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Basics of offset printing & other printing methods

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XXNewspapers

XXXBooks

XXXPackages, wrappers

XXXXXDirect marketing

XXBrochures, annual reports etc.

XXXMagazines, catalogues

ColdsetHeatsetSheet-fed

DigiFlexoGravure

Offset

XXNewspapers

XXXBooks

XXXPackages, wrappers

XXXXXDirect marketing

XXBrochures, annual reports etc.

XXXMagazines, catalogues

ColdsetHeatsetSheet-fed

DigiFlexoGravure

Offset

Products

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Why ?

A better understanding of the first end use of our products (to be printed) will allow you to better advise your customers and thus give added value to your sales

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SummaryOffset printing

What is colour ?

Principle of offset printing

Sheet-fed offset printing

Heat-set web offset printing

Other printing methods

Conventional methods

Gravure

Flexography

Silk screen

Digital printing methods

Laser (electro photography)

Ink jet

Conclusion

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Placement of printing methods

100 1 000 10 000 100 000 1 000 000 10 000 000

Run length

Qu

alit

y

Lo

wM

ediu

mH

igh Ink-jet

Electro photo-graphy

GravureHeatset offsetSheet-fed offset

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SummaryOffset printing

What is colour ?Principle of offset printingSheet-fed offset printingHeat-set web offset printing

Other printing methodsConventional methods

GravureFlexographySilk screen

Digital printing methodsLaser (electro photography)Ink jet

Conclusion

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What is colour ?

ColourWithout light, no object has a colour

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What do we need to see colours ?A light source (Sunlight, Light Bulb, Candle)

An object to interact with the light source

A receiver and processor (eye + brain)

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Human eye

• light enters the eye through the lens and is focused on the retina

There, an array of photoreceptor cells respond to light and pass the signals on to the brain which translates into colour sensation

There are 2 types of cells :

-> rods = black & white -> cones = colours

3 different types of cone cells sensitive to the wavelength of=> red light=> green light=> blue light

Cells light stimulates the eye three dimensionally!!

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Spectrum of electromagnetic radiation waves

400 500 600 700

nm

Radio

0,001

UVX-rayGamma

0,01 0,1 1,0 10 10³100

IR Radar

104 105 10101010106 10 11

12 13Wavelength

nm

violet blue green yellow orange red

Spectrum of the visible light

400 - 500 nm = blue 500 - 600 nm = green 600 - 700 nm = red

different wavelengths = different colours

photon

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How to create colours ? : 2 methods

SubtractiveAdditive

MagentaCyan

Yellow

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How to create colours : additive method

Additive Lights : Red + Blue + Green = White

For example a Television or a Computer Monitor

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How to create colours?

Based on this (Green + Blue = CYAN, Green + Red = YELLOW, Blue + Red = MAGENTA) , we can reproduce every colour based on 3 selected colorants

Each colorant will have to act on a primary colour of the spectrum (Red, Green or Blue)

For example to act on Red, we need a colorant which absorbs only Red and not Green and not Blue. This is the CYAN

Ink which absorbs red light reflects blue light reflects green light

blue + green = cyan ink

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How to create colours : subtractive method

Selective absorption of certain wavelength by the ink layer.

Magenta inkreflects redreflects blueabsorbs green

Cyan inkabsorbs redreflects bluereflects green

Yellow inkreflects redabsorbs bluereflects green

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Primary colours in printing

Mixing the 3 primary colours theoretically results in black.=> In practice the result is brown!

=> To remedy this, blacK is used as the fourth primary colour.

CMY Black CMYK

+ =

CMY blacK CMYK

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SummaryOffset printing

What is colour ?

Principle of offset printing

Sheet-fed offset printing

Heat-set web offset printing

Other printing methods

Conventional methods

Gravure

Flexography

Si lk screen

Digital printing methods

Laser (electro photography)

Ink jet

Conclusion

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Principle of offset printing On a printing plate there are two areas:

• Areas accepting ink and being repellent to water = Image areas (= hydrophobic)

• Areas accepting water= Non-Image areas (= hydrophilic). The areas are occupied by water.

