The Munken Guide to Uncoated Paper

Transcription

1 The Munken Guide to Uncoated Paper

2 MUNKEN PURE ROUGH 120g/m² The Munken Guide to Uncoated Paper A workshop of common understanding for better results At the mill in Munkedal, we believe every part of the production process is equally important for achieving a quality result. And while we all know the definition of quality may vary from one person to the next, the way we see it, clear communication is key to achieving a common goal. This guide contains detailed information that will help you avoid taking chances when it comes to producing printed matter on uncoated paper. By having a single reference document, there can be better communication in each step of the process, ensuring all parties have the same expectations on the final artwork. The guide is divided into four workshop sessions. While some of you may know the contents inside out, others may be understanding it for the first time. The aim is that by using this guide, all the questions such as, Am I using the right ICC profiles?, Is the image adjusted for uncoated paper?, Does the printer need additional information? may be better answered to obtain the desired results. So, equip yourself with the knowledge provided in this guide, and use it to enhance your next project. Because, as you probably agree, when it comes to producing great artwork, the common language we should all be speaking is quality.

3 MUNKEN PURE ROUGH 120g/m² Table of contents Art Workshop Session 1 The paper Session 1:1 The make-up of paper Pulp Fillers Water & moisture content Chemicals Session 2 The repro Session 2:1 Main paper issues in repro p. 34 How does the repro affect the result? p. 34 Screen/Raster p. 36 Dot gain p. 36 Total ink coverage Session 1:2 Production p. 11 Production Session 1:3 Paper properties p. 12 Wood-containing & woodfree paper p. 12 Grammage, thickness & bulk p. 13 Roughness p. 13 Brightness & shade p. 14 Opacity p. 14 Porosity & absorption Session 1:4 Fibre direction & format p. 16 Fibre direction p. 16 Format p. 16 Reel formats p. 16 Sheet formats & sizes Session 1:5 How to choose paper p. 22 The paper choice p. 22 The choice of shade p. 22 Opacity in your printed matter p. 22 The paper feeling p. 23 Images on different paper p. 23 Text on paper p. 23 Lifespan p. 24 The environment & recycling Session 1:6 Storage handling & conditioning p. 25 Temperature & humidity Session 2:2 Image p. 40 Image types p. 40 Image format & quality p. 40 Image resolution & pixels p. 41 RGB & CMYK p. 41 File types p. 42 Paper & colour space p. 43 Spot coloured areas & images p. 43 Black & white images p. 43 Six-colour separation p. 43 Image preparation p. 43 Short summary Session 2:3 ICC profiles p. 46 Overview p. 46 Calibration p. 46 Characterisation p. 46 Conversion p. 46 Paper perspective when creating an ICC profile p. 48 A paper ICC profile p. 48 Different types of ICC profiles p. 49 Basic & special ICC setting Session 2:4 Production flow p. 50 Production flow & colour management p. 50 CTP & paper types p. 51 ISO Standard p. 53 Image references p. 54 Image optimization p. 56 ICC uncoated profile p. 58 ICC coated profile p. 60 Colour separation p. 66 Screens p. 68 Black & white/ duplex/ triplex p. 70 Ink drying 2 Art Workshop Table of contents

4 MUNKEN PURE ROUGH 120g/m² Table of contents Art Workshop Session 3 The printing Session 3:1 Printing methods p. 80 Offset p. 80 Sheet offset p. 80 UV sheet offset p. 82 Web offset heatset & coldset p. 82 Flexographic printing p. 82 Digital printing p. 83 Inkjet, digital printing with ink p. 83 Digital printing with toner/xerography & laser p. 83 Pre-print in a conventional press Session 3:2 Offset printing & uncoated paper p. 90 The offset principals p. 90 Water, ink & paper interaction p. 90 Different inks p. 91 Ink density & uncoated paper p. 92 Back pressure p. 92 Print powder & after print stocking p. 92 Ink drying & paper p. 93 Press varnish p. 93 Print quality on paper Session 4 The post production Session 4:1 Effect enhancing & post production p. 100 Foil blocking p. 100 Embossing p. 100 Varnishing p. 100 Relief printing p. 101 Laminating p. 101 Die cutting p. 101 Hole punching p. 101 Perforation Session 4:2 Binding p. 102 Bookbinding p. 102 Folding & scoring p. 102 Binding p. 102 Trimming Summary Bear in mind p. 114 Key points p. 115 Summary charts p. 116 Your own keypoints p. 126 Munken Design Range Table of contents Art Workshop 3

5 4 Art Workshop Session 1 The paper

6 Session 1 The paper Paper is an extremely broad concept. The variations are almost limitless, and it is not only the visual impression but also the tactile qualities that make each paper unique. This is why paper is far more than just a surface. Paper can convey feeling and add an extra dimension to your printed message. Our aim is to give you the knowledge to be able to distinguish one paper from another, but with particular reference to uncoated paper. Uncoated simply means that the paper has no added layer on the surface and is completely natural. We hope this knowledge will help you select the best paper for your particular end use by being able to recognise the individual qualities and properties of the paper you see. Session 1 The paper Art Workshop 5

7 MUNKEN PURE ROUGH 120g/m² 6 Art Workshop Session 1 The paper

8 MUNKEN PURE ROUGH 120g/m² The paper master Leif Lundgren A great mill is nothing without its paper machines. And the paper machines are nothing without the skilled craftsmen running them. Leif Lundgren is a paper master at the mill in Munkedal. The 80-metre-long paper machine he is in charge of during his shift produces some metres of paper every hour. Being a bit of a control person, Leif and his team hang up a huge sheet of paper on the lightbox every twenty minutes and slowly search the surface to make sure the paper is up to Munken s high standards. Session 1 The paper Art Workshop 7

