Thermal Contraction in Flexo Oct. 2014

An operator moves a sleeve that was just unpacked from a hot delivery
truck.

Hot & Cold

Thermal Contraction Can Dramatically Affect Press Registration

Carey Color

It’s January. Outside the window, the snow is falling.
On the schedule: a reprint of 400,000 impressions
for your biggest customer. You order the sleeves
from the warehouse and when it comes time to
print, the pressman calls. He can’t get the job to
register and you know the last time the job was on in
October, it printed flawlessly.

You walk down to the press and examine the results. Baffled, you lean
against the cold metal girder as it stretches from the ceiling to the floor.
You wonder, “What is going on here?” And then it dawns on you…

Temperature affects printing. One of the biggest ways is through
the expansion and contraction of polymer and elastomer plates and
sleeves. Material changes across temperature ranges can produce
registration and quality issues with your flexographic printing process,
which has tolerances in the thousandths of inches. Knowing how
different materials change with temperature is the key to planning a
printrun and mitigating mounting, registration and quality issues.

THE THEORY

The rate that a solid material changes in length with temperature
change can usually be calculated as the fractional change in length
per degree of temperature change. This number is called the linear
coefficient of thermal expansion and is often expressed in 10-6 m/m
K (or degrees Celsius), or in the U.S., degrees Fahrenheit. As long as
one stays in the same units for their fraction, it becomes possible to
convert this to inches and thousandths of inches easily. It is also
important to note that, with solids, the effects of air pressure differences
on thermal expansion are negligible.

But the theory becomes complicated in the flexographic printing
process, because printing plates and sleeves are not uniform solids.
They are compound materials bonded to other compound material.
Sleeves are usually composed of elastomer or photopolymer compounds
layered around a fiberglass core, bonded to the core with
adhesive. Plates are usually photopolymer plates mounted to fiberglass
cores using stickyback cushion tape. Because fiberglass formulations
generally tend to have a lower coefficient of thermal expansion than
most elastomer and polymer formulations, the materials mounted
around them cannot expand and contract without strain effects of
being attached to the fiber cores.

Because of the difficulty of studying
every single type of compound and
core, we decided it was important
not to study these separate
materials in a lab, but to study the
effects of thermal expansion on
plates and sleeves in real world
scenarios: on warehouse floors,
in storage rooms, and mounted on
engravers and presses.



Further complications arise because different formulations of elastomers,
polymers and fiberglass have different thermal expansion
characteristics, and it may be impossible to characterize every type of
material bonded to every type of core. In addition, not all materials
are isotropic (they do not expand at the same rate in every direction).

Often, the formulations and thermal expansion characteristics of the
materials are trade secrets held by the different manufacturers.

Because of the difficulty of studying every single type of compound
and core, we decided it was important not to study these separate
materials in a lab, but to study the effects of thermal expansion on
plates and sleeves in real world scenarios: on warehouse floors, in
storage rooms, and mounted on engravers and presses.


These four thermographic pictures, taken in three minute intervals, depict sleeves being placed on a cold warehouse floor. Notice how, within three minutes,  the bottoms of the sleeves start to get colder. The heat transfers away from the sleeves via conduction (direct contact between solids) with the cold floor, a much more efficient method of thermal transfer than convection (heat transfer from a liquid or gas to a solid). This illustrates why it’s a bad idea to storesleeves in direct contact with a cold warehouse floor.


THE STUDY

For the purposes of this study, we standardized on 48-in. sleeves
of different elastomermaterials and on 50-in. photopolymer plates
mounted to a fiberglass core. In addition, given the difficulty of
measuring diameters of bendable plates and sleeves not on a mandrel,
we decided to only measure sleeve lengths at this time.

The outside temperature was around 29 degrees Fahrenheit, so
cooling was as simple as leaving materials outside for a couple
of hours. Materials were first acclimated for 24 hours
off the ground to insure they were the same temperature head to tail.
Then the following procedures were performed, and temperature
and length were measured at each step:

  • Engrave the material with a measurement
    scale in the material 61-in. long
  • Set material outside for two to four hours
  • Set material inside and allow to acclimate
    for two hours
  • Allow material to sit overnight
  • Heat sleeve

All length measurements had a tolerance of plus or minus
0.002-in. All temperature measurements are accurate to within
plus or minus 3 degrees Fahrenheit.

ELASTOMER VERSUS POLYMER

Several trends were readily apparent. First, 90 percent of
elastomer is a formulation of EPDM (ethylene propylene diene monomer).
We found negligible differences in the variations of EPDM sleeves
with respect to thermal expansion and contraction. They
performed very similarly to each other.

Second, we noted that polymer versus elastomer plates had very
different coefficients of thermal expansion. Polymer tended to be
more sensitive to thermal expansion than elastomer. Below room
temperature, a 61-in. polymer plate mounted to a core tended to
contract (and/or correspondingly expand) around 0.003-in. per degree
Fahrenheit, while a 61-in. elastomer tended to contract
around 0.001-in. per degree Fahrenheit. Standard 0.25 undercut was used.
Polymer plates had a 0.015-in. stickyback, a 0.040-in.
base and a 0.067-in. plate.

