Flexography
Definition:
Frequently used for printing on plastic, foil, acetate film,
brown paper, and other materials used in packaging, flexography-printing uses
flexible printing plates made of rubber or plastic. The inked plates with a
slightly raised image are rotated on a cylinder, which transfers the image to
the substrate. Flexography uses fast-drying inks, is a high-speed print
process, can print on many types of absorbent and non-absorbent materials, and
can print continuous patterns.
The
two major types of fluid inks used in flexography are water-base and
alcohol-base. Alcohol-base inks are most often used to print on nonabsorbent
substrates such as film and foil. Water-base inks most often are used on
absorbent substrates such as paper and paperboard. Although, with changing
technology, water-base inks are also used on nonabsorbent substrates.
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In
order to meet the often rigid specifications of today's flexible packaging requirements,
the inkmaker should know:
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Flexographic presses are less complicated than those used
for other printing processes. The typical flexographic press includes the
four rollers shown here. A rubber fountain roll turning in an ink pan
delivers ink to a steel or ceramic ink metering roll and then to the plate
cylinder. The paper or other substrate to be printed then passes between
the plate cylinder and a polished metal impression cylinder.
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The
purpose of the fountain roll is to pick up a heavy flow of thin ink from the
ink pan or fountain and deliver it to the metering roll. The fountain roll
and metering roll are set to rotate under pressure against each other. The
fountain roll is driven slower than the metering roll causing it to squeeze
away excess ink from the surface of the metering roll.
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The
typical ink metering roll is referred to as the anilox roll, seen
here. The anilox roll surface is engraved with tiny uniform cells that carry
and deposit a uniform ink film onto the plate. The cells are so small that
they must be magnified to be seen. Anilox rolls with varying sizes and
configurations of cells carry different amounts of ink depending on printing
requirements.
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The
purpose of the anilox roll is to pass on a thin, even layer of ink to the
plate. The pressure and speed difference between the anilox roll and fountain
roll leave the ink primarily in the engraved cells of the anilox roll.
The plate cylinder is
a metal cylinder placed between the anilox roller and the impression cylinder.
The anilox roll contacts the plate that is attached to the plate cylinder and
deposits its ink onto the raised image areas. Plate cylinders come in various
sizes. To change the number of times an image prints or repeats around a plate
cylinder, the existing plate cylinder can be removed and another easily dropped
into position.
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The material to be printed, or substrate,
passes between the plate cylinder and the polished metal
impression cylinder. The impression cylinder backs up and supports
the substrate as it contacts the printing plate. The gap, or
nip, between these two cylinders must be just right to give the
proper printing pressure.
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