Offset printing
The offset printing (from English set off ' set off ' or offset ' set off ' in the sense of 'transfer') is an indirect flat printing process and the most widespread printing technology in book, newspaper, advertising and packaging printing. In the case of indirect printing processes, the printing plate and the printing medium do not come into contact with one another. The color is first transferred to a rubber blanket cylinder and then onto the substrate . This protects the printing plate and a wide range of substrates can be printed, for example paper, cardboard, plastic (foils), glass and ceramics, metal sheets and, thanks to waterless offset printing , DVDs as well .
Functional principle of offset printing
In offset printing, the printing and non-printing areas are on the same plane. The printing elements on the printing plate - i.e. raster dots, lines or areas - are prepared in such a way that water pearls off them. The non-image parts, in turn, are prepared in such a way that water is retained here. The principle is that fat and water repel each other.
Specifically: The printing plate is first moistened by dampening rollers with every revolution of the cylinder, and then provided with ink by the inking rollers. The areas that previously accepted water remain color-free, i.e. white on the paper. The others, on the other hand, take on color, they are colored. These will later be all color-bearing elements on the printing material such as texts, lines or grid points.
The pressure plate is usually made of aluminum , the surface of which is anodized . The color-bearing points are formed, for example, by a photopolymer. The non-image areas, on the other hand, conduct moisture through the anodized surface in which the dampening solution is held. As soon as enough dampening solution is available, this thin layer of water blocks the ink's access to the non-image areas. Without a dampening solution, the entire surface of the plate takes on color; “The plate tones over the entire surface,” they say in the printing trade.
Products created in offset printing can be recognized primarily by the following features: a sharp printout without squashed or frayed edges and a smooth paper back without embossing or shading. Web offset heatset prints also show typical paper waviness (parallel to the grain direction) and a uniform gloss (fat gloss).
In offset printing (as well as in relief and screen printing , in contrast to gravure printing ) no real halftones can be printed. The following applies: color or no color. Therefore, images, intermediate tones or color nuances have to be reproduced with the help of simulated halftones. This is done by screening in periodic ( autotypical AM ) or non-periodic ( stochastic FM , random) rasters.
Color printing
From the three basic colors cyan, magenta and yellow (technical language: yellow) as well as black (technical language: depth) for the additional contrast, many colors can be represented on the paper. A machine for four-color printing therefore consists of four printing units . If spot colors are printed, they each need their own printing unit. Four or more printing units are located one behind the other in the corresponding printing lines and print each sheet one after the other. Often there are other works for painting (overprinting with a colorless, clear lacquer layer), for effect colors (metallic or mother-of-pearl) or for embossing or punching on the same printing line.
History and development of offset printing
Offset printing is a further development of lithography (also: stone printing ) invented by Alois Senefelder in 1796 . At that time, the playwright Senefelder was looking for an inexpensive duplication process for his sheet music. First, he made relief printing forms from limestone for his prints by covering the printing areas with grease ink and etching the areas without drawing into the smooth stone surface with a slightly acidic solution of gum arabic . By wetting the freshly etched non-image areas with water, these were not covered by the color and thus only the greased image areas were colored. This made etching to the letterpress form unnecessary. With this discovery he laid the foundation for the planographic printing principle on which today's conventional offset printing is based. In the following years he further developed his invention and first constructed the so-called "bar press" and then the cylinder press, which made it possible to print sheets of paper using a stone plate and impression cylinder.
With the introduction of the zinc plate at the end of the 19th century, the slowly moving stone could be replaced by a rotating cylinder with a clamped metal plate. The offset printing developed from this is attributed to two inventors independently of one another: the American Ira W. Rubel and the immigrant Cašpar Hermann, who lives in the USA . Around 1904, both of them constructed indirectly printing machines - that is, from the printing plate via a rubber blanket cylinder onto the sheet of paper. After his return to Germany in 1907, Hermann planned numerous further developments such as the web offset presses. However, the realization of his ideas could only be implemented in 1910 together with the Vogtländische Maschinenfabrik AG (VOMAG). The first completed web offset press was then demonstrated in Leipzig in 1912 .
