Teoria de Impresion Offset Basico
Teoria basica para la impresion Offset...
Press Stage Centro de Capacitación para la Industria Gráfica de ACOACEIG de R.L.
1. Paper 1.1. Modern Paper manufacturing process
Paper has been used for thousands of years as a tool for communication. At first, they used natural fibers such as papyrus, cotton or other materials for its production. It was not until mid-19th century that began the widespread use of wood as raw material for paper. Technically, there are over 25,000 types of plants with wood fibers, but conifers and deciduous trees, by its structure, are the most suitable for the manufacture of paper. Conifers have much longer fibers that are transient, and thus achieve much stronger paper. The most used coniferous three are the pine and the spruce. The eucalyptus tree is the most widely used of the deciduous family for wood free paper such as machine-coated papers. Pulp manufacturing The pulp is the base from which paper is made. It's made from wood fiber, water and chemicals, chemicals , depending on their final use. For the manufacture of pulp, trees are transported to large processing plants. There, the first step is to remove the bark from the trunks of trees. Once without its bark, the wood must then be reduced to chips and fibers, either through a mechanical or Chemicals process. The mechanical process involves the application of large amounts of water (98% of the material is now water), while the wood is ground against a rotating stone. The result is is called mechanical pulp. The chemical process is much more complicated, having to cook on the wood under pressure with acid, which generates a cleaner pulp, which is called "wood-free pulp”. In this section, we’ll focus only on the mechanical pulp.
The resulting pulp is now brown and has to be bleached before it can be used to make paper. Modern techniques have replaced bleaching acid and chlorine, with an oxygen process, motivated by the environmental benefit of avoiding the use of toxic chemicals. Once the pulp has been bleached, it passes to the paper mill for its final conversion.
Paper Composition Upon entering the paper mill, the pulp is refined (in the following figure, steps 1 through 4), passing by huge rotating cylinders whose surface is composed of several blades, which, moving in or out, generate different types of paper, depending on the desired structure. Already refined, the pulp is processed with cellulose c ellulose from an external source to obtain the final form of the paper. Additives, such as calcium carbonate and clay are added to produce a more opaque paper and a more closed surface (Step 5). The calcium carbonate in particular helps the paper to be more resistant over the years. Components like these makes up to 30% of the final mass of the paper. In addition, you can add other types of components to the paper, such as dyes, optical brighteners, etc. Computers carefully regulate this whole process. But undoubtedly, the most important material in the manufacture of paper is water. Up to a thousand liters are used per kilogram of paper. Because of this, paper mills are always located near rivers or other water sources, and are obligated to recycle up to 90% of the water they use. 2
When the pulp is placed on the bands of the machine (step 8 and 9), these begin to vibrate to create a smooth surface, it's in this step that the chips and paper fibers are aligned and create the paper grain direction. It is during this step, going through the press that the paper takes the form of a sheet, and it's extracted most of the water (steps 12,13 and 15), and we have what might be called paper. However, for the paper to be used, it must be prepared according to its next use. Among these preparations is the coating, (as in couche paper). The coating of the paper (for example, with starch) helps the paper to avoid absorbing dirt from the environment, and the printing looks clean and even throughout the whole surface. To a same type of paper you can apply different coating processes, as agents regulating its surface (natural like starches or synthetic such as latex), bleach (calcium, carbonate, clays) and dyes, as well as special additives that each manufacturer uses to distinguish its paper from the competition.
