TECHNICAL GUIDE - Papyrus
Transcript of TECHNICAL GUIDE - Papyrus
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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.
WoodfreeWFC = woodfree coatedWFU = woodfree uncoated
Mechanical coatedMWC = medium weight coatedLWC = light weight coatedMFC = machine fi nished coated
Mechanical uncoatedSC = super calenderedMFS = machine fi nished specialNews
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.
News
MFS
SC
MFC
LWC
MWC
WFU
WFC
Rela
tive
pric
e
Relative quality
Mec
hanic
al un
coate
d
Mecha
nical
coate
d
Woo
dfree
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Different printing surfaces
Smoothness and porosity of the paper surface affect the printing ink performance. The ink is wetting the surface, spreading and setting in different ways. The rougher and more (macro) porous the paper surface, the more the printed ink spreads 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 and dot gain during printing. Dot gain is the average spreading of a single colour dot on the paper.
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 to excessive ink coverage. In this case, the screen ruling should be lowered to achieve discrete dots and a wider tonal range.
Coated 54 lines Uncoated 54 lines
Uncoated 40 lines
Dot increase
Coated
Uncoated SC
Uncoated MF
The more even and dense the surface,the better the dot coherence.
Lowering the screen ruling helps separate individual dots.
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Adjusting screen rulings for different papers
Screen rulings in AM screening
Printing conditions have some effect on screen rulings, but the main factor is the paper grade and its impact upon dot gain.
Dot gain, screen rulings and the target print density have an overall impact upon print quality.
Recommendations for screen rulings for different paper grades: lpi lpcm
News 85 34
MFS 100 40
SC 120 48
MFC 120 48
LWC matt 133 54
LWC gloss 150 60
MWC matt/silk 150 60
MWC gloss 175 70
WFU matt 133 54
WFU gloss 150 60
WFC matt/silk 150 60
WFC gloss 175 70
LPILPCM
8534
10040
12048
13354 68
17015060
20080
Newsprint
Uncoated
Coated matt
Coated silk
Coated gloss
254100
6526
LPILPCM
8534
10040
12048
13354 68
17015060
20080
News
MFS
SC
MFC
LWC matt
LWC gloss
MWC matt
MWC gloss
WFU matt
WFU gloss
WFC matt
WFC gloss
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High screen ruling – high dot gain
There are two main factors which increase dot gain:- square screen dots- higher number of dots
When screen ruling is high, dot gain is high. Dot gain is 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.
Typical dot gain of different papers
40
35
30
25
20
15
10
5
00 10 20 30 40 50 60 70 80 90 100
1st order FM AM 150 lpi AM 120 lpi
Dot area %
Dot
gai
n %
16% 31% 50% 68% 84%
0.181 mm 0.471 mm 0.116 mmEdge lengths of screen dots in different tones withscreen ruling of 60 LPCM.
Different sized and amount of dots in AM (left) and FM (right) screening.
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Choosing target print densities
The amount of ink transfer is dependent upon the target densities of each process colour within a given 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.
Target print densities for the solid covered printed area.
LPILPCM
8534
10040
12048
13354 68
17015060
20080
News
MFS
SC
MFC
LWC matt
LWC gloss
MWC matt
MWC gloss
WFU matt
WFU gloss
WFC matt
WFC gloss
Print densitiesCyan Magenta Yellow Black
1.10 1.15 0.90 1.20
1.15 1.20 0.95 1.25
1.25 1.30 1.05 1.40
1.25 1.30 1.05 1.40
1.35 1.35 1.10 1.50
1.50 1.45 1.30 1.65
1.50 1.45 1.30 1.65
1.60 1.55 1.40 1.80
1.35 1.35 1.10 1.50
1.55 1.50 1.35 1.70
1.55 1.50 1.35 1.70
1.65 1.60 1.45 1.90
The directive target print densities for different types of papers are as follows:
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Cyan +
Magen
ta + Yello
w
Black
Black Cyan Magenta Yellow0% 60% 100%
100%
Cyan +
Magen
ta + Yello
w
Black
Black Cyan Magenta Yellow0% 60% 100%
100%
60%
The recommendable maximum Total Area Coverages with different types of paper:
LPILPCM
8534
10040
12048
13354 68
17015060
20080
News
MFS
SC
MFC
LWC matt
LWC gloss
MWC matt
MWC gloss
WFU matt
WFU gloss
WFC matt
WFC gloss
Print densitiesCyan Magenta Yellow Black
1.10 1.15 0.90 1.20
1.15 1.20 0.95 1.25
1.25 1.30 1.05 1.40
1.25 1.30 1.05 1.40
1.35 1.35 1.10 1.50
1.50 1.45 1.30 1.65
1.50 1.45 1.30 1.65
1.60 1.55 1.40 1.80
1.35 1.35 1.10 1.50
1.55 1.50 1.35 1.70
1.55 1.50 1.35 1.70
1.65 1.60 1.45 1.90
Total Area CoverageCyan + Magenta + Yellow + Black
240%
250%
260%
260%
270%
280%
280%
300%
270%
300%
290%
320%
Total Area Coverage (TAC) can be decreased
Under Colour Removal (UCR) and Grey Component Replacement (GCR) are tools for controlling the total ink coverage.