Machine types of offset printing
Basically, a distinction is made between two types of offset printing machines:
- Sheet-fed offset printing machines
- Web offset printing machines
The designation of these machine types results from the type of printing material in use. In sheet-fed offset, individual sheets of printing material run through the machine one after the other, while in web offset, the web to be printed is unwound from a roll. There are many different configuration options, depending on the area in which the printing machines are used.
Sheetfed offset
| Format classes | Print format (approx.) |
|---|---|
| 00 | 35 cm × 50 cm |
| 01 | 46 cm × 64 cm |
| 0b | 52 cm × 72 cm |
| 1 | 56 cm × 83 cm |
| 2 | 61 cm × 86 cm |
| 3 | 65 cm × 96 cm |
| 3b | 72 cm × 102 cm |
| 4th | 78 cm × 112 cm |
| 5 | 89 cm × 136 cm |
| 6th | 100 cm × 140 cm |
| 7th | 110 cm × 160 cm |
| 8th | 124 cm × 180 cm |
| 9 | 140 cm × 200 cm |
Sheet-fed offset printing offers high print quality and a wide range of production. The areas of application range from simple business card and letterhead productions to high-quality and extensive advertising brochures, annual reports and catalogs. Depending on the machine configuration, single-color or multi-color printing as well as double-sided printing ( front and back printing ) is possible in one printing process. The sheet-fed offset printing machines are divided into format classes based on their maximum printable paper formats (see table opposite).
Sheetfed offset presses basically consist of the feeder , printing unit and delivery assemblies . The feeder is used to separate and feed the printed sheets into the first printing unit. Depending on the design, additional printing units can follow, which include several cylinders as well as dampening and inking units . After the sheets have passed through all the printing units, they are sent to the delivery. This is used to stack the printed sheets. The fastest printing machines reach a speed of 18,000 sheets per hour. So far, four manufacturers have achieved this peak value with a total of five models. The largest sheet-fed offset printing machine KBA 205 has a printable area of 1490 mm × 2050 mm.
Investors
Before starting the printing process, a stack of paper must first be fed into the machine's feeder. The system then has the task of separating the sheets, transporting them from the system stack to the system table and feeding them to the first printing unit. Depending on the format of the class offset press either be single-sheet feeder or stream feeder used. The former can be found in small-format sheet-fed offset presses, but their importance has decreased considerably with the advent of digital printing systems. With the single sheet feeders, each sheet is first separated pneumatically on the system stack, then gripped at the front edge, guided onto the system table and transferred from there to the first printing unit. The following sheet is only transported to the feed table when the previous sheet has been transferred to the first printing unit. Due to ever larger formats and higher printing speeds, these single sheet feeders reached their mechanical limits. In order to achieve smooth sheet travel and maximum accuracy, stream feeders are used today on large-format multi-color sheet-fed offset machines. These enable the simultaneous transport of several sheets over the feed table to the printing unit. Due to the scale-like overlap, the following sheet has to cover a shorter distance to the printing unit. In this way, a significantly quieter transport and thus higher speeds can be achieved compared to the single sheet feeder. The sheet then passes the drawing and front lays; this means that it is precisely positioned before the transition to the printing units.
Printing units
Every conventional sheet-fed offset press has at least one printing unit, which consists of a printing forme, blanket and counter-printing cylinder as well as inking and dampening units . Typically, the major printing press manufacturers (such as Heidelberg , manroland , KBA , Komori ) build the multi-color offset presses in the so-called series design. Each printing unit consists of a three-cylinder system. This means that for each color there is a complete plant with its own plate, blanket and impression cylinder. The number of works thus determines the maximum number of colors to be printed in one run. In the case of multi-color machines, there are also transfer drums between the individual printing units, which transport the sheet from one unit to another. The inserted schematic representation of a sheet-fed offset printing machine shows two printing units in a row construction with the most important assemblies.