1.2 • •
Paper properties Basis Weight: Weight: the weight in grams of an area of one square meter. Brightness: Brightness: the percentage of reflection at a wavelength standard. Is measured primarily on the white ideal. This property affects the contrast, c ontrast, tones, images, and the overall appearance of the printed product. Gloss: Gloss: the percentage of light that is reflected from the surface, by firing a beam of light at a certain angle. Roughness: Roughness: is the difference between a perfectly flat surface and the surface of the paper under review. Opacity : This is the amount of light that the paper lets (transparency) or not (opacity) go through. Lower opacity lets more light trough than a high opacity. Relative humidity : the amount of water suspended in the air between the sheets of paper (and not the water inside the paper). For offset printing, the relative humidity should be between 50% and 55%, and for office paper, 35%. Acidity or Acidity or alkalinity (pH): alkalinity (pH): from an average of 1-14, where the more acidic is close to one; and close to 14 is alkaline. The paper should have a pH of 7 or near a neutral value for printing. Bulk: Bulk: Relations between papers papers of the same weight, expressed expressed in cubic centimeters per gram
1.3 Paper Structure Each sheet of paper has a defined structure, structure , which it's the direction in which the fibers that compose them are placed. As we saw in the previous sections, when the trees are shredded into fibers and chips, these fall into the bed of the machine and by vibration are aligned in one direction. To identify the direction of a sheet of paper, you can use several methods: Tear the paper: if the resulting line is straight, that is the direction of the paper, otherwise the cut c ut will be diagonal, or irregular. Fold the paper: take a piece of paper and fold it with some degree of force (without breaking it, but fold it as tightly as possible) the paper will tend to curl if it is bent against its structure. Dampen the paper: when applied a slight amount of water to a sheet or piece of paper, paper fibers absorb water and expand, generating a slight ripple, to indicate the direction the paper. •
Importance of the Grain Direction or Structure For Pos Press: Depending on the final work to be done, it is necessary that the sheets be folded or cut. It is imperative to know the direction of the paper before printing, because it must done be in the right direction as not to damage the work done in the Post Press. For example, a leaflet should be in the direction of the grain, so that the fold is perfectly straight. For the final material: If the material produced will be used as media (ei letterhead pages) pages must be printed with the grain, so that when the page goes into a digital printer or copier, it will not damaged by being bent. To print: If the pressure of an offset machine is badly calibrated, the force exerted on the paper can damage (as seen in the picture), stretching the material as it passes through the machine, causing register problems, damage to the paper, etc. For carton the direction must be parallel on the cylinder. If not, the carton will not fold easy with the cylinder. Register problems and •
2. Ink 2.1 Ink Composition The ink used in printing is made from three basic ingredients: Pigments: the chemical substance that gives the ink its color. These make up about 20% of the total volume of ink. The pigments are made especially for each of the Ink manufacturers, by the most important laboratories in the world, given that only 3 or 4 are capable of faithfully reproducing the tones necessary for consistent ink through the ages. Varnishes: Varnishes (not be confused with varnishes that are applied at the end of printing) are the bases of the ink. They are composed of various types of oils and resins and are about 70% of the volume of ink. Additives: Known as the salt and pepper from each producer. These additives change the properties of the ink, which allows the production of different types of ink for each paper type and application.
2.2 Ink fabrication The first step in manufacture of ink is the mixture of oils and resins to form the main base, or varnishes, and then applying the desired pigments.
The second step is to transport the ink to a separated tank to add the additives that makes each ink different, and mix thoroughly.
The final step is to mix the final formula, and pack it in cans, ready to ship to the printers.
Every single ink is different according to its production process and the additives that each manufacturer uses. It’s impossible to say that one brand is better than others, but it’s a fact that to mix inks from different brands can lead to troubles in the printing jobs. Each manufacture creates a chemical balance in their inks that translates in they physical characteristics (remember, inks are translucent). If two inks aren’t in balance, it can lead to different and unexpected results. 2.3 Ink drying process From the last paragraph, we take that all inks are different, and they are made to match each other, and the material in which they will be applied. There is great importance in this aspect, and one of the main factors is the drying process. There are different ways for the ink to dry:
By Setting: With uncoated papers, most of the ink gets absorbed into the material, and dry within it.
By setting and chemical drying: In this method, part of the ink is absorbed, but not all of it, and the rest reminds on the surface of the material. Over this, most of the time a chemical dryer is applied to keep the ink on the material.
100% Chemical Ink: there are materials that don’t have the ability to absorb any of the ink, like plastics and polymers, in which case, a special chemical dryer is used.
Infrared Reactive Ink: this type of ink reacts to infrared light, to aid in the drying process.
3. Printing Technologies 3.1 Background Modern Printing was founded with the Gutenberg Press, from a German inventor who created the system of interchangeable type printing. Since then, there has been a continuous evolution of printing machinery. Book Printing Platten Press: This type of technology, that in our country is still alive in machines like the Minerva, is a flat-to-flat surface system. While the types are fixed on a stationary bed, ink is applied to them and another bed moves, applying pressure on the whole surface while leaving the impression on the material. This type of technology was useful for small formats and small amounts, but is severely limited by the strength that is necessary for the printing (up to 4 tons). These machines had, most of the time, a maximum size of 24 x 38 cm. Increasing the format would require to apply almost 16 tons of pressure, something completely out of the possibilities for a machine of this size.
Cylinder Press: The next step in evolution was the cylinder press. This replaces the movable bed by a pressure cylinder. By significantly reducing the area where the force is exerted to print, this machine could use a bigger paper format; it was faster, with less effort than the old systems (one ton with a larger format.) However, this technology was still far from ideal. The flat form was now the moveable part, which wasted a lot of strength, and, being made mostly of lead and other metals, it was incredibly heavy, limiting the size of the format and quantity by the sheer force that was needed to move the platform.
