offset_printing_basics

• • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • Offset Printing Technology • • • • • • • • • • • • • • •

Sheet-fed Offset Printing Basics

© Heidelberger Druckmaschinen AG

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Print Media Academy

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Printing process: Sheet-fed offset printing • Offset printing is a planographic process: Printing and non-printing elements of the printing form lie next to one another in one plane • The printing form – usually a printing plate – statically stores the information to be printed as image areas and non-image areas • The image areas are inked up either conventionally or without water – Conventional = two phases: 1. Dampening the printing form 2. Inking up the printing form – Waterless: Inking up the printing form using a special printing ink

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Print Media Academy

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Image areas and non-image areas in conventional offset printing • Important prerequisite for inking up the image information on the printing plate:

"e“ = printing element: Image area on this offset printing plate

Good wetting with dampening solution and printing ink Micrographs of an offset printing plate (aluminum)

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• Image area = ink-accepting area: polymer et al.

• Non-image area = moisture-accepting surface: aluminum oxide

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Printing process: Sheet-fed offset printing Conventional offset printing Inking unit

2nd phase:

1st phase:

Phases of the inking-up process • 1. The dampening solution wets the nonprinting elements on the printing form, but is repelled at the printing areas

Plate cylinder

• 2. Rollers transfer printing ink from an ink fountain to the printing form • Dampened non-image areas reject printing ink, while non-dampened image areas accept the printing ink

Dampening system Printing material, e. g. paper Blanket cylinder Impression cylinder Blanket

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Printing process: Offset printing •

Offset printing is an indirect printing process, which means: the inked up information on the printing

Information is ... ... right reading on the printing form.

form is first transferred to an intermediate cylinder, the blanket cylinder, and from

there to the printing material.

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... wrong reading on the blanket cylinder.

In sheet-fed offset printing, individual sheets are printed consecutively.

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In web offset printing, on the other hand,

the printing material is unwound from an "endless” reel and guided through the printing units as a web.

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... right reading on the impression cylinder and the printing material.

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Influences on print quality in sheet-fed offset printing • Ink/dampening solution balance • Inking unit • Dampening system • Printing ink • Dampening solution • Color sequence

• Sheet guidance, gripper setting • Register setting

• Sheet separation and alignment

• Powder application • Sheet delivery

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Design of the printing units Setting of the entire sheet travel path Cylinder packing and rolling Printing pressure adjustments (contact pressure) • Printing material • Climatic conditions in the pressroom © Heidelberger Druckmaschinen AG • Name of author • Date • File name

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Color printing using autotypical color mixing • In offset printing a printed image is made up of evenly inked up binary picture elements: – halftone dot – no halftone dot.

• Autotypical = variable-area dots, always having the same distance from midpoint to midpoint.

• For a screen ruling of 60 l/cm there are 60 x 60 = 3600 halftone dots per cm² • The dot size determines the tone value (lightness) on white paper – small dot = high light reflection = light – medium dot = medium light reflection = medium – large dot = low light reflection = dark

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Print Media Academy

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Color printing using autotypical color mixing • A multicolor image is usually printed with the process colors cyan, magenta, yellow and black • Screen angling the individual color separations for the process colors produces a rosette pattern of the individual halftone dots • The halftone dots of the four printing inks can be printed next to, partly or even completely on top of one another • The human eye "sees“ a combination of additive and subtractive color mixing

• Correct color-to-color register in the print • Register deviation of more than ¼ of the screen definition (e. g. 40 µm) has a negative effect on the image quality

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Color printing: Four printing inks – one picture!

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To produce a color print, each printing ink requires a special printing form.

Each printing form is printed using the relevant process color: black (K) – cyan (C) – magenta (M) – yellow (Y) Printing is mainly in the color sequence K-C-M-Y, wet-on-wet in four-color offset printing presses: the printed product is the finished color image!