9 MUNKEN PURE ROUGH 120g/m² 8 Art Workshop Session 1 The paper

10 MUNKEN PURE ROUGH 120g/m² Session 1 The paper Art Workshop 9

11 MUNKEN PURE ROUGH 120g/m² Session 1:1 The make up of paper In simple terms, paper consists of pulp, filler, water and chemicals. The ingredients are combined according to the unique recipe for each particular paper, and the grades are then produced in a way that ensures they have the desired properties. Pulp In school, we used to learn that paper comes from trees. This is of course true, but also an extreme simplification. It would be more correct to say that pulp consists of cellulose fibres that usually come from pulp wood which in turn comes from trees. Cellulose can also be extracted from cotton or grass, but here we will be focusing on pulp made from wood. The most common wood types used for pulp are hardwood as aspen, eucalyptus and birch, and softwood as pine and spruce. Different types of wood are used because the properties of the fibres vary. For example, hardwood fibres are shorter and give the paper good opacity and formation, while fibres from softwood trees are longer and make for a strong paper. There are two main ways of producing cellulose fibres from wood: a chemical method and a mechanical method. With the chemical method, which produces chemical pulp, cellulose fibres are released from the other constituents in the tree trunk by first being ground into chips and then digested using chemical additives. This method produces strong, almost pure cellulose fibres which are highly resistant to ageing. Since this method almost exclusively uses the tree s cellulose around 50-55% of the tree s volume the resulting product is often called woodfree pulp. The other constituents in the tree are instead used as many different kind of raw materials or energy, which also means that many modern pulp mills are more than self-sufficient in terms of energy. The production of mechanical pulp, on the other hand, involves grinding the tree in a mill or between grindstones until the cellulose fibres have been separated. Thus the name mechanical pulp as the fibres are extracted using a mechanical method. Moreover, unlike the chemical method, mechanical pulp production also utilises other parts of the tree such as lignin and resins so that almost 95% of the tree s volume is exploited. Mechanical pulp is therefore often called wood-containing pulp, as all the wood-containing substances are still there. There are several different variations of chemical and mechanical pulp. Also, combinations of the two methods are used to produce a third type of pulp, semi-chemical pulp, one example being Chemo Thermo-mechanical pulp, known as CTMP. In CTMP production, the tree is turned into chipwood in the same way as in the chemical method, and the chips are then partially treated with heat and chemicals before the fibres is separated mechanically. Fillers Pulp made of fibres therefore represents the most important ingredient in paper, although it alone is not enough. Additives are also required, one being filler. As the name suggests, the task of the filler is to fill in the gaps in the complex fibre network. A paper made with filler is softer and more even. It has better formation, higher opacity, better inksetting properties, a smoother and more flexible surface all of which make for better printing characteristics. There are various types of fillers, the most common being calcium carbonate, e.g., limestone, and clay, e.g., kaolin or china clay. The choice of filler depends both on the desired paper parameters and on the type of production system. The filler will affect certain paper parameters like shade, opacity, porosity, stiffness, etc. For this reason, different types of calcium carbonate are also used. They vary depending on how they are ground, original brightness and if they are treated in different ways. Some common names are GCC and PCC. Each mill has a very sensitive production system with process water, and this can be acid or alkali/ neutral. Calcium carbonate is only used in a alkali /neutral system and results in a paper with a high ph value, i.e., an alkali/neutral paper. Kaolin filler is normally used in an acid system and produces a paper with a low ph, i.e., an acid paper. A paper s life span partly depends on whether it is manufactured in a basic/neutral or an acid system; if it s acid-free or not. This is because a paper made in an acid system is broken down from within by the low ph value, while the opposite applies for a basic /neutral paper: the high ph value helps the paper better withstand external acid stress. This is a requirement for an age-resistant paper. Water & moisture content Paper also contains some moisture in the form of water, commonly between 3 7% of its weight. The moisture level depends on the application and printing process the paper is intended to be used in. The water content of paper is expressed in two measurements: absolute humidity (AH) or relative humidity (RH). Absolute moisture refers to water content as a proportion of the paper s weight, for example 5.5% of the total paper weight. Relative moisture is a quota figure that defines existing water and the paper s (water) saturation at one and the same temperature, e.g., 50 55% at 20 C. If the temperature or air humidity in the paper s surroundings change, the paper will either release or absorb moisture. This in turn means that the paper may change format or become wavy during exposure resulting in poorer printing properties. It is therefore essential to handle and store paper in the correct way. This is discussed later in this session. Chemicals Paper production also requires other chemicals in addition to fillers. These are necessary primarily for the paper to be made, but also to ensure that the finished product has the desired properties, such as extra strength, better water resistance and the right shade. For instance, stock-sizing is used to stop the paper absorbing too much moisture, while the manufacturing of surface-sized fine paper also uses a starch additive to make the paper more print-friendly and durable. Furthermore, shading dyes are also usually required so that the paper maintains a consistent colour from one production run to the next, as the shade of the pulp frequently varies. In order to achieve higher brightness, an OBA, or Optical Brightening Agent, is also used. The OBA and dying inks will also interact to gain a final perfect shade. Production chemicals will always be necessary to obtain a stable chemistry in the production process, and then a stable product quality. Paper is almost a 100% natural product, but as always when chemicals are used, it is essential to use the right type of chemicals to avoid environmental impact and also get a final product that is harmless. For this reason there are very strict controls today both by local authorities and the EU via the REACH system. Paper grades can also be tested for compliancy with food contact as well as for safety of toys, and these should be the best guarantees for a safe paper choice. 10 Art Workshop Session 1 The paper

12 MUNKEN PURE ROUGH 120g/m² Session 1:2 Production Once the pulp has been produced, paper is manufactured in two stages: stock preparation and on the paper machine. Production In stock preparation, the fibres in the pulp are beaten to give the paper special properties, such as optimum strength and fibre binding. In addition to pulp, the stock ingredients usually comprise of stock size, calcium carbonate, shading dye, and 99% water. Paper production begins with the stock mixture being fed into the head box, which is the starting point of the paper machine, and sprayed onto what is called the wire. The wire section is basically the paper machine s first dewatering process. Using one or two wires or straining cloths, the water in the stock is removed with dewatering elements and vacuum, and the moisture content of the mixture decreases considerably. It is also in the wire section that the paper begins forming, and the fibres orientate themselves mainly in the direction of the fast moving paper web. From the wire, the still wet pulp moves on to the press section, where it is dewatered further using pressure between cylinders and felts. The pressure in the press section also influences the finished paper s bulk, stiffness, opacity, strength and roughness. The next stage is the drying section, where the paper passes over a number of steam-heated cylinders, and the majority of the remaining water is removed. Only now, after the drying section in which almost all the moisture has been removed, can the word paper be used in its true sense. As the paper comes out of the pre-dryer, it is often surface sized. Surface sizing means that a thin film of a sizing agent, most often starch, is applied to the surface for extra strength and printability. The paper then goes on to the after-dryer still at the same high speed, where it is dried after starch has been added. This stage is omitted if the paper is an unsized quality. Before the paper is completely ready and wound onto a large reel called a tambour reel, it is often calendared, pressed between cylinders, were pressure and friction are applied to reach the desired surface smoothness and the final thickness. In the final stage of the process, the finished paper is wound onto large tambour reels, and then cut down into smaller reels or sheets. The entire process in the paper machine takes very little time, between 10 and 30 seconds from stock to finished paper. During this brief sequence of events, the stock is dewatered and becomes paper. The paper machine is therefore, quite simply, a large dewatering machine. WIRE SECTION PRESS SECTION DRYING SECTION Head box Upper wire Pre-dryer Surface sizing After-dryer Calendering/smoothing Tambour reel Lower wire Session 1 The paper Art Workshop 11

