by Ron Ellis
Being able to maintain a press calibration is as important as the initial calibration procedure itself. G7 has been a great tool for US printers because in addition to providing a calibration method it provides a target and set of aims for all the devices in the plant. Prior to G7 most US printers used the press as the target. This approach was problematic because the press is normally the least stable device in the print workflow due to mechanical variation. Also multiple presses seldom print the same unless they are identical types and are well maintained. The aims provided by G7 are based on color, and can be used on any cmyk print devices in the workflow. This concept of a reference print condition is very important because without a target you cannot control your process. You have to have something to reference and compare against. The G7 calibration is a procedure used to align the printing machine to these G7 aims. This calibration is not a one-time procedure, as press conditions are constantly changing, and presses will drift away from the G7 print condition over time.
The Importance of Efficiency and Manufacturing
The recent recession has made the US printers who survived much more lean and efficient. It is clear that print margins are not increasing and the biggest gains are to be made by improving the process through automation and efficiency. In printing workflows this means reducing make-ready and quickly getting up to color so that you are printing sellable sheets. Once we know the target print condition, we can then tell when we are and are not achieving it. This allows us to know when the press is deviating, and to know when we need to correct it. In the past many printers would periodically fingerprint their press and then run with that calibration until the press drifted to the point where they could no longer match proofs – or even worse, until they would lose a customer over the poor print quality. In the hyper-efficient world of contemporary printing this will no longer be sufficient. Printers need to control their process – or risk going out of business.
The Importance of Measurement
To control your process you need to measure constantly, and compare against your reference print condition. As Peter Drucker said, “What gets measured gets managed.” If you are not measuring, then you have no idea how you are printing. Not measuring means you are running blind – it means you don’t fully know what you are doing in the pressroom. Fortunately there are devices and software that allow you to measure and track every sheet pulled from the press. For example with a Techkon SpectroJet or SpectroDrive, every sheet scanned can go into a database for immediate viewing, as well as trending and export to a number of external apps as well. The cost of these devices is lower than they ever have historically been –automated press sheet measurement can now be done for 30% of the cost of a comparable system a few years ago. Measurement is important. It is how we see when a press is drifting and the way we can predict if you will be able to get a proof to press match. With that in mind remember G7 is a temporary print condition based on the mechanical conditions at the time of calibration – the press conditions will change.
Comparing to the G7 Reference Print Condition
Once the calibration is achieved we can use G7 metrics as well as the recorded mechanical print attributes to monitor the print condition. G7 provides us with different metrics than previous methods. The key difference is that with G7 we are measuring color and not mechanical print attributes such as density and dot gain. Mechanical print attributes vary across different printing machines based on substrate, mechanical conditions, ink and other variables. While mechanical print attributes are valuable for telling us if anything has changed on that specific press, they are not useful for telling us if we have achieved a color match. To assess conformance we will compare these new G7 metrics against the G7 reference print condition. The reference print condition we will be comparing to (such as GRACoL or SWOP) is a description of color. Basic G7 aims that describe the color and weight of the gray balance which are also contained in the reference print condition.
What Do We Measure for G7
For G7 we measure different attributes than many press and production staff are used to. It is important to understand LAB and how to measure LAB as many of the key G7 metrics are based on LAB measurements. The basic G7 metrics are based on a gray measurement and refer to a print condition called G7 Gray, which is only concerned with the gray color and weight. G7 Targeted is another G7 print condition, which has additional metrics, related to the complete colorspace. For example GRACoL and SWOP are G7 colorspaces, which have paper, solids and overprints, specified in addition to the gray aims. These additional metrics make it easier to match the complete colorspace in addition to matching the gray.
G7 Gray Metrics
Here is a summary of what we measure to assess the basic G7 gray print condition:
- NPDC Curve: The Neutral Print Density Curve specifies the weight of the G7 curve. Patches such as the HC (Highlight Range, 25. 19, 19), HR (50, 40, 40) , and SC (75, 66, 66) can be measured to assure that the NPDC for both the CMY and K channels is correct.
