Recommendations for Prepress and Printers Different Paper Types Have Different Information Capacity

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TECHNICAL GUIDE

Recommendations for prepress and printers Different paper types have different information capacity

2 There are certain factors which defi ne the quality of a paper grade. Among these are the raw material, processing level and surface fi nish. Paper quality steps range from newsprint to the fi nest woodfree double coated papers.

oodfree W WFC Relative price

WFU

MWC Woodfree WFC = woodfree coated

Mechanical coated WFU = woodfree uncoated LWC

Mechanical coated MFC MWC = medium weight coated LWC = light weight coated SC MFC = machine ﬁ nished coated

Mechanical uncoatedMFS Mechanical uncoated SC = super calendered News MFS = machine ﬁ nished special Relative quality News

All papers have a different information capacity; the ability to reproduce an ink dot. The highest capacity is with woodfree coated papers. Because the surface characteristics are different, the papers’ absorption properties are different. To obtain the best possible reproduction with a particular grade, it is important the paper’s characteristics are taken into account during the pre-press stage – and subsequently throughout the entire printing process. To ﬁ nd out more about the papers in different quality steps, please refer to the UPM product catalogue or Paper Search.

3 Screen dot behaviour on different types of paper

4 Dot increase Different printing surfaces Smoothness and porosity of the paper surface affect the printing ink performance. The ink is wetting the surface, spreading Coated and setting in different ways. The rougher and more (macro) porous the paper surface, the more the printed ink spreads

Uncoated SC and penetrates into the paper. Similarly, the heavier the paper coating is, the sharper the image dot. This has an inﬂ uence in attaining an achievable print density

Uncoated MF and dot gain during printing. Dot gain is the average spreading of a single colour dot on the paper. The more even and dense the surface, the better the dot coherence.

Compensation of dot gain The quality potential of different papers should be taken into consideration and compensated for during pre-press. Dot gain can cause blurring and loss of printable tones due Coated 54 lines Uncoated 54 lines to excessive ink coverage. In this case, the screen ruling should be lowered to achieve discrete dots and a wider tonal range.

Uncoated 40 lines

Lowering the screen ruling helps separate individual dots.

5 Adjusting screen rulings for different papers

LPI 65 85 100 120 133 150 170 200 254 Screen rulings in AM screening LPCM 26 34 40 48 5460 68 80 100 Printing conditions have some effect on screen rulings, but the Newsprint main factor is the paper grade and its impact upon dot gain. Uncoated Dot gain, screen rulings and the target print density have an Coated matt overall impact upon print quality. Coated silk

Coated gloss Recommendations for screen rulings

LPI 85 100 120 133 150 170 200 for different paper grades: LPCM 34 40 48 5460 68 80 lpi lpcm

News News 85 34

MFS MFS 100 40

SC SC 120 48

MFC MFC 120 48

LWC matt LWC matt 133 54

LWC gloss LWC gloss 150 60

MWC matt MWC matt/silk 150 60

MWC gloss MWC gloss 175 70

WFU matt WFU matt 133 54 WFU gloss WFU gloss 150 60 WFC matt WFC matt/silk 150 60 WFC gloss WFC gloss 175 70

6 Typical dot gain of different papers

High screen ruling – high dot gain There are two main factors which increase dot gain: - square screen dots 16% 31% 50% 68% 84% - higher number of dots 0.181 mm 0.471 mm 0.116 mm Edge lengths of screen dots in different tones with When screen ruling is high, dot gain is high. Dot gain is screen ruling of 60 LPCM. highest in the middle tones because of the overlapping nature of the screen dots. Dot gain is higher in FM screening compared with AM screening due to a higher number of dots (pixels) and their higher combined borderline value.

Different sized and amount of dots in AM (left) and FM (right) screening.

Dot gain % 30

0 0102030405060708090100 Dot area %

1st order FM AM 150 lpi AM 120 lpi

7 Printing paper effect on screen rulings

8 Choosing target print densities The amount of ink transfer is dependent upon the target densities of each process colour within a given printed area.

Target print densities for the solid covered printed area. The target density depends on the surface properties of the chosen paper, its smoothness, porosity, gloss etc. The higher the roughness and porosity value, the higher the ink demand and dot gain due to higher ink penetration and absorption. For a given volume of ink, the higher the roughness and porosity, the lower will be the resultant print density.

