Unwinding and Infeed Units (Continued)
Unwinding Brake and Tension Control With rotogravure printing machines, it is important that the tension between each of the printing units between unwinding and rewinding is always stable. If tension is not maintained during printing, registration and pitch will not match and it will be difficult to ensure the required quality. As such, the brake system that feeds the plastic film, for example, at a stable tension from the unwinding unit to the printing unit is one of the most important systems for tension control.
The unwinding brake system can be a simple device that uses a leather band to tighten a flywheel, which is either directly or indirectly connected to the unwinding shaft, or more complex devices, such as manually adjusted magnetic powder brakes or automatic tension control systems. These more complex systems combine the brake with a tension detection roller that feeds back tension fluctuations to the magnetic powder brake, DC motor, or AC vector motor. In addition, there are air brakes that use pneumatic pressure, which are particularly used at locations that must be explosion proof, but because the low-torque performance of these is not as good as that of the powder brake, these are not used on flexible packaging gravure printing machines.
If we look at the causes of tension fluctuation during plastic film printing, we get the following:
1. If we assume that the roll of plastic film is printed at a constant brake force, as the unprinted roll diameter gradually decreases, a larger tension force will be required to unwind the smaller roll. In other words, as the roll diameter decreases, the tension will increase (the base film roll diameter and tension are inversely proportional).
2. The thickness of the plastic film to be printed is not uniform or has a tendency to sag. Further, if the slipperiness is not uniform, there is a greater chance of tension fluctuation.
3. When the machine accelerates or decelerates, the degree of friction between the plastic film and rollers will change, causing tension fluctuation.
4. Changes in elasticity and slip characteristics caused by variation in the indoor environment (temperature, humidity) will cause tension fluctuation to occur.
Therefore, to continuously stabilize these conditions, we first set the optimal tension for each of the plastic films at the unwinding unit prior to entering the printing unit. Next,
if there is a change compared to this reference tension, the change must be detected, fed back, and corrected. The system that automatically detects and corrects such changes is
called an automatic tension control system. Conventionally, the brake force in the tension control system was operated manually based on the feel of the brake and the operator’s intuition to determine the appropriateness of the tension. Today, however, we use a feedback method that relies on a tension signal, or a feedback method that relies on a dancer roller position signal. Because the latter is more effective in reducing tension fluctuation than the former (particularly during splicing and when transience occurs during acceleration and deceleration), it achieves a good control result, so has become a widely adopted method. Common actuators include powder brakes, DC motors, and AC vector motors.
Here, let us look briefly at the torque required for tension control.
Brake force, in other words, unwinding torque, is expressed by Equation 2.
T0: tension (N), R: unwinding roll diameter (m)
When a constant tension is applied to draw the unwinding roll, the roll diameter will gradually decrease. In addition, torque will decrease in proportion to the roll diameter.
In short, the braking force applied to the unwinding shaft must gradually be reduced in response to the unwinding roll diameter. For example, at a tension of 100 N, if we express
the change in torque from a roll diameter of 400 mm to 100 mm as an equation, we get 100 NÅ~(0.4/2 m to 0.1/2 m)=20 N·m to 5 N·m
In this way, to maintain a constant tension, we can see that it is necessary to decrease the unwinding torque in response to the decrease in unwinding roll diameter.
If we summarize the unwinding brake automatic tension control methods, we get the following:
(1) Powder Brake
This is the most basic, least expensive, and widely adopted system, but the tension control range and speed control range are relatively narrow, and when splicing is required, a run-up motor must be used.
(2) DC Motor
This motor allows for high-precision control over a wide range, from low-speed to high-speed, and from low-tension to high-tension. Because DC motors are used for run-up and
brake control, a power supply line is required and the system is large.
(3) Powder Brake + DC Motor
This control method combines a powder brake and DC motor to make up for the shortcomings of Methods (1) and (2), using the powder brake for control at high-tension and using the DC motor for control at run-up and low-tension.
(4) AC Vector Motor
This method is the same as Method (2), but replaces the DC motor with an AC vector motor.
