COLOURS
White light
The eye may distinguish a shape or an
object if those are lightened by a luminous source.
The main one is the sun, which emits a
white light.
Decay of the white light
With
a Newton’s prism, it is possible to decay the white
light into numerous coloured lights. The white light is
consequently the superposition of all that coloured
lights.
Electromagnetic spectrum
Every coloured light moves with the shape
of a sinusoidal wave.
A certain wavelength «»
corresponds to each coloured light.
Our eye may only distinguish a very small
part of the lights.
Wavelengths of the visible part are
between:
400 nm
and
700 nm.
In the current circumstances, we may find
this decay effect in the following cases:
- Rainbow: decay of the light by the
raindrops.
- Shell nacre
- Laser C.D.
- Oil spot on the ground.
Primary coloured lights
Inversely, the superposition of all these
coloured lights reconstitutes a white light.
Our eye will get the same feeling with
the superposition of only 3 coloured lights:
Blue
Green
Red
These 3 coloured lights are : primary
coloured lights
The additive synthesis:
The superposition in various proportions
of 2 or 3 primary coloured lights allows reconstituting
all the coloured lights of the spectrum.
Superposing them two by two, we get
secondary coloured lights.
Superposing the 3 primary coloured lights
we obtain the white light.
The principle is used for:
-
T.V. screens with the addition of
enlightened red, green and blue dots,
-
Process printing dies manufacturing:
chromatic analysis with filters of the original
image, for the spread of the different colours in
primary colors.
Primary colours :
The
subtractive synthesis :
So far we have spoken about coloured
lights. A printer uses inks, that is colours on a
support to be printed, that absorbs a part of the
spectrum of the white light
The three primary colours are the colours
that allow reproducing all the colours thanks to an
appropriate mixing:
Magenta
Primary yellow
Cyan blue
For process printing, we use these
primary colours to reproduce all the other colours.
The chromatic circle allows finding most
of those colours with the same principle of a hue list
(the Pantone).
On a chromatic circle 2 diametrically
opposite colours are complementary. The mixing of 2
complementary colours produces a black one.
Eye and colour
Basically speaking, the eye contains 3
types of sensitive cells : 1 for blue, 1 for red and 1
for green.
Any lightened object with white light
absorbs a part of the spectrum and reflects the other
part.
The eye sees cyan blue: that is to say a combination of
blue and green light.
The eye sees magenta: that is to say a combination of
blue and red light.
The eye sees yellow: that is to say a combination of red
and green light.
The eye sees black: no light.
A colour-blind person has one or several type
of those cells damaged.
Some optical effects:
Focusing on a colour, the eye will compensate
in generating the complementary colour at the side, or when
taking off the rosy colour, after a long observation with
white background.
PRINTING
PROCESSES
The 3 main printing principles are:
- hollow printing (hollow printing die)
Rotogravure
- flat printing : Offset (flat die)
-
relief printing : Letter press and Flexo
printing (relief printing die)
Rotogravure
*
Use of greasy ink
*
For large series only, this process is costly
because the cylinder is engraved with the design to print.
Example of use: wall paper
Offset
*
Use of greasy ink : high viscosity
*
Very good quality printing
*
Example of use : newspaper, posters,
magazines
Letter press
*
Origin of the flexography
*
Use of greasy ink
*
Not used for corrugated board
Flexography
*
Embossed printing process
*
Use of water ink.
FLEXO GROUPS
Different components of the flexographic
group.
The MARTIN flexo groups is composed of:
-
an anilox roll
-
a printing cylinder
-
a rubber roll or a chambered doctor
blades
-
an impression roll
-
a inking circuit
-
a washing circuit
-
stereos or printing plates
According to the Martin’s machine, 2
different configurations are possible: top printing or
bottom printing:
*
Top printing : 616, 618, 718, 1224.
*
Bottom printing : 924, 1228, 1628, 1636,
DRO.
Advantages
of the bottom flexo:
- “natural” removal of the dust before printing
- no need to turnaround the boards
- the ink cannot overflow on top of the
boards
- top die cutting on inner side of the
boxes (better stripping of the waste)
Advantages of the top flexo:
-
The dust does not fall on top of the inking system
-
Folding is easier because of the gravity.
The anilox roll
(screened cylinders)
The surface is composed of many little cells
that determine the volume of ink to be transferred to the
printing die.
There are 2 classes of Anilox rolls :
*
with chromium coating after a mechanical
engravment
*
with ceramic coating that is then laser
engraved.
The chromium plated screened cylinders
The
steel cylinder is engraved by milling. Sometimes, the steel
is first covered with a copper coatingand is then engraved
by means of a diamond or by milling.
