The core part of gravure printing is tension control. The gravure printing here mainly refers to rotogravure printing, because there is no tension control for single sheet gravure printing. Sheet-fed gravure printing is the same as sheet-fed offset printing. It mainly relies on gripping teeth to grip the paper, and then is positioned under the action of the front and side rules. The paper is still at the moment of printing. Rotary gravure is different. The printing material advances at a stable speed. The rubber pressure roller presses the printing material at a synchronous speed. These two movements are synthesized in a relatively static state, so the key to multicolor overprinting The printing material is progressive and stable, and the tension of the material roll is stable during the transfer process.

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

1. Tension zone and control requirements of each area

The tension control of the gravure printing machine is generally divided into 4 areas, namely the unwinding tension zone, the feed traction tension zone, the discharge traction tension zone and the winding tension zone, which are separated by a belt-driven rubber pressure roller. The printing area from the feed drive roller to the discharge drive roller, the requirement for the tension of the printing area is constant tension, because only under constant tension conditions can ensure registration. The unwinding tension serves 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 transmitted tension can be kept constant regardless of the size of the roll diameter, otherwise the tension fluctuation will be directly Affects the constant tension of the printing area, which in turn affects the registration. Rewinding tension is different. In this area, multi-color printing has been completed, just smooth rewinding. According to the different quality requirements of winding, there are constant tension winding and variable tension winding. The common decreasing tension winding (also called taper winding) refers to the latter. Since the variable tension winding may affect the registration accuracy of the last printing color group, the printing area is effectively separated from the winding area, and the driving material roll is advanced from the printing area to the winding area, so that the printing area remains Constant tension and decreasing tension in the winding area are realized by the rubber roller mechanism driven by the belt. Both segmented control and overall coordination make the tension of each area serve the multi-color registration, which is the key point of rotary printing in tension control, as is rotary gravure.

2. Tension detection and feedback in each area

There are generally independent detection devices in each area, such as a tension roller detection device composed of a resistance strain gauge sensor, a capacitance sensor, etc., and a position detection control device composed of a floating roller and a potentiometer. The functions of these differently constructed detection circuits in the entire system are obviously different. The tension roller detection device responds quickly, but the ability to control the system stability caused by the quality problem of the roll itself is still lacking, while the position control device does not The tension is directly detected, but by controlling the relative position of the printing material in the paper path of the floating roller under normal tension, the tension is controlled to be constant. The position control device can use the floating roller paper path to alleviate tension fluctuations caused by raw material quality problems such as the roll diameter being not round, which cannot be achieved by the tension roller detection device.

3. Tension actuator

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

The traditional gear transmission system also includes a miniature actuator motor on a 360 ° gearbox. It is worth noting that according to the theory of automatic control, the clutch brake mechanism belongs to the first-order linear link, but because the linear segment occupies only the middle segment in the entire curve area, and the high-end and low-end of the curve area are mainly imitative linear, the same linear control The requirements still have a distance, so this executive function is still flawed. The servo motor belongs to the second-order link. Under the existing control theory, both stability and follow-up can be considered. Therefore, advanced tension control systems generally use servo motors for direct drive control. [next]

4. Compensation for mechanical synchronous speed loss in the process of material roll transfer

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 caused by the inflexible bearing of the guide roller.

The main role is the pressure exerted by the rubber pressure roller of each color group on the printing material. The pressure that must be applied during this transfer process is the largest in all printing processes. According to experience, for different printing materials, the hardness of the rubber pressure roller used is different, and the applied pressure is also different. For film-based 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-based printing materials, rubber rollers with a Shore hardness of 70 to 80 degrees are used. The pressure is 1800kg / m; for the cardboard printing materials, a rubber pressure roller with a Shore hardness of 80 to 90 degrees is used, and the unit pressure is 4500kg / m. Using rubber rollers of different hardness and applying different pressures obviously affects the synchronous speed in the transfer of printing materials. The softer the rubber roller, the greater the applied pressure and the greater the loss of mechanical synchronous speed.

The traditional approach to mechanical synchronous speed loss compensation is to increase the diameter of the active roller (ie, plate roller) of each color group. As we all know, the linear speed V = 2Ï€rn, if you want to adjust the linear speed of the plate roller during printing, you can adjust the plate roller speed n (that is, the current method of increasing the motor speed of each color group plate roller used in the servo gravure press), or The diameter of the plate roller can be adjusted (that is, the traditional method of increasing the diameter of the plate roller for more than 20 years). However, this compensation is based on experience accumulation rather than accurate calculation. Obviously, the characteristics of the printing material are different, the width of the applied pressure is different, and the loss of the mechanical synchronization speed is different, so the precise compensation value should also be different. However, the method of increasing the diameter of the plate roller by 0.02 to 0.03 mm (or even increasing by 0.04 mm) is now based on the "fuzzy" algorithm.

When the domestic printing press industry developed a servo-type gravure press, it was initially thought that the rule of increasing the diameter of the plate roller could be eliminated, because when setting the rotation speed of the servo motor of the plate roller for each color group, the increase in the diameter of the plate roller could be brought in advance The amount of magnification is calculated, so the servo type gravure printing machine does not need to be able to exchange printing color sequence like the traditional gravure printing machine. However, the printing practice proves that even the servo-type gravure printing machine has increased the rotation speed of the plate roller motor of each color group. If the plate roller continues to keep increasing in diameter, the printing registration effect is much better than when the plate roller does not increase in diameter. This shows that the current gravure servo system can only be limited to the scope of experience in compensating for the loss of synchronization speed, and there is still a lot of distance from the real compensation control. [next]

5. Register control

The registration control of gravure printing must be based on stable tension control. Only after the above four problems are basically solved, accurate registration of gravure printing is possible. There are some misunderstandings in the industry about the registration problem, especially for servo-type gravure printing machines, which believe that the accuracy of the encoder can reach millions of times per second, and the registration is certainly no problem. In fact, in the actual process of registration, the computer is not tracking the cursor itself, but the trend of cursor movement. This is like hitting a bird with a gun. Instead of pulling the trigger when you see the bird, you calculate the advance based on the bird's trajectory. It is reliable to hit the bird in this way. The registration system of the gravure printing machine is also the same. The trend of cursor movement is the premise of registration. If the cursor movement trend printed 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 The quasi-error is also likely to exceed the standard, and the registration loss may be unexpectedly large.

6. Compensation for deformation of printing materials

The synchronous speed compensation of the printing material in operation can be guaranteed by the servo model, but there is currently no reliable compensation method for the deformation of the printing material itself. Taking film printing as an example, different types of films have different expansion and deformation under the same thickness and width conditions; for the same film with different thicknesses, such as 20μm and 40μm thick BOPP films. Under the same drying conditions, the expansion and contraction of the two are also different. Taking paper as an example again, under the same drying conditions, the deformation data of paper with the same material and thickness of 5% moisture content and 8% moisture content are different. For these differences, it is necessary to establish a mathematical model and load it into the mathematical model of register control to effectively control the printing register accuracy. If this is not the case, the registration accuracy of gravure printing can only be guaranteed by using thicker, less deformable printing materials, such as PET film. At the gravure production site, we often hear front-line operators complaining about poor printing plates or poor printing materials, resulting in inaccurate color registration or excessive registration errors in the printing process, and resulting in cost losses, but check the printing plate and The printing materials can't really give any reason why they can get on the desktop. According to the author, the main reason lies in this.

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