The directive target print densities for different types of papers are as follows:

LPI 85 100 120 133 150 170 200 Print densities LPCM 34 40 48 5460 68 80 Cyan Magenta Yellow Black

News 1.10 1.15 0.90 1.20

MFS 1.15 1.20 0.95 1.25

SC 1.25 1.30 1.05 1.40

MFC 1.25 1.30 1.05 1.40

LWC matt 1.35 1.35 1.10 1.50

LWC gloss 1.50 1.45 1.30 1.65

MWC matt 1.50 1.45 1.30 1.65

MWC gloss 1.60 1.55 1.40 1.80

WFU matt 1.35 1.35 1.10 1.50

WFU gloss 1.55 1.50 1.35 1.70

WFC matt 1.55 1.50 1.35 1.70

WFC gloss 1.65 1.60 1.45 1.90

9 Controlling Total Area Coverage 10 100% Total Area Coverage (TAC) can be decreased Under Colour Removal (UCR) and Grey Component Replacement (GCR) are tools for controlling the total ink agenta + Yellow coverage. Different papers can tolerate different total ink coverage Cyan + M – for the same reasons that affect the target densities. The Black maximum coverage is naturally 400% in CMYK printing, but 0% 60% 100% Black Cyan Magenta Yellow only the very best, multicoated glossy papers can handle that much. 100% The ability of papers to increase density with the amount of ink can be measured and quantiﬁ ed as ink mileage. Ink

60% mileage can give a guideline as to the total maximum coverage

agenta + Yellow and print densities of different papers. The most common tools in the reproduction phase to Cyan + M control and lower the Total Area Coverage are Under Colour Black Removal and Grey Component Replacement. 0% 60% 100% Black Cyan Magenta Yellow

The recommendable maximum Total Area Coverages with different types of paper:

LPI 85 100 120 133 150 170 200 Print densities Total Area Coverage LPCM 34 40 48 5460 68 80 Cyan Magenta Yellow Black Cyan + Magenta + Yellow + Black

News 1.10 1.15 0.90 1.20 240%

MFS 1.15 1.20 0.95 1.25 250%

SC 1.25 1.30 1.05 1.40 260%

MFC 1.25 1.30 1.05 1.40 260%

LWC matt 1.35 1.35 1.10 1.50 270%

LWC gloss 1.50 1.45 1.30 1.65 280%

MWC matt 1.50 1.45 1.30 1.65 280%

MWC gloss 1.60 1.55 1.40 1.80 300%

WFU matt 1.35 1.35 1.10 1.50 270%

WFU gloss 1.55 1.50 1.35 1.70 300%

WFC matt 1.55 1.50 1.35 1.70 290%

WFC gloss 1.65 1.60 1.45 1.90 320%

11 Paper type effect on HSWO fount and plates

12 Fountain solution The fountain solution should be clean and regulated within a speciﬁ ed temperature range to attain proper emulsiﬁ cation and plate dampening. When printing paper grades containing calcium carbonate (many UPM coated grades e.g. UPM Finesse, UPM Star, UPM Ultra), the following guidelines should be adhered to: - acidity: pH 4.5 – 5.5 - conductivity: 1500 – 2500 mS/m - hardness: 5 – 15 °dH - temperature in tray: 10 – 15 °C

The volume of fount should be as low as possible for all paper grades. The following procedure should be adhered to: - clean and correctly set rollers - 10% overfeed during start up, and 3% over the toning limit during the printing run

CTP plates vs. conventional plates Due to demands of the manufacturing process, CTP plates differ in behaviour from conventional photomechanical printing plates. The main issue relates to the hydroscopic nature (dampening) of the plate due to the different surface grain structure. CTP plates are more sensitive therefore towards different levels of fount solution. Paper grades with very high water absorption rates (such as matt coated papers) tend to dry much quicker causing toning and piling problems. Right amount of fount at start-up helps to control the situation.

13 Paper type effect on HSWO printing and drying

Drying in HSWO 1,80 1,60 1,40 1,20 ensile strength at

T 1,00

folded paper kN/m No cracking 0,80 0,60 0,40 Cracking Typical behaviour of tensile strength at folded coated 0,20 paper vs. moisture content. 0,00 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 Temperature °C

1,60 1,40 1,20 1,00 0,80 ensile strength at

T No cracking

folded paper kN/m 0,60 0,40 Cracking Typical behaviour of tensile strength at folded coated 0,20 paper vs. temperature. 0,00 4,9 4,7 4,5 4,3 4,1 3,9 3,7 3,5 3,3 3,1 2,9 2,7 2,5 Abs. humidity of paper %

1,80

1,60

1,40

1,20 ensile strength at T

folded paper kN/m 1,00

0,80

0,60 80 g

0,40 90 g 100 g 0,20 115 g Tensile strength at fold vs. web temperature. 0,00 90 100 110 120 130 140 150 160 Temperature °C

14 For all HSWO paper grades, there is one basic rule for drying: it should occur within a time interval of one second, and within a web temperature range of 120-140 °C. For example, a 10m long dryer should be used when printing a copy length of 64 cm at 60,000 cph. Increasing the drying temperature can lead to quality problems such as surface roughening and loss of gloss. Runnability problems can also occur with folder cracking and blistering. It is possible to assure the cracking strength and blistering strength of papers provided the web temperature does not exceed 155 °C.

