How to choose a correct uncoated paper for your publication
A set of advices by Michał Büthner-Zawadzki from Arctic Paper.
Over the past few years, uncoated papers have strengthened their market position. They can now be found in these applications which till not long ago used to be the domain of coated papers. A few justifications may be identified for this trend to use uncoated papers, especially those of top-shelf. On one hand, it is seeking for new solutions, the desire to draw attention and the endeavour to be distinguished, as well as boredom of graphic designers with coated papers. On the other hand, it is the fact that printers gain more and more access to better know-how, tools and printing press, thus they are more and more eager to take the challenge of high quality full-colour printing on uncoated papers. And the quantity of these papers on the market is really vast. They differ among one another with raw-material composition, bulk, roughness and shade. Let us then try to arrange them according to their applications.
Books with texts but without illustrations
Here we require that the paper demonstrates appropriate opacity - at least 89-90% - so that printed text will not be seen on the other side, making reading difficult. Another issue is paper bulk (also called volume), i.e. the quotient of paper thickness and grammage - the bigger is the bulk, the thicker is a paper with the same grammage. In other words, a book will have a broader spine with the same weight, making its appearance more impressive. Let us also remember that texts, printed on papers of lower whiteness and in creamy colour, are easier to read for their not so high contrast.
High opacity, a high bulk (1.5-2.0) and lower whiteness are among the typical parameters of wood-containing papers, the manufacturing process of which is based on mechanical pulps and recycled pulps. Their weak features include dusting tendency and low resistance to ageing for the lignin content - their strength deteriorates in time, as well as they get yellow. If quality is what we care about, let us select wood-free papers or papers with a slight addition of mechanical pulps. The most common grammage values are 60-80 g/m2.
Books and printed materials with a text and single colour or duoton illustrations
With regards to prints of this type, the qualitative requirements are a bit higher. We obviously further insist on opacity and bulk but also on qualitative reproduction of graphic arts or black & white photos. There are good reasons to opt here for surface-sized papers, less absorbing the printing ink, what will ultimately result in a smaller dot gain. Surface-sizing will also reduce the dusting tendency of paper. How to check if a given paper has been surface-sized? Apply a water droplet on the paper surface - it should stay there for a longer time. Whereas, in case of non surface-sized paper, it will immediately be absorbed. The grammages we choose are usually within the range 80-120 g/cm2, while the volume values range from 1.5 through 1.8.
Books with full-colour reproductions, albums, wall calendars, exclusive advertising materials, etc.
In such cases, we shall seek for top quality papers, designed for full-colour print. Paper smoothness is here a very important parameter. The smoother is the surface of the paper, the more details of a photo can be reproduced on it. The dot gain will also be smaller, what means a better colour reproduction. High quality uncoated papers are surface-sized and woodfree, manufactured from cellulose fibres - the most noble raw material. Those papers are characterised by optimal strength parameters, high smoothness, resistance to ageing and low dusting tendency. Their relatively low opacity may be regarded their only weak point. When photos are printed, opacity should be 95%, while it should be 97%, when printing solids. Therefore, we have to seek for slightly higher grammage lavels (paper opacity increases with its growing grammage) – minimum 100 g/m2. The volumes of these papers are usually 1.1-1.4.
As I have mentioned before, high quality, full-colour works can successfully be printed on uncoated papers. However, it requires a proper setting of materials for printing – the prepress processes.
Let us start from screen ruling selection. The maximum ruling with which printing is possible on a given paper, depends on its absorptive capacity and surface roughness. The rougher and more absorbent paper we take, the lower screen ruling should be applied. Regarding uncoated papers, the usually selected ruling values range from 133 through 175 lpi. The screen ruling selection is usually done by the printing house but it is always advisable to consult and ask the printer why such a particular ruling has been applied.
