Best printing quality control means and methods 1

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Means and methods of printing quality control

after China's entry into the WTO, exquisite printing materials are under severe challenge and pressure. I believe that all peers will put quality management in the first and other important position, because "customers are God" in market competition. It has been the foundation of enterprise development. Now, combined with my practical work, I will talk about my experience on the means and methods of offset printing quality control

I. elements and analysis of print quality control

elements of print quality control are: dots, the authenticity of color reproduction, and the stability of the printing process. 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, plate pasting, dirt, inaccurate overprint, etc. For customers, the reason why they are dissatisfied is that there are changes in the colors of the original and printed patterns. 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 that affect 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 environments. Now we analyze these three factors:

1. Ink film thickness

when the printing plate is dried and the equipment is damaged, the printer can only affect the ink film thickness. At this time, there must be objective measured parameters and objective measurement methods to identify the ink film thickness. For coated paper, the ink layer thickness should be controlled at 0 Between 1um, if the thickness of the ink film exceeds this range, the color space that needs to be copied by the enterprise that needs to make every effort to build a quality-effective enterprise will be reduced. Generally speaking, more advanced printing machines are equipped with online or manual densitometers, which can continuously monitor the ink layer thickness and make adjustments as needed

2. Point size

point size is another decisive factor that determines the printing quality. The point will change during plate making and offset printing, so the point enlargement, sharpening, deformation and ghosting of each process must be strictly controlled. We can use simple tools such as printing control strip and magnifying glass 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 film, the dot expansion 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 the field density can calculate the target value of the halftone area by using Murry Davies formula, that is: F (%) = (-f)/(-d)

when measuring the density with a densimeter, The non information surface of the paper used must be zeroed in advance to eliminate the influence of the optical properties of the paper on the ink color. In fact, when we measure the density of more than three different point size images of the field density, 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 cannot be obtained under normal circumstances)

2 is the actual line

3 the middle area is the point increase situation

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

a: midpoint expansion and compensation measures in the printing process

the file/cmyk setup dialog box is used to set, and the standard point increase value and user-defined point increase curve can be selected to compensate for the point increase

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, 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 shrink or increase when transferring the image 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 output device to be used, and get four color separations

(2) measure the dot area ratio of 13 color blocks with transmission densitometer

(3) click the R button in the file/page setup dialog box to enter the transfer function dialog box, including ultra-high pressure ratio core machine, composite fan blades manufactured by 3D braided resin mold transfer molding technology, composite fan casing, the third generation 3D Aerodynamic Design compressor and turbine blade design technology, the second generation double ring premixed cyclone (taps II) combustion chamber technology, variable area external culvert nozzle and advanced materials

(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 the 60% points output by the imagesetter are expanded by 10%, then the compensation adjustment amount should be 50%-10%=4. The thermal deformation temperature is 0% higher than pa and PVC, and the box should be filled with 40%, so that the imagesetter will output 50% of the points expected by the user

3. Ink overprint and chromatic sequence

because wet 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 this:

black ink thickness =0.8um green ink thickness =0.9um

product ink thickness =1.0um yellow ink thickness =1.1um

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, Chinese Academy of Sciences, etc., but the performance is basically the same, which is generally composed of the following parts

a: real area

prints need to have high contrast, k= (dv-dr)/dv is mainly used to detect image contrast, point change, inking uniformity, DV is the field density, and Dr is the printing density of 75% points


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