Opacity & White Ink

White ink is used extensively in printing plastic films for packaging. The role of the white print is to totally or partially block the package ingredients and provide an opaque print contrast for line and process colors. Colors on the press are controlled to an appearance standard with a spectrophotometer. This level of instrumental control also fixes the application rate, and as a direct result, the cost of ink use becomes predictable for the colors.

Management of white ink application can be a different story. With white ink, the usual measure of graphic performance is opacity. Other factors, such as gloss or whiteness, also can be important, but opacity is the primary determiner in cost analysis. Technically, opacity is the ability to block light coming through the package. However, it also governs how the color graphics stand out in contrast and the overall vividness of the image.

Often, white ink printed on film is not well controlled even when more than half the ink coverage on the design is white. Where opacity is measured instrumentally with an opacimeter, the cost ramifications of the limits set are not evident at face value. Then again, there may be some instances where visual appearance is the only control in use. Somehow an understanding needs to be gained on the relationship among white ink appearance, print properties, and the amount and cost of ink being applied.

The above graph shows how opacity readings change with coating weight (pounds/ream) for a typical white flexo ink printed on transparent film. The data points are generated easily from lab or press prints, and the actual curve will vary from ink to ink but follow the same general trend.

The Hierarchy of Value model was presented in my October 2007 column. “Ink Cost per Printed Unit” is the second level on the Hierarchy of Value. White ink opacity and ink cost/unit are linked. Opacity values in packaging range from 48-55, but some higher and lower numbers are seen. In this range it takes about 10% more ink to produce an increase of 1 pt in opacity, and the visual difference is barely perceptible. However, this imperceptible difference is costly; for a converter using $100,000/yr of white ink, it amounts to an additional $10,000.

It is evident that it pays to measure opacity and to run at the low end of the specification range. Occasionally, attempts are made to print white ink at very high opacity readings to provide exceptional whiteness and hiding power. To achieve an opacity of 60, it would take 3x the amount of ink than at a reading of 50. The $100,000 white ink bill then goes to $300,000. This might be acceptable if the converter builds this cost into the package price, but there are other performance implications.

In my November 2007 column, “Cost of Ink Use” is the third level in the Hierarchy of Value. Here we already have a handle on the ink cost per unit and, in addition, now need to factor in the press performance.

In general, as coating weight increases, solvent retention increases, and at some point, blocking or offsetting may occur in the rewind. One way to counteract these effects would be to slow down the press, which is rarely a good idea. These effects can be quite dramatic. For instance, on a job with a full background white, the following scenario could happen:

Opacity 55 58
Ct. Wt. #/ream 1.2 1.8
Cost Standard +50%
Solvent Retention In Spec 2x-5x higher. Possibly out of spec.

There are several lessons in this example. The difference in going from 55 to 58 opacity may not be apparent visually but will cost dearly and may cause odor problems in the packaging material.

Conclusions

Measure opacity instrumentally. Set tight opacity limits at a level that is acceptable and manageable. Run to the numbers.

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; djvargent@sbcglobal.net.


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