Customization of Inkjet Head Technology
The future for inkjet printhead development in terms of technology and business-management trends
The advancement of inkjet technology has taken the customization of graphics imagery to higher levels for large-format graphics. Increased speed, resolution, and software advancement for variable data have opened these doors to a multitude of marketing opportunities.
The next phase of development is the customization of the actual inkjet heads to optimize the manufacturing of specific applications. Printhead development leads to more throughput (faster), more up time (nozzles are staying open), pushing through the particular ink that makes large format special.
Piezo and thermal heads are the mainstays of the inkjet printhead world for the drop-on-demand market. The fine tuning of thermal heads resulted in an explosion in the consumer markets for desktop inkjet printers during the last 20 years, and it continues to this day. Continuous tone appearance can be had for under $100 in desktop units for use at home. This is quite an amazing occurrence, when our clients have first-hand experience in our digital print product, albeit in a non-production desktop device. Any way you look at it, print clients know what looks good.
Thermal heads, as the name suggests, use heat to create the droplet. Used by manufacturers such as Canon and HP (not Epson), this method is commonly referred to as bubble jet. In a thermal inkjet printer, small resistors create heat. The heat vaporizes ink to create the bubble. As the bubble grows, some of the ink is pushed out of a nozzle onto the substrate. When the bubble pops (collapses), a vacuum is created, pulling more ink into the printhead from the cartridge.
A typical bubble jet printhead has 300 or 600 tiny nozzles, and all of them can fire droplets simultaneously. The inks used are usually water-based (aqueous) and use either pigments or dyes as the colorant. Latex ink is in the aqueous category. The inks used must have a volatile component to form the vapor bubble (water), otherwise droplet ejection cannot occur.
The advantages of thermal inkjet technology include the potential for very small drop sizes and high nozzle density. High nozzle density leads to compact devices, lower printhead cost and the potential for high native print resolution. The disadvantages of the technology are primarily related to fluid limitations. Not only does the ink have to contain a material that can be vaporized (usually an aqueous or part-aqueous solution), but it must withstand the effects of ultra-high temperatures. Because no special materials are required, the printhead is generally cheaper to produce than in other inkjet technologies.
Relating to cleaning and clearing the head, the thermal head can use significant force to create a spurt to keep the head open. The piezo head does not clear in this manner.
Piezoelectric, patented by Epson, uses piezo crystals. A crystal is located at the back of the ink reservoir of each nozzle. The crystal receives a tiny electric charge that causes it to vibrate (resonate). When the crystal vibrates inward, it forces a tiny amount of ink out of the nozzle. When it vibrates out, it pulls more ink into the reservoir to replace the ink sprayed out. When nozzles refuse to fire, a technician may ask you to increase the voltage to the particular color/head.
Piezoelectric inkjet allows a wider variety of inks than thermal inkjet because there is no requirement for a volatile component, and no issue with kogation (buildup of ink residue). However, these printheads are more expensive to manufacture as a result of the use of piezoelectric material (usually PZT, lead zirconium titanate). UV and solvent inks tend to be the most common inks put through these heads. This electric charge is not enough to create that spurt alluded to earlier. With UV inks, UV lights are used to cure; therefore, care must be used to prevent ink curing inside the heads through reflection off the substrate.
There are two types of inkjet printing systems: scan- and single-pass printing. In scan printing, the printhead carriage moves in the sub-scanning direction (laterally in respect to the print media), making several passes to complete the image printing process. Piezo and thermal heads are applicable for this process. In single-pass printing, the printhead carriage remains in a fixed position and completes image printing in a single pass while the print material is conveyed underneath. Because of their high-speed printing capability, single-pass printheads are the piezo version, and systems are actively in use in the field of industrial printing where high productivity is required.
Trends in inkjet heads
Where are the trends going with inkjet heads? Talking to insiders, the goal of the digital print or inkjet world is to encroach continuously on the analog markets of litho, flexo, and screen printing.
Ray Work, Ph.D., of Work Associates, Inc., suggests that the ceramic tile industry will eventually be handled by digital printing. Inkjet printing is all but replacing the analog methods for decorating tile, and the return on investment with inkjet can be in just months, not years. The advancement is the relationship of inkjet heads and the inks to create a durable product. In essence, printheads can handle the specialized ink needed for ceramic printing.
Piezo heads can be, categorized as industrial inkjet heads. The cost at present is in the range of $1000 or more for each head. With arrays of heads in the hundreds, you can do the math to prove why some of the most robust presses cost more than hundreds of thousands of dollars.
Visually seeing the dots and noticeable banding are the elements that frustrate the digital print producer. Screen printing has crossed that bridge of resolution vs. production many times. As a solution to this issue, printheads are now able to vary the spot size within the jetting output, which fools the eye to simulate greater resolution, and hide banding. The result is the ability to keep speeds up, to give a continuous tone look, and to reduce banding.
