Printing technology : problems sheetfed offset

| [Printing problems Sheetfed Offset] 1

Printing Technology

Printing Problems Sheetfed Offset Technical Support

Sappi Fine paper Europe


Content

- Questions and check list in case of complaints

- Doubling effects

- Misregister

- Ghosting effects / Yellowing effects

- Rub resistance / Pick outs

- Mottling

- Scuff resistance / Carbonize

- Ink drying


Important questions in case of complaints

What is the problem?

Is printing still carried out or was the paper completely processed?

Was the printing process interrupted and another job taken into the

machine?

Is a further printing possible:

With a less good printing result?

With a lower production speed and/or more interruptions?

What are the estimated costs?

Is the entire job processed at once?

Is the supply of additional paper necessary?

In connection with the visit: how long is printing still carried out?


Doubling

Doubling characteristics:

The question is of doubling if an unintentional, shadow-shaped

colour point with small dimensions can be found at the artificial

printed screen point. Doubling is generated by the non-congruent

re-transfer of colour through the following rubber blanket.

Doubling:


Passer differences

Passer differences are caused by:

– Hygro-expansion of the printed stock

– Lack of plane position of the printed stock

– Technical error or failures of the printing machine

– Unfavourable climatic conditions

1. Printing colour 2. Printing colour

Printed stock

expansion


Passer differences

The following points should be checked:

– Longitudinal direction of the paper (use small web)

– Check fountain solution and reduce it, if possible

– Check the paper run of the printing machine (feeding)

– Check the winding-off


Ghost effect

Characteristics of the ghost effect:

A ghost effect is caused by an interaction of paper, printing colour

and production conditions. The printing pictures distinguish

themselves by released cleave products on the back of the sheet

lying above in the stack. We differ between matt-glossy-effect and

contact yellowing.

Printing colour

Paper Cleave

products


Matt-glossy-effect

Characteristics of the matt-glossy-effect:

The cleave products change the back of the sheet lying above in

the stack. If this back is now printed the colour drying is changed

at this point. By the different colour dryings, the matt-glossy-

effect is generated.


Contact yellowing

Characteristics of the contact yellowing:

The cleave products taken from the back react with the paper.

Strongly brightened papers are especially sensitive to contact

yellowing.


Ghost effect

The ghost effect appears completely accidental and

unexpected. A ghost effect may be avoided by the following

processes:

– Printing of bigger colour surfaces in the second form printing

– Run of smaller stacks and provision of sufficient aeration during drying (manual, stack changer)

– Use of concentrated colour to keep the colour guidance as low as possible


Picking / parts breaking off

“Picking” is a tearing-out of coating particles or fibres from the

paper surface due to too high forces during colour cleaving.

Rubber blanket cylinder

Bulb formation

Printing colour before cleaving

Printed

sheet

Colour cleaving Printed sheet tear-off angle

Impression cylinder gripper Printing zone width

Impression cylinder


Picking / parts breaking off

Reasons for the “picking” are:

– Poorly sized paper

– Too quick absorption of the printing colour

– Too continuous printing colour

– Printing room and machine too cold

– Too high printing speed


Irregular printing / mottling

This error appearance in the printing is caused by a non-uniform

absorption of the printing colour in the paper surface and, thus, a

non-uniform re-cleaving of the following printing material to the

rubber blanket (backtrap mottling).

By changing the colour sequence, the printer may reduce or

prevent the mottling effect.

Printing

colour

Coating

Basis paper


Scuffing / carbonising

Definition of the “scuff strength”:

Scuff strength is the resistance of a printing colour layer against

the mechanical stress due to pressure load and friction.

– Scuffing: colour soiling due to mechanical abrasion of

the printing colour film – Carbonising: colour soiling due to the pressure load onto

the unprinted back of a printing object

in the stack or during cutting



Difference:

Scuffing

Average pressure with

relatively long scuffing path

(e.g. collating machine, folding

machine, gather-stitcher)

Carbonising High pressure with very small

strokes

(e.g. guillotine-type cutter, three cutter

machine)



Carbonising

Silk

Silk Matt

Matt

Scuffing



Especially in case of matt papers, the scuff strength is

influenced by the following factors:

Scuff strength

Colours

Oxidative drying

colours

Fresh colours

Fountain solution

Optimal 5-8% ISO ph 5.5

Acid water, much ISO

Paper

Smooth surface

Coarse surface

Time

Drying time, incl.

aeration

Quick processing

Powder

Starch powder

Mineral powder

Printing conditions

Stable continuous printing

without many stops good

climate

Drying e.g. with IR

Repro

UCR

400 % colour

at 1 point

Book bindery

Upright sheet feeder

Horizontally laying

sheet, pulled from the

bottom



Little or not influenced :

1. Placing of single-coloured and multicoloured figures.

2. Use and arrangement of special colours.

3. Realisation of an enamelling.

4. Realisation of a laminating.

5. Planning of a bigger paper size for co-printed colour bars.

“Scuff strength” What is the printer capable to do?


Scuffing / carbonising “Scuff strength” What is the printer capable to do?

1. Selection of the printing colour – Use of so-called „scuff-resistant“colours – No use of “box-fresh” printing colours

2. Determination of the colour sequence

– The colour printed last should always be yellow (function as lacquer printing)

3. Selection and dosing of printing auxiliaries – Principally, no printing auxiliaries (oils, pastes, etc.) should be used – The use of paint driers could be useful (dosing according to the manufacturer’s

recommendation)

4. Regulation of the fountain solution – Alcohol share 8 - 10 % – pH - value 5.3

5. Optimisation of the colour-water balance – Damping adjustment slightly above the “lubrication limit” – Perfect condition of colour and damping rolls – Precise adjustment of colour and damping rolls – Stabilisation of the colour-water balance by co-printed colour bars in case of poor

printing forms

6. “Correct” printing-dry-spraying – Graining of the powder analogue to the basis weight of the paper – No powder with “edged” structures (mineral powder) – Use of powder with “round” structures” (starch powder) – No powder on sugar basis – The less powder, the better the scuff strength

7. Drying time

– The drying time between printing and further processing should amount to at least 48 hours


Drying of the colour

The drying of the printing colour is a criterion of the printability. In the sheet

offset printing drying colours are mostly used in an absorbing or oxidative

way.

The most important parameters are:

– The penetration or the physical absorption behaviour of the paper.

In this first phase, the coating of the paper shall provide the absorption of

the liquid parts of the colour binding agents in the paper, similar to the

function of a screen.

– The acidity or the alkalinity of the paper surface (expressed as pH-value),

which influences the chemical drying process of the binding agent

(oxidative drying).

The surface pH-value should be in the neutral or slightly alkali range in

case of coated paper ( 6.0).

– The relative stack humidity of the paper (approx. 50 % rel.).

– The topographic nature of the paper surface


Drying of the colour

The printing colour drying depends on the interaction with the humidity in the

offset printing.

Critical are:

– Too high fountain solution guidance

– Printing forms with poor colour consumption

– Wrongly dosed fountain solution binding agents

The colour absorption behaviour of the paper should not be adjusted too

quickly or too slowly.

In case of printing machines operating at a high speed, a deposit in the stack

should not occur, on the other hand, the colour absorption should not run too

quickly in view of 10 and 12 colour offset machines (danger of accumulation

and picking).

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