Page tree
Skip to end of metadata
Go to start of metadata

Flexo plate media generally have limited highlight resolution. Because halftone dots become smaller in highlight areas, a point is eventually reached where the halftone dots no longer develop properly. This is caused by insufficient UV energy reaching the plate. For flexo plates digitally imaged on a ThermoFlex imaging device, the effect of oxygen inhibition during UV exposure causes the dot on plate to be smaller than the exposed mask opening. For instance, the size of the mask minimum dot may be equivalent to a 5% dot, but the size of the resulting dot on plate may be as small as 1%, due to the effects of oxygen sharpening.

An experienced media representative must perform systematic exposure tests to establish minimum plate exposure values. It is not the responsibility of Kodak workflow specialists to determine minimum exposure values, but those values must be established before the Kodak specialist can proceed with implementing flexo screening.

Without a screening technology like Maxtone, it is necessary to have a strategy for dealing with the values below minimum dot value. Traditionally, a bump curve is applied at output to adjust values that are below minimum dot to force the smallest halftone dots to print at the minimum dot value or higher.

Three common methods are used to create bump curves:

  • Overall etch
  • Highlight etch
  • Highlight limit value

Figure 1. Three methods of creating bump curves 

You should normally use the overall etch method, because it provides for the preservation of the maximum number of tones with the most dynamic range. It also provides for maximum separation of tonal values, which contributes to better image quality. All discussion of bump curves in this document assumes that an overall etch method is being used.

To make a bump curve in Harmony, create a derived calibration curve by combining a current curve with a target curve. The final calibration bump curve will look something like this:

Figure 2. Calibration bump curve (bumped to 5%)

There are two ways to create a final calibration bump curve for use in Prinergy and Prinergy Evo.

Method 1: Target bump

Figure 3. Current bump curve + Target linear curve = Calibration bump curve 

Method 2: Current bump

Figure 4. Current linear curve + Target bump curve = Calibration bump curve 

Although the results appear similar, there is a slight difference. Method 1, which places the bump in the current curve and uses a linear target curve, is the recommended method.
Method 1 (bump in current) moves all values from 0.1% up to the bump value. Method 2 (bump in target) has the limitation that the minimum bump value is 1%. Method 2 is unable to adjust values below the original 1%, and a minimum dot size must be also be set in the output process template to clip off the values to prevent scum dot.


See Also

  • No labels