Paper Analysis - Uniformity is Key to
Efficient Operation .
The uniformity of paper is of fundamental importance
to the final product quality. Of the different types of uniformity, the
basis
weight and caliper uniformity are the most important. Without this
uniformity a host of problems arise, including wrinkles, cockling,
breaks, and
misregister in the pressroom.
On-line scanners used to measure these properties
have a number of limitations. Since
the paper travels through the scanner in the machine direction while
the scanner moves across the machine, the scanner is really measuring
the paper in a triangular fashion. The algorithms in the scanner then
determine the MD & CD components, to the best of their ability.
Another limitation of the scanner is the low frequency response
inherent in the radioactive (Beta) source. Different scanners filter
the signal differently, and use different algorithms for separating the
MD from CD components. This makes it difficult to compare the
uniformity from one machine to the next.
To measure paper without the limitations of the
on-line scanners, it is recommended that the basis weight and caliper
of your paper be tested in both the MD & CD on an off-line
profilograph, or TAPIO. The TAPIO unit is the closest instrument to a
standard at the present time. Because of this, a benchmark of your
paper uniformity compared to other machines can be obtained.
The CD strips, collected from the top of the reel,
represent a true cross-direction profile. There is no possibility that
MD components will confuse the analysis. A machine direction component
is also obtained, due to the spacing between the strips. Finally a
residual component that cannot be related to the MD and CD is
determined. This component is used to determine headbox performance and
is used by manufacturers to warranty the headbox.
To overcome the low frequency response of the Beta
gauge in the mill sensor, MD butt rolls are slit from the parent reel
on the winder. When these rolls are measured in the off-line scanner at
low speed, the frequency response normalized to paper machine speed is
very good. This gives a frequency response reading greater than 200 Hz,
much higher than on-line scanners, immediately showing the frequencies
at which basis weight barring is occurring.
A paper analysis including the MD and CD TAPIO
testing is recommended on an annual basis.
If, however, there is any suspicion of a problem, or after any major
equipment change or machine speed-up, a paper analysis is also
recommended.
If the variability is high a wet-end survey will be
required to determine the source of the problem.
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