(5) Powder Brake + AC Vector Motor
This method is the same as Method (3), but replaces the DC motor with an AC vector motor.
With gravure printing machines, the stability of the infeed tension has the greatest influence on the precision of printing registration. Therefore, the infeed unit is the most important control unit.
Conventionally, this type of tension control adjusts tension based on a mechanical method using a primary drive shaft that drives the machine via a stepless speed change gear. As long as the speed ratio of the stepless speed change gear remains the same, there is no issue even if the input revolutions (printing speed) is changed. In practice, however, the speed ratio changes slightly. Therefore, the printing unit tension changes between the initial adjustment period and stable running period, so registration defects occur during printing, which prevents high-performance. Further, because the gravure printing roller diameter differs depending on the content of the job, this method uses the stepless speed change gear to readjust the infeed rollers so that they match the circumferential speed when the gravure printing rollers are changed out. This adjustment is extremely difficult and depends highly on the technical sense of the operator, so requires a high level of experience. As a means of decreasing the dependence on the operator’s technical sense, there are methods that adjust the stepless speed change gear while ignoring the tension meter, but these are rarely used today.
The different types of automatic tension controllers in the infeed unit are as follows:
(1) Powder Clutch + Brake
The infeed rollers are connected to the powder clutch output shaft and a powder brake while the input shaft of the powder clutch is attached to an induction motor. The induction
motor’s revolutionary speed is synchronized to the printing speed using an all-purpose inverter. Tension control is made via feedback to the powder clutch and powder brake.
Although this method is inexpensive, its responsiveness to tension fluctuation during splicing, acceleration, and deceleration is poor.
(2) DC Motor
Using printing speed as the standard, this method controls tension using a DC motor to which the dancer roller position signal is fed back. However, the dancer roller mechanism
has mechanical loss and inertia, which can lead to tension fluctuation, so one must be aware of this problem. Further, there are also methods that feed back the tension signal using an infinitesimal displacement tension detection roller. Although this method has good tension responsiveness, because it cannot absorb tension fluctuation during splicing, acceleration, or deceleration, fluctuation is large during these operations. The tension control method that handles these problems is a method that feeds back both the dancer roller position signal and the tension signal.
(3) AC Vector Motor
In this control method, the DC motor is replaced with an AC vector motor. The control characteristic is the same as that for the DC motor, but AC vector motors are maintenance free and the unit’s explosion proof configuration is simple.
Tension Detection Methods in the Unwinding and Infeed Units
Two methods include infinitesimal displacement tension detection and dancer roller tension detection. The details of these will be provided in the section on the rewinding unit.
As mentioned previously, the most important point in eliminating registration and pitch defects in rotogravure printing machines is to maintain a constant tension for the web being printed.
With printed plastic films, the most important condition for stability is the rate of plastic film elongation. This rate determines the printing machine tension. In other words, the elongation or contraction in percent (elongation rate) of the printing roller pitch (length) compared with the product pitch (length) at which the product is printed determines
whether the tension must be increased or decreased. For example, when the product pitch is shorter than the required pitch, the tension is lowered, and when the pitch is longer, the tension is increased. The tension differs depending on the type, thickness, and width of the web, as well as the surrounding temperature and humidity, however, so it is difficult to maintain a constant tension. Equation 3 is the relational expression for the plastic film elongation rate and tension.
T: tension (N), E: plastic film modulus of longitudinal elasticity (Young’s modulus), N·mm2
…is the change caused by temperature and humidity, t: plastic film thickness (mm), w:
plastic film width (mm), ΔL: plastic film elongation length (mm), L: plastic film length (mm), ΔL/L: elongation rate
There are various factors in stabilizing the elongation rate of plastic film, but from Equation 4 we can see that tension is the most important factor.
Splicing in Fixed Unwinding Units
In manually operated splicing units, the operator draws out the web from a new roll, attaches an adhesive tape to the end, feeds the new web between the old web and guide
roller without stopping the machine, and cuts the web from the old roll.
