Note: Descriptions are shown in the official language in which they were submitted.
2~.~
1 OFFSET PRINTING FORM AND METHOD OF MAKING IT
The invention relates to an offset printing form
comprising a metal foil and a photoresist layer placed
thereupon, in the form of the pattern to be printed, and a
method of making an offset printing form from a record
carrier inscribed with an electro-erosion printer.
Usually, such record carriers for electro-erosion
printers, hereinafter called record carriers, consist o~ a
thin metal film applied on a substrate. At least that
substrate portion which is adjacent to the metal film is
non--conductive. An offset printing form is a flat printing
form where the image-wise differentiation is effected in
practically one plane, and implemented chemophysically
through oleophilic and o:Leophobic areas on the printing
; form. An offset printing form consists of at least two
layers, i.e. a mechanicallv stable carrier foil and a thin
layer thereupon which has a design corresponding to the
pattern to be printed. The exposed surface of the substrate
which in the present case consists, e.g., of aluminum is
preferably oleophobic, and the layer material which in the
present case consists of a resin layer is oleophilic, i.e.
ink-attractive. However, there also exist offset printing
forms where the carrier ma-terial which in that case
consists, e.g~, of a synthetic foil is oleophilicl whereas
the surface of the layer which in that case preferably
consists of aluminum represents the oleophobic areas.
There already exist offset printing forms made of record
carriers. The production of such offset printing forms is
GE9-81~018
l described e.g. in DE-AS l 496 152, and DE-OS 24 59 055, 25
14 682, and 24 33 448. The thus made offset printing forms
are each of the type which is used less frequently where the
carrier foil is oleophilic and the applied layer, e.g. an
aluminum layer is oleophobic. The offset printing forms
made in accordance with prior art are available either after
printing, or after a protective layer covering e.g. the
aluminum has been removed during printing. Furthermore,
high resolution patterns cannot be transferred by means of
electro-erosion printers onto the record carriers used
there. This is due to the fact that the layer or layers to
be printed on have to be relatively thick to make sure that
the offset printing form is suEficiently stable from a
mechanical point of view.
A record carrier without these disadvantages is
structured of a metal foil carrying a layer only as few ~m
thick which consists of a high impedance, light-absorbing
resist and which is of a defined micro-roughness. Onto this
resist, a very thin mekal layer is vapor-deposited. In that
connection, "thin" means less than lOO nm, and "high
impedance" means greater than lOO MQ/ ~ . When this record
carrier is inscribed, current is guided through the thin
aluminum layer by means of a stylus; a light arc forming at
the point of contact after the aluminum has melted there,
that light arc effecting the evaporation of the melted
aluminum. Owing to the above mentioned roughness in
combination with the small thickness of the metal layer to
be inscrihed, highly resolved patterns can be transferred
onto the record carrier~ Attempts have been made to use the
above mentioned record carrier after printing directly as an
offset printing form. However, it has turned out that the
GE9-81-018 2
1 highly oleophobic qualities of the aluminum, together with
the highly oleGphilic characteristics of the resist could
not be reached. This wettability difference would have been
the prerequisite for using the above mentioned record
carrier as an offset printing form. Furthermore, the
inscribed record carrier did not have the mechanical stabl-
lity needed for an offset printing formD
It is the object of the invention to provide an o:Efset
printing form that can be made of a record carrier which is
inscribed by means of an electro-erosion printer, and which
has a micro-rough, high impedance, light-absorbing .resist
deposited on a metal carrier foil and a thin metal layer
vapor deposited on said resist, as well as a method of
making such an offset foil of the above menti.oned record
carrier.
