Structural Units (continued)
Outfeed and Rewind Units
Rewinding aims to rewind the printed web being continuously fed from the printing units in a stable manner so that there is no loss to the product quality.
Rewinding tension is determined based on the conditions of the plastic film being printed (thickness, edge sagging, etc.), the type of plastic film, the ink being used, and the printed image, for example. In all cases, rewinding is the final step in the printing process, so if pitch defects or blocking occur, the fi nal printed matter will be defective no matter how good the accuracy of the printing precision. To prevent this situation, we must set and control the appropriate rewinding tension. This rewinding tension is typically believed to have no effect on the printing units, but in practice, rewinding tension has a significant influence on the registration of the final printing unit. One method of preventing this tension from affecting the printing units is to install an outfeed roller (draw roller).
Outfeed Roller
As mentioned above, the outfeed roller is used to prevent registration problems in the printing units caused by rewinding tension changes resulting from rewinding tension control (taper tension, etc.). The outfeed roller also prevents the infl uence of tension variation occurring in the rewinding unit as a result of transitory phenomenon during acceleration and deceleration of the machine and during splicing.
The tension control methods for the outfeed roller are basically the same as those for the infeed roller, and often use mechanical speed control that drives the machine using a stepless speed change gear through a main transmission shaft. As mentioned in Session 2 (March/April 2014), the stepless speed change gear has speed variation and is not able to absorb transitory phenomenon caused by disturbance, so has poor performance. Moreover, the printing cylinder circumferential speed must be calibrated each time the printing cylinder is changed. In this way, this approach places a high demand on the level of operator skill. For these reasons, this method is almost never used now.
Rewinding Methods and Drive Systems
Rotogravure printing machine rewinding units consist of the following types. Of these, the paper outfeed unit in commercial rotogravure printing machines uses a sheeter as a folding machine or paper wrapper in place of the rewinding unit.
Folding Machine The folding machine cuts the printed web and outfeeds the web in a folded manner before binding.
Sheeter The sheeter is an outfeed unit that cuts the printed web into sheets using a rotary cutter.
Surface Drive Rewinding (surface rewinding) In this method, the rewinding core is placed over a roller or is placed between two parallel rollers, which drive the surface of the roll to wind the web.
Center Drive Rewinding (center rewinding) This method drives the rewinding shaft itself to wind the web.
Surface Drive Rewinding Methods
This rewinding method is often adopted for the rewinding systems in paper making machines. In flexible packaging gravure printing machines, this method presses the roll too
strongly against the surface drive roller to rewind the web, so the roll is wound tightly, which often results in blocking and damages the plastic film. As such, this method is not
used in this field. In the case of paper printing, however, where the roll diameter is large and it is necessary to wind the roll tightly with a strong tension, this method is used.
Standard type of surface drive rewinding. The web wound on the core (rewinding shaft) comes in contact with the two drive rollers as it is rotated. As the roll diameter increases, the rewinding shaft rises. Because the weight is low at the start of rewinding, the contact pressure with the drive rollers is insufficient, so a load is applied to the roll core to prevent slip. On the other hand, as the roll becomes larger and heavier, the contact pressure increases exponentially compared to the increase in diameter of the roll, so the roll may deform. Once the roll diameter reaches a certain point, the contact pressure is manually or automatically adjusted so that it gradually decreases in the counterload direction against the rewinding core within a range that maintains the necessary contact pressure for rewinding. This contact pressure adjustment often uses pneumatic or hydraulic pressure.
Of the two drive rollers, the trailing contact roller is set to rotate at a slightly higher circumferential speed (using a difference in diameter) to wind the web more tightly. Therefore, depending on the system, a mechanism that uses a slip clutch (powder clutch) is used so that the circumferential speed of the trailing roller can be varied.
In surface drive rewinding, the torque of the drive roller does not change much even as the roll diameter increases, which makes this an efficient, low power consumption rewinding method for large diameter rolls. On the other hand, it is disadvantageous in that splicing for continuous rewinding operations is difficult.
In Surface drive rewinding methods that use one drive roller bring the roller into contact with the rewinding core and move the roll upwards in a parabolic curve as the roll diameter increases.
There are various methods for setting the contact pressure with the drive roller, including using air pressure and using the weight of the rewinding roll itself. One typical rewinding system that uses this method is called the Pope Reel method, where some systems have an automatic splicing unit.
