Offset product quality control (on)

I. Data digitization and standardization of offset printing

The quantitative standards for quality of offset printing products must be based on data and standardization. In a broad sense, data is the basis for statistics, calculations, scientific research, engineering design, etc.; norms are summarized through practice as a standard prescribed by the text. The combination of the two is standardization.

Second, standardization in offset printing production process

In the offset printing production process, from the electricity points → → repair → printing → proofing → printing → finished product inspection → cutting, etc., are in accordance with certain procedures. Standardization is the place where all available data is expressed. Through testing means, the data under various conditions are measured and recorded, and they are summed up to formulate formulas, curves, charts, etc. that can guide the production laws, and the data and curves are used to formulate the procedures. The operating standard. Then, organically unite the interdependencies and interactions among the various processes in the whole process into a unified whole, and fix or relatively stabilize the process relationship with the relevant data and specifications, so as to be able to follow To guide production practice.

1. The standardization of the method. The rules for offset printing methods include procedural descriptions and instruction manuals. The program description is to specify the order in which to work, determine what procedures, what materials to use for inspection, how to check, and specify the responsible person. It is the comprehensive information of various standard instructions. The instruction manual guides the specific work of each process after the operation sequence is determined.

2. Standardization of materials. Various materials such as ink, paper, blanket, dampening solution, printing plate, etc., are determined as standard equipment. If it can be unified into a single best, but in view of the requirements of customers, according to the standards and special requirements of the product quality, the material will be developed into standard parts.

3. The standardization of the machine. The same type of equipment is best to purchase devices with similar process data. If the size of the mouth of the offset press is the same, it will facilitate the standardization of the printed version.

4. Standardization of personnel. Printing technology often depends on human skills. If you do not understand the technical level of the operator, you cannot print the same quality product even if you use the same machine. Therefore, it is necessary to identify the technical level of each person, formulate corresponding appraisals, formulate corresponding training standards, and establish post responsibility systems and assessment standards.

5. Environmental and other standardization. In addition to the above standards, the standardization of temperature, humidity, and lighting safety management is also important, and management standards should also be established.

The process of performing, checking, and processing this sequence of operations in the production process is called quality management, as shown in Figure 14-1.

Proofing is the inspection of the plate in the platemaking department, and the printing department becomes the printing standard. Printing is to check the product according to this standard. The quality management of printing is actually for the standard (proofing), execution (printing) inspection, and the result is fed back to the next proofing and printing.

Proofing is a check on the plate making. It is a sample of the product for the customer, but from a printing point of view, it should always be the standard. Therefore, if there is a situation where the proofing and formal printing are inconvenient, as a standard proof, it is not correct. For example, in the case of a cyan ink density of 1.4 and a magenta ink density of 1.6, although the product sample is desirable, the ordinary magenta ink has a density of 1.6 at the time of the printer printing. In the case of printing, it is too high. Although the actual part is good, the outlets are thicker and all are printed in red. If the dots are not thickened, the density of the solid part will decrease.

Fourth, the main parameters of offset printing quality control

1. Ink layer thickness control. The density of printed matter is closely related to the thickness of the ink layer transferred to the paper. The thickness of the ink layer is not only closely related to the density of the printed material, but also has a close relationship with the dot gain, overprinting, printing failure in the field, and various other printing problems. In general, the thicker the ink layer printed on the paper is, the darker the ink is; the thickness of the ink layer is the same, and the darkness of the ink is also comparable. Therefore, controlling the change in the thickness of the ink layer also controls the darkness of the ink and the uniformity of the ink color. However, it is difficult to determine the thickness of the ink layer on the paper. Generally, the thickness of the ink layer is characterized by the solid density of the printed matter. Figure 14-2 shows the relationship between the density of the solid and the thickness of the ink layer. It is an experimental curve obtained by changing the thickness of the ink layer on a printability tester and then measuring the reflection density of ink layers of different thicknesses. The curve shows that the in-situ reflection density of the print increases with the thickness of the ink layer of the print. When the ink layer thickness increases to a certain value, the ink's on-site reflection density also reaches a maximum D saturation, and even if the ink layer thickness continues to increase, the on-site reflection density no longer increases. The in-situ reflection density of various inks varies with the thickness of the ink layer and is similar to the curve shown in Figure 14-2. However, different colors of ink are printed on the same paper, and the maximum value of the maximum reflection density obtained is different. For example: printing ink, cyan, magenta, and yellow inks on coated paper. When the thickness of the ink layer reaches 1.1 μm, the thickness of the ink layer is increased. The field reflection density no longer increases and the maximum reflection density value measured is obtained. For: black: 1.80-2.00; blue: 1.45-1.70; magenta: 1.25-1.50; yellow: 0.9-1.05. Different papers, because of the different penetrative distance and oil absorption degree of the pigment, although the thickness of the ink layer is the same, the density of reflection in the field is also different. Such as the general coated paper up to 1.5; offset paper 1.2-1.4; ordinary newsprint is below 1.0.