13 MUNKEN PURE ROUGH 120g/m² Session 1:3 Paper properties All papers are unique and have different properties properties that are crucial to the final result of a piece of printed material, how it is perceived and the feeling it conveys. Choosing the right paper is therefore important, and requires some knowledge. Quite simply, you have to understand the preconditions and properties of the paper in order to make the correct choice. Wood-containing & woodfree paper Earlier in the section, we explained the two principal ways of manufacturing pulp, the chemical method and the mechanical method. The chemical method removes practically all wood-containing substances from the wood, apart from the cellulose. This is why paper that contains woodfree pulp, i.e., chemical pulp, is called woodfree. Woodfree paper properties include high strength and good performance. A woodfree paper can fulfil the requirements of permanent paper, the ISO 9706, if certain parameters are fulfilled. The parameters are maximum permitted lignin content, an alkaline buffer of calcium carbonate and high enough strength. The production of mechanical pulp exploits the majority of the tree volume, which means that parts other than the pure cellulose are added to the pulp mixture. A paper with mechanical pulp is therefore called wood-containing paper. The properties of a wood-containing paper include good strength, high opacity and a natural feel. Paper grades, which mainly comprise of semichemical pulp (CTMP), change the traditional distribution of woodfree and wood-containing paper because benefits are gained from both types of pulp. By definition, these are often referred to as wood-containing paper, although they are often more similar to woodfree grades, depending on the desired function. when choosing a paper, although it is often used somewhat carelessly to refer to a paper s thickness, which is misleading as thickness actually depends on the paper s bulk. Thickness=grammage x bulk A paper s thickness is measured in micrometres (µm) thousandths of a millimetre and is the distance between the paper s two surfaces. Thickness is in many ways an important consideration in paper selection, as it affects the stiffness, stability, feel and of course thickness of the final printed product. This is particularly true in books, which can contain hundreds of pages of sheets. Bulk=thickness / grammage As the above formula shows, bulk defines the relationship between a paper s thickness and its grammage. The dimension refers to the volume or compactness of a paper which is then cm3/g. The correct definition of bulk is in fact the reciprocal value of density, (g/cm3). These formulas show that the bulk is actually the volume divided by the weight and not the thickness divided by the grammage. However, when the weight and the volume of the paper are both per m², the calculation will give the same result. A paper with a low bulk is more compact than one with a high bulk, and contains less air. A low-bulk paper is therefore thin and heavy, while a highbulk paper is light, airy and thick. bulkier paper dramatically increases stiffness and rigidity. When comparing an uncoated paper with a coated, the coated is, in general, more floppy than its uncoated equivalent in grammage. The definitions of woodfree and wood-containing paper differ between standards, but the most common is that woodfree paper can consist of up to maximum 10% mechanical fibres. Paper made to meet the requirements of the various standards for age-resistant paper (such as ISO 9706) can, in principle, not contain any mechanical fibres at all. Grammage, thickness & bulk Grammage, thickness and bulk are three important and common paper properties. The three are often mentioned in the same breath, for the simple reason that they are mathematically linked. Thickness and grammage are defined independently of one another, while bulk, sometimes also referred to as volume, is the relationship between grammage and thickness. This section defines each term more closely. Grammage=thickness / bulk Grammage is the most common measure of a paper s weight, and is given as weight per square metre, e.g., 130 g/m². The concept is fundamental Compare bulk 1.4 and 1.13 on pages Bulk is a very important factor when producing books, and is crucial to how the final result is perceived. If the aim is to convey an impression of a thick book rich in content, a high-bulk paper should be used. If the idea is to fit in a lot of text but still keep the book thin, a low-bulk paper is more appropriate. Bulk can also, in many cases, be a question of saving costs. For example, when setting up a mail campaign and using a paper with a slightly higher bulk, a lower grammage can usually be used without detracting from the feeling. This could save a lot of money in distribution costs. We often talk about what yield we get per m² and the result is that you get a higher number of printed books, brochures, DMs, etc. with same weight of paper, a better yield of paper and money. We recommend that you compare different papers according to the image and see for yourself that a This sheet has a high bulk it is airy, light and thick. Bulk 1.8 x grammage 130 g/m² gives a thickness of 234 micrometres. This sheet below has a low bulk it is thin and heavy. Bulk 1.1 x grammage 130 g/m² gives a thickness of 143 micrometres. 12 Art Workshop Session 1 The paper

14 MUNKEN PURE ROUGH 120g/m² Session 1:3 Paper properties Roughness In some way, the surface of a sheet of paper is like the topography of a landscape, with peaks, troughs and minor bumps. The term used to define a paper s deviation from an absolutely smooth surface is roughness. Roughness is expressed in ml/min, referring to the volume of air that passes between the paper and the flat surface of a measuring device in one minute. This measurement is called the Bendtsen value. In general, uncoated paper has a higher roughness more peaks and troughs on the surface than coated paper. In order to reduce roughness, the paper is compressed and smoothed in a calendar. This makes for a certain link between bulk and roughness, as a smooth, compressed paper will have a lower bulk. So to achieve a higher bulk, the evenness of the paper surface will have to be compromised to some degree, and the paper will be rougher. And this is, of course, significant to the final result. In some cases the aim is a nice, even surface, while in others the printed matter is intended to convey more of a coarse, robust impression. Surface roughness also affects the printing process, especially in high-roughness papers where irregularities can affect the graphics. There are also other standards with similar methods to measure the paper surface roughness as Bekk, Sheffield and Parker Print, etc., and when measuring coated paper the smoothness is normally defined according to Parker Print Smoothness PPS. Brightness & shade Just like the Innuits have many different words for the colour and consistency of snow, so paper has a broad spectrum of brightness and shades. All with one thing in common: they enhance the impression of the finished printed product. Because as we know, there is a great difference between white, natural and coloured paper. However, all pulp of the same type, basically has a similar brightness and shade. For this reason, different chemicals and dye colours are added to distinguish the finished papers brightness and shade. Adding OBAs (Optical Brightening Agents) to the mixture also increases the final brightness of the paper. OBA works by converting reflected invisible UV light into a visible blue white light, which makes the paper look brighter than it actually is. This effect can easily be verified by using a UV-lamp. optical parameters are with the CIE whiteness, ISO brightness and the shade. Although whiteness, brightness and shade are visual properties that may seem hard to distinguish at first sight, they do in fact describe different things. Brightness is expressed as a percentage of how much light of one particular wavelength, in the blue area of the daylight spectra, the 457nm, that is reflected from the paper. Whiteness is a related parameter, although this is gauged over several wavelengths, also in the blue area, it gives a value closer to what the eye actually perceives. The fact that these two most common parameters more or less express the different levels of blueness in the paper, we often need some supporting information when papers differ in other colours. In fact, two papers with the same whiteness or brightness can have differences in yellow, red or green. Shading dyes are also added to most papers. For example, a blue shade is often added to a paper when the aim is to give a whiter impression, while a touch of yellow will make the paper appear more natural. Therefore, it is common to describe the shades of the paper in words such as white, natural white, bluish white or cream. The shade of a paper can actually be well-described in a common colour model called CIE Lab, which gives the level of the blue, yellow, red and green shades and also the lightness. This is often used when controlling quality during production, but is not as common for explaining paper parameters. As you have already understood, the colour of paper is a science in itself. However, the information provided in technical specifications is very helpful. But two fundamental things are needed to make a good visual comparison between paper shades: samples and the right lighting conditions. Samples with different shade levels and references are normally needed when visually determining the paper s optical parameters. Without comparing references, it is very hard to do a proper evaluation. When evaluating shades, you should also use the correct lighting conditions. In the paper industry, we use D65 as an illuminant, while in the evaluation of printed colours D50 should be used. In most cases, these specific lights are not available at hand, and then the best practice would be to use and compare the samples with several different light sources. Compare paper shades on pages The most common way to describe a paper s Session 1 The paper Art Workshop 13

