 |
 |
| Pressroom Automation = Faster Makeready + Reduced Waste + Consistent Color |
By The Numbers: What Different Measurements Mean and How They Can Help
By Ron Ellis
With the emphasis on specifications like GRACoL and standards like ISO, measurements have taken on a new
importance. These standards and specifications have defined targets, which means it is easier than ever to tell if you
are ‘there yet'. Instead of wondering if it is ‘good printing' we can use measurements and standards to know your
process is under control. In addition to being helpful for setting up our process, measurement can be even more
helpful for making sure we can repeat the same process over and over again. These same metrics can become key
indicators to show that we can print predictably or conversely that something has gone terribly wrong with our print
process. Knowing we are printing the same way as the day before is helpful, and knowing if things are changing can
be important, especially if we have critical jobs on press.
What are the numbers we should learn and know about for pressroom measurement? Depending on what we are
doing these are often the same metrics we have always used such as density and dot gain. Contemporary color
measurement in the pressroom does not rely exclusively on the old school metrics, although they are still important
indicators. Because much of color measurement today is based on measuring color, rather than just density, there are
new measurements and inexpensive measurement devices that tell us what is happening on our press and proof.
In other industries, great investment is made and attention paid to making sure every single manufactured item
matches — even when mass marketing. In printing it is not the same, and granted the printing process is much more
variable than other forms of manufacturing such as making plastic molded toys or products. If a light bulb factory gave
you a box of bulbs with a 25w, 150w, 72w and 38w bulbs they probably wouldn’t be around long. But for a printing plant
this same variation may almost seem normal. For most plants repeatability is the goal — the idea that you can
reproduce the same printed piece over and over. In order to achieve this, you need to be able to measure the printed
condition, and tell where one stands. This is the essence of process control.
When I leave a customer after a press calibration I leave them with a set of metrics for process control purposes, as
well as for troubleshooting. These metrics are based on the data received during the final qualification runs on press.
Too much information tends to just confuse people so while I collect much more information, I leave just basic
information for each press. Here are some of the basic metrics that I leave with the customer and how they can be
used. All of these can be used in daily production and can normally be found on a standard color bar and normally do
not require separate press runs.
How Much to Measure?
The above measurements are very helpful in maintaining good process control. Obviously not every sheet can be
measured, nor would it be desirable due to the wide variety of stocks used. There are several different aspects of
process control.
The first is daily process control – how you run. In general you should attempt to run your jobs to the reference
densities +/- .05. (Keep in mind with out of spec papers you densities will change slightly). The idea is to start at these
target densities and make whatever adjustments are required to match the proof. At the same time operators should
be comfortable checking the dot gain or NPDC curves when at density to make sure everything is running as required
and that there are no major mechanical issues occurring.
The second is long term process control – the ability to check and adjust for the inevitable changes that occur in
pressrooms before they become a crisis. This can be done with a simple spreadsheet such as that supplied in the
GRACoL Qualification Kit, or a more powerful application built for this purpose such as Spot On!, ColorMetrix, GMG
RapidCheck and others. While it would be great if every sheet could be read it is not practical, and while some shops
read every job and log them, many shops simply read a few jobs a week from each press and log the results. As the
press falls out of spec they will then make note of the new condition, and correct via mechanical adjustment on press
or gain curves as soon as possible. The big issue is not how much you are measuring, but simply to be measuring and
monitoring your printing conditions.
About the author: Ron Ellis is a New England-based consultant specializing in color management, graphic arts
integration, and press calibration. He has provided installation and training services to dealers, manufacturers, and
content creators since 1986. An IdeaAlliance G7 Expert and chair of the GRACoL Committee, Ron has qualified
numerous G7 Master Printers in the United States. In addition to calibrating pressrooms, Ron also specializes in
creating internal working spaces for brands and agencies that allow them to work more efficiently with vendors, saving
both time and money. Ron produced training materials for numerous printing industry vendors and publishers.
By Ron Ellis
With the emphasis on specifications like GRACoL and standards like ISO, measurements have taken on a new
importance. These standards and specifications have defined targets, which means it is easier than ever to tell if you
are ‘there yet'. Instead of wondering if it is ‘good printing' we can use measurements and standards to know your
process is under control. In addition to being helpful for setting up our process, measurement can be even more
helpful for making sure we can repeat the same process over and over again. These same metrics can become key
indicators to show that we can print predictably or conversely that something has gone terribly wrong with our print
process. Knowing we are printing the same way as the day before is helpful, and knowing if things are changing can
be important, especially if we have critical jobs on press.
What are the numbers we should learn and know about for pressroom measurement? Depending on what we are
doing these are often the same metrics we have always used such as density and dot gain. Contemporary color
measurement in the pressroom does not rely exclusively on the old school metrics, although they are still important
indicators. Because much of color measurement today is based on measuring color, rather than just density, there are
new measurements and inexpensive measurement devices that tell us what is happening on our press and proof.
In other industries, great investment is made and attention paid to making sure every single manufactured item
matches — even when mass marketing. In printing it is not the same, and granted the printing process is much more
variable than other forms of manufacturing such as making plastic molded toys or products. If a light bulb factory gave
you a box of bulbs with a 25w, 150w, 72w and 38w bulbs they probably wouldn’t be around long. But for a printing plant
this same variation may almost seem normal. For most plants repeatability is the goal — the idea that you can
reproduce the same printed piece over and over. In order to achieve this, you need to be able to measure the printed
condition, and tell where one stands. This is the essence of process control.
When I leave a customer after a press calibration I leave them with a set of metrics for process control purposes, as
well as for troubleshooting. These metrics are based on the data received during the final qualification runs on press.
