Tension control in gravure printing

The core part of the gravure printing is the tension control. The gravure printing mentioned here mainly refers to gravure gravure printing, since there is no tension control in the gravure printing of a single sheet of paper. Sheet-fed gravure printing, like sheetfed offset printing, relies mainly on tusks to grip the paper, and then it is positioned under the action of the front gauge and side gauge, and the instant paper in the printing is stationary. Rotary gravure is different, the printing material advances at a constant speed, and the rubber roller presses the printing material at a synchronous speed. The two movements are combined in a relatively static state, so multi-color printing The hub is a continuous process of printing materials, and the tension of the material roll is not disturbed during the transfer process.

The gravure tension control system analysis generally includes the following content.

1. Tension zoning and control requirements for each area

Gravure press tension control is generally divided into four areas, namely the unwinding tension zone, the feed pull tension zone, the discharge tension zone and the winding tension zone, separated by a belt-driven rubber roller. From the feed drive roller to the discharge drive roller in the printing area, the tension in the printing area is required to be constant, since the registration may be guaranteed only under a constant tension. The unwinding tension is to serve the constant tension of the printing area. The requirement for the unwinding area is to release the material roll into the printing area to ensure that the tension of the transmission can be maintained regardless of the size of the roll diameter, otherwise the tension fluctuation will be directly Affects the constant tension in the printed area, which in turn affects the registration. The winding tension is different. In this area, multi-color printing has already been completed, and you only need to roll it smoothly. According to different winding quality requirements, the winding method has constant tension winding and variable tension winding. Common decreasing tension winding (also known as taper winding) refers to the latter. Because the variable tension winding may affect the registration accuracy of the last printing color group, the printing area and the winding area are effectively cut off, and the driving of the printing material from the printing area to the winding area advances the printing area. Constant tension, and decrement of tension in the winding area, is achieved by a rubber roller mechanism with a belt drive. Both the sub-control and the overall coordination, so that the regional tension is multi-color registration services, which is the hub of rotary printing tension control, rotary gravure is so.

2. Tension detection and feedback in each area

There are generally independent detection devices in each area, such as tension roller detection devices made up of resistance strain gauge sensors and capacitance sensors, and position detection control devices made up of floating rollers and potentiometers. These different configuration of the detection circuit in the entire system is significantly different role, the tension roller detection device response is fast, but the control of the system caused by the quality of the roll itself is still lack of control capability, and the position control device The tension is not detected directly, but by controlling the relative position of the printing material under the normal tension state in the floating roller paper path, the purpose of controlling the tension is constant. The position control device is capable of slowing the tension fluctuation caused by the quality of the material such as the unwinding of the reel due to the floating roller path, which cannot be achieved by the tension roller detection device.

3. Tension actuator

Commonly used tension actuators include electromagnetic or magnetic clutch/brake, pneumatic clutch/brake and servo motor

The conventional gear transmission system also includes a miniature actuator motor on a 360° gearbox. It is worth noting that according to the automatic control theory, the clutch brake mechanism belongs to the first-order linear link, but because the linear section only occupies the middle section in the entire curve area, the high-end and low-end sections in the curve area are still mostly linear and linear. There are also gaps in the control requirements, so this execution function is still flawed. The servo motor belongs to the second-order link. Under the current control theory, the chaos and followability can both be taken into consideration. Therefore, the tension control system used to improve the seniors generally uses the servo motor direct drive control.

4. Compensation for loss of mechanical synchronous speed during transfer of material

There are three reasons for the loss of mechanical synchronous speed: the printing pressure applied to the width of the entire web, the surface friction of the printing material in contact with the guide roller, and the resistance due to the inelastic roller bearing.

