The core Department of gravure printing is tension control. The gravure printing here mainly refers to rotary gravure printing. As sheet fed gravure printing does not have tension control. Sheet fed gravure printing, like sheet fed offset printing, mainly relies on the teeth to pick up the paper, and then locate under the action of front and side gauges. The paper is stationary at the moment of printing. However, rotary gravure printing is different. The substrate material advances forward at a constant speed, and the rubber pressure roller presses the substrate material at a synchronous speed. The two movements are combined in a relatively static state. Therefore, the key of multi-color overprint is that the printing material is progressive and the tension of the material roll is not disordered in the transmission process. Gravure tension control system analysis generally includes the following content. 1. The tension control of gravure press is generally divided into four areas: unwinding tension area, feeding traction tension area, discharging traction tension area and winding tension area, which are separated by rubber rollers with drive. The printing area from the feed drive roller to the discharge drive roller requires a constant tension in the printing area, because only under the constant tension can the registration be guaranteed. The unwinding tension serves for the constant tension in the printing area. The requirement for the unwinding area is to release the roll into the printing area to ensure that the transmitted tension is constant regardless of the size of the coil diameter, otherwise the tension fluctuation will directly affect the constant tension in the printing area, and then affect the registration. The winding tension is different. In this area, multi-color printing has been completed and only smooth winding is needed. According to different winding quality requirements, there are constant tension winding and variable tension winding. The common decreasing tension winding (also known as taper winding) refers to the latter one. Because the variable tension winding may affect the registration accuracy of the last printing color set, the printing area is effectively separated from the winding area, and the material roll is driven from the printing area to the winding area, so that the printing area still maintains constant tension, and the decreasing tension in the winding area is realized by the rubber roller mechanism driven by the belt. This is the key point in the tension control of rotary printing, and the same is true of rotary gravure printing. 2. Tension detection and feedback in each area, there are generally independent detection devices in each area, such as the tension roller detection device composed of resistance strain gauge sensor and capacitance sensor, and the position detection and control device composed of floating roller and potentiometer. The function of these different detection circuits in the whole system is significantly different. The tension roller detection device has fast response, but it is lack of control ability for the system disorder caused by the quality problem of the roll itself. Although the position control device does not directly detect the tension, it can ensure the relative position of the printing material in the floating roller paper path under the normal tension state So as to control the tension constant. The position control device can slow down the tension fluctuation caused by the raw material quality problems such as the diameter of the coil is not round, which can not be achieved by the tension roller detection device. 3. The commonly used tension actuators are electromagnetic or magnetic powder clutch / brake, pneumatic clutch / brake and servo motor. In the traditional gear transmission system, the micro actuator on 360 ° gearbox is also included. It is worth noting that, according to the automatic control theory, the clutch brake mechanism belongs to the first-order linear link. However, because the linear segment only occupies the middle section in the whole curve area, and the high-end and low-end parts of the curve area are mainly imitative linear, and there are gaps with the requirements of linear control, so this executive function is still defective. The servo motor belongs to the second-order link. Under the existing control theory, both the stability and the follow-up can be taken into account. Therefore, to improve the tension control system of the predecessors, the direct drive control of the servo motor is generally adopted.4. There are three reasons for the loss of mechanical synchronous speed in the process of coil transfer: the printing pressure applied to the width of the whole coil, the surface friction between the printing material and the guide roller, and the resistance caused by the inflexibility of the guide roller bearing. Among them, the main effect is the pressure exerted on the substrate material by the rubber roller of each color group. The pressure required in the transfer process is the largest in all printing processes. According to experience, for different substrate materials, the hardness of the rubber roller is different, and the pressure is also different. For example, for the film substrate, the rubber roller with a shore hardness of 65-70 degrees is generally used, and the unit pressure is 900kg / M; for the paper substrate, the rubber roller with the shore hardness of 70-80 degrees is used with the unit pressure of 1800kg / M; for the paperboard substrate, the rubber roller with the shore hardness of 80-90 degrees is used with the unit pressure of 4500kg / m. The softer the rubber roller is, the greater the pressure is, and the greater the loss of mechanical synchronous speed is. The traditional way to compensate the loss of mechanical synchronous speed is to increase the diameter of active roller (plate roller) of each color group. As we all know, the linear speed v = 2 π RN. If you want to adjust the linear speed of the plate roller during printing, you can adjust the speed of the plate roller n (that is, the method of increasing the motor speed of each color group plate roller adopted by the servo gravure press) or the plate roller diameter (the traditional method of increasing the plate roller diameter for more than 20 years) can also be adjusted. However, the compensation is based on experience rather than accurate calculation. Obviously, the characteristics of the substrate material, the width of the applied pressure is different, the loss of mechanical synchronous speed is different, so the accurate compensation value should be different. However, the method of increasing the plate roller diameter by 0.02 ~ 0.03mm (some even increase by 0.04mm) is only based on the "trance" algorithm. When developing the servo gravure press, the domestic press industry initially thought that the law of increasing the plate roller diameter could be eliminated. When setting the rotation speed of the servo motor of each color group plate roller, the amplification caused by the increasing plate roller diameter can be calculated in advance. Therefore, the servo gravure printing machine does not have to exchange the printing color sequence like the traditional gravure press. However, the printing practice has proved that even if the servo gravure press has increased the speed of the plate roller motor for each color group, if the plate roller continues to keep the diameter increasing, the printing registration effect is much better than that when the plate roller diameter does not increase. This shows that the current gravure servo system can only compensate the loss of synchronous speed in terms of experience, there is still a lot of gap from the real compensation control. 5. Registration control gravure registration control must be based on the tension control. Only after the above four problems are basically solved, the accurate registration of gravure printing is possible. There are some misunderstandings about the registration problem in the industry, especially for servo gravure press. It is believed that the encoder accuracy can reach millions of times per second, and the registration is certainly no problem. In fact, in the actual process of registration, the computer does not track the cursor itself, but the movement trend of the cursor.It's like shooting a bird with a gun. Instead of pulling the trigger when you see a bird, you can calculate the lead time according to the bird's trajectory. Only in this way can the bird be shot reliably. The registration system of gravure press is also the same. The cursor movement trend is the registration condition. If the cursor movement trend of printing [encyclopedia Weibo] on the printing material cannot be controlled, even if the servo motor is equipped with an encoder of 4 million times per second, the registration error is likely to exceed the standard, and the registration loss may be too large to be expected. 6. Compensation for the deformation of the substrate material, the synchronous speed compensation of the printing material in operation can be guaranteed by the servo type, but there is no reliable compensation method for the deformation of the substrate material itself. Taking film printing as an example, different kinds of films have different stretching deformation under the premise of the same thickness and width; for the same kind of film with different thickness, such as BOPP film with 20 μ m and 40 μ m thickness. Under the same drying condition, the expansion deformation of the two is different. Taking paper as an example, the paper with the same material and thickness with moisture content of 5% and 8% has different deformation data under the same drying condition. For these differences, it is necessary to establish a mathematical model and load it into the registration control mathematical model in order to effectively control the printing registration accuracy. If not, gravure registration accuracy can only rely on the use of thick substrate, such as PET film to ensure. In gravure production site, we often hear the front-line operators complain that the printing plate is not good or the substrate material is not good, resulting in inaccurate color registration or registration error in the printing process, and thus resulting in cost loss. However, the inspection of the printing plate and substrate materials does not give any reason to put it on the table. According to the author, the main reason lies in this.
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