Technology
News on Digital Print Technology
| Monday, 01 September 2008 15:20 |
It’s A Head CaseWhat benefits do the new developments in print-head technology afford?Sophie Matthews-Paul provides the answers. The inkjet technique extends far beyond the boundaries of display print and the advances being made into print-head technologies are finding their uses being extended into any area where a liquid has to be jetted onto a material. Visitors to Drupa who walked past the Fujifilm Dimatix stand were able to enjoy eating biscuits that had an inkjet element in their decoration. It might seem to be a far cry from the wide-format sector, but the basic principles of application are much the same. Currently there are two main types of print-heads which dominate. First, there are thermal print-heads which incorporate a heater or resistor at the base of the ink channel, close to the nozzle. Once a voltage pulse is applied, the ink heats quickly to the point where it transforms from liquid to a vapour and a drop is forced through the nozzle. These print-heads are most commonly used with aqueous-based inks because of the heat requirements necessary, although HP’s latex ink also uses the company’s own wide scan thermal technology. Thermal print-heads are normally an integral part of the ink cartridge and have to be replaced every time the ink expires although this principle doesn’t apply to HP’s latest offering. More popular in the wide-format market, piezo-electric drop-on-demand print-heads use the generation of an electric charge to trigger the volume of ink to be forced through nozzle; the droplets are ejected mechanically from a nozzle in the ink chamber when a current impulse activates a piezo crystal. Unlike their thermal counterparts, these print-heads are suitable for all types of chemistries, including solvent-based and UV-curable inks, plus textile dyes. Piezo-electric print-heads are independent of the ink cartridge and, thus, do not need to be replaced when the ink expires. As well as drop-on-demand technology, continuous inkjet was employed in some of the earlier wide-format machines but, now, it is now found more commonly in the marking industry for product and packaging coding. A highly pressurised pump directs the ink from a reservoir through to a nozzle, creating a continuous stream of droplets. These ink droplets are exposed to an electrostatic field and, once charged, are directed to the material or to an area where the unused droplets are recycled. This method is also based on piezo-electric technology. With inkjet printing in all sizes being a thriving industry it has been essential for print-head developers and manufacturers to stay abreast of demands by being able to handle higher speeds and continual expectations in the quality stakes. Reliability is also of paramount importance and can be costly for the end user, with versatility requirements including the need to be able to handle all ink types ranging from aqueous-based, through all the varieties of solvents, to UV-curable. Additionally they’re expected to work with whites and varnishes. The numbers of physical print-heads present in a single machine varies tremendously and a good example at the high industrial end is the Inca Onset which has arrays of 24 print modules of six/colour with 24 Fujifilm Dimatix print-heads in each module, addressing a total of 73,728 nozzles. Because quality is determined by the performance of the inkjet nozzles, the extremely high speeds of the Onset need to take into account a high level of fault tolerance in order to maintain sustained performance to ensure optimum output standards. Most printers in the wide-format sector have more conservative numbers of arrays with the balance of speed and quality being made against cost and, even, size of the end unit. Marriages between machine, head and ink manufacturers and suppliers all play an important role in maintaining the relationships essential to keep equipment working as it should and future print-head developments rest largely on assessing what demands the market will make in years to come. The two differences which tend to come to the surface when questions are asked by types of print-heads are whether they are binary or greyscale. With the latter, variable sized droplets of ink are fired with the difference in the drop sizes working well across a variety of materials and ink types. Unlike binary print-heads, where the droplet size is consistent, the variable drops produced using greyscale technologies allows for an apparent resolution which is higher than the native resolution. Dot gain can be managed to get optimum results for different applications, depending on the size and type of the end display, and the various levels of greyscale determine the range of drop sizes in a given configuration. This also means that users can print with fewer passes without compromising the quality of text and other details. It’s the development of variable drop technology which has led to advances currently being made in single-pass printing. Xaar’s Mark Alexander has produced an excellent technical white paper on his company’s approach to single-pass printing and it’s available on the Xaar web site. Everyone is aware of the advances being made in this side of print-head technology and how it is now being incorporated into industrial and specialised ink-jet machines. Putting wider arrays into larger printers might not come under the single-pass category but, even so, this technique is being scrutinised by manufacturers because of the advantages to be gained. Fewer passes leads to higher speeds and, overall, lower amounts of general wear and tear engendered by any piece of equipment which has moving parts. An