Large-format roll-to-roll and flat bed printing took off with the advancements of the piezo heads. The physical drop size of the Xaar 1001 heads are in the 6 to 42 picoliter range, which is more than visually acceptable within the event and POP categories that we now print. This head has a print swathe of 70.5 mm (approximately 2.8 inches), and fires at 5kHz. Therefore, this seems to position itself in the high resolution, less production market for UV or solvent inks. Whereas Xaar’s Electron head is focused in the wide-format indoor/outdoor market with a faster 6.5kHz firing, and a 70 pl drop size— more speed, larger drop, and specifies solvent ink only.
Chris Lynn, VP sales and marking of Xaar USA, expanded on the company’s perspective on inkjet printhead developments. Xaar believes that piezo has several inherent advantages over thermal inkjet for industrial strength imaging, she says. They believe the heads are more durable and have a longer MTBF (mean time before failure), with life times measured in years and not weeks. They can impart more energy to a drop of ink, so larger drops and longer throw-distances are possible. And they can handle a greater variety of ink types and viscosities, making for a more flexible machine, which can handle a greater range of substrates.
Lynn continued saying that the crucial developments in printhead technology in recent years have been greyscale (variable drop size) printheads, which Xaar calls XaarDOT, and the advent of side-shooter printheads in which the ink flows past each nozzle orifice (Xaar’s TF technology). The Xaar 1001 uses both of these technologies. The through-flow of ink across the back of the nozzles provides a number of benefits: easy printhead fill, increased reliability and self-recovery, minimal maintenance, and therefore reduced ink wastage through purging. In addition, the internal design of the printhead for TF Technology facilitates easy cleaning and color/fluid changeover. The ink recirculation design allows the print head to self-prime and self-recover from jet-outs, leading to the high reliability needed for single-pass printing. It also allows inks with high solids (whites and metallics, for example) to be printed in a single pass.
Greyscale printing gives the high resolution and smooth tones of a small-drop print head while providing the high productivity of a large-drop head. We have seen this optical illusion of variable dot size in the analog world of halftone printing. Many will remember second order stochastic dot patterns in color separations.
Lynn continued, “Xaar expects the market to continue to demand low-price, low-volume wide-format printers, as well as high-productivity machines with an attractive total cost of ownership.” The first category will be mainly thermal inkjet technology with water-based inks, and the second will be ever-faster piezo technology with UV-curable inks.
Konica Minolta’s view is their piezo inkjet heads are able to handle a wide range of inks to meet the requirements of various industrial applications. Their principle of “shear mode piezo actuation” with shared wall structure allows lower power consumption as well as a high-density nozzle array. On-demand dot size modulation, sometimes referred to as grey-scale mode, enables the printing of high quality images and functions to increase productivity. All the development work is based on advanced computer simulation techniques that can precisely predict the droplet formation process resulting from differential ink characteristics.
The KJ4A Series technology by Kyocera, is designed for UV inks and other inks with high viscosity. The unique technology powering Kyocera’s piezoelectric actuator enables 600dpi x 600dpi image quality with up to five levels of grayscale (visually 1,200dpi) by controlling drop size.
Variable Size Droplet Technology (VSDT) is the description of Epson’s Micro Piezo printhead technology to eject ink droplets of different sizes by varying the electrical charge given to the piezoelectric elements. This enables Epson’s printers to achieve variable sized ink droplets as small as 1.5 picoliters. The trend for greyscale/variable dot technology is also a mainstay for Epson.
FujiFilm’s recently released that the Dimatix heads also show diversity for applications. Variable drop sizes assist in simulating higher resolutions. Q Class Polaris of Dimatix heads has the ability to jet two colors simultaneously with picoliter drop size of 15 to 200.
“Polaris supports aqueous ink formulations in addition to UV-curable, and aggressive organic solvents, making it suitable for a growing range of commercial and industrial printing applications,” the company said. Dimatix has touted their variable dot technology with VersaDrop. The Spectra line has the VersaDrop technology and repairable advantages, while the Performa line is used in single-pass applications.
As an example of producing heads that advance specific industries, FujiFilm/Dimatix created the Inspira product for larger particle size applications, such as direct-to-screen devices, ceramic printing, and textiles. Samba technology also incorporates VersaDot with native 1200 dpi resolution for single-pass applications.
Inkjet head advancements are creating more opportunities within the industries that are more related to litho and flexo, in the manner that affected the screen process a number of years ago. These are the collateral, label industries, and electronic boards. As for wide-format printing, the thermal heads are pushing its way back
on the radar screen with the advent
of latex inks.
