Gravure printing process quality control
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First, the overprint system optimization settings
With the rapid development of electronic technology and computer technology, the combination between modern gravure printing machine and computer technology is getting closer and closer. Especially for the modern advanced high-speed gravure printing machine, the overprinting system mostly adopts the microcomputer to control the processing, and adjusts the overprinting deviation of the printed image through signal feedback.
1. Set the position of the overprint color according to the shape of the printed pattern
The overprint identification color code generally adopts a trapezoidal pattern. For the overprinting system to identify more effectively, when setting the color mark position, the similarly shaped pattern in the printed pattern should be avoided as much as possible, that is, do not let the printed pattern The similar shape and the color mark appear in the same scanning area adjacent to each other. Otherwise, it is easy for the operator to misidentify the waveform on the display screen, which brings a series of unnecessary troubles to the registration adjustment, resulting in unnecessary Waste, when the situation is serious, can also cause the overprinting system to work in a stable manner.
2. Optimized selection of overprint reference objects based on different types of originals
Two types of representative manuscripts are discussed separately below.
The first type of manuscript: the main body of the print is the line and the ground. The characteristics of such manuscripts are that the pictures are mainly in the field and the lines, and the interprinting relationship between the images and the images is simple and the requirements are not high. This type of manuscript can be divided into two categories:
2.1 Each spot color pattern has only an overprint relationship with a certain color pattern, then the color code of the print can be set to be overprinted with the key color version, that is, the reference color code of each color version is Key version color code.
2.2 The two field versions (A version, B version) have higher requirements for overprinting and have an overprint relationship with key color plates (C version), while other color plates (D version, E version) only have key color plates. (C version) There is an overprint relationship, and there is no overprint relationship with the first two versions of the field, then the overprint relationship of such an original can be set as: A → B → C; D → C;
F→C (“A→B” means “A” is overprinted with reference to “B”, and so on).
The second type of manuscript: an image, a line, and an original that coexists in the field. The feature of such an original is that the image part is rich in layers, and the color reproduction is formed by color overlapping of a plurality of color groups, and there is a high-precision overprint relationship between the image and the image. If the overprinting precision is insufficient, the image portion will be There are a series of overprinting problems such as unclearness and ghosting. Assume that the image part is made up of the patterns printed by the four spot colors of A, B, C, and D. E is a line text version, and there is no obvious overprint relationship with the image part. The F version is the field reference version, and There are overprint relationships between the other versions, so the overprint relationship between them can be set to A→B→C→D→F; E→F. The printing color sequence of the four color versions of A, B, C, and D can be interchanged according to specific conditions.
Second, gravure tension control
During the pre-press unwinding process, the paper or film needs to maintain a certain tension during the multi-color printing process or during the post-press process. The tension is too large to cause longitudinal wrinkles, and the tension is too small to cause lateral wrinkles. In short, the fluctuation of the tension will affect the accuracy of the overprint, thus affecting the stability of the quality of the print, so the tension must be controlled.
1. Pressing control of various parts of the printer
(1) Tension control from the feed shaft to the feed draw rolls
Due to the action of the pulling roller, the printing material is pulled out from the feeding shaft at a certain speed and tension and sent to the printing portion. The tension of this portion is greater than the tension of the printing portion, and the tension is usually controlled by a brake attached to the paper feeding shaft. . It is well known that the diameter of the work is gradually reduced at the same time. In recent years, due to the increase in speed, the use of materials has diversified, and the coil diameter has changed greatly. Therefore, manual mechanical brakes used in the past are unlikely to obtain high quality prints. Therefore, the use of automatic control methods, especially the use of tension measurement tables with data management becomes a necessary practice. The detection methods include spring-shaking roller type and micro-displacement type detection. Mechanical brakes have been used from an early age and are inexpensive, but the torque values are not accurate. So there are gradually other forms. The magnetic powder brake uses a magnetic powder as a friction medium to generate a braking force by a current, and is widely used because of its linearity of torque-current characteristics. Air brakes, although some printing machines are used more, they are gradually replaced by large-capacity magnetic powder brakes due to non-linear torque characteristics and large wear. The tension of the feedstock is determined by the following factors: 1 tensile change due to uneven thickness; 2 slippage of the material and even slowing down; 3 rolls not loaded, material and shaft eccentric; 4 turning device in rotation The resulting peripheral speed changes; 5 the reaction force of the pressure roller and the cutter when the paper is picked; 6 the roundness of each roller is insufficient.
