GB2114057A - Printing process - Google Patents

Abstract

A web of heat-sensitive adhesive-coated paper is printed with conventional ink and subsequently coated with a water-based overvarnish so as to prevent setting off. The apparatus may include a plurality of letter-press or lithographic print stations 10, 12, 14 followed by a flexographic print station 16 which serves to apply water-based overvarnish immediately downstream of the last of the print stations. Printed material is then subjected to a high velocity, air-impingement dryer 18 and is found finally rewound on a spool 22. The overvarnish, when dry, is permeable to oxygen to allow oxidation drying of the underlying ink. Heating to a temperature which might affect the heat-sensitive adhesive is avoided. <IMAGE>

Description

SPECIFICATION Printing process This invention relates to a printing process for paper or other thin flexible material in which setoff of printing ink immediately after application thereof to the paper is substantially avoided.

It has always been a problem in the printing industry, to avoid the setting-off of printing inks from a a recently printed sheet or printed web to the next adjacent sheet or turn of the web. The problem is a fundamental one of compromise between the rapidity of drying of the printing ink and the need to ensure a clear print impression which latter is at risk if the volatile components of the ink have an excessive volatility which results in drying or at least partial drying of the ink on the printing plate.

To overcome this problem it has already been proposed to make use of water-based overvarnishes and such varnishes have been successfully used for many years when printing carton board. Such water-based overvarnishes have, however, given rise to problems when applied to thin sheets of paper or similar material since they induce curl in the final product.

With conventional letterpress and lithographic printing inks it has been the practice to avoid setting-off by application of heat immediately after printing and it has also been proposed to use ultra-violet light for the same purpose with special inks which are rapidly cured by such light.

However, it is clearly not possible to make use of these curing processes for paper, synthetic paper and card materials which have a coating of a heatsensitive adhesive, since even ultra-violet light sources generate considerable quantities of heat by radiation, with the result that the adhesive will be inadvertently activated.

Hitherto, therefore, it has only been possible to print heat-seal adhesive coated paper and card stock with relatively low quality flexographic printing which involves little or no application of heat, or by the gravure process which gives rise to competitive costs only for very long runs.

According to the present invention there is provided a method of litho or letterpress printing of a material treated with a heat-sensitive adhesive, the method comprising the steps of printing the material with ink, immediately coating the material with a water-based overvarnish while the ink is wet so that the overvarnish forms a coherent film thereon, and drying the printed and coated material at a temperature below that at which the heat-sensitive adhesive is activated, the film of overvarnish having sufficient permeability to oxygen to allow the ink to dry by oxidation.

The invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawing in which: Figure 1 is a diagram illustrating a first embodiment of apparatus for carrying out the process in accordance with the invention; and Figure 2 is a diagram illustrating a second embodiment of apparatus for carrying out the process in accordance with the invention.

Referring now to the first embodiment illustrated in Figure 1 ,the apparatus typically includes at least four letterpress or lithographic print stations 1 0, 12 and 14 followed by a flexographic station 1 6 which serves to apply water-based overvarnish immediately downstream of the last of the print stations.

Subsequently the web of printed material is passed through a high velocity, air-impingement dryer 1 8 from which the web is taken to a tool station 20 for punching and cutting the web prior to rewinding on a rewind spool 22.

The web will normally incorporate on the nonprinted surface a layer of heat-activated adhesive which must not be activated during the printing process.

The high velocity air-impingement dryer 1 8 is of conventional construction and similar remarks apply to the punching and cutting station 20 downstream of the dryer. Again the rewind spool is entirely conventional in itself. It will be understood that the number of print stations can vary according to requirements.

At the overvarnish station the varnish is applied by means of a coating head capable of applying a wet coating of varnish to the wet ink and if desired the varnish need only be applied to the print characters themselves and for this purpose a flexographic or gravure process may be employed.

Auxilliary devices, not shown, will ensure that there is constant web tension throughout the process. This application of tension ensures that curling and other problems arising with paper and synthetic paper materials after application of water-based varnish will not arise.

The drying station will be generally standard but will incorporate controls to ensure that at no time does the temperature reach a level at which the heat-sensitive adhesive might be activated.

Referring now to the apparatus illustrated in Figure 2 the three, in-line, printing stations 10, 12, 14 of the first embodiment are replaced by corresponding printing stations 30, 32, 34 of a common impression print head arrangement.

Again at the end of the printing process a flexographic or other apparatus 36 will be employed to coat the just printed characters with a layer of water-based overvarnish. Downstream of the flexographic station the various steps will be identical to those of Figure 1 and the units are given the same reference numerals. The manner of operation of the second embodiment is substantially the same as the first embodiment.

The choice of the correct water-based overvarnish will depend on the particular conditions such as the printing ink volatility and one suitable varnish is marketed in the United Kingdom by Mander-Kidd Limited under their trade name -- SURESEAL. Another overvarnish which as proved effective in the process according to the invention is marketed by Coates Brothers Limited under their trade name -- HYPERSEAL The volatile component of the overvarnish must be removable by a stream of turbulent air at a temperature below that at which the heat-seal adhesive is activated at most equal to 500C and preferably at room temperature. The non-volatile components of the varnish must form a coherent film the oxygen permeability of which is sufficient to allow the underlying ink to dry by oxidation.

Claims (10)

1.A method of litho or letterpress printing of a material treated with a heat-sensitive adhesive, the method comprising the steps of printing the said material with ink, immediately wetting the printed material with a water-based overvarnish so that the overvarnish forms a coherent film thereon, and drying the printed, coated, material at a temperature below that at which the heatsensitive adhesive is activated, the film of overvarnish having sufficient permeability to oxygen to allow the ink beneath the coating to dry by oxidation.

2. A method according to claim 1 wherein the coating with overvarnish is effected by a flexographic or gravure process.

3. A method according to claim 1 or claim 2, wherein the drying is effected by a high-velocity air stream.

4. A method according to any one of the preceding claims, wherein the overvarnish is applied only to the printed impression.

5. A method according to any one of the preceding claims, wherein the material is kept under tension as the method is carried out.

6. A method according to any one of the preceding claims, wherein the printing is carried out at a plurality of print stations arranged in line.

7. A method according to any one of the preceding claims wherein the drying is effected at room temperature.

8. Apparatus for litho or letterpress printing of a material treated with a heat-sensitive adhesive, said apparatus comprising the means for printing the material with ink of at least one printing station, means downstream of the printing station(s) for applying a coat of a water-based overvarnish so that the over-varnish forms a coherent film thereon and means for drying the coated, printed material at a temperature below that at which the heat-sensitive adhesive is activated.

9. Apparatus according to claim 8, wherein the overvarnish applying means is a flexographic or gravure printing means.

10. Apparatus according to claim 8 or claim 9, wherein the drying means is a high-velocity air impingement dryer.

1 A method of litho or letterpress printing a material treated with a heat-sensitive adhesive, substantially as hereinbefore described with reference to Figure 1 or Figure 2 of the accompanying drawings.