Among them, the main function is the pressure applied to the substrate by the rubber roller of each color group. The pressure that must be applied during this transfer process is the largest in all printing processes. According to experience, the hardness of the rubber roller used for different substrates is different, and the applied pressure is also different. For thin film printing materials, rubber rollers with a Shore hardness of 65 to 70 degrees are generally used with a unit pressure of 900 kg/m. For paper printing materials, rubber rollers with a Shore hardness of 70 to 80 degrees are used. Pressure 1800kg/m; For paperboard printing materials, rubber rollers with a Shore hardness of 80 to 90 degrees are used with a unit pressure of 4500kg/m. The use of different hardness of the rubber roller and the application of different pressure, the impact of the synchronization speed on the transfer of the printing material is obviously different, the softer the rubber roller, the greater the pressure, the greater the loss of mechanical synchronization speed.

The traditional way to compensate for the loss of mechanically synchronized speed is to increase the diameter of the active roller (ie, the platen roller) for each color group. As we all know, the line speed V = 2πrn, if you want to adjust the linear speed of the printing roller, you can adjust the roller speed n (that is, the method used by the current servo-type gravure printing machine to increase the motor speed of each color roller), also The diameter of the printing roller can be adjusted (ie, the traditional method of increasing the diameter of the printing roller for more than 20 years). However, this compensation is based on accumulated experience rather than accurate calculations. Significantly, the characteristics of the printing materials are different, the width of the applied pressure is different, and the loss of the mechanical synchronization speed is different, so the exact compensation value should also be different. However, the method is now compensated by using a method in which the diameter of the plate roller is increased by 0.02 to 0.03 mm (even if it is incremented by 0.04 mm), and only the “恍惚” algorithm can be used.

In the development of a servo-type gravure printer, the domestic printer industry originally thought that it would be possible to remove the law that the diameter of rollers was increased. This is because when setting the rotation speed of the servo motor of each color combination, the diameter of the roller can be increased in advance. The amount of amplification is calculated in, so the servo-type gravure printer does not have to interchange printing color sequences like a traditional gravure printer. However, the printing practice has proved that even servo-type gravure printers have increased the rotational speed of the roller motors of each color, and if the roller has continued to keep the diameter increasing, the printing register effect is much better than the diameter of the printing roller. This shows that the current gravure servo system can only be limited to the scope of experience in compensating for the loss of synchronous speed, and there are still a lot of gaps from the real sense of compensation control.

5. Register control

The gravure registration control must be based on the uncontrolled tension control. Only after the above four problems are solved, the accurate registration of gravure is possible. The industry has misunderstood the subject of the registration, especially for servo-type gravure printers, assuming that the accuracy of the encoder can reach several million times per second, and registration is certainly not a problem. In the actual process of registering, the computer is not tracking the cursor itself, but the cursor movement trend. It is like hitting a bird with a gun. Instead of seeing the bird before pulling the trigger, it is based on the trajectory of the bird to calculate the lead-in amount. It is reliable to use this method to beat the bird. The same is true of the gravure printing machine's registration system. The trend of cursor movement is the condition of registration. If the trend of cursor movement on printing materials cannot be controlled, even if the servo motor is configured at 4 million times per second, For encoders, the registration error is also likely to be exceeded, and the register loss may be unexpectedly large.

6. Compensation for deformation of printing materials

The synchronous speed compensation of printing materials in operation can be guaranteed by using servo models, but there is no reliable compensation method for the deformation of the printing materials themselves. Taking thin film printing as an example, different types of films have different telescoping deformations under the same thickness and width conditions; for different thicknesses, such as 20μm and 40μm thick BOPP films. Under the same drying conditions, the expansion and contraction of the two are not the same. Taking paper as an example, the same material and thickness of paper with a moisture content of 5% and a water content of 8% have different deformation data under the same drying conditions. For these differences, mathematical models need to be established and loaded in the mathematical model of register control in order to effectively control the accuracy of the printing register. If not, the accuracy of gravure printing can only be ensured by using a thick, non-deformable printing material such as PET film. At the gravure printing production site, we often hear that the front-line operating staff complained that the printing plate is not good or that the printing materials are not good, resulting in inaccurate registration or registration errors in the printing process, and resulting in cost loss, but checking the plates and Printing materials do not provide any reason why they can get the desktop. According to the author, the main reason is this.