15 MUNKEN PURE ROUGH 120g/m² Session 1:3 Paper properties Opacity Another word for opacity is non-transparency. Therefore, the higher a paper s opacity, the less translucent it is. A paper with 100% opacity is therefore completely non-transparent, while one with a low opacity, such as greaseproof paper or tracing paper, lets a much higher proportion of light through. Opacity depends on how well the paper surface can scatter and absorb light. The perceived opacity is altered during printing, after which we refer to print opacity. This is an important factor to consider, as printing ink penetrates the paper and reduces its opacity, which could result in the print being visible through the sheet. This is especially important in double-sided printing, particularly on low-grammage paper, when images or text on one side can disturb the other. It is therefore important to think about where on the page the text and graphics should go Porosity & absorption Porosity and absorption are two important factors where coated and uncoated paper differ. These two parameters have a major influence in the way paper absorbs the printing ink, and we would therefore like to spend some time reflecting on these. Porosity and absorption are not normally considered when choosing a fine paper, and therefore these parameters will seldom be found in technical specifications. They are more related to the way ink is absorbed and how images will turn out on the paper. It is not easy to imagine that a paper, which is based on cellulose fibre, is a porous material of which 70% of the volume could be air. As we explained earlier, the cellulose fibres form a web with gaps and pores in between. Even if the gaps are filled with fillers, the paper will still be a porous material, where air can pass through and liquids can be absorbed. The way paper absorbs liquids is due to the porosity, but also the way the paper is surface sized and the surface is treated. On uncoated paper, the actual fibres absorb liquids as well. This is often called the micro porosity, which affects the way liquids are absorbed into the paper. The total absorption on uncoated paper occurs both in the paper and on the surface, creating a bleeding effect in all directions. On coated paper, the surface coating will cause a totally different absorption behaviour. The coating layer consists of pure minerals like calcium carbonate and/or china clay with a certain amount of binders. This layer is much more compact than the fibre network, but this layer is also highly porous on a fine micro porosity level. This porosity is created by the gaps between the mineral particles, which are very small. The coating layer is developed to interact with the offset printing inks. It will have a kind of filtering effect that keeps the colour pigment, together with some amount of ink binders, on top of the surface. But the main part of the solvent and liquids will be absorbed into the coating layer. The colour pigment particles should be bigger than the micro-porosity gaps to be kept on the surface. The absorbtion of liquids is very quick with the main part soaking into the surface with a minimal bleeding effect. To summarise the absorption of ink; on uncoated paper, both the solvents and pigments will be absorbed into the paper s surface. This will also cause the ink dots to grow and more colour pigments to be absorbed into the paper, resulting in a lower ink gloss. The coated paper will, instead, very quickly absorb the ink straight into the coating layer, keeping the pigment more or less on top of the surface, providing a high ink gloss. Uncoated and coated surfaces are fundamentally different. The surface of uncoated paper is made of pure fibres, and the porosity and absorption are caused by the gap between fibres and the fibres own absorption. The coated paper has a surface of mineral coating on top of the base paper, and absorption is caused by the micro porosity in this coating layer. 14 Art Workshop Session 1 The paper

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17 MUNKEN PURE ROUGH 120g/m² Session 1:4 Fibre direction & format In much the same way as fish in a school orientate themselves to swim in the same direction, most fibres point in the same direction as the paper web during production. The fibre direction is a crucial parameter in the printing and in post production but most of all important in printed matter. A book or brochure with the wrong fibre direction will immediately give a feeling that something is wrong, therefore getting the fibre direction right, is one of the first things that needs to be learned when working with paper. Fibre direction The direction the fibres orient to during paper production is simply called the paper s fibre direction and sometimes the machine direction (MD), the opposite direction is called the cross direction (CD). In the fibre direction, the paper will be stiffer and thereby harder to bend across. This has implications in the feeling of a printed matter. For example, a brochure with the wrong fibre direction feels less stable, and in a book, the pages feel stiff and are harder to open. It is easy to find out in which direction a paper s fibres run by laying a sheet over the edge of a table. Across the fibre direction, the paper will be stiffer and harder to bend and fold. Another way is to drag hard with your thumbnail and index finger along the paper s edges. The edge that distorts most runs across the direction of the fibres. The paper supplier s size information also reveal the fibre direction, as the figure given first (e.g., the 210 in 210 x 297) refers to the length of the side running across the fibre direction. There are several other ways to note fibre directions, as for example, noting the fibre directions size in bold or adding a terms like long grain when a paper sheet is longer in the fibre direction or short grain when it is shorter in the fibre direction. Format As the paper comes off the paper machine, it is wound onto a large reel called a tambour reel. If the paper is to be used in a web offset press it is cut down into smaller reels, otherwise it is cut Fibre direction. Sheets can be cut so that the fibre direction runs along the short or long edge of the paper, thus influencing the way the size figures are given. into the required sheet sizes. Sheets and reels are the main paper formats, but there are a great many different formats within each primary group that are either standard or customised depending on the customer and intended use. A paper buyer can be relatively certain that standard formats are in stock with the supplier. However, if a customised format is required there may be a slightly longer delivery time, as it first has to be ordered and then produced at the mill. Reel formats Like sheets, reels are cut into the diameters and widths ordered by the customer, and also according to weight and length. However, standard formats are less common in reels, although they do exist. All paper reels also have a core in the centre so that they can be brought into the printing press. Common standard core diameters are 70, 76 and 153 mm. Sheet formats & sizes The sheeting machine cuts the paper into sheets according to the customer s ordered dimensions or a standard format, and the paper is then either delivered on pallets usually in a bulk pack where all the sheets are stacked up or in smaller packs, where sheets are wrapped in reams of 500, for example. As we mentioned before, the sheet dimensions are given depending on fibre direction. The first figure is the length of the side that runs across the direction of the fibres. There is also a wide assortment of formats in standard use in different countries. Suppliers normally offer these as standard stock at a fixed number of sheets per pallet. Two common standards are the A series and the B series, and below you can see the similarities and differences. There are also versions of these standard sizes that are adapted for local demands or for special applications B7 B8 B5 52 A7 A8 A5 125 B6 B3 105 A6 A3 250 B B1 210 A B2 420 A2 Direction through paper machine A1 297x x Art Workshop Session 1 The paper

