Flexo plates become slightly larger when they are wrapped around a printing cylinder because of the soft photo-polymer stretches. To have a correct size of the printing image a shrink factor has to be applied in the direction along the printing cylinder. This is sometimes referred to as flexo distortion or dispro.
In the Imaging Engine "Image to Screened Separations" ticket and in the Automation Engine Pilot this is called "flexo distortion". Since there is a built-in assumption in Imaging Engine and other Esko prepress tools that the vertical direction is the direction along the printing cylinder and the horizontal direction is the direction across the printing cylinder, a value smaller than 100% has to be specified in the Vertical distortion control.
But what values should be used? Can these values be calculated automatically in Automation Engine?
Plate Vendor Calculators
There are flexo plate distortion calculators provided by flexo plate vendors like DuPont and Asahi. In these plate distortion calculators, plate type and the intended print repeat length has to be specified. The tool will give you the flexo distortion value.
The tool is typically a Microsoft Excel or a Windows application. Since these calculators are endorsed by the plate vendor, you can assume that these calculators give reliable results.
This gives you the values that need to be used. But it doesn't answer the question for plate types that are not listed.
This calculator uses the gear pitch, number of teeth and plate thickness as parameters and returns the flexo distortion factor.
The gear pitch x number of teeth is same as the printing drum circumference.
On the same page you can find tables with results of this calculator.
In this calculator and also in the tables there is an undocumented assumption that the base layer (or polyester base thickness) of the plates is 0.005" (0.127mm). So these tools only give reliable results for plates with a 0.005" base layer.
Flexo Distortion Formulas
The correct formula to use to calculate the flexo plate distortion is:
K= 2 x π x (Plate thickness - Base layer thickness)
Flexo distortion = (print repeat length - K) / print repeat length.
e.g. For a plate with thickness 1.70 mm, base layer thickness 0.127mm and a circumference (vertical size) of 500mm, the flexo distortion factor becomes 98.021%.
This formula is used by the plate vendor calculator and is also the formula used in flexo distortion tables mentioned in the previous paragraph.
When searching for flexo distortion formulas on google, all kinds of results are found. Don't have blind trust in these formulas.