Dot gain correction in color separation of the hot

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Correction of dot gain in Flexo color separation

what is dot gain? Mile Southworth defines it as the change of the size of the photosensitive film and the printing point of the printing plate. In the whole production process from scanning to printing, the point gain is everywhere. Printing test is a tool for evaluating the point gain, and the point gain compensation is realized by adjusting the characteristic curve of the final output film. In order to improve the product quality and shorten the product life, the point gain must be found, measured, adjusted, monitored and controlled in time. There are two types of point gain: the mechanical gain caused by the physical growth of the point and the optical expansion of the apparent density change due to the scattering of the reflected light from the point. We will discuss the mechanical point gain more deeply

adjust the mechanical point gain

mechanical gain is the result of the joint influence of many prepress variables. The variables that affect the dot gain of flexo include: plate material, lining (model and thickness change), ink viscosity, timing ink supply system, dot radial runout of the plate, pressure, service life and stamping conditions, number of lines and wear of the embossing roller, film condition and humidity. For the whole process of flexographic printing, the accurate measurement of point gain is relatively strict. The first step in the measurement of flexographic point expansion is to conduct printing test on your printer

at present, there are many commercial and non-commercial test methods, which have their own characteristics in specification, design and appearance. These tests are to give the color separation machine a color separation information, which generally includes a set of specific color separation information required for a specific printing, including printing color sequence, field density, point gain, optimal line number, gray balance, film opacity. Before printing test, the variables such as plate material, lining, density and embossing roller must be determined. You can rest assured about the manufacturer of photosensitive resin. Moreover, each plate brand has many models, and each model has unique printing characteristics. We should choose the printing plate with better ink residue and tolerance. The next variable is the lining. In the ideal situation of the printing press, printing plate and paper, the printing plate of the rigid cylinder has a small dot expansion and does not need lining. But in practical work, chromatic printing must be lined. The field density is an important characteristic not only in the color separation setting, but also in the printing contrast. The printing contrast affects the overall color characteristics of the print. We generally set the final density between 120 and 135, which requires ink with high color saturation

finally, it is more important to choose the wrinkle roll. You can choose any test in the product catalogue. Generally, you choose the chrome roll of line 360 or the chrome roll of line 440. You can adjust the wrinkle roll according to the test results. When printing test variables on the machine, we must calibrate more variables and measure and record them, especially the printing speed and pressure. The printing speed will cause an interesting dynamic balance; You need high-speed in the construction period, but high-speed will affect the printing quality. In the test, we use three speeds to observe the influence of tortuosity modulus measurement speed on the printing quality and point gain. When testing the pressure setting, two different pressures can be set. We can see the tolerance of plate material and lining to the influence of pressure, as well as the change of point gain

when setting the pressure, you can ignore the point gray scale first, and then set the pressure at a place where the point gain progression is clear for testing. So we can analyze the best and more compromise pressure settings. You can take a few examples, and then combined with the parameters in front, I can improve the technical level and core competitiveness, and we can get the effects of the roller, printing speed and pressure. After the above three results are collected and labeled, we can analyze the data. Before that, we have to use the transmission densitometer to measure the film of the printing plate. The transmission density can measure the amount of light passing through the half color negative film and display it in the form of dot percentage. The name and pressure adjustment of the tested object must be measured and recorded. Next, the paper must be measured with a reflection densitometer. The operator tests the field, blank and halftone areas respectively. The densitometer calculates the percentage of points after analyzing the three information. The next step of point gain is to draw a copy curve. After drawing the characteristic curve according to the test results, the curve will show the increase in the percentage of point gain from film to print. In order to evaluate each separation variable independently, we draw a curve for each variable

when analyzing the test results, we can't ignore the optical gain. Although the optical gain can't be controlled, as a visual phenomenon, you should identify it and include it in the total point gain. The optical gain is an illusion caused by the human eye can't see all the reflected light of the print. This illusion is mainly caused by the scattering of the reflected light of the halftone point. The optical point enlargement is not physical, but the enlargement of the area of the point missed by the human eye instrument and not measured

the printing test results provide several duplicate curves for the color separation workers. In the selection of the best curve, the scanning workers will evaluate the curve, and then compensate for the phenomenon caused by the point expansion such as gray balance and intermediate tone expansion. We must determine the highlight, extreme highlight, minimum point, virtual light mask and GCR. GCR is a technology that uses black to replace the neutral gray overprinted by trichromatic ink. The digital information obtained by the scanner can be output to the film or saved for later operation. The test results are input into the system and point generation table, and then added based on the test results. For example, if 35% points are printed and 10% points are expanded at 25%, the point generation table will read 25% points for output. The color electronic prepress system with curve compensation function can revise the version, arrange the pictures and texts, and adjust the overprint image

because the electronic file has no design size and point structure, we can adjust the point size, line number and curve characteristics at will. Whether electronic operation or not, the final result we get is to check the accuracy of the film. At every step of the production process, we need to pre proofing. If the film is electronically corrected, the curve compensation proofing sheet is compared with the original scanning film proofing, so as to evaluate the curve compensation and obtain the ideal point gain. There are many types of pre proofing, each of which has its own advantages and disadvantages. Each color separation worker must find a suitable type. The widely used pre proofing method is the toner method, that is, the toner is adhered to the adhesive layer to make various color plates, which can be mixed and matched to different ink hues and densities. For those printers with small color gamut and ink density less than 130, strive to continue to expand the lithium salt industry in 2 to 3 years; In the downstream brush, this has great advantages. Because the hand coated toner is easy to produce unevenness, the automatic adjustment device is generally used, and the overexposure method is used. This proofing method can simulate the point gain. Another kind of pre proofing is the pre toner layer. This method distributes swop ink and is constant in size and density. Although it is limited due to lack of flexibility, this method has good uniformity. In addition, there are color overlay pre proofing, electrostatic method and inkjet method. The last one is flexo proofing, which uses flexo ink, scraper, spinning roller, central embossing roller and rubber plate or photosensitive resin plate to proofing on paper. Each time there is only one color, and each time there is only one complete maximum resolution. But by making different flexographs and changing the ink hue, viscosity and embossing pressure, you can better simulate the actual printing

copper engraving in point gain analysis, we only focus on photosensitive resin. In fact, copper engraving is also related to the printing quality of chromatic printing. When engraving copper plate, the copper plate must be tested. After the test, we will conduct printing quality analysis and point gain analysis. The photosensitive resin plate is compensated during color separation, but the copper plate is not. It is compensated during engraving. Due to the inherent point loss during deep engraving, the pre proofing color is too dark. After engraving, we usually use overprint ink for proofing to check the color balance, and then the copper plate can be etched to change the point size to better match the color, and then proofing for customer proofing; Finally, for the gain compensation of printing time point, it is generally necessary to conduct black-and-white proofing of each color group before etching to observe the expansion of printing time point

in order to ensure print quality (color, registration, etc.), point enlargement must be defined and compensation controlled. Remember that point gain compensation must be based on printed test results

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