Dr Sanjiv Kamat & Shyam Phadke
Kothari Infotech Limited
In the understanding of digital printing, it is an essential step to get basic knowledge of printheads. In the first two articles of this series, we have seen the importance and historical aspects of digital printing of textiles. In this article we shall study the essentials of printhead technology. Two key methods type of printheads namely; Continuous Ink Jet (CIJ) and Drop On Demand (DOD) are discussed in detail.
THE Printhead is one of the important parts of the digital printing machine. It would not be an exaggeration, to say that, the printhead is the heart of the digital printer and the software the brain. Digital printing has made significant progress in many market segments like DTG, soft signage, sportswear and recently apparel. One of the main reasons for this success is the performance of the printhead. The printhead determines the speed of production and overall print quality. Undoubtedly the other components like software, ink, etc are important but the one that has paramount value is the printhead. The functioning of the printheads is based on two fundamental principles namely, Continuous Ink Jet (CIJ) and Drop-on-Demand (DOD). Fig. 1 depicts ink jet technology tree.
Continuous Ink Jet
In CIJ, the ink is passed through nozzles at a constant speed by applying a constant pressure. The jet of ink is naturally unstable and breaks up into droplets shortly after leaving the nozzle. The drops are left to go to the substrate or deflected to a gutter for recirculation, depending on the image being printed. The deflection is usually achieved by electrically charging the drops and applying an electric field, to control the trajectory. Since the drops are ejected at all times, hence the name continuous. In the traditional CIJ approach, a piezoelectric transducer is coupled to the print head to provide the periodic excitation. The oscillations are therefore mechanical in nature. There are two ways of deflecting the drops in piezoelectric-driven CIJ. In the binary deflection method the droplets are directed either to a single pixel location in the substrate or to the recirculating gutter. In the multiple-deflection method the deflection is variable so the drops can address several pixels. Fig. 2 demonstrates schematic of CIJ setup.
In CIJ the jet of ink generated by each nozzle breaks up into droplets shortly after exiting the nozzle. Without any other intervention, the breakup would occur randomly and would result in droplets of variable sizes. The combination of the jet velocity and frequency of the excitation determines the droplet size, which can be controlled to very high accuracy.
About the Authors
Dr Sanjiv Kamat a post graduate from UDCT Mumbai and a PhD from the University of Leeds, worked with Sandoz and then with Clariant in various capacities. He then was with Pidilite as Chief Marketing. Dr Kamat has been honoured with the Gold Medal of the Society of Dyers and Colourists, UK, of which he was the President for the year 2012-13. He is also a member of the editorial board of Colour Index, for Pigments. Presently he is the Vice President with Kothari Info Tech Limited.
Mr. Shyam Phadke , a Post Graduate from Institute of Chemical Technology, is presently Marketing Officer at Kothari Info Tech Limited. Kothari Info tech Limited, (KITL) is a technology company based in Surat and has been in this business for over a decade and known for our innovative products. KITL have made significant investments in R&D covering the field of ink chemistry, RIP and colour management software. Kothari Infotech Limited started its journey in the technology space with the Print Pro™ Software which is patronised globally by leading players in digital printing of "fabrics as well as garments". We specialise in Charu series of water based inks, our Reactive inks are GOTS version 3 certified and are suitable for almost all the major industrial print heads and have been accepted by OEMs. Furthermore, with our Disha pre-coats, we are able to offer a complete chemistry solution to the textile digital printer. In other words KITL is an organisation having the capacity and capability of handling all problems in Digital Printing of textiles.
The drops not intended to reach the substrate are charged and deflected to a gutter. The printing drops are given a smaller charge to prevent them from merging in flight.
Drop-on-demand piezoelectric ink jet
Fig. 1: Ink Jet Technology Tree
In DOD, drops are ejected only when needed to form the image. The two main drop ejector mechanisms used to generate drops are Piezoelectric ink jet (PIJ) and Thermal Ink Jet (TIJ).