Vince Cahill, President of VCE Solutions, has a great feel for the jumps in technology within the industrial inkjet sectors. He mentioned many industries taking advantage of the improvements, especially textile and label arenas, as well as 3D prototype applications. Cahill suggests that the speed advances are assisted by single-pass technology and the large array of heads.
The resultant speed is achieved by adding multiple heads in synchrony to create arrays of heads. For example, the HP TJ8600 has 150 heads to produce over 5000 square feet per hour of production with a 42 picoliter drop size. While the Inca Onset S70 is designed to perform at more than 6000 square feet per hour with 576 Dimatix heads (128 nozzles per head, therefore a whopping 73,728 nozzles that fire). The Vutek’s GS3250 press uses a 24 picoliter drop, and a speed of over 2400 square feet per hour.
HP introduced their latex printer a number of years ago and has continued to refine the ink formulation to create the required durable product for POP and outdoor imagery.
“It is the position of HP (Scitex wide format) that the utilization of both piezo and thermal technologies has its place,” says Tomas Martin, Scitex worldwide segment manager, large-format production, HP. Martin said that both technologies have a specific application for usage in large-format printers. The thermal heads fit high quality/high resolution printing with the dye/water-based inks, and stretch to added durability and medium volume printing with the latex technology within the water-based sector. Replacement thermal heads are said to be easily snapped into position by the print operator, though the piezo heads require a service visit for replacements.
In the HP products (and others), piezo is used for fast speed/longer run applications. The UV and solvent systems cannot print through thermal heads; therefore piezo must be used. HP makes their own heads, and focuses their technology for specific print applications. The thermal inkjet printhead for the latex printer contains two colors within one head; therefore three heads are required. The cost for a replacement head would be in the $600 ($1800 for a full complement) range, while the piezo head can run higher than $3000 ($18,000 to replace all six).
Technology keeps on jumping for inkjet heads as Konica Minolta announced recently that it has successfully developed a high-accuracy inkjet head capable of 1-picoliter drop size, the first for printed electronics applications. The new printhead uses the company’s proprietary MEMS (Micro-Electro-Mechanical Systems) technologies for the first time.
This development does not affect the classic large-format digital print market, though it shows where the technology can go.
My quest to find advancements in inkjet head technologies kept swaying the focus to the ink. As Work reiterated, screen printers tend not to print with one ink system for all applications and substrates, and we are asking inkjet to perform this minor miracle either, while going through a very small nozzle. Of all these trends, arguably the most important for adoption of inkjet technology, is increasing the range of jettable fluids.
As an example, in textile printing applications the use of sublimation dyes that volatilize at high temperature to migrate and bond strongly to the textile fabric to produce a water-washable image are less than desirable due to the ancillary heating and washing processes required. Using pigmented textile inks can remove some of these process requirements. Other examples are printing of ceramic inks, direct printing of conductive patterns by using metallic nano-particles, and printing of 3D plastic structures.
As technology advances, reality steps in. Using smaller and more accurately placed drops, clever image processing/ manipulation, and greyscale techniques, inkjet has reached the limit of what the human eye can differentiate.
If vanity is the resolution (drop size) of the print, producing a quality product on time and on budget is the true beauty of large-format printing. This product must have the durability required with the visual attributes for the viewing distance intended. Sound familiar? Screen printing battled the same issues with dot size for halftone printing. How do we produce a truck graphic that appears in continuous tone like litho printing? The race for fine-line printing was the vanity of screen printers. To produce such results, prepress costs rose, and the actual durability of the printed ink was in question with such small dots. Sounding more familiar? The new inkjet printheads have incorporated the variable drop size in their architecture.
Large-format inkjet printing seems to be at that same crossroads, though learning from its senior sister. Jettable inks for large format possess the similar attributes of the screen process, namely durability and the need for intense colors. White (and now metallics) compound the issue with their large particle sizes that must go through these minute nozzles. As with many processes, smaller requires advance engineering. As heads can produce smaller drops, the cost to manufacture such inks increase, as does the cost to manufacture the print heads. The smaller drop size opens markets for the equipment manufacturers, though it has little affected to the wide-format printer, other than that insatiable vanity.
Over the past number of years, the resolution of inkjet printers across all classes of productivity has been very good. The newer head technology for variable dot size within the print enhances the resolution. So, we can say that we are just fine with resolution, and now the push is the ability to reduce the cost of operation. Fast is great, though up-time, maintenance time and cost, ink cost, etc., are the next steps that the production houses have on the radar. As the drop sizes drop, the expense increases, and keeping those nozzles open is even tougher.
Cost of operation is being addressed. Heads that can handle the different ink technologies without added problems is important. Speed and smaller drop sizes are targeted to encroach on other analog methods—flexo, offset, and other industrial applications. Our large-format world lives and dies with up time, consistency, product durability, and ease of manufacturing. The more new head advances address cost of production, the more large-format digital printing will continue to thrive.