Paper Standards Measur Umschlag - Sappi Mobile

Vl Absorption
Types of paper for offset printing should have a certain
absorption to give the ink a good adhesion and to store the
fountain water (temporarily). The absorption level should not
be so high that printing ink and fountain water show through
the paper. The absorption is an important property of the
paper during the printing process and is a crucial factor for
paper-ink-fountain water interaction studies.
Water absorption Cobb ISO 535
Dependent on the intended purpose, different measuring
methods for the determination of the water absorption ability
were developed. An old method is with the help of a pulling
feather/spring to apply a coloured ink on the paper with a
specific increasing width and observe if the ink spreads out
or shows through. With the ink swimming test the measure
of gluing is likewise tested. Both testing methods are limited
to printing and writing papers.
For papers which are printed in offset the absorption is often
determined with the Cobb test. The water absorption is determined
by measuring the amount of water, which is taken
up by the paper within a specific time. The Cobb test gives
only reliable values, if the paper is not fully soaked through
with water.
Cobb Sizing Tester
Oil absorption Cobb-Unger FOGRA
The Cobb test can also be used for the measurement of oil
absorption instead of water and is called the Cobb-Unger.
Ink absorption / Set-off
The set-off of paper describes the absorption of ink on the
paper. The speed of absorption is determined by the time
that the ink needs to penetrate into the paper. For the offset
process this property is very important. An absorption which
is too slow, may result in smearing because the ink does not
dry fast enough. An absorption which is too fast, may result
in a reduction of the dry ink properties.
The set-off of paper can be measured with a test printing
machine like Prüfbau or IGT.
A sample is printed with a standard ink. After several time
intervals, a part of the printed sample is countered against a
blank counter sheet of the same paper (top on bottom in
order to simulate a pile). The density of the transferred ink of
each area on the counter sheet is measured and plotted
against time.
The shorter the time from high ink transfer to no ink transfer
of the counter sheet the faster the set-off.
Water absorption – Drop test (Prüfbau)
The drop test is carried out using the Prüfbau printing
machine. A drop of a defined Isopropylalcohol solution is
applied to the paper sample by means of a micro-pipette
and the paper is immediately printed with an ink. The ink
densities in the pre-wetted and dry printed areas are measured.
The ink density in the pre-wetted area is expressed
as a percentage of the ink density in the dry printed area.
The higher the value, the lower is the ink repellence of the
paper.
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