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i = ARCTIC PAPER RECOMMENDS
We are now rapidly approaching an important milestone in the production of a printed product – the printing itself. The repro and film work has been done, the colour proof adjusted and approved, so it’s time to make an imposition – i.e. arrange the pages so they are printed in the right order – and produce the film and plate, or possibly go direct to plate using Computer to Plate (CTP), to make stereotypes or print direct on a digital press.
Below are some common printing methods, and we have highlighted some of the most important factors to consider when printing on uncoated paper. |
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Offset is the predominant printing technique in use today, and there are several variations on the method. Whatever the variation though, all offset is based on the lithographic principle. This principle in turn is based on the opposition – or seen from another perspective, the interplay – between water and ink. The method is therefore also sometimes called wet or water offset.
Offset printing
The top cylinder has an printing plate which takes up damping solution and printing ink. The ink is offset onto the rubber blanket on the cylinder iIn the middle, which in turn transfers the ink onto the paper. At the bottom is a back-pressure cylinder.
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The process basically works as follows: ink is transferred onto the paper via a rubber blanket, which in turn has picked up the ink from a printing plate. However, the printing plate has first been exposed to make certain areas repel water but absorb ink, and others to do the opposite, i.e. absorb water but repel ink. The ink attaches to the dots, and is transferred
onto the rubber blanket and then onto the paper.
Damping/fount solution therefore plays a vital role in lithographic printing, as it ensures that ink is only transferred to the printing surfaces on the plate, thus assuring a problem-free run. For this to work the damping solution has to have a low surface tension to make it flow out over the non-printing surfaces, and various chemicals are therefore added, often alcohol. Among other things, the chemicals govern the solution’s pH value and hardness.
An alternative method is dry or waterless offset, where the damping solution is replaced with a chemical material on the printing plate’s surface.
i As we mentioned before, the various stages of the printing process give rise to dot gain, and again this is greater on uncoated than on coated papers. Dot gain must therefore be taken into account in repro and film adjustment, as mistakes in repro cannot be put right by the printer. However, some rectification is possible if you over-compensate in the repro stage by increasing the back pressure and ink coverage on the printing machine. Back pressure refers to the pressure between the rubber blanket cylinder and the impression cylinder the paper passes between. Reducing the distance between the two increases the pressure, pushing the ink into the paper and its irregular surface.
Ink is always an important factor. All four-colour printing uses process colours, CMYK. These are transparent, and are part of what is known in the industry as the subtractive colour system. As they are printed layer on layer, the various inks show through all the transparent layers, and in combination they reveal the different shades of the image.
The inks are of course adapted for each individual printing method, although they should still be checked to make sure they are suitable for use with uncoated paper and set well on it.
Well, there you have the basics of the lithographic principle. We will now describe two different offset methods: sheet offset and web offset. |
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Sheet offset is, together with web offset, the most common printing method, and it is used with all kinds of paper grades in the modern industry. Sheet printing can be run in everything from small to very large volumes, but always with high-quality results. Sheet printing is therefore popular for various kinds of advertising, posters and books, especially as the technique allows advanced post-production.
Briefly and in greatly simplified terms, sheet offset involves placing a pallet of sheets at one end of a printing press, feeding them into the machine, and collecting the printed sheets at the other end.
Of course, all kinds of things happen in the process. The suction nozzle picks up one sheet at a time and feeds it into the press. The sheet then passes through one or more printing units.
The number of printing units varies, but in four-colour printing there are of course four. However, there is also often a further printing unit for applying an extra indexed colour or lacquer. The press is then called a five-colour press.
The sheets are usually printed on one side at a time, although certain multi-colour presses can print on both sides simultaneously – these are called perfector presses.
The printing units are set to determine how much ink should be transferred to the printing plate. The back pressure is also adjusted, i.e. the pressure between the two cylinders the paper passes through.
i As mentioned previously, always use plenty of ink when printing on uncoated paper. Also, the back pressure should be increased to make sure the ink really penetrates into the slightly uneven paper surface. However, the process presupposes that the repro and films have been correctly prepared, otherwise there is a risk of compressing and thereby expanding the dots. And we know what the results of that are: set-off, smearing, poor contrast in dark areas, and so on. On the other hand, too little ink can produce flat, lacklustre images.
