Offset
Offset printing offset (also adapted as offset; /i>) is a printing method (reproduction of documents and images on different supports) that consists of applying an ink, generally grease, on a metal plate, generally made of an aluminum alloy; It is similar to lithography process.
The development of offset printing occurred through two inventors in different parts of the world: In 1875, by the Englishman Robert Barclay, who developed a version for printing on metals (tin), in 1903 by the American Ira Washington Rubel, for printing on paper.
The operating principle is similar to that of lithography: The plate is wetted with water or a polar solution so that it repels the ink in the non-image areas (hydrophilic area), so that the rest of the plate takes the ink in the areas where there is a hydrophobic or apolar compound (also known as lipophilic) with the shape of the motif to be printed previously engraved on the plate, either by manual methods or by photo-engraving; The difference with traditional lithography is that the image or text is transferred to the surface to be printed not directly, but through a cylinder covered on its surface by a flexible material (blanket), generally rubber or silicone, which receives the image to transfer it, by pressure, to the printed surface, usually paper.
It is precisely this characteristic that gives this type of printing an exceptional quality, since the rubber coating of the printing roller is capable of impregnating, with the ink that it has adhered to, surfaces with irregular or rough textures. All this due to the elastic properties of rubber, which metal rollers do not have.
Features
The offset printing method is one of the indirect printing systems, since the substrate (generally paper) does not have contact with the matrix plate to transfer the image. The ink passes from the aluminum plate to the rubber holder cylinder or blanket and then passes to the paper (or other substrate), exerting pressure between the rubber holder cylinder and the pressure cylinder.
Offset printing is done using monochrome plates, so a plate must be created for each color to be printed; in the case of photochromy, for each of the four colors of the CMYK color model (cyan, magenta, yellow and black), which is also known as four-color printing; In addition, a fifth color intervenes in the process: the white of the paper. By using different amounts of these five colors, almost any color achievable through the subtractive color model can be reproduced, with the exception of "metallic" colors like gold and silver, and colors that are phosphorescent or outside the range of the CMYK model. which, if necessary, can be applied with inks of these colors directly using additional plates, using the method known as flat inks.
All photographs (color or grayscale) may be reproduced lithographically using the halftone diffusion process. Currently, photographic film is digitized (by means of a high-resolution scanner), or the image is obtained by means of digital photography, and in the pre-press process it is subjected to a color separation process, which creates a different representative image of each channel. color.
A photolithium is created from the separated image, which is projected with ultraviolet light onto a sensitive lipophilic emulsion spread on a metallic substrate, generally made of anodized or monometallic aluminum or other alloys, which is then revealed; Another method of creating the plate is printing on a plastic plate using a laser printer (toner acts as a lipophilic substrate), or direct transfer of the digital image through a laser, using a device known as Computer to Plate, also known by its acronym CTP. Each of the halftone graticules is printed at a different angle, to minimize overlap between the dots; however, the inks used have a reasonable level of transparency, to correct for halftone blending imperfections so that during normal viewing the illusion of blending between the four separate inks is produced.
View of the rollers in an ofset printing press, where you can see the plate for printing.
Serial-ordered prints, usual setting for cuatricromy.
Printer MAN Roland, example of rotary ofset printing.
Later, the plate is first passed through a dampening roller, impregnating it with water, and then through an inking roller. As the ink is a fatty compound, it is repelled by water, and is deposited exclusively on the treated parts, that is, with image. The water often contains other substances to improve its reactivity with the plate and ink.
Finally, the already inked images are transferred to a rubber lining another cylinder, this rubber being the one that comes into contact with the paper to print it, aided by a counter-pressure cylinder, or platen.
This type of printing is the most used in large volume runs, due to its obvious advantages of quality, speed and cost, which allows large volume printing jobs at very low prices. Although modern digital presses (for example HP's Indigo series or the Xerox iGen3) come close to the cost/benefit ratio of an offset press, they are still not capable of producing the huge quantities that are required, for example, for the circulation of a widely distributed newspaper. In addition, many state-of-the-art offset printers use computerized printing plate systems instead of the older film-based systems, which further increases their quality.
Over the past two decades, flexo has become the dominant way of printing on packaging due to its low quality expectations and significantly lower cost compared to other forms of printing.
Plot angles
A halftone screen is made up of small dots arranged in lines. The size of the dots varies depending on the tones that are to be simulated. For example, in the light areas these dots are small and the dark ones are larger. Frame dots are comprised of exposure dots on a movie camera; A camcorder's resolution is measured in dpi (dots per inch). The exposure dots are within a grid pattern or graticule called a halftone cell.
Hatch frequency is a measure of the number of halftone cells per line. It is expressed in lines per inch (lpi).
Appropriate ruling according to paper qualities and printing techniques:
- Paper
- Paper paper 65-85 lpi
- Not tattooed 100-133 lpi
- Stucado, mate 133-170 lpi
- Bright coat 150-220 lpi
- Printing methods
- Ófset 65-220 lpi
- 120-240 lpi
- Screen printing 50-100 lpi
- Flexography 90-120 lpi
The brain can easily perceive patterns of angles between 0 and 90 degrees. In four-color printing we have four plots -one per ink-; these must be placed at four different angles, with a difference of 30° between each one to avoid the appearance of moiré; which is always generated with yellow, but since it is less noticeable, it hides the effect that is produced due to a wrong angle in the orientation of the plot. This can vary in its angulation, given the measures that must be taken to prevent the appearance of the undesirable moiré effect in an image:
- Cian: 15°/75o
- Black: 45°/90(0)o
- Magenta: 75°/45o
- Yellow: 90°/15o
It should be taken into account that, to prevent the appearance of the moiré effect, Cyan and Black should never be left close by in their plot twist.
Advantages
- A consistent high quality image, clearer and more defined than with other printing systems.
- It can be used in a lot of surfaces other than cellulosic supports with different surface finishes, suitable for complementary operations
- The foils (plank or matrix) are quick and easy to produce.
- The length of the sheets is greater than in direct lithography prints, because here there is no direct contact between the template and the contact surface.
- In large production strips the price of each printing is lower than any other system.
The offset system presents undoubted advantages for printing on paper supports, some of these advantages over other systems are:
- High lineage.
- No trace on the back.
- No squash effect.
- Big print speeds.
- Low cost of printer form.
- Retouchability.
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