The ink is supplied via the inking units, whose task is to permanently supply the printing areas of the printing forms with the required amount of ink. The ink layers transferred to the printing material are only about 1 µm thick (1 µm = 0.001 mm). The highly viscous (very viscous) paints are supplied via the paint box, which is divided into several zones with a width of 25 mm to 35 mm. The required amount of ink in the circumferential direction is regulated via the individual zones, since the color profile of a printed sheet is usually not built up uniformly and the range of colors must therefore be adapted to the profile. How much color should be added to the factory can be set individually for each zone. The ink is transported from the ink fountain to the plate cylinder by around 15 to 20 rollers, which are alternately covered with hard special plastic and soft rubber material. The large number of rollers is necessary, among other things, to obtain a streak-free, uniform ink film over the entire printing width.
The dampening solution required for the process is supplied via the dampening units. The dampening solution, which consists of water and various additives, has other functions in addition to keeping the non-printing areas free. Among other things, it is responsible for the stability of the printed emulsion. (One speaks of a stable emulsion with a dampening solution content of approx. 15-25%.) Since even minimal fluctuations in the ink-water balance have enormous effects on the print quality, the continuous supply of the dampening solution is of great importance. Due to the cold that occurs when the dampening solution evaporates, it also contributes to a stable temperature balance within the inking and dampening system.
The dampening solution and ink are first transferred to the printing plates, which carry the image information of the respective color separation. These are clamped onto the plate cylinder of the respective printing unit. In order to be able to fix the thin sheets on the cylinders, there are so-called plate clamping channels. The channels form interruptions in the circumference of the cylinder, in which slide rails are housed. By means of these rails it is possible to clamp the plates firmly onto the cylinders. With multi-color printing, it is very important that all plates are precisely clamped. Since the print image is composed of several colors, even slight inaccuracies in the overprint can lead to unusable results. By moving the plate cylinder axially and radially, the printing units can be precisely adjusted to one another. Current semi-automatic or even fully automatic plate clamping systems achieve a high level of precision when clamping the plates from the outset.
Offset printing is an indirect printing process. This means that the ink or emulsion is not transferred directly from the plate cylinder to the substrate, but first on a rubber blanket . These blankets, made of elastic material and layers of fabric, are stretched over the rubber blanket cylinders of the printing units. Because the print image is transferred to the paper through the rubber blankets, their quality is important for the print result. However, aging or damage can seriously affect the quality of the wipes, which is why they must be replaceable. For this reason, like the plate cylinders, the blanket cylinders also have a channel in which the clamping devices for fastening the blankets are located.
The print image transferred from the plate to the rubber blanket is passed on to the printing material. This is done with the help of the impression cylinder, which guides the sheet of paper through the printing unit. The task of the impression cylinders is to fix the sheet in place, guide it through the printing zone and exert the necessary pressure on the blanket cylinder for perfect image transfer. The fixation takes place with the help of grippers, which are housed in the channel of the cylinder. These grippers take hold of the sheet at the leading edge, guide it through the respective printing unit and then transfer it to the grippers of the transfer drums. These in turn forward the sheet to the next plant.
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After the sheets have run through all the printing units, it is necessary that they are laid out exactly on a stack. However, since the sheets arrive at a very high speed, they have to be slowed down, tightened and straightened using various guide elements. This is achieved, among other things, through controlled air flows, baffles, sheet brakes and straight bumpers. A smooth-edged delivery stack is of great importance, especially in the subsequent print processing , in order to be able to feed the sheets precisely to the subsequent machines. Another problem in the delivery arises from the drying principle in conventional offset printing. The printing inks used are not yet completely dry when they arrive in the stack, but are still sticky and sensitive to smearing. In order to avoid smearing or being deposited in the stack, the distance between the last printing unit and the delivery stack is used to install drying units and powder devices . Since the distance is very short, there is not enough time for the paint to dry completely. The fine powder granules, which are distributed over the entire sheet, however, ensure that there is a gap between the non-dry surfaces and the subsequent sheet, thus minimizing the risk of being deposited, smeared and blocked.
