Best printing quality control means and methods 0

2022-07-27
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Means and methods of printing quality control

after China's accession to the WTO, exquisite printing materials are under severe challenge and pressure. I believe that all peers will put quality management in the first place, because "customers are God" in the market competition. It has been the foundation of enterprise development. The following is my experience on the means and methods of offset printing quality control based on my practical work

I. elements and analysis of quality control of printed matter. The target value (point size) of the overprint area is the most important control factor. Of course, there are other factors that affect the printing quality, such as circular white spots, paste, dirty, inaccurate overprint, etc. For customers, the reason why they are dissatisfied is that the colors of the original and printed patterns change. Generally speaking, the main reason for the difference between the original and the printed matter in the printing work is the improper printing process. In color printing, ink film thickness, dot size, ink overprint and chromatic order are the main factors affecting the faithful reproduction of color. Of course, it should be noted that the judgment of color deviation is often affected and restricted by various external bad environments. Now we will analyze these three factors:

1 Ink film thickness

when the printing plate is dried, the printer can only affect the ink film thickness. At this point, it will not be eliminated. There must be objective measured parameters and objective measurement methods to identify the ink film thickness. For coated paper, the ink layer thickness shall be controlled within 0.7-1.1 μ M, if the ink film thickness exceeds this range, the color space to be copied will be reduced. Generally speaking, the printing press is equipped with an online or manual densitometer to continuously monitor the ink layer thickness and make adjustments as needed

2. point size

point size is another decisive factor to determine the printing quality. Both plate making and offset printing processes will change the points, so the point enlargement, sharpening, deformation and ghosting of each process must be strictly controlled. We can use the tools of printing control strip and magnifying glass cylinder to monitor the change of points. During offset printing, point enlargement will occur. What we need to do is to control the point enlargement within the standard range before the printing process. Generally speaking, for offset printing film, the dot enlargement of printed matter is:

ff: film dot size: 20%, 40%, 80%, 100%

fd: print dot size: 30%, 55%, 90%, 100%

sometimes we use a densimeter to measure the density value of the halftone area, And field density the target value of the halftone area can be calculated by using Murry Davies formula, that is:

f (%) = - cost reduction section d/-d

when measuring density with a densimeter, the non information surface of the used paper must be zeroed in advance to eliminate the influence of the optical properties of the paper on the oil ink color. In fact, when we measure the field density and the density on three or more images with different point sizes, we can use Murry Davies formula to calculate the actual target value of the printing sheet, and according to these data and the point size value on the corresponding film, we can draw a characteristic curve, that is, the printing characteristic curve

① is the ideal line (which can not be obtained under normal conditions)

② is the actual line

③ the middle area is the point enlargement condition

in order to control the point enlargement within the standard range, two different forms of point enlargement compensation in the plate making and printing process can be used. Photoshop adopts different methods to compensate the point enlargement caused by plate making and printing

a: midpoint enlargement and compensation measures in printing process

what type of file to use depends on the use itself/CMYK setup dialog box settings. You can select standard point increment value and user-defined point increment curve to compensate for point increment

standard point enlargement value: the standard point enlargement value designed by the software according to different paper and ink matching and experience

point increase curve: measure the density value of the corresponding color block through the densimeter, calculate the target value and the corresponding expansion value with the MD equation, and select curves from the dotgain pull-down menu to specify the point increase value of up to 13 points. Of course, the expansion value of the point size at other positions is automatically interpolated and calculated by the software

b: use transfer function to compensate point increase

if the image output device used is not calibrated properly, the point may be reduced or increased when the image is transferred to the film. At this time, the transfer function of postscript technology can be used to compensate

its steps:

(1) output an image containing 13 gray blocks of C, m, y and K with the required point size on the planned output device to obtain four color separations

(2) the dot area ratio of 13 color blocks was measured by transmission densitometer

(3) click the transfer button in the file/page setup dialog box to enter the transfer function dialog box

(4) calculate the required adjustment amount and enter them into the corresponding box

regardless of the compensation method, the adjustment amount is as follows: if the pixel value is 127, 50% of the points should be output, but 60% of the lo points output by the imagesetter are expanded by 10%, then the compensation adjustment amount should be 50% - 10% = 40%, and the box should be filled with 40%, which means that the imagesetter will output 50% of the points desired by the user

3. Ink overprint and chromatic sequence

since mixed printing is used in color printing, the chromatic sequence has been standardized, generally K → C → m → y. in order to facilitate ink overprint, it is beneficial to arrange the ink layer thickness from small to large. We stipulate that:

black ink thickness = 0.8 μ M green ink thickness = 0.9 μ M

ink thickness = 1.0 μ M yellow ink thickness = 1.1 μ M

II. The role of the printing control strip in the process of offset printing quality

as mentioned earlier, the printing control strip is composed of various units used to measure and control color separation, plate exposure, color restoration, proofing and printing effect. Now there are many kinds of measurement and control strips, such as Bruner, Fogra, GATF and China Institute of science and technology, but the performance is basically the same. It is generally composed of the following parts

a: high contrast is required for real area

printed matter. K=dv-dr/dv

is mainly used to detect image contrast, point change and ink uniformity. DV is the field density and Dr is the printing density of 75% points

b: the overprint area

mainly checks whether the first color accepts the second color and the third color, which affects the overprint rate and thus the ink layer thickness

c: the exposure control area of the printing plate

is used to check the tightness between the film and the printing plate, and determine the exposure of the given image resolution of the printing plate. Generally, it is required to keep 2% dots for the proofing plate and 5% dots for the printing plate. Exposure control: generally, the exposure is 3 to 4 levels higher than the corresponding exposure of the original resolution of the printing plate

d: ghosting and deformation

the deformation of the detection point has the identification area of longitudinal, transverse and bidirectional deformation, and can indirectly detect the pressure, whether the rubber blanket is loose, and whether the ink volume and water volume are too large

e: halftone area

detects the expansion of the intermediate point

f: the gray balance area

is used to detect the color restoration. Realizing neutral gray balance is the key to the reproduction of color fidelity

it is worth mentioning that the measurement and control strip cannot be copied. The point edges of the measurement and control strip provided by the manufacturer are clear. After copying, it is found that the edges are fuzzy and slightly enlarged

I am glad to see that China's printing industry has gradually embarked on the road of digitalization and standardization management. Although there is some gap in the standardization of printing materials, it will be improved as soon as possible through the efforts of people of insight. Of course, for the control of offset printing quality, the experience of technicians is also a very important factor. I believe that with the joint efforts of many parties, the application of offset printing quality control, quality management and color management in China will certainly bring the printing quality to a higher level

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