Thermal Ink Jet
In TIJ an electric heater is typically built inside the nozzle, usually by microelectronic device fabrication techniques. A current pulse is allowed to flow through the heater to quickly raise the temperature of the ink in its vicinity to over 300°C. This causes a vapour bubble to violently nucleate and expand, ejecting an ink droplet through the nozzle orifice. Water tends to cause more explosive bubble growth than other solvents. For this reason, TIJ favours water- based inks. The TIJ process resembles an explosion. Once the bubble nucleates and starts expanding, there is no point in continuing to provide power to the heater because the bubble is a poor thermal conductor. Thus, the pulse is usually tailored to stop shortly after bubble nucleation. As the bubble expands it cools and its pressure (which starts at over 70 atmospheres in water based inks) drops quickly. The bubble reaches its maximum size and then, just as violently, it collapses, retracting the meniscus to a region inside the channel. After the bubble collapses, capillary action drives the refill process, which continues until the channel is full again, ready to fire. Because of its explosive nature, there is little control over the process beyond the pulse length and power applied. Techniques of providing a short pre-pulse (or train of pre-pulses) to pre-warm the ink in the vicinity of the heater are sometimes used. With these techniques, one can control or modify in a limited way the total ejected ink volume. There are several configurations of TIJ drop ejectors, the most common being the roof-shooter' and side-shooter' types. These are as demonstrated by Fig. 3 . In the roof-shooter' type shown, the plane where the heater resides is parallel to the nozzle plane. In the side-shooter' type, the nozzle plane is perpendicular to the heater plane.
Fig 2: CIJ Set Up
Piezoelectric Ink Jet
In PIJ, the volume of an ink chamber inside the nozzle is quickly reduced by means of a piezoelectric actuator, which squeezes the ink droplet out of the nozzle. In piezoelectric ink jet, mechanism used to generate the droplets is a piezoelectric element, typically made of lead zirconium titanate (PZT). Depending on the architecture of the head, the piezoelectric transducer could be attached to a membrane that forms an ink chamber wall or could actually constitute the chamber itself. In either case, when a voltage is applied to the electrodes of the piezoelectric element, the volume of the chamber is typically reduced, which results in a droplet of ink being squirted out of the nozzle. PIJ print heads are sometimes subdivided in different classes according to the geometry of the drop ejector and/or how the piezoelectric element operates. The classes, shown in Fig. 4 , are shear mode', bend mode', push mode', squeeze mode'.
Fig. 4: DOD (a) Shear Mode (b) Bend Mode ( C ) Push Mode (D) Squeeze Mode
In shear mode ink jet, the electric field is perpendicular to the poling direction of the piezoelectric material. In bend mode piezoelectric ink jet, the electric field and poling directions are parallel. In the push mode piezoelectric ink jet, the electric field and polarization vectors are also parallel but the membrane is placed in the expanding direction of the piezoelectric material. In the squeeze mode the drop ejector is a hollow tube of piezoelectric material. Upon the application of an electric field, the inside volume of the tube (firing chamber) decreases its radius and ejects the ink in the direction of its axis.
(a)(b)Fig 3: (a) DOD Thermal Ink Jet Roof Shooter (b) DOD Thermal Ink Jet Side Shooter
Fig. 5 shows how some of the printheads would look like. As discussed above though there are predominantly 2 main technologies for drop formation used by the print heads, namely Thermal and Piezo; the textile industry generally uses water based piezo heads. Since textile printing is an industrial application these heads need to be robust. These heads from different makers may differ from each other on the number and size of nozzle, their placement, method of drop creation and jetting frequency.
Future developments in printheads:
Line head has long been discussed but challenges remain in the implementation. To ensure print quality a lot of nozzle redundancy needs to be built in, coupled with a vision based inspection system, which surely would come at a cost. Comparatively, the scan method is robust, in order to improve speed, the one pass method is gaining importance. The speed could be increased by increasing the number of nozzles per colour or per head. An increase in the swath size / length of the print-head will also help to increase the printing speed. Additionally print heads capable of operating at higher jetting frequencies will help increasing the speed of printers.
What is a picolitre(pL)?
A picolitre is 1/trillionth of a litre, or 1 x 10(-12)litre. If you were to think of a 1 litre bottle, and divide it into 1 million drops, then divide each of THOSE drops into 1 million drops, then the single newly formed entity is a picolitre. An average raindrop would be equal to hundreds of thousands of picolitres.