Note: Descriptions are shown in the official language in which they were submitted.
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PCT/RU99/00407
ENGRAVED SHAFT AND METHOD FOR MANUFACTURING
THEROF
Technology field
This invention relates to the field of producing high-precision rolls with
engraved relief surface, which are used in printing equipment, in the
production of
textiles, for making wallpaper, and for applying lacquers, glues or
suspensions to
sheet materials, fabrics etc. In particular, the invention relates to screen
(anilox) rolls
for flexographic and offset lithographic printing, which have the finest
engraved
relief surface.
Prior Art
In the course of printing, a layer of ink is applied to the screen surface of
the
roll, consisting of regularly disposed recess-cells. The excess is removed by
a doctor.
Thus, the cells receive a dosed volume of ink, which is then transferred to
the surface
of the printing form or inking roller. To obtain high quality printing, high
precision
is required in the positioning of the cells on the cylindrical surface of the
screen, and
also in the shape and depth of the cells themselves. The depth of the cells is
from 10
to 50 p.m. Constant contact with the hardened steel doctor causes wear of the
screen
surface. The mechanical wear process is augmented by the corrosion effect of
the
inks, which include solutions of salts, organic solvents etc., reducing the
serviceable
life of the engraved roll.
In practice, the most widely used screen rolls are those made of construction
steel. The cells are rolled onto the cylindrical surface by a knurling roller
made of
tool steel. The high cost of the knurling roller (which has to be made to a
higher
order of precision than the screen roll cells) and its low strength make it
necessary to
use slow, undemanding rolling regimes, and cause the productivity of the
process to
be low.
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To increase resistance to wear and corrosion, the steel rolls are chromium-
plated. An example of such a roll may be found in US Patent No. :3,613,578.
The
thickness of the chrome plating should not be more than a few microns (maximum
15
pm), since increasing the thickness of the plating alters the geometry and
volume of
the cells in the screen surface. Power contact with the doctor in the course
of
operation leads to rapid wear of the thin chromium layer and greatly reduces
the
serviceable life of the roll.
A more modern method of imparting wear and corrosion resistance to the
screen surface of the steel roll is the ion nitriding process described in US
Patents
Nos. 5,514,064 and 5,662,573. The chemical conversion of the steel surface to
iron
nitride forms a hard well-bonded protective layer (Hv 700-1000). However,
there is
a serious problem with this method, namely the high temperature of the
nitriding
process, leading to distortion of the roll due to thermal deformations, which
means
that a correcting operation has to be carried out on the roll. Furthermore,
the use of
high-chrome steel for this process makes it more difficult to knurl the screen
and
reduces the life of the knurling tool.
Attempts to increase the wear resistance of the screen roll by applying a
ceramic coating to the screen surface by the plasma spraying process (US
Patents
Nos. 4,009,658, 4,601,242 and 4,912,824) proved unsuccessful; too fine a
ceramic
coating did not protect the roll against premature wear, whereas the
application of a
thicker coating filled the screen cells to a considerable degree, reducing
their size
considerably.
The heavy weight of steel rolls is a drawback in using them. A roll about one
metre long weighs about 70 kg. This causes difficulties in fitting and
replacing rolls,
requiring special experience and tools. There is a high probability of chance
damage
to the screen surface.
Furthermore, during use at relatively high revs., the slightest imbalance in
steel rolls causes dynamic vibrations, making the rolls "judder", which
aversely
affects the quality of the printing.
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PCT/RU99/00407
The use of lightweight rolls made from aluminium alloys, which are just as
rigid as steel ones, has considerable advantages. Aluminium alloys are easy to
machine at high cutting speeds and to high precision. Thanks to its high
precision
and light weight, an aluminium alloy roll has law dynamic inertia and low
dynamic
imbalance. This enables the roll to rotate more uniformly in operation, and
reduces
or eliminates vibration and "judder".
There are known processes for producing screen rolls of aluminium alloys
with protective coatings (US Patents Nos. 5,411,462 and 5,548,897), in which
the
following sequence of operations is proposed:
- making a high-precision roll from an aluminium alloy;
- applying a protective corrosion- and wear-resistant ceramic layer to the
external cylindrical surface;
- finishing the cylindrical surface of the roll by polishing;
- engraving the screen cells in the ceramic surface by using a laser beam.
The proposals include two types of protective coating. The first type consists
of a layer of chromium oxide or aluminium oxide 200-250 pm thick, applied by
plasma spraying. The second type is a layer of aluminium oxide 25-SO pm thick,
formed on the cylindrical surface of the roll by anodising in sulfuric acid
electrolyte.
In this case, the depth of the screen cells cut by the laser beam should not
exceed the
thickness of the anode-oxide coating.