This object is achieved with an o~fset printing form of
the type and method specifie~ below a.nd in the claims~
Offset printing forrns according to the preamble of
patent claim 1 are known e.g~ from the article "Neue
Technologien zur filmlosen Herstellung von Druckformen" (New
technologies for the film-less making of printing forms~ by
H.W. Vollmann, published in Angewandte Chemie, 92,
pp~95ff~(1980)o Such offset printing forms are made from an
original transparent photographic image is made through
which a photoresist layer prQvided on an aluminum foil is
e~posed and subsequenily developed, with the offset printing
form being obtained as a result~ In this manner, it is also
possible to make an offset printing form of a pattern
30- inscribed by means of an electro erosion printer in a record
carrier. The generation of the offse~ printing form in
G~9 81-018 3
2~L7
1 accordance with the invention ho~ever does not require the
production of a transparent photographically made original,
nor the photolithographic transfer of the pattern made
thereon into a photoresist layer. In fact, -the offset
printing form as disclosed by the invention can be made
directly from the record carrier inscribed by the
electro-erosion printer, and of the kind defined with the
object of the present invention, involving only a very small
amount of time and apparatus. Contrary to the offset
printing forms produced through the direct inscribing in the
record carrier, in accordance with the above mentioned prior
art, the offset printing form as disclosed by the invention
is mechanically stable, and patterns of ~ery high resolution
can be transferred thereby.
The method of making the of~set printing ~orm in
accordance with the invention has been developed on the
hasis of the surprising fact that in a uniform blanket
exposure of a photoresist layer those areas on this layer
which are on a micro-rough, reflecting surface are exposed
to a radiation dose greater than twice that of those areas
of the photoresist layer that are provided on a
non-reflecting surface. If the photoresist layer is exposed
to a radiation dosage of only about one third of that
specified per se for the photoresist selected for a
predetermined layer thickness, the differences of radiation
intensity to which the individual photoresist areas will
have ~een exposed is sufficient to ensure that during
development the photoresist (with a positive resist being
assumed for the present case) will remain over the
;
. G~9-81~018 4
1 non-reflectin~ substrate, and will be removed over the
reflecting substrate.
A record carrier which is structured of a metal foil
carrier, a superimposed soot-filled resist layer with a
micro-rough surface, and a thin metal layer vapor deposited
thereon, also havin~ micro-rou~hness, shows (after havin~
been inscribed by means of the electro- erosion printer,
with the thin metal layer beiny selectively removed)
micro-rough reflecting surface portions where the thin metal
layer still exists, as well as non-reflecting surface areas
where the metal layer has been removed. If therefore such a
record carrier is coated with a photoresist layer, and
subsequently exposed to a uniform blanket radiation it is
possible, provided the procedures are executed as speclfied
above, to generate in the photoresist layer a pattern which
corresponds to the pattern inscribed by means of the
electro~erosion printer. If in addition thereto it is made
sure that those area~ of the thin metal layer and of the
soot-filled layer which are not covered by the pattern in
the photoresist are removed, the finished offset printing
form in accordance with the invention is available, provided
the metal carrier foil has been rendered oleophobic prior to
the coating with the soo$ filled resist. It is pointed out
in this connection that although micro-roughness increases
the differences in radiation intensity it is not exclusively
responsible therefor, iOe. there would be radiation
differences in the above used record carriers with inscribed
patterns a~so in those cases where the thin metal layer is
completely smooth. It is true that the radiation
differences would be smaller than if there were no
GE9-81-018 5
1 mlcro-rough metal layer~ but if a suitable photoresist and
process conditions are selected :it should suffice for
effecting useful differences in developing speed.
Advantageous embodiments of the offset printing form of
the method of making them in accordance with the invention
; are described below.
The invention will be described with reference to
drawings illustrating embodiments in accordance with the
invention. Figs. 1 to 4 illustra~e in schematic
cxoss-section the offset pri.nting form in accordance with
the invention, in sequential stages of its manufacture, an
inscribed record carrier.
In the following, the production of the offset printing
form as disclosed by the ;.nvention will be described in
detail, with reference to Fiys. 1 to 4.