Web offset
In web offset printing presses, a basic distinction is made between two different processes: on the one hand the heatset process and on the other hand the coldset process. The former machine technology is used, among other things, for the production of magazines, catalogs and brochures, while newspapers, paperbacks and the like are mainly produced with coldset printing machines. In contrast to sheet-fed offset printing, in which face-to-face printing is only optional in one print run, the paper web is always printed on both sides in web offset printing. In principle, web offset presses consist of the following components: roll carrier / changer, tensioning unit, printing unit, folder superstructure and folder. In heatset printing machines, a dryer and a cooling roller unit are also integrated between the last printing unit and the folder superstructure. The paper web is unwound from the roll and fed to the first printing unit with constant web tension regulated by the pre-tensioning unit. Depending on the configuration, the web then passes through other printing units and, in heatset offset printing, reaches a dryer after the last unit. This ensures that the colors dry quickly. Since the paper web gets very hot during this process, it is then passed over cooling rollers. The web then runs - both in heatset and coldset printing - into the fold superstructure with the former. In this area, among other things, the longitudinal cut of the web, the first longitudinal fold and the superimposition of the partial strands obtained in this way can be provided. The prepared strand package then enters the folder. This cuts the web transversely and ensures the necessary folds of the printed product. In contrast to sheet-fed offset printing, in which the sheets must first be processed into the desired end product in a number of additional steps after printing, web offset products are predominantly processed further directly inline to produce the end product.
Roll changer / pre-tensioning unit
The paper web wound on a roll is fed through the roll changer in both heatset and coldset web offset printing. There are basically two types of reel splicer. On the one hand the so-called autopasters, which enable a flying roll change, and on the other hand the standstill roll changer. Both processes have in common that the printing process does not have to be interrupted to change rolls. Machines without a roll changer are rarely found in production practice.
The on-the-fly roll change can take place using two or three-armed roll stands with swiveling support arms and is used in both newspaper and commercial printing . If the running paper roll is running out, a new roll is clamped and accelerated. The acceleration continues until the circumferential speed of the new roll corresponds to the path speed of the path currently running out. When a certain predetermined residual roll diameter is reached, bonding is initiated. For example, a flexible roller is used to press the outgoing web against the previously applied adhesive points on the new roll. Then a knife cuts the old paper web. While the new web is being fed in, the remaining roll is braked and ejected.
Idle reel splicers are mainly used in commercial printing. In contrast to the flying roll change, with this variant of the paper feed the new web is glued on when the paper rolls are completely at a standstill. In order not to have to interrupt the printing process during the roll change, a paper web memory is required. This is located directly behind the paper rolls that are firmly stored one above the other in the frame. Several guide rollers, between which the web is looped, ensure that the paper web is stored. The further these guide rollers are moved apart, the greater the web storage supply. To change the roll, the expiring roll is braked and the new roll is clamped into the integrated gluing device. While the two rolls are at a standstill, the webs are glued to one another and the outgoing web is cut with a knife. Meanwhile, the machine is being supplied with paper from the web store. The store is emptied by moving the guide rollers together. After successful gluing, the new roll is accelerated, the web is fed to the machine and the paper web storage is refilled by moving the guide rollers apart.
Between the roll changer and the first printing unit there is usually a pre-tensioning unit (also: feed unit) to regulate the web tension. An even and constant web tension is of great importance for the printing process in order to be able to produce without disruption. However, paper irregularities and roll changes can lead to fluctuations in the web tension, which must be compensated for by the pre-tensioning unit. Due to the permanent scanning of the paper web, the smallest changes in tension are recognized immediately. The infeed mechanism ensures tension compensation by means of a draw roller and pressure rollers.
Printing units
As a rule, web offset printing machines each have four double printing units (cyan [C], magenta [M], yellow [Y] and black [K]) for 4/4 color printing and offer pages from 8 pages to A4 96 pages DIN A4. A distinction is made between “standing” and “lying” machines.
With stationary machines, the pages are arranged in portrait format, the paper direction is parallel to the later bundle of the printed products (based on DIN A4 productions). DIN A4 products that are manufactured on stationary machines are usually glued inline in the machine, or are manufactured without binding for later processing (saddle stitching or adhesive binding).
With horizontal machines, the pages are arranged in landscape format, the paper direction is transverse to the later bundle of the printed products (based on DIN A4 productions). A4 products that are manufactured on horizontal machines are usually stapled inline in the machine, or are manufactured without binding for later processing (saddle stitching). The production of A3 products on horizontal machines is particularly interesting. Here it is possible to glue DIN A3 products inline.