Most of the above changes occur in a short period of time, and all changes to the tension are transmitted to the printing department. From the viewpoint of mechanical structure, it must be noted that the inertia and manufacturing error of the guide roller between the feed shaft and the traction roller are both causes of the increase in tension, and in particular, the low tension of several kilograms or less must not be ignored.
(2) Change and control of tension between traction roller and plate roller (traction)
The traction roller in the gravure press is generally adjacent to the first plate roller and is provided with a peripheral speed synchronizing mechanism. This is different from the feeding situation, the diameter does not change, so the tension is controlled by other principles.
Printed materials are as elastic and plastic as normal objects, and printing is done within elastic limits. Therefore, the tension is controlled according to the law of elasticity. This is to understand the characteristics of different materials. In order to obtain the same elongation during use, the tension can be determined in various ratios of the width and thickness of the material.
There are two ways to get the right tension:
a. Make the diameter of the pinch roller slightly change; correct the number of revolutions of the pinch roller. The time required for the printing material to reach the set tension from the no-tension state during operation is a one-time delay response of the distance between the pressure rollers and the speed reduction value as a time constant, which is usually called tensile control. For the basic form of the gravure press: between the paper feed pulling roll and the plate roll, the tension between the plate rolls, the plate roll and the take-up draw roll is based on this principle.
b. In addition, there is a method of controlling the tension by the pendulum roller. The principle is that the tension changes with the change of the weight and the position of the weight. In any case, the pendulum roller has the effect of absorbing the change of the tension, and the pendulum roller generally adopts the manufacturing error. Small, low-inertia, light-weight, and large-diameter rolls (such as hollow aluminum tubes) are typically removed in this manner by a short period of time between the feedstock and the draw rolls to minimize slight tension changes. This method also reduces damage when picking up paper.
(3) Tension control of the receiving traction part
From the plate roll to the receiving line for drying, the distance is usually increased. Due to the drying and heating, the extension of the car changes, causing the material to change in expansion and contraction, which are the reasons for the inaccurate overprinting. Therefore, as with the feed traction section, a better and more accurate tension control is required. Materials with low tension on the ultra-thin paper are also subject to the adhesion of static electricity by the wind blown by the dryer, which causes minor changes, which must also be considered.
(4) Coil tension control
The tension control of the coiling section (between the receiving take-up rolls and the receiving shaft) is also an important issue. In this part, the printed material is used as the final product and sent to the next process of compounding, slitting, and bag making.
At the same time, care should be taken to avoid scratching and wrinkling of the uniformly wound product. It is worth noting that in plastic films and aluminum foils, care should be taken to prevent variations in thickness. This is not just a matter of how much tension is taken up. It is important to adjust the tension while increasing the diameter of the coil, whether it is adjusted with a constant tension or by dimensional tension.
There are two types of driving methods in the winding section: surface driving and center driving. Although the general center drive type is used more, the surface take-up type also has good properties, and is good when the roll speed is constant. Therefore, it is used more in some industries. Compared with the feeding part, the receiving part should pay more attention to the taper tension and the inertia compensation error. The center drive is to directly take up the material on the paper tube. In the past, the friction control method was used. Now, the height, automatic and high-performance electrical control method is adopted. The receiving and winding method is as follows: 1 using the magnetic powder clutch; 2 using the torque motor ; 3 use a DC motor.
