Note: Descriptions are shown in the official language in which they were submitted.
~.13375~
The pre~ent invention relate~ to ~ew o~-
set plates whose hydrophilic surface has been given a
"dull finish" by way of a thin chromium layer.
It is known that offset printing proces~e3
use plates having a surface of hydrophilic nature. Said
surface should advantageously be given a dull finish,
as said dull finish advantageously helps the control
work at printing time, on the one hand, and the
adherence to ~aid surface of a photosensitive layer,
on the other hand.
Said dull finish can be obtained by treat-
; ing the plate support or the ~urface made from a hydro-
philic material with proce~ses wher~by the said surface
is etched either mechanically, or chemically, or electro-
chemically.
The use of chromium to produce the hydro-
philic surface of an offset plate has also been widely
described. The chromium layers used have considerable
advantages where hardness, water-acceptance and
appearance are concerned. Indeed, 3aid chromium layers
can present a dull surface. Two methods are known to
produce~ electrochemically, a chromium layer with a
dull surface~ one consisting in using an electrolytic
bath at low temperature (between 5 and 80C for example)
and the other consi~ting in using an electrolytic bath
. at a temperature below normal (between 25 and 35C) but
with an electrical current cut when the deposit is
e*fected. But in all the known ca es, the chromium layer
; (whether with a shiny or a dull ~urface) which is to
play the part of the water-accepting layer in o-ffset
plates should have a sub~tantial thickness (definitely
over 1 u and~generally between 1.5 and 2.8 ~).
It has already been suggested to depo3it
onto support3, hydrophilic layers of chromium of le~
than 1 ~u thickness. But con~iderins the examples
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furni~hed, speciali~ts are well aware that the very
thin layers proposed cannot act as water-acceptin$
hard layers as they 3cratch too easily and are often
porous, thus expo~ing a sub-jacent surface the proper-
ties of which are unacceptable in offset printing.
It has been found and thi~ i~ preciselythe object of the present invention, that it is
possible to dull the shiny surface of a hydrophilic
material, which surface is to be used as hydrophilic
surface for an offset plate, by depositin~ thereon a
layer of dull and porous chromium le~s than 1 ~ thick,
and pre~erably less than 005 p thick
The invention therefore consi~ts in depo -
iting over a surface showing the hydrophilic propertie3
re~uired in offset printinS, a very thin layer of dull
chromium the relative porosity and fragility of which
will be accepted precl~ely because the 3ub-jacent
surface is a water-accepting and inl~-refusing ~urface,
but which will be u3ed to give A "dull finish~' to the
surface of the final material.
Hydrophilic surfaces which can be "dulled"
accordi~S to the invention are all surface~ which,
heretofore, have been con3idered as hydrophilic surface~
suitable for offset platesO
For example, it i~ po~ible to ~dull~
with a fine layer of chromium~ ~hiny or semi-shiny
surfaces of hard materlals ~uch as stainles3 steel,
nickel-tin, chromium itself, chromium-chromium oxide
surfaces ~used a~ a coatin$ on steel in certain type~
of cans), phosphoru~-nickel, tin alloys and nickel
; alloy~.
It is al~o possibla, if neces3ary, to
~dull~ the surface~ of softer shiny material~ such as
tin or ~inc; but considering the "30ft" nature of the~e
metals (and in particular of tin~ it i5 preferable
to dull only the surf~ce of the thin layers of tin (i.e.
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of less than 1 }I thickness) deposited on a hard sur-
face.
In order to obtain a dull coating of
chromium according to the invention, the already known
electrochemical techniques are used, which techniques
involve either the use of a cold bath or the u~e of
a current cut during the deposit operation. It has
however been found that it t~as possible to obtain the
dull chromium deposit directly when using for the baths
a temperature below normal (25-35C) and without any
current cut, whenever the chromium coating is applied
to a chromium or tin surface.
The following examples are ~iven non-
restrictlvelv to illustrate the invention.