Rotary Press: The rotary press was the most important step towards the development of offset. It finally removes the use of a movable m ovable bed, and instead placed two cylinders which are completely solid, rotating in opposite directions. This type of system hardly used anymore.
3.2 Letter Press The transition from flat to cylinder press systems could be achieved only with the development of malleable forms. Initially, forms and types were placed in a wooden mold to create the words and paragraphs. These types were originally made from an alloy of lead, copper and antimony, and exchanged every time you wanted to print something new. But manufacturers and printers were still in their desire to improve their processes, at higher speeds and with larger formats, like newspapers and magazines. Which was impossible to achieve with flat beds systems. To remedy this situation, engineers at the time devised a way to pass the flat figures to cylindrical shapes. The first step was to create a non-readable form of what you wanted to print, with the types in a wooden mold. With this completed form they imprinted it on a cardboard-like material, which created a readable form. The board, being malleable, was then given the cylindrical shape required. Over this mold, it was applied a mixture of lead and copper, to create the final and non-readable plate, ready to print on the desired material. The end result was a plate like this: These new forms allowed printing larger quantities in less time, revolutionizing the way information was tr ansmitted. However, the system was not entirely perfect, as these plates were made of lead, a material that eventually became known as toxic, and that even at the time was heavy, and soft in comparison with other metals. Its use was limited because the letters and forms wore out. The speed was also limited by the weight of the form; therefore, at higher revolutions, the piece tended to separate from the cylinder c ylinder by centrifugal force, and could generate large damage to machines or operators.
The typography presses operated with the pressure between the lead plate (higher cylinder in the photo), against a soft cylinder (bottom cylinder), usually made of rubber, this extended the life of the plate, and achieved a better impression, for the paper printed was less damaged than if both cylinders were completely solid.
3.3 Flexography In the early twentieth century, flexography was born in two different places almost simultaneously: in France, invented by engineer Houleg, and Great Britain, Liverpool, patented by the printing house 'Bibby, Baron and children', in 1905. This system changes the way the production of plate, by replacing lead with rubber, and the soft cylinder and a solid cylinder. The new plate was lighter, allowing faster printing; it was easier to replace, increasing the number of copies, and it allowed to print materials once considered impossible, such as plastics and metals. The inks also had to evolve, becoming less dense. The quality, however, suffered with this step. As the ink grew thinner, and the materials couldn't absorb ink, it started to generate an effect called 'squash', as the ink tends to run down and created an uneven impression. As technique evolved, these problems were declining. 3.4 Offset Printing Offset Printing is, perhaps, the most important development in the industry, becoming the most used printing technique throughout the world by the 50s, and it's still to this day. Offset printing was created in 1875, with Robert Barclay, whose development allowed the transfer of an aluminum plate to a rubber body. It was 1907, when a printer of New Jersey, Ira Washington Rubel discovered that when transferring the image of a plate to the rubber, and then to paper, the image was of much better quality. Today, all printers are familiar with the Offset Printing Process, which uses three Cylinders (plate, rubber and steel) to generate an impression on the paper. Plate: an aluminum foil on which it generates a chemical emulsion, thus transmitting a positive image (of a negative or directly with CTP), which is intended to print. For color prints, you create a plate for each color used, and then have the combination to generate the final print. The offset printing process depends on the perfect combination on this cylinder of water and ink. Rubber or blanket: On this, the plate makes a non-readable impression. The blanket is one piece, usually made of rubber or other soft material, which is responsible for transmitting the image to the media. Steel: This cylinder is responsible for generating the pressure between the blanket and the material to achieve the final print. Besides the three main cylinders, Offset machines have two separate batteries of rollers: A battery of ink rollers and another of water rollers, which are in contact with the plate. Offset printing relies on a precise balance between water and ink. The emulsion on the plates should only receive ink, so the rest of the plate must be covered by water. Not only water is responsible for ensuring the sharpness of the impression (for which it must have a precise chemical composition), but also fulfills the function of cooling the machine to prevent it from overheating and damage.
This technique solved the problems of the past: the materials used were light (aluminum plates, rubber for the blanket and the steel one was the only completely solid cylinder), c ylinder), allowing to print at higher speeds, the pressure needed for printing was smaller, which saved energy and did not harm the material. Today’s offset Printing can print up to 18,000 sheets per hour, on most modern machines with A3 or A2 format without problems. 9
3.5 Gravure Printing The rotogravure is a very specialized technique, as its use is limited to high volume runs. Like flexography and letterpress printing, rotogravure uses only tw o cylinders, one hard solid that acts as a plate and a soft one to apply the pressure. However, this method does not use a mold or plate but it physically engraves the image onto a copper cylinder. This process is expensive because it uses a cylinder for each color for each design, and they aren't as replaceable as in typography. Therefore, this technique is only used in high volume runs, and with paper reels, not sheets. The rotogravure printing is generally used for magazines sold nationally or globally.