In other methods, the rewinding shaft is locked at a fixed position, and an air cylinder is used to apply pressure to the surface drive roller (rewinding contact roller) to wind the
web. There are also rewinding methods that drive both the rewinding shaft and the contact roller. In general, the drive system drives rewinding using a stepless speed change gear through a main transmission shaft. Recently, however, there are many examples of sectional drive methods in which DC motors and AC vector motors are used to drive each individual unit.
Center Drive Rewinding (center rewinding)
The circumferential speed of the roll in surface drive rewinding does not need to be changed even if the diameter of the rewound web increases, as long as the printing speed is the same. In the case of center drive rewinding, however, if the rewinding shaft rotates at a continuous speed, as the rewound roll diameter increases, the rewinding speed will increase equal to the increase in the roll diameter, which will cause problems with rewinding tension and workability.
There are several mechanisms used for solving these problems.
(1) Friction clutch based mechanism
(2) Constant tension rewinding stepless speed change gear based mechanism
(3) Torque motor based mechanism
(4) Powder clutch based mechanism
(5) DC motor based mechanism
(6) AC vector motor based mechanism
Before explaining these mechanisms, let us look at rewinding tension and roll hardness. There are three approaches to rewinding tension. The first is constant tension rewinding, which maintains the tension at a constant regardless of the roll diameter. The second is taper tension rewinding, which gradually decreases the tension in relation to the increase in roll diameter. The third is constant torque rewinding, which decreases tension in an inverse proportion to the roll diameter ratio.
In the case of rewinding in plastic film gravure printing machines, taper tension control rewinding is a good method that is adopted. In this case, the tension near the roll core is
set relatively high, but is gradually reduced as the roll diameter increases. This approach is taken to prevent problems that occur when the web is rewound at a constant tension from the start to the end of rewinding. Some of these problems include roll hardening caused by contraction resulting from residual stress in the rewound film, wrinkles from occurring near the roll core, printing pitch defects, blocking, and a loss of balance in the winding forces between the inside and outside of the roll that causes the inside of the roll to slip sideways and result in telescoping.
With taper tension control, some methods detect the diameter using a contact roller while other methods make a ratio calculation of the printing speed and rewinding shaft speed to determine the diameter.
The taper ratio of taper tension differs depending on the plastic film, the ink, and the printed image, for example. In general, taper tension is used in the range of 30 to 50%, but depending on the rewinding range and rewinding speed, it can be as high as 80%.
Taper tension behavior is generally linear taper, but recently there are also control methods that can set an arbitrary taper tension curve. The taper ratio (%) is defi ned by the following equation.
A=(1-Tmax/Tmin) X100 (7)
A: taper ratio [%], Tmax: tension at maximum roll diameter [N], Tmin: tension at minimum roll (core) diameter [N]
The printed plastic fi lm must be rewound to a soft degree that does not result in the aforementioned phenomena. Naturally, there must be no edge shifting during rewinding.
Moreover, the web must be rewound so that the layers do not shift during transport of the rewound web. In contrast, in the case of paper, there is a tendency to wind the web tightly, so the troubles mentioned above are relatively rare.
An air layer (air boundary layer) adheres to the top and bottom of the fi lm as it passes through the air. The air density increases directly before the film contacts the rewound web, and at least some of this air is entrained within the rewound web and remains between the fi lm layers. The greater the amount of entrained air, the more loosely the web will be rewound, but this air also makes it more likely for the edges of the fi lm to shift during rewinding or for the film layers to shift during transport of the roll. Edge shifting during rewinding or layer shifting during transport of the roll also depend on the extent to which the film surface has been treated.
These phenomena can be reduced to some extent by using taper tension during rewinding tension control, but the entrainment of air decreases the coefficient of friction between the film layers and makes it more likely for edge shifting and layer shifting to occur. As such, these problems cannot be fully prevented. In general, a commonly used method for preventing edge shift during rewinding is to attach a circular side plate with a diameter greater than the maximum roll diameter to either side of the rewound web. Holes are opened in several locations in these side plates so that the entrained air between the fi lm layers can easily escape.
If the rewinding tension is increased, air entrainment will decrease, so edge shifting can be reduced somewhat, but it is not desirable to increase the tension more than necessary. A typical method for adjusting the amount of entrained air is to use a contact roller or a near roller.