The main problem with the laser engraving (burning-out) process is that it
requires the use of very expensive equipment for the automatic control of the
laser.
Furthermore, the recesses engraved by the laser beam are not always of the
correct
shape. Differences in the roughness of the walls and bottom of the cells lead
to the
retention of a certain volume of ink and its random release, which is
detrimental to
the quality of the printing.
But in spite of this, the use of "ceramic" rolls on steel and aluminium bodies
is increasing all the time, due to the absence of any alternative which would
provide
high wear resistance and durability in the operation of the rolls. "Ceramic"
rolls last
5-10 times as long as steel chromium-plated rolls, but cost 4-6 times as much.
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Anode oxide coatings consist mainly of amorphous phases of aluminium
oxides, so their strength and microhardness are not great. The coatings are
hydrated
to a considerable degree (their water content exceeds 10%), and also contain
in their
composition 10-20% of electrolyte anions forming part of the structure of the
coating.
When the roll heats up in use, the electrolyte components and water are
removed
from the structure of the coating, leading to fracturing and breakdown of the
anode-
oxide layer and degradation of its protective properties.
There is also a known process for making an aluminium screen roll for
lithographic printing proposed in US Patent No. 4,862,799. The screen cells
are first
made in the precision cylindrical surface of the roll by engraving (pricking)
with a
diamond needle. This surface is then anodised to form a fine, relatively hard
layer, 1
3 p.rn thick, and finally, a relatively soft layer of copper, 5-8 Nxn thick,
is applied on
top of the anode layer. This is done to impart to the screen surface the
surface
properties required for lithography as regards the attraction of oil
(lipophily) and the
repulsion of water (hydrophoby).
The problem with this process is the low thickness of the copper oxide layer
on the screen surface of the roll. This layer cannot withstand the mechanical
and
chemical stresses occurring in a corrosive medium with the screen in friction
contact
with a steel doctor. Increasing the thickness of the anode-oxide layer to 15-
20
microns leads to unacceptable changes in shape and dimensions of the screen
surface
cells. This is because in anodising, no less than 50% of the thickness of the
oxide
layer grows out from the surface being treated. Allowing for the thickness of
the
copper layer applied on top of the anode-oxide layer, it is virtually
impossible to
obtain screen cells of an acceptable volume. The considerable problems with
the
anode-oxide layers themselves have already been described.
Substance of the Invention
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The main aim of this invention is to create a light and relatively cheap
engraving roll with virtually no inertia and a long serviceable life for use
in various
systems for the dosed transfer of liquids and suspensions.
Another aim of this invention is to develop an efficient process for producing
an engraved roll, including high-precision and high-productivity processes far
applying the screen cells, and up-to-date technologies for hardening the
screen
surface by forming wear- and corrosion-resistant coatings on them without
significantly altering the set volumes and shapes of the screen cells.
These and certain other aims of the invention will be explained in the course
of the detailed description of the invention.
The engraved roll proposed in this invention is made in the form of a high-
precision base cylinder of a deformable aluminium alloy. A screen surface with
a set
disposition, shape and volume of recess-cells is engraved on the working
(external)
cylindrical surface. A hard wear-resistant oxide-ceramic coating 15-50 p.m
thick with
microhardness 700-1500 Hv is formed on the screen surface of the roll by the
plasma
electrolytic oxidation method. The oxide layer bonds strongly with the
aluminium
base and is applied uniformly in thickness, adequately repeating the
configuration of
the screen. To impart various functional properties to the screen surface,
further to
improve the strength and corrosion resistance of the coating, and also to
produce a
smoother surface from which ink will wash off easily, a fine (1-~ p,m) layer
of
metallic or organic materials is applied to the porous oxide-ceramic surface.
And
finally, to create a smooth external screen surface consisting of the boundary
ribs
between the cells, which is as symmetrical as possible relative to the central
axis of
the roll, this surface is subjected to a finishing treatment in the form of
fine circular
polishing with an allowance of 2-10 pm per side.
Although the plasma electrolytic oxidation (PEO) process is known, it has
never been used in the manufacture of engraved rolls. The PEO process makes it
possible to produce uniform-thickness coatings on complex-shaped surfaces,
since,
unlike anodising, the greater part (80-90%) of a coating produced by PEO
builds up
inwardly from the surface.
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PCT/RU99/00407
The selection of optimal oxidation regimes for the screen surface of the rolls
ensures the production of a hard, relatively thin coating which is sufficient
for
prolonged operation.
The oxidation is carried out in ecologically safe, weakly alkaline aqueous
electrolytes at a temperature of 15-55°C. Impulse voltage of 50-1000 V
(amplitude
values) is supplied to the components. The pulse repetition rate is 50-3000
Hz. The
current density is from 2 to 100 A/dmz.