3.6 Screen Printing Screen-printing is a printing technique that uses a woven mesh to support an ink-blocking ink -blocking stencil. The attached stencil forms open areas of mesh that transfer ink as a sharp-edged image onto a substrate. A roller or squeegee is moved across the screen stencil, forcing or pumping ink past the threads of the woven mesh in the open areas. Screen-printing is also a stencil method of print making in which a design is imposed on a screen of silk or other fine mesh, with blank areas coated with an impermeable substance, and ink is forced through the mesh onto the Printing, printing surface. It is also known as Screen Printing, silkscreen, silkscreen , serigraphy, serigraphy, and serigraph. serigraph. A screen is made of a piece of porous, finely woven fabric called mesh stretched over a frame of aluminum or wood. Currently most mesh is made of man-made materials such as steel, nylon, and polyester. Areas of the screen are blocked off with a non-permeable material to form a stencil, which is a negative of the image to be printed; that is, the open spaces are where the ink will appear. The screen is placed atop a substrate such as paper or fabric. Ink is placed on top of the screen, and a fill bar (also known as a flood bar) is used to fill the mesh openings with ink. The operator begins with the fill bar at the rear of the screen and behind a reservoir of ink. The operator lifts the screen to prevent contact with the substrate and then using a slight amount of downward force pulls the fill bar to the front of the screen. This effectively fills the mesh openings with ink and moves the ink reservoir to the front of the screen. The operator then uses a rubber blade to move the mesh down to the substrate and pushes the squeegee to the rear of the screen. The ink that is in the mesh opening is pumped or squeezed by capillary action to the substrate in a controlled and prescribed amount, i.e. the wet ink deposit is proportional to the thickness of the mesh and or stencil. As the squeegee moves toward the rear of the screen the tension of the mesh pulls the mesh up away from the substrate (called snap-off) leaving the ink upon the substrate surface. There are three common types of screen printing presses.: The 'flat-bed', 'cylinder', and the most widely used type, the 'rotary'. This technique will be used for printing large and many ink volumes on paper and to print on materials like plastic, carton, glasses, wood, iron and so on up to the thickness of 5 cm. In other words: every material which is not easy to fold around cylinders.
4. Color in Printing Offset printing, and all types of printing, depends on creating an image from thousands or even millions of small dots. For colors, most printers are limited to create from the combination of basic colors, Cyan, Magenta, Yellow and Black. From these four colors, a printer can create most of the color gamut, a process that is explained below. The color sequence will almost always be Cyan, Magenta, Yellow and Black. 4.1 Color Mixing The color is a property of each object, which reflects a specific color when light falls upon it. For computers or printers to generate color, they must deceive the human eye with a mixture of colors. Computers, televisions and any type of display used additive mix, while printers use the subtractive.
4.1.1 Additive Color Mixing In additive color mixing, light of different colors is superimposed. If all the colors of the spectrum are superimposed, this results in the color white, in the absent of them all, black is the result. The additive primary colors are red, green and blue. Each of these represents one-third of the visible spectrum. The principle of additive color mixing can c an be clearly illustrated using three slide projectors. Each projector generates a circle of light on a screen in one of the three additive primary colors. Additive color mixing is used in television broadcasts, lcd computer screens, etc. 4.1.2 Subtractive Color Mixing Subtractive color mixing removes different color components from white light. When all color components have been removed, the result is black. The subtractive primary colors are cyan, magenta and yellow. Each of them represents two-thirds of the visible spectrum. Subtractive Color Mixing: Cyan + Yellow = Green Yellow + Magenta = Red Magenta + Cyan = Blue Cyan + Magenta + Yellow = Black No color = White
They can be generated by removing one additive primary color from white light (e.g. with a filter) or by superimposing two additive primary colors. Printing inks are translucent substances that function as color filters. Which color do you get if you print a blue-absorbing substance on a white paper? Blue is removed from the white light; the other components (green and r ed) are reflected. The additive combination of these two colors results r esults in yellow: This is the color we see.
The ink has thus subtracted a third (blue) from the white light (consisting of red, green and blue). Assume two transparent substances are overprinted. For example, let us take the inks yellow and cyan. These two substances first filter the blue component from the white light and then the red component. We see the resulting light as green. The two inks have subtracted two thirds of the color components from the white light. If cyan, magenta and yellow are overprinted, all the incident light is absorbed (i.e. there is no reflection). We see black.
Color images are printed with cyan, magenta, yellow and black inks. The black ink improves the definition and feeling of depth in images. Also, the black that is produced by combining cyan, magenta and yellow is never a really deep black due to the pigments used in the inks.
