Yellow 3
Magenta 4
SpottySpot 5
*3
Calculating web growth
This topic gives examples for calculating the percentage of web growth
from measurements on press sheets or traditional page-shifts.
The standard process for determining web growth compensation
parameters is to print press sheets with no compensation, and then
measure the discrepancies between the separations at different points
on the surface. The growth percentage for one separation relative to
another is then calculated by adding the discrepancy at the farthest left
pair of marks, to the discrepancy at the farthest right pair of marks, and
dividing by the distance between the pairs.
Once calculated, the percentages are entered into the web growth
profile file. Prinergy uses the percentages from the web growth profile
in an algorithm to compensate for web growth.
Example 1: Measuring the displacements from the yellow
separation across a grid
Table 1 is a table of measurements from a press sheet of grids printed
with the ink order KCMY. Each column of numbers corresponds to the
vertical grid line at a particular horizontal position. (This is only an
example. In reality, a grid would contain far more lines.) Yellow is the
last ink down, so it is the base separation against which other
separations are measured.
Table 1: Displacements from the yellow separation across a grid
Distance in mm
Negative numbers indicate distances to the left.
Non-negative numbers indicate distances to the right.
Page
Growth
X -886.
8
-584.
0
-252.0 -12.7 12.7 315.5 584.
0
886.8 N/A
K-Y -1.38 -.82 -.36 .1
.13 .64 .95 1.40 (1.38 + 1.40)/(2 x 886.8) = .
157%
C-Y -1.04 -.66 -.36 .10 .10 .30 .46 .78 (1.04 + .78)/(2 x 886.8) = .
103%
M-Y -.52 -.30 -.08 .10 .08 .12
.23 .40 (.52 + .40)/(2 x 886.8) = .
052%
X measures the horizontal distance (across the web) of the vertical
grid line from the horizontal center of the sheet. The first column
736
Chapter 12—Outputs
