Kiss Impression in Flexo July 2014

Pucker Up

A Kiss Impression Is Key in Preparing And Executing an Ideal Pressrun

As the industry introduces new presses,
inks and substrates, it is important to
remember that it is often simple best
practices that determine the success
of a printrun. Methods for consistent impression
are some of the most important best practices a
printer can establish and the “kiss” impression is
one of the most essential.

What is a kiss impression? Simply put, on an ideal printrun, the
printing sleeve or plate should just “kiss” the substrate. Technically, a
kiss impression is the minimum impression needed to transfer all text,
solids and halftones to the substrate. The advantages to a kiss impression
are numerous. The kiss allows pressmen to hold a fine dot while
increasing graphic fidelity. It allows for printing at faster speeds, with
less wear, and will protect the most fragile dot: the highlight.

But a kiss impression is an ideal. In most “real world” environments, it
is difficult to achieve. Lots of things in a print environment can ruin the
perfect impression, so every printer has to look at the printing process
from the center of the press outward and ask questions like these:

  • Is the substrate a perfect thickness on the roll and on every
    different roll?
  • Are the mandrels perfect in total indicated runout (TIR) without
    tapering?
  • Does the bridge mandrel have low spots?
  • Is there any “play” or movement in the mandrels from one unit
    to the next?
  • Do you trust that the plates are mounted to the cushion tape
    without waves?
  • Is the printing sleeve perfect in TIR and without tapering?
  • Have you ensured proper and consistent ink acetate levels and
    viscosity?
  • Have you ensured consistent surface tension that is within the
    substrate’s specifications?
  • Is the corona treatment of the substrate working correctly?

It is much easier to have a customer
approve a job that’s over impressed
with too much gain than it is to have
a customer approve a job that is
missing graphics because it was
under impressed.

Print managers have to be risk managers. Most pressmen and print
managers will err on the side of over impressing a printrun to
compensate for those real world problems. It is much easier to have
a customer approve a job that’s over impressed with too much gain,
than it is to have a customer approve a job that is missing graphics
because it was under impressed. And the difference between a perfect
impression and an under impression may be as small as 0.001-in.



In addition, substrates vary in thickness. A 40-μm. variation in a
substrate may produce a “wave” pattern of impression variances in
a printrun. And every printer knows that even if a substrate has a
rated thickness tolerance specification, it can still receive substrates
that are out of tolerance. Many substrates are also prone to stretching
during the print process. Printers regularly adopt a “make it
work” mentality, because they do not have time to deal with a manufacturer
on an out of spec substrate. It is easier to squeeze it and get
the job done with some over impression than it might be to change
out a stock or miss a press date.

Finally, plate and sleeve defects may necessitate over impressing.
Plates, by their nature, have more variables than sleeves:

  • Mounting tape
  • Cushion adaptors
  • Mounting errors

These can all change the pressure needed across the web. Heavy impressing
may be necessary to mitigate low spots, press bounce and/or
maximize the usable print area across the web when using plates.



…While many modern presses
take care of these practices with
automation, there is no substitute
for an experienced pressman
operating with a company’s unique
set of best practices for its specific
press.

Mitigating Consequences of Over Impression

As we’ve shown, there are a lot of reasons printers feel the need to slightly
over impress a job. But there are strategies for mitigating the effect
and need for over impression. Gray Component Replacement (GCR) in
the file prep stage can help immensely to minimize the negative effects
of over impression. GCR is the removal of most of the multichannel
whites and grays from a 4-color (or more than 4-color) process. Those
grays are then seamlessly added back as a full range black, maintaining
the shape and depth of the original. Neutrals that were once three colors
are now mostly black. With less ink behind grays there is less variation
in color throughout, as pressure is increased. GCR also produces better
highlights because a majority of colors are created by only one color (in
the case of a spot) or two colors (CMYK or expanded gamut) for hue,
and then black is added for shading.


Ice Cream

One of several GCR advantages (pictured,  black channel) is a reduced color ink 
consumption, as composite grays are replaced with less expensive black ink. 

Ice Cream

UCR — black channel

 

 

 

 

 

 

 

 

 


Engraved elastomer sleeves mitigate the effects of over impression and
allow for wider tolerance in impression pressures. Elastomer compounds
can be specifically tailored to printers’ ink types, which allow
for optimized ink transference. A key component to direct ablated
elastomer sleeves is known as “below surface imaging.” By taking the
highlight dots below surface (1 percent to 5 percent) we can alleviate
dot gain and problems with damaging highlight dots, especially at
higher pressures. Laser engraved sharp shoulders on the edge of text
and borders help to minimize fat text and fill in on reverse text.

Engraved sleeves can also offer increased uniformity across the web,
especially when compared to flat plates. This can mitigate the need to
over impress and can greatly increase the life of a sleeve.

Another reason to use engraved elastomer sleeves is their durability.
Elastomer is inherently more durable than polymer. You can expect
two to four times the run length without degradation in highlight dots
compared to polymer. What does this mean? Elastomer decreases the need
to over impress to achieve the kiss impression at the later stages of
a typical printrun. There is no breaking down of cushion layers like a
cushion adaptor with polymer ITR or sticky back with flat plates.

As the industry introduces new presses, inks and
substrates, it is important to remember that it is
often simple best practices that determine the
success of a printrun.

We’ve developed a set of best practices for establishing kiss impression
when running fingerprints, discussed in Adam’s article
(see sidebar on page 26). It is important to note that while many
modern presses take care of these practices with automation, there
is no substitute for an experienced pressman operating with a
company’s unique set of best practices for its specific press. This
way, the press operator has the ability to manually make adjustments
for best impression, if conditions change during the course
of a printrun.


About the Author: Carey Color Inc. is a full service digital imaging
company headquartered in Sharon Center, OH, with locations in
Illinois, Wisconsin and the U.K. It employs more than 75 experienced
prepress craftsmen. Carey Color specializes in manufacturing laser engraved
plates and elastomer sleeves for the flexo, dry offset, emboss and
intaglio industries. Carey also provides prepress services for direct mail
catalogs, packaging, flexo and dry offset in addition to commercial photography
and offset platemaking. To learn more about how Carey Color
can help you, contact us at 800-555-3142 or visit www.careyweb.com.

 

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