High-speed inkjet production printers all use a form of single-pass printhead technology. Mark Alexander from Xaar explains how this new technology will help to make single-pass ubiquitous.
Until recently digital printing has not been able to achieve the speed, quality and reliability delivered by commercial printing techniques such as offset, litho or flexography. This is changing rapidly as a new generation of innovative inkjet printing systems come to market. They incorporate variable drop piezoelectric inkjet printheads that can deliver – in a single pass – both quality and high speed.
The new generation of piezoelectric inkjet printheads, such as those produced by Xaar, are having a major impact on the market and many new high-quality, single-pass inkjet printers featuring Xaar printheads for a wide range of applications were launched at drupa and Labelexpo earlier this year.
What these fast inkjets have in common is that they use single-pass configurations, with fixed, non-reciprocating arrays of heads that cover the whole print width, so the paper or other media is simply moved underneath them and the complete image is built up in the same pass. This has potential for very high speeds, but it has huge challenges in terms of reliability and print quality.
The most vital factor in image quality is the intended viewing distance and the point of resolution for the application. Resolution can mean slightly different things to different people. Here it refers to the ability to see, or “resolve” the individual dots or pixels that make up an image.
In addition to resolution it is necessary to consider tonal range, the number of shades or tones that go into making up a print. Desktop and wide format photo-quality inkjets all need a scanning printhead with multiple passes to build up their high resolutions. This is not productive enough for commercial printing, or digital overprinting in-line with a conventional press.
Xaar’s greyscale technology solves the quality issue by varying the size of each ink droplet. Even with only six grey levels one can see the difference in print quality, and latest Xaar 1001 head can print up to 16 levels. However, we reckon that only six to eight levels of greyscale are needed to achieve the high quality and smoother tones needed for single-pass printing applications at 360 dpi.
To reproduce a true greyscale image it is necessary to vary the printed dot size. This is done by building up each printed dot from a variable number of sub-droplets generated by the printhead. We use an acoustic wave method to achieve high frequency multi-pulse drop formation. By modifying the waveforms and therefore the droplet formation, inkjet printing can be finely tuned to suit the particular application. Smaller droplets also mean faster drying, an important issue in single-pass printing.
An unique capability of our technology is what we call XaarDOT, which allows integrators to choose whatever droplet size best suits their applications, ranging from very small droplets needed for close-up viewing or true photographic quality, through to larger droplets for high speed printing of work that isn’t intended for close-up viewing, such as billboards.
Long life and reliability
With a multiple-pass printer, the overlapping swathes of ink tend to cover up the effect of any blocked nozzles so one can keep on printing until he can clean the heads. With single-pass printing, a single missing nozzle might leave a noticeable lighter stripe or line in the printed image. At present the solution adopted by most printer manufacturers has been arrays of multiple heads to provide redundancy and compensate for blocked nozzles. And Xaar’s solution being an unblockable nozzle, there’s less need for back-up.
Resistance to blocking is achieved with the Xaar TF Technology (Through Flow) which is part of the architecture of latest Hybrid Side Shooter (HSS) head design. TF Technology provides ink through-flow and when combined with the side shooter architecture is a significant step forward towards running inkjet systems reliably for a full shift with minimal maintenance. The first HSS head in production is the 1001, being built in our new Huntingdon factory since last year.
With HSS the ink flows right past the back of the nozzle. Two acoustic waves moving through the channel meet in the middle and cause pressure changes which fire a drop out of the side of the channel i.e. downwards through the inkjet nozzle. Due to the TF Technology, any air or particles are moved away from the nozzles, keeping the channel primed and operational.
The key with single-pass printing is the ability to achieve full coverage in just one pass. The beauty of variable drop is that one can achieve the high image quality with fine detail and good text one needs combined with the productivity benefits of sheet or web fed production. The first single-pass, narrow web, inkjet systems are coming onto the market for label and transaction applications. Once they are operational, the true potential of inkjet printing can be exploited: high quality full colour variable data printing for personalised marketing, transactional and version applications and high- speed, short run production.