp (995). 4. M. Usui, Pro. IS&T s 2 th International Conferene on Digital Printing Tehnologies, (996). 5. T. Kitahara, Japan Hardopy 99, pp (999). Biography Figure. Driving waveform of MSDT In the VSDT, the drive waveform is designed so that the droplets with different sizes would have about the same Takao Mimura is a general manager in the Inkjet Printer Design Dept. of SEIKO EPSON Corporation. He worked on a printer hardware design of SIDM(Serial Inpat Dot Matrix Printer) for ten years and urrently is working on a LFP (Large Format Inkjet Printer) produts. His primary responsibilities are produt planning and design management of LFP. His reent interest is a tehnology shift of photographi market. 234