Direct Printing of Barcodes on Corrugated: Part 1
Direct printing of barcodes on corrugated material is one of the larger challenges in the supply chain. The several methods of printing directly to corrugated each have their own issues and the verification report doesn’t express itself in printing language. When the direct printed barcode on corrugated is downgraded due to the ISO parameter Modulation, how should a flexo process be adjusted to improve the grade? How should an ink jet process be adjusted?
Low reflectance is a problem common to all barcodes on corrugated
One problem common to all direct printing processes is the low reflectance of the bare kraft, which should provide the high reflectance background for the barcode. Barcodes in the supply chain are all subject to the PCS or Print Contrast Signal method of measuring (and evaluating) the low reflectance features (bars) and the high reflectance features (quiet zones and spaces) in a barcode. One aspect of this bears emphasis: the PCS system requires the background (quiet zones and spaces) to be the high or light reflectance value; the bars are required to be the low or dark reflectance value. This is because scanners calibrate themselves for the light reflectance value in the quiet zones. This also points out another problem with some bare corrugated: when the natural color is not consistent, it worsens verification grades and scanning problems.
In general, the industry recognizes that direct printed barcodes on bare corrugated will not achieve ANSI grades above D because of low print contrast grades.
Press gain—the tendency for the ink image to wick into the corrugate and spread—is also a problem in most direct printing processes. The problem is compounded in flexo, where printing plate durometer and roller pressure also spread the image. This is further complicated when photopolymer plates are mounted along with rubber plates, making it impossible for roller pressure adjustments to uniformly affect the entire impression.
Barcodes on corrugated: identify and control the variables
Direct printing with ink jet is not immune to these issues. Print contrast may be an even bigger problem here because ink jet isn’t really “ink” so the low reflectance blacks aren’t always very opaque or black. The “ink” is relatively thin to accommodate the spraying process, so wicking and spreading can also be a concern. Other factors include spraying pressure and distance of the spray heads from the corrugated surface.
Printing plates and design software to drive ink jet systems can easily compensate for press gain using a bar width reduction factor. If the process anticipates a press gain of .020”, the widths of the bars in the barcode can be narrowed by .020” so that they gain on press into their nominal width, rather than through—and beyond—their plus tolerance. Critical to this compensation is the repeatability of the anticipated gain. There are a lot of things involved beyond the factors mentioned above. Others include the viscosity of the ink, characteristics of the bare corrugated, reliability of the ink color, pH characteristics of both the ink and the corrugated (and how they react), and flatness and absorbency of the corrugate surface. It must also be considered whether these factors present themselves uniformly throughout the stack of corrugate feeding into the press.
Barcodes on corrugated: tilt the playing field to your advantage
This is not a gloomy picture—this is reality for all direct printing on corrugated. It is the level playing field for everyone in this business. Those who identify and control these variables tilt the playing field to their advantage.
In the next postings we will look at the ANSI verification report. I will relate the scan grade to the direct printing process and show you how to improve those grades.