Structural Units (continued)
Rewinding and Splicing Units
The rewinding unit is composed of nearly the same elements as the unwinding unit. In order to meet the requirements of increased printing speeds and operability, the switch to turret type rewinders from carriage and fixed types (parallel dualshaft mounting type and vertical dualshaft mounting type) followed the same course as that for the unwinding unit. See Session 1 (January/February 2014) for details.
Once the printed substrate has been rewound to the specified length, an adhesive tape is wound on the surface of a second core that has been mounted on the second rewinding shaft, the traveling web cut, and the rewinding switched to the new core. In this case, it is necessary to run-up the second prepared rewinding core to prevent the occurrence of tension instability at the start of rewinding. The reason many rewinders use two drive motors is to improve the operability of the above operation. Moreover, with this method
splicing is manual.
During rewinding switchover, the rewinder can rotate the second core to a position that is convenient for the start of rewinding and the splicing unit is relatively easy to install. As such, today all printing machines use a turret rewinder except in special instances.
With turret rewinding switchover, once the roll diameter has reached the specified diameter, the rewinder attaches an adhesive tape to the new core and the turret arm rotates to bring the new core just up to the traveling web. In this case, the new core is run-up to synchronize it to the printing speed. At some arbitrary point, a contact roller is used to press the web against the new core at the same time a saw blade is used to cut the web from the old roll. This method does not require much experience and operations are simple, so it is also suited to high-speed printing. In addition, along with the cutting time being arbitrary, automated methods can be used that link the rewinder to a counter to cut the splicing tape by linking this action with the splicing action of the rewinding unit.
The saw blade can also be heated to heat-cut uniaxially oriented films and thick films.
As with the unwinding unit, the splicing unit is sometimes used together with an accumulator unit.
Rewinding splicing is the general term used to refer to the set of actions that cut the traveling web once the rewound web has reached the indicated roll diameter and start rewinding the web around a new core.
Cutting of the web often uses a saw blade type cutter, as with unwinding splicing, to cut the web perpendicularly to the travel direction of the web. The methods generally used to wind the cut fi lm edge around the new core are listed below.
Contact Roller Method
In this method, which is the simplest rewinding method, an adhesive tape is wound around the new core surface ahead of time, and a contact roller or brush is used to press the cut
edge of the fi lm against the new core as it is cut.
Air Jet Method
As with the contact roller method, an adhesive tape is wound around the new core, but compressed air is blown from a nozzle at the same time the web is cut, which blows the cut
edge of the fi lm against the new core to start rewinding. Compared to the contact roller method, this method applies a smaller load to the core during splicing, which allows for a
smoother splicing action and less tension variation.
In this method, an adhesive tape is not wound around the core. Instead, the web is wrapped three-quarters of the way around the new core before the web is cut, while the web is nipped using the cut edge as the nip point. In all cases, the new core circumferential speed must be matched to the speed of the traveling web.
With rotogravure printing machines, the individual printing unit printing cylinders, the doctor oscillation, and if used, the furnisher rollers, must each be driven. The infeed
and outfeed rollers installed to stabilize the tension of the printing units must also be driven. In special cases, rollers located between the printing units, such as cleaning rollers,
are also driven. In addition, the unwinding shaft in the unwinding unit and the rewinding shaft in the rewinding unit are driven.
Because gravure printing machines are used with paper and plastic films having different properties, as well as the different inks for these, it is necessary to determine the printing speed based on the drying conditions. With flexible packaging, in particular, the diameter of the printing cylinders can be freely chosen within a certain range, so a drive mechanism with a wide range of speeds becomes necessary. For this reason, variable speed motors are often used.
Conventionally, a single drive source was used to drive all of the driven units in the gravure printing machine, but recently, sectional drive systems are being used instead to control the speed of the infeed and outfeed rollers independently. In addition, the variable speed motor drive source is seeing a shift from VS coupling motors and DC motors to AC vector motors, and from analog to digital control methods.
Main Motor: the motor used to drive the printing units
Three-phase Shunt Commutator Motor (AS motor)
Although this type can easily be controlled within a variable speed range of 20:1 (typically), because the speed is varied by changing the brush position, there are constraints and the speed cannot be rapidly changed. Given that brush maintenance is necessary and the explosion proof structure is complex, this type is not often used today.
Coupling Motor (VS motor)
The coupling motor integrates an eddy current coupler and a three-phase induction motor. This type is used relatively widely for printing machines. This motor is easy to control, is maintenance free, and has a simple explosion proof structure, but the variable speed
range is relatively narrow and the motor generates speed fluctuations resulting from load variation.
This motor requires brush maintenance and has a complex explosion proof structure, but the variable speed range can be controlled over a wide range of greater than. DC motors also have an excellent control property.
AC Vector Motor
The primary coil current delivered by a three-phase induction motor can be divided into an excitation current that generates a magnetic field and a torque current (secondary current) that generates torque. Vector control is a method that individually sets and controls the excitation current and torque current by controlling the size of the primary coil current, the frequency, and the phase.
