|
Trapped air bubbles are one of the leading causes of print
defects in areas of solid print and process collors. They
cause isolated "donuting" in vignettes and voids
in areas of solid print. It is estimated that approximately
50% of air bubbles go thoughout the print run, leading to
what gravure printers refer to as "flexo quality"
results.
Most, if not all, flexo printers take a reactive
approach to air bubbles, removing them with syringes in the
mounting department and razor knives on press. The results
is increased prepress costs and excessive pressroom downtime
- not to mention damaged photopolymer plates. Mounting operators
are continually are called to the pressroom to axtract air
bubbles onpress or to remount individual cylinders that were
riddled with air pockets in the image area.
Stretching the Problem
The stretching of the tape during the mounting
process leads to caliper and register inaccuracies. There
is direct telationship between caliper variation in stickyback
and impression settings required to print effectively: The
greater the variation in your stickyback, the greater the
printing impression settings. This creates halos at the edge
of solid graphics, in addition to producing excessive dot
gain.
Taking this one step further, we find that the
excessive squeeze causes compressible adhesive to take a "compression
set", meaning the foam starts to break down and loses
its abibility to recover after each impression. This is a
progressive process that occurs over time and leads to station-to-station
register problems. How quickly it occurs depends on the varaition
in the compressible adhesive and in the recovery properties
of the base foam.
Print defects directly related to air bubbles
are fairly easy to uncover.
|
|
They will typically show up a blemish in process color, and
are especially noticeable in skin tones and highlight areas.
Unfortunetely, the very nature of this problem prevents such
defects from showing up in the proof. When the printed web
is viewed under magnification, distinct dot donating is apparent,
corresponding to the area on the printing plate that has an
air bubble trapped beneath it. (see Fugure 1). The image area
directly above a trapped air bubble is often printing with
0,005" - 0,010" of excessive impression. In addition,
it is most important to understand that this portion of printing
plate is applying excessive squeeze to the anilox roll. This
compounds the problem, as an ink-film thinkness that is not
consistent with the one being delivered by the unaffected
image area.
Applying stickyback to the cylinder by hand is often accompanied
by lifting the stickyback in an attempt to bring one edge
of the stickyback precisly to the other edge. Thos procedure
does not only create air bubbles, it also creates overstretching
of the stickback.
Symply put, air bubbles and stretching are having a negativ
impact on the evolution of flexographie printing.
|