When proofing, the actual reflection density values ​​of the just-printed proof-quality proofs are measured. As the printing standard, the ink color of the two can be approximated. At the same time, in the printing process, the spot reflection density of the printed matter markings can be measured by sampling to check whether the ink supply is adequate and whether the ink supply of the entire print is uniform. However, between the printing press and the proofer, the adjustment of ink, printing pressure, ink temperature and speed are all different. Therefore, under normal circumstances, even if the thickness of the ink layer is the same, the dots of the printing press and the proofs of the proofer are increased. As a result, the printing machine's printing plate is generally lighter than the printer's printing plate. In the printing press, if you want to control the dot gain and reduce the dirt on the back, the thickness of the ink layer should be less than 1.2μm. Generally, the density of yellow ink is about 1.0, and the ink layer thickness is printed within 1.2 μm. If the magenta ink is printed with a density of 1.5 or less, its dot gain and back blemishes will be reduced. Blue ink should be controlled below 1.6.

In addition, according to different models (monochrome, two-color, multi-color), printing color sequence is also very important for printability and print quality, this issue is not discussed here.

2. Dot gain value (ZD). The dot enlargement value ZD refers to the difference between the dot area (FD) of a part of the printed matter and the area (FF) of the corresponding part of the original plate, that is, ZD=FD-FF. The dot gain is measured to control the change in dot size.

Density meter can be used to determine the density value of the solid part (or color mark) of the print, and the density value of the print dot part (or the test piece mesh block) can also be measured. which is:

FD=1-10-DR/1-10DV

Where: DR - dot density value;

FD - percentage of outlets;

DV - The field reflection density of the printing ink.

The above formula can be used to calculate the actual percentage of dots on printed sheets.

In general, the percentage of dots on the original is fixed. It determines the reproduction of a print tone and the reproduction of colors. It is a parameter that provides absolute quality. This parameter is determined by color screening and proofing. The actual percentage of outlets is changed due to the influence of various factors in printing, and the percentage of increase in outlets is smaller, indicating that the closer the percentage of actual outlets on the printed sheets is to the percentage of original outlets, the better the printing quality.

The thickness of the ink layer has a great influence on the dot gain percentage. The proper control of the ink thickness has a very practical significance for controlling the dot gain of printed products.

3. Print contrast. Print contrast, also called print contrast, is the ratio of the difference between solid density and dot density to solid density, expressed as K:

D=1-DR/DV

The value of K is generally between 0-1. The larger the value of K, the smaller the ratio of dot density to field density, the larger the printing contrast and the smaller the dot gain. Conversely, the smaller the K value, the more serious the dot gain, and the smaller the print contrast. Figure 14-3 shows the relationship between printing contrast and ink thickness.

When the ink layer thickness is δ0, the print contrast K value is the largest. When δ<δ0, although the dot edge expansion does not occur, the amount of ink is insufficient. When δ>δ0, the ink layer is too thick and the edge of the mesh is expanded. Only at the maximum value of K, there is sufficient ink, and at the same time the minimum dot gain is the best ink supply point.