Fig. 1 shows in a schematic cross-section the record
carrier of which the offset printing form in accordance with
the invention is made. The record carrier consists of a
metal carrier foil 1 onto which is applied a layer 2 of a
soot-filled resist, i.e. a resist which preferably contains
a filler consisting of soot and if necessary calcium car-
bonate. Onto layer 2, a thin metal layer 3 is vapor de-
posited~ Metal carrier foil 1 consists preferably of
aluminum, but other metals are suitable, too, which can be
rendered oleophobic. Aluminum can be rendered oleophobic,
or example, by means of anodizing. Metal carrler foil 1 is
between O.l and 0O3 mm thicko Onto metal carrier foil 1 for
GEg~81-018 6
2~
1 instance onto a foil of anodi~ed aluminum the soot-filled
resist layer is applied which on an average is approximately
2 ~m thick. The resist material used is taken preferably
from the group containing cellulose nitrate, cellulose
butyrate, e.g. an acetomonobutyrateO As a plasticizer the
resist preferably contains phthalic acid esters in quanti
ties between approxi~ately 8 percent and 30 percent by
weight of the resist portion. The soot portion in the
resist is approximately less than 15 percent by weight of
the resist portion. The size of the primary soot grains is
approximately 0.05 ~m, and that of the secondary grains bet-
ween approximately 2 and approximately 8 ~m. The secondary
soot grains effect a micro-roughness of the resist layer in
the order of approximately 5 ~m. The resist is deposited in
that a dispersi.on of the components of resist layer 2 in an
organic solvent, as e.g. ethyl acetate, with a solids
content between approximately 20 and approximately 25
percent by weight i.s applied onto metal foil 1, and in that
; subsequently the solvent is evaporated. Thin metal layer 3
which preferably consists of aluminum and is approximately
30 nm thick, is vapor deposited onto dried resist layer 2.
Metal layer 3 is of approximately the same roughness as
resist layer 2. The roughness of metal layer 3 ls necessary
for inscribing the record carrier with the electro-erosion
printer, and it is of advantage in the production of the
offset printing form.
Fig. 1 depicts a structure after having been inscribed,
i.e., after part of aluminum layer 3 has been selectively
removed. Inscrlbing can be effected by means of the
electro-erosion printer in such a manner that the portion to
G~9-81 018 7
l be inscribed is contacted with a stylus, and that subse-
quently there flows a current through the stylus and metal
layer 3 which melts the metal at the spo-~ to be inscribed,
so that an arc is formed which causes the evaporation of the
melted metal. A contributory factor for the forming of the
arc is the above discussed micro-roughness of metal layer 3
- t.he micro-roughness effects very high local field
intensities.
In the next process step, a photoresist 4 is applied
onto the structure of Fig. 1, e.g. by spraying or
immersion. Since usually the pattern inscribed in the
aluminum layer is to be transferred later with the offset
printing form, the photoresist will generally be a positive
: resist. However, it is also possible to transfer with the
offset printing form the negative of the inscribed pattern,
i.e. where the pattern inscribed in aluminum layer 3 is not
dyed on the printed pages. In those cases a negative resist
is used. In the following specification it is assumed that
the photoresist used is a positive resist. Photoresist
layer 4 is between approximately 1 and approximately 5 ~m
thiclc. The thickness of photoresist layer 4 is
substantially determined by the solubility of the exposed
photoresist in the solvent for soot-filled resist layer 2.
Photoresist layer 4 is dried at temperatures of less than
approximately 150C, and subsequently exposed. The blanket
exposure preferably carried out with a broad band tungsten
.~ lamp takes approximately 2 to approximately 5 minutes. The
process involved is the following: Photoresist layer 4 over
;~ 30 those regions where aluminum layer 3 has been removed during
. inscribing, i.s traversed by light and subsequently the
:`
GE9-81-018 8
l radiation is completely absorbed in soot-filled resist layer
2~ i.e. these por-tions of photoresist layer 4 are traversed
by radiation only once. However, where under photoresist
layer 4 ther~ is still aluminum layer 3, the radiation is
reflected, after having passed -through photoresist layer 4
at the surface of aluminum layer 3. Owing to the
micro~roughness of aluminum layer 3, the radiation is
reflected in very different directions and also causes total
reflections on the one hand at the interface between
photoresist and aluminum layer 3, and on the other a-t the
interface between the photoresist and air. The effect of
the above mentioned ref].ection of the radiation is that in
those areas of photoresist ~ which are based on aluminum
layer 3 a beam tra~erses photoresist layer 4 in its entire
thickness at least two to three times on an averaye, with
the consequence that those pho-toresist reyions which are
based on aluminum layer 3 are irradiated with a hi.gher
intensity than the re~ions placed directly on soot-filled
resist layer 2.
