Quality Control of Printed Products in Offset (I)

Simply speaking, the printing process is the process of copying the original textual information on the substrate surface. In this regard, to evaluate the quality of printed matter, the first thing to look at is the faithfulness of the printed matter to the original. We know that due to the limitations of equipment, materials, printing processes, etc., it is impossible to completely faith in the manuscript. Therefore, the high-quality copying we have mentioned means that the printed matter is very close to the original. At the same time, because of the difference between print and manuscript, we not only require a small difference between print and manuscript, but we also put forward the stability of this difference, that is, the stability of copying. This stability is similar to printing. is very important.

In addition to the standards for quality evaluation of prints mentioned above, we have extended the surface quality standard (empirical quality standard) of prints from the aesthetic point of view of prints as follows:

a. The ink is bright and the depth of the screen is uniform;

b. The ink layer is thick and glossy;

c. The outlets are clean, clear, and free of burrs;

d. Conforms to the original and has a clear color tone;

e. Accurate printing;

f. Text is not missing;

g. The printed sheet has no wrinkles, no imprints, stains and fingerprints, and the product is neat;

h. The back is clean and free of dirt.

From the time when the manuscript is subjected to gradation compression, color correction, color separation, printing plate making until the end of printing, each step in the process will affect the reproduction quality of the printed matter. In the next article, we will focus on the quality control of prints in the printing process.

As we mentioned earlier, due to the limitations of various conditions in actual production, our print copy cannot be completely consistent with the original. Let's make a hypothesis. When all the materials, equipments, and processes that we use to participate in printing reach our desired ideal state, our printed matter will be very close to the original. Through this hypothesis we will easily think that what we need to do in printing is to narrow the gap between the conditions and processes that we can control and change, and the ideal state. The smaller the gap, the better the quality of the prints we can get.

Below we discuss these conditions and processes separately.

First, the choice of printing supplies.

(1) Paper.

The nature of the paper will directly affect the quality of the reproduction of the printed matter. Therefore, in the design of the printing process, a trade-off must be made between the quality of the paper and the cost of printing (often, low cost and high quality cannot be obtained at the same time). The most ideal paper for print reproduction should have the following properties:

1 has absolute whiteness, that is, it is completely non-absorbable for various types of light. In this way, high image contrast can be printed. In addition, light can be prevented from entering the paper near the dot, and is absorbed by the dot from the inside of the paper, resulting in unnecessary optical dot gain.

In fact, paper with absolute whiteness does not exist at all, and none of the papers can completely absorb external light. In the printing process design engineering, what we need to do is to select as much whiteness as possible within the allowable cost range (except for products with special process requirements, of course) in order to meet the requirements for image reproduction in order to achieve richer level.

2 has a good gloss. Similar to the above, but the difference is that the gloss is not as great as possible, too much glossiness hinders the normal reading of printed matter.

3 smoothness. The smoothness of the paper refers to the roughness of the paper surface, and the ideal paper surface smoothness should be at least 500 seconds or more. If the smoothness of the paper during printing is too low, the high-profile dots will be easily lost, the low-key dots will be merged, and the real part will be empty.

• Also for the consideration of printing costs (sometimes due to the special effects of printed materials, such as textured paper), it is not possible to use high-quality, high-smoothness paper for each type of print. When encountering paper with low smoothness, the number of printing screens should be appropriately reduced during the process design, and the printing pressure should be increased during the printing process to make up for the surface smoothness of the paper.

4 has a neutral pH. Acidic paper can cause the ink to be unable to dry in the printing process. This can lead to over-emulsification of the ink in the printing process. As a result, a series of quality problems may occur in the printed product, such as blemishes of the printed material, blurring of the edges of the dots, and enlargement of dots. Deformation and other issues. When the pH value of the paper itself is alkaline, it reacts with a large number of pigments of the ink, which causes the printed product to fade after the printing is completed. This phenomenon is particularly evident in the printing of gold and silver inks.

· In the choice of paper, try to choose a paper that is near neutral, but if the paper has been selected, PH value has been determined, we must pass the pH of the dampening solution to the pH of the paper during the printing process. Make some degree of compensation. The general effect is also more obvious.

5 paper shape stability. The ideal paper should not be deformed, either under the influence of the printing pressure or under the dampening of the fountain solution. This ensures overprinting during the printing process and avoids a series of problems due to paper deformation.

• What we need to do is not to select paper that is not deformed at all, but to make trade-offs in the cost of the process and the highly stable paper. Such as the printing of newspapers, due to the low requirements and quality requirements are not very high, so when the choice of paper, stability is not very important, and newsprint can be accepted. For printing such as maps and other printed matter, we must choose papers with good stability to avoid overprinting problems caused by paper deformation and changes in the finished product size.

6 paper surface strength. The ideal paper requires no stripping and lint-free during the printing process, avoiding the wear of the printing plate due to this, and the reduction of image quality due to the adhesion on the blanket, printing plate, and ink roller.

· The paper we use in the printing process will generally fall off more or less during the printing process. In addition to selecting the paper before printing, we must judge the surface strength of the substrate and clean it in time during the printing process. Foreign material such as paper powder on the plate and blanket.

7 paper silk. The ideal paper is no silk direction, and the strength of the paper is large enough. However, as long as the cost is taken into consideration, we must not use paper with a lot of direction. In the printing, we must pay attention to the direction of the paper and the direction of the printer's paper, which can greatly improve the level of overprint.