Note: Descriptions are shown in the official language in which they were submitted.
21380~
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention is directed to a printing form for letterpress printing, in particular for
flexographic printing.
2. Description of the Prior Art
Le~el~ress printing processes include the flexographic printing process which makes
use of elastic printing forms, rubber plates or photopolymer printing forms. The elastic
printing form enables printing not only upon absorbent substrates such as paper or cardboard,
but also on nonabsorbent m~teri~ls such as plastics, films or metal. Flexographic printing
forms are also used for in-line v~rni~hing in offset printing machines. For surface v~rnishing
with blank portions, cut out rubber blankets are used, although the latter are not suitable for
spot v~rni~hing and ornamental v~rni~hing for which flexible photopolymer flexographic
printing plates are used as v~rni~hing plates.
Conventional techniques for manufacturing flexographic printing forms include the
production of rubber plates which are made from molds. The process step for producing a
mold is omitted in the production of photopolymer flexographic printing plates.
Flexographic printing plates are produced by exposing, washing out the non-image regions,
subsequent drying and post-exposure or burning-in. Further, there are also laser-engraved
rubber plates. These printing processes are known from "Technik des Flexodrucks"
("Flexographic printing technique") (edited: J. Paris), St. Gallen, 1986, second edition.
A:\410~50.APP -2-
21380~
Conventional methods for manufacturing printing forms for letterpress printing or
particularly for planographic printing are accordingly based on the removal of non-printing
areas. The processes often require a plurality of process steps, e.g., production of a mold,
or a plurality of photographic steps and are therefore costly.
Expandable coatings are used in various areas of industry, e.g., in the manufacture of
textiles, leather, wallpapers, flooring materials and the like. For instance, an expandable
polymer, polyvinyl chloride, combined with a propellant (azodicarbonamide) and an activator
suitable for the production of a floor covering is known from DE 19 37 474.
A:\410~50.APP 3
213806~
SUMMARY OF THE INVENTION
The object of the present invention is to provide a novel possibility for the use of a
letterpress process, in particular the flexographic printing process.
Pursuant to this object, and others which will become apparent hereafter, one aspect
of the present invention resides in a device and method for producing an erasable letter press
printing form on a printing form cylinder. Coating means coat the form cylinder with at
least one layer of an expandable coating m~teri~l which contains a polymer paste and a
propellant. The coating has printing locations that can be raised, as well as non-printing
locations. Image forming means then form an image on the coating and, subsequent to
printing, erasing means remove the printing form from the form cylinder.
Pursuant to another embodiment of the invention, heat is applied to the coating
m~teri~l to raise the printing locations. An absorbent m~teri~l is contained in the coating
m~t.qri~l and the heating means can be provided in the form of either a heatable pin electrode
or a laser.
Pursuant to a further embodiment of the invention, an inhibitor is mixed with the
coating m~teri~l at the non-printing location so that only the printing locations which do not
have the inhibitor are expanded by the application of heat.
In yet another embodiment an activator is mixed with the coating at printing locations
so that only the locations where the activator is mixed are expanded by the heat application.
A particular advantage of the invention consists in that a printing form which is
produced according to the invention can be used in a varnishing mechanism for producing
surface varnishes with blank portions as well as spot varnishes and ornamental varnishes.
A:\4100-50.APP -4-
213806~
Such a device can be used inside a printing machine because the form cylinder need
not be removed from it.
The various features of novelty which characterize the invention are pointed out with
particularity in the claims annexed to and forming a part of the disclosure. For a better
understanding of the invention, its operating advantages, and specific objects attained by its
use, reference should be had to the drawing and descriptive matter in which there are
illustrated and described preferred embodiments of the invention.
A:\4100-SO.APP -5 -
213&0~
BRIEF DESCRIPrION OF THE DRAWINGS
Figs. 1 to 4 show expandable layers prior to and after expansion;
Fig. 5 shows a device for producing and erasing a printing form on a form cylinder;
and
Fig. 6 shows the layer construction of a printing form on a substrate.