The unrolled and tensioned paper web is first fed to the first printing unit. Basically, every printing unit of a web offset press consists of the components inking unit, dampening unit, plate cylinder, blanket cylinder and, with certain machine configurations, also of an impression cylinder. However, the number and arrangement of these elements differs depending on the design.
The printing units of the heatset machines are mostly I-printing units with a horizontal web run. In order to enable simultaneous printing of the front and back of the web, double printing units are used, each consisting of two plate cylinders and two blanket cylinders as well as inking and dampening units. In contrast to sheetfed offset, this 4-cylinder design does not require a special metal impression cylinder, as the blanket cylinders each act as an impression cylinder for one another.
A distinction is made between different technologies for the blankets to be used. Depending on the machine, conventional blankets with tensioning rails, blankets with sleeve technology or with minigap technology are used. The use of blankets with a clamping rail requires a clamping channel on the cylinder. Among other things, this results in a relatively wide, non-printing area and, in the case of small cylinder circumferences, can lead to vibration strips in the print image that are induced by channel impact. To circumvent this problem, blanket sleeves can be used in single-circumference printing machines. The sleeve concept is characterized by the fact that the rubber blanket is applied seamlessly to a sleeve-shaped carrier. This sleeve is pushed sideways onto the cylinder when changing the blanket. The system has the advantage that the vibrations triggered by the canal overrun are avoided and there is also only a pressure-free area of around 2.3 mm. When using the minigap technology, blanket plates are clamped onto special cylinders with a very narrow channel. The plates consist of a metal carrier on which the rubber blanket is vulcanized. This variant makes it possible to reduce the non-printing strip to around 6 mm. Advantages include fast blanket changes, the ability to compensate for changes in the length of the blanket during the printing process and lower costs compared to sleeves. Either conventional printing plates, printing form sleeves or plate cylinders with minigap technology are used, depending on the type of blanket.
The coldset web offset presses for newspaper printing differ from the heatset presses mainly in the printing unit construction and the web guide. The I-construction used in heatset web offset printing with upright double printing units and a horizontal web guide is unsuitable in newspaper printing, since large numbers of pages are usually printed and therefore multi-web operation is usually necessary. In order to be able to guarantee an undisturbed web run and good accessibility, vertical web guidance has become established in newspaper printing presses. The number and arrangement of the cylinders in the printing unit varies depending on the design. A distinction is made in particular between the following types of construction: 8-cylinder (H or bridge printing unit), 9-cylinder (satellite printing unit), 10-cylinder (semi-satellite printing unit). Currently the 8-cylinder H printing units and the 9-cylinder satellite printing units are mainly being built. The illustration shows the four different machine configurations.
Web offset systems with a conventional structure with four double printing units are very widespread on the market and are suitable for print jobs with high numbers of copies (approx. 35,000-20 million copies). The disadvantage of these machine types is the increasing costs with a high number of variants or version changes.
WAZ printing unit , still without plates
With plates, colored, not washed
In rotation (during printing)
Vertical web guidance
In addition to the usual 4-tower web offset systems, there are other machine types with 5 or 6 double printing units, which enable large orders to be individualized at low cost. This makes it possible, for example, to subdivide a large total print run into many small partial editions. B. differentiate by an individual company imprint or individual prices. This type of production is suitable for. B. for personalization, individualization, regionalization for specialty trade cooperations or franchise companies.
These machines are either equipped with full-fledged printing units or with simpler imprinting units. With the help of the imprinting technology, z. B. individual texts, dealer prints, company prints or prices without machine downtime at full production speed limited 1/0 color (for machines with five printing units) or 1/1-color on all sides (for machines with six printing units) can be changed (on-the-fly plate change) . When production is full, one plant is printed and the second plant is equipped with new printing plates. To change, the impression units are switched over and the first unit is prepared again. While normal 4-tower web offset systems have to be stopped completely when a version is changed, which means higher setup costs and fluctuations in quality can be expected, with 5- or 6-tower web offset systems, version changes during operation can minimize costs through less waste and quality fluctuations.