example can be seen in the Meital 3000-10 which was launched at Drupa and, to the uninitiated at first glance, seemed to be printing remarkably slowly. But this printer has been designed to use wide arrays of Xaar greyscale 1001 print-heads with the result that its operation is deceptively fast, with variable drop sizes ranging from six to 42 picolitres. Although this print-head, which incorporates Xaar’s proprietary hybrid side shooter technology, wasn’t really intended for the wide-format market, it has proved its versatility in this sector. It’s also to be found in single-pass web machines, such as the FFEI/Caslon label printer and the EFI Jetrion 4000, amongst others. Xaar says that much of its success has come from the flexibility and benefits of XaarDOT (drop optimisation technology), its range of drop formation options and its ability to fine tune them to specific applications. This gives printers the choice of drop size or resolution to use for a specific job, both in terms of image quality and substrate flexibility. It also means that, by tuning the options which are available, anything can be produced, from high quality photographic results for close-up indoor viewing, through to high productivity larger drops for exterior viewing as wide- and superwide-format signs and displays. The other name which tends to dominate the wide-format sector is Fujifilm Dimatix whose print-heads have been used widely over the years in a variety of printers and, again, this is a company which has been heavily involved in developing single-pass heads. Specifically for Drupa, the company built its XP-200 so that it could demonstrate the possibilities of printing direct in a single pass to unusual materials, as well those with irregular surfaces. These included sponge, sand-paper, leather, wood and PCBs (printed circuit boards). Slightly reminiscent of Bruce Forsyth’s “The Generation Game”, different objects passed on a moving belt and under the print-head at more than 60 m a minute, the idea being to promote the versatility of its print-heads which are just as at home working on unusual substrates as they are being used for the materials we’re all used to in the wide-format inkjet sector. Single-pass technology has also come to the fore with Fujifilm Dimatix with its Samba that uses the company’s own MEMS (microelectromechanical systems), along with its VersaDrop multi-pulse jetting and meniscus replenishment. Together, these allow the nozzles in the print-head to be arranged in a matrix array with improved ink recirculation. The Samba has already been announced as a strong reason behind the introduction of Fujifilm’s prototype B2 ink-jet press which uses a full width print-head array of 720 mm. Fujifilm Dimatix’s Q-Class platform also incorporates MEMS and is a hybrid construction comprising durable silicon and laminate carbon which is designed to make jetting inks at temperatures of up to 70 per cent degrees, making them suitable for both graphics and fluid deposition applications. HP started promoting its X2 piezo-electric print-heads for use with UV-curable inks back in 2006, wanting to bring through the benefits of using MEMS for high tolerance manufacturing, leading to uniformity and performance. The company assesses this as being important when using multiple arrays and its X2 is planned to appear in many future printers. This print-head has a chip with 128 nozzles which are fixed to an integrated circuit board. Ink is ejected from both sides (64 nozzles/side) at a rate of up to 30,000 drops/second/nozzle, and with drop volumes up to 50 picolitre. The nozzles have a native resolution of 100 nozzles/ inch and can be assembled into multi print-head configurations bringing resolutions of up to 800 dpi. HP says this is achievable because of the X2’s slim profile; the delivery end is 8 x 64 mm, making it suitable for use with multi print-head modules with very high nozzle density. The head assembly uses no adhesive, but relies on anodic bonding for fixing the silicon and glass, forming a permanent chemical bond and hermetic seal between the constituent materials. MEMS is a word which has become linked increasingly with inkjet printing although there are many industrial and consumer applications where it’s been making its presence felt for some time. Microelectromechanical systems comprise miniscule components and integrate elements, such as sensors and electronics, usually onto a silicon wafer which has properties to give high resistance to stress. These include mechanical tension and high temperatures, providing good overall tolerance and robustness. Incorporating MEMS into print-heads means that the resulting bigger, denser arrays can speed up throughput and improve quality and reliability, and there are several companies currently involved developments, such as HP, Fujifilm Dimatix, Kodak and Memjet. As vice president of marketing for Memjet Wide Format, Kevin Shimamoto has firm beliefs in the future of MEMS. “Memjet is a new technology that enables high-quality, high speed colour printing at break-through prices for both print engines and consumables,” he states. “The Memjet technology and related components – print-heads, ink, driver chips and software – is very flexible, making it applicable to many printing markets. We believe the exceptional combination of speed, quality and cost can create a significant impact within the wide-format market. “While there are other page-width printing technologies in the market, none of them delivers high-quality, 1600 dpi printing at 60 pages/minute in a low-cost format. One of the differences of the Memjet technology is the density of its nozzles and the < 1 mm print path. This simplifies paper handling complexity