Quality Control & The Role of Color Management
Hello all,
I throught to provide this area in case you would like to comment on my articles published on GravurEzine and FlexoGlogal.
http://www.gravurexchange.com/gravurezine/0803-ezine/ploumidis.htm and/or
http://www.flexoglobal.com/pdfs/01-May%202008.pdf
I chose the topic of Quality Control as a means to emphasize the importance on color management and to provide an outline of the topics i'll be discussing.
Regards,
-D
3 comments:
Dimitrios,
The term inherent keeps coming up and this is often an excuse to not improve the "system".
There is no inherent variation in processes. One of the great efforts in the Quality Movement is to reduce variation not to accept unacceptable variation.
You have indirectly stated the solution to the problem of unacceptable variation in printing in your article. You have related it to the variation of ink film thickness printed during a run. The ink going out of the press in the ink film is a mass flow quantity. Based on the principle of Conservation of Mass the mass flow of ink going out of the system MUST be equal to the mass flow of ink going into the system on average. That is a scientific principle.
The solution to the problem of ink film variation is therefore control of the ink infeed into the press. Provide a consistent ink feed into the system and the result will be a very consistent printed ink film during the whole run. Obtaining a consistent ink feed can then result in print density predictability by means of the calibration of the ink feed rate relative to the ink requirements of the printed design that provides the desired density.
Erik,
Based on the available technology and equipment the variation is inherent. However, I have long accepted your point of view, and wish for an improvement in this aspect. But in a manufacturing environment of print and not presses, we have to find ways to determine the magnitude of the variation, the causes, and subsequently minimize it, since eliminating it is impossible due to the reasons you explained. I hope press manufacturers accept the shortcomings of their products and present a solution in drupa 2008.
To provide some numbers to illustrate the magnitute.
For sheetfed offset printing there is variation across the sheet, on the circumferential direction of the press and from sheet to sheet. What makes things more complicated is that sometimes you cannot be sure whether the variation occurs in one of these dimensions or in two of them simultaneously.
The variation acorss the sheet, assuming the press operator controls the ink/water balance adequatly, should be not more than 0.03 density points. This is doable.
From sheet to sheet, the variation is about 0.03 points as well, but there needs to be a sampling plan, every 500 sheets or depending on the press, and subsequent adjustments either in the ink feed rate, water, or ink keys. That is also controllable.
On the circumferential direction, the variation can be as high as .08density points. (I am talking only about solids; in dot gain the variation can be different due to the same principles applied in the feed of water). For the circumferential direction the only control is the ink surge, or rainbow gear as some printers call it. This is a totally unscientific means to adjust for variations from top to bottom of the sheet and it solely depends on the press operator having experimented enough to know what to do.
If you compound these three variation types, you get a mess. The most serious difficulty I have experienced would be during a fingerprint, when you need to have certain target densities at the profiling targets, and certain densities at the color bar. If your density at the profiling target is different than the one in the color bar, then you end up with a wrong color gamut of the press.
A printer has to deal with the available technology. The best way to minimize these sources of variation is to have a concrete roller maintenance system. The rollers need to be checked regurarly. I would suggest a weekly or biweekly roller audit for the ink form, water form and metering rollers, during which if the ink stripes are found to be excessive the rollers should be reset or replaced. Once per quarter, the printer should open up the printing units as wel and check the rollers throughout the ink train.
Keeping the rollers healthy and in perfect condition is the only way we have to tackle the inadequcies inherent in press manufacturing.
-D
What you say about the multiple sources of variation is very true but much of it is related to the specific design of the press. There are some possibilities in the press that can be modified to make some improvement. Maintaining the press helps if the variation is due to poor conditions but does not help if the variation is due to the design of the press itself.
I can see that you are a lot more aware of the problems now than before. That's good. Hopefully later you will be thinking about solutions to some of these variability problems. Press manufacturers and the graphic arts institutions do not understand these problems well enough to solve them. You might as well think about the solutions on your own because they will not be of much help.
In order to solve a problem you must believe that a solution is possible. These problems are not as difficult as you might think but they do require a lot of thinking. Good luck.
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