Due to the different surface tension water and ink do not mix.

The behaviour of the image and non-image areas is caused by a complex chemistry.

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How do we get printed and unprinted areas?

Function of the offset plate

NON-IMAGEIMAGE

Ink = hydrophobic Water = hydrophilic

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How do we get printed and unprinted areas?

Magnified (15x) of an offset plate

Image

Non-Image

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How do we get printed and unprinted areas?

Magnified (1000X) picture of an offset plate

Image

Non-Image

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How do we get printed and unprinted areas?

Function of the offset plate

NON-IMAGE IMAGE

= direct transfer

Substrate

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How do we get printed and unprinted areas?

Function of Rubber blanket = indirect transfer

NON- IMAGEIMAGE

Rubber Rubber

1st transferSubstrate

2nd transfer

IMAGE NON-IMAGE

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Offset printing unit

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The printing plate

Light sensitive coating

Aluminium oxide coating

Grained aluminium base

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The printing plate

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Basic composition of offset inkPigments ~10-20 %

organic and inorganic

Binders ~30-50 %

Hard resins

Alkyds

Vegetable oils

Solvents ~20-30 %

Mineral oils

Vegetable oils

Additives ~0-5 %

waxes, rheology modifiers, antioxidants, fillers...

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Offset blankets

Oil-resistant synthetic

elastomers attached to

textile fabrics

Reproduction properties

Release properties

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The fountain solutionSurface tension of liquid has to be lower than surface energy of solid to ensure quick spreading

•Water 85-98 %

•IPA (Isopropanol) 0-20 %

•Additives 2-4 % weak organic acid(s), buffers, film formers, surfactants, chelating (sequestering) agents, biocides, anti-foams, humectants, glycols, corrosion inhibitors

pure water,72 mN/m

surface energy of non-image area, 50 mJ/m2

water + 6% IPA,45 mN/m

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Screening

One half-tone image may contain thousands of different shades………

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Different screening

Conventional screening

distance between the centers of

the dots is constant

Stochastic screening

tone is formed by altering dot position and spacing, number of dots

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Stochastic screen

Standard dot size, but the amount of them vary.

Dot size and amount of dots vary.

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Screen ruling – conventional screening

Tonal value 75%

Tonal value 25%

Tonal value 50%

Tonal value 100%

Screening ruling (L/cm or L/inch)

Light tones ~ small dotsDark tones ~ bigger dots

The dots are so small that the human eye sees them as a single colour.

40l/cm

60l/cm

80l/cm

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Strongly magnified screen surfaceNormal screen surface

In case of the 4-colour printing the colours are mixed in the eye when looking at them, as the eye cannot differ between the small, close picture elements.

Human eye

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Print densityPrint density (darkness) is created by increasing the inked area

(halftone percentage)

Black 20% Black 40% Black 60% Black 80%

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Print colourPrint colour (tone) is created by mixing halftone percentages of process colours

C80%/M20% C60%/M40% C40%/M60% C20%/M80%

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Examples of screening methods

Staccato 25µm Conventional 60 l/cm

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SummaryOffset printing

What is colour ?Principle of offset printingSheet-fed offset printingHeat-set web offset printing

Other printing methodsConventional methods

GravureFlexographySilk screen

Digital printing methodsLaser (electro photography)Ink jet

Conclusion

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Sheet fed offset press

Anti set-off powder system

Margin - FeederInk rollers & Damping rolls

Settings of pressure

and register

Delivery of the printed

sheets

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After the 4 printing units

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Sheet fed presses5 colour units + 1 varnish unit

FeederDelivery Varnish Ink units

4/4 colour units, change of the printing side of the sheet in the perfecting system

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Different formats

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Feeder

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Ink Rollers

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Sheet Transfer

Transfer cylinder

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Spray powderto avoid ink set-off

distance holder between the sheets.

increases the amount of oxygen to improve the oxidative drying process.

Spray 10 to 80 micronscalcium carbonateorganic, vegetable based powder of natural starch

300x - CaCo3

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Varnish Why ?

