PRINCIPLES OF FLEXOGRAPHY PRINTING PROCESS

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PRINCIPLES OF FLEXOGRAPHY PRINTING PROCESS

7 June 2018

in flexographic (Relief) printing the printing elements i.e., image area are in raised
form. When the printing plate is inked, the ink adheres to the raised image area (printing
parts) and is then transferred under pressure onto the printing substrate. In flexography a
flexible, soft rubber or plastic plate is employed. – see Figure.

 

Figure 1: Flexography (Relief) printing
The principle on which a flexographic printing unit works is illustrated in figure-1. The
low-viscosity ink is transferred to the printing plate via an anilox roller that is evenly screened
with cells, the so-called screen roller/anilox roller (screen width 200–600 lines/cm, ceramic or
hardchromed metal surface). The rubber or plastic plate is attached to the printing plate
cylinder. Ink is transferred to the printing substrate by the pressure of the impression
cylinder. The use of a blade (together with the ink supply system) on the screen roller has a
stabilizing effect on the printing process resulting from even filling of the cells on the screen
roller.

Using the flexible (soft) printing plate and the appropriate ink (low viscosity) for the
printing substrate, it is possible to print on a wide range of absorbent and non-absorbent
printing substrates. With the rubber plates in exclusive use earlier, only a low to moderate
printing quality of solid motifs and rough line drawings could be achieved. For today’s higherquality
requirements, especially in the printing of packaging, photopolymer wash-off plates
are used, such as “Nyloflex” from BASF and “Cyrel” from DuPont. These allow screen
resolutions of up to about 60 lines/cm (150 lines/inch).

Flexographic Printing Process:
Flexography is a process in which the printing image stands up in relief. Liquid inks
are used which may be solvent-based, and dries mainly by solvent evaporation. Waterbased
inks are also widely used, and UV-cured systems for printing with UV inks are being
introduced. A low printing pressure is essential to the process because of the combination of very
fluid inks and soft, flexible printing plates that are used. The process has several distinctive
features.

MAIN SECTIONS OF FLEXOGRAPHY PRINTING MACHINES (PRESSES)

Multi-cylinder flexographic printing press
All flexographic presses are made up of four basic sections typically mounted in
succession between sturdy side frames.
1. Unwind section
2. Printing section
3. Drying section
4. Rewind section

1. Unwind Section :
Most of the substrates come in the form of roll or webs. Firstly they are fed through
infeed draw rolls, which pulls the web into press section. Now the speed of the web and
press speed should be synchronized ‘to provide correct tension & register control. If the
speed is more in unwind section, it is controlled by unwind breaking. An unwind section may
also include a nest of internally heated steel rolls, or the rolls used for infeed tension control
may be heated for a secondary purpose. This purpose is to ‘open’ the surface of heavily
glazed or ‘tight’ papers by preheating, thus rendering the surface more receptive to printing
ink. Preheating in this manner is also beneficial with some plastic materials, as it ‘normalizes’
the web, making it flatter and reducing the tendency to wrinkle
2. Printing Section:
A single color station with the four essential rolls are fountain roller, inking roller,
printing plate cylinder and impression cylinder – is sufficient to constitute a press. The
majority of printing presses are multi-colour; from two to eight colors in printing section. In
some presses these color units are arranged horizontally, in-line, similar to a rotogravure
press. Much over common is an arrangement, unique of flexography, in one or more
‘stacks’with a single stack of two to four color units, each color unit arranged vertically one
above another. An arrangement of color units similar to a rotary letterpress, around a single,
large, common impression cylinder is also common. This arrangement is called a central
impression (CI) press.
The printing unit consists of the following three basic parts:
a. Inking unit
b. Plate cylinder
c. Impression cylinder
a. Inking Unit:
The function of the inking system is to meter out a fine and controlled film of liquid
ink, and apply this to the surface of the printing plate.
It typically consists of an ink trough, a rubber-covered fountain roller, and a screened
(Anilox) inking roller into which cells of uniform size and depth are engraved. The fountain
roller lifts ink to the nip position, where it is squeezed into the cells in the screened inking
roller and by a shearing action, ink is removed from the roller surface. The ink in the cells is
then transferred to the surface of the printing plates. To regulate ink film thickness in printing,
screened ink (anilox) rollers are available which have screens ranging from 40 to 200
cells/cm. These may be engraved or etched on metal or ceramic. The engraved cells are
generally square in shape (although many other shapes are available now) with sloping side
walls. When printing halftones, the cells per centimetre of the anilox roller needs to be
about 3.5 times the halftone screen ruling. The number of cells and their size regulate the
volume of ink transferred. Further regulation of the ink is achieved by varying the surface
speed of the fountain roller, by altering the pressure between the fountain roller and
screened roller, and also by altering the hardness of the rubber covering on the fountain
roller. Despite these controllable factors it is still the basic characteristic of the anilox roller
which determines the ink supply to the plate. The anilox roller is a crucial factor in achieving
good-quality flexo printing.

