With the development of society, material life is becoming more and more abundant, and people's demand for spiritual culture is also increasing. Because of its artistic, decorative, and beautified characteristics, packaging and printing products are more impacted and affected.
If printing enterprises want to be invincible in the fierce market competition, it is very necessary to establish a new concept of quality competition, improve technology, strengthen inspection, and establish a unique quality assurance system. Taking cigarette and wine packaging as an example, today's products are more gorgeous and exquisite than before, and the technology is becoming more mature and complex. Gone are the days when the previous printing method could meet the requirements. A molded product often needs to be cut, offset, gravure, silk screen, bronzing, film glazing, adhesive box ... multiple processes before it can be put on the market. So to solve some problems that arise in the actual production process of multiple printing methods, each printing manufacturer must face it.
In the matching production process of screen printing and offset printing, the phenomenon of color misregistration due to the mismatch of the two printing methods is one of the more common problems. There are many reasons for its emergence. Through careful analysis and careful process control and automatic adjustment technology, it can minimize losses and improve benefits.
1. Pre-press design
As a designer, you must not only 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 be targeted in product design, give full play to strengths and avoid weaknesses. Only works that have both high aesthetic value and good printability can be made relatively long. In four-color screen printing, offset printing is tolerated by its high resolution and low cost. The screen printing can also reflect some special effects that other printing methods cannot achieve, such as: cloth pattern, ice flower, frosted and so on. Gravure printing has the characteristics of high-speed stability of offset printing and thick ink layer of silk screen, etc., which can be applied to large-area background printing.
In the production of film, the silk screen is stretchable because of its own. When making 1: 1 color film with offset printing, the main color pattern should be made slightly larger than the offset film. In this way, in the actual printing, the characteristics of thick screen ink layer and strong covering can effectively avoid the phenomenon of bottom leakage and air leakage. In the arrangement of process color, according to the characteristics of speed and stability of the printing machine, the principle of offset printing followed by screen printing should be followed as much as possible.
2. Control before offset printing
In the production process of multiple processes, problems in any one process may lead to disastrous consequences for product quality. Therefore, it is particularly important to do prepress control to eliminate hidden dangers in the bud. Here we mainly control the imprint of offset printing. The length of the offset print can be adjusted by adjusting the thickness of the offset paper wrapped in the offset roller. Use this method to adjust the offset print to the standard of film 1: 1 and then start printing. At the same time, it is necessary to ensure that the printing needle position is consistent and the gauge is good during the printing process, which lays a good foundation for the subsequent work.
3. Semi-finished product control
Paper expansion and contraction 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 way to reduce paper expansion and contraction. The first is the control of temperature and humidity in the printing workshop. Most printing companies are now equipped with central air conditioning equipment. The temperature and humidity of different workshops should be as consistent as possible. A temperature of 20 ° C to 23 ° C and a relative humidity of 40% to 60% are more suitable for printing. The cut paper is first placed in the workshop for pre-stretching to fully adapt to the environment. The Sichuan cling film is sealed when the printed semi-finished product is stored. Shorten the production cycle between the production processes. In the selection of raw materials, paper with low expansion and contraction rate is selected according to its own characteristics. Generally speaking, imported paper is better than domestic paper, and wood pulp paper is better than straw pulp paper. But the cost is relatively higher.
Fourth, screen production
Scientific and reasonable production of screen printing plates can also effectively improve overprinting. Screen tension is an important control element. A screen with too low a tension will cause the screen to detach slowly from the printed surface, affecting the stability of the color. At the same time, the continuous expansion and contraction reduces the service life of the screen. Moreover, the pattern is deformed, the print is elongated in the longitudinal direction, and even different degrees of elongation appear in the transverse direction. However, the use of deformed screen frames will cause flicker in printing, which will affect the accuracy of overprinting. Therefore, choose a good frame. According to the actual situation of the pattern, choose polyester mesh with different meshes. The tension is adjusted according to the number of days. When the mesh number increases, the tension decreases, and when the mesh number decreases, the tension increases, so as to avoid tearing the net due to excessive tension. For example, 300-mesh wire mesh is used in cigarette packs, with tension controlled between 20 and 23 N / cm. For a wire mesh of about 170 mesh, the tension should be above 30N / cm. Use the stretched net for a period of time to prevent it from rebounding.
V. Adjustment of screen printer
Through a series of adjustments to the pressure, angle, and screen pitch of the screen printing machine before printing, the best printing effect can be achieved at the lowest expansion. Squeegee blades usually choose 70 ~ 75 ° medium hardness rubber strip. The screen pitch is adjusted to 3 ~ 5mm, and the angle between the scraper and the screen is 60 ~ 75 °. Grind the squeegee into a flat edge, and the edge is on the center line of the (automatic machine) printing cylinder. Control the pressure of the scraper so that the pressure is the lightest when scraping out the best effect. The method is also very simple: reduce the pressure to the lightest, pour ink on the plate, and gradually increase the pressure through the scraping of 4 to 5 bad films until the desired effect is achieved. The pressure of the ink-returning knife should not be too heavy, so as not to lift the bar at the bite.
Six, silk screen film pre-shrink
After the above series of control and adjustment, for the platform type semi-automatic screen printing machine, the color registration problem is not very big. However, for the fully automatic screen printing machine that uses the cylinder as the printing platform, the phenomenon of inaccurate overprint still occurs. This has a certain relationship with the flat and round printing method designed by the automatic machine.
Adding the thickness of the printing material to the circular cylinder directly leads to the increase of the diameter of the circle. In addition, the characteristics of the screen itself are destined to be inevitably elongated, and the circumference of the corresponding circle will also change. Thus, longitudinal elongation of the print is inevitable. Here you can apply the formula of the circumference of the circle: C (circumference) = π (circumference) × D (diameter), roughly calculate the elongation of the print.
Ï€ = circumferential D = printing cylinder diameter A = paper thickness X = extended length
Calculation formula: X = π (D + A) -πD
After simplification, it is: X = πA
Assuming that the thickness of the paper measured with a micrometer is 0.3 mm, it is substituted into the formula to calculate X≈3.14 × 0.3 = 0.942≈1 mm, and the imprint elongation is about 1 mm.
Through the above data, the film is pre-shrinked in advance before plate making. Taking the twelve-pack cigarette pack as an example, the film is divided into four parts horizontally. Based on the second couplet, the first couplet moves up by 0.25mm, the third couplet moves down by 0.25mm, and the fourth couplet moves down by 0.5mm. Re-print trial printing, in most cases, has basically met the printing requirements. Fine-tune the shortcomings to achieve the best overprint effect.
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