Paper web temperature The recommended maximum web temperature during drying for all HSWO grades is 155 °C. This can be achieved with low TAC under optimised printing conditions. Chilling of the paper web has an impact upon the quality and runnability performance of the web. The temperature of the paper web should be less than 23 °C after transit through the chill rollers. This will ensure the coating binders are cooled correctly to maintain the required gloss levels.

15 Conditioning and handling of paper reels before printing 16 Conditioning and unwrapping policy of papers The conditioning time of paper depends upon: • reel diameter (conditioning from the edges) • volume of pallet (sheets) • temperature difference between warehouse / transportation and print room

Example (see chart): A conditioning time of 24 hours is required when transporting a 1 metre (1000mm) diameter reel directly to the print room maintained at a temperature of +23 °C at an outside temperature is +13 °C. Paper reels should be conditioned with the edge covers removed but with the body wrap in situ until preparation of the roll splice.

17 Printing ink behaviour

Mineral vs. vegetable oil based inks Emulsiﬁ cation

Printing inks with a predominantly oil based composition There are three important properties of an ink which defi ne are often referred to as ‘Eco Inks’. In some cases, they are its interaction with water or fount: more diffi cult to clean and problematic in terms of water • amount of ink and fount required for emulsifi cation in a emulsifi cation, but otherwise the quality is normally better balanced situation than with mineral oil based inks in terms of print gloss. • how well the ink and the fount are released from the Due to the problematic ink-water balance, vegetable oil emulsifi cation based inks can cause problems such as piling, trapping, • residual content of fount in the ink after release smearing and unevenness, especially with coated papers. These properties can be measured on a laboratory scale. There are some general rules about the properties of HSWO Ink setting inks which are: The speed of ink setting on paper during printing has an • emulsifi cation saturation point should be between infl uence on the following print characteristics: 25–65% • back trap piling and mottling • water / fount should be released from the ink during one • formation of print gloss turn of former rollers • trapping of consecutive inks • residual water / fount content after releasing should be as • mechanical ghosting low as possible, below 5% • smearing Effective emulsifi cation should create small, even sized Inks can be slow or quick setting which applies equally droplets without a solid water interface between the ink layer to differing paper grades. Typically, papers using calcium and paper during printing. carbonate as the coating pigment and SB latex as a The pH value, surface tension, viscosity and temperature binder are quicker setting. If the setting speed is too high, of fount are important in achieving the correct ink-water problems can be encountered with blanket release, piling, balance during printing. ghosting, back trapping, loss of gloss etc. In this case, either the printing inks should be replaced by a slower setting ink, or the setting speed of the ink can be reduced by lowering the amount of applied fount.

Good ink-water balance. Poor ink-water balance. Very poor ink-water balance. 18 Basic guidelines for the main paper types

Newsprint and MFS papers Coated papers

• uncoated matt papers with mechanical fi bres • coated matt, semi matt and glossy papers with mechanical • rough and porous papers, newsprint rougher than MFS fi bres and chemical fi bres • supercalendered, soft calendered or machine calendered - ink demand/mileage is double in comparison to papers coated grades - dot gain is 50% higher compared to coated grades - ink demand/mileage is in coated glossy papers from - all extra fount/water, especially in CSWO printing 1.1 to 1.7 g/m² from multicoated WFC to single is causing problems: ink setting, print quality, coated LWC for target densities of maximum print runnability (web behaviour) contrast - piling problems occur with too tacky inks and with - ink demand of coated semi matt papers is 20% too much dampening higher in comparison to coated glossy papers - papers are roughening in printing due to water - ink demand of coated matt papers is 33% higher in and heat comparison to coated glossy grades - dot gain of coated glossy papers in middle tones is SC uncoated papers from 15% to 25% from WFC papers to LWC papers - dot gain of coated semi matt papers is 25% higher • uncoated glossy papers with mechanical fi bres and with coated matt papers 33% higher than with • supercalendered papers, smoother and less porous than the coated glossy papers newsprint and MFS papers - all extra water/fount is bad for the quality with the coated papers also - ink demand/mileage is 50% higher in comparison - all extra heat in HSWO drying is bad for the quality to coated grades in printing - dot gain is 33% higher in comparison to coated - printability and print quality problems with coated grades papers are very often connected to ink setting - all extra water/fount is off from print quality, can be parameters in a printing nip a reason for piling, too - LWC and MWC (mechanical papers) are - printability problems occur with too tacky inks and roughening in printing due to the water (moisture) with too much dampening and heat - papers are roughening in printing due to water and heat

19 Printed in April 2004. Cover: Maxisatin 250 g/m2, made by UPM. Inside pages: Maxisatin 130 g/m2, made by UPM.