More gloss

Protect

Less drying time before converting

Sometimes two varnish unitsHigh gloss and matt varnish at the same runPremier and UV- varnish when using conventional inksBetter gloss than one unit applying

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SummaryOffset printing

What is colour ?Principle of offset printingSheet-fed offset printingHeat-set web offset printing

Other printing methodsConventional methods

GravureFlexographySilk screen

Digital printing methodsLaser (electro photography)Ink jet

Conclusion

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Basic design of heatset press

Web widths: ~50- (over) 200 cm

Web speed: ~4-15 m/s

Web tension: ~600-800 N/m

Drying (web temp): ~110-140 °C

Folding: usually on-line

reelstand

infeedprinting units

dryer

chill rolls

siliconization unit

folder

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Heatset presses - different designs

additional colour varnishing unit (offset varnish)

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Operating pressDriving the press

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Infeed

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Heatset press – an example

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Plate cylinder

Blanket cylinder

Plate cylinder

Blanket cylinderPaper web

Paper web in one unit

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Oven

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Oven

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Heatset dryerWeb delay in the dryer is ~1 s

dryer length is determined by the press speed (10 m/s 10 m)

Dryer has multiple modules

Hot air from the nozzles carries the web through the dryer

Exhaust air is either circulated back or is let to atmosphere through treatment (e.g. afterburner)

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Temperature profiles in the dryerWeb exit temp. usually 100-140 C (90-180)

Usually declining temperature profileweb temperature rises faster and solvent evaporation starts earlier

1st 2nd 3rd 4th

°C

100

200

300

130°C

195°C

255°C

165°C

drying air temp

web temp

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Chill rollsThermoplastic ink binder is in soft form after the dryer

chill rolls cools down the web and solidifies the binder (better rub resistance)

Steel cylinders (3 - 6) with cold water pumped through

Web tension has to be high enough to ensure good contact

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Silicone application

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Center Cut

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Aligning ribbons

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Different kind of folders

Former fold

Cylinder/Jaw folder

Chopper folder

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Example of fold 16 pages

Formerfold

Jaw foldChopper/jaw fold

630 mm

880 mm

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Folding possibilities

310 x 440 mm

310 x 220 mm

155 x 110 mm

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Imposition and plate makingImposition

Done usually at the printerDepends on what press is used

16/32/48/64 pagesSpecial software used

Plate makingFrom imposition layout

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Press layouts

Run length

Pag

inat

ion

8-page

16-page

24-pageSingle perimeter

Double perimeter32-page

48/64-page

72/80-page

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Manual delivery

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Post-press

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Perfect bindingOperations

signatures are collated together in conveyor belt signatures travels down the belt to the saw saw trims off of the bind edge of so the binding

glue can be applied between the individual pages

magazine travels to the covering station magazine is trimmed with 3-knife cutting

Separate cover is needed

Long grain=MD

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Saddle stitcherOperations

signatures are pulled open and dropped in proper order onto a moving "saddle". assembled signatures receive a cover in the same manner and then pass under the stitching heads where wire staples are applied magazine is trimmed with 3-knife cutting

Usually used only product with paginationunder 96 pages

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SummaryOffset printing

What is colour ?Principle of offset printingSheet-fed offset printingHeat-set web offset printing

Other printing methodsConventional methods

GravureFlexographySilk screen

Digital printing methodsLaser (electro photography)Ink jet

Conclusion

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Gravure

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Gravure

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Gravure - applications

Packaging & convertingFlexible packagingLabels & wrappersGift wrapsWall coveringsVinylDecorative laminatesFloor coveringsTissue productsStamps

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SummaryOffset printing

What is colour ?Principle of offset printingSheet-fed offset printingHeat-set web offset printing

Other printing methodsConventional methods

GravureFlexographySilk screen

Digital printing methodsLaser (electro photography)Ink jet

Conclusion

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Flexography

paper

sleeve

Counter pressure cylinder

blade

ink

Double face tape

plate

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Applications

• Plastic bags

• Paper sacks • Milk carton• Pocket Books

• Labels• Aluminium • Corrugated Board • Carton cups • Napkins • Tissue • Envelope • Bussinessforms • News Papers • Flexible packaging • Pocket Books