~0
If in the next process step the structure obtained
after irradiation and depicted in Fig. 2 is exposed to a
developer suitable for the photoresist, the different
effective intensity with which the individual regions of
photoresist layer 4 have been irradiated suffices to permit
complet~ removal of the photoresist ~here aluminum layer 3
s-till exists, whereas in those regions where the photoresist
is placed on soot-~illed resist layer 2 d.irectly,
photoresist layer 4 is not removed or to a very small extent
only. Since the usual developers of the most fre~uently
used positive resists ~polymers of the phenoiic resins of
G~9-81-01~ 9
l the Novolak-type) are buffered a~ueous-alkaline solutions,
aluminum layer 3 is completely removed, too, during
developing. Fig~ 3 depicts in a schematic cross~section the
structure ob~ained after developing. The areas of
photoresist layer 4 which are left untouched by development
reproduce the pattern inscrib~d by the electro-erosion
printer in the record carrier, or in metal layer 3,
respectively.
rrhe next process step removes those areas of
soot-filled resist layer 2 which are not covered by
photoresist layer 4. Depending on the resist used, a more
or less concentrated acetic acid is employed. To give an
example: For a cellulose nitrate resist ace-tic acid of over
80 percent concentration is used, for a resist on the
cellulose acetomonobutyrate basis an acetic acid of
approximately 50 percent to 60 percent concentration is
used, and for a resist which contains cellulose butyrate as
resist material which is marketed by Chicago Molted Products
under the model designation B-120 an acetic acid of 30
percent is used. The removal of resist layer 2 takes
between approximately 3 to 5 minutes. Fiy. 4 shows in a
schematic cross section the structure thus obtained which
represents a finished offset printing form in accordance
with one aspect of th~ invention. In the following, the
invention will be described in more detail with reference to
a specific example.
.~
This example is based on an inscribed record carrier
which consists of a 0.2 mm thick anodized aluminum foil~ a
superimposed micro-rough~ soot filled resist layer 2 with an
GE9~81-018 10
1 average thickness of approximately 2)um which contains as
resist material the above mentioned cellulose butyrate
marketed by Chicago Molted Products under the model designa-
tion B-120, and as a plasticizer phthalic acid esters and a
filler in the form of calcium carbonate and soot, the
phthalic acid ester and the filler being provided in a
quantity of approximately 20 percent by weight of the resist
material portion, and oE an approximately 30 nm thick
aluminum layer 3 vapor deposited onto resist layer 2. By
means of blanket spraying onto layer 3 or ~, respectively,
the record carrier is coated with an approximately 2 ~m
thick layer 4, which consists of a phenolformaldehyde resin
of the Novolak type, a positive resist which contains a
diazonaphthoquinone as a sensitizer. A photoresist of the
above rnentioned type is marketed e.g. hy Shipley under the
trade name 1350 H. The photoresist layer is dried at a
temperature of less than 150C. Subsequently, photoresist
layer 4 is blanket exposed with an 1000 Watt broad band
tungsten lamp for 3 minutes approximately. The thus
obtained structure is developed with a buffered
aqueous-alkaline developer solution for several minutes,
with the photoresist over layer 3 and subsequently layer 3
itself being removed. The resulting structure is processed
for three to five minutes with 30 percen-t acetic acid to
remove those areas of resist layer 2 which are not covered
by photoresist layer 40 The thus obtained structure can be
used as an offset printing form without any additional
processing~ This offset printing form is characterized by a
high mechanical stability, and by the fact that metal foil 1
and photoresist layer 4 differ distinctly with respect to
their wettability with ink.
GE9-81~018 11
