January 2017
e-VIBES
January 2017
TVF , alias DOT GAIN
Half tone dots are the basic building blocks in offset printing . The dots , though tiny , they are tricky . Lack of understanding of dots may cause serious problems in the reproduction quality . One of the inevitable consequences of transfer of these half tone dots is Dot Gain . Most printers treat dot gain as an evil and is the result of a bad printing machine . But dot gain is a natural phenomenon in any photochemical and printing process . “ I print , therefore I have dot gain ” is the better perspective . Every printing process from laser , rotogravure to offset will have dot gain .
Dot gain is one of the most discussed variables in offset printing . Dot gain impacts the quality of reproduction , especially in color printing .
Dot gain is technically called TVF - Tone Value Increase . In the print applications the tonal values are the effect of half tone dot patterns . In the digital workflow the dot patterns are generated by RIP - Raster image processing . This in turn is imaged on a printing plate and then inked on the machine . These inked dots are transferred under pressure to the blanket and then to the substrate . During each stage the dots get altered in shape and size . As a result the tonal values on the print differ from the original . The change in the size of the half tone dots , occurring during these processes is called Dot Gain . The half tone dot sizes increase even during the imaging of the content in film and also during the plate making . Also , while these dots are transferred from one surface to another , some pressure must be applied for the transfer . During these pressure transfers , ink is squeezed outword of the dot ’ s original boundaries . The degree of increase in size depends also on the viscosity of the ink , its tack and stickiness . Ink with higher tack resists the squeezing effect , while lighter inks show more gain . In printing there is a tendency for the ink to form a tiny extra circumference around the dots . The tendency predominates and persists with incorrect fount solution .
Press problems contribute to dot gain . The printing cylinders running out of circle will distort and increase the dot size . Improper blanket packing will make the gain greater . The slur , effect of badly kept machine can make dot gain grow excessively . The dot gain depends very much on the paper . Coated papers resist the gain , while unquoted papers , due to higher absorption , show more gain . The pressmen trying to apply more ink to extend the limit of the print color gamut or to compensate low ink densities , are instrumental causing more dot gain .
Few other factors influencing dot gains are :
Ink and water balance , blanket properties , blanket height , roller settings , bearer pressure , cylinder gaps , plate wear , LPI employed and ink temperature . Paper lint accumulation on blanket and plates will show more gain . The gain will change with the speed of printing , also ! The positive point is that dot gain is measurable , predictable and controllable . The pre press professional carry more responsibility for measuring , controlling and compensating the press related dot gain , during pre printing digital processes . The performance of the press , on dot gain is tested ; analyzed and corrective measures are applied in digital files . For this test files are created and the printing is done . Dot , at various half tone percentages are plotted . The check points in high light , shadow and mid tone are given more attention . Excess dot gain makes the dot pattern solid in the shadow areas . High pressure may be one of the major causes . Loss of details in the highlight areas indicates plate exposure and plate processing defects . Look for reasonable dot patterns in highlight 1 to 4 % and in shadows of 95 %. Consequent of thorough study the pre press engineers are to generate appropriate correction curves to compensate the press dot gain during RIP . The curves are to be built for every definite combination of press , paper , ink and applied accordingly .
Color printing is really the printing of four or more monochromatic half tone dots of different color inks . Each ink will induce different dot gains and hence the compensation curves are to be generated and saved for each color . This brief introduction to dot gain , we are sure will give a paradigm shift in the readers perspective about dot gain and will shift their focus from worrying on the dot gain in their press to employing the corrective application in pre printing arena .
- KEYYEN
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