An intercrystalline oxide layer of 700-1500 Hv microhardness, 1 S-50 pm
thick, is created on the screen surface of aluminium alloy rolls under the
effect of
plasmo-chemical reactions.
The oxide-ceramic coating formed on the surface of the aluminium
components consists mainly of a composition of different crystalline phases of
the
oxides of aluminium (alpha, beta, gamma etc.), Therefore, in spite of their
great
hardness, they possess a certain plasticity, and compared with ceramic
coatings
formed by plasma spraying, they are less liable to micro-chipping and flaking
on the
surface.
The porous structure of oxide coatings forms an ideal matrix for the creation
of composition coatings by filling this matrix with compounds possessing
specific
functional properties.
For this purpose, this invention makes use of various metals and organic
compounds (depending on the functional properties required).
Such materials, penetrating into the pores and capillaries and forming a film
on top 1-5 ~m thick, protect the oxide coatings, while hardly changing the
volume of
the screen cells or smoothing out their rough surface at all. Unimpregnated
oxide-
ceramic coatings absorb ink so intensively that difficulties arise when
'washing off the
rolls for a change of ink in the system.
The strongly developed surface of the porous structure of the oxide layer
gives excellent adhesion between itself and the impregnating compound, and
consequently, excellent cohesion strength for the entire composition.
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Oxide rolls impregnated with one of the metals of the series Ni, Cr, Mo or a
compound of one of these metals with its oxides and carbides can be used
successfully in systems for letterpress book printing, photogravure printing
and
flexography, i.e. in those cases in which liquids on aqueous, oil or synthetic
bases are
being transferred.
In offset lithographic printing systems, where inks on an oil base are used in
the presence of water, the surface of the engraved rolls is required to
attract oil
(lipophily) and to repel water (hydrophoby). It is known that copper is such a
material by its nature. Therefore, in lithography, it is efficient to use
engraved rolls
with oxide-ceramic coatings impregnated with copper.
The application of thin protective layers of metallic compounds may be done
by chemical or electrochemical precipitation from aqueous or organic
solutions, by
chemical precipitation from the gas phase, or by physical precipitation
methods.
Organic substances for the impregnation of the microporous structure of an
oxide-ceramic coating must have good adhesion to ceramic surfaces, or should
at
least be gripped by the cavities in the ceramic coating. In the reaction
process
(heating, ultraviolet irradiation, etc.), they form a hard smooth corrosion-
resistant
layer.
Since organic substances must penetrate as deeply as possible into the porous
structure of the ceramic coating, it is preferable to use low-viscosity
diluted solutions
or ultra-disperse suspensions.
To apply the organic materials, simple technologies of the immersion of a
rotating roll in a liquid, spraying on a solution with an atomiser, and the
method of
precipitation from a gaseous phase can be used.
Apart from corrosion resistance, the organic layers may possess special
properties (lipophilic, hydrophilic or hydrophobic) and may accordingly be
used in
different printing systems.
For the impregnating compositions, the most suitable are the widely known
self vulcanising elastomers: butadiene-styrene, butadiene-nitrite, acryl-
nitrite, and
also paired regulating epoxy and formaldehyde resins and modified elastomers.
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Others can also be used: polymethyl rnethacrylate, chlorosulfonated
polyethylene,
ethylene-propylene elastomer and the like.
The external surface of the screen, consisting of the projecting intervals
(ribs)
between the cells, is subjected to fine circular polishing on a precision
circular
polishing machine. The polishing depth is 2-10 wm. The aim of the operation is
to
ensure the maximum symmetry of the external surface relative to the central
axis of
the roll and the smooth operation of the printing system. Furthermore, due to
the fine
polishing, the initial period of working in the oxide-ceramic external surface
of the
roll with a steel doctor, during which the doctor may vibrate severely and
wear
intensively, can be eliminated.
Example of the Implementation of the Invention
This invention can be used for making engraved deformable aluminium alloy
rolls of different dimensions and designs, and is illustrated by the drawings,
which
show:
Fig. l: Design of small engraved rolls of length up to 500 mm. The
monolithic body (1) is machined entirely from rolled aluminium alloy rod.
Fig. 2: Sectional design of medium-sized engraved rolls of length up to 1000
mm with apertures (3) machined into the endfaces, into which are pressed
journals
(shanks) (2) of aluminium alloy or steel.
Fig. 3: Sectional design of large engraved rolls of length more than 1000
mm, consisting of a thick-walled aluminium bush (5) pressed onto a roll (core)
(4) of
hardened steel.