A distinction is made between impression units in short units with flexographic printing technology or simple offset printing units and full-value offset printing units. Simple impression units are ideal for individualizing small areas as easily as possible and offer limited print quality. Fully-fledged imprinting units do not have any area or quality restrictions and can also be used for special color printing. Particularly unusual machine configurations allow inline stapled productions of up to 96 A4 pages due to the machine and folder structure or make it possible to manufacture different products on different papers and to insert them into one another inline.
Dryer / chill roller unit
In heatset web offset printing, drying systems and chill roller units are required after the last printing unit, since printing inks that dry through heat are used. In contrast to this, in the coldset process, the printing inks dry purely physically by knocking away and neither a dryer nor a group of cooling rollers is required.
The heatset inks are mainly dried by evaporation of the mineral oils they contain, which act as thinners. For this purpose, hot air dryers are used that direct heated air to both sides of the paper web. In order for the mineral oils (boiling range 200 to over 300 ° C) to be expelled strongly enough, air temperatures of around 250 ° C must be reached in the dryer. These high temperatures cause the paper web to heat up to around 110 ° C to 120 ° C. Not only do the mineral oils evaporate from the paint, but also some of the water contained in the paper. This side effect leads to the drying out of the paper web, which, depending on the nature of the paper, can lead to various defects such as wave formation, bubble formation and static charge. The heat also causes the binder resins contained in the heatset inks to melt. As a result, the paint film is still soft and sticky when it leaves the dryer. The curing only takes place during the subsequent cooling of the paper web in the cooling roller unit. There, the web is suddenly cooled to 20 ° C to 30 ° C on the bright chrome-plated roller surfaces. The ink becomes hard and has a gloss typical of heatset printing. Following the chill roller unit, the paper runs through a silicone system, which applies a water-silicone mixture. This mixture ensures, on the one hand, that the paper is re-moistened and, on the other hand, that the surface is more scratch-resistant, which is of great importance for low-damage transport through the folding unit.
Due to strict environmental protection regulations with regard to the resulting emissions of the evaporating mineral oils and increasing energy costs, drying systems with heat recovery are widely used today.
Folder superstructure and folder
After printing, the paper web is fed into the folder superstructure and then into the folder. These units ensure that the printed web is processed into the desired final format. First of all, in the rebate superstructure, the lengthwise cutting of the web and the superimposition of the resulting partial strands using turning bars take place. The combined strands are then fed to the so-called fold former, which creates the first longitudinal fold. The strand package is then cross-cut with a knife. The further processing of these cut sheets then takes place in the folder. In principle, a distinction can be made here between four basic fold types from which various fold products can be developed. First the first cross fold, followed by the parallel second cross fold. In addition, a second longitudinal fold and a so-called post fold can be produced. This fold is important in newspaper production to make the products ready for dispatch. In addition to the folds, longitudinal and transverse gluing, gluing, trimming and numbering can also be carried out in the folding unit.
Consumables in offset printing
In order to produce a print product using the offset process, consumables such as printing forms, printing materials, printing inks and dampening solution are required in addition to the machine. Since the printing process is very sensitive to the smallest discrepancies, it is of great importance that all parameters are precisely coordinated. The selection of the required aids should therefore not be underestimated, because an excellent print result can only be achieved through an optimal selection.
Printing plates
The information stores or image carriers in offset printing are thin printing plates , usually made of anodized aluminum. Sheets on polyester film or paper are also processed. One plate is required for each color to be printed. In the past, the printing forms were illustrated using analogue photographic processes. Today it is mainly done digitally through point-by-point exposure from data.
In spite of the different manufacturing variants, the basic principle has remained the same: Exposure and development cause targeted changes in the plate surface that differentiate between the ink and water-bearing areas of the printing form. In this way, films can be exposed from data ( computer-to-film ), printing plates in separate platesetters ( computer-to-plate ), or plates directly in a machine (computer-to-press). All of these variants have one more thing in common: The printing forms are static and do not allow any dynamic changes to the print motif during the print run, as is the case, for example, in digital printing .
Substrates
In addition to the optical properties that are extremely important for the end product, the printing material must also have good printability and printability. Printability refers to the running properties of the substrate in the machine, while printability mainly refers to the acceptance of the ink and the inherent visual effect. Due to the nature of the process, the substrate is subject to relatively high loads during the printing process. Particularly important for paper are, therefore, among other things, pick resistance, low paper dust , abrasion resistance, absorbency, smoothness, uniformity in the coating and fiber structure, and with cardboard the adhesion of the layers to one another. The paper used should also be insensitive to the formation of bubbles and waves (caused by the overdrying of the paper in the heatset dryer), paper web breaks , wrinkles and excessive stretching due to the influence of tensile force and moisture.