On MARTIN machines, all the chromium plated
cylinders are engraved by direct millng on the steel.
The cells may be :
*
pyramidal
*
truncated pyramidal.
The cylinders are finally covered with a
chromium coating (15 microns thick) that increases the wear
strength.
Ceramic SCREENED cylinderS
The cells have a hemispherical shape or a
cone shape. The cylinder surface has been polished to avoid
irregularitiesand get a smooth surface.
The ink return may be improved because of no
retaining edge in the cells. The wear strength is higher and
allows returning the same ink quantity even longer.
Screen angles
The cells are lined up with a specific angle
according to the generatrices of the cylinder.
The two main screen angles are :
*
45° (on chromium plated cylinders and
occasionnally on ceramic coated cylinders)
*
60° (common value on ceramic anylox rolls)
The 60° angle is well adapted for the solid
printings. It gives a better ink transfer for a similar cell
volume of the anilox.
The number of lines per cm
The screen is expressed in :
*
number of lines (or cells) per cm
*
number of lines (or cells) per inches
For current jobs (solid, text), the use of
cylinder with a screen between 80 and 100 1/cm allows to
obtain good results in most cases.
For process printing, it is recommended to
use cylinders with a screen 4 times finer than the one of
the printing die. This is to guarantee an uniform inking of
the printing die dots.
The use of an anilox cylinder with 120 to 160
lines / cm allows to keep this ratio for higher definition
printing plates and to get a quality printing.
Ink volume
This is the theoretical quantity of ink that
fills the cells of a reference area.
It is measured in cm3/m2
or in BCM (Billion Cubic Microns).
The printing quality depends on the volume.
If the anilox roll get clogged, the ink volume decreases and
the printing tint decreases as well.
The choice for an anilox :
Depending on the requested printings : solid,
lines or process printing, the ink, the metering system and
the support : white, Kraft, coated paper, the anilox roll
should be defined according to the following chart.
For a common quality of corrugated board, the
standard definition of the anilox roll is:
10 cm3/m2
with 100 l/cm and a 60° angle.
The
print cylinder
Different dies mounting :
To decrease the set-up time, it is better to
position the printing die on the polyester base according to
the holding flaps and the distance of printing.
For the lateral positioning, the axis of the
plate must correspond with the centreline of the machine
engraved on the print cylinder.
2 systems are generally used to hold the dies
on the print cylinder :
- MATTHEWS system.
-
FIX system.
- Mixed system with Fix and Matthews devices.
MATTHEWS system:
The die partially surrounds the print
cylinder. It is hold by means of rubbers. The print cylinder
may have intermediate holding grooves.
FIX system:
The die completely surrounds the print
cylinder. A mechanical system strains the die.
Advantages:
easy fitting of the dies on small machines
(width smaller than 2 m.),
no more damage to the dies and to the anilox
and rubber rolls due to the rubbers breaking.
Disadvantages : even for the small printings,
we must have a large surface of
Polyester backing.
Mixed system Fix/Matthews
With this system, it is possible to use a mixed die,
combining a Matthews hanging bar and a Fix tension bar.
The Matthews hanging bar gives the following advantages:
- More
rigidity for the stereo,
- Easier
to fix on the polyester
- Safer
fixation on the polyester
The cylinder diameter according to the die
thickness :
The different dies used on the same machine
must always have the same thickness. : the one requested by
the customer. The diameter of the print cylinder is machined
accordingly to this value. The printing diameter corresponds
to the machine repeat diameter.
THE Rubber roll
use:
The purpose of the rubber roll is to wipe the
anilox by means of pressure and speed difference. It is
better to use it for solid or lines printing.
It is not recommended for process printing
because the ink volume may vary with the machine speed.
Shape and composition:
It is a steel cylinder rubber coated with a
thickness of 10 to 15 mm and a hardness of approximately 65
shores.
The shape is a little bit convex to
compensate the bending due to the pressure against the
anilox.
drive:
It is equipped with an independent motor,
running at constant speed, which is lower than the minimum
speed of the anilox.
When the ink circuit is running but the
printer does not print, this motor drives the anilox. When
it is printing, the anilox is driven by the print cylinder
and at higher speed than the rubber roll.
- overinking:
When the machine speed increases, the ink
pressure tends to spread out the rubber roll from the anilox
and create an ink film around the anilox roll. The quantity
of ink will therefore be more important at higher speed.
This phenomenon is known as “aquaplaning”.
THE chambered doctor blades
USE:
They are used for fine printings, rather than
rubber roll.
Positive
and negative blades functions:
1
: positive
blade:
it actuates like a seal for the chambered doctor blades.