Cyan

Magenta

Yellow

Black

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Printing plate systems in offset printing: Conventional – waterless As a general rule, offset printing presses can use – conventional offset printing plates – waterless offset printing plates.

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Print Media Academy

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Printing plate systems in offset printing: Conventional – waterless Inking up the picture information on the printing form

2nd phase: Inking up

Inking up

1st phase: Dampening

Conventional offset printing

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Waterless offset printing

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Printing plate system: Waterless offset printing •

Waterless offset printing plates have a special coating structure of - silicone - polyester or aluminum

Silicone (ink-repellent)

Polyester (ink-receptive) Example of waterless offset printing: Heidelberg Quickmaster DI 46-4

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Waterless offset printing Advantages of waterless offset printing •

Lower dot gain than with conventional offset printing

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In the shadow areas the halftone dots remain more open

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Halftone dots print sharper, producing greater color contrast and brilliance in the printed image

Data or film

9%

50%

90%

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Waterless offset print

Conventional offset print

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Conventional dampening system for alcohol-free dampening • The two dampening form rollers and the

Inking form rollers

vibrator roller are covered with plush material (Molton).

• In some cases the pan roller is also

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covered with textile

• Metering is determined by the speed of

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the pan roller and/or the vibrator roller cycle

• Plush covers hold a lot of water • Disadvantages are a delayed reaction in the dampening solution control system and the formation of lint

Cylinder gap 1.

2.

3.

4.

5.

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2 3

1

Dampening system

Pan roller Vibrator roller (surface: plush-covered) Distributor roller Dampening form rollers (plush-covered) Printing plate cylinder

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Conventional dampening system in comparison to a continuous-type dampening system

100 times magnification of halftone dots, printed with conventional dampening

Molton/plush

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100 times magnification of halftone dots, printed with the Alcolor continuous-type dampening system

Rubber

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Conventional dampening system in comparison to a continuous-type dampening system •

% In conventional dampening the start-up phase takes longer due to the variations in the ink/water balance

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Graph: Water content in the ink measured at the fourth inking form roller from the start of printing up to about the hundredth print sheet Conventional dampening system Continuous dampening system *)

0 Continuous-type dampening system *) During production the ink/water balance remains stable, once it has been quickly reached by special predampening during the start-up phase

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Time

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Continuous-type dampening system: The Alcolor dampening system •

Alcolor is a 5-roller dampening system, which is directly connected to the inking unit via an intermediate roller

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Pan roller and metering roller have independent drives, which are electronically controlled and speed compensated

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Dampening distributor and dampening form roller run at press speed

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Print Media Academy

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Continuous-type dampening system: The Alcolor dampening system •

The variable speed (slippage) between the metering roller and the dampening form roller produces a very thin dampening solution film

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This extremely thin dampening solution film is worked into the ink on the dampening form roller and into the inking unit and transferred with the printing ink to the printing plate

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The ink/water balance is quickly achieved and remains constant

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Print Media Academy

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Continuous-type dampening system: The Alcolor dampening system • The distributor cylinder C (distribution roller) in the dampening system is driven directly

F

by the printing press

• Its matt chrome-plated surface ensures that the dampening solution is optimally worked into the printing ink on the dampening form roller, producing a suitable emulsion

Intermediate roller for separating

E B

D C

A

• A predampening of the system during the start-up phase optimizes the printing process • The connection between inking unit and dampening system can be interrupted in the event of excessive emulsification and the formation of an unstable emulsion

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A B C D E F

Pan roller (surface: rubber) Metering roller (surface: chrome-plated) Distributor cylinder (surface: matt chrome-plated) Dampening form roller (surface: rubber) Intermediate roller (surface: Rilsan) Inking form roller 1 (surface: rubber)

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Continuous-type dampening system: The Alcolor dampening system Functions in detail • The speed of the pan roller and metering roller is continuously adjustable

F

E

• Basic setting of the dampening solution film occurs between A and B • Fine setting of the dampening film occurs between B and D due to the variation in speed (slip) between the metering roller and form roller. A very thin, even film of dampening solution is produced, which can be controlled exactly by varying the speed

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B A

D C A-B:

Pre-metering and formation of an initial dampening film in the nip B-D: Formation of a very thin dampening film on the printing ink C-D: The dampening solution penetrates the printing ink.