A:\4100-50.APP -6-
~13~0~
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Polymer coatings, in particular thermoplastic polymer coatings such as layer 1 (Fig.
1) on substrate 8, are rendered expandable by the addition of unstable substances 3. These
substances decompose accompanied by a marked increase in volume when excited physically
or chemically or in particular physically with chemical reinforcement. Raised regions 4
forming the printing areas occur when these substances 3 are excited and subsequently
decomposed corresponding to the image to be formed. The unstable substances, henceforth
referred to as propellants, are preferably excited by thermal energy. The application of
thermal energy for the purpose of forming images can be effected, for example, by means of
a laser 5 (Fig. 1, Fig. 3) or by a heatable pin electrode 6 (Fig. 4). For example, CO2 lasers
or YAG lasers are used for the purpose of image-oriented expansion. YAG lasers are
distinguished by their improved resolution due to their improved focussing ability. However,
they require the addition of carbon black particles 7 (Fig. 3). To achieve optimum
dispersion and accordingly optimum absorption, the carbon black particles preferably have a
diameter of less than 1.0 ~m. However, the addition of carbon black particles 7 can have a
negative impact on the ~tt~in~ble degree of expansion. Therefore, it must be decided
whether the degree of expansion or the resolution of the image to be printed has the greater
importance. But other types of light sources can also be used.
The expandable polymer in layer 1 is polyvinyl chloride, for instance. Generally, a
pre-gelated thermal paste, i.e., a thermoplastic polymer, is required. Beside direct
application of thermal energy for the purpose of forming images and subsequent
decomposition for the purpose of forming images, such polymer coats according to the
A:~410~50.APP -7-
~1380~
invention can also be expanded by the inhibitor process or the activator process. In the
inhibitor process, a stabilizer (kicker) is added to the polymer in addition to a propellant.
The outer surface 10 of the layer 1 (Fig. 1) is coated, corresponding to the locations to be
printed, with an inhibitor-containing paste which deactivates the stabilizer and accordingly
inhibits expansion at the printed locations. The application of the inhibitor-containing paste
corresponding to the image to be formed is effected, e.g., by an ink jet method. The
expansion of the coating at the locations to be printed is achieved by applying heat over the
entire surface, e.g., by means of infrared radiation. The layer 1 does not expand at the
locations printed with the inhibitor, since this inhibitor deactivates the kicker.
In another process, the layer 1 is coated with an activator corresponding to the image
area. The activator is applied, e.g., by means of the ink jet method. The pre-gelated
propellant-containing thermoplastic paste forming layer 1 is accordingly printed with an
activator (kicker) so that the layer will expand to a greater extent at the printed locations than
at the non-printed locations when heat is subsequently applied to the entire surface area, e.g.,
by means of an infrared radiator.
The printing form shown in Fig. 6, which is produced by one of the processes
described above, can be made more resistant to mechanical or chemical action, e.g., solvents
or abrasion or splitting of pores in the region below the surface 10, by applying a cover layer
9 on top of layer 1 before layer 1 is expanded by heat. The cover layer 9 preferably
contains the same binder systems as layer 1, but may also be constructed in a different
manner for special printing requirements. An adhesive layer 2 can be arranged between
layer 1 and substrate 8 to increase bonding strength.
A \4100 50.APP -8-
213~06~
Adhesive layer 2, layer 1 and cover layer 9 can be applied to the substrate 8 by
means of known coating techniques. A device for applying the different layers by means of
rollers which are rotatably supported in a rotatable drum or cylinder is particularly suitable.
Such a coating device can be advantageously arranged within the printing machine, e.g.,
within a web-fed printing machine 11 (Fig. 5). The web-fed printing machine 11 has a
printing form cylinder 12 which is adjusted relative to an impression cylinder 13 in order to
print on a printing stock web 14. Printing ink is applied to the printing form cylinder 12 via
a short inking mechanism with a doctor blade 15 and a screen applicator roller 16. At the
conclusion of a printing process following the printing run of the printing form arranged on
the printing form cylinder 12, this printing form can be removed by an erasing device 17.