18 MUNKEN LYNX 150g/m² Session 1 The paper Art Workshop 17

19 MUNKEN LYNX 150g/m² 18 Art Workshop Session 1 The paper

20 MUNKEN LYNX 150g/m² The selector Örjan Öhman Most printers have their house paper. Still, keeping updated and informed about what paper can do is what every successful printer continuously needs to do. Örjan is a classic jack-of-all-trades character. With eleven years in the craft, he is involved in processes spanning from paper purchasing to overseeing the post production work. This holistic view on the production and the in-depth understanding of all stages is invaluable when choosing and purchasing paper to the printing house. Being a keen golfer, he also understands the virtues of keeping up quality every single step of the way. Session 1 The paper Art Workshop 19

21 MUNKEN LYNX 150g/m² 20 Art Workshop Session 1 The paper

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23 MUNKEN LYNX 150g/m² Session 1:5 How to choose paper In some cases, the paper is only a means of carrying information, and very little attention is given to the paper itself. So the standard house paper will be the obvious choice. But often, the ambition of the total impact of printed matter is higher. The paper then becomes a key part of the whole message the feeling, shade, environmental aspects and the way images are reproduced need to interact. The paper choice Deciding whether printed material actually is needed is not as obvious today as it was in the past. These days, there are many alternative ways of spreading information and reaching the target group. However, the decision to use paper as the material is often an easy choice, due to many obvious reasons. The actual choice of a specific paper grade will affect the impact of the printed material in many ways, and therefore needs to be very carefully considered. If chosen correctly, the paper can boost the impact, but interpreting different technical paper parameters into the effect in your own printed matter requires some reflection and knowledge. If the paper choice is not well made, the whole concept can be lost. On the other hand, the perfect choice of paper can support the purpose of the content and the total sum may be greater than the sum of each individual part. When choosing paper for a graphical production, there are two main perspectives to consider; the technical criteria required for functionality and, the more subjective, how the paper suits the idea of the end product. There are, of course, other perspectives, such as price and availability, etc., but the aim of this guide is to focus on the paper itself. Earlier, we explained the technical parameters of paper. In this part of the session, we will aim to link them to a more subjective perspective in the paper choice. If we, from a subjective perspective, were to divide the paper choice, we would start with visual appearance in shade, the tactile paper feeling, how paper can reproduce images, readability in text and finally the lifespan of the paper. When you have obtained the basic knowledge on how paper can be perceived, your experience of how paper characteristics can be used will add to this knowledge for many years to come. So don t hesitate on exchanging experiences with others. The choice of shade For some paper grades, standard shades has been developed, as for example, standard offset grades or A4. But for some grades a unique shade is developed so that the paper will stand out. Therefore, the first decision is whether to choose a unique shade or not. Whites and creams are very common paper shades. But the most common papers today are white, which normally have different levels of a slightly bluish tone. The blue tone in white paper will often give a slightly colder impression than a cream shade paper, which has a warmer, yellow tone. White paper will give more contrast in images. Whereas a cream shade paper is recognised as friendlier to read on, and that s why most novels are printed on a cream shade. The paper shade we see is actually a reflection of the surrounding light on the paper. This means that the lighting conditions will heavily affect the shade we perceive. So, it s always a good idea to consider where the printed matter will be used and to test the paper in different lighting conditions, for example, daylight, tube light and bulb light. You will notice a big difference when changing the light. Note that the paper shade measurements, such as ISO brightness and CIE whiteness, will not cover all the colour aspects of a paper and therefore the visual comparison between paper should support your final decision. Opacity in your printed matter Another obvious visual effect when something is printed on paper is the opacity. Remember that on a computer screen, there is no disturbance from the backside, however on paper, the text and images on one side of the sheet may show through, creating unwanted effects on the other side. Therefore, it s important to choose a paper with an opacity level that s high enough for your needs. The opacity level is influenced by the grammage the higher the grammage, the higher the opacity. Therefore, sometimes there needs to be a compromise between the grammage and opacity you choose. However, the measured paper opacity is only part of the visual opacity in printed matter. How ink-heavy the images are and how the layout is structured both play large parts in the final, so called, print opacity. So we strongly recommend that you consider the layout and image types, and the show-through effect they will have, and select the grammage and opacity level based on these. The paper feeling One of the most important and talked about aspects of paper is the physical feeling. The paper feeling can, of course, mean different things to different people. But in simple terms, it is the way paper feels when you touch and hold it. It can be warm or cold, smooth or rough tactile, soft or stiff, thick or thin. The first thing you will react on when touching a paper is whether it s warm or cold. This is an unconscious, physical reaction to how different paper surfaces draw heat away from your fingertips and although obvious, is not often considered. Uncoated paper is a wood fibre material, and will therefore feel warmer. Whereas a coated paper, which has a surface of minerals, will feel colder. Natural feeling, tactile, robust and abrasive are words we use to describe the feel of paper. However, it s the paper roughness that we usually try and put words to. A smooth surface is often considered to give a more exclusive touch, whereas a rougher surface has a more tactile and natural feeling. The surface roughness will also, to some extent, affect printed images and it will be slightly harder to get the best result on a really rough paper. However, a rough surface will often give the image a very special character. Stability, stiffness and the perceived thickness will all come from the actual paper thickness. Papers containing the same grammages can be produced in different thicknesses, and this is defined as bulk levels. A bulkier paper has a higher thickness, at a given grammage, and will feel more stable and robust than a low bulk paper. The thickness of a paper will also give a different impression on how solid the printed matter feels. If the thickness is too low, it will give a floppy and sub-standard impression. If it is too high, it can have an unpleasant feel and can result in pages in a book or brochure not opening in a nice way. Very often, the format of the printed matter, in terms of height and width, is well defined. However, the thickness also greatly affects our initial impression. A thick, heavy brochure or a thin light one will communicate different things. There are great opportunities to make the best first impression by choosing the right grammage, thickness and bulk. 22 Art Workshop Session 1 The paper