(1) Contact Roller
During rewinding, this contact roller is continuously in contact with the web, which prevents the entrainment of air as the film is drawn in. When the contact pressure is high, there is no air entrainment and the web is rewound tightly. As such, it is necessary to adjust the contact pressure so that it is low, in other words, so that it is in a kiss touch position with the web. Although the contact roller must be selected depending on the film material, in general, rubber or metal rollers are used.
(2) Near Roller
This method maintains a fixed gap without bringing the near roller into contact with the web, while automatically shifting the position of the roller in response to the roll diameter as the web is rewound.
Movement of the near roller is automatically controlled by an AC servo motor that responds to the change in roll diameter as feedback.
An important point for all rewinding methods is control of rewinding tension, as mentioned earlier. A simple method that has been used since early on is to manually adjust the drive torque through a friction disk, but it has become standard to use automatic control of rewinding tension to prevent problems, such as wrinkles caused by inappropriate rewinding tension, web damage from edge shifting, and pitch defects.
To prevent blocking caused by the characteristics of the plastic film or printing ink and to prevent wrinkles and pitch defects caused by roll tightening, there are some cases in which the web must be rewound at low tension. In this way, as with the unwinding shaft, the bearing unit must have a mechanism that allows it to rotate at low torque. The basic equation for rewinding tension during stable operations is as follows.
T4: rewinding tension [N], R: rewound roll radius [m]), τE: externally applied torque [N . m], τM: breaking torque from mechanical loss [N . m]
As can be seen from Equation (8), the low tension range easily becomes unstable from size (mechanical loss volume). This factor is particularly important in center drive rewinding methods.
1. Friction clutch based mechanism
2. Constant tension rewinding stepless speed change gear based mechanism
3. Torque motor based mechanism The above three methods are almost never used today for center drive rewinding, so we will omit their explanation here.
4. Powder clutch based mechanism
This is the most basic system, and in principle belongs tothe category of friction disk clutch mechanical slip mechanisms mentioned earlier. The drive source is an induction motor that is connected to the input side of the powder clutch, while the rewinding shaft is connected to the output side of the powder clutch. A general purpose inverter is used to control the induction motor so that the input shaft rotational speed of the powder clutch rotates faster than the output shaft rotational speed by a constant degree. This method has a relatively narrow tension control range, but is inexpensive, so is often adopted for simple printing machines.
5. DC motor based mechanism
This mechanism allows for high precision control in all ranges, from low speed to high speed, and from low tension to high tension. To improve the stability of the tension and roll form, as mentioned before, this method is used together with taper tension control, contact pressure control using a contact roller, and near roller control. In particular, the tension curve for the roll diameter with taper tension differs depending on the web and printing conditions.
6 AC vector motor based mechanism
This method is the same as item (5), but replaces the DC motor with an AC vector motor. Given the superior control characteristics, this method is adopted as the rewinding method in almost all gravure printing machines. The performance of tension control of the above methods is determined by the web tension detection method. There are several systems that directly detect and control tension, as listed here.
(1) Micro-displacement Tension Detection System
This method directly detects and controls the tension of the web itself, so allows for high precision tension control during stable operations. During transient phenomena, such as acceleration, deceleration, and splicing, however, it is not able to absorb tension fluctuations.
(2) Dancer Roller Tension Detection System
This method is able to absorb tension fl uctuations during the above mentioned transient phenomena. The approach uses an air cylinder as the actuator and uses a potential meter to detect the dancer roller position. So that the dancer roller does not affect the system through rotational mechanical loss or inertia, it is necessary to use large, light rollers. It is also necessary to adopt an air cylinder with low mechanical loss and mechanical hysteresis, as well as a high precision, high response pneumatic regulator for setting the tension.
(3) Current Control System
Unlike micro-displacement tension detection and dancer roller tension detection, this system does not use a direct detection sensor for the web, but indirectly detects the tension from an analog signal (current signal) from the drive motor. In addition, when taper tension is required, this system uses a roll diameter detection mechanism which reads an electrical signal proportional to the roll diameter to make pressure adjustments using an electro-pneumatic converter.
This article is reprinted with permission from the International Association of Diecutting and Diemaking’s monthly magazine, The Cutting Edge, June 1999.
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