- Gray Balance: The gray balance metric specifies the color of the gray across the G7 curve. Patches such as the HC (Highlight Range, 25. 19, 19), HR (50, 40, 40) , and SC (75, 66, 66) can be measured to assure that the NPDC for both the CMY and K channels is correct
- The values for both of the above are calculated based on the substrate and the maximum neutral density on that print combination. This means these G7 metrics change based on the paper. The Techkon SpectroDens is one device that automatically calculates the new aim based on the combination of the paper and substrate.
When reading these patches it is fairly obvious if the weight and gray balance are off. An example would be a GRACoL proof, of which the HR value should always be L*57, A* 0 B* -1. The tolerance to set for each varies depending on the print process. Proofs are very repeatable, while offset, flexo and other print processes all have varying degrees of variation across the print run.
G7 Targeted Metrics
For G7 Targeted we have additional metrics that are of value. These are as follows:
- ISO Solid Ink Values: ISO solid ink values are specified in LAB. Matching the ISO or other G7 target solid ink value assures that the ink is the correct color. The tolerance for the solid ink value is 5 deltaE. A device such as the Techkon SpectroDens has a mode that will tell you the deltaE as well as possible ink moves to correct the solid ink value.
- ISO Overprint Ink Values: ISO overprint ink values are specified in LAB. Matching the ISO of other G7 target solid ink value assures that the overprints are the correct color. While ISO does not require that the overprint values are met, G7 Targeted does make this a requirement. The overprint is actually more important than the solids because this indicates the color achieved when ink is printed with another ink. The tolerance for the overprint is 5 deltaE. Devices such as the Techkon SpectroDens have a mode that will tell you the overprint as well as the LAB value, which you can use to correct the overprint.
Mechanical Print Attributes
We also want to measure the following mechanical print attributes:
- Density: Density will assure us that the sheet is in the proper condition as compared to our benchmark. If we are not at the density achieved during our calibration (with slight variation allowed for paper color changes) then the LAB values referred to above will not be valid.
- Dot Gain: Dot gain, when compared to the benchmark calibration dot gain will show us if a specific unit is printing different our out of spec. Typically this is at least the 50% tint, and often the 25% and 75% tints as well. This information can be used for guidance by a press operator to correct a press issue.
Using the Measurements to Maintain G7 Print Conditions
A colorbar with the above elements is essential. Using the above we can measure and monitor all of the key metrics necessary to assess G7 compliance as well as machine consistency.
For example, XYZ customer based in XXX, has a Techkon SpectroDrive. When they pull a sheet they lay it on the console and scan the sheet. In 8 seconds they have all the information about that sheet and if it matches the G7 aim. The data is live and is used by the pressman for process control, and also can be used offline for trending purposes. (Additional software can automatically create new press curves if needed.) Here is a typical production scenario.
- The pressman loads plates and begins running up to color.
- Even though he is at the right density he notices that the Y solid is failing with a deltaE of 8, as well as the YC overprint, with a deltaE of 9.
- Gray balance as measured on the HR patch is also off, showing the sheet as being slightly too Blue.
- All of the above measurements can be seen on his Techkon Expresso console with a few clicks on the touchscreen.
- Looking at the Y ink value he notices it shows the Y as being too blue.
- The press operator goes to the fountain, noticing the Y ink is contaminated.
- After scooping out the ink and replacing it he begins to run again.
- This time his Y solid comes in correct, and his CY overprint is also passing.
His gray is still slightly off based on paper color. He moves the ink density slightly to correct for paper color, and then begins running in production mode.
Take Control of Your Print
Other typical scenarios involve detecting problems with a specific unit requiring adjustments to blankets, rollers. In some cases mechanical or environmental conditions have changed to the point where ink will need to be re-targeted or plate curves adjusted. These same live measurements can be used with a variety of software to perform these adjustments automatically.
The important message is to measure – all the time. Once measuring against a G7 reference print condition you will know when you are moving away from the ability to have quick make-readies and efficient production. The first step to taking control of your print is to know how you are printing. Without it you are just printing to chaos.