Today's printing industry is becoming more and more competitive, and offset printing technology is becoming more and more mature. In order to improve the book printing quality, it is often necessary to start with printing details, and consider its production accessories and process technology meticulously and thoughtfully. Offset printing has the characteristics of low printing pressure, small printing deformation value, good original reproduction effect, soft tone, clear dots, rich printing layers, fast printing and less loss, so it is widely used in China photo book printing and other colorful printing projects. It is especially suitable for printing color transition dot version, lines, small characters, etc. Now, based on my years of work experience, I will discuss with my colleagues in the industry on the control of offset printing process technology.

First. Design plate making process technology control

1. The printing solid layout should avoid small characters, lines and transitional dots in the same color version, so as to avoid the phenomenon of smearing, dot expansion or insufficient ink layer due to the inability to take into account pressure and ink volume, which will affect the product quality.

2. If there is overprinting between the printing plates, the overprinting edge of one of the plates must be properly expanded and contracted to overcome the phenomenon of "whitening" of the edge caused by inaccurate overprinting.

3. When printing with four colors, pay attention to the angle of the network cable when making the plate and outputting the film. Under normal circumstances, the angle of the screen cable is Y90°, M45°, C75°, K15°, and the screen cable angle of the main color version is generally 45°.

Second. Process technology control in printing

1. Temperature and humidity control of the printing environment.

Controlling the temperature and humidity of the production workshop is an important part of improving the overprinting accuracy of the product. The temperature and humidity of the offset printing workshop should be kept relatively balanced and the temperature should be kept between 20 and 25°C, and the humidity should be kept between 50% and 60%.

2. Reasonable selection of offset PS version.

PS plate plays an important role in offset book printing. It is mainly composed of photosensitive layer and plate base. The thickness is generally 0.15~0.3mm. The photosensitive layer after exposure is required to have good lipophilic and the plate base should have good hydrophilic. The PS version should meet the basic properties of flat layout, uniform thickness, stable size, not easy to deform, clean surface, good dot reduction, high printing durability, good ink balance during printing, and not easy to get dirty.

3. Reasonable selection of blankets.

As a medium for imprinting transfer, blanket has a great influence on the quality of offset printing products. Correct selection of blanket plays an important role in improving product quality. The blanket is composed of synthetic rubber and textile cloth, and the textile cloth layer is usually made by laminating 3 to 4 layers of fabric to the synthetic rubber. The selection of the blanket should be based on the principles of good appearance quality, high flatness, good elasticity, good ink transfer, small compression deformation, high tensile strength, small elongation, and good surface rubber properties. In addition, different types of blankets should be selected according to different printing materials, inks and graphics.

4. Control of offset printing ink balance.

There are many factors that affect the balance of ink and water in offset printing, including the thickness of the imprinted ink layer, the properties of the printing paper, the performance of the ink, the temperature and humidity of the workshop, and the fountain solution. Among them, the characteristics of the fountain solution have a great influence, especially the pH value and conductivity of the fountain solution directly affect the quality of the printing plate, the drying speed of the ink, and the interfacial tension. Under normal circumstances, the conductivity of the fountain solution should be between 800~1200μΩ/cm, preferably between 800~900μΩ/cm, if it is higher than 1200μΩ/cm, the emulsification of the ink will be aggravated, resulting in blurred dots; If it is lower than 800μΩ/cm, it will cause a dry plate and make the blank part of the printed product dirty. The pH value of the fountain solution should be kept between 4.5 and 5.5, when the dots are the clearest. If it is lower than 4.0, it is easy to drop, and if it is higher than 5.5, it is easy to paste. Therefore, to control the balance of ink and water, it is necessary to use the smallest amount of fountain solution supply as possible, so that a sufficient amount of ink is attached to the printed product, and the balance method of "large ink and large water" should be avoided.

5. Control of offset printing color sequence arrangement.

At present, the arrangement of the printing color sequence is not uniform. In actual production, the printing color sequence should be determined according to the characteristics of the samples, ink, paper, etc. and the specific conditions of the production workshop. Under normal circumstances, the printing color sequence arrangement should follow the following principles: the ink color with strong covering power is printed first, and the color with high transparency is printed later; the ink with a small amount of ink is printed first, and the ink with a large amount of ink is printed later.

6. Protection of printing semi-finished products and control of production cycle.

A high-quality product is often produced by a combination of multiple processes according to its product characteristics. During the production process, multiple printing and combination processes such as bronzing, embossing, and die-cutting will inevitably occur. If the temperature and humidity of the production workshop are unstable, the semi-finished products should be sealed with plastic film at this time, so as to avoid the expansion and deformation of the printed products, resulting in inaccurate overprinting in the subsequent process. At the same time, according to the production progress of the product, the production cycle should be reasonably formulated, and the various processes should be closely connected to reduce the occupation of the production workshop and improve the production efficiency.

Third. Check imposition quality

Layout inspection is the last process before the printing plate is put on the machine. If this process is not strictly controlled, it will bring great economic losses to the printing factory. In addition to checking whether the text and pictures on the layout are broken, less drawn, and false, the layout of the layout should also be checked in detail. If it is a continuous printing plate, it is necessary to check the deviation of the first and second printing plates of the four CMYK printing plates. Most of the printing workshops rely on visual inspection to inspect the quality of the printing plates. However, due to the small contrast between the graphics and the blank parts of the printing plate, long-term inspection will cause fatigue, and some errors are not easy to detect. A better inspection method is to pick up the ink in time for the washed printing plate and increase the contrast of the printing plate to adapt to the observation habits of the human eye.

The traditional prepress plate-making process is complicated. At present, most book printing factories have basically realized the upgrade of digital plate-making technology. The traditional plate-making technology has become a supporting role. It will definitely withdraw from the book printing process and be replaced by CTP imposition technology.