In-Line Color Measurement Spectrophotometer Improves Press Control and Customer Satisfaction
An article by Brian Gamn, Flexo Magazine, November 2016
The demand for color measurement is expanding rapidly in the flexographic printing industry today. Improved information about color quality appeals to both consumer products companies (CPCs), who demand ever more detailed quality reporting, and to printers who want better process control. Many flexo printers still rely on the well-calibrated eye of a veteran operator to judge color quality. While visual judgements are vital to evaluating print quality, differences in color vision between operators and variations in viewing environments make visual judgements less than ideal for precise process monitoring.
Spectrophotometers measure target colors reliably and repeatedly, without removing the value of an operator’s judgement of overall image quality. Today, printers can choose from inline and offline color measurement systems. Offline color measurement systems require the removal of samples when the press is stopped, followed up by later measurement using a handheld spectrophotometer. This significantly limits the color information available. On the other hand, inline color measurement systems use spectrophotometers installed on the press. These systems can measure color on the moving web. So, color is checked more frequently, and checked while the press is printing at production speeds.
Offline vs. Inline?
Traditional color measurement practices in flexography have been defined by the limitations of the printing technology and of the color instrumentation. Printers often employ personnel, separate from the operators, specifically to make and record measurements. These specialists are trained in the use of spectrophotometers and the color management applications. Specialists, or the printers themselves, may also be responsible for entering color measurement data into different applications, such as a color management application for internal quality monitoring and a different print quality application for sending data remotely to a CPC.
Meanwhile, during production, thousands of feet of material pass between each recorded color measurement. Sometimes, samples are pulled more frequently during makeready, whether from short rolls or cut from the web itself. But, generally, continuously pulling samples and measuring color mid-roll during production has proven too labor intensive and impractical.
More Measurements Please!
Printers cannot adequately characterize their processes with so few color measurements and so few opportunities to correct changes in color that occur during production. Increasing the number of color measurements during a job increases the printer’s ability to quickly respond to changes in color and provides them with a more detailed map of color behavior. Inline color measurement has emerged in the market in recent years as a solution that drastically increases color measurement efficiency. This enhanced efficiency reduces waste through early detection of color drift and reduced quality rejection after the product is delivered.
The foundation of all inline color measurement systems is a spectrophotometer installed on a printing press. Mounted on a traverse to allow lateral movement, the spectrophotometer can measure patches anywhere in the artwork on a moving web (Figure 1). The position of the spectrophotometer is driven either by a 100% inspection system or by a camera mounted inside the spectrophotometer.
Figure 1. An inline spectrophotometer measuring color on the moving web of a wide-web packaging press.
Inline color measurement systems may employ different measurement backings that double as web-stabilization systems. Color measurement requires precise positioning of the target in space. A black roller is commonly used for measuring prints on opaque substrates, such as carton board. The roller keeps the substrate stable as it passes by the spectrophotometer.
However, white backings are required for the measurement of translucent and transparent materials. White rollers are used in some systems but require frequent cleaning due to contact with the material. A white roller also raises the system’s cost because the cost of a white roller may be very high (especially if using a ceramic roller).
Measuring reverse-printed films presents even more challenges. When handling reverse-printed webs, the ink, in contact with the roller, may leave the roller permanently stained, rendering measurements wildly inaccurate. A more effective approach is to suspend the substrate over a white backing by a few microns. This arrangement is superior for measuring films because the substrate is not in direct contact with the white backing and the small gap does not adversely affect the accuracy of the color measurements.
More Measurements Allow Early Warnings
Perhaps the most direct benefit of an inline color measurement system is the ability for a printer to detect and correct changes to color before the end of a roll. Operators can view color measurement data press-side and monitor the consistency of color within a roll. While using inline color measurement data to resolve disputes with customers is advantageous, it’s better to avoid disputes in the first place. Inline inspection systems can provide alarms that alert the operator any time printed color drifts outside an acceptable tolerance, giving operators the opportunity to take corrective action.
Inline color measurement systems provide measurements to the operator in seconds, allowing them to quickly respond to color shifts. Without inline color measurement, the operator is blind to color changes that occur at the start of a roll or within a roll. The data shown in Figure 2 illustrates the change in ΔE00 between the measured L*a*b* values of a Blue “spot color” and the respective Standard L*a*b* values. The solid line represents the trend of 150 inline color measurement collected over six rolls and the diamonds indicate the respective offline measurements collected at the end of each roll.