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SummaryOffset printing

What is colour ?Principle of offset printingSheet-fed offset printingHeat-set web offset printing

Other printing methodsConventional methods

GravureFlexographySilk screen

Digital printing methodsLaser (electro photography)Ink jet

Conclusion

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Silk screen

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SummaryOffset printing

What is colour ?Principle of offset printingSheet-fed offset printingHeat-set web offset printing

Other printing methodsConventional methods

GravureFlexographySilk screen

Digital printing methodsLaser (electro photography)Ink jet

Conclusion

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What is digital printing ?

DIGITAL printing means printing directly from digital dataoriginal is in digital formno films are neededno plates are needed

Slide 81

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Pro or Contra digital printing ?Pro

easily changeable infodesign and colourstext

no make-ready-> small runs with reasonable costspersonalisationversioningon-demand printingdecentralised printingeasy to operate vs. offset”unlimited” repeat length

Contra Print quality still lower than in offset Low printing speed compared to conventional presses Format limitations (small reels/sheets) Limited or non-existing spot colours and metallic Conventional further converting Costs (toners etc)

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Applications

Production variable data

direct marketing

individualized statements

customized catalogues, brochures etc

Commercial printing

on-demand and short run books

short run brochures

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SummaryOffset printing

What is colour ?Principle of offset printingSheet-fed offset printingHeat-set web offset printing

Other printing methodsConventional methods

GravureFlexographySilk screen

Digital printing methodsLaser (electro photography)Ink jet

Conclusion

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Electro photographic principle

1. Photoconductor is charged

++

+

+

-+

-

---

--

- --- - -- - -

---

- -

---

++ ++

4. Toner (image) is transferred to the paper in electric field, opposite charge

attracts

5. Toner is fixed to the paper by heat = fusing

6. Remaining toner

is wiped off

2. Latent image is

formed by discharging the drum by laser or

LED

3. Latent image is

developed by toner

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SummaryOffset printing

What is colour ?Principle of offset printingSheet-fed offset printingHeat-set web offset printing

Other printing methodsConventional methods

GravureFlexographySilk screen

Digital printing methodsLaser (electro photography)Ink jet

Conclusion

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Ink jet : continuous1. Continuous stream, CS, CIJ

printing droplets are selected from the continuous stream by charging them selectively

droplets have similar size

high speed

e.g. Scitex VersaMark

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Ink jet : drop on demand2. Drop-on-demand, DOD

every drop is printing

high resolution

lower speed

e.g. Aprion

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SummaryOffset printing

What is colour ?Principle of offset printingSheet-fed offset printingHeat-set web offset printing

Other printing methodsConventional methods

GravureFlexographySilk screen

Digital printing methodsLaser (electro photography)Ink jet

Conclusion

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Print definition depends on paper quality

Coated Fine,70l/cm

SC,54l/cm

LWC,60l/cm

Newsprint,48l/cm

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Different printing process, different results

Flexography(20%, 60 l/cm)• Ink is pressed to

edges, print density inside dot varies

• Edges are smooth• Shadow in the edge

of text

Gravure(~20%, 100 l/cm)• Broken shape of dots and

missing dots are typical for light tones

• Doughnut shaped dots• Text is screened, serrated

edge

Offset(20%, 75 l/cm)• Dots are uniformly

covered with ink• Edges are ragged

(uncoated papers)• Uniform text

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Examples of halftone dots and text

Inkjet(20%)• Dots are quite uniformly covered with ink• Dot is formed by using several

droplets• Edges are ragged or even

depending on paper• Satellites, escaped droplets

Electrophotography(~20%)• Dots are quite uniformly covered with toner• Edges are ragged• Separate toner particles

(dry toner methods)• Uniform text

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Questions ??Will be happy to answer your technical questions :

Françoise Accou, +32 492.582.287, francoise.accou@sappi.com