- December 01, 2007, By David Argent Contributing Editor
There are some basic problems that crop up over and over again, with costly results in wasted time and materials. Below are examples, in capsule form, from my own experience in flexo printing.
- Viscosity Management
We all know the importance of running a job at the correct ink viscosity, but the most common instrument used for measurement — the efflux cup — has limitations. Because there is variation among suppliers and within batches of cups from a specific supplier, a calibration procedure has to be in place to make sure each cup is standardized. Also, use the correct cup for the correct viscosity range in use. Efflux cups are subject to wear and damage, so check cups periodically and replace those out of calibration.
Another point to remember on viscosity is that the reading is dependent on temperature, so always make sure the ink is at operating temperature before measurement takes place.
- pH Control
If you're using water-based inks, pH control is critical. Normally, pH will drift lower due to loss of amine from the ink. This will cause the ink viscosity to climb. Therefore, pH must be in the correct range before making any viscosity adjustments. Do it the other way around and you risk spoiling the ink. If pH is adjusted too high, the ink can become so alkaline that some pigments, such as rhodamine, can burn out.
- Color Control
This area can be a minefield of potential problems. Besides the obvious issues caused by running off-shade product, there are other problems lurking from inappropriate use of pigments. In the Januray 2007 issue, the column on Color Measurement and Control explained that pigments are discrete chemical entities. For example, Lithol Rubine is “Red 57:1” and is a unique molecular structure. The molecular structure defines the actual color and other properties associated with the pigment. If you lose track of pigment management, you run the risk of conditional color matches, color fading, and other physical and chemical instability. Work with your suppliers to become familiar with pigments in the inks you are using. When a packaging end use changes, make sure the pigments are up to the job.
- Humidity Effects
Ink suppliers can advise on appropriate solvents for use in summer conditions. However, attention to chill roller settings and dew point often are overlooked, and the web is rewound with condensate on the ink surface. Blocking and sticking can occur, particularly on treated films.
- Film Treatment
Corona discharge treatment can be overdone, and over-treatment can hurt ink adhesion by degrading the film surface. Thus, it is important to control the treatment power applied to the film.
- Ink Additives
There may be two sets of ink additives in use, the official set that management knows about and the secret set in actual use! Each shift may have its own protocol for additive use. Audit ink additive use regularly and correct as needed.
- Process Capability
There are occasions when a print job is approved by the customer only after numerous machine and ink adjustments. The danger here is that this job always will be difficult to run at acceptable levels of quality, waste, and line speeds.
I'm sure you could add to this list from your own experience. Converting “sins” are many and they're out there, waiting to happen. Be careful!
Process improvement expert David Argent has 30+ years of experience in process analysis with particular emphasis on ink and coating design and performance. Contact him at 636-391-8180; firstname.lastname@example.org.