CA1282278C - Method for printing objects having curved or polygonal surfaces - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
The method for printing objects having curved or polygonal surfaces, more particularly cylindrical objects, which method improves the quality of the printing, is easy to carry out, and has a high throughput capacity, 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, is brought to bear closely against the surface of the object as a strip, at least the said strip being heated to above the sub-limation temperature of the printing ink, and being secured, during the heating, closely to the surface of the object, and the means used to secure the said strip, and then the strip itself, being finally removed.

Description

'12B22~

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In recent times the three-piece can is being superseded more and more by the two-piece can which has no welded or soldered seam either in the cylindrical part or between it and the bottom.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 ,, ~

.

~282;~78 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.

~Z8Z278 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 ~28:~

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.

- ' --~28Z278 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 ", 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 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.

Claims (6)

1. A method for printing objects having curved or polygonal surfaces, especially of cylindrical objects, by coating the surface of the object with a plastic which has an affinity for dyes and which inhibits migration, placing a flexible auxiliary carrier printed with printing inks which sublime under heat as a strip closely against the surface of the object with the printed side facing the plastic coating, heating at least said strip to above the sublimation temperature of the printing inks, the strip being held during the heating closely to the surface of the object and finally removing the means used to secure the said strip and then the strip itself, characterized in that the strip is secured to the surface of the object electrostatically or by securing the ends of the strip by means of an adhesive or adhesive strip.

2. A method according to claim 1, characterized in that at least the said strip is heated at a temperature of 200 to 350°C.

3. A method according to claim 1, in which at least the strip is heated at a temperature of 250 to 300°C.

4. A method according to claim 2 or 3, in which at least the strip is heated for 5 to 40 sec.

5. A method according to claim 2 or 3, in which at least the strip is heated for 0.01 to 20 sec.

6. A method according to claim 1, 2 or 3, in which the ends of the strip are united by means of an adhesive or adhesive strip which, upon being heated, loses its adhesiveness, the said strip being thus released from the object in the heating stage.