Because this method only requires an easy to mange roll position, it is extremely economical, but because the new web has no run-up (in practice, run-up is provided manually, but run-up speed differs depending on the operator) tension fluctuation is large and there is often printing registration loss. As mentioned in the section on the unwinding unit, the bearing mechanism must have a structure that reduces rotational resistance, but because the roll has weight and uses an unwinding shaft, determining the degree of run-up
and the timing of brake changeover is difficult. As such, splicing requires experience and the results differ depending on the operator. Even if splicing goes smoothly, because there is a momentary increase in tension, the plastic film is stretched and there is an impact on the printing units, resulting in a tendency for registration instability and pitch loss. Further, the amount of material remaining on the old roll depends on the operator, so is not consistent.
Splicing in Turret Unwinding Units
(1) Manual Method
As with the previous fixed type, in this method the end of the new roll is attached with an adhesive tape and the turret arm is rotated so that the web and new roll come into contact, or the end of the new web is fed between the old traveling web and guide rollers to splice the material. In this case as well, tension behaves similarly to that in the fixed method, so there is much printing loss.
(2) Semi-automatic Method
Although this method is almost the same as the manual method, the difference is that run-up of the new web uses center drive (Fig. 13) with an AC vector motor, for example. The speed is automatically synchronized to that of the old traveling web while the operator visually determines the amount of material remaining on the old roll. Pressing the splice command button causes a pressing roller to press the new web against the old traveling web, which completes splicing.
Next, a cutter is used to cut off the old web at a point just after the new web and old web overlap and briefly travel to gether, while at the same time the brake is switched over to complete the splicing operation. Because this method runsup the web, there is less tension fluctuation than with the manual methods, but because the new web and old web overlap, this section becomes a cause of tension fluctuation. As such, this section must be made as short as possible.
In addition to center drive, run-up drive methods also include surface drive, but with flexible package printing, center drive is commonly adopted. Methods for detecting the speed synchronization of web run-up include those that measure the speed by placing
a touch roller against the web, those that synchronize the speed by measuring the roll diameter from the stop angle of the turret arm using a potential meter, and those that measure the new roll diameter ahead of time to preset the speed.
In addition, the cutter blade used to cut the web is often a saw blade.
(3) Fully-automatic Method The fully-automated system is simply started by pressing a splicing command button during operation. The system automatically detects that the remaining web on the roll has
decreased and rotates the turret arm to the desired position. At the same time, the system runs-up the new roll, which has been attached with an adhesive tape ahead of time, and synchronizes the speed to the traveling web. Further, once the system detects that the old roll is almost finished, it splices in the new web while cutting the old web at the same time.
Automatic splicing must ensure splicing 100% of the time without fail and must minimize the remaining web. As such, the following mechanisms are necessary:
a) Pre-drive mechanism (roll run-up method)
b) Remaining web detection mechanism This mechanism uses a revolutionary speed detector attached to the unwinding shaft and infeed rollers to compare the pulse from each with the reference remaining web roll diameter in order to detect the unwinding roll diameter.
c) Glue position detection mechanism
When the touch roller contacts th e-glue position, there is a high chance that splicing will fail. As such, it is necessary to detect the glue position and offset the position contacted by the touch roller and the glue position. The position of the glue is detected by placing a reflector on the unwinding axle at a position with the same phase as the position of the glue, which is detected by a photocell.
Because the shape of the paper core is often non-uniform and because mounting on the paper core is often eccentric, detection of the remaining web length tends to be inconsistent when this method is used for plastic film. Moreover, when erring on the side of safety, there is often more web length left on the roll than when judged visually by the operator. As such, although many machines are equipped with such a mechanism, it is rarely used.
Web End Detection Splicing
This automatic web splicing method is used with both fixed and turret systems. This system automatically detects the final end of the web wound on the paper core when splicing the old and new webs together. Although no web is left remaining on the old roll, because the end of the web must be detected, the web tension drops momentarily to zero and there is no run-up, so there are still factors that cause inconsistency in registration.
