The case for process control

By Ron Ellis

               Though not common in many printing plants today, process control is destined to become a part of our future. Printing has long been undergoing a transformation from craft to manufacturing. In addition to this march from craft to manufacturing, the tight economy also is pushing printers to become more efficient and requiring more process control.

“What are you printing to?” asks Gerry Gerlach, chair of GRACoL. “If you don’t know, you are living in chaos. How could possibly match or repeat something if you don’t actually know how you are printing?” The case for process control hinges on being able to repeat the same thing over and over.

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 that 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.

 

Record all the metrics

There are several metrics that are very important to measure. The key idea is to record all the metrics from proof to press. This includes recording data from the proof condition, the plate condition, and the press condition. Unless we record our benchmarked condition, we have no way to tell if printing is correct or not. Starting with the proof condition, we want to record LAB data of our proof when properly calibrated. This LAB data can be compared with data on a proof to make sure that the proof is still valid. This data is typically recorded from a color bar on the proof. The tolerance can vary from less than 1 deltaE to as much as 2 deltaE or more depending on how tightly you are monitoring your proofing system.

While this can be done using common software such as XRite’s Measuretool, it is more often done using dedicated quality control software. This software will give you a pass or fail condition on the proof, and will typically allow you to print a label indicating the proof status for inclusion on the proof. The key idea here is to read the proof and make sure the proof has not changed and is valid.

While proofing systems such as the Epson are typically stable and do not change much over a few months, simple issues such as a clogged nozzle can cause a proof to become invalid. How many proofs need to be read? Shops that care about maintaining high quality read every single proof, while other shops that are less willing to maintain these standards may read a proof every day to verify that the proofing condition has not changed.

The next area to record is the printing plate. We typically want to record dot gain on the plate. Because all plates may vary by color, it is important to pick a specific plate color as the benchmark value. For example, always reading the 50 percent on the black plate can tell us if the plate is changing. As with the proof, the big idea here is to make sure that the plates are not changing. In many high quality shops they read each plate as it comes out as part of their quality control procedure. Other shops that are less stringent will read a plate each day to makes sure they are not having any consistency problems with their plates.

One thing you find out if you do this for a living is that very often the proofing systems are not moving much (with the exception for the famous Epson nozzle clogs, which are worth tracking for on their own), and that platesetters normally are not moving either. Usually all the big movement is coming from the pressroom.

The press is the most complicated and mechanical part of the process, and because of this it is the part of our manufacturing process that deserves the most attention. In most plants press conditions are constantly changing. If it is not the season and the weather, then it is wear and tear on machines and the differences in operators. And with the presses being the most expensive pieces of equipment in the plant, as well as the key moneymakers, it should not come as a surprise to find out that the press deserves a lot of attention.

Recording data on the press is very important, because if the press is drifting or has changed, then matching the proof can become virtually impossible. There are several easy-to-record indicators that the press is changing. Probably the most simple is to record the dot gain at a specific density for each cylinder. If the amount of dot gain changes over a number of jobs from the recorded dot gain, then it is an indicator that something has changed on press, and you need to be on alert.

For example, if you suddenly have problems matching proofs, you can read the proof and ascertain whether or not the proof has changed, read the plate and see if the plate has changed. If you stop to read the press gains and notice that the Y gain has gone up 10 percent, then you will have determined that you may have something wrong with the Y unit. Typically a press operator can use this type of information to go check their press and make sure there is not an issue on the cylinder in question. Of course a press is not that linear so one job being out of spec is not enough to mean there is a problem on press.

There are a number of variables that can make a press print out of spec, and you need to see the same behavior on multiple jobs before it means there is actually an issue. Other metrics such as LAB values for solids, and trap information can also be valuable in determining if there is an issue on press. They key idea here is that unless you are recording this information, you will have no idea if you are printing correctly or not. And if you can’t tell how you are printing then as Gerry Gerlach says, you are living in chaos.

There are a number of tools and procedures we can use to monitor our printing process, and turn printing from being a crafty emotional game into a manufacturing process.

 

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 co-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. He can be contacted at 603-498-4553 or through his web site at www.ronellisconsulting.com.