and lowers the overall cost of the technology,” Shimamoto continues. Xaar’s CEO Ian Dinwoodie has his own views: “When people talk about inkjet MEMS these days they are generally referring to silicon MEMS – ie the fabrication of inkjet actuators from a silicon wafer. Silicon is the substrate material used for thermal ink-jet heads, and is a very elegant fit for that technology. “However silicon is not a piezo-electric material; hence if you wish to create a piezo print-head you need to marry piezo elements with silicon processing,” he continues. “Xaar investigated this technology more than five years ago and concluded that the cost advantages claimed were dubious in all but mass volume markets. We also believe that the performance advantages claimed could be achieved via alternate methods. Hence we decided to continue to develop our ceramic MEMS technology; our Xaar 1001 product is presently being utilised in more than 50 developments world-wide.” Dinwoodie concludes: “Whilst there continues to be lots of talk about silicon MEMS, I don't see too many product launches. We don’t think that customers really care what substrate material print-head manufacturers use; I think they care simply about price and performance.” The introduction of its latex printing technology has also put HP in the news with its wide scan print-heads which stick with the company’s thermal technology, as demonstrated in its Designjet L65500. In this instance, it wasn’t chicken and egg as such because HP designed and developed both its latex inks and its wide scan print-heads together, complementing the company’s proprietary optical media advance sensor which maintains accuracy with longer material moves between swaths. Its wide scanning print-head arrays comprise five units containing two colours and ink and 1,200 nozzles/inch – or 10,560 nozzles/print-head. Manufacturers don’t necessarily stick with a single print-head supplier, however. Using Agfa as an example, a new platform was developed the piezo-electric Xaar Omnidot and Afga UPH (universal print-head) now in use in the company’s M-Press and recently launched Anapurna XLS. As well as being a Xaar official ink partner, Agfa’s Dotrix uses single-pass systems based on Toshiba Tec print-heads whilst the Anapurna XL and XL2 has Fujifilm Dimatix Spectra heads and the entry-level Anapurna M features Konica Minolta products. Others work with print-head manufacturers and tend to remain using the products around which their printers have been developed and, whilst some machine producers are happy to divulge whose technology is used others, for some reason I’ve never been able to understand, prefer to keep what’s under the bonnet as a guarded secret. It’s true that some machines might start off with one manufacturer’s heads and change mid-stream to another but, with today’s inkjet systems, it shouldn’t really make any difference to how the output is handled. When I took a first proper look at the Gerber Solara Ion at Drupa, there were pursed lips and head-shaking when I asked about the print-heads. When the engineer opened the lid, however, it wasn’t difficult to recognise the Konica Minolta KM512 as the product of choice to use with Gerber’s new cationic UV-curable flat-bed machine. Many of the print-head manufacturers out there today come under the title of being Xaar licensees. When the company was formed 18 years ago its intellectual property rights comprised four granted patents, plus a further 84 which were in the application process. Typically, its licensees are multi-nationals which manufacture large numbers of print-heads for use either in their own products or for OEMs, with names including Seiko, Konica Minolta and Toshiba Tec. In fact, it wasn’t until 2003 that Xaar actually introduced its first full production print-head, the Xaar 318, which incorporated patented greyscale technology and was manufactured in Japan by Toshiba Tec. This was followed by a raft of developments including the Xaar 760 which was designed to provide of choice of binary or dynamic variable drop sizes. What happens next? In terms of print-head developments, many of these are running parallel to printer manufacturers breaking into new market areas and complementing other products, such as inks, which require more refined jetting properties. One example is Epson which has announced that it is currently working on its Type-D print-head, moving the company into the UV-curable inkjet market. This follows through from its proprietary TFP (thin film piezo) technology which benefits from low vibration meniscus control to promote very accurate dot structures and shapes, as small as 3.5 picolitres, complemented by high precision impact-point control for absolutely accuracy of dot placement. It also benefits from a new ink repelling coating to prevent clogging of the nozzles. Whilst the principles of inkjet printing haven’t altered much during the past few years, the technologies behind it are continuing to change and improve. Markets are beginning to feed off one another with different developments, and the techniques used for packaging, labelling, proofing and others are learning as much from wide-format demands as the display sector is benefiting from advances being made in other print related and industrial segments. In the wide-format industry, the people using the machines probably aren’t too concerned about which print-heads are being used. What they do expect is consistent quality, reliability and speed. The print-heads of today have been instrumental in making inkjet the accepted process it has grown to be; tomorrow’s variants can only see improvements on what we’re achieving currently. 
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