Magnetic powder clutch can obtain excellent performance at a low price; torque motor is not easy to use when the winding diameter changes greatly; DC motor is used for large capacity and automatic paper feeding. Table 1 lists the tension values of several reel substrates in different parts of the press.
2. Taper tension
In general, the tension is taken up by the constant tension, and as the roll increases, the moment relative to the inner material becomes larger, resulting in slippage, that is, take-up shrinkage. Further, due to the shrinkage of the material and the pressure of the air to the center, the material is crushed or extruded laterally, resulting in a so-called bamboo shoot phenomenon. Wrinkles occur near the core, making the surface uneven. To solve these problems, the tension should be gradually reduced when the winding diameter becomes larger, that is, the taper tension is used. The initial tension at the time of winding determines the tension at the end of the winding. The degree of reduction is called taper. Generally, a taper of 10% to 50% is used. For example, with a 30% taper; that is, starting with a tension of 10 kg, the tension is terminated from 7 kg. The ratio of change at this time is different depending on the properties of the material and the mechanism. The manufacturer should test various types of test curves for the production control. Adjustment.
3. Inertia compensation
The so-called inertia compensation is to compensate for the change in tension caused by the inertia of the coil during the start of the machine, during the operation, when the speed is changed, or when the stationary speed reaches the predetermined speed. In practice, the nonlinear moment is nonlinearly proportional to the diameter and the material density is proportional to the material width. The printing manufacturer uses the formula to calculate the average value and accurately calculate the compensation curve to guide the production.
Third, printing pressure control
The force of interaction between the embossing bodies during the printing process is called the printing pressure. Gravure is the same as lithographic printing and embossing. It is necessary to use the printing pressure to transfer the graphic on the surface of the printing plate to the printing substrate. Printing pressure is not only a fundamental guarantee for the printing process, but also largely determines the quality of the printing. To improve the quality of gravure, the printing pressure must be controlled, then what factors should be considered to determine the printing pressure:
1. The hardness of the blanket
The pressure of the gravure is caused by the deformation of the blanket. As the printing pressure increases, the amount of deformation of the blanket increases, but they are nonlinear. Generally, when the pressure is less than 10 kg/cm, the relative deformation of the blanket changes in proportion to the pressure. After that, as the pressure increases, the relative deformation increases slowly. When the deformation increases to 25%, the pressure increases again and the deformation increases. It can also be seen from Fig. 1 that in the case where the deformation of the blanket is the same, the blanket I having a high hardness is more stressed than the blanket II having a lower hardness.
2. Substance properties
For substrates of different thicknesses and different surface smoothness, the amount of pressure required for printing is different. For papers with poor smoothness, it is necessary to overcome the rough surface to obtain a strong imprint. For this reason, only the printing pressure is increased to make the substrate fully contact with the surface of the printing plate. Table 2 lists the range of printing pressures required for various substrates.
3. Printing speed
Practice has proved that when the printing speed increases, the printing pressure also needs to increase accordingly. This is because the speed is high and the contact time of the printing surface is reduced. In order to avoid the complete contact of the printing surface, the ink is easy to be printed from the printing plate. When the surface is transferred to the surface of the paper, the printing pressure must be increased.
4. Number of printed materials
During the printing process, as the number of prints increases, the printing pressure will decrease, as shown in Figure 2. This is because the blanket and the liner are plastically deformed during the printing process to make the thickness thinner and the pressure lower, which in turn reduces the transfer rate of the ink. Therefore, in order to obtain a stable quality print, a certain number of prints are printed. After the product, you should check the change of printing pressure. Make some adjustments as appropriate to keep the pressure stable.
5. Print quality requirements
Different prints have different reproduction requirements, such as the solid version, which can use a slightly larger pressure to achieve the requirements of blotting, while for fine mesh images, it should be emphasized that the use of the least possible pressure under the conditions of clear imprinting .