The cliche is the carrier of the print motif. Varying types of cliches are used depending on the desired print quality and quantity.
The motif is recessed, etched into or washed out of the cliche. In order to assure smooth fitting and printing, the motif should be at a distance of at least 25 mm (1 inch) from the outer edge, and centered on the plate. In the case of long or fine lines, the motif must be at an angle of approx. 5° to 15° to the blade track in order for the doctor blade not to dip too low and scoop out too much ink. Numerous trials have shown that an etching depth of 25 μm is optimal for pad printing. Since the pad can only transfer a limited quantity of ink, a greater depth is pointless. At an etching depth of 25 μm, the pad only picks up an ink film of 12 μm. The rest remains in the depressions of the cliche. Because the ink film consists of approx. 40 % to 60 % thinner, which evaporates during the transfer and drying, the actual thickness of the ink film left on the substrate comes to around 5 μm – 8 μm, in some cases, as little as 2 μm. As an exception to the above statement, very fine fonts are etched at approx. 15 μm, and very large motifs, at approx. 30 μm. For very large surfaces, the dip of the doctor blade can be avoided by additionally copying a fine screen into the motif.
STEEL STRIP CLICHE
This is a strip of spring steel, 0.5 mm (0.02 in.) in thickness, with a very fine surface, and a hardness ranging between approx. 48 and 54 Rockwell. In the press, the steel strip is held in the ink well by a metal plate. The advantage of steel strip cliches over photopolymer ones is that within one cliche, sections of a motif can be screened while other sections are not. As well, etching can be done in stages, i.e., certain portions of a motif can be etched more or less deeply than others. The quantity of prints which can be made depends on the type of press used and the settings, as a steel strip cliche is somewhat softer than a doctor blade. In practice, the serviceability allows between 20,000 and 50,000 printing runs.
STEEL CLICHE
This classic cliche is produced from special high-grade, abrasion-proof steel. Its area of application is found primarily in the industrial sector. Steel cliches are especially suited to high quantity printing runs and closed ink systems. The quality of these cliches is unequaled. This is evident from their absolute sharpness of contour, multi-stage etching, utilization of screens of all types, very fine surface, low susceptibility to physical damage and consistency in high quantity printing runs. The number of prints which can be achieved using this cliche comes to at least 2,00,000 doctoring operations.
CHROME CLICHE
A chrome cliche is made up of a brass film with an upper layer of durochrome which is photo-coated. In a printing press, a chrome cliche can only be used with a vacuum plate or double-stick tape. Production of such cliches is relatively expensive, and results in the formation of substances which must later be disposed of. The advantages of the chrome cliche lie in its capability to produce large numbers of prints at approx. 100,000 doctoring operations, and its multi-stage etching.
PHOTOPOLYMER (PLASTIC) CLICHE
Photopolymer cliches consist of a UV light sensitive layer of plastic which, if necessary, is bonded to a metal carrier by an adhesive coating. Single as well as double-layer materials are used for photopolymer cliches. With the latter, the upper layer of approx. 25 μm is generally carried off during the developing process. These cliches are unsuited to screened motifs. The thickness of the single-layer material is up to 400 μm, and the etching depth is controlled by the exposure and the use of a screen. The number of prints which can be obtained using this cliches lies in the range of some 10.000 doctoring operations. Under optimal conditions runs up to 100.000 prints are attainable. Areas of use for this type of cliche include (progressive) proofs, pre-series runs and smaller jobs.
OTHER CLICHE TYPES
Two further types of cliche are used for pad printing. For rotary presses, steel rollers are necessary. Since the printing may sometimes be done in a scope of
360°, the film montage and production of the rollers is very expensive. Special etching equipment is also required.
In many areas of industrial production, transitional codes are to be printed, such as the date of production, batch number, model number, etc. In order to avoid the constant production and changeover of new cliches, it is advisable to work with encoding cliches or encoding rods. These cliches are the same thickness as steel cliches, but are polished on the two long sides. Thereby, the rods lie very close together and can be pushed against each other in the ink well. This shift permits any combination of digits and letters to be printed without the necessity of interrupting the production process.