When I first posted the measurements from my previous post on LinkedIn, I pointed out the fact that brand colors are only measured in the control strip. Which doesn’t give any guarantee that the rest of the sheet/web will be within the specified tolerances. A few people replied to that statement, advertising their ability to measure print quality inline, so within a sheet/web, not just in the control strip. At first sight, that might seem a nice innovation. But not necessarily when think about it a bit longer: it can also be a very dangerous tool. Printers should think twice before investing in this kind of options. Please, read on!
CONTENTS: The hypothetical sheet | The sting… | A dangerous tool… | Why is this important?
First things first: when I talk about inline print control in the rest of this article, I’m talking about those systems that have the ability to measure within the print image, any spot you want within that print image, not just the control strip. Please keep this in mind!
Let’s do a small exercise. I measured in total 28 spots in two Ypra packages (see my previous post). Now, let’s consider one of those measurements as the reference, the exact representation of the brand color. And calculate the difference between that reference and all other measurements. Now, distribute these measurements evenly on a hypothetical printed sheet. Then you could get the picture below. With all measurements in the control strip (the bottom row) within a 2 dE00. And almost all other measurements outside a 2 dE00, with even differences up to 5,2 dE00… The control strip is within that tiny tolerance the print buyer demanded, so he would accept the job. But most of the rest of the sheet is not within that tiny tolerance. And guess what: that’s the part of the printed sheet that the print buyer gets delivered, what he will use in his business.
The sting…
And here’s the thing – or should I say the sting? It’s nice that you can measure all these different spots within a sheet, but can you also control every individual spot? Meaning: can you influence that cm² that is out of tolerance, without having an effect on the rest of the printed sheet?
Take a look at the middle spot of column 2, which has a 4,5 dE00: can you do something on your press to get that specific spot below 2 dE00, without influencing the top spot in column 2, which was already below 2 dE00? Can you do exactly that?
On a digital press, you can, obviously. But what about a conventional press? You can influence ink zones on an offset press, but that’s the complete column in the example above… If you would change something to that 4,5 dE00, by changing the ink key settings, that will also influence the 1,4 dE00 spot: it’s in the same ink zone. And by lowering that 4,5 dE00, you might increase the 1,4 dE00. Which is something you don’t want to do: it’s within that tiny tolerance.
You should also consider the feedback loop: how long is that? How many sheets or linear meters will pass before ‘corrective measures’ are in place? When the measurement is done (e.g., in offset presses, that’s a camera after the last printing unit), that sheet has already been printed in full. You can’t change that one anymore. So, corrective measures will only be applied to the next sheet (or rotation) or after x sheets (rotations). Now, let’s take the cause of the deviations in the Ypra samples as an example: mottling. Chances are that the next sheet and most sheets after that, don’t have a 100% identical surface structure as the ‘mottled’ sheet. These might have a closer to perfect surface structure. Then, the ‘correction’ would be applied to a sheet that does not have the issue that measure is meant to correct… Resulting in a higher-than-expected color deviation. [sigh] Did I ever mention printing is something complex?
In inkjet, issues like failing nozzles can be detected and fixed instantly, but I’m not sure if other color-related issues, like the example of mottling, can be fixed instantly. If you have the answer, please share it in a comment below!
A dangerous tool…
And that’s where a good idea becomes a dangerous tool: if you can measure any spot, any cm², but you can’t influence the color in every individual spot, it’s useless. Even more: it will be a dangerous tool in the hands of brand owners: they will judge a print job on all those measurements, not just the control strip. And they will reject good print jobs. Jobs that would have been within tolerances if only the control strip had been measured. Giving that extra information to brand owners and print buyers will get printing companies into trouble.
The deviations in the samples I used are due to mottling. One of the causes is the roughness of the substrate, as I stated above. As far as I know, that’s something you can not control on the printing press and certainly not on a ‘spot’ level. Please correct me if I’m wrong! Share how it could be controlled, if there is a way.
And I’m not telling something hypothetical. A long time ago, a large package printer told me the story of a print buyer who came in for a press check. There were about a dozen identical packages on the sheet. And she pointed to every one of them, saying: “I like that one, not that one, that one is OK, but not that one…” The printer was flabbergasted. And had to explain how printing, how a printing press works… Why did she reject part of that sheet? Probably because she could. Because that was her job: checking print quality visually, without any knowledge of printing or color. As I stated before, it’s often a power game.
Why is this important?
It seems like a nice innovation, that inline color control where you can measure any spot in the print image. For some, it may even be a ‘must have’. But you should think twice: if you can’t control/influence every individual spot on a sheet / on a web, it will be a dangerous weapon in the hands of brand owners and print buyers. And rest assured: they WILL ask you for the full reports once you have that tool.
Only if the specified tolerances are sufficiently broad to accommodate deviations like those due mottling, printers could consider investing in such a tool. But not with tiny tolerances like 2 dE00.
PS: for the record, inline systems that measure the control strip on a sheet or along the web, and only that control strip, are very useful. These should be a no-brainer. These systems will decrease the time for the feedback loop I talked about, which can be very helpful to get within a certain tolerance and stay within that tolerance.
Unless there has been a major revolution in measuring technology “inline” measurement does not mean that the printer measures different spots on the sheet during production all the time.
Inline simply means that the colourbar is measured constantly during the run – but the colourbar is in a fixed position, – either at the back of the sheet – or (which is in fact the best location imo, – in the middle of the sheet).
I have not seenor heard of such dE00 differences within a sheet – say if we are printing a 70 cm long sheet.
To get a physical dE00 of more than 1 from top to bottom, there must be something seriously wrong.
But as I mentioned before, if colour consistency is important, brand owners should use premium paper, – not cheap, bulky paper. It does cost more but the quality will be higher.
In order to help prevent problems of the type you have described, there are tricks that printers have used for the longest time. One of them is to use the first print unit, not for printing but to clean dust and particles from the paper, and to stretch it (to avoid misregistration between the print units).
By the way, printers that get certification to ISO 12647-2 only need to ensure that the colourbar is within range – whether it is at the front, in the middle of the sheet or at the back. The images are just evaluated visually – see https://fogra.org/en/certification/offset-printing/pso-certification
The CEO of Koenig & Bauer Vision & Protection claims otherwise… It’s part of their Qualitronic Colour Control.
Sound good, and definitely it happens, but how will be doing the press correction? the presses has tools to reduce these differences in Y axis, the differences in X axis are controlled by ink keys, please let me know,