D-E-F: Inking unit and dampening system are connected with one another

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Continuous-type dampening system: the Alcolor dampening system The entire process is controlled automatically •

Rapid attainment of an ink/water balance through a special predampening process during the start-up phase

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Speed compensated dampening: Supply of the required quantity of dampening solution in accordance with the speed of the printing press

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The dampening system is cleaned together with the inking unit

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The entire system is operated pneumatically

Predampening

Production

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Separated: washup

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Printing process: Technique to prevent the formation of hickeys during production The Vario function works on the principle that the dampening form roller D runs at a slightly slower relative surface speed to the printing plate cylinder The resulting slippage means that dirt particles (hickeys) on the printing plate are simply wiped away or do not even occur D

The greater surface roughness of the matt chrome-plated distributor roller achieves the required frictional resistance, without affecting the transfer of the dampening solution The dampening form roller D is driven indirectly by the rough C D distributor roller C

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C

Separately driven distributor roller Indirectly frictionally driven dampening form roller - in contact with the roughly chrome-plated distributor roller

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Dampening solution in conventional offset printing Regular mains water is unsuitable for offset printing: The surface tension is so high that water droplets would form.

This causes problems in correctly dampening the non-image areas.

The water must be conditioned for the printing process and improved with further additives. An important additive is isopropyl alcohol to reduce the surface tension.

Air

Liquid Surface

No wetting

Wetting

Spreading

Wetting angle Surface tension of the solid (printing plate, carrier) Surface tension of the liquid (water)

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Printing plate

Wetting behavior on identical surfaces for various liquids

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Dampening solution in conventional offset printing The use of isopropyl alcohol (IPA) offers advantages in the printing process IPA ...

- reduces the surface tension of the dampening solution

- evaporates quickly, thus cooling the printing plate and the inking unit - in the dampening solution increases the viscosity of the printing ink - prevents the growth of microorganisms in the dampening solution - very quickly stabilizes the ink/water balance, - helps the printing plate to run clear more quickly, - ensures an even dampening solution film, - evaporates rapidly and pollutes the environment!

Aim: Only use as much IPA as is absolutely necessary!

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Spindle for alcohol metering: Accurate metering is important!

Attention: Evaporation of IPA due to higher temperatures!

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Printing process: Temperature in the printing press at 5 % IPA content •

The reduced quantity of IPA means less alcohol evaporates. This reduces the cooling effect.

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The temperature in the printing press increases during production. The printing ink tends to scum.

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When operating at a room temperature of > 30 °C and with a desired IPA concentration of less than around 10-8%, the inking unit will need to be temperature controlled

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The ideal setting for the temperature control of an inking unit is 26 °C. The temperature of the dampening solution should be around 10 °C

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Dampening solution: Too much – not enough

• Insufficient dampening solution Results in: • filling in of reverse type • filling in of half and three-quarter tones • vertical ink streaks at the edge of the print

• Too much dampening solution Results in: • water drips • reduced contrast in the image • underinked strips in the print image

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Printing process: dampening solution emulsifies into the printing ink Tiny droplets of dampening solution are evenly dispersed in the ink . The water content of a "stable" emulsion is up to 20%.

Stable emulsion in offset printing: Optimum emulsion of printing ink and dampening solution

Emulsion of printing ink and dampening solution: Shortly before turning into an unstable emulsion

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Unstable emulsion: Cause of printing problems

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Printing process: Conditioning of water for production

• An unsuitable pH value of the dampening solution leads to printing problems • pH value well below 5: More difficult ink drying.

Oxidation of metallic inks.

Shorter service life (durability) of the printing plate • pH value above 7: Lower surface tension.

