Home' FLEXO Magazine : October 2014 Contents 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 tem-
perature, a 61-in. polymer plate mounted to a core tended to contract
(and/or correspondingly expand) around 0.003-in. per degree Fahren-
heit, 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 con-
tracted. 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 tem-
perature 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 mate-
rial 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 tempera-
ture 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.
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
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.
60 FLEXO | OCTOBER 2014
Links Archive September 2014 November 2014 Navigation Previous Page Next Page