b. Plate Cylinder:
The plate cylinder is usually made from steel. The printing plates, which have a
thickness of up to a few millimetres are secured to the cylinder with double-sided selfadhesive
material.
c. Impression Cylinder :
The impression cylinder is also made from steel. The substrate passes between the
plate and impression cylinders, which generate light printing pressure. The ink is transferred
from the cells in the screened ink roller to the plate surface, and then to the substrate, during
which it reaches virtually a uniform film.
For high-quality flexographic printing the components of the printing unit must be
engineered to very tight tolerances (measured in tenths of thousandths of an inch). The
ability to manufacture to these standards is one of the factors which has contributed to the
growth of flexographic printing to produce higher-quality products.
3. Drying Section:
The Drying section require an after-drier to remove the remaining solvent from all the
colours before the web can be wound in to a roll. The drying section may also require
between printing units on multi color presses to permit the necessary printing of color on
color. The removal of solvents can be accomplished in several ways, hot air drier is the most
common. However revolutionary method of drying are being investigated.
An exhaust system conjunction with the after dryer prevents a build of solvent laden
air that might become an explosive hazard. In between color hot air dryers it is essential that
the exhaust exist the warm air supply, otherwise the location of these dryers in the very
minimal space between color units would result in warm air being blown on to the inking
rollers and plate cylinders. Premature ink drying would seriously interfere with the inking of
the plates and printing of their image on to the web.
4. Rewind Section:
This section is identical to the unwind section in most respects but with some
significant differences. It need to be nothing more than a shaft in plain bearings holding the
winding roll by means of core chucks. However, there is one important difference. The
unwind shaft is braked to add necessary tension as the press pulls the web off the roll. The
rewind shaft must be driven.
Drying Systems
The most common type dryer is the forced hot air system. There are various sources
for heating the air but usually it is natural gas. Steam and electric heat exchangers are also
used. It is important to define ink drying in order to better understand the function of a drying
system. The drying of ink on any substrate is basically the process of attempting to eliminate
the solvents from the ink.
Water or solvent molecules are held together by their potential energy bond. These
molecules are not static but are always moving at a high speed and colliding with each other.
The higher the temperature of these molecules, the greater their kinetic energy, and
the faster their speed of travel. If these molecules can absorb sufficient energy from hot air
or other heat sources, it is, possible for them to break their potential energy bond and at this
point start to evaporate. Once evaporation has started, a new set of conditions arise. The
molecules now in a gaseous state, must pass through a laminar layer of air that is present
with a moving web.

These gas molecules must be removed quickly to prevent their return to
the ink surface; and heat and fresh air must be continually fed to the surface to continue the
evaporation process. The basic purpose of most drying systems is to induce a faster
evaporation rate of the solvents by first heating the solvents, and secondly by continuously
supplying a fresh supply of non solvent laden air to the ink surface in order to absorb the
evaporating solvents.
Whether the flexographic press be a stack central impression cylinder, or in-line type,
all drying systems are designed to dry the ink between each color station as completely as
possible before the next layer of ink is applied. After the web has been printed it travels to a
final drying oven to complete the solvent removal. On earlier drying systems, it was common
to use one heat source for heating the supply air, one fan source for blowing and supplying
the air to both the between-color and final oven, and one exhaust fan to exhaust the solvent
laden air from both the between-color and final oven. Today, dual drying systems are
generally used. There is a separate burner, control, and supply fan for both the betweencolor
and the final oven drying section. The advantage of the dual drying system becomes
apparent when printing is done on cellophane materials. After the initial trapping at the color
station the final oven must perform the added function of providing ink adhesion by fusing
the ink to the cellophane coating. This can be accomplished readily in the dual systems by
increasing the oven heat.
There are many different styles of between color drying covers, but all aim at
delivering the maximum amount of air with the highest possible jet velocity over the longest
web travel possible. There is, however, an optimum goal for designing a jet in order to get
the maximum heat transfer with high velocity air. This relation is between the air velocity, the
jet opening, jet spacing, and the distance of the jets to the web. High velocity air can be
defined as air movement that is always higher than 10,000 FPM when measured at a
sufficient distance from the jet orifice to simulate the position of the web.
Many presses combine different drying methods. For instance, in the after-oven
various temperature and velocity zones may be combined. Also, different heat sources can
be combined for different drying applications. Chill rolls are used to cool the printed web
back to room temperature to prevent blocking. Single or multiple rolls in either single wall or
double wall construction are presently used. It is also common to cool the non-printed side of
the web first.