Printing quality control methods and methods

Faced with China's accession to the WTO, fine prints have been subjected to severe challenges. It is believed that all the peers will place quality management at the top of the list because the “customer is God” in the market competition. It is already the foundation of enterprise development. The following combination of practical work, talk about my experience in the means and methods of quality control of offset printing.

I. Elements and Analysis of Print Quality Control

The elements of the print quality control are: the true degree of the dot, color reproduction, and the stability of the printing process. The mesh value (the dot size) of the overprint area is the most important control element. Of course, there are other factors that affect the printing quality such as ring white spots, stencils, dirt, overprints, etc. For customers, the reason for their dissatisfaction is that there is a change in the colors of originals and printed patterns. In general, the main reason for the difference between this manuscript and the print in the printing job is: an inappropriate printing process. In color printing, ink film thickness, dot size, ink overprinting, and color registration sequence are the main factors affecting faithful reproduction of colors. Of course, what needs attention is whether or not the color cast is often influenced and restricted by various external environments. The three major factors are now analyzed:

1, ink film thickness

When the printing plate is finished, the printer can only affect the ink film thickness. At this point, there must be objective measured parameters and objective measurement methods to identify ink film thickness. For coated paper, the ink layer thickness should be controlled between 0.7-1.1um. If the ink film thickness exceeds this range, the required color space for copying is reduced. In general, more advanced presses equipped with on-line or manual densitometers can continuously monitor the ink layer thickness and make adjustments as needed.

2, network size

The dot size is another deciding factor in determining the printing quality. 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 print control strips and magnifying glass simple tools to monitor the changes in the dot, offset printing phenomenon occurs in the offset printing process, all we need to do is to control the dot gain in the standard range before the printing process. In general, offset film is used to expand the printed dots:

FF: Film size: 20%, 40%, 80%, 100%

FD: Printing outlets size: 30%, 55%, 90%, 100%

Sometimes we use the density meter to measure the density value of the halftone area, and the field density can use the Murry-Davies formula to calculate the mesh value of the halftone area: F(%)=(1-10-F)/(1- 10-D)

When measuring the density with a densitometer, it is necessary to zero the non-informative surface portion of the paper in advance to eliminate the influence of the optical properties of the paper on the ink color of the ink. In fact, when we measure the density on an image with more than three different dot sizes in the field, 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 (not available under normal conditions)

2 is the actual line

3In the middle area, dot gain

In order to make the dot gain control within the standard range, two different types of dot gains can be compensated for in the plate making and printing process. The dot gain caused by photoshop to plate making and printing is compensated by different methods.

A: Dot enlargement and compensation measures in the printing process

Using the File/CMYK Setup dialog box settings, optional standard dot gains and custom dot gain curves compensate for dot gain.

Standard dot gain value: Standard dot gain value based on experience based on different paper and ink matching.

Dot gain curve: Density meter to measure the density of the corresponding color block, using MD equations to calculate the mesh value and the corresponding expansion value, select Curves from the DotGain drop-down menu to specify the dot gain of up to 13 points Of course, the enlargement of the dot size in other locations is done by the software's automatic interpolation calculation.

B: Compensate dot gain with transfer function

If the image output device used is improperly calibrated and has not been calibrated, the dot 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 a C, M, Y, K image of 13 gray blocks with a desired dot size on the planned output device to obtain four color separations.

(2) The dot area ratio of the 13 color patches was measured with a 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 adjustments and enter them into the appropriate boxes.

Regardless of the compensation method, the amount of adjustment is as follows: if the pixel value is 127, 50% of the dots should be output, but the dot matrix machine output 60% of the dot network is expanded by 10%, then the compensation adjustment volume should be 50%-10. %=40%, 40% should be filled in the box, so that the imagesetter will output at the user's desired 50% outlet.

3, ink overprinting and color sequence

Since wet printing is used in color printing, the color registration sequence has been standardized. It is generally KCMY. In order to facilitate ink overprinting, the thickness of the ink layer is sorted from small to large. It is useful to provide:

Black ink thickness = 0.8um Green ink thickness = 0.9um

Ink thickness = 1.0um Yellow ink thickness = 1.1um

Second, the role of printing control in the quality of offset printing

As mentioned above, the print control strips are used to measure and control color separation, plate exposure, color reproduction, proofing, and printing effects. There are various measurement and control strips such as Bruner, FOGRA, GATF, and China Academy of Sciences. , but the performance is basically the same, generally consists of the following components.

A: Real area

Prints need 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 a print density of 75% dots.

B: Overprint area

The main inspection of the first color to accept the second color, the third color of the situation, affect the overprint rate, thus affecting the ink layer thickness.

C: Plate exposure control area

It is used to check the closeness of the film and the printing plate to determine the exposure of the resolution of a given plate. It is generally required that the proofing plate retains 2% of the small outlets, and the printing plate reserves 5% of the outlets. Exposure control: Generally 3 to 4 levels higher than the original exposure of the plate.

D: ghosting and deformation

There are vertical, horizontal, and biaxial deformations in the deformation of the inspection outlets, and the pressure can be detected indirectly. Whether the blanket is loose or not, and whether the amount of ink or water is too large.

E: Halftone area

Detecting the expansion of intermediate tone points

F: Ash balance area

Used to detect color reproduction and achieve neutral gray balance is the key to color faithful reproduction.

It is worth mentioning that: the measurement and control strips cannot be copied, the edges of the factory-supplied strips are clear, and the edges are blurred and the dots are enlarged after copying.

I am delighted to see that China's printing industry has gradually embarked on the road of dataization and standardized management. Although there are some gaps in the standardization of printed materials at present, they will surely be perfected as soon as possible through the efforts of people of insight. Of course, for the quality control of offset printing, the experience of technicians is also an extremely important factor. I believe that under the joint efforts of many parties, the quality control of offset printing in China, the application of quality management and color management, will surely make the quality of printed products reach a higher level. .