By Mike Todryk
My great grandmother taught me to read at a very early age. She always had a love of books. Our living room had one wall filled with hundreds of mystery novels; her favorite type. While other kids were reading Dick and Jane, I was reading Agatha Christie and Sherlock Holmes. I even wrote Mr. Holmes a letter when I was eight. He wrote me back (ask me about it sometime).
I recently started reading Sherlock Holmes again. One of the things I love about the stories is how he would always see and deduce things that others missed. These things were usually in plain sight, but we didn’t perceive them. When Mr. Holmes would reveal his deductive reasoning to Watson, you would shake your head and say, “How did I miss that?”
Just like those great stories of old, many inkjet color issues are hiding in plain sight as well. We can see and troubleshoot the obvious things, but we can sometimes get stumped by inkjet issues hiding just below the surface. Let’s put on our deerstalker hat, take up our pipe, and look at three hidden inkjet color issues.
Mystery #1: The Case of the Chameleon
As I’ve talked about before, we use process control software to monitor all of our inkjet devices. We are taking multiple readings a day from every machine on all kinds of stocks. We were starting to see a disturbing pattern on several of our inkjet devices, especially on one particular paper stock. We would be running well, with everything passing in process control, then suddenly we would get several readings slightly outside of tolerance. After a little while these would seem to clear up. It was a mystery. The issue didn’t seem to be in any one color or area, just a general change in tone and strength. From all appearances, we were doing the right things. Our color digital area was climate controlled. Preventive maintenance was being performed. A truly puzzling case.
After scratching our heads, we put on our thinking caps and started to see some patterns emerge. It often happened when one or two machines were on very long print runs. After studying the way that material was moving into the staging area in the room, we realized that because of the high volumes, the paper was not moved into staging in time to acclimate properly, with one stock being particularly troublesome. Depending on how you are storing and staging your blank paper, your paper needs time to adapt to its new environment. We put tighter controls in place to adhere more strictly to our paper manufacturers’ acclimation recommendations, and began to see our drifting readings fade away. The Case of the Chameleon was solved.
Mystery #2: The Case of the Yellow Headed League
This case was unusually baffling. Print seems to be light on yellow in places, but the test print all looks fine in the yellow bars and the yellow solids all measure within tolerance. We had a real mystery on our hands. As we began to look at this more, the issue wasn’t uniform across the sheet. Cleaning didn’t seem to help. We couldn’t see any visual issues on the test print. Interesting.
After remembering a trick that we had learned in the past, we used a handheld UV light and shined it on what appeared to be solid yellow test bars. What we found were dozens of jetouts in the yellow that were undetectable to the eye, but showed up under the UV light. After some head replacements, we issued UV lights to the operators to monitor their print more closely. Another case solved.
Mystery #3: The Case of the Errant Archer
This case is less about the details and more about the deductive reasoning process. When looking at measurement data, it can often be difficult to understand how to interpret it. Here are some general guidelines that can prove helpful.
There are two main ways that I will look at measurement data: how accurate it is and how precise it is. Thinking about our errant archer, how accurate it is means how close to the bullseye the shots are. How precise it is means how tight the shots are to each other. When judging shots on a target you will usually see four possible scenarios:
- Accurate and precise
- Accurate but not precise
- Precise but not accurate
- Neither precise nor accurate
Any of the four can help you troubleshoot issues. Obviously, if you are accurate and precise, you are good to go. If you are neither precise nor accurate, you have big problems that start with stabilizing the whole system. What do the other two tell us?
If something is precise but not accurate, it means the system is stable, but has drifted. Case #1 would be an example of this. Look for universal issues like a paper stock or a calibration drift.
If something is accurate but not precise, it usually means the system is starting to become unstable. Case #2 is an example of this. Unnoticed deterioration in the yellow was causing a general lack of precision. Inconsistent maintenance and measurement device drift can all cause things to remain fairly accurate, but not precise. And don’t forget about personnel. Operators improperly trained in taking readings can introduce lack of precision into a process.
As we have seen, sometimes the answer is right in front of us. We just need to look at it in a different way. Let these cases show you some different ways to troubleshoot hidden inkjet color issues. Now come Watson, the game is afoot.
Mike Todryk is a Color Technical Specialist for IWCO Direct. He has more than 20 years of printing industry experience and has specialized in Color Management for the last 18. Ask Mike a question here.