Category: ScreenPro Core

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What causes banding in inkjet? (And the smart software solution to fix it.)

Banding, or non-uniformity, is a common problem in inkjet printing that can often result in print production downtime and loss of revenue. In this post, I’ll discuss the challenges printer OEMs and print service providers face when trying to reduce banding and provide an insight into the work we’ve been doing at Global Graphics Software to remove banding and streaking artifacts from the print output, enhancing print quality and raising productivity.

What causes banding in inkjet?

Inkjet printheads produce variable density output both across an individual printhead (known as the inkjet ‘smile’) and when comparing output from one printhead with another. The output from a printhead can also change with time, as the printhead wears or ages. Additionally, the overlapping stitch area between printheads in a single-pass printer, or between overlapping passes in a multi-pass printer, can also cause density variations. Such variable density becomes visible in the printed output as ‘banding’ and ‘stripes’, and means that it is not possible for print providers to digitally print jobs with certain image features (such as flat areas or gradients), or that they must sell the lower quality output produced at a significant discount.

Why is uniformity in inkjet a challenge?

Fixing banding or streaking in inkjet is not without its challenges:

  1. In the printer design phase, the use of interlacing in the printing process can be effective at reducing banding and improving uniformity, but significantly impacts the speed and/or cost of the printer. This approach is especially undesirable in single-pass systems, where the only option to interlace is by doubling the quantity, and hence cost, of printheads in the printer.
  2. Currently most OEMs attempt to correct uniformity issues with hardware solutions such as drive voltage tuning, but these give only limited improvement and are slow, complex and costly to implement. Most printheads have only limited voltage adjustment for banks of many nozzles together, or even the entire printhead as a whole, and do not allow adjustment of drive voltage for individual nozzles – such adjustment does not have the granularity necessary to really eliminate banding. Additionally, adjusting drive voltage to balance output density (drop volume), is undesirable as this is likely to negatively impact drop velocity, printing reliability (jetting stability) and even printhead lifetime. As the printer performance changes over time, and when printheads are replaced, service and support engineers must spend a significant amount of time onsite re-making these complex adjustments to achieve quality that is, at best, a compromise.

A solution in software

Global Graphics Software has been working with printer OEMs and print service providers to significantly enhance the quality of their inkjet output, one such company being Ellerhold AG, a leading poster printing house and press manufacturer in Germany.

Ellerhold wanted to enhance the printing quality of it’s large-format posters. Specifically, the printheads on its digital printing machine showed variation in printed density both between the heads and across each head, which produced clearly visible bands within some types of printed output.

Together with Ellerhold we were able to enhance the quality of the printed output using our ScreenPro™ screening engine with PrintFlat™ technology. ScreenPro is a very fast and efficient multi-level screening engine that mitigates artifacts such as banding or streaking and mottling from the inkjet print process and can be used in any print workflow, including Adobe®, Caldera, Esko, EFI and Sofha, with any combination of inks, substrates, printheads and electronics. In ScreenPro every nozzle can be addressed separately on any head/electronics to achieve very fine granularity. The PrintFlat technology adjusts the density within ScreenPro to produce uniform density across a print bar, thereby optimizing print quality.

The project brought many technical challenges: As it was a multi-pass process we needed to efficiently capture repeating density variations across the entire print area in an unbiased way. We carried out tests, analyzed the scanned prints and created a PrintFlat calibration workflow for the press designed to compensate for the non-uniformity in output across the print bar. The team also used a variant of Global Graphics Software’s Advanced Inkjet Screens™, available with ScreenPro and the Harlequin RIP®, which they adapted specially for scanning-head systems. These proved very effective.

You can watch the short case study film here:

PrintFlat technology provided the ideal solution, giving smooth, uniform tints and accurate tone reproduction via a simple ‘fingerprint’ calibration of the screening process, where the density compensation is then built into the screen halftone definition. This means that the PrintFlat calibration is applied during the screening process at runtime and enhances the quality of your output without any compromise on speed. The PrintFlat approach addresses every individual nozzle, has no negative effect on other printing parameters, and allows drive voltage to be used to maximize printing stability and reliability instead.

A valuable additional benefit is in increasing overall productivity. Achieving higher quality with fewer print passes allows for greater use of faster print modes. Jobs that require 4-pass quality can be printed in 2-pass mode with PrintFlat.

The process can be automated for closed-loop correction and, unlike correction by adjustment of voltages, there is no effect on jetting stability or head lifetime, nor ink pressure and timing/drop speed variation.

