Note: Descriptions are shown in the official language in which they were submitted.
'12B22~
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The invention relates to a method for printing objects
having curved or polygonal surfaces, more particularly
cylindrical objects, by transferring the printing ink from a
flexible auxiliary carrier placed upon the surface of the
object and coating the surface thereof with plastic.
It is known to print tin-plate cans made in three pieces,
for beverages and preserves, by the silk-screen, flexographic
or offset process, the flat tin-plate being first printed, and
being then bent round and welded or soldered at the overlap and
10 to the bottom.
In recent times the three-piece can is being superseded
more and more by the two-piece can which has no welded or
soldered seam either in the cylindrical part or between it and
the bottom. The advantage of such two-piece cans is that
leakage is no problem and there are no traces of lead in the
contents of the can. It is also possible to use materials other
than tin-plate, for example untinned sheet or aluminum sheet.
Cans in which the seamless cylindrical part and bottom
consist of one piece may be produced by various known methods,
20 for example by ~rawing stamped-out circular sheet-metal blanks
or by extrusion. The latter process is used more particularly
for aluminum, in which the case the starting material used is
tablet-shaped circular aluminum blanks.
Conventional printing methods cannot be used for two-piece
cans since there is no flat surface to be printed. Instead, the
object to be printed is cylindrical. To this end an offset
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circular-printing process was developed in which the matter to
be printed is offset-printed onto a rubber mat from which it is
reprinted onto the cylindrical wall of the can. After drying,
the printed surface of the cylinder must be lacquered and again
dried in order to stabilize the printed matter against
mechanical action.
With this method, however, printing quality is relatively
poor since reprinting from the rubber mat inevitably impairs
the quality. Furthermore, the method is relatively cumbersome
lO and it is impossible to achieve large throughputs. In the case
of cans for the beverage industry, this is a major disadvantage
since very large numbers are involved and large throughputs are
required.
It is also known, for example from German OS Nos. 28 06 892
and 28 17 566, to print rigid material, such as sheet-metal,
by the transfer-printing process. In this case the sheet-metal
is first of all coated with a plastic which has an affinity for
dyes and which inhibits migration. A flexible auxiliary carrier,
printed with sublimatable printing ink, is then placed upon
20 this coated surface and the auxiliary carrier is heated under
pressure, causing the printing ink to be sublimated into the
plastic coating. This transfer-printing process has hitherto
been used only for the printing of flat objects since, during
printing, the said objects, with the printed auxiliary carrier,
must be passed, while heat is being appliedl through printing
rolls or a printing press.
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It was thus the purpose of the invention to provide a
method for printing objects having curved or polygonal surfaces,
more particularly cylindrical objects, such as cans, the said
method providing good printing quality, being as simple as
possible to carry out, and providing the largest possible
throughputs during printing.
The method according to the invention, for printing objects
having curved or polygonal surfaces, more particularly cylindri-
cal objects, by transferring the printing ink from a flexible
10 auxiliary carrier placed upon the surface of the object and
coating the surface thereof with plastic, is characterized in
that the surface of the object is first coated with a plastic
which has an affinity for dyes and which inhibits migration,
whereupon a flexible auxiliary carrier, which is printed with
printing ink adapted to be sublimated under heat, which may be
made of paper, and which has the printed side facing the plastic
coating, i5 brought to bear closely against the surface of the
object as a strip, at least the said strip being heated to above
the sublimation-temperature of the printing ink, being held,
20 during the heating, closely to the surface of the object, and
the means used to secure the said strip, and then strip
itself, being finally removed from the surface of the object.
According to this method, the known outstanding ~uality
of the transfer-printing process is obtained for the first time
on non-flat surfaces and without the use of external pressure
such as that provided by presses, heated rolls, or the like
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which ha~e hitherto always been necessary with the known
transfer-printing process. Furthermore, it is possible, with
the method according to the invention simply to pass objects
carrying the strips through a heating zone in which they are
brought to the required temperature for reprinting. Surprisingly
enough, the dyes sublimate onto the plastic coating and are
taken up thereby. In existing transfer-printing processes,
heating was always carried out only on the side of the auxiliary
carrier, whereas now the whole object may be heated. This
10 provides the possibility of higher throughput velocities by
means of a hot-air duct in which the objects are brought to the
necessary temperature.
Another advantage of the invention is that it does not
cause pollution since transfer printing is a dry process and
produces no waste-water containing harmful substances. It is
also an advantage that the objects need not be lacquered again
after printing since, in the transfer-printing process, the
dyes penetrate from the surface into the previously applied
plastic coating where they are fixed. Coating with plastic
20 prior to printing is more satisfactory than lacquering after
printing since transfer-printing units are often not equipped
with lacquering units, whereas precoating may be carried out
during production of the objects, especially in the case of
cans~
The strip may be secured to the surface of the object in
various ways.
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According to one form of the invention, the ends of the
strip may be joined together.
In this case, the length of the strip may be such that the
ends overlap where they can be secured to each o~her with the
aid of an adhesive. Instead of this, the length of the strip
may be such that the ends thereof abut without overlapping,
in which case the said ends must be united by means of an
adhesive strip or the like. Even if the ends of the strip
overlap, an adhesive strip may, of course, be used to join
10 them together.
