Measuring inkjet print resolution
Defining the quality of output from inkjet technology.
Over the past few years, digital inkjet technology has continued to push the boundaries of innovation with a steady stream of technological advances that has given inkjet the same levels of performance, cost effectiveness and efficiency of other digital and more traditional methods of print. As a result, high speed piezo drop on demand inkjet has become the most significant front-runner in the digital print sector in recent years, with several companies launching digital inkjet presses offering highly efficient, high quality output. What methods and technologies are being used within digital label presses to improve the quality of the printed labels?
This article was written by Philip Easton
Fundamentally, the basis of the print quality and productivity achieved from a new inkjet printer is largely determined by the type of print head that is integrated within the press and with that has emerged a new language to help define the quality of output. Typically, the most common methods of defining output quality relies on dots per inch (dpi), effective dpi and greyscale.
Dots Per Inch (DPI)
Print resolution, although not the only measure, remains one of the key methods of measuring printing quality. For those more familiar with flexographic technology, this is commonly quoted in the form of lines per inch. In contrast, with inkjet technology, whether measuring printing resolution for desktop devices or full scale commercial UV-curable presses, it tends to be quoted in Dots Per Inch or dpi.
Dpi defines the density of dots that can be printed in one inch to form printed text or graphics. The more dots printed per inch, the higher the print resolution of the image.
Closely related to dpi in terms of impacting and measuring print quality is the range of droplet size that is used in the printing process often referred to as greyscale capability. Today, all the latest inkjet print heads support greyscales. However, generally lower resolution (lower dpi) print heads will be printing more greyscales, with on average larger drops being used in order to create the required ink coverage.
Vertical v Horizontal
Printing resolution can also be different in the vertical and horizontal axes of the image. For an inkjet press, the dpi running perpendicular to the travel direction of the printed substrate is fixed. In this case, the dpi is defined by the number of nozzles per inch within the print heads printing the image. In contrast, when reviewing the dpi of the substrate travel direction, this is determined by a number of different factors including the jetting frequency of the print head, the number of greyscales being used as well as operational speed.
In instances where there are fewer greyscales being printed, while higher running speeds are possible, printing density and the consequent availability of colour range may be compromised. In addition, if the print resolution is halved in the material travel direction, it can be possible to double the speed recognising the print head has a defined jetting frequency. For this reason in some cases, dpi is defined in two axes for example as 600x300dpi. This means it prints at 600dpi in one axis and 300dpi in the other axis.
Print head technology
Two inkjet print head technologies being used within digital label presses are the Xaar 1001 and the Kyocera KJ4A.
The Xaar 1001 print head is used by most inkjet label press manufacturers. This print head has a native print resolution of 360dpi, typically operating at 25m/m with 360x360dpi and eight greyscales. Xaar has combined the greyscale capability with print resolution and quote “Effective” or “Apparent” print resolution. This is broadly calculated as the square root of the number of greyscales multiplied by the native dpi. Therefore, although the print head doesn’t actually produce 1,000 drops per inch, Xaar claim an “Effective” resolution of over 1000dpi. The print head has eight greyscales (a blank droplet where no ink is ejected is counted as one greyscale) with a drop size between 6pl and 42pl (pico litre).
The Kyocera KJ4A print head, and has a native resolution of 600pi, operating at 50m/m with 600x600dpi and four greyscales (five including the “no drop”). This would imply an ‘Effective’ resolution of 1,340dpi. Within one square inch, therefore, the Kyocera print head can deliver up to 360,000 individual drops compared to just 129,600 typically larger drops from the Xaar 1001 print head. The Kyocera KJ4A print head has four greyscales with a drop size of between 6pl and 14pl. With more droplets available for printing, the average drop size is much smaller. As a result, it is reasonably claimed that fewer greyscales are really required given the higher native resolution available.
There are, of course, many other factors that will determine the final quality of the printing output including registration control, printing density and ink flow for example. But ultimately, it is the underlying technology and capabilities of the print head that will determine print resolution and the level of detail that can be achieved in the printed image. Generally speaking, the higher the native resolution of a print head, the greater the detail that can be reflected in the image, although it is possible for lower native resolution print heads to partly compensate for this through offering more greyscales.
When reviewing print quality and considering what print resolution is best, it is recommended to print a range samples of images, text and graphics onto a number of different substrates to provide a full picture of what can be achieved and whether it matches the print quality required.
This article was first published in NarrowWebTech 1-2014