Different papers can tolerate different total ink coverage – for the same reasons that affect the target densities. The maximum coverage is naturally 400% in CMYK printing, but only the very best, multicoated glossy papers can handle that much.
The ability of papers to increase density with the amount of ink can be measured and quantifi ed as ink mileage. Ink mileage can give a guideline as to the total maximum coverage and print densities of different papers.
The most common tools in the reproduction phase to control and lower the Total Area Coverage are Under Colour Removal and Grey Component Replacement.
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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.
Paper type effect on HSWO printing and drying
Drying in HSWO
Typical behaviour of tensile strength at folded coated paper vs. moisture content.
Typical behaviour of tensile strength at folded coated paper vs. temperature.
Tensile strength at fold vs. web temperature.
1,80
1,60
1,40
1,20
1,00
0,80
0,60
0,40
0,20
0,00Te
nsile
str
engt
h at
fold
ed p
aper
kN
/mTemperature °C
20 30 40 50 60 70 80 90 100 110 120 130 140 150 160
Cracking
No cracking
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
1,60
1,40
1,20
1,00
0,80
0,60
0,40
0,20
0,00
Cracking
Tens
ile s
tren
gth
atfo
lded
pap
er k
N/m
Abs. humidity of paper %
No cracking
1,80
1,60
1,40
1,20
1,00
0,80
0,60
0,40
0,20
0,0090 100 110 120 130 140 150 160
80 g
90 g
100 g
115 g
Tens
ile s
tren
gth
atfo
lded
pap
er k
N/m
Temperature °C
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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.
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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.
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Printing ink behaviour
Good ink-water balance. Poor ink-water balance. Very poor ink-water balance.
Emulsifi cation
There are three important properties of an ink which defi ne its interaction with water or fount:• amount of ink and fount required for emulsifi cation in a balanced situation• how well the ink and the fount are released from the emulsifi cation • residual content of fount in the ink after release
These properties can be measured on a laboratory scale. There are some general rules about the properties of HSWO inks which are:• emulsifi cation saturation point should be between 25–65%• water / fount should be released from the ink during one turn of former rollers • residual water / fount content after releasing should be as low as possible, below 5%
Effective emulsifi cation should create small, even sized droplets without a solid water interface between the ink layer and paper during printing.
The pH value, surface tension, viscosity and temperature of fount are important in achieving the correct ink-water balance during printing.
Mineral vs. vegetable oil based inks
Printing inks with a predominantly oil based composition are often referred to as ‘Eco Inks’. In some cases, they are more diffi cult to clean and problematic in terms of water emulsifi cation, but otherwise the quality is normally better than with mineral oil based inks in terms of print gloss. Due to the problematic ink-water balance, vegetable oil based inks can cause problems such as piling, trapping, smearing and unevenness, especially with coated papers.
Ink setting
The speed of ink setting on paper during printing has an infl uence on the following print characteristics: • back trap piling and mottling• formation of print gloss• trapping of consecutive inks • mechanical ghosting• smearing
Inks can be slow or quick setting which applies equally to differing paper grades. Typically, papers using calcium carbonate as the coating pigment and SB latex as a binder are quicker setting. If the setting speed is too high, problems can be encountered with blanket release, piling, 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.
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Newsprint and MFS papers
• uncoated matt papers with mechanical fi bres• rough and porous papers, newsprint rougher than MFS
- ink demand/mileage is double in comparison to coated grades - dot gain is 50% higher compared to coated grades - all extra fount/water, especially in CSWO printing is causing problems: ink setting, print quality, runnability (web behaviour) - piling problems occur with too tacky inks and with too much dampening - papers are roughening in printing due to water and heat
SC uncoated papers
• uncoated glossy papers with mechanical fi bres• supercalendered papers, smoother and less porous than newsprint and MFS papers
- ink demand/mileage is 50% higher in comparison to coated grades - dot gain is 33% higher in comparison to coated grades - all extra water/fount is off from print quality, can be a reason for piling, too - printability problems occur with too tacky inks and with too much dampening - papers are roughening in printing due to water and heat
Basic guidelines for the main paper types
Coated papers
• coated matt, semi matt and glossy papers with mechanical fi bres and chemical fi bres• supercalendered, soft calendered or machine calendered papers
- ink demand/mileage is in coated glossy papers from 1.1 to 1.7 g/m² from multicoated WFC to single coated LWC for target densities of maximum print contrast - ink demand of coated semi matt papers is 20% higher in comparison to coated glossy papers - ink demand of coated matt papers is 33% higher in comparison to coated glossy grades - dot gain of coated glossy papers in middle tones is from 15% to 25% from WFC papers to LWC papers - dot gain of coated semi matt papers is 25% higher and with coated matt papers 33% higher than with the coated glossy papers - all extra water/fount is bad for the quality with the coated papers also - all extra heat in HSWO drying is bad for the quality in printing - printability and print quality problems with coated papers are very often connected to ink setting parameters in a printing nip - LWC and MWC (mechanical papers) are roughening in printing due to the water (moisture) and heat