We can also see this from the basic equation for tension generated by the inertia of the roll.
T: tension (N) resulting from web inertial force, W: roll weight (kg), V0 : printing speed (m/s) when time t=0, V: printing speed (m/s) several seconds t later, g: acceleration of gravity (m/s2 ), t: time (s)
In other words, the larger and heavier the roll, the faster the printing speed, while shorter acceleration times mean higher tensions, so registration becomes unstable. This method, however, does not require an operator for splicing and allows for the potential to automate lines and reduce labor.
As shown in, this method uses an accumulator unit located between the unwinding unit and the infeed unit. The machine automatically or an operator visually determines when the old roll will become empty and stops the supply from the old roll. During this time, the web is fed from the accumulator to the infeed unit without stopping the main machine and the switchover work to the new roll is conducted manually or automatically.
This method is not often adopted for flexible package printing machines, but is adopted for flexible PVC film, PVC leather, and special printing unwinding units.
Preheater Unit (seasoning unit)
With rotogravure printing machines, preheating of the plastic film is used to improve the printability of the film and the ink adhesion, whereas paper seasoning is used to stabilize
printing registration by ensuring a constant water content ratio and removing internal stress via drying.
By sufficiently seasoning the web (plastic film, paper, etc.) immediately prior to printing, this system works to prevent web trouble during operations and is particularly necessary for multicolor printing.
Although both heating and cooling are used, heating can rely on blowing hot air in the chamber method or rely on a heating roller in the contact heating method. The former is
often used for plastic film, whereas the latter is often used for paper and PVC film. Although standard heating temperatures range form 40 to 120°C, settings must be made depending on the printing speed and condition of the web. Heating rollers are driven using the same drive source as the infeed rollers and are sometimes used as the tension control unit discussed previously. For the chamber method, heat sources include steam, electricity, and LPG, which is often the same heat source for the drying system in the printing unit.
In addition, infrared heaters have good efficiency and are often used, but for reasons related to the Fire Services Act in Japan, are used less often today.
Heating rollers also generally use steam and electricity, but induction heating rollers with uniform roller surface temperatures and high precision are often used.
In either case, an automatic thermostat must be incorporated into the system to maintain the temperature setting at a constant, while precautions must be taken to ensure a uniform temperature is applied across the width of the web. Next we will explain the importance of the preheating unit (seasoning unit).
(a) Most plastic films contain a variety of additives. For example, polypropylene contains anti-static agents while moisture resistant film (polyvinylidene chloride coated film, etc.) contains slippery agents and plasticizers. If these additives seep out to the film surface, the ink will lose its adhesiveness to the web and the printing capability will decrease. In the case of moisture-resistant film, problems occur especially easily in the winter.
(b) Webs such as nylon, standard cellophane, and vinylon are strongly affected by the humidity and the degree of moisture absorption will cause ink adhesion to drop.
For these reasons, the use of a preheater unit is effective in preventing these types of problems.
When printing on paper, hysteresis becomes a problem. Figure 18 shows an example of a measurement of the elongation and contraction of paper in the length and width directions caused by changes in humidity.
During printing, if the paper dimensions remain the same, the registration of the printing units ought to match. Therefore, as long as the water content of the paper during printing is constant, there is no problem, but when printed, the paper absorbs moisture (particularly with water-based ink) and loses moisture when dried in the drying unit. As such, the paper elongates and contracts, so is not uniform and registration becomes difficult.
When paper is made, the paper fibers orient in the travel direction. Although the fibers may expand in either the length or width directions, fibers tend to expand more in the radial direction than the length direction when they absorb water. As such, when paper is moistened, elongation in the width direction can be 2 to 8 times that of the length direction.
Given these reasons, we use a heating roller, for example, to over dry the paper to a water content of 1 to 3%, which is lower than usual (5 to 6%). The goal here is to minimize
and stabilize the elongation and contraction of the paper by printing the paper after minimizing the absolute value of water content variation.