Plastic silk screen combined color
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With the development of society, material life is becoming more and more abundant, and people's demand for spiritual culture is getting higher and higher. Because of the artistic, decorative and beautifying life, the printed printed matter is more shocked and affected.
In order to be in an invincible position in the fierce market competition, it is necessary for a printing enterprise to establish a new concept of quality competition, improve the process, strengthen inspection, and establish a distinctive quality assurance system. Taking cigarette and wine packaging as an example, today's products are more gorgeous and beautiful than ever, and the process is becoming more mature and complicated. Gone are the days when a previous printing method could meet the requirements. A molded product often has to be cut into the market after cutting, offset printing, gravure printing, screen printing, bronzing, glazing, and adhesive packaging. Then, to solve some problems that appear in the actual production process of various printing methods, it is also a must for every printing manufacturer.
In the production process of screen and offset printing, the color misregistration caused by the mismatch of the two printing methods is one of the more common problems. The reasons for its emergence involve many aspects. Through careful analysis and careful process control and automatic adjustment technology, it can 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 be targeted in the design of products, to play their strengths and avoid shortcomings. Only works that have both high aesthetic value and good printability can be made longer. In the printing of four-color mesh, offset printing is at a high resolution and low cost. Screen printing can also reflect some special effects that other printing methods can't achieve, such as: cloth, ice, scrub. The gravure printing has the characteristics of high-speed stability of offset printing and thick ink layer of the screen, which can be applied to large-area ground color printing.
In the production of film, the screen has a stretchability. When making and printing 1:1 sets of film, the main color pattern should be slightly larger than the offset film. In the actual printing, the thickness of the screen ink layer and the strong cover are utilized, and the leakage of the bottom leakage can be effectively avoided. In the arrangement of the process color, according to the characteristics of the speed and stability of the printing machine, the principle of the screen after the offset printing is followed as much as possible.
Second, offset printing prepress control
In the production process of multiple processes, problems in any one process may lead to catastrophic consequences of product quality. Therefore, it is especially important to do pre-press control to eliminate hidden dangers in the bud. Here we mainly control the imprint of the offset printing. The length of the offset print can be adjusted by adjusting the thickness of the blanket cylinder liner. In this way, the offset print is adjusted to a standard 1:1 print with film. At the same time, the printing process should ensure that the printing needle position is consistent and the gauge distance is good, laying a good foundation for the later work.
Third, semi-finished product control
Paper stretching is also one of the factors that affect the accuracy of color registration. Strengthening process control of semi-finished products is a more effective means of reducing paper stretching. The first is the control of temperature and humidity in the printing workshop. Most printing companies now have central air conditioning equipment. The temperature and humidity of different workshops should be as consistent as possible. The temperature is 20 ° C ~ 23 ° C, relative humidity 40% ~ 60% is more suitable for printing. The cut paper is placed in the workshop for pre-stretching to fully adapt to the environment. When the printed semi-finished products are stored, the Chuanbao fresh film is sealed. Minimize product production cycle between production processes. In the selection of raw materials, paper with a lower expansion ratio is selected according to its own characteristics. In general, imported paper is superior to domestic paper, and wood pulp paper is superior to straw paper. It's just that the cost is relatively higher.
Fourth, the 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 too low tension will cause the screen to detach from the printed surface slowly, which affects the stability of the color. At the same time, the continuous expansion state 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 occur in the lateral direction. The use of deformed frames can cause cornering in printing, which affects the accuracy of overprinting. Therefore, choose a good frame. According to the actual situation of the pattern, choose a different mesh of polyester mesh. The tension is then adjusted according to the number of days. When the number of meshes is increased, the tension is reduced, and the mesh is increased, so that the tension is increased, thereby avoiding tearing the mesh due to excessive tension. For example, the 300 mesh screen in the cigarette pack is used more, and the tension is controlled between 20 and 23 N/cm. For a screen of about 170 mesh, the tension should be above 30 N/cm. Stretch the net for a while and then use it to prevent the net from rebounding.
Fifth, the adjustment of the screen printing machine
Through the pre-press calibration of the screen printer pressure, angle, and mesh distance, the best printing effect can be achieved under the minimum expansion. The squeegee usually selects a medium-hard strip of 70-75°. The mesh distance is adjusted to 3 to 5 mm, and the angle between the squeegee and the screen is 60 to 75°. The squeegee is ground into a flat mouth with the cutting edge on the centerline of the (automatic) printing cylinder. Control the pressure of the scraper so that it is the lightest when it is best. The method is also very simple: reduce the pressure to the lightest, pour the ink on the plate, and gradually increase the pressure by scraping the 4 to 5 bad pieces 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 mouth.
Sixth, screen film pre-shrinking
After a series of control and adjustments above, for the platform-type semi-automatic screen printing machine, the color registration problem is not very large. However, for the automatic screen printing machine with the roller as the printing platform, the phenomenon of overprinting still occurs. This has a certain relationship with the flat-press printing method designed by the fully automatic machine.
The thickness of the substrate is added to the circular roller, which directly leads to an increase in the diameter of the circle. In addition, the characteristics of the screen itself are also destined to be inevitably elongated, and the circumference of the corresponding circle also changes. Thereby, the elongation in the longitudinal direction of the print is inevitable. Here, the circumference formula of the circle can be applied: C (circumference) = π (pi) = D (diameter), and the elongation of the imprint is roughly calculated.
π = pi
D=printing roller diameter
A = paper thickness
X = elongation length
Calculation formula: X=π(D+A)-πD
After simplification, it is: X=πA
Assume that the paper thickness is 0.3mm with a micrometer and is substituted into the formula.
X≈3.14×0.3=0.942≈1 mm, and the stretch of the print is about 1 mm.
Through the above data, the film is pre-shrinked before the plate making. Take the 12-piece cigarette pack as an example, and divide the film into four in the horizontal direction. Based on the second joint, the first joint moves up by 0.25 mm, the third joint moves down by 0.25 mm, and the fourth joint moves down by 0.5 mm. Re-printing test overprinting, in most cases, has basically met the printing requirements. Fine-tune the shortcomings to achieve the best overprinting effect.