Printing plate tends to scum

pH value 10 -0 Hydrogen ion concentration

• The quality of mains water can vary considerably.

Therefore the water used in the printing process must be conditioned

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ideal pH value 10 -4 8 9 10 11 12 13 14 10 -7 1 2 3 4 5 6 7 pH

10 -10 neutral

10 -14

increasingly acidic

increasingly alkaline

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Printing process: Water hardness – the dH value • The hardness of the water in 0dH is determined by the quantity of calcium and magnesium salts • Hard water with an extremely high concentration of calcium and magnesium salt can form fatty and insoluble suds in reaction with fatty acids in the printing ink

0dH

= German degree of hardness

mg CaO/l H

300

soft

medium

hard

• Suds react both hydrophilically and lipophilically • These different properties lead to printing problems: – glazing of the inking rollers – ink piling on the rollers and blanket – irregular feeding of the dampening solution – tendency to ghost

200

100

4 8 12 16 20 24 28 32 o dH 4 to 15°dH are recommended

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Printing process: Functions in an inking unit • The printing process requires a constantly even application of ink on the printing plate • The distribution of ink from the ink fountain occurs via many rollers, which also store the ink • Inking form rollers ink up the information on the printing form Ink feed: The ink fountain supplies the required quantity of ink

Ink splitting: Most important transport function for the transfer

Ink storage: Many roller surfaces store printing ink

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Ink application: Inking form rollers transfer the ink to the printing plate

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Printing process: Remote-controlled ink control • The ink film thickness is achieved through interaction of the ink fountain roller and ink metering cylinder (adjusting eccentric) • For ink control only the eccentric metering cylinder rotates, allowing a larger or smaller gap for transferring the ink to the ink fountain roller Ink fountain liner Printing ink Ink fountain roller

Ink metering cylinder with lateral supporting collars

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Printing process: Inking unit Typical inking unit with 18 rollers, of which 4 are oscillating inking form rollers of different diameters •

Ink fountain

Continuously driven ink fountain roller

Ink fountain roller

Speed compensated inking The speed can be set in percent via the control console •

Lateral oscillation can be set between 0 – 35 mm

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Setting the application timing of the lateral oscillation

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The majority of ink is transferred to the printing form by the first inking form rollers

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Adjustable oscillation of the form rollers from 0 to 8 mm

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Distributor roller 8% 37% with Rilsan 7% 48% coating Rubber rollers

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Printing process: Manual setting of the timing and the lateral oscillation travel

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Printing process: Timing of application and travel of lateral oscillation An optimized example: Ink fading from the lead edge to the rear edge of the sheet in offset printing The ink fading can be counteracted by setting the oscillation timing Ink zones 1 2 3 4 5 6 7 8 9 1011121314151617181920212223242526272829303132

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Printing process: Example of an inking profile on the ink form rollers With “zero oscillation option”: inking profile is retained - Better print quality - Less waste during press start-up

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Without “zero oscillation option”: Leveling out of the inking profile during interruptions, such as pile change or blanket washup

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Current inking profile in the inking unit or on the inking form rollers

During the production run Current inking profile on the ink fountain roller

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Ink zones

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Printing process: Energy and heat – cooling the printing unit Around 45 % of the energy is converted into heat and released by convection and radiation

Around 50 % is converted into heat and dissipated via peripheral equipment

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Around 5 % is converted into kinetic energy

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Printing process: Energy and heat – cooling the printing unit

Temperature curve in the printing unit

• Elimination and reduction of heat in the

press and in the pressroom promotes print quality and productivity

45º C 40º C 35º C

• The increase in temperature changes the climatic conditions in the 30º C pressroom and causes printing and 25º C quality problems

Temperature of the ink distributor rollers

Uncooled press Printing plate temperature Temperature of the ink distributor rollers

Cooled press Printing plate temperature

20º C 5,000 10,000 15,000 Sheets during production

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25,000