24 MUNKEN LYNX 150g/m² Session 1:5 How to choose paper Images on different paper So how will images turn out on different paper? This is a wise question to consider at an early stage. And you will get the best answers by looking at good printed references for the specific grade. These should be confirmed as having top quality in the repro and print, otherwise you might get the wrong impression of the possibilities. When you are searching for the paper feeling you want, it s important to know how different papers will affect the printed images. For example, coated and uncoated papers will show colours in very different ways. This will be explained in more depth later in this guide. Another factor that will affect images is the paper shade. The base shade of the paper will give a certain tone to the image. For example, a yellow toned paper will give a slightly warmer feeling, and a blue tone a slightly colder one. At the same time, the blue will increase the feeling of white and often give a higher contrast. Text on paper We already mentioned that cream shade paper is considered to be more reader friendly. Several studies have shown this to be true, and black text with good contrast on cream shade paper enables you to keep concentration levels up for a longer period of time with your eyes becoming less tired. Specular reflections from paper cause major disturbances when reading text. Coated paper has a certain gloss level, which you will find in the technical specifications. This value gives information about the actual reflection at a specific angle the higher the value, the higher the reflection. Uncoated paper tends not to reflect directly due to the light being scattered so much. The absence of reflections enhances readability to a greater extent. In most printed material, there is a combination of text and images. Therefore, you should consider if the importance of the text is greater than that of the images, and if it is, then readability is a vital parameter. Lifespan Is the lifespan in a paper a subjective parameter? We would say yes, because a paper will not just break or die of old age after a certain point. However, all paper will age over time, and this will become obvious from two main aspects, yellowing and decreased strength. The effect of this will be that at some point, you will think the printed matter is not as fresh as you would like it to be. In your planning of a production, you should normally consider the time the paper is required to stay fresh. For example, a direct mail just needs a few days, but an advertising brochure may need a year or two. Whereas a schoolbook, a cookbook or a novel should be able to be passed on to future generations. Yellowing is an aspect that usually needs to be considered when selecting a wood-containing paper. However, all paper will change colour to some extent. For example, the OBA in white paper will fade somewhat, and even woodfree paper will become slightly more yellow over time. Whereas, while the effect on wood-containing paper can start after just a few days, depending on what lighting conditions it is exposed to, with woodfree we re talking decades or even several hundreds of years. Regarding strength in the paper, there are two issues to consider: the binding and how the paper will withstand the handling it is supposed to endure. In the binding, the paper strength plays a great part actually keeping it all together. A few basic rules can be applied: woodfree paper is stronger than a wood-containing one, higher grammages are stronger than lower grammages and uncoated paper will be stronger than a coated one at the same grammage. There is, however, an additional strength aspect how the paper changes when it is exposed to moisture. Paper can remain strong in wet conditions from different sizing and surface treatments. Therefore, it s important to consider whether the paper will be exposed to humidity in some way. Finally, it can be interesting to reflect over the lifespan of your information when choosing a long life paper. You probably no longer use the content of a floppy disk created 15 years ago, but you can still read a book published then. Do you think you will be able to read the information you store digitally today in years? Perhaps, but today there is still no future-safe digital media. By choosing the right paper, however, you can ensure your information lasts several hundreds of years. Session 1 The paper Art Workshop 23