It is worth repeating that it is best to over-compensate for dot gain in the repro stage, and increase the back pressure and ink coverage in the press.
Also bear in mind that the ink in the sheet press must be designed for printing on uncoated paper. Ideally use vegetable inks, which set oxidatively, i.e. dry from the oxygen in the air. It is also worth mentioning that the drying time can be slightly longer for uncoated than coated paper.
As the sheets leave the press one by one they have to dry. And since the drying time can vary by up to 24 hours or more – and in order to avoid set-off – spray powder should be sprayed onto the sheets to ensure they do not come into contact with each other. This also creates an air column between the sheets which helps the ink to dry. It is important to use quite a coarse-grain powder, otherwise the fine granules can get stuck in the rough structure of the uncoated paper – this eliminates the air column while also potentially producing a dull image. When the printed sheets are moved onto pallets to dry, there is a risk of ink setting off if the sheets are piled too high. It is therefore vital to keep the pallets as low as possible until the ink has completely dried. |
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Flexographic printing works in much the same way as a stamp, i.e. with raised areas and depressions between the parts that print and those that don’t. Plastic or rubber stereotypes with different level variations attached to a cylinder are used. Flexography is a typographic printing method.
i The flexographic method is used for reel formats and prints at very high speeds. It can print on most materials and is therefore often used in the packaging industry and book production. Flexographic printing is also an excellent method for uncoated papers. |
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For high-volume printing, such as brochures, daily newspapers and weekly magazines, web offset is the most common printing method. However, printing ‘from the reel’ does not really produce the same high-quality results as sheet offset, and lower grammages are often used. Moreover, web offset does not offer the same opportunities for advanced post-production. The main advantage is that the unit cost for large volumes is far lower than sheet printing. In web offset, a large reel of paper is fed into the press in a long web, and it runs through the entire printing press at very high speed. There are two main types of web offset press: Heatset and Coldset. Common to both is the fact that printing and often also post-production – such as folding, cutting and stitching – are done in the press.
i The speed of the process places great demands on short drying times so that the ink doesn’t set-off. A Heatset press therefore uses a heat-dryer or ultraviolet light to dry the ink. The results are of a high quality and there is no problem printing on uncoated grades.
The Coldset method uses a different setting process – drying by absorption. Coldset uses fast-setting ink, and it is essential to check that the chosen paper is compatible with the ink and allows it to set quickly.
In web offset, both sides of the paper are usually printed simultaneously in a perfector press. With this method the opposing printing unit’s rubber blanket is used to provide back pressure.
i As with sheet printing, the ink coverage should be increased when printing on uncoated paper. However, there is less opportunity to increase back pressure in a web offset process, so the ink cannot be pressed into the paper in the same way as sheet printing. |
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Gravure is like flexographic printing, a rotary printing method for reel formats. The difference is that gravure is a complicated and therefore expensive method which really only comes into its own when printing extremely high volumes. The complexity and expense of producing the printing cylinder is the reason for the high initial investment.
Gravure involves etching or engraving a large cylinder to form cells of varying depths and sizes, eliminating the need for printing plates. When printing, the cells are filled with ink which is then transferred onto the paper as the paper is pressed between the printing cylinder and a back-pressure cylinder.
i Gravure produces a very high print quality even with low grammages, but places tough demands on the paper’s smoothness, surface and ink absorbency. Uncoated papers rarely perform well in gravure printing, and coated papers are generally recommended. |
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Digital printing is a collective term for a range of new printing methods that have appeared since the early 1990s, and new machines and methods are being added every year. In the past – and still to some extent today – digital printing has mainly been used for low volumes, but as technology and quality has improved, so have the opportunities.
The aspect that all digital printing has in common is the fact that the entire process from computer to paper takes place without interrupting the digital flow of text and graphics. Quite simply, no interim media such as films and printing plates are required to transfer the colour to the paper. Moreover, information is freshly transferred for each individual print, which means that sections of text and images can be replaced each time without compromising the printing speed. The altered information is called variable data, and used correctly this dramatically increases the value of printed matter.
Below, we describe the two main types of digital printing – using toner in xerographic and laser printing, and using ink in inkjet printing – and clarify the issues that are particularly important to consider when printing on uncoated paper. Another technique we outline here is pre-print, a combination of digital and conventional printing.