Fig. 4: Section of the screen surface of aluminium roll (1) with engraved
standard cells (6) of set shape and volume.
Fig. 5: Section of the same roll as in Fig. 4, with an oxide-ceramic coating
(7) formed on the screen surface.
Fig. 6: Section of the same roll as in Fig. 5, with a layer (8) of metal or
organic compound applied on top of the oxide-ceramic layer (7).
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Fig. 7: Section of the same roll as in Fig. 6, after the finishing circular
polishing operation.
Fig. 8: Photograph with 1000-fold magnification of the screen surface of a
flexographic roll after the plasma electrolytic oxidising operation.
The rough surface of the oxide-ceramic layer at the boundaries (ribs} of the
cells can be seen clearly.
Rolls of aluminium alloys are easy to machine. Therefore, using high-
precision metal-cutting equipment, it is possible to make high-precision rolls
with an
external surface close to the cylindrical and co-axial with the central axis
of the roll.
Such lightweight, low-inertia, dynamically balanced rolls do not require
further
balancing after they have been made.
However, the rolls must be strong enough and rigid enough to prevent
deformations due to the forces arising from hydrodynamic pressure in the ink
wedge
between the roll and the printing form during operation.
The appropriate designs of rolls and grades of aluminium alloys from which
the rolls are to be made are selected depending on the required dimensions
(diameter
and length) of the roll and the pre-calculated strength.
Small rolls of length up to 500 mm (Fig. 1 ) are machined completely
(monolithically) from rolled rods of deformable aluminium alloys of grades of
the
SAE 5000 series (5082, 5086, 5056, 5356), 6000 series (6061, 6063, 6067,
6082),
2000 series (2021, 2024, 2018, 2618) and 7000 series (7075, 7175, 7475). Rolls
of
medium length up to 1000 mm (Fig. 2) are made sectionally from an aluminium
cylinder with deep apertures (3) machined co-axially into the endfaces, into
which
journals (shanks) (2) of steel or aluminium alloy are pressed. Large rolls of
length
more than 1000 mm are also made sectionally (Fig. 3). They consist of a high-
precision thick-walled bush (5) of aluminium alloy pressed onto a high-
precision roll
(core) (4) of hardened steel.
In making rolls of the medium and large sizes, only high-strength heat-treated
aluminium alloys of the 2000 or 7000 series are used.
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A significant advantage of this invention is the fact that the cells of the
screen
surface can easily be knurled at low specific pressure with high productivity
and
precision. This ensures the long life of the knurling roller. But the best
results for
precision of the screen and the productivity of the process are achieved by
high-speed
electronically-controlled engraving with a diamond needle (at about 3000 cells
per
minute}.
Another advantage of this invention is the possibility of a calculated
increase
in the volume of the cells when they are engraved to 10-25% more than the
necessary
final volume of the cells in the finished cylinder. This is necessary to
compensate for
a certain reduction in the volume of the cells in the course of the oxidation,
impregnation and finishing treatment of the screen surface.
Corrosion tests in a hydrochloric mist chamber of PEO-oxidised specimens
of the aluminium alloys from which engraved rolls might be made, without
additional impregnation, showed that the guaranteed time before the appearance
of
traces of corrosion were: for specimens of alloys SAE 5082 and 6082 - more
than
2000 hours; for those of alloy 7075 - about 700 hours; and for those of alloy
2024 -
about 200 hours.
The example given below illustrates the practical implementation of this
invention. A high-precision roll 165 mm long and 38.6 mm in diameter was made
from heat-treated SAE 6082 alloy. A screen surface with cell volume exceeding
the
required volume by 20% was applied to the cylindrical working surface by the
diamond engraving method. The lineature (density) of the screen was 100 lines
per
centimetre. The roll was then subjected to plasma electrolytic oxidation. The
roll
was placed in a bath of an aqueous solution of an alkaline electrolyte (pH
11.5) at a
temperature of 30°C. Electrolysis regimes: pulse repetition rate 1000
Hz, current
density 40 A/dm2, amplitude value of voltage at the end of the process - anode
900
V, cathode 250 V. Oxidation time 15 min. Thickness of oxide-ceramic coating on
the screen and shanks was 25 t 2 pm. An enlarged photograph of a fragment of
the
oxidised screen is shown in Fig. 8. The roll thus produced was subsequently
subjected to chemical nickel-plating with the application to the oxidised
surface of a
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uniform nickel layer 2-3 pm thick. The shaft was then polished on a circular
polishing machine, taking off an allowance of 2 pm per side.
The roll was fitted to a flexographic printing press for printing packing
materials. The roll demonstrated excellent printing qualities. The prints
produced
after making 3,000,000 copies and 6,000,000 copies were virtually identical.
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