4.2 Color Separation As we have seen, to print a color image, it must first be separated into the four basic colors: Cyan, Magenta, Yellow and Black. Even if the previous three could form the black, it never generates an image as sharp as when used separately. As shown in the image, when each color is separated, and then we print them in a set order, you create a color image of high quality. In order to separate the colors we use filters. As explained in the previous paragraph, for each CMY color their opposite RGB color is used as a filter, for example, the filter for Magenta is green, for cyan is red, and for yellow the blue filter is used. Once you have four separate colors, a plate is created for each, and you can proceed to printing.
4.3 Ink Thickness The thickness is the measure of the amount of ink that has been applied to a sheet of paper. It’s I t’s measured by height of the ink layer on a sheet. Inks are translucent rather than opaque. Light penetrates into the ink, when passing through; it strikes pigments that absorb a greater or lesser part of certain wavelengths. Depending on the pigment concentration and thickness of the ink, the light strikes a larger or smaller number of pigments; this absorbs different portions of the light. The light beams finally reach the surface of the substrate and are reflected by it. The light must pass through the ink film again before it reaches the eye. A thick layer of ink absorbs more light components and reflects fewer than a thin layer; the observer therefore sees a darker and more saturated color tone. The light component arriving at the viewer’s eye thus forms the basis for assessing the relevant color. As shown in the image, measuring only the density as a quality setting can lead to waste of ink, set-off problems (ink passing from sheet to sheet), drying problems (the more ink we use, the more time it takes to dry), etc. 12
4.4 Densities It’s the degree to which an ink layer is impermeable to light. On a theoretical level, this is the relationship between a measurement on unprinted paper and a measurement on printed-paper. The density and ink thickness relationship is very important to maintain the quality of the printings. The more ink we use, the higher density value, but, as the ink film grows thicker, it creates a polluted color. Remember that ink is translucent, tr anslucent, and the more ink the light has to go though, the less light will come out, and the color will look dark and dirty (see image).
5. Offset 5.1 Paper feed The first stage in offset printing, it is the correct feeding of paper into the machine. On this aspect, which at first glance may seem simple, depends the good print quality, maintaining the material safe, getting a good register, and many more. As a rule, in modern machines offset, three guides to be used for the entry of the paper. One side and two front guides. Although the machines have up to 8 front Guides, but only two must be used, placing them a fourth in from each end of the paper. This way we ensure that the paper gets it straight, without bending or leaning to one side or another.
The space between the side guides and the paper should be 3-4 mm, to give the paper enough space to accommodate their movement. Also, this is the same distance that must exist between the stack of paper and the suction cups on the tray.
5.1.1 Single sheet feeder Examples of machines using this system are the Heidelberg GTO or Kor, older offset technologies, which can print up to 8,000 sheets per hour. This is due in part to the type of feeding they use. In this system the paper enters the machine one sheet at a time, where a suction cup lifts the paper from the stack, and delivers it to two mechanical clips that take you inside the printer. As the sucker is in the edge of the machine, it has to wait that each sheet ends its journey to start moving the next. Then the sheet is taken by the feed gripper, which executes a pendulum movement, which at one time carried the paper to the press, and the next should return to take the next, wasting half the energy. We must also note the number of steps that are necessary, with 5 moving parts being responsible for transporting the paper, there are five times in this activity where something can go wrong.
5.1.2 Stream feeder A more modern system for feeding paper in offset printing presses, is the stream feeder, as used by the Heidelberg SOR and Speedmasters, Mitsubishi, and other modern press manufacturers. This system places the suction cup to the end of the pile and "pushes" the sheets; it doesn't wait for the sheet to be completely gone, and send the following. It has also replaced the feed gripper with a conveyor belt, where rollers apply a bit of force to the material and push it into the press.
This system allows the machines to operate at speeds up to 18,000 sheets per hour. It also has the advantage that, by the way the cylinders are placed internally, allows the paper to enter more easily, preventing it from bending and damage. As can be seen in the figure: In the first figure, the transport cylinder receives the paper, instead of the steel. This cylinder is not perfectly round, allowing space for the swing gripper to transport the paper without damaging it. This picture below describes the operation of a machine SpeedMaster system. This is what regulates the supply of paper into the machine. It is a very complex system consisting of several steps:
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Air is blown to separate the sheet from the pile The first sucker takes the sheet The governor foot applies pressure on the rest of the pile, preventing them from joining the first and blows air to completely separate the sheet above. The second sucker takes the sheet, sheet, and brings it to the feeding table, which it's then taken by the rollers and the moving belt
5.2 Offset Machines Heidelberg – Speed Master 4-color press, with a stream feed system. This is a classic example of a modern printing machine. It has a stream feed system, and it can be seen as the input and output trays have been elevated to allow a continuous work without stopping the machine every time it runs out of paper. Its speed reaches 15,000 sheets per hour. Each color towers have a three-cylinder configuration: plate, blanket and steel, and a set of three transporters cylinders between each tower. The main problem with this machine is the size of transporter tr ansporter cylinders, being so small, the paper bends too much in its passage through the machine, preventing the use of thicker materials such as cardboard or others.
