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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 and thereby cool the roll.
This hotter temperature causes the roll end 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
edge of the sheet resulting in soft edges that may be cracked. |
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 are over insulated 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 at 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
thermal and distortion 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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