Quality Control & The Role of Color Management

Part I of an ongoing Color Management Series
By Dimitris Ploumidis, Pacific Southwest Container

 

Color management in a manufacturing environment is a tool in the mix of processes that assures that the product conforms to the established quality standards. This article will deal with the general concept of color management as applied to quality control purposes, and subsequent articles will discuss the issues mentioned here in more detail.

Printing companies are defining their mission with a focus on the customer, realizing that what constitutes success is not printing by itself, but printing to meet customers’ requirements. These requirements are communication requirements, and quality should be defined under this perspective. In a competitive environment where print is more often than not considered a commodity, being able to be responsive to the customer’s needs and positioning one’s business as an integral part to the customer’s communication strategy is necessary in establishing long-term relationships that allow for the value-added benefits of providing service.

The importance of measurement with regard to quality
To be efficient, sometimes even to survive, we need to control the manufacturing processes to assure quality. The foundation of such control is measurement. Alan Greenspan argued that one of the main problems of the former Soviet Union was their inability to accurately measure their productivity—define the processes to be assessed, determine the best way to assess them, and capture that information. The inability to identify and act upon those areas where improvement was needed resulted in an inability to resolve economical issues.

The first consideration is defining what quality is and what is its cost. If quality is based upon the expectations of the customer, then the quality standards should be established within that scope. However, every manufacturing process has inherent variation, and the establishment of quality standards should take this variation into account. There is unavoidable variation in color, in the degree of slide angle for packaging coatings, in the moisture content and ply bond of the stock, and so on. The printer should establish good communication with the print buyer to precisely determine what the purpose of each product is and manufacture it within the tolerances that assure conformance. Without accurate measurement of the process, it is impossible to define these tolerances or how capable and consistent the process is and, thus, let the customer know what is feasible and what is not. The capability of the process, therefore, imposes constraints on the customer’s expectations; but, without it, the demanded quality would be unattainable and the business would be put at risk. The real determination of quality is what degree of variation is acceptable to the customer. A discussion on these expectations will provide the basis of a reliable and healthy relationship.

Measurement should be able to establish the tolerances defined by the inherent variation, and at the same time monitor the process for any deviation from these tolerances that would be attributed to unsystematic causes. Human errors, equipment malfunctioning, raw materials that don’t meet the established standards or require different processing methods are all causes of poor quality. If the process is not measured, it is very likely that these errors would go unnoticed, only to be captured at the customer’s warehouse. Then, the cost of quality, or poor quality, is significantly higher, as is the cost of correcting it. Monitoring such non-conformances allows a facility to determine, in effect, how systematic these unsystematic errors are. For example, if “human error” is a frequent cause of nonconformance, then it is fairly reasonable to say that if those costs outweigh the cost of training, the manpower needs to be trained.

Measuring every aspect of manufacturing is not an easy task, though, and is restricted by the labor cost involved or automation systems required to capture the information. A manufacturing facility is a complicated environment where the various production line processes form a subtle web with possibly unstable links. Finding out in advance the various threats to each link, or even the whole of the web, is an almost impossible task. Even the cost of trying to measure everything can be prohibitive, and most likely it would drive both management and personnel to insanity. It would, thus, make sense to use qualitative means to evaluate the processes that need closer monitoring, or to initially use broader measurement processes to pinpoint general areas that are not at maximum operating efficiency, and then utilize more resources in a concentrated effort to capture analytical information in those areas.

Furthermore, not knowing the capabilities of the process is like not knowing how good or how bad we actually are. We cannot possibly be seeking improvement if the present state is unknown. How would we know if we need to improve, what to improve, and at what costs that improvement would incur? How would we even know that we’ve actually improved? Implementing quality in a competitive environment is tied up with improvement and change—as well as resistance to change. In that, quality assessment based on measurement provides the most objective and most compelling argument towards change.

Prerequisites to Quality Control
STANDARDIZATION
The industry is working diligently to create standards that allow printers to implement best practices and provide the foundation for repeatability in production across different platforms. Standards enable reliable communication with the customer and the supplier by creating a common understanding of what is acceptable and calibration methodologies that assure the final print product will adhere to the same quality standards.

Standardization can be broken down into internal and industry-wide or international. An internal standard would be a sampling plan, or the standard densities that are used as targets for a particular customer, or the tolerances of the process. Standardizing these parameters allows for operational repeatability. A broader set of standards is the industry-wide or international standards. Two of the most important standards are the ISO 12647 and 2846 series. The ISO 2846 series defines the ink colors for the various printing processes, and it should be up to the ink supplier to deliver inks that meet these specifications. The ISO 12647 series provides standards for the process itself, covering the ink colors on different substrates, tonal value increase, and the respective tolerances. Working together with the suppliers to make sure that the incoming materials adhere to these standards is the starting point for any successful operation. There are also plenty of standards about different paper properties, on the metrology to be used, the viewing conditions and testing methods, as well as standards related to each printing process.