Another issue is the compensation of dot gain. Printing on rough and absorbent substrate is associated with a considerable increase in dot size. This will result in detail losses in darker picture areas and in general reduction of printed picture contrast. Therefore, the dot gain should be compensated. It is even more complicated since it does not depend on paper type only but is also associated with screen ruling, the printing press performance, the ink quality, etc. The best solution is to enquire the printing house for information which profile should be considered for printing on uncoated paper. The printing house may sometimes provide us with their own profile. If the printing house recommends the use of ISO Coated - we should seek for another partner.
In order not to apply excessive ink volumes on paper in picture separation, the maximum ink coverage should be determined. It regards the sum of percent coverages of particular CMYK model colours at a given area of the print. We can decrease the proportion, replacing the use of CMY model colours by a bigger share of black ink. Assuming that the printing press has been accurately calibrated and the materials properly set up, the printing ink volume on uncoated paper may amount to 280% maximum. The value of 260% is recommended in practical applications, as higher values rarely bring better effects, while additional problems may occur, related to ink drying, setoff and smudging.
The use of general ICC profiles, offered by paper manufacturers, seems to be a good solution. Such a profile ensures proper compensation of dot gain, resulting from the characteristic features of the paper itself. This profile also contains the algorithm of total ink coverage. The Arctic Paper Company has designed a portfolio of general profiles for all their uncoated and coated matt papers. The profiles, together with instructions for use, can be downloaded from the web: arcticpaper.com/pl/icc.
Let us, however, remember that the most important tool in the colour management process is not a spectrophotometer but... a telephone. Good communication, held among the customer, the designer and the printing house at an early stage of task planning, will ensure that misunderstandings and numerous problems are avoided after production is triggered, when the problems may then turn out to be rather expensive. Therefore, at the earliest opportunity, we head to the printing house for approval of our project ant try to pal up with the printer.
Glossary of terms:
coated paper - paper enriched by a layer which consists of a pigment-adhesive mixture.
woodfree paper - paper without lignin, produced mainly from chemical pulps.
wood-containing paper - paper with lignin content, produced mainly from mechanical pulps.
mechanical pulps - fibre pulps, obtained by mechanical wood defibering, semi-product for paper production
chemical pulps - fibre pulps, obtained by wood delignification, i.e. chemical solution and removal of the lignin majority. Another term is a cellulose. Semi-product for paper production.
surface-sizing of paper - application of adhesive layer on the paper web. Modified starch-based adhesives are mainly used. Surface-sizing increases paper resistance to abrasion, reduces the absorbency of ink and decreases dusting tendency.
paper grammage - paper weight (expressed in grams) of one square meter of a paper product.
paper volume - the paper thickness (expressed in micrometres or microns) to paper grammage (in g/m2) ratio. Also called bulk. Extremely smooth, glazed, uncoated papers present the bulk of 1.1. High bulk papers may even reach the volume of 2.0.
paper opacity - defines the level of absorption of light as it passes through the paper.
solid - a surface printed with single, non-screened colour.
screening - the technique that is used in printing to simulate continuous-tone images such as photographs using tiny elements: dots or lines.
amplitude modulation screen (AM screen) - a screen, in which tonal differentiation results from size changes of regularly laid dots. Other terms: traditional screen, conventional screen.
screen dot - an element of screened image. It is a point which is covered with ink during printing.
screen ruling - in AM screen, it defines the number of rows of dots (or lines), configured in parallel to one another, per length unit. The screen ruling is expressed in lines per inch - lpi or - more rarely - in lines per centimetre.
CMYK model colours - process inks in four-colour printing: Cyan, Magenta, Yellow and Black (Key colour). Combined together on a printed image, they provide the final colour.
maximum ink coverage - a parameter which determines the maximum volumes of all CMYK inks, which can be used for printing In offset printing. There is 0 - 100% coverage for each of the four CMYK colours. Thus theoretically - we can obtain a maximum value of 400%. In other words: Total Ink Limit, Total Ink Coverage.
ink setoff - printing defect characterized by the transfer of wet ink to the reverse side of the sheet lying on top of it in the press delivery tray. In extremal cases, it may lead to sticking of sheets in stack.