If you print on plastic films (BOPP, PVC, PE), film properties and colors must be coordinated so that the print adheres well. In addition, static electricity often occurs, mainly caused by the frictional contact of the sheets. Printing machine manufacturers therefore offer special equipment to compensate for static charges.
Printing inks
The highly viscous (very viscous) printing inks used in offset printing essentially consist of pigments , binders and additives.
The pigments mainly influence the color effect, the hiding power and the resistance to various influences such as light, solvents or acids. Basically, you can use practically the same pigment body and concentration in different printing processes, provided that the same coloration is intended. The only exceptions are special requirements in terms of wettability, solvent sensitivity and other process engineering requirements.
The task of the binders is to encase the pigments so that they can be printed evenly. The composition of the binder influences, among other things, the anchoring of the pigments on the substrate, the gloss and the drying behavior of the ink. It is essential to ensure that a color can be printed using a certain printing process; must therefore be coordinated with the subtleties of the printing process and sometimes even the edition.
The additives are used to make colors ready for printing and to adapt them to the respective production conditions. They include, for example, surfactants for pigment wetting, siccatives to accelerate drying and waxes to increase the rub resistance of the paint layer.
Due to the process, offset printing inks must be able to absorb a certain amount of dampening solution in dynamic equilibrium (constant exchange in and out) as an emulsion during the printing process.
Since color perception in four-color printing is based on the mixture of primary colors printed on top of one another, the printing inks / pigments must be translucent (here: translucent / transparent).
Dampening solution
The dampening solution consists not only of water, but has to fulfill a number of tasks, which are finely tuned to the machine, substrate and print run conditions. It should keep the non-printing areas of the plate color-free. To do this, it must be able to be partially emulsified into the paint, but it must not impair the flowability under the machine conditions. It is intended to compensate for disruptions during the print run that may result from contamination from the printing material, ink or tap water used. It must be stabilized against biological disturbances (algae, mold, slime) and must not foam.
The printer can only check that the specified concentrations are maintained well enough; an experienced specialist must do the recipe work for him.
Further variants of offset printing
Waterless offset printing
The waterless offset printing is started an evolution of conventional offset printing, the commercial distribution in the 1970s. Initially, the process was only used in sheet-fed offset printing, but since around 2000 web offset presses have also been built for newspaper printing. The principle is based on inking the printing form without the use of additional dampening solution. To implement this process, among other things, special printing plates and inks are required. In order to separate the printing areas from the non-printing areas on the plate, materials with different surface tensions are used. The ink-carrying areas of the printing form mostly consist of a polymer layer that has a relatively high surface tension, whereas a silicone layer with a significantly lower surface tension is applied to the non-image areas. The printing ink only wets the printing areas because it has a lower surface tension than the polymer layer. The silicone layer has a lower surface tension than the paint and is therefore not wetted with printing ink. One advantage of waterless offset printing is, among other things, an extremely precise and sharp printout due to the lack of ink / dampening solution competition, which means that significantly finer screens can be printed than in conventional offset printing. However, it also has some disadvantages, such as the high prices for the printing plates and inks.
UV offset printing
The UV offset is a special form of offset printing . This process requires special equipment in the offset printing machines and special printing aids. The core is formed by UV-reactive inks and UV lamps. UV inks contain no solvents or thinners and contain special binder compositions. The basic components are acrylate monomers , oligomers and photoinitiators . Under the action of UV rays, the photoinitiators trigger a polymerization within the printing ink, which causes the freshly printed ink film to harden in a flash. Dryers with UV lamps are used to stimulate this reaction. Furthermore, both the UV inks and the cleaning agents required for them require the machines to be equipped with special rubber blankets and inking rollers, whose rubber compounds have suitable resistance properties. The method has several advantages. Due to the fast drying , for example, immediate print processing is possible. It also offers the option of printing on paper as well as on other substrates such as polyester, PVC, PET, metallic foil and other closed and non-absorbent surfaces. Furthermore, the UV technology is characterized by the possibility of achieving the best matt or gloss effects and their combination. In addition, the prints have good resistances, e.g. B. in the abrasion resistance, the solvent resistance and with carefully selected raw materials also in the low migration .