KBA – Rapida 105 In response to the Heidelberg problem, the competition developed ways to improve and differentiate themselves from others. The printing system is essentially the same, a configuration of three cylinders for each color tower. However, the shape of the steel and transportation tr ansportation cylinders, by replacing the three used in the SpeedMaster with just one, allows a freer flow of paper through the machine. This type of machine can printed cartons and all types of papers. The shape of the transportation cylinder allows it to move two sheet for each revolution, saving energy and improving the process.
Heidelberg – Speed Master CD In response to the previous machine, Heidelberg developed this new system, based on the design of a single transport cylinder, trying to improve it by increasing the size of this, and the number of sheets transported per revolution. With this, the company had a machine that could print on any material, with the same printing system as the one previously used.
Man Roland – 4 colors Leaving the 3 cylinders configuration, Man Roland (since the development of this machine has been separated into two different companies) sought to differentiate their products with a 5-cylinder configuration: two plates, two blankets and one steel cylinder. Thus, it printed two colors on each tower, reducing the number of moving parts, and making the process more efficient. See as each transport cylinder is connected to the next tower by a system of chains, which are responsible for mobilizing the sheets of paper. Today Roland no longer makes these machines, focusing on the traditional configuration of three cylinders configuration.
Heidelberg GTO 52-2 This press will go down in history as the machine that saw the industry grow. Though now seen as an inefficient machine, with a maximum speed of 8,000 sheets per hour, this printer at the time allowed for the growth of large printing houses, while cemented the dominance of Heidelberg in the field of small formats. A two-color machine, you can see how the input and output piles are shorter, and used the same set of three cylinders to carry the sheet from one tower to another.
5.3 Alternative Technologies Heidelberg and the other major offset press manufacturers were competing not only against each other, but also against the digital revolution. Gradually digital printing was taking over small printing print ing jobs, leaving printers without customers, and therefore, making larger presses ineffective. The office works were becoming more common, and large printing runs, as magazines and advertising, were declining. It was the duty of manufacturers to develop new forms of print, to cope with this change. While they may always be classified as offset, these new printing techniques radically change the method of work. Some were successful, others not. 5.3.1 Quickmaster DI 46-4 In response to color copiers, c opiers, Heidelberg development development offset machines capable of printing small format in full color without taking up too much space, since its intention was that for the machine to be inside an office. The Quickmaster occupied a Direct Imaging System, or DI for short. The machine should be connected directly to the computer, so this would send the design to be printed. Laser equipment would engrave the image on each special prepared cylinder, which was not in a traditional plate, but a cylinder coated with a layer reactive to lasers, which could have to be completed removed to implement a new design. The Quickmaster used a satellite configuration; it occupied a single steel cylinder for four blankets and four plate cylinders (9 cylinders in total). The main factor against this machine was the high cost of operation. If a plate was damaged, the production should be stopped while the printer erased the cylinder, and engraved on a new image, a process that took quite some time. This drawback ended up taking it out of the market, as it never achieved a competitive advantage against digital Systems.
5.3.2 Printmaster QM 46-2 A new Heidelberg attempt, to compete against digital technology, led to the development of the Printmaster. This time, the German company's engineers returned to work the traditional way, with interchangeable plates, but in this case, made of a plastic polymer, similar to the omega plates. The Printmaster is able to automatically exchange plates, allowing continuous work. 16
Its limitation was the quality: Using a 4-cylinder configuration, with two plates on a single blanket, the combination of both colors on the plastic material of the blanket causes color contamination in the more complex designs. This machine was designed for simple jobs, such as custom paper, color prints or presentation cards, for which the Printmaster was efficient and effective. This technology is still marketed, enjoying success in small and specialized office work.
5.4 Drying Systems in offset machines. The development in printing technologies follows two main criteria: improving quality and reducing time. time. For these reasons, manufacturers began to place large drying systems in their printers. Thus, at the same time that the sheets pass through the machine they get dry and are ready to be handled in the Post Press. For example, the image #1 is the Heidelberg Speedmaster 52, 52, a four-colors machine that has a drying system attached at the end. In the second image one can appreciate the added space, between the last transfer cylinder and the output tray. A set of Infrared lights (represented as the red line in the images) has been placed to accelerate the drying as the paper goes through, led by the grips in the chains. Other systems use a set of UV lights, to achieve the same effect. These equipments also facilitate the work of varnishing, especially in machines of 5 or 6 towers, where one can make the whole process (printing, coating and drying) in a single run. The drying systems can be applied to many types of machines, according to the needs of the printer. One can have a drying system in a six-color machine, as well as in a one-color machine. These systems are parts that must be ordered directly from the factory, before the machine is assembled, it is not possible to add these types of systems according to one own schedule.