BELOW & ABOVE ROOM TEMPERATURE

Third, the thermal expansion characteristics of sleeves and plates did
not appear to be linear across all temperature ranges. What we
noticed was that from 29 degrees Fahrenheit to room temperature
(72 degrees Fahrenheit), thermal expansion of plates and sleeves
appeared to be fairly consistent. However, as materials were heated above
room temperature, we found they did not expand at the same rate they
contracted. Above room temperature, a 61-in. polymer mounted
to a core tended to expand 0.0015-in. per degree Fahrenheit, while
elastomer tended to expand 0.0005-in. per degree Fahrenheit. The expansion
numbers were not nearly as consistent as the contraction numbers.

Fourth, sleeve sizes of 48-in. or more seemed to expand the same
per degree as sleeves of 51-in., 53-in. and 61-in. This supports
a hypothesis that after a certain length, elastomer or polymer
adhered to a fiber core, has finite expansion capabilities and,
with elastomers, the rate of contraction is 0.001-in. per degree
Fahrenheit, regardless of whether a sleeve is 51-in. or 61-in. (20
percent longer). This is clearly an area that requires more study,
as much of this was based on rough estimates.

The previous two observations would support a hypothesis that
the strain of the material being bound to the fiber core has a
limiting effect on how much the elastomer or polymer can
contract or expand, and has a limiting effect above
and below room temperature.

TIME TO RETURN “TO SPEC”

Another observation that was readily apparent, was that it took much
less time for polymer plates and sleeves to return to ambient
temperature than it did for elastomer sleeves. This is due to two
reasons that both have to do with Specific Heat Capacity (commonly
called “Specific Heat”)—the amount of heat per unit of mass
(or volume) required to raise the temperature of a material by
one degree. Specific heat is usually measured in joules
per kilogram per degree Kelvin, J/kg-K, or kilojoules per
kiloliter per degree Kelvin 103 J/m3-K.

The first reason polymer plates contract and expand more quickly
is that polyester, the base for most photopolymer sleeves and
plates, has a lower specific heat than EPDM elastomer
(1 kJ/kg-K versus 2 KJ/kg-K). It takes more energy to increase
the temperature of elastomer. Again, this is general, as
EPDM and photopolymer formulations for flexography are proprietary.

The second reason is that elastomer sleeves usually have much
thicker walls, which means there is more material to return to
temperature. As can be seen from the formulation for specific
heat, the more material you have, the more energy it requires to
raise its temperature—it takes longer to return to room temperature.

In our observations, it took most elastomer sleeves 12 hours to come
back to spec, while it took polymer only three hours. Fiberglass cores
took two hours to come back to spec. It is important to note that “back to spec”
means the material returned to its room temperature dimensions or file dimensions.
We did not base this timeframe off a measure of temperature, because only the surface
heat of the elastomer sleeve could be measured. We believe that a “return to spec”
meant that the sleeve’s core temperature, not just the surface reading
had returned to 72 degrees Fahrenheit. More study is needed and
ways to measure the core temperatures of sleeves is something that
manufacturers may want to examine.


In this time lapse, the sleeve was photographed every minute for 40 minutes,
cooling down to room temperature via convection (the air of the room in
contact with the solid sleeve). Note that the ends cool faster than the middle,
and that even after 40 minutes, the sleeve is not down to room temperature.


FUTURE DEVELOPMENTS

There is still much to learn when it comes to how thermal expansion
affects polymer plates and sleeves. We feel that this rough study is
just a starting point, and that extensive research needs to be done to
improve our knowledge of all the effects of thermal expansion and
contraction in flexography.

Accurate linear and volumetric thermal expansion and contraction
statistics should be provided by plate, sleeve and core manufacturers,
and it is up to end users to make manufacturers aware of the issues.
Studying these issues in the lab will help increase the real world
knowledge for engravers, pressmen and production managers.

Despite the need for increased communication, manufacturers are
constantly making strides. New materials are continually under
development. One elastomer we studied had one third the thermal
coefficient of expansion as EPDM, putting it very close to the
thermal expansion characteristics of the steel bases of rotogravure.
We are also working with manufacturers and printers on methods
of acclimatizing plates and sleeves more quickly, and are exploring
the concept of using embedded devices to report core sleeve
temperatures.

Knowledge is power. Knowing how temperature affects the printing
process allows us all to continue to capitalize on the economic
advantages of elastomer and polymer over rotogravure, and the run
length benefits of elastomer. We look forward to continuing to study
thermal expansion issues and working closely with manufacturers to
best address them.


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 plate making. To learn more about how Carey
Color can help you, contact 800-555-3142 or visit www.careyweb.com.

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