EDUCATION
Education of the workforce is another crucial quality prerequisite. Decision making takes place at every step of the process, but it can only be efficient if each individual has the requisite technical knowledge of the process and a clear understanding of the product’s quality requirements. A skilled craftsman is more reliable than any system or piece of machinery. What is different today than in the past is that press operators need to have a thorough understanding of color measurement and management as part of their arsenal, and must utilize these “scientific” methods to augment their productivity and deliver quality work. The initial reaction, however, may be one of resistance to change. For people who have been working for a couple decades without a closed-loop color measurement system, for example, it may be hard to view the application of science as something other than unnecessary extra work. These people need to be empowered for the transition from a craft-based process to a more scientifically driven industry. Untrained operators are a hazard to the process; their work involves higher operating costs and the constant threat of non-conformance. An investment in training allows management to declare that every employee is an integral part of the business, and the pay-off should be increased employee motivation and productivity.

MAINTENANCE
The role of maintenance and good housekeeping is integral to delivering quality product. Each press and auxiliary system needs to be working properly. The older the press is, the less its ability to deliver consistent product. There are, nevertheless, important reasons that fight against this assumingly “common sense” issue. A poorly maintained piece of machinery may still deliver what it is meant to deliver, but at a decreased productivity rate and with higher costs translated in downtime and waste and with possible defects that do not conform to the given quality standards. The costs, however, might not be measured accurately enough for the realization to occur that they are actually more than the costs of maintenance. Moreover, in order for the press to be taken care of, it has to be taken off production. It is a vicious cycle; trying to fix mechanical problems as they come along is like chasing one’s tail. For that, maintenance should be as preventive and proactive as possible. Maintenance needs should be established through detailed assessment processes that allow the machinery to be taken off production at a scheduled time. This will prolong the life of the equipment and allow for higher operating efficiencies.

Color management as a means of quality control
Having discussed quality in general, we can move on to the role of color management and see how it should be implemented under this scope. The technical topics that we’ll go through will be discussed in further detail in following articles.

CRAFT OR SCIENCE
Printing has traditionally been considered a craft. Press operators pride themselves in making a complicated piece of machinery produce the desired output. To do that, they rely on the hard-earned understanding of the mechanical and chemical elements of the process and their interactions. What makes their craft even more challenging is that they need to do so on a job-to-job basis. Contrary to other manufacturing processes where the output involves a rather stable input, in printing, every job has different graphics and layout, and, thus, different needs. Moreover, the interactions of the raw materials are still not fully understood and cannot be accurately predicted. The amount of ink emulsification is one example; the absorbency of different stocks, the effect of temperature, and press speed are a few more.

Still, even if we lack an exact scientific prediction of the materials’ interactions, we can set guidelines for each material by itself by utilizing the available standards and then assessing through density or color readings the printed output as an overall quality metric. In doing so, we’ll be able to detect any deviations from the normal having excluded part of the equation’s unknowns. This makes detection and elimination of root causes easier. What could be more reasonable than to expect a quantification of the process’ input and output to reduce the degree of uncertainty involved?

SUBJECTIVITY IN COLOR VISION
The human eye is a perfect discriminator of color variation. A color shift from a neutral grey, for example, is easily detected by the observer. However, our eyes are incapable of determining with precision the amount of variation. We are able to see that one grey is different from another, but we do not know if the difference is acceptable. Furthermore, there are other constraints in relying purely on vision. Human error and lack of attention, eye fatigue, and differences in the viewing conditions are some of the most common reasons why our eyesight should not be the sole judge of conformance.

Color is not an objective property of the object. It depends on the object itself, the observer, and the viewing conditions. Two people see and interpret color differently; this allows for variability in determining what is acceptable and also in trying to correct those differences. Even the color vision of a single observer is not constant. The most common cause of shifting is fatigue. A pressman views color differently at 11:00 p.m. when the shift starts and 6:00 a.m. when the shift is about to end. Detecting and acting upon what is perceived as right and wrong is hazardous when we understand that color vision is subjective. Moreover, inspecting color under a light booth versus room lighting is also going to result in slight color differences. It is likely to have differences even between two light booths, if the bulbs of one are old and emit different wavelengths. Do we even need to mention that we have no certainty in knowing where the customers inspect the supplied press sheets or proofs? Or what they define as “reddish”? We need objective measurement to avoid these uncertainties. Whether it is density or color, solids, tints, or grays, the printed sheet has to be measured at a fixed sampling rate to detect variations in the output.