25 MUNKEN LYNX 150g/m² Session 1:5 How to choose paper The environment & recycling Caring for the environment has become a natural part of our everyday life. An increased number of reports about global warming, waste mountains and polluted waters has made us more and more environmentally conscious. We are faced daily with large and small environmental choices and as a producer of printed matter, the paper choice is one of them. Essentially, paper is a product with many environmental benefits and to use paper as a media carrier is, most of the time, the obvious and most environmentally friendly choice. Paper is a renewable, recyclable and biodegradable material. What makes paper so unique from an environmental perspective is that it is produced from cellulose fibre. The cellulose fibre used in paper has a life cycle in balance with nature and it is an amazing material for many reasons. First of all, it is chemically built up by carbon, which the trees, as they grow, take up from the atmosphere in the form of carbon dioxide. In return for the carbon dioxide the trees give back oxygen, which is essential for all life on earth. The fact that the trees take up carbon dioxide makes the cellulose fibre a carbon-neutral raw material and this is one of the main reasons why the carbon footprint of paper and printed matter is very low. One other advantage with the cellulose fibre used in paper is that it can be recycled and re-used many times. These days, paper can be almost 100% recycled, and to minimise the paper industry s impact on the environment, it is essential that the fibres are recycled to the greatest extent possible. In the paper manufacturing process, the paper waste from quality changes and trimming is efficiently taken care of by recycling it directly back into the process. This is possible because this paper is clean which is to say that it has no printing ink on it it is ground down, added to the pulp stock and used again. This type of re-used surplus paper is called broke, and it ensures that everything in the process is exploited to the maximum. But maybe the greatest gain in terms of reduced environmental impact is the collection and recycling of all sorts of printed matter and packaging paper in our community. Paper that has been printed on or used in some other way nearly always contains impurities that a normal paper mill cannot deal with. In this case, it is sent to a special recycled-paper mill that has the necessary cleaning systems. The paper undergoes several stages of cleaning before being used to make new paper. Recycled paper is primarily used for newsprint or cardboard, but is also found in some other grades of paper. quality and each time the fibre is recycled, to use it in paper grades with lower quality requirements. In this way, the fibre will be used to its maximum potential with minimal input of chemicals and energy for the recycling process. So, for high quality printed matter it is a good environmental choice to use paper made of virgin fibre when it is used and recycled, it will be a necessary input of fresh fibre into the paper recycling system and can be re-used for production of new paper. Even this guide, which has been made from virgin fibres and broke, is on its way into an eco-cycle. It may be recycled and become a newspaper in just a few years time, when you think you know everything about paper and printing. But what we really want is for you to keep it for many years to come, which is why we have made it from woodfree grades. If you do choose to keep it, it can last for several hundred years. Everything has an end and so does the life cycle of the cellulose fibre. Even though it can be recycled many times, it will eventually reach its end-of-life phase. But here the benefits of being a natural material are also evident. Paper can be incinerated to generate electricity or central heating for houses. The carbon in the cellulose fibre is then released and goes back into being the carbon dioxide it once was before taken up by the growing trees. If the paper by accident ends up in nature, the same thing will happen as the cellulose fibre degrades naturally that is the advantage of a biodegradable material. Indeed, paper is essentially an environmentally friendly material, but to make the best paper choice from environmental perspective there are a number of aspects to consider. What are the impacts on water from the paper production? What is the carbon footprint of the paper? Does the wood raw material originate from sustainable forestry? What environmental certifications does the paper have? All of these are relevant questions to ask. To analyse and compare in detail all different environmental parameters is however a complex task and it does not always give an easy answer. One paper grade can be better from one perspective a second paper grade from another perspective. Therefore it could be a good recommendation to focus on a limited number of environmental aspects or certifications, the ones which feel most important and relevant for you, and make the comparisons and choices based on those. So, what are the most common environmental aspects and certifications for paper? We can recommend looking for the following: environmental criteria from a life cycle perspective and with a wide range of environmental aspects taken into account. TCF and ECF means that the paper has not been bleached with chlorine gas. The bleaching actually takes place during the pulp production and nowadays most pulp mills have stopped using chlorine gas for bleaching. Instead oxygen, ozone, hydrogen peroxide or chlorine oxides are common methods. TCF stands for Total Chlorine Free and means that the pulp has been bleached without any chlorine compounds. ECF stands for Elemental Chlorine Free and means that the pulp is bleached without chlorine gas but that chlorine oxide is used in combination with, for instance, oxygen and hydrogen peroxide. ISO and EMAS these are Environmental Management Systems (EMS) which many mills have in place in order to work with environmental issues in a structured way. One important principle in such work is to set and follow up environmental targets in order to continuously improve the environmental performance. Paper Profile this is an environmental product declaration (EPD) for paper where the most important environmental parameters are given, such as emissions to air and water, waste to landfill and purchased electricity. Declarations according to Paper Profile follow a well-defined method in order to provide comparability of the data. So there are many certifications and tools to help you find the most environmentally friendly paper. For us, as a paper producer, the most important thing is probably to have an open approach in the contact with our customers; it is through continuous dialogue and openness around environmental issues that you can feel confident that you have made the right paper choice from an environmental perspective. Since the cellulose fibres are processed and circulated several times, they eventually wear out. This is why new or virgin fibres will always be necessary in the paper industry. From an environmental perspective, it makes most sense to use the virgin fibres in paper grades of highest FSC or PEFC certification certifications that guarantees that the forests have been managed in a responsible way. Nordic Ecolabel (Svanen) or EU Ecolabel paper which is certified or approved according to these environmental labels meets strict 24 Art Workshop Session 1 The paper

26 MUNKEN LYNX 150g/m² Session 1:6 Storage, handling & conditioning Like almost all other materials, paper will change due to ageing and the surrounding environment. Paper is a natural living material that creates the final artwork. The understanding of these fundamental changeable parameters is vital both during production and in the final printed product. Temperature & humidity All paper contains a small amount of moisture, and is highly sensitive to changes in temperature and humidity in the surrounding air. In humid environments, the paper s fibres expand and make the sheet swell. If on the other hand the environment is dry, the opposite occurs and the paper releases moisture. These phenomena can cause dimensional changes and waviness, which in turn can lead to printing problems. In order to reduce this external influence, and to protect the paper against dirt, damage and other wear before it reaches the customer, the paper is wrapped. However, the wrapping should be kept on even after the paper reaches the customer, as it will begin to interact with the surrounding humidity level as soon as the wrapping is opened. If the premises do not have the right temperature and humidity, this can lead to the problems mentioned above. maintains the existing moisture level while keeping water out. Corrugated board end-discs are used to cover the ends of the reels. There are also several types of plastic packaging. One of these is stretch film, whereby thin plastic wrap is wound several times around a pallet or reel to ensure a stable parcel. The film also provides protection. Another plastic wrapping is shrink film. The pallet is wrapped in plastic polythene which then shrinks after passing through an oven. Finally, any air is removed by compression. Following this process, the pallet is practically vacuum-packed and very steadily supported and well protected. Below are suggested conditioning times, in hours, of original wrapped paper depending on temperature differences between paper and environment. To reduce the risk of dimensional change, the paper should be stored in the same environment as it will be used in, to make sure it adapts to the right temperature. This is of course especially important if the paper has been transported over a long distance or has previously been stored in a cooled warehouse or over the winter. Temperature differences between paper and environment Volume of paper (m3) 0,2 0,4 0,6 1,0 2,0 Humidity, after temperature, is the second major factor which influences the behaviour of paper. The suggested maximum difference between the relative humidity of paper and the printing hall, shouldn t exceed 10%. Otherwise, paper processing problems might occur. In the case of standard offset paper, the suggested humidity of the printing house environment should be in the 40 60% range at a temperature of C. Also bear in mind, that once the dimensional changes caused by excessive humidity differences have occurred, the paper will never return to its original shape, even if the correct air humidity level is restored. There are several different kinds of wrapping, the most common being paper and plastic. Paper wrapping usually consists of a special type of kraft paper with a thin embedded plastic film, and this is wrapped around the reel of paper. This 5 C C C C C C C C C Conditioning time (h) Session 1 The paper Art Workshop 25

27 MUNKEN LYNX 150g/m² 26 Art Workshop Session 1 The paper

28 MUNKEN LYNX 150g/m² Session 1 The paper Art Workshop 27

29 28 Art Workshop Session 2 The repro

30 Session 2 The repro Not so many years ago, the graphical reproduction of images, as we normally just refer to as the repro, was a manual craft. The repro was very complex work that needed years of experience and very special equipment. Today, the complex part of the work is done under the hood of any modern computer with the right software. The repro is today possible for anyone to do. However, to reach the level of quality that is needed in a high quality production, craftsmanship and skilled art workers are needed more than ever. A well done repro and a repro adapted to the paper will enhance the images and will use the full capacity of each paper grade. This session covers the most general aspects that occur in prepress from a paper perspective and particularly for sheet fed offset. The topics explained evidently occur in all printing techniques, but to a different extent. The recommendations given are mainly for sheetfed offset in order to achieve the highest quality. Session 2 The repro Art Workshop 29