5.5 Water in Offset Printing As it was seen in the 3.4 point, the Offset Printing depends on a delicate balance of water and ink. The water must cover all the surface of the plate where there isn’t any emulsion (which must be covered with ink). To make this happen, the water must have a perfect Chemicals composition, measured by a pH from 4.5 to 5.5.
The water must form a uniform layer over the plate, as can be observed in the third figure. If the Chemicals composition of the water isn’t adequate, the water won’t flow through the surface, creating little drops, ruining the balance, causing ink-water contamination, drying problems, etc.
Following this explanation, in every plate there are two zones, the emulsion, that accepts the ink from the ink rollers, and the ink-repelling zone that must be covered by water. But, in the natural process of printing, little by little the ink start to accumulate on the plate, causing ink contamination on the water rollers, and this stains the printing. This can be averted by stopping the presses every two or two and a half hours to clean the rubber and the plate cylinders, eliminating the residual ink.
Water tends to be the cause of most problems in offset printing, therefore, the industry is moving forward to waterless offset. In the figure ‘a’, there is a close up of the typical plate, covered in water and ink, while in the figure ‘b’; there is only ink on the plate.
The waterless offset generates great benefits, the quality is always better, there isn’t a drying problem, there is no contamination, the printers uses less chemicals, so it’s better for the environment, among others. But, to use it, there is a very complex preparation process. Only the newer offset machines can print without water, and using a special type of plate, made from a polymer instead of aluminum. At the same time, the machines must be capable of cooling itself without the traditional water rollers. All conditions must be perfect to print without water. Although, at the moment, there are very few print shops around the world those prints in this manner, it’s a trend that is growing in Europe, and in the rest of the world.
5.6 Sheet transfer systems The paper sheet transportation from one cylinder to the next, works through a complex system of tweezers or grips. In the process, there are three steps: Step one: both cylinder moves in different direction, taking their grips to meet with each other. In this moment, the grip from the transfer cylinder is closed, with the paper sheet; the grip from the receiving cylinder is open. 18
Step two: Both set of grips meet (the grips are not in front of each other, as it may look like in the picture, but placed in different places across the cylinder) and at that moment, both set of grips are closed with the paper sheet. Step three: The cylinders continue to turn, and the transfer cylinder’s grips open to let the sheet go through. The synchronization and calibration of the machine are vital to this process, because, if a grip is a second too late to open or it closed a second too early, the sheet can be damaged; lost in the machine, give a bad register, etc.
5.7 Powder sprayer Known as powder sprayer, this is a process to apply a fine layer of anti set-off powder or talc over the printed materials. The main function of this powder is to create a small space between the printed sheets, avoiding set-off, and to help the drying process while air can circulate between the sheets. It works with a pump of compressed air, pushing the powder particles through a series of tubes to distribute the powder across the sheet. However, if the conditions aren’t ideal, like when it’s to humid, the powder particles can stick to one another, taking a granular size, that can be difficult to move along the tubes, getting them stuck, or, in case they reach the sheet surface, they create an uneven layer, that let the smaller particles fly through the air, contaminating the print shop atmosphere, wasting a good part of the material, and dirtying the work place.
In this example, the small particles escape f rom the surface of the paper.
5.8 Common problems with pressure pressure This image #1 reflect a good impression, all the points are uniform, and perfectly round, the lines are constants, meaning that there have been a good press calibration.
In figure #2, and taking into account that the direction the paper into the machine is up and down, too much pressure between plate cylinder and blanket cylinder has caused be long points, distorting the print. Vertical lines remain nearly the same, but the change in horizontal lines can be distinguishes. It is also possible that excess ink is the cause of the problem.
In this case, there is a horizontal deformation, contrary to the direction of printing. This problem is caused by excessive pressure between the blanket and the steel cylinder, causing a deformity of the paper (it stretches it).
This doubling effect is caused by a blanket that has been improperly adjusted, leaving it with some freedom for movement. The blanket, being free to move, ripples with the movement of the cylinder, touching twice on the plate, and ink charging twice. Conveying to the material in the form of a double impression.