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Inkjet, Digital Printing With Ink |
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As the name implies, inkjet printing involves spraying liquid ink onto the paper in drops, and this forms dots in roughly the same way as in off-set printing.
There are two main types of inkjet printer. One is called ‘Continuous inkjet’, whereby high pressure is used to form drops and electrical charges are used to guide them. The other is called ‘Drop on Demand’, whereby the drops are added one by one. The latter method is most common in office and text printers, while continuous inkjet is more suitable for faster, more advanced productions. Inkjet technology is developing rapidly, and there are many different machines on the market today, from inexpensive home printers to very high-quality large-format printers and extremely high output machines. But progress is not likely to stop there.
i Most uncoated grades with surface sizing are suitable for the various inkjet methods. Since the ink is liquid and relatively thin, it has to be absorbed – soaked up – and set on the paper in the right way. A normal uncoated paper has quite a high absorbency, but because the drops flow out, there is roughly the same effect as dot gain in offset printing. In order to achieve truly fine results, the paper surface should therefore be treated in some way to allow it to quickly absorb and set the ink.
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Digital Printing With Toner/Xerography and Laser |
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Digital methods based on xerography are often used in laser printers and photocopiers. They involve using a dry toner, i.e. a powder of the colour to be printed. The number of colours in the press varies, some using only black (‘monochrome’ in the trade) and others using several colours.
The xerographic process begins with the toner being electrically charged. When for instance an image is being printed, the areas on the paper that are to be printed are also given an opposite charge to the toner’s, so that the powder particles are electrically attracted and attach to the charged areas on the paper. Once the powder is in place it is fixed onto the paper, normally using heat which melts it onto the paper surface. Toner rests on the surface of a paper rather than being absorbed, and you can actually feel this if you run your finger across the surface.
Because heat is a central factor in the process, the moisture content of the paper is very important, as too much moisture means that a lot of moisture leaves the paper in the fixing process, resulting in a warped, curled paper. Too dry a paper on the other hand can cause problems with static.
The paper’s conductivity is also very important. If the conductivity is wrong, the toner will not attach to the paper. Conductivity is also influenced to a certain degree by the paper’s moisture content.
i Bear in mind that the printing temperature can vary widely between different types and makes of machine, and that some presses use heat in direct contact with the paper while others do not. This means that the effects of incorrect moisture levels can vary according to the machine being used.
In order to retain the proper moisture level – i.e. the level to which the paper has acclimatised at the mill or with the supplier – it is extremely important to reseal the packaging and to maintain a controlled humidity level in the printing workshop. Also make sure that the paper’s moisture content is compatible with the type of machine being used. Sometimes there are even ‘script files’ for each paper grade which ensure the press has the correct settings.
Also bear in mind that this type of digital printing method dries the paper out slightly, and this should be taken into account before post-production.
Digital methods based on dry toner are ideal with uncoated paper, as the toner powder is not absorbed into the paper so dot gain is not an issue. It is also possible to print with a higher ink coverage than in, say, offset printing, which can produce better image reproduction.
However, it is not a good idea to use uncoated paper grades that are too rough, especially not in colour printing, as with current technology it is difficult to achieve an even layer of toner on too rough a surface.
There are also digital methods that use wet toner, i.e. liquid ink. The process is however much the same as with dry toner – the liquid ink is transferred by electrical charges, but first to rubber blanket and then into the paper. |
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Pre-Print in a Conventional Press |
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You are probably familiar with balance statements and invoices etc. that contain ‘variable data’, i.e. text and figures customised for your particular message. These documents are usually produced in a monochrome digital press, where the black text and figures are printed on sheets or reels which are pre-printed with colour – usually logos or similar – in a conventional printing press. This method is called pre-print.
i When using pre-print, it is extremely important to choose an uncoated paper that can deal with both printing methods. The ink must also be compatible with both methods. As with several other digital techniques, the moisture content of the paper is important. Pre-printing in offset adds damping solution and ink to the paper, thereby increasing its moisture content. Therefore if the original moisture content of the paper is high – before the damping solution is added – this can cause problems in the next stage, i.e. digital printing.
We therefore recommend that the proportion of damping solution in the initial offset run be kept to a minimum, even if the paper is designed for pre-print. |
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