Screen printing and offset printing production process

With the development of society, the material life is becoming more and more abundant, and people's demand for spiritual culture is also increasing. Because of its artistic, decorative and beautifying nature, packaging prints are more impacted and influenced.

For a printing company to be invincible in the fierce market competition, it is necessary to establish a new concept of quality competition, improve technology, strengthen inspection, and establish a distinctive quality assurance system. Taking cigarettes and wine packaging as an example, today's products are more beautiful and sophisticated than before, and the process is becoming increasingly sophisticated. The days when a previous printing method could satisfy the requirements are gone forever. A molded product often goes through the cutting process, offset printing, gravure printing, silk screen, bronzing from the convex, coating film polishing, sticking boxes ... ... multiple processes, in order to enter the market. Therefore, solving some problems that arise in the actual production process with various printing methods is also a must for each printing manufacturer.

In the matching production process of screen and offset printing, color misregistration due to the mismatch of the two printing patterns is one of the more common problems. The reasons for its emergence involve many aspects. Through meticulous analysis, careful process control and automatic adjustment techniques can be used to minimize losses and improve efficiency.

First, prepress design

As a designer, not only must have a certain basic knowledge of color science, but also have a better understanding of the advantages and disadvantages of various printing methods, so as to target the design of the product, give full play to their strengths and avoid shortcomings. Only works with high aesthetic value and good printability can be done for a long time. In the printing of four-color meshes, offset printing is not tolerated by its high resolution and low cost. The screen printing can also reflect the special effects that can not be achieved by some other printing methods, such as: cloth pattern, ice flower, frosted and so on. The gravure printing combines the high speed stability of the offset printing and the thick ink layer of the screen, which can be applied to a large area of ​​the background color printing.

In the production of film, because of its own stretch of the screen, in the production and offset 1:1 color film, the main color pattern should be slightly larger than the offset film. In this way, in the actual printing, the screen ink has a thick layer and strong hiding characteristics, which can effectively avoid the occurrence of leaking at the bottom. In the process of color arrangement, according to the speed, stability and other characteristics of the printing press, try to follow the principle of first screen after offset printing.

Second, offset printing prepress control

In a multi-process production process, problems in any one process may lead to catastrophic product quality. Therefore, it is especially important to do a good job in prepress control to eliminate hidden dangers in the bud. Here we mainly control the print of offset printing. By adjusting the thickness of the offset cylinder liner, the length of the offset print can be adjusted. In this way, the offset print was adjusted to a standard 1:1 printing restart with the film. At the same time, during the printing process, it is necessary to ensure that the printing needles are consistent and that the gauge distance is good, which lays a good foundation for the subsequent work.

Third, semi-finished product control

Paper stretching is also one of the factors that affect the accuracy of color registration. Strengthening the process control of semi-finished products is a more effective means of reducing paper shrinkage. The first is the control of temperature and humidity in the printing shop. Most printing companies now have central air-conditioning equipment. The temperature and humidity of different workshops should be as consistent as possible. Temperature 20 °C ~ 23 °C, relative humidity 40% ~ 60% is more suitable for printing. The cut paper is first pre-stretched in the workshop to fully adapt to the environment. The printed semi-finished products are sealed with cling film. Between production processes to shorten the product production cycle as much as possible. The choice of raw materials, according to their own characteristics of the use of low expansion rate paper. In general, imported paper is superior to domestic paper, and wood pulp paper is superior to straw paper. Only the cost is relatively high.

Fourth, screen production

Scientific and reasonable production of screen printing plates can also effectively improve overprinting. The tension of the screen is an important control element. A screen with low tension will cause the screen and the printed surface to depart slowly, affecting the color stability. At the same time, the continuous expansion and contraction state reduces the service life of the screen. In addition, the pattern is deformed, the print stretches in the longitudinal direction, and even in the transverse direction, different degrees of elongation occur. The use of a deformed mesh frame will cause the phenomenon of rollover in printing and affect the accuracy of overprinting. Therefore, use a good frame. According to the actual situation of the pattern, select different mesh mesh screens. The tension is adjusted according to the height of the mesh. The number of meshes increases tension and the number of meshes decreases, the tension increases, thus avoiding tearing the mesh due to excessive tension. If the cigarette bag is used in more than 300 mesh screens, the tension is controlled between 20 and 23 N/cm. The screen mesh around 170 mesh should have a tension of 30N/cm or more. Stretch the mesh for a period of time before use to prevent it from rebounding.

Fifth, screen printing machine adjustment

Through a series of adjustments of the pressure, angle, and net distance of the screen printing machine before printing, the best printing effect can be achieved at the lowest expansion and contraction. The squeegee usually chooses a 70-75° middle hardness strip. The pitch adjustment is 3~5mm, and the angle between the scraper and the screen angle is 60~75°. Grind the squeegee into a flat mouth with the cutting edge on the centerline of the (automaton) impression cylinder. Control the pressure of the blade so that the pressure is lightest when the best effect is achieved. The method is also very simple: reduce the pressure to the lightest, pour ink on the plate, and then increase the pressure by 4 to 5 bad films, until the desired effect is achieved. The pressure of the ink-returning knife must not be too heavy, so as to avoid biting at the mouth of the graphic.

Sixth, screen film shrink

After the above series of controls and adjustments, the problem of color registration for platform-type semi-automatic screen printers is not very large. However, overprinting is still unacceptable for fully automatic screen printers that use a cylinder as a printing platform. This has a certain relationship with the printing method of the flat press circle designed by fully automatic machine.

Adding the thickness of the substrate to a circular cylinder directly results in an increase in the diameter of the circle. In addition, the properties of the screen itself are also destined to be elongated, and the circumference of the corresponding circle will also change. Therefore, it is unavoidable that the longitudinal extension of the imprinting occurs Here we can apply the circle's perimeter formula: C (perimeter) = π (circumference) × D (diameter) to approximately calculate the elongation of the imprint.

Ï€ = pi

D = cylinder diameter

A=paper thickness

X = length of extension

Formula: X=Ï€(D+A)-Ï€D

After simplification, it is: X=Ï€A, assuming that the thickness of the paper measured with the micrometer is 0.3m, which is substituted into the formula

X≈3.14×0.3=0.942≈1mm, and its imprint stretch is about 1mm.

Through the above data, the film is pre-shrinked before the plate making. Take the 12-pin fight cigarette packet box as an example, horizontally divide the film into four equal parts. Based on the second joint, the first joint is moved up by 0.25 mm, the third joint is moved down by 0.25 mm, and the fourth joint is moved down by 0.5 mm. Re-plating test overprinting, in most cases has basically met the printing requirements. Fine tune the deficiencies to achieve the best overprint effect.

Source: Print Magazine Hubei Guangshui Guangcai Printing Co., Ltd. (Wu Qiang)