Ghosting is one of the strangest phenomena in the world of prints, while it is almost impossible to prevent. The ghosting occurs in very specific situation: when we place a text the same color as a solid box in the direction that will be printed, and whose distance is the same as the circumference of the ink roller. When these conditions are met, what happens is: the ink roller loads the plate where the text will go, leaving a mark on the roll, that keep on turning to load the space where the solid background anger, but there is a difference between the amount of ink on its surface, having less in the area where the previous turn left the ink for text. As the printing dries, the image is noted as a lack of ink, which is read as the text. There is no way to prevent ghosting, rather than try to avoid it in the designs.
5.9 Digital control systems As printers evolve and become more and more automated, improving the quality of work and its speed, it's also progressing the way they control it. The control panels have moved away from the machines, to the point where we can now control the offset machines from somewhere in an office, with buttons to adjust the pressure, the amount of ink, the water level. At the same time, it will centralize the tools, allowing measuring density, and other records from the machine, or controlling points. Now a printer doesn't have to wait to finish the orders to check their quality. Each manufacturing company has named their controls in different ways, Heidelberg calls it CPC, but in general, all are essentially the same, methods of a command and control computer.
6. Rasters A Raster system is one that transforms the design you want to print into millions of points and space that can be converted into printing plates. There are two basic systems of screening: AM (Amplitude Modulation or analog raster) and FM (Frequency Modulation or stochastic or crystal raster). AM Method: The AM method is currently the most used in the country. It works separating the images in a uniform series points. In one square millimeter nine points are placed in a 3 by 3 formation. This limited number of points is the main constraint on the print quality of a machine. Just as in a picture in a newspaper, photos and images become a visual effect, fooling the human eye to see more of what really is there.
Each printing starts from a image, which must be separated into the four basic colors (CMYK) and adjusted to be printed in a press. When printing four colors, each frame of each color should have its own direction, to prevent items from falling over each other, creating a series of black dots, or ‘moire’. Usually managed as standard: Cyan: 15 degrees Magenta: 75 degrees Yellow: 90 or 0 degrees Black: 45 degrees. The end result is a 'thread' color, which in proper combination, produce the desired colors, and the final shape is the original design was expected.
FM Method: The FM method replaces the order of the fixed points, by a smaller set of points placed "random" (each manufacturer has a specific order, but that at first sight does not have a definite shape.) These points are much smaller, enabling to place hundreds in a single square millimeter.
The main limitation for using this method is that it should be employed by CTP (computer to plate) and eliminate the use of films to develop the plate. This is because when using the film points on the plate tend to increase in size, which in the AM system is no big deal, but as FM, if the points gain size, these are mixed with each other, generating a giant black spot. With FM technology eliminates the use of 'threads' colors, and the problem of Moire. The quality is much better, allowing to show more details in each image.
7. Post Press Once the printing process is finished, it is important that each pressman, and all who are in the graphic arts industry, know what happens to their work, when it comes to the Post Press.
7.1 Folding There are two basic systems of folding; the Buckle fold principle, and Knife fold principle. The Buckle system works on the principle of friction. The rollers run the paper in the direction desired, until bumping into the guide, once there, the bender roller pulls the sheet down, generating a fold while passing through the narrow space between the two rollers. The knife system is simpler, the paper moves on a tray until it bumps into the guide and then a blade pushes the paper into two rolls that force him to fold to pass between them. There are hybrid systems that occupy both technologies. These have the advantage that, when folding the paper its thickness increases, and the buckle system becomes unable to continue folding, the hybrids are able to assist in the process with a knife.
The Buckle folding machines offer a variety of options, not only fold, but, with the right tools, the machine can cut, puncture, perfore, etc. Making a good plan from the design and Pre Presspreparation, with the necessary knowledge of the tools available in Post Press, workshops allow printers to save time and resources, and improve all its processes, and thus its quality.
7.2 Die Cutting The die cutting works the same way as the old Platten presses, with two surfaces pressed together. In this case, the shape of the types is replaced by a mold made of wood and metal blades that can cut the perimeter of a shape in a single movement, or create a crease in thick materials such as cardboard or card stock. Modern machines use molds dies with laser, creating a positive effect on the wood, which will w ill be attached to metal blades by hand. Around the blades are placed in small blocks of rubber, which prevent the material adhering to the form.
7.3 Collate Collate is the act of preparing the sheets of a book to make its final shape. Typically, various pages of a book are printed on one single sheet of paper, folding them correctly, it creates a section of the book, ready to be cut. A book usually consists of several sheets, which must be assembled in the corr ect sequence, to obtain the proper sequence of pages. To avoid errors, in the stage of pre-press marks are placed in the sheet, which, when folded, indicating their place in the order of the book. As shown in the image, this allows to see, quickly and effectively if all the sheets have been properly armed.