PrintFlat can increase the added value of your service engineers’ visits, producing a much higher quality result in less time. Alternatively, the print service provider can operate the PrintFlat calibration process to maximize their output quality themselves.

Sunflower web image before PrintFlat is applied Sunflower web image after PrintFlat is applied.Before and after images illustrating how effective PrintFlat technology is at improving print uniformity.

 

For more information about PrintFlat technology visit: https://www.globalgraphics.com/technologies/printflat

For further reading about the causes of banding and streaking in inkjet output see our related blog posts:

  1. Streaks and Banding: Measuring macro uniformity in the context of optimization processes for inkjet printing

  2. Where is screening performed in the workflow

About the author

Jimmy Fox, Inkjet Printing Technologist, Global Graphics Software
Jimmy Fox, Inkjet Printing Technologist, Global Graphics Software

Jimmy Fox is an inkjet printing technologist with 25 years’ experience of developing inkjet printers, inks and applications.

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How to accurately calculate the ink costs for your digital press

There are many costs that can impact your profitability when running a production digital press, from power consumption to the substrate you’re printing on. One of the most variable costs is ink consumption, which often varies from job to job and therefore can be difficult to estimate. As you might expect, the content to be printed is the key determining factor, but you also need to consider the resolution, screening method, drop sizes and choice of colorants. This can bring quite a challenge for a press shop when quoting for a job, especially if the client is open to hearing a range of options.

Even with a static job that might be suitable for a test print run to get a cost that can be multiplied for the number of copies, it’s still not ideal to have to spend any time or other resources using the actual press. It’s much better to be able to get an accurate ink cost estimate away from the press, which is where our Job Cost Estimator comes in. It’s available as part of our Direct™ software range as well as our Harlequin Host Renderer™ and ScreenPro™ products. It uses the same setup that drives your printer, calculating a very accurate estimate of the ink cost for a specific job. Self-contained, it doesn’t require any connection to your printer, which makes it ideal when you want to give a job cost indication away from the print shop.

The screenshot shows a calculation performed using our Job Cost Estimator for a 1200x1200 dpi version of our two-page Direct brochure, screened with 4-drop pearl.

The screenshot above shows a calculation performed using our Job Cost Estimator for a 1200×1200 dpi version of our two-page Direct brochure, screened with 4-drop pearl. Under Cost Per Page, this is the average cost per page per colorant based on the two pages that were analyzed, with a final row showing the total (All). This is then multiplied by the total pages and the number of copies to get the Cost Per Job for each row.

Obviously, no costs can be determined without knowing how much the inks cost per liter, so you can set these within the application. Similarly, you will need to configure your printhead(s) to specify how many picoliters of ink are used per drop size.

As you can see from the left image above, we have assigned a different printhead for Black called Budget_PrintHead, which will have fewer picoliters per drop size than the Default_PrintHead shown on the right, to represent a possible response to a hypothetical jump in the price of black ink.

The Job Cost Estimator has been designed to be extensible, so it would be possible in future to incorporate other costs, such as paper, or factor in ink used periodically for nozzle refreshing, for example.

If you’d like to know more about the Job Cost Estimator, watch my short demonstration here:

For more information visit the Direct pages on our website: globalgraphics.com/direct

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About the author:

Ian Bolton, Product Manager, Direct
Ian Bolton, Product Manager – Direct

Ian has over 15 years’ experience in industry as a software engineer focusing on high performance. With a passion for problem-solving, Ian’s role as product manager for the Direct range gives him the opportunity to work with printer OEMs and break down any new technology barriers that may be preventing them from reaching their digital printer’s full potential.

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Where is screening performed in the workflow?

In my last post I gave an introduction to halftone screening. Here, I explain where screening is performed in the workflow:

 

Halftone screening must always be performed after the page description language (such as PDF or PostScript) has been rendered into a raster by a RIP … at least conceptually.

In many cases it’s appropriate for the screening to be performed by that RIP, which may mean that in highly optimized systems it’s done in parallel with the final rendering of the pages, avoiding the overhead of generating an unscreened contone raster and then screening it. This usually delivers the highest throughput.

Global Graphics Software’s Harlequin RIP® is a world-leading RIP that’s used to drive some of the highest quality and highest speed digital presses today. The Harlequin RIP can apply a variety of different halftone types while rendering jobs, including Advanced Inkjet Screens™.