There are various possible methods for erasing the printing form, e.g., melting, dissolving or
embrittling. For example, the printing form is first softened by applying heat or chemicals
and is then removed by means of high-pressure water jets. In another method the printing
form is embrittled by cooling and then removed mechanically, e.g., by means of CO2 dry ice
jets, which includes bombardment with dry ice pellets.
If the new printing form to be applied has adhesive layer 2, layer 1 and cover layer 9,
a coating device 18 with three applicator units 181, 182, 183 is necessary. Each of the
individual applicator units 181 to 183 is preferably constructed as a rotatable roller which
dips into a container filled with the material of the layer to be applied and can be adjusted
relative to the respective surface of the printing form cylinder 12 in order to coat the latter
with the material in question. Each applicator unit 181 to 183 in turn is arranged on a
rotatable cylinder 180 so that the applicator units 181 to 183 can be adjusted relative to the
A:\410~50.APP 9
, 2l3sa64
surface of the printing form cylinder 12 one after the other by rotating the cylinder 180. The
applied layer can be dried by a drier 19, e.g., an infrared heat source. If an ink jet
applicator device 20 is provided in addition to the applicator units 181 to 183, an activator or
an inhibitor can be applied to the outer surface of layer 1 by this ink jet applicator device 20
as described above so that this layer 1 can be expanded subsequently by the drier 19. Instead
of expanding layer 1 by the inhibitor method or activator method, this layer 1 can also be
expanded at the locations to be printed by means of a laser.
The thickness of the applied expandable paste depends on the desired raising of
m~teri~l The raising of the m~teri~l must correspond to the standard relief depth in
flexographic printing which is at least 0.39 mm to a maximum of 3.5 mm. Relief depths of
0.39 to 1.10 mm are standard for printing smooth surfaces. In very uneven printing stock
such as corrugated cardboard, relief depths of 3.0 to 3.5 mm are standard. Commercial
propellants have minimum grain sizes of approximately 10 ~m and expansion factors of 20 to
200 ml/g. Accordingly, it is possible to raise the material by roughly three to six times the
application thickness of the pre-dried material. Consequently, in order to achieve differences
in height between the printing locations and non-printing locations on the outer surface 10 of
layer 1 amounting to approximately 0.8 mm, which is standard for flexographic printing, an
average thickness of 200 ~m of the pre-dried material is necessary.
To produce the layer 1, the coating contains one or more polymer film formers as
chief components. These film formers are expanded by adding suitable propellants. The
m~teri~l properties of film formers and propellants must be carefully matched to enable the
greatest and most uniform possible expansion of the propellant and accordingly of the
A:\4100-SO,APP - 10-
21380~
coating. Thermoplastics such as polyvinyl chloride, polyolefins, ethylene/vinyl acetate
copolymers, polystyrene, polyamide, acrylonitrile/butadiene/styrene copolymers or
thermoplastic elastomers (plastics) or elastomers (rubber) are preferably used for the coating
according to the invention. In addition to the desired favorable image-forming and printing
characteristics, another characteristic of these polymer groups is that they can easily be
erased by the device 17 by melting, dissolving or embrittling.
Propellants are added to these coating materials in a proportion to the total coating
compound such that the excited expansion of the coating achieves a uniform printing form
height conventional for flexographic printing. To produce finely structured printing areas
with optimum stability of the expanded areas, the distribution and particle size of the
propellant is a very important. The smaller the gas bubbles in the coating, the tighter and
more stable the binder mesh. The distribution of the propellant must therefore be very
uniform and very fine particulate propellants with particle diameters between 5,um and 10 ~m
are preferably used.
Suitable propellants are, in particular, solid chemical propellants, e.g.,
azodicarbonamide, azoisobutyrodinitrile, toluenesulfonhydrazide and 4,4'-
oxybisbenzenesulfohydrazide. Such organic propellants based on hydrazides and azo
compounds are exothermic.
Inorganic propellants such as sodium hydrogen carbonate or ammonium carbonate in
combination with weak organic acids, e.g., citric acid, react endothermally. Therefore, the
exothermic organic propellants are particularly suitable for the present invention. The
proportion of propellant in the coating material is between 1% and 25 % .
