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Printing technology : problems sheetfed offset


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Printing technology : problems sheetfed offset

  1. 1. Printing Technology Printing Problems Sheetfed Offset Technical Support1 | [Printing problems Sheetfed Offset] Sappi Fine paper Europe
  2. 2. 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 drying2 | [Printing problems Sheetfed Offset]
  3. 3. 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?3 | [Printing problems Sheetfed Offset]
  4. 4. 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:4 | [Printing problems Sheetfed Offset]
  5. 5. 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 Printed stock expansion 1. Printing colour 2. Printing colour5 | [Printing problems Sheetfed Offset]
  6. 6. 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-off6 | [Printing problems Sheetfed Offset]
  7. 7. 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. Paper Cleave products Printing colour7 | [Printing problems Sheetfed Offset]
  8. 8. 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.8 | [Printing problems Sheetfed Offset]
  9. 9. 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.9 | [Printing problems Sheetfed Offset]
  10. 10. 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 possible10 | [Printing problems Sheetfed Offset]
  11. 11. 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 Colour cleaving Printed Printed sheet tear-off angle sheet Impression cylinder gripper Printing zone width Impression cylinder11 | [Printing problems Sheetfed Offset]
  12. 12. 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 speed12 | [Printing problems Sheetfed Offset]
  13. 13. 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). Printing colour Coating Basis paper By changing the colour sequence, the printer may reduce or prevent the mottling effect.13 | [Printing problems Sheetfed Offset]
  14. 14. 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 cutting14 | [Printing problems Sheetfed Offset]
  15. 15. Scuffing / carbonising Difference: Scuffing Carbonising Average pressure with High pressure with very small relatively long scuffing path strokes (e.g. collating machine, folding (e.g. guillotine-type cutter, three cutter machine, gather-stitcher) machine)15 | [Printing problems Sheetfed Offset]
  16. 16. Scuffing / carbonising Scuffing Silk Matt Carbonising Silk Matt16 | [Printing problems Sheetfed Offset]
  17. 17. Scuffing / carbonising Especially in case of matt papers, the scuff strength is influenced by the following factors: Colours Fountain solution Paper  Oxidative drying  Optimal 5-8% ISO  Smooth surface colours ph 5.5  Acid water, much ISO  Coarse surface  Fresh colours Powder Time  Starch powder  Drying time, incl. Scuff strength aeration  Mineral powder  Quick processing Printing conditions Repro Book bindery  Stable continuous printing  UCR  Upright sheet feeder without many stops good  400 % colour  Horizontally laying climate sheet, pulled from the  Drying e.g. with IR at 1 point bottom17 | [Printing problems Sheetfed Offset]
  18. 18. Scuffing / carbonising “Scuff strength” What is the printer capable to do? 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.18 | [Printing problems Sheetfed Offset]
  19. 19. 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 hours19 | [Printing problems Sheetfed Offset]
  20. 20. 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 surface20 | [Printing problems Sheetfed Offset]
  21. 21. 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).21 | [Printing problems Sheetfed Offset]