Too much information tends to just confuse people so while I collect much more information, I leave just basic
information for each press. Here are some of the basic metrics that I leave with the customer and how they can be
used. All of these can be used in daily production and can normally be found on a standard color bar and normally do
not require separate press runs.
- LAB Values for Ink Solids. LAB measurements of the solid ink values are helpful to have. While not used for
daily production they are very helpful for troubleshooting. First they indicate whether we have achieved ISO
compliance with our ink set and corresponding density. Second, these values can tell us if our ink is changing. If
the LAB value between the current ink and the reference measurements are great enough it means that the ink
has changed. This could indicate a press side problem such as contamination, or it could indicate consistency
problem from the manufacturer. When the press and proof are not matching a simple LAB measurement of the
inks is an easy way to tell if the ink has changed. This test will also indicate if the ink is ISO compliant at the
densities being run. The same ink at the same density on the same paper should be relatively consistent and
not change by more that a few delta E.
- Density Values for Ink Solids. Although density for ink solids is a tried and true measurement, once we have
determined our proper ink density it is important that we maintain them. Both accurate printing, as well as the
value of LAB and other measurements is closely tied to the press operator being able to get to the same point
(usually) through density. Tolerance for this on a specific media and print condition is generally 0.05.
- Dot Gain Values. Dot gain is also an old-school metric, but it is very important for diagnostic purposes. If at the
same density and on the same paper the dot gain shows a large fluctuation, it will indicate a potential problem
with the press or platemaking. For example, if the reference dot gain is 16% and after learning that proofs are
not matching we read it and see that one cylinder is now gaining 27% we will know exactly where to look – and
why we can no longer match a proof. It is a simple metric that can usually be determined on almost any job
simply by reading the color bar with a densitometer that has a dot area or dot gain feature. Often if the issue
cannot be corrected by mechanical adjustment to the press, a simple plate curve can be used to pull color back
into spec. Tolerance for this varies depending on printing process but generally can start at +/- 4% and may be
loosened from there if needed for daily production.
- LAB Solids for Overprints. LAB Solids for Overprints are often overlooked but can be valuable tools. When
operating to a specification such as GRACoL, or standard such as ISO, the overprint values can be used to
tweak your ink aim points so that you achieve the best possible ink film thickness as well as overprint colors (i.e.
blue sky instead of purple sky). Once in daily production, a large deviation can indicate press conditions have
changed and need to be examined. While there is no official tolerance for LAB measurements of overprints, a
tolerance of 5 delta-E is sometimes used.
- LAB Values for c50/m40/y40. The LAB values for the neutral patch are often a great overall indicator of the
presses calibration state. While it can be difficult to use gray balance as a metric for daily operation, it can be a
helpful metric to see exactly where the press is and if the press can maintain the ideal calibration condition.
Ideally this patch will read L*57 +/-, A*0 +/-1 B* -1 +/-1. In daily production you will probably have to nudge your
solid ink densities to achieve these values, but if the calibration conditions have stayed the same you should be
able to come close to this. A quick glance at this patch can tell you where your system is, and confirm any color
bias you are seeing. (Once you are familiar with LAB readings any deviation from the above will tell you exactly
where the color is headed.)
- LAB Values for Paper. LAB values for paper are valuable as a reference point. When using tight process control
the above values are valid when run in the same conditions, meaning the same paper and ink. Of course many
papers are fine and can be made to work by simply adjusting the solid ink densities as printers have done for
decades. For all the other values to be useful for troubleshooting you will need to be on the same or similar
paper. The LAB reading will tell you if that paper has changed, or if it is a completely different paper. Even if you
are checking this during runs you can detect paper changes as an operator throws in a supposedly similar
paper because they have run out of the original paper
How Much to Measure?
The above measurements are very helpful in maintaining good process control. Obviously not every sheet can be
measured, nor would it be desirable due to the wide variety of stocks used. There are several different aspects of
process control.
The first is daily process control – how you run. In general you should attempt to run your jobs to the reference
densities +/- .05. (Keep in mind with out of spec papers you densities will change slightly). The idea is to start at these
target densities and make whatever adjustments are required to match the proof. At the same time operators should
be comfortable checking the dot gain or NPDC curves when at density to make sure everything is running as required
and that there are no major mechanical issues occurring.
The second is long term process control – the ability to check and adjust for the inevitable changes that occur in
pressrooms before they become a crisis. This can be done with a simple spreadsheet such as that supplied in the
GRACoL Qualification Kit, or a more powerful application built for this purpose such as Spot On!, ColorMetrix, GMG
RapidCheck and others. While it would be great if every sheet could be read it is not practical, and while some shops
read every job and log them, many shops simply read a few jobs a week from each press and log the results. As the
press falls out of spec they will then make note of the new condition, and correct via mechanical adjustment on press
or gain curves as soon as possible. The big issue is not how much you are measuring, but simply to be measuring and
monitoring your printing conditions.
About the author: Ron Ellis is a New England-based consultant specializing in color management, graphic arts
integration, and press calibration. He has provided installation and training services to dealers, manufacturers, and
content creators since 1986. An IdeaAlliance G7 Expert and chair of the GRACoL Committee, Ron has qualified
numerous G7 Master Printers in the United States. In addition to calibrating pressrooms, Ron also specializes in
creating internal working spaces for brands and agencies that allow them to work more efficiently with vendors, saving
both time and money. Ron produced training materials for numerous printing industry vendors and publishers.
|
JSA prides itself upon providing exceptional prepress-to-press solutions, software and systems. Leading-edge innovations and superior product quality ensures value that will provide usefulness throughout the product life. An adept product research function, industry leading support and applications expertise allow us to meet and exceed our customers’ expectations. As a full-service organization, JSA is committed to providing best-in-class solutions in ink delivery, color management, ink preset, color measurement, closed loop ink control and integration solutions that you can rely upon to deliver a strong return on investment. |