15 - EXAMPLE 1
A stainle3s steel plate of 35/100 thicknes~
with a shiny 3urface i used. Said plate is anode
~coured, rinced and then immersed in a conventional
chromium bath (250 g/l of chromicanhydride and 2.5 g
of sulfuric acid per litre, temperature : 280).
The plate is connected to the cathode; the current
A dcm2) is switched on 15 minutes after the
immer3ion of th~ plate and it is kept on until the
chromium deposit has reached a thickness of 0.3 ~,
the current being cut 15 seconds after the beginning
of the chromium depositing operation.
A plate with a dull finish i9 obtain~d.
Said plate is covered with a commercial photosensitive
printing layer (PCAS~ of approximately 1.8 ~ thickness~
3 After drying (3 mins. at 450C and 5 mins. at 850C) the
plate is ready~
The plate i~ thereafter uied in the
converltional manner in offsQt printing processes :
insolation, removal of soluble parts, sizing, etc
Then the plate is used on an of~set
printing machine~ It is then noted:
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- that the printing layer adheres solidly to the dull
chromium surface,
- that the exposed hydrophilic parts are defi~tely
ink-refujing,
After a printing run of 60,000 sheets, ~t
was noted that the dull chromium had been scratched
by a hard object contained in the paper, said scratch
exposing the stainless steel but the printing had not
been affected by it.
EXAMPLE 2
An offset quality steel plate produced by
the company USINOR i9 used, of 35/100 thickness.
After scouring, rinsing and an etchin~
rinse, a layer of o~6 ~ thick of hard and shiny
chromium is applied on both side~ of the plate in a
- bath composed of:
- 250 gr/l of chromic anhydride
- 2 ~/1 of sulfuric acid
- Temperature : 450C
- Density : -15Adcm2.
The plate is thus coated on both its
faces uith a smooth, hard and shiny layer of chromium
; which will protect adequately the plate during
manipulation~.
Said plate is then immersed in an electro-
lytic bath of chromium of ~imilar composition to that
described above, but the operational temperature is
280C and a layer of 0.4 ~ thickneqs of chromium i~
deposited, under 30 per dcm2, on only one of the
faces of the plate.
There is thus obtained a plate which is
protected over all its faces by a smooth layer of
chromium, and which is coated on one face with a layer
of dull chromium for use in off4et printin~. Then a
photosensitive printing layer is deposited on the dull
surface and a ready-to-use off~et plate is obtained.
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EXAMPLE 3
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A chromium plated plate sold by the com-
pany SOLLAC is used as starting plate.
After hot scouring t650c) in a sodium
bicarbonate bath (100 g/l), the plate being made
a cathode, said plate is introduced in a chromium
bath with a view to depositing on one of ita faces,
a thin layer (0.3 ~ for example) of dull chromium.
Similar tests have been conducted u~ing as
starting material:
- a steel plate (35/100 thick) produced by the company
USINOR, on which has besn applied beforehand a layer of
0.9 ~ of tin nickel alloy,
- a plate of gal~anized steel (thickness 35/100,
reference El, sold by the Société de Fer Blanc)~
with 2u8 S of tin per m2 over each one of its faces,
- a steel plate of 35/~00 whose surface haq been sanded;
it is possible on such a plate to apply the dull chro-
mium coating according to the invention (of less than
1 ~ thickness) without using a current cut during the
electrolysis ~ . -
- a steel plate (35/100 thickne~s) produced by the
company USINOR on both sides of which has been coated
a chromium, o.6 ju thick of the micro-cracked typs
supplied by Etablissements WALBER~, reference W~S.A.
2300 and workin~ at 420. This type of chromium i3
very resistant to corrosion~
- a steel plate (35/100 thickne 8) produced by the
company USINOR on both sides of which has been applied
a crack-free chromium layer o-f o.6 ,u thickness supplied
by Etablissements WALB~RG~ refersnce W~S~Ao 2650 and
working at 650. This type of chromium is very
resistant to corrosion~
All the plates prepared this wa~ ha~e proved
to be very good offset plates, capable, without any
baking of the photohardened layer~ of reaching an
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output of over 40,000 impre~ion~.
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