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Printing technology web offset printing
- 1. Printing technology Web offset printing Technical Support Sappi Fine paper Europe
- 4. Printing technology Web offset printing Printing units
- 5. Printing technology Web offset printing Drying oven Hot air Inlet temperature > 200°C Web temperature approx. 120°C The length of the oven is very important
- 6. Printing technology Web offset printing 1 Automatic storage web changer 2 Drawing-in apparatus 3 Floating indent apparatus 4 Rubber against rubber printing apparatus 5 Continuous main shaft 6 Paper catching device 7 Dryer 8 Cooling unit 9 Numbering apparatus 10 Silicon-coating 11 Perforating apparatus 12 Wet coating unit 13 Plough fold 14 Turning bars 15 Former MAN-Roland Octoman
- 7. Printing technology Web offset printing Reel with join
- 8. Printing technology Web offset printing Reel in the autopaster
- 9. Printing technology Web offset printing Rotary Press: 1 Unit
- 10. Printing technology Web offset printing Paper web in one unit Plate cylinder Blanket cylinder Plate cylinder Blanket cylinder Paper web Colour absorption Water absorption Moisture resistance pH5 No deformation Resistant against cracking of the colour
- 11. Printing technology Web offset printing Ink drum
- 12. Printing technology Web offset printing Hot-air dryer
- 13. Printing technology Web offset printing Re-moistening
- 14. Printing technology Web offset printing Re-moistening Chill roll assembly
- 15. Printing technology Web offset printing Paper web on the chill roll
- 16. Printing technology Web offset printing Silicone applicator
- 17. Printing technology Web offset printing Center cut
- 18. Printing technology Web offset printing Paper on the turner bars
- 19. Printing technology Web offset printing Paper on the turner bars
- 20. Printing technology Web offset printing Fold softening
- 21. Printing technology Web offset printing Paper on the former fold
- 22. Printing technology Web offset printing Cylinder folder
- 23. Printing technology Web offset printing Cylinder folder
- 24. Printing technology Web offset printing Chopper folder
- 25. Printing technology Web offset printing Stacker
Hinweis der Redaktion
- Ovens; drying principle : A Heatset offset printing machine is provided with a hot air oven to let the colour dry more quickly. The modern ovens are divided in two or three segments with respectively different temperatures , from approx. 250°C to approx. 100°C. At the end of the oven, the paper has a temperature of approx. 120°C, depending on the paper quality and the gram weight. The colours include pigment, fixing agents and solvents. By evaporating the solvents out of the colour, the colour dries. The length of a drying oven is very important. As rule of thumb it can be assumed that the paper has to be 1 second in the oven to let the colour dry, this means one press running with 10 m per second requires an oven length of 10 m.
- Paper guidance in the press: With the Heatset Rotation Offset printing is carried out from a web. The section of the machine, in which the web is suspended and unwound is called unwinder or “paster". It is possible to suspend two webs at the same time, in order to be able to change nonstop while printing. In order to obtain the correct tensile stress for the paper web, the unwinding unit is provided with a brake which is adjusted depending on the weight. The paper is running from the web via a storage chamber (this is required for a nonstop-change) to the feeding system, where the web is kept in the centre by means of web controls. Once the paper web is running slightly to the left or right, a correction is immediately carried out. Afterwards, the paper web is reaching the printing units where the paper is printed on both sides at the same time. After printing, the web with wet colour is running through an oven, where the colour is dried by means of hot air. The paper web heated up in the oven is afterwards cooled down by several chill rolls. By varying the speed of the chill rolls, it is possible to adapt the tensile strength of the web also in this section. After the chill rolls, a thin silicon coating is applied on the printed web, which is afterwards folded.
- Web profile : The web profile of the reel must be optimal in order to guarantee a good paper guidance. The word sound hardness, meaning the winding hardness of a reel, will surely be familiar to you. Important is that the winding hardness is uniform over the entire width. A simple control method is to tap off the web on the width with a wooden slat with the sound nearly being the same everywhere. If hard and soft sounds are audible the web profile is not good. In Germany, this phenomenon is called water pocket. A professional measuring method of the control is the one with a Schmidt hammer, with which the winding hardness can be measured and shown in the width in a graphic. Reels with bad web profile, among other things with soft edges, often create passer and doubler problems. The part of the web with a low winding hardness is not drawn tightly by the web tension control and will start to hurl. In extreme cases, this problem may even generate cockles which my sometimes lead to a web cracker. Doubling is a phenomenon which occurs if one or several colours are double printed so that a shadow picture is generated at the same time. Passer difference means that the colours are not above each other. The general rule for this type of problems is to produce 2 to 3000 sections and if the problem still occurs, the reel has to be taken out of the press and has to be sorted out.
- Rotary press: 1 unit In the prepress phase the original picture is separated into 4 single-colour pictures by means of filters. In the four colour offset printing there are thus 4 plates: black, cyan, magenta and yellow. The printing process described above is therefore passed through four times in order to receive a full colour print. In case of the sheet printing presses this is carried out in one or several passages depending on the number of printing units in the press. With web presses this is principally carried out in one working phase. The printing unit on the picture is a Blanket-to-blanket-type: the paper web is printed at both sides at the same time. The two rubber blankets rest on each other. The paper web is led through the two rubber blankets. Rotary offset therefore means: Printing from the web, according to the offset process. With the high rates in the rotary offset on coated paper types, self-drying of the colour is insufficient. Additional drying is necessary. This additional drying is carried out by means of a hot-air oven coupled to the press. The colours are matched hereto. The full term for the printing process we mostly use for our web qualities is thus : Heatset Rotation Offset .
- Relation colour / water / paper : The paper quality plays an important roll in relation to the water and the colour absorption capability in the printing result and has to meet several requirements, among which you can find: A good absorption to support drying and adhesion of the colour. The paper has to absorb the water quickly and uniformly to avoid that the following colour is badly printed after printing the first colour as the water film coming onto the paper when printing the single colours is not completely absorbed by the paper (repelling of the colour). If insufficient water is absorbed by the paper, the colour may start to emulsify (= to mix with water), influencing the printing quality and the drying properties extremely in a negative way. The paper has to be resistant against fountain solution with a pH of approx. 5 (the coating layer should not detach). In case of a contact with moisture the paper should practically not deform . The paper surface has to be resistant against the forces being released during cracking of the colour if the paper loosens from the rubber blanket.
- Folding and folding problems : A rotation press is provided with a folding unit in which the printed paper is folded in line to the folds. The first fold is mostly carried out on a funnel , the so-called former. After this former, the paper web is cut-off. An A4-size machine is mostly cut off in a length of 63 cm. The second fold is a cylinder fold , the third is a knife fold . Now we have a completely ready fold . The paper web is fairly dried out in the drying ovens. This has a negative influence on the solidity of the paper which becomes fragile. During folding this may lead to a breaking of the paper on the fold which becomes so thin that it even tears. During the quality control in the paper mill the paper is checked among other things for the remaining break load, i. e. the strength which the paper shows after drying and folding. The breaking on the fold may be the result of a too low remaining break load; however, often it can be put down to the strong drying in the pressure press, to too sharp folding or to a faulty stitching on the gather stitcher.