2 : negative blade: it wipes the anilox before the transfer of the ink
onto the die.
samples of blades:
Material
|
Thickness
(mm) |
Use. |
Life time
(h) |
Finger cutting danger |
|
Stainless steel |
0.2 |
neg. |
4/40 |
yes |
|
Plastic |
1.6 |
neg. |
80/140 |
no |
|
Plastic |
0.4 |
pos. |
average |
no |
|
Plastic |
0.8 |
pos. |
good |
no |
|
Fibre
glass + resin |
0.6 |
neg. |
50/100 |
yes |
The blades may be differently positioned:
STANDARD chamber: the efficient scraping of
the ceramic anilox roll ensures the deposit of a controlled
quantity of ink.
The wear of the blade may produce a thin
overinking.
PRINT PLUS chamber: this chamber has been
designed to leave a greater ink film between blade and
anilox roll to get a greater quantity of ink. This
overinking may vary between 3 to 6 cm3/m2,
according to the speed..
The PRINT PLUS system allows to interchange a
standard chambered doctor blades with a «print plus» one.
The dual system is a flexo equipped with
chambered doctor blades and a rubber roll as well.
The target is to have two inking levels. The
principle is the following :
-
a single ceramic cylinder corresponding in
volume to fine jobs requested by the customer
-
2 interchangeable chambered doctor blades on
a same support
-
one of them, the standard one, gives an
efficient wiping of the cylinder and leaves only ink into
the cells. The ink quantity is directly linked to the
cylinder cells volume
-
the « print plus » chamber with a different
wiping lets an ink film to pass between the blade and the
ceramic cylinder. The result will be an higher ink quantity
on the cylinder, that is to say, the cells volume plus the
film volume (average 4cm3/m2)
Print Plus system advantages:
As we may see on the following charts, the
overinking lightly varies with the speed but is almost
constant over 200m/mn.
If we compare this system with a rubber
roll :
-
the overinking is more regular in the middle
and according to the speed.
-
There are less scratches and the solid
colours have the quality of the blades printings with neat
edges.
-
even the
lines with a
0.2-mm
width and text with 10 points can be very well printed.
-
the dot gain with a 17 l/cm screen is just
higher than with a standard blade chamber. This allows very
good monochromatic screen-printings.
Advantages for the operators:
-
It is a single system on the machine and the
maintenance is divided by 2, in relation to the former
double inking system.
-
Only one technology (chamber blades) is used,
easier to understand for the operators.
-
No change of anilox needed for two different
orders, lines or solid or brown, and white paper as well.
-
The maintenance in concealed time of the
chambers and a better control of the system will be achieved
because the chambers will more often come out of the machine
and may be better maintained out of the flexo.
Difference between the chamber installed at
the inlet and outlet of the unit:
Inlet:
-
possibility to fit STANDARD chamber and
PRINT PLUS chamber (several inking
levels)
-
a single inking system: better
understanding for operators
-
washing system simplification
-
better accessibility for anilox removal
-
better control for the chamber
maintenance (in concealed time)
-
with a Print Plus chamber, the ink excess
recovery is
posible by the positive blade : recovery
by gravity
Outlet:
-
the ink transfer onto the printing die is
shorter ; less risk of anilox clogging
-
possibility to add a rubber roll
Difference between a chambered doctor blades
and a rubber roll printing:
The wiping is different : with a chambered
doctor blades there is no longer a capillary phenomenon,
generating lines on the prints.
THE
Impression roll
The steel impression roll that applies the
board against the printing die gives the printing pressure
(3).
The printing pressure
This adjustment is important:
if the pressure is too high:
*
Bad printing of the edges of the solid
prints.
*
The board is crushed: decrease of the box
compression test.
*
Clogging on the reverse text prints
*
Increase of the dot gain
*
Distortion of the thin lines printed.
pressure too low : lack of pressure in the
flute hollows.
THE Inking and washing circuit
The inking circuit with rubber roll:
A diaphragm pump conveys the ink
The ink is pumped up from the bucket and
conveyed between the anilox and the rubber roll where it
will naturally flow from the centre to the edges of
cylinders. The side scrapers guide the ink to the recovery
pan and to the bucket.
Filters in the circuit avoid the ink
pollution created by paper and dust.
The ink circuit with chamber doctor blade:
The ink is conveyed to the chamber on a side
where it will cross it up to the opposite side. A drain hole
will allow the ink to return to recovery pan.
Washing with rubber roll:
The washing is simply done by water
circulation instead of ink. The rubber roll is pressed
against the anilox.
Washing with chamber doctor blade:
The water circulates instead of ink. In the
chamber the nozzles spread out water with pressure on all
the length.
In case of double inking, the rubber roll can
be used during the washing to clean any throwing and to
better rinse the anilox.
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