After passing through the heating zone, and after the
printing process occurring therein, the strip must be removed
from the object, for example by slitting the strip and removing
it from the object by air-suction. Simpler, and therefore
preferable, is a method whereby the ends of the strip are joined
together by means of an adhesive, or an adhesive-coated strip,
which decomposes or in some other way loses its adhesiveness
when it is heated during the transfer-printing process. The
strip is thus automatically released from the object and can be
20 removed upon leaving the heating zone without being slit. In
this connection, the adhesive may be arranged to lose its
adhesiveness at the temperatures used, only after the dyes have
been completely sublimated over, i.e. by at least 90~, so that
no further pressure is necessary. However, it is also possible
to use an adhesive which loses its adhesiveness only at a `-
temperature above the transfer-printing temperature used in the
~282~8
heating zone. In this case, the objects carrying the strips
must be passed through an additional heating zone in which the
temperature is higher than that used in the printing zone.
In a first example of the present method, the strip is
paper and shrinks during heating for the printing process, as
a result of the rapid evaporation of the natural water-content
of the paper. As a result of this, the said strip bears firmly
against the surface of the object until it is released.
According to another desirable embodiment, the strip may
10 also be held electrostatically to the surface of the object. It
is thus pressed firmly to the object, with no need for mechanical
pressure. In this case, the strip is released simply by
eliminating the electrostatic field, at which time the said
strip automatically falls from the object,
It is in any case important that the strip be pressed -
during the heating, without the use of external mechanical
means such as presses, rolls, or the like, until the desired
amount of dye has been transferred - so firmly to the surface
of the object that the dye cannot sublimate way laterally.
20 Furthermore, it must be possible to release the strip easily,
so that it may rapidly be removed from the object after the
printing. This permits high throughput velocities, thus making
the process suitable for printing mass-produced articles such
as beverage-cans.
In order to ensure rapid transfer of the dye to the surface
of the object and, in the case of the first example of the
present method mentioned hereinbefore, to ensure rapid
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128Z278
evaporation of the water in the paper and thus cause the said
strip to shrink before the dye can sublimate away laterally, it
is desirable to heat the object rapidly to a relatively high
temperature, as soon as the strip has been secured to the
surface thereof. The object is therefore heated in the heating
zone to a temperature of 20Q to 350, prefera~ly 250 to 300C.
The period of resi~ence in the heating zone may be between 0.01
and 40 seconds, at tlle most 30 seconds, preferably between 0.01
and 5 seconds. At these temperatures, which create extreme
10 heat-shock, spontaneous vaporization of the moisture contained
in the paper occurs as soon as the object, carrying the strip,
enters the heating zone. The strip is therefore shrunk onto
the object in a fraction of a second and the pressure required
for transfer-printing is produced automatically. Upon further
heating in the heating zone, the rate at which the dye
sublimates into the underlying plastic coating depends upon the
molecular weight and thus the sublimation temperature.
The strips may be applied to the objects with conventional
labelling machines. Such machines usually draw one strip per
20 object from a stack of labels cut to size. For the purpose of
increasing the throughput velocity it is desirable, in this
connection, to use labels in the form of continuous strips.
In the case of the method according to the invention, the
objects, such as cans, to be printed are first passed through
a conventional plastic-coating unit which applies, to the out-
side of each can, a coating of plastic which has an affinity
~2~32;~3
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for dyestuffs and inhibits migration. After this coating has
been chemically or physically dried, the coated objects pass
to a labelling machine where strips are taken from a stack, or
from a continuous strip, are placed around the ohject, and are
secured by means of an adhesive, an adhesive strip, an electro-
static field, or the like. If an adhesive is used, it dries
in a few seconds without distortion. The labelled cans then
pass, with the printed side inwards, in synchronism with the
installation as a whole, continuously to a closed transfer-
10 printing zone which is heated, by means of hot air or otherheating devices, to the temperature necessary for printing, for
example 250 to 300C. After leaving the printing zone, the
strips are removed from the objects either by slitting and
applying air-suction, by air-suction alone if a decomposable
adhesive is used, or by eliminating the electrostatic field.
Suitable plastics for the surface-coating of cylindrical
ob~ects are those already known in the state of the art as
having an affinity for dyestuffs and as inhibiting migration.
They may be thermoplastic or thermosetting, as described in
20 various publications dealing with the transfer-printing process.
Examples of such plastics are: epoxy resins, silicone resins,
phenolic plastics, amino-plastics, acrylate resins, alkyd
resins, polyether-sulphonic resins, polyamide-imide resins and
others. Particularly resistant to migration are cross-lir.ked
thermosetting plastics and these are therefore particularly
preferred when high-quality printing is required.
~28Z~
g
Examples of cross-linked thermosetting plastics are:
silicone resins, radiation-hardened unsaturated acrylate resins
or radiation-hardened unsaturated polyester resins.
As regards dyes, sublimatable dyes known to be used in the
transfer-printing process or also dyes not hitherto described,
which can be sublïmated at the desired temperatures, may be
used. Since cans for beverages or preserves do not have to
withstand high temperatures, it will be possible to use
relatively low-molecular dyes for such cans. These have the
10 advantage of sublimating at relatively low temperatures which
makes it possible not only to use relatively low temperatures,
for example 200, in the heating zone, but also to use high
throughput velocities, for example 5 seconds or less, in passing
the cans through the said heating zone.
The method according to the invention is not restric~ed to
specific groups of dyes. However, suitable groups are, for
example, anthraquinone, monoazo, and azo-methine, the molecules
of which may heavily occupied with amino-, alkoxy-, oxalkyl-,
nitro-, halogen- and cyano-groups. These groups of dyes are
20 defined, for example, in the Colour Index, Vol. 1, pages 1655
to 1742. Other suitable groups of dyes are the diazo-dyes,
nitroacrylamine, quinophthalone and styrene dyes. These may be
processed into printing pastes or inks and may be printed onto
a suitable flexible carrier-material by the silk-screen,
flexographic, offset or rotogravure process. Suitable flexible
carrier-materials are, in particular, paper or plastic foils.