In an offline color measurement workflow, the operator can check only those measurements at the end of each roll to decide whether or not the Blue color was printed accurately. The background of the plot in Figure 2 is highlighted to illustrate “good,” “acceptable,” and “unacceptable” ΔE00 tolerances. The color measured on the sample at the end of Roll 2 was clearly within the “good” tolerance. Yet, the inline color measurements reveal that most of the roll was unacceptable. The offline measurements at the end of Rolls 4-6 suggests that color varied just a little and was within the “acceptable” tolerance window. Yet, the inline color measurements show an increased process variability from the previous rolls.
Figure 2. Process plot for the ΔE00 color difference between the standard L*a*b* values and inline L*a*b* values for a solid Blue target. The green, yellow, and red background marks “good,” “acceptable,” and “unacceptable” color tolerances.
More Measurements Inform Better Corrections
While color difference trends sound the alarm, they do not provide enough information to put out the fire. ΔL*a*b* plots, like that shown in Figure 3, help operators diagnose the nature of a problem mid-roll and choose the right corrective action more quickly, reducing waste and the possibility of a returned roll.
The position of a measurement point in ΔL*a*b* space can indicate whether the problem is with the ink itself (a blue that is too red) or perhaps the ink’s viscosity. In a case like this, switching to a different anilox, or adjusting the corona treatment is not likely to improve the situation. A look at the ink’s formulation is the right first step.
Figure 3. Scatter plot for the ΔE00 color difference between the standard L*a*b* values and inline L*a*b* values for a solid Blue target. The green, yellow, and red background marks “good,” “acceptable,” and “unacceptable” color tolerances.
What Else Can Inline Measurement Do for You?
The primary benefit of inline color measurement systems is the ability to collect a much larger number of measurements than offline color measurement systems without stopping the press to manually measure printed samples. But the advantages of inline measurement don’t end there.
In one case, a printer reported that, prior to implementing inline color measurement, he employed three people dedicated to collecting color measurements. These employees collected samples from every roll of every job, measured the color bars using an offline spectrophotometer, loaded the data into a color management application, and generated summary reports for both internal quality control and for reporting to their customers. After implementing inline color measurement, the number of employees needed for color assessment was reduced to one. The inline color measurement system collected the color measurements and exported them to a central location where one specialist could load them into a color management application and generate a report.
In other cases, several printers have reported, collecting color measurements throughout each printed roll allowed easier resolution of color-related disputes with a customer. The printers now investigate customer complaints of inaccurate color by reviewing color measurement logs from the inline system. Often, these logs clearly show that the printed color was within the specified tolerance throughout most, if not all, of the supplied rolls.
Some customers require their printers to upload print quality data to print quality reporting applications. These applications compile a wide range of print quality attributes, such as color measurements, registration, printed defects, visual comparisons, UPC grades, gloss, and opacity, to name a few. These applications assist customers with remotely assessing the quality of printed product. Furthermore, because so many people are involved in the print supply chain, automating the import of data into print quality applications can establish a shared reference that improves confidence and communication about the data. Inline color measurement systems are especially powerful when coupled with in-line inspection as they can not only automate the capture of color measurement data, but also export many kinds of print quality data in the same data stream. This process minimizes the handling of printed samples, reduces color measurement operator error, ensures the integrity of the data, and enables careful and insightful print process analysis.
Color measurement is vital for monitoring and maintaining consistent color in flexographic print production. Inline color measurement offers printers the ability to collect much better color data quickly and efficiently. The press can run continuously during production, without the need to stop and pull samples. Printers can make better use of their employees by minimizing the number of personnel required for the collection of color data, and both the printer and CPC can automatically receive data that accurately reflects the color of a job throughout each roll. Inline color measurement systems are an important innovation for the flexographic printing industry that will help streamline the color measurement process and upgrade print quality monitoring.
Brian Gamm is a Color Scientist with AVT (Advanced Vision Technology), a global leader in print process control, quality assurance, and press control for the packaging, labels, and commercial print industries, focusing on the development of inline color measurement systems for the packaging and labels markets. He is a graduate of the Rochester Institute of Technology with an M.S. in Color Science and B.S. in Imaging and Photographic Technology. He can be reached at BrianG@avt-inc.com