31 MUNKEN LYNX 150g/m² 30 Art Workshop Session 2 The repro

32 MUNKEN LYNX 150g/m² The magician Hans-Olof Axgren Understanding how a specific paper behaves in a printing machine and how different inks act in the same process is key to successful retouching and repro. Having an interest in photography helps Hasse to do a better job as a retoucher. But retouching for printing is something completely different from improving your personal images in Photoshop. In printing, the magic lies in meeting the attributes of the chosen paper, the printing press and inks. Still, the most important aspect of Hasse s job is to be accurate and never, ever cut corners. And the biggest reward? A great result, of course. Session 2 The repro Art Workshop 31

33 MUNKEN LYNX 150g/m² 32 Art Workshop Session 2 The repro

34 MUNKEN LYNX ROUGH 120g/m² Session 2 The repro Art Workshop 33

35 MUNKEN LYNX ROUGH 120g/m² Session 2:1 Main paper issues in repro Often the smallest changes have the greatest impact. The essence of years of experience with uncoated paper could be condensed into a few main issues. The handling of these main issues; dot gain, total ink coverage and screen can be the difference between a true artwork and a disappointment. Of course, other parts in the process are also important, but they might not differ between paper types. How does the repro affect the result? The aim of a good repro is to reproduce an image, most often printed on a paper, in the best possible way. The repro should handle the most important parameters that will affect the final result. Keep in mind that it is the actual repro that sets the main quality level on the end result, not the actual printing. The repro will affect two important things: the printability and the runability. The printability, or as we normally say, the print quality, is the actual visual quality of the image. To some extent, this is a subjective term whether it s visually similar to the original image, or it s a certain desired effect in print. But it is also measurable on certain parameters, such as dot gain, density, misregistration, etc. When the repro is done incorrectly for uncoated paper, what you often see is that images are darker then expected. Dark areas can also lose details and contrast. Another common effect is that colours are not as clean and bright as they could be. The main impact on print quality in connection with uncoated paper and repro is due to dot gain, and the chosen screen ruling and screen type. This will be described in more detail later in this session. Runability is when we talk about the actual production in the press, which involves stability in the press, ink drying time and problems like smearing and set off. It also consists of problems in post production, like binding. Different papers absorb ink very differently, meaning the possible amount of ink that can be applied is different. If a paper is over inked, it may cause several problems, such as prolonged drying, smearing and set off, both in press and in binding. The biggest impact the repro has on the runability is in the Total Ink Coverage (TIC), the density value for each process colour and the balance between the TIC and density. A few small changes in the repro will have a huge effect on the entire production and end result, and will also be the difference between a true artwork and a disappointment. It is not more difficult to perform a correct repro, but to understand cause and effect might require some study and experience. Therefore, we recommend that anyone working with production of printed matter tries to understand the basics, so that each person knows what is important to communicate and when. used terms in the graphic design and printing industry, although, these days, we tend to refer to the screen, screening, screen ruling and screen density of a digitally processed image. The word raster comes from the Latin word rastrum, meaning to divide. A normal, classic photograph consists of continuous tones or gradients of colour. However, these tones cannot be reproduced in print quite simply, a printing press cannot handle the task. In order to make the images printable, they are therefore screened, i.e., divided into a multitude of microscopically small dots. The dots in a screened image therefore give the eye the illusion of a single continuous image, with tone values of differing strengths, just like a normal image. Screen dots are created in a computer called Raster Image Processor (RIP), and then exposed to the printing plate in the CTP, which will be further explained later in this session. The size of conventional screen dots ranges from 1 to 100%, creating a perception of half tones. Tones of colour can be obtained by changing the size of the dots. A 5% dot is just a very small dot giving a very light hint of colour on the paper. 50% dots cover half of the paper surface and then give a mid tone of colour. When the dots are at 100%, for instance, they have completely run over into each other to produce a fully covered surface, which is called a full tone area. A full tone area will not show any details, because it s only when there is space between the dots, the screen is open and it s possible to show details. Offset inks are transparent, so the paper always acts as the fifth colour. But it s when a screen is open that the paper surface and colour will really give brightness and detail to the image, between the printed ink dots. On a conventional screen, also called AM screen, the screen dots are arranged in lines or rows. The density of these lines is called the screen ruling or screen density, and is normally measured in lpi (lines per inch) or l/cm (lines per centimeter). The denser the screen ruling, the harder it is to distinguish between the dots and the detail of the image improves. When the ink dots representing each colour are printed on paper, they jointly make up the final image. In order to avoid unwanted pattern distortion the Moiré effect the dots need to be placed at specific screen angles. with different frequencies in darker and lighter areas. A dark area means that a lot of dots are added and in light areas just a few. The benefit of stochastic screen is that it can reproduce very fine detail and eliminate the Moiré effect. Screen/Raster Raster dots and raster density are traditionally In a stochastic screen, also called FM screen, the dots are all the same size. They are very small, microns, and they are placed randomly and 34 Art Workshop Session 2 The repro

36 MUNKEN LYNX ROUGH 120g/m² Session 2:1 Main paper issues in repro When talking about screen dots, resolution must also be mentioned. This is because screen dots are represented by the small pixel dots in the digital file, whose proportional size define the printout resolution. This is usually measured in dpi (dots per inch), or ppi (pixels per inch) Producing a high screen density will also require a high printout resolution. This will be further explained later on but it should be mentioned here that the higher the screen ruling, the higher the number of pixels needs to be, and stochastic screen normally demands much higher resolutions due to the very small dots in the raster. Today, a new type of screen, called a hybrid screen is becoming more commonly used. The hybrid screen combines the conventional AM screen and the stochastic FM screen into one. Normally, the stochastic FM screen is used for lighter tones and the conventional AM is preferred for darker tones. This enables a higher screen resolution and finer reproduction of details in print, all of which enhance the overall print image quality. When printing four colours, four different screens are printed on top of each other. Therefore, to ensure each of the four screens is positioned correctly, there are several control lines or crosshairs of each process colour in the edges of the printed sheet. These lines should perfectly fit over each other, otherwise there will be a certain degree of misregistration and the image will appear to be slightly blurry. Conventional screen Stochastic screen A screen rose. In a conventional screen the lines are placed at different screen angles, one for each process colour. The colour tones are varied by varying the size of the dots. When the dots are laid overlapping one another, a new colour is created. The illusion of different colours is also created when the screen dots are close to each other. Compare different screens on pages Session 2 The repro Art Workshop 35