Islam Instructor tech amp Head of the department Printing principles of offset printing In the offset printing process the printing and nonprinting areas of the plate are practically on one level The printing areas of the printing plate are ID: 1020048
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1. Welcome to Session
2. MD. Mofakharul IslamInstructor (tech) & Head of the department Printing.
3. principles of offset printing In the offset printing process the printing and non-printing areas of the plate are practically on one level . The printing areas of the printing plate are oleophilic / ink-accepting and water-repellent, that is hydrophobic. The non-printing areas of the printing plate are hydrophilic, consequently oleophobic in behavior. This effect is created by physical Phenomena at the contact surface. The dampening system covers the non-printing areas of the printing plate with a thin film of dampening solution. This dampening solution (water plus additives) spreads over the non-printing areas. To achieve good wetting, surface tension has to be reduced by means of dampening solution additives. In extreme cases, reducing the surface tension of the dampening solution too much can result in too great an emulsification of printing ink and dampening solution, leading to a situation where an exact separation of printing and non-printing areas on the plate is not achieved when inking.
4. The perfect offset printing process depends on many chemical and physical specifics of the materials and components involved in the process. The most important are given in the following list:Influence of the printing plateSurface tension of the ink accepting areas,Surface tension of the dampening solution accepting areas,Surface roughness, especially of the ink-free/ non printing surface,Capillary attraction, microstructure of the non-image surface,Type of materials,Production methods in making the offset plate(mechanical or electrolytic graining,
5. Influence of the inking rollers Characteristics of the roller coverings, Surface tension of the roller materials, Surface roughness, Viscoelastic properties of the rubber coverings, Type of materials, Throw-on (pressure in the nip), adjustment,Concentric running,Influence of the blanketSurface tension of the blanket, Surface roughness, compressibility, ink acceptance and ink transfer behavior, tone value transfer behavior , dimensional stability,
6. Influence of the inkSurface tension, contact surface tension in relation to the dampening solution , rheological properties (viscosity, tack, etc) temperature behavior , ink composition, drying behavior , Influence of the dampening solution Water hardness / impurities, dampening solution additives (alcohol, detergents, buffer agents) pH value, Surface tension, rheological properties (viscosity, tack, etc)Dependence on temperature of the rheological characteristics,
7. Influence of the substratePrintability properties (smoothness, absorption capacity, wet ability) , pH value of the substrate, work ability properties (tension/ stretch behavior, picking, tearing), Influence of the printing press ( on print quality and process stability)Design of the printing unit (accurate, stable, vibration absorbent, etc, Design of the inking unit, Design of the dampening unit, temperature control This short overview shows that the offset printing technology must be viewed as a multi parameters system. Changing just one of the parameters can have an immediate effect on the printing process.
8. Color measurement & control system
9. Color measurement & control system (continue)
10. Color measurement & control system (continue)
11. Color measurement & control system (continue)
12. Color measurement & control
13. Computer to press/ direct imaging system
14. Offset plate graining structure
15. Color management Process/system for calibrating the individual devices and machines that are involved in the workflow from Color image processing right through to the finished product. Serves to guarantee correct color rendering from the input (e, g, scanner) right up to the output on various media and with various printing technologies (e, g, monitor, through printing equipment or printing presses based on various technologies on various substrates)
16. Why color management? When a prepress expert produces files for offset printing in his usual environment, that is, using his familiar scanner and screen, he generally achieves a quality of color reproduction that can scarcely be improved up on by employing color management. However, if this expert is asked to produce images once a year for news paper printing, he is no longer likely to achieve the desired color quality at the first attempt. His skill in controlling color quality also suffers if he suddenly has to work with a different monitor. He would also not find it possible to deliver optimum images immediately if he was told that a poster is going to be produced on a large format ink jet printer. Color management comes into play at precisely that point where the accuracy of the expert in controlling color reproduction ceases. Color management ensures optimum color reproduction irrespective of the input device, monitor, and output device used as long as the device is characterized by an ICC profile (International color consortium). Therefore, an important reason for using color management today is the certainty that the correct output result will be achieved at the first attempt.
17. Color Management CONTD…The importance of color management Color management used to be a task only the most exclusive publishing houses, service bureaus and print shops dealt with. As designers, we accepted the results those companies gave us, because we had no control over it, nor the knowledge or tools to do it ourselves. But today, color management is not only easier to do, it is almost a requirement for designers to maintain control over the appearance of their work once it’s published and, ultimately, keep clients happy.
18. Color Management CONTD…The future of color management. We can be confident that reliability in color reproduction will not remain a dream forever. Existing color management systems will continue to improve in terms of both the quality of gamut mapping and user-friendliness. Color management will then finally become transparent and easy to use for everyone. Of course, these changes will be accompanied by improvements in hardware. Color copiers are already available which identify changes in humidity and temperature and adjust to them so that they always achieve the best possible results. Other devices such as scanners or image setters will become more color-stable so that they do not need to be calibrated as often. Standards will increase in popularity. Many more applications will be available which will be capable of generating or using ICC profiles. We will undoubtedly achieve a situation where color information can be transported just as reliably and easily as fonts are today. Then, there will be no more boundaries to worldwide cooperation in the reproduction of color images.
19. Mottle patches(qc strips)Printing : spotty, mottled appearance of prints due to differing ink absorption into the paper. Particularly noticeable in multicolor prints.Tone: The degree of lightness or darkness in any given areas of print.Overprint: (1) A color made by printing any two of the process ink (yellow, magenta, and cyan) on top of one another to form red, green, and blue secondary colors. (2) In lithographic plate making exposing a second negative onto an area of the plate previously exposed to a different negative. This is a method of combining line and half tone images on the plate. (3) Solid or tint quality control image elements that are printed over or on the top of previously printed colors. Overprint patches are used to measure trapping, saturation, and overprint color densities. Like other quality control elements, overprints may be measured from a color bar in the team of a press sheet or from the printed image itself.
20. Over trapping In process color printing the transfer of an excessive amount of one color over another.Gray scale A reflection or transmission film strip showing neutral tones in a range of graduated steps. It is exposed alongside originals during photography and used to time development, determine color balance, or to measures density range, tone reproduction, and print contrast. Gray scales can also be used to check focus and resolution. (Alternative terms : step wedge, gray scale, step tablet)
21. Highlight: The lightest or whitest area of an original or reproduction, represented by the densest portion of a continuous-tone negative and by the smallest dot formation on a halftone and printing plate. Ink mileage: The surface area covered by a given quantity of ink coating material.Mid tone: The range of tonal values between halftone highlight and shadow areas (Alternative terms : middle tones, quarter tone )Skeleton black: A black printer that enhances detail an contrast in the mid tones and shadows of a four color reproduction. (Alternative terms : half scale black, ghost key)Screen angle: The position of the rows of dots on half tone screens in relation to a reference grid with horizontal and vertical lines. The most dominant color screened is positioned at a 45 º angle to the reference grid to avoid moire patterns.
22. Gray balance chart: Near- neutral yellow, magenta, and cyan dot values printed in a grid pattern. A halftone black Gray scale is used as a reference to find the three- color neutral areas. The dot values that compose these areas represent the Gray(color) balance requirements for a set of color separations. The gray balance should be produced in- house under normal production conditions using the process ink, paper, plates, and that will be used for the job.
23. Problem & solution of offset printing
24. Thank You