A common misunderstanding in implementing metrology is that people tend to regard the measured value as an absolute attribute. There is a difference between what is, and what is measured. For example, there can be differences in the absorbency characteristics of certain substrates that can result in different density or color readings, even if the same amount of ink is transferred. Dot gain is an ambiguous metric, since the interaction of light with the substrate is not precisely undefined. Likewise, certain differences in colors are more forgiving than others—despite all the applied color science. The same amount of measured color difference between a dark and a light yellow color would constitute the dark yellow acceptable and the light one unacceptable. Creating a process that would have different clauses for each of these different scenarios would lead to over-engineering. The best solution is to train the manpower adequately enough to realize the basic concepts and be able to exercise their judgment on applied metrology. In determining conformance, human judgment needs to go hand in hand with quantification.

INSTRUMENTATION
Technology advances are making instrumentation affordable and predictable. However, there is deviation among instruments and variation within the same instrument. Manufacturers specify the instruments’ repeatability, and the devices should be calibrated on their white point as per the manufacturer’s specifications, and their precision should be verified on a standard with known densities or color to determine that it reads correctly. Inter-instrument agreement is a more complicated issue. Differences in optics and lack of standardization from the instrument manufacturers’ side may disallow two instruments to provide the same measurement. There isn’t much to do here, since we are not in the business of manufacturing instruments, but we could still use the same instrument—as long as its repeatability is verified—as a reference for process control purposes. We should also be clear on the exact measuring specification used; two density measurements of the same patch can be different if we are using absolute or relative density. As such, it is important to decide upon and implement the preferred metrology and educate the press operators in using the different instruments functions.

PROCESS CONTROL
We’ve talked about systematic and unsystematic process variation in all aspects of the manufacturing process. When it comes down to color management, however, we should narrow down the field to ink film thickness. The amount of ink film that is laid down on the substrate is what varies throughout a press run, resulting in visible or not differences on the output. It is, thus, the major determinant of the end product. Either density or color can be used to measure ink film thickness. It would be unrealistic to imagine that the ink film would be constant throughout a press run. The question is how capable the process is, or how consistent the ink film remains throughout the press run, and what amount of variation is acceptable. Certain jobs can be considered acceptable with a variation of 0.10 density points. Other jobs call for significantly tighter tolerances at the range of 0.05 density points. As long as the process is capable of delivering an output within these tolerances, then there is agreement between what is acceptable and feasible. Defining the standards of quality on these premises is a best practice.

The application is also a determining factor on the tolerances specified. For packaging, for example, one of the most important areas that needs monitoring is where the two sides of a box overlap. Our eyes are not precise enough to detect the amount of difference in color when the areas of interest are across the two sides of the printed sheet. However, when the sheet folds, these differences become obvious. This is an example of the need to establish particular process control parameters for the application at hand.

In addition, depending on the printing process, there are different ways in which it can systematically vary. Gravure, for example, lays down an even ink film across the sheet, where offset printing has more variation due to the ability to modify the ink keys across the ink key fountain. A capability study of the process must provide a detailed determination of the parameters that should be controlled and their frequency.

Knowing the capability of the process allows for a determination of a trend in which the process can systematically vary. Any variation larger than the one that has been established as systematic should be attributed to a different cause that impacts the process. Having such insight allows the printer to search for the cause and eliminate it, thus making the process more consistent. This detailed analysis could further allow the printer to define a sampling plan. The frequency of different unsystematic errors cannot be accurately determined; they are, after all, unsystematic. However, it is possible to determine a sampling plan at a frequency rate that provides assurance that an unsystematic error is detected timely.

Furthermore, there are different metrics that allow for a more detailed insight into what goes on inside the press. Dot gain, or tonal value increase—as it has been recently termed, provides a more sensitive metric to the interactions of ink and water than solid ink density, thus allowing a more precise process control metric for the interaction of ink and water and tint reproduction. Print contrast has traditionally been used to define the overall quality of the reproduction. Trap provides insight into how the different inks are being laid on the paper with relation to each other. Grey balance is a metric of the overall acceptability of the image, of whether all the inks are balanced enough to achieve a pleasing reproduction. It is also the easiest means by which we can detect deviation in any of the inking units. Colorimetric values are used to verify that the inks and stocks adhere to international standards and to verify that spot colors are what the customer expects them to be. Usually a combination of such metrics should be used to assess quality, taking into account the specifics of the job and the printing process.

