Novel Micro Piezo Technology for Ink Jet Printhead

Micro Piezo technology is offering superior features in ink ejection performance, compatibility with a wide variety of inks, and durability. As a result, this technology has already being utilized effectively in various commercial and industrial fields such as photo lab printing, digital printing, textile printing, and color filter manufacturing. However, new printheads that have a larger number of nozzles and higher performance are constantly required to improve printing speed or productivity, both for commercial products and industrial applications. We have succeeded to develop a next-generation Micro Piezo printhead, which has 360 nozzles-per-inch and piezo elements of 1 micrometer of thickness. In spite of the nozzle density doubled, this printhead retains all the advantages of the current Micro Piezo printheads, including the ability to produce variable sized droplets and the capability to eject ink droplets at high frequency. The installation of next-generation Micro Piezo printheads in printers and other devices will make it possible to design speedier and more compact inkjet equipments than ever before. In this paper, the key to improve the printhead performance and increasing nozzle density is studied. Then, the structure and characteristics of the next-generation Micro Piezo printhead are described. Introduction Today, ink jet technologies are currently attracting a great deal of attention as they are being put to use more and more frequently in printing and manufacturing processes. Ink jet systems have the advantages of being ideally suited to small print loads and of being gentler on the environment. Compared to other inkjet systems, Micro Piezo technology is offering superior features in ink ejection performance, compatibility with a wide variety of inks, and durability. As a result, this technology is already being utilized effectively in various commercial and industrial fields such as photo lab printing, digital printing, textile printing, and color filter manufacturing. The Micro Piezo printhead is not only suitable for functions ranging from consumer applications to commercial and industrial applications, but it also offers outstanding cost effectiveness due to its high degree of reliability and production volume efficiency. However, new printheads that have a larger number of nozzles and higher performance are constantly required to improve printing speed or productivity, both for commercial products and industrial applications. Structures of current Micro Piezo printheads Piezoelectric ink jet printheads have a mechanism that transfers piezo-striction to a volume change of the pressure chamber and this volume change induces oscillating ink flow that generates droplets from a nozzle opening. We had two types of the piezoelectric ink jet printheads in terms of the mechanism of the piezo transducer [1]. MLChips (Multi-Layer Ceramic with Hyper Integrated Piezo Segments) type MACH Figure 1. Structure of MLChips type MACH To meet the market demand for low-cost and high performance ink jet printers, EPSON developed MLChips MACH in 1995 [2]. The principle of the transducer, shown in Figure 1, is similar to the conventional piezo printhead, the piezo strips are not independently manufactured but processed and sintered together with the flow channel substrate as a single layered ceramics structure. It is strongly required to make both of the vibrating plate and the piezo strip thinner to improve the ink jet printhead for jetting fine droplets. By taking the process that unifies the core parts of the printhead without machining and gluing, the MLChips could develop the high cost performance printers by reducing the printhead dimensions. MLP (Multi-Layer Piezo) type MACH Figure 2. Structure of the MLP type MACH Another current Micro Piezo ink jet printhead is called MLP type MACH (Multi-layer ACtuator Head) developed in 1992 [3]. The MLP type MACH, shown in Figure 2, has multi-layer piezo 314 Society for Imaging Science and Technology elements sliced to thin pillar shape, one end of them is fixed to the base and the other end is connected to the vibrating plates of the pressure chambers. This piezo element gets shorter to extend the pressure chamber by the transverse piezoelectric effect when voltage is supplied. The MLP actuator can transfer the piezostriction directly to the displacement of the vibrating plate. Because of the displacement of the piezo element is proportional to the supplied voltage, the volume changing of the pressure chamber is also proportional to the supplied voltage. Therefore, ink droplet's property can be controlled easily using waveform control method [4]. Nozzle density of MLP type MACH is 180 nozzlesper-inch or npi. The key to improve printhead performance Basically there are 2 ways to improve printhead performance, one is increasing nozzle number and the other is improving the performance of each nozzle. Nozzle number can be increased relatively easily by making printheads bigger, or putting multiple printheads in a row. But these methods make a production cost higher and application products bigger. The way around the problem is putting more nozzles into a printhead by increasing nozzle density in each row. However, increasing nozzle density of MLP type MACH was difficult, because machine tools are used to slice piezo elements. On the other hand, MLChips had problems to increase nozzle density in terms of a complicated cavity structure and ceramics materials. Figure 3. Cross sectional diagram of an ink jet printhead. Cross-sectional area change is important parameter to measure nozzle performance. The piezoelectric ink jet printhead uses a mechanism that transforms piezostriction into a volumetric change in the pressure chamber. This volume change generates droplets from a nozzle orifice, and the volume change is determined from a crosssectional area change (shown in Figure 3) and an actuator length. In the case of increasing nozzle density simply, the displacement and the cross-sectional area change reduces certainly, because the cavity width gets narrow. Reducing the cross-sectional area change means reducing the performance of each nozzle, therefore the total performance of the printhead doesn’t improve. The key to improve the printhead performance is increasing nozzle density without reducing the performance of each nozzle. Actuator characteristic change by dimensions To estimate the effect of actuator dimension change, FEM model calculation was done. At first, the actuator performance by a cavity width change was estimated. The main condition used to this calculation is shown in Table1 and results are shown in Figure 4. When the cavity width becomes half, the displacement becomes approximately 1/4 and the cross sectional area change becomes approximately 1/8. It means the performance of each nozzle becomes about 1/8. Therefore, for the purpose of making nozzle density higher, there is no point to just making the narrower cavity or the actuator, without any other changes. Table1: Condition to simulate the cavity width property. Piezo elements thickness 4.0 micron Vibrating plate thickness 4.0 micron Electrode thickness 0.1 micron Cavity width Parameter Applied voltage 30 V Figure 4. Actuator characteristics vs. cavity width: When the cavity width becomes half, the displacement becomes approx. 1/4 and the cross sectional area change becomes approx. 1/8. Next, actuator performance by the thickness change was estimated. The main condition used to the calculation is shown in Table2 and results are shown in Figure 5. When thickness becomes a quarter the displacement and the area change become approx. 12 times. Accordingly, performance of each nozzle can be kept while making the cavity width narrower, by making the thickness thinner. Table2: Condition to simulate the thickness property. Piezo elements thickness Parameter Vibrating plate thickness Parameter, same as above Electrode thickness 0.1 micron Cavity dimension W55*D70*L840 micron Applied voltage 30 V 0 50 100 150 200 250 300