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Oxbow Effect - Caliper Uniformity at the Edge of the
Reel
When calender rolls are heated the roll ends tend to be hotter
as there is no sheet to absorb the heat, thus cooling the roll at
this location. This hotter temperature causes the roll to increase
in diameter. With the inside of the roll hotter, and the core iron
having a higher coefficient of thermal expansion than the surface
chill, the ends tend to grow even more.
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This growth at the ends is known
as the oxbow effect. This results in over-calendering the edges,
soft edges, and cracks in the paper on the reel. |
To counteract this tendency, the ends of calender rolls are insulated.
With well-designed insulation the calender will run with no problems
at its design temperature and trim. If the ends have too much insulation
in them, the roll will be too cool at the ends with too small a
diameter. This results in high caliper paper at the edges or hard
edges on the reel. This reel build problem can again cause breaks
in the paper edge.
The solution is to ensure that the calender rolls have the correct
amount of insulation to minimize the distortion. To accurately determine
the amount of insulation required, an accurate model of the calender
roll must be created. This requires the knowledge of its temperature
profile and boundary conditions. Typically the temperature profile
of the calender roll is measured along with the sheet temperature,
water temperature and air temperature. This temperature profile,
along with the calender stack speed is used to calculate accurate
temperatures throughout the calender roll and the thermal distortion
of the calender roll. Different configurations of insulation can
then be modeled to determine the effect on the distortion profile.
The insulation configuration that yields minimal distortion is the
one selected for use.
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