But an inkjet press vendor may also build their system to apply screening after the RIP, taking in an unscreened raster such as a TIFF file. This may be because:

  • An inkjet press vendor may already be using a RIP that doesn’t provide screening that’s high enough quality, or process fast enough, to drive their devices. In that situation it may be appropriate to use a stand-alone screening engine after that existing RIP.
  • To apply closed loop calibration to adjust for small variations in the tonality of the prints over time, and to do so while printing multiple copies of the same output, in other words, without the need for re-ripping that output.
  • When a variable data optimization technology such as Harlequin VariData™ is being used that requires multiple rasters to be recomposited after the RIP. It’s better to apply screening after that recomposition to avoid visible artifacts around some graphics caused by different halftone alignment.
  • To access sophisticated features that are only available in a stand-alone screening engine such as Global Graphics’ PrintFlat™ technology, which is applied in ScreenPro™.

Global Graphics Software has developed the ScreenPro stand-alone screening engine for these situations. It’s used in production to screen raster output produced using RIPs such as those from Esko, Caldera and ColorGate, as well as after Harlequin RIPs in order to access PrintFlat.

Achieve excellent quality at high speeds on your digital inkjet press: The ScreenPro engine from Global Graphics Software is available as a cross platform development component to integrate seamlessly into your workflow solution.
Achieve excellent quality at high speeds on your digital inkjet press: The ScreenPro engine from Global Graphics Software is available as a cross platform development component to integrate seamlessly into your workflow solution.

The above is an excerpt from our latest white paper: How to mitigate artifacts in high-speed inkjet printing. Download the white paper here.

For further reading about the causes of banding and streaking in inkjet output see our related blog posts:

  1. Streaks and Banding: Measuring macro uniformity in the context of optimization processes for inkjet printing

  2. What causes banding in inkjet? (And the smart software solution to fix it.)

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What is halftone screening?

Halftone screening, also sometimes called halftoning, screening or dithering, is a technique to reliably produce optical illusions that fool the eye into seeing tones and colors that are not actually present on the printed matter.

Most printing technologies are not capable of printing a significant number of different levels for any single color. Offset and flexo presses and some inkjet presses can only place ink or no ink. Halftone screening is a method to make it look as if many more levels of gray are visible in the print by laying down ink in some areas and not in others, and using such a small pattern of dots that the individual dots cannot be seen at normal viewing distance.

Conventional screening, for offset and flexo presses, breaks a continuous tone black and white image into a series of dots of varying sizes and places these dots in a rigid grid pattern. Smaller dots give lighter tones and the dot sizes within the grid are increased in size to give progressively darker shades until the dots grow so large that they tile with adjacent dots to form a solid of maximum density (100%). But this approach is mainly because those presses cannot print single pixels or very small groups, and it introduces other challenges, such as moiré between colorants and reduces the amount of detail that can be reproduced.

Most inkjet presses can print even single dots on their own and produce a fairly uniform tone from them. They can therefore use dispersed screens, sometimes called FM or stochastic halftones.

A simple halftone screen
A simple halftone screen.

 

A dispersed screen uses dots that are all (more or less) the same size, but the distance between them is varied to give lighter or darker tones. There is no regular grid placement, in fact the placement is more or less randomized (which is what the word ‘stochastic’ means), but truly random placement leads to a very ‘noisy’ result with uneven tonality, so the placement algorithms are carefully set to avoid this.

Inkjet is being used more and more in labels, packaging, photo finishing and industrial print, all of which often use more than four inks, so the fact that a dispersed screen avoids moiré problems is also very helpful.

Dispersed screening can retain more detail and tonal subtlety than conventional screening can at the same resolution. This makes such screens particularly relevant to single-pass inkjet presses, which tend to have lower resolutions than the imaging methods used on, say, offset lithography. An AM screen at 600 dots per inch (dpi) would be very visible from a reading distance of less than a meter or so, while an FM screen can use dots that are sufficiently small that they produce the optical illusion that there are no dots at all, just smooth tones. Many inkjet presses are now stepping up to 1200dpi, but that’s still lower resolution than a lot of offset and flexo printing.

This blog post has concentrated on binary screening for simplicity. Many inkjet presses can place different amounts of ink at a single location (often described as using different drop sizes or more than one bit per pixel), and therefore require multi-level screening. And inkjet presses often also benefit from halftone patterns that are more structured than FM screens, but that don’t cluster into discrete dots in the same way as AM screens.

 

The above is an excerpt from our latest white paper: How to mitigate artifacts in high-speed inkjet printing. Download the white paper here.