COLOR-MANAGED WORKFLOWS
We’ve been mainly talking about the use of color management as a process control tool to assure quality. However, color management answers also the questions of color predictability and portability. Establishing a color-managed workflow from the digital file to the printing press is the most advanced application of color management. An ideal pressroom should be able to print every job targeting standard densities and within certain tolerances on density, color, grey balance, and dot gain. There are significant benefits in relation to waste and productivity if this is achieved. But for that, all the elements that affect the repeatability of the process should be addressed; otherwise, the science cannot be applied.

Color portability refers to the ability to move a job from one press to another and still achieve the same visual appearance. This is quite often a necessity in a production environment. However, it is feasible only if the inks are standardized and the behavior of each press is repeatable. If these requirements are met, then the differences between the presses could be compensated with plate curves that would force each press to print the same. Color predictability refers mostly to the ability to predict the color of the press during the proofing stages, and in doing so create proofs that match the press sheet. In order for this to be effected, the press needs to be characterized and through one of the various color management software packages that rely on ICC profiling or proprietary color calculation algorithms, color manage the output of the proofing system to match the color of the press in terms of gamut and the tone values.

The technology advancements in color management software are providing features that address the particular needs of different applications. Each software developer uses different algorithms to color manage a workflow, and there might be slight differences among the different packages. However, the main goal of such packages—matching color among different print platforms and providing a precise hardcopy or softcopy prediction (what we would call “proof”) of the printed output—is achieved by all of them. The printer needs to decide which software offers the features that best fulfill the needs of the applications at hand. Then, it is important to apply the same principles of consistency and documentation of the elements that affect the workflow to have good results.

Today, print buyers frequently request a printer to determine the metrics that allow the best assessment of quality and request a proof of conformance based on those metrics. This can actually be a necessity for customers that print and sell their products in international or regional markets, and a printer needs to meet this expectation to get that business. The best practice to satisfy this demand is to agree on the methodology to be followed on calibrating the presses and implementing a color-managed workflow. If all the steps were followed correctly, the resulting product would share the same visual appearance and characteristics. The metrics and their tolerances would then verify that the methodology was properly applied, providing quality assurance. The industry is working in this area as well, trying to decide on a common methodology that would be best for all the international markets. Even if being similar to others seems like a competitive disadvantage, it is a fact that not all printers are able to apply such sophistication to their workflow, yet, their customers demand it, and there are important benefits in terms of productivity.

AUDITING
Defining the capability of the process, training manpower, taking care of the machinery, agreeing with the customer upon what is feasible and acceptable, assuring conformance of the raw materials, and establishing color-managed workflows are all best practices that are nevertheless subject to wear and change due to a variety of reasons. Expecting that the processes in place would assure everlasting quality could be a grave mistake. In order to help prevent any degradation of these processes, they need to be monitored and systematically audited. If any of the elements that were in effect during the characterization of the presses changes, then color cannot be predicted. Thus, it is critical to systematically audit and follow up on the implementation of these systems.

Conclusion
We have discussed the different prerequisites and means to achieve a conforming product in a print production environment to provide the expected quality as defined by the customer and the capability of the process. Controlling the quality in complicated manufacturing environments is not an easy task, and it entails a variety of systems that allow the assessment, prediction, and correction of the production parameters. The one thing that remains constant is the need to quantify the processes so that management can objectively evaluate and improve as demanded by a constantly changing environment. Color management is, in this perspective, another quantification and quality control tool. However, its efficient utilization has the same needs for consistency and understanding of the science and technology that are found in using every other tool. Moreover, color management allows for advanced applications that allow color to be predicted and be reproduced on different print platforms. This series of articles will cover in detail the particular aspects discussed rather broadly here, focusing mostly on their technical aspect.

About the Author:
Dimitri Poumidis finished his Master of Science in Print Media from the Rochester Institute of Technology, with a concentration on Color Science. His thesis dealt with the consistent reproduction of spot colors. During his studies, he worked in the Color Management System’s labs at the School of Print Media and did an internship with Graphics Microsystems. Upon graduation, Dimitri moved to California to work for Pacific Southwest Container as a Color Assurance Engineer (www.teampsc.com). Prior to his studies at Rochester, he completed a Bachelor’s of Science in Marketing in Greece, and worked as a printer, designer, and photographer.

Please, submit any comments, questions, or topics you would like to discuss on printcolor.blogspot.com under the post of the respective article. Dimitri can also be contacted on dxp3756@gmail.com.

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