A:\4100-50.APP
213~0~
In the activator method, an activator (kicker) is additionally added to the coating
m~teri~l. Suitable activators are compounds of lead, zinc and cadmium such as PbO, ZnO,
cadmium oxide, cadmium acetate and/or isophorone diamine and/or dodecylamine.
Normally, 0.5% to 5% stabilizer is added.
For the inhibitor method, inhibitors such as fumaric acid, maleic acid, oxalic acid,
hydroquinone, thiourea, trimellitic anhydride or methyl ethyl ketone are used in combination
with azodicarbonamide. The use of such substances is known, e.g., from DE 19 37 474, GB
2 076 005 or DE 30 43 202.
Moreover, additional additives can be added to the coating material. For example,
carbon black increases sensitivity to lasers, since it absorbs the laser light very well within a
wide range of wavelengths. Depending on the type of laser used, the carbon black content
can fluctuate within a range of 0.1% to 10%. The wavelength absorption in the coating
material and accordingly the propellant decomposition can be accurately controlled in a point-
by-point manner by adding carbon black. Comparison between the laser source 5 in Fig. 3
and the pin electrode 6 in Fig. 4 shows that the relief 4a achieved for the printing areas with
excitation by means of laser light is much steeper than the relief 4b produced by heating with
the pin electrode 6. In the latter case, the heat acts on the surface of the layer so that heat is
uniformly distributed in every direction by heat conduction in the layer 1. With the use of
laser 5, the thermal excitation acts uniformly in the entire irradiated region given optimal
distribution and concentration of the absorbent m~teri~l, e.g., carbon black particles.
Excitation of the propellant by heat conduction is minimi7ed when using laser light. Because
of this, steeper relief edges 4a and more uniform pore sizes result from the propellant. This
A:\4100-50 APP -12-
213~0~
in turn leads to advantageous characteristics of the expanded coating with respect to stability
and resolution capacity.
Furthermore, the layer 1 can also contain other inorganic fillers which increase the
mechanical stability of the porous expanded regions. l~tçri~l~ which improve the surface
characteristics of the printing form are added to improve receptivity to ink.
A printing form produced according to this principle can be used for letterpress
processes other than the flexographic printing process if m~teri~l~ having the required
hardness are used.
According to the invention, an expandable m~tçri~l which contains a polymer paste
and a propellant and is contained in a layer 1 or in at least one layer 1 in a series of layers 9,
1, 2 arranged on top of one another is used to produce a printing form for a letterpress
process, in particular for the flexographic printing process, by raising the locations to be
printed. The process can preferably be applied within a printing machine 11 by means of a
coating device 18 having a plurality of applicator units 181 to 183. The printing form which
is accordingly produced on the surface of a printing form cylinder 12 can be raised by
expansion, e.g., by means of a laser 20, at the printing locations corresponding to the images
to be printed. After concluding the printing run, the printing form can be removed again at
the conclusion of the printing process by means of an erasing device 17.
The printing form which can be produced and erased according to the invention can
also be used in particular on a printing form cylinder 12 of a varnishing mechanism which is
constructed in the manner of the device shown in Fig. 5. The final printing mechanism of a
A:\410~50.APP -13 -
- ~1380S~
printing machine for direct or indirect printing of a web or sheet of printing stock is used as
the varnishing mechanism.
A varnishing mechanism with a printing form cylinder for selective varnishing which
varnishes only a certain proportion of the surface of printing stock, e.g., in folding
cardboard, is known from DE 39 06 648 Al.
The varnishing mechanism according to the invention which has the devices shown in
Fig. 5, the coating device 18, erasing device 17, and a laser 20 for forming images, is
suitable for producing a surface varnishing with blank portions as well as for spot v~rni~hing
and ornamental v~rni~hing.
The invention is not limited by the embodiments described above which are presented
as examples only but can be modified in various ways within the scope of protection defined
by the appended patent claims.
A:\4100-50.APP - 14-
