Means and methods of printing quality control
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Faced with China's accession to the WTO, fine prints are under severe pressure. I believe that all peers will put quality management at the top of the list, because in the market competition, "customers are God." It is already the foundation of enterprise development. Let's talk about some of my experiences and methods of offset quality control.
First, Elements and analysis of print quality control
The elements of print quality control are: dot, the true degree of color reproduction, and the stability of the printing process. The mesh value (mesh size) of the overprint area is the most important control element. Of course, there are other factors that affect the quality of printing, such as ring white spots, paste, dirty, and inaccurate overprinting. The reason for the customer's dissatisfaction is that the color of the original and the printed pattern changes. In general, the main reason for the difference between such manuscripts and prints in the printing work is the improper printing process. In color printing, ink film thickness, dot size, ink overprinting, and color registration order are the main factors affecting the faithful reproduction of color. Of course, it is necessary to pay attention to whether the judgment of color deviation is often affected and restricted by various external environments. Now analyze these three factors:
1, ink film thickness
When the printing plate is finished, the printer can only affect the thickness of the ink film. At this time, it is necessary to have objective measured parameters and objective measurement methods to identify the thickness of the ink film. For coated paper, the thickness of the ink layer should be controlled between 0.7 and 1.1 um. If the thickness of the ink film exceeds this range, the color space to be reproduced is reduced. In general, more advanced presses, equipped with on-line or manual densitometers, continuously monitor the thickness of the ink layer and make adjustments as needed.
2, the size of the outlet
The size of the dot is another decisive factor in determining the quality of the printing. The plate making and offset printing processes will change the dot. Therefore, the dot enlargement, sharpening, deformation and ghosting of each process must be strictly controlled. We can use the printing control strip and the simple tool of the magnifying glass to monitor the change of the dot. During the offset printing process, the dot enlargement phenomenon occurs. What is required is to increase the dot gain within the standard range before the printing process. In general, the film for offset printing and the dot enlargement of printed matter are:
FF: film dot size: 20%, 40%, 80%, 100%
FD: Printed dot size: 30%, 55%, 90%, 100%
Sometimes we use a densitometer to measure the density value of the halftone area, and the solid density uses the Murry-Davies formula to calculate the mesh value of the halftone area: F(%)=(1-10-F)/(1- 10-D)
When density is measured with a densitometer, it must be previously zeroed on the non-information surface portion of the paper used to eliminate the effect of the optical properties of the paper on the ink color. In fact, when we measure the density on the image of three or more different dot sizes in the field density, we can use the Murry-Davies formula to calculate the actual mesh value of the printed sheet, and based on these data and the dot size value on the corresponding film, we can draw A characteristic curve, the printing characteristic curve.
1 is the ideal line (can not be obtained under normal circumstances)
2 is the actual line
3 The middle area is the increase of the dot
In order to make the outlets expand and control within the standard range, the two different forms of outlets existing in the plate making and printing processes can be expanded and compensated. Photoshop uses different methods to compensate for the expansion of the dots caused by the printing and printing.
A: Network expansion and compensation measures in the printing process
Using File/CMYK The Setup dialog box is set to select the standard dot gain value and the custom dot gain curve to compensate for the dot gain.
Standard dot gain value: The standard dot gain value designed by the software according to different paper and ink matching.
Dot increase curve: The density value of the corresponding color block is measured by a densitometer, and the mesh value and the corresponding expanded value are calculated by the MD equation. From the DotGain pull-down menu, choose Curves to specify the dot gain value of up to 13 points. Of course, the expanded value of the dot size at other locations is calculated by software automatic interpolation.
B: compensate the dot gain with the transfer function
If the image output device used is not properly calibrated and not calibrated, the image may be reduced or enlarged when the image is transferred to the film. In this case, the transfer function of PostScript technology can be used (Transfer).
Function) to compensate.
Its steps:
(1) Output an image of C, M, Y, and K containing 13 gray blocks of the desired dot size on the output device to be used to obtain four color separation sheets.
(2) The dot area ratio of 13 color patches was measured by a transmission densitometer.
(3) Click on File/Page
In the setup dialog, the Transfer button enters the Transfer Function dialog.
(4) Calculate the required adjustments and enter them into the corresponding boxes.
Regardless of the compensation method, the adjustment amount is such that if the pixel value is 127, 50% of the dots should be output, but if the 60% of the output of the photo machine is expanded by 10%, the compensation adjustment should be 50%-10. %=40%, the box should be filled with 40%, so the photo machine will output at 50% of the network points that the user wants.
3, ink overprinting and color sequence
Since wet printing is now used in color printing, the color registration sequence has been standardized, generally K-C-M-Y. In order to facilitate ink overprinting, it is beneficial to arrange the thickness of the ink layer in order of small to large, which we stipulate:
Black ink thickness = 0.8um green ink thickness = 0.9um
Product ink thickness = 1.0um yellow ink thickness = 1.1um
Second, the role of printing control strips in the quality of offset printing
As mentioned above, the print control strip is used to control the color separation, plate exposure, color reproduction, proofing, and printing effects. Now there are various control bars such as Brunel, FOGRA, GATF, and Chinese Academy of Sciences. , but the performance is basically the same, generally consists of the following parts.
A: Real area
The print needs to have a high contrast, K=(DV-DR)/DV is mainly used to detect image contrast, dot change, ink uniformity, DV is solid density, and DR is 75% dot print density.
B: Overprint area
Mainly check the case that the first color accepts the second color and the third color, affecting the overprinting rate, thereby affecting the thickness of the ink layer.
C: Plate exposure control area
It is used to check the tightness of the film and the printing plate to determine the exposure of the predetermined plate resolution. Generally, the proof plate retains 2% of the small dots, and the printing plate retains 5% of the dots. Exposure control: Generally, it is 3 to 4 levels higher than the corresponding exposure of the original resolution of the plate.
D: ghosting and deformation
The deformation of the detection dot has a discriminating zone of longitudinal, lateral and bidirectional deformation, and can indirectly detect the pressure, whether the blanket is relaxed, whether the amount of ink and the amount of water are too large.
E: halftone area
Detecting the expansion of intermediate adjustment outlets
F: gray balance area
Used to detect color reproduction and achieve neutral gray balance is the key to faithful reproduction of colors.
It is worth mentioning that the control strip cannot be copied. The edge of the measuring strip provided by the manufacturer is clear. After copying, the edge is blurred and the dot is enlarged.
I am delighted to see that China's printing industry is gradually embarking on the road of dataization and standardization management. Although there are some gaps in the standardization of printed materials, the efforts of people of insight will definitely improve as soon as possible. Of course, for the control of offset quality, the experience of technicians is also an extremely important factor. I believe that under the joint efforts of many parties China's offset printing quality control, quality management and the use of color management will definitely make the quality of printed products a step.