In addition to these positive features, there are also disadvantages. Among other things, higher investment, production and energy costs are to be expected. Furthermore, suitable protective measures must be taken to prevent the skin from coming into contact with unhardened binders from paint mist or contamination ( GHS symbol GHS07, H phrases 319, 335, 315, 317 skin-irritating and sensitizing). The impurities in the binder and photoinitiator often lead to a typical odor. On the other hand, the oxygen molecules react with the free radicals under UV light to form ozone . Therefore it is also necessary to suck out the air in the drying area. Furthermore, the printer must guarantee that the inks cure sufficiently so that they do not leave any residual monomers or oligomers that can migrate in the ink film. You would endanger the end user.
In addition, UV-crosslinked inks (including LE and LED UV inks) cause considerable problems in paper recycling , since they harden as a plastic film (not or only poorly deinkable ) and microplastics are also suspected of introducing microplastics into the recycling process; Corresponding print products are therefore excluded from environmental labels such as the Blue Angel .
Offset printing with electron beam curing inks
A second variant of the offset with radiation- hardening inks uses electron beams for color hardening. In contrast to UV light (electromagnetic radiation with photons), these are so-called corpuscle rays, which consist of emitted electrons, e.g. B. as with the old television tube. Such rays are considerably harder than electromagnetic rays (transmit more energy) and can harden the binders of the paints without the help of photoinitiators. You have practically no risk of penetration, so you can be sure to harden the color film in its depth. This and the thoroughness of the hardening predestine them for z. B. Food packaging, because the hardened ink film is more or less inert, i.e. extremely low in migration. Unfortunately, systems for electron beam curing are much more extensive than those for UV radiation: The emitters are related to the cathode ray tube. The hardening reactions in the paint are sensitive to oxygen. This means that you have to work in a vacuum or under an inert gas .
Continuous form printing
Narrow-band web offset printing machines are built for table and form printing, which are usually operated with sheet-fed offset inks that are retarded to the skin. They are often combined with various further processing facilities.
Small offset printing
Compact offset printing machines are built up to DIN A3 format, which are still used for smaller print jobs. In this respect, small offset printing is a separate division within the sheetfed and web press sector. This market field is being taken over more and more by digital printing and copying techniques that are constantly being developed .
See also
- Driography
- Paint waste
- Total paint application
- Commercial web offset printing
- Iris print
- Reproductive technology
- Thermal relief printing
- Tone value increase
- Process standard offset printing
- Media standard printing
literature
- Helmut Kipphan (Hrsg.): Handbuch der Printmedien. 1st edition. Springer, Heidelberg 2000, ISBN 3-540-66941-8 .
- Helmut Teschner: Specialist dictionary digital and print media. 1st edition. Christiani, Konstanz 2008, ISBN 3-86522-384-2 .
- Helmut Teschner: Printing and media technology 13th edition. Christiani, Konstanz 2010, ISBN 978-3-86522-629-7
- Kaj Johansson, Peter Lundberg, Robert Ryberg: Print production well done! 3. Edition. Mainz 2008, ISBN 978-3-87439-731-5 .
Web links
- Literature on offset printing in the catalog of the German National Library
- Offsetdrucker.net German-speaking forum for offset printers
Individual evidence
- ↑ Helmut Kipphan (Ed.): Handbuch der Printmedien. 1st edition. Springer, Heidelberg 2000, ISBN 3-540-66941-8 , p. 347.
- ↑ The fastest sheet-fed offset printing machines, compiled by the printing industry portal print.de
- ↑ The largest sheet-fed offset printing machines, compiled by the printing industry portal print.de
- ↑ INGEDE press release of July 3, 2015, accessed on November 27, 2018: Cross- linked colors are difficult to remove when deinking
- ↑ f: mp. News - LE (D) UV printing from an ecological point of view. Retrieved January 15, 2020 .