Although control of this motor is relatively complex, the variable speed range can be controlled within a range of greater than 200:1. AC vector motors also have excellent
control properties. The control properties are equal to or better than those of DC motors, and these motors are maintenance free given that they have no brushes. Moreover, the
explosion proof structure is simple, so these are increasingly used as main motors in place of DC motors.
For details, see the drive related sections for infeed, outfeed, unwinding, and rewinding units.
Printing Cylinder Shaftless Drive
In this method, each unit is equipped with a motor, where AC servo motors and AC vector motors are primarily used to handle synchronized control. Although registration can use
either a printing cylinder phase calibration method or compensator roller method, in general, printing cylinder phase calibration methods are standard.
On-machine Interface (in-line machine)
Flexible packaging production processes typically require more than two separate processes to produce one product. One option to satisfy the demands of a market for short run production of many products is to switch to an in-line process that combines the printing, laminating, and coating processes in one machine. The benefits of in-line machines include:
1 reduction of loss from process startup times and materials,
2 reduction of cost and labor for transporting the roll to the next process,
3 low facility costs including machine installation space, and
4 reduction of storage and stock.
As can be seen from this list, in-line machines rationalize the process by shortening the production line, but also have several shortcomings.
1 Difficult to manage the materials for each process.
2 It is necessary to match the process speed for each step.
3 Depending on the product, not all of the process steps are necessary, so such machines do not necessarily reduce costs.
Other options are to adopt a cassette replacement type head for one of the printing units of the printing machine so that the machine can be used as a dry laminator, wet laminator, hot melt laminator, or coater, but from the stance of productivity, this is not ideal. There are also printing machines in which the printing units can use different types of printing cylinders. Because these allow for the production of unique, highvalue products, this approach is also an important option for converters on a level with rationalization. Although there is a question over whether these types of concept facilities will become common, there is sufficient room for consideration given that these machines that can handle short-run production and meet diverse market demands.
• Examples of combined printing method machines
1 Attach a gravure printing unit at the end of an offset rotary line.
2 Attach a fl exo printing unit to a gravure printing machine.
3 Attach a rotary screen printing machine to a gravure printing machine.
4 Attach a letter press unit to a gravure printing machine.
5 Exchange a gravure printing cylinder with a flexo printing cylinder in the same printing unit.
• Examples of In-line machine systems
1 Die Cutter
With in-line printing machines for thick paper, such as those for milk and juice cartons, a creasing unit that applies a ruled line and punches the material along the printed image, and die cutter units are connected, which produces carton blanks from the substrate in one pass.
2 Coater, Laminator
With this in-line machine, a coater and laminator are connected before or after the printing units to produce the item in one pass. Laminators and coaters include dry laminators (solvent, non-solvent), wet laminators, hot melt laminators, lacquer coaters, primer coaters, and PVDC coaters.
3 Film Extrusion Line
This in-line machine conducts simple printing by attaching a printing machine to a blown extrusion film line or a T-die extrusion film line. 4 PVC Wallpaper Production Line This in-line machine connects a laminator unit and an embossing unit to the printing machine, or an embossing unit to a coater to produce the wallpaper in one pass.
Conventionally, the quality of gravure printing was based on the skill of the operator or a sample, and there were few devices for effectively evaluating print quality during operation
of the machine. As such, it was often the case that the operator would decide whether or not to make corrections. In contrast, there are now closed-loop control systems that automatically analyze the print quality during operation of the gravure printing machine, provide this as feedback, make an evaluation, and automatically make corrections.
• Example of a closed-loop management system
1 Tension Control (unwinding, infeed, outfeed, rewinding) Here, a tension detection roller (dancer roller) detects the tension, which is compared to a reference value in the control unit, which sends a feedback signal for the deviation to an actuator to make corrections. In addition, the system repeatedly detects the tension and compares this to the reference value until the deviation disappears.
2 EPC Unit
This unit determines the reference edge position of the web, and when the substrate shifts from the sensor position (reference position), the EPC sends a feedback signal to the actuator, which corrects the web position. This action is repeated until the web returns to the reference position.
3 Automatic Registration Unit (length/width directions)
4 Ink Viscosity Control
5 Automatic Drying Temperature Adjustment Unit
6 Printing Pattern Static Imaging Unit
7 Defect Detection Unit
Until now, methods that provided a still image for viewing printed matter smudging, printing gaps, and register misalignment in a static manner included mechanical methods that used a refl ective mirror that was synchronized to the printing machine and methods that flashed a stroboscope synchronized to the printed matter. Both methods relied on the image retention property of the human eye, but there were many problems with observing the pattern as a fully static image. Recently, observation systems that use a CCD camera have come into common use, but because a processed image is viewed by eye and corrections made by the operator, it is incomplete as a closedloop management system.
This closed-loop system has problems including how print quality confirmation (printing smudges, printing gaps) should be processed, how these should be corrected, how artificial intelligence functions should be adopted, and what their capability is. These will be the keys to whether an image processing unit is included in the closed-loop system.
8 Ink Density Observation Unit
This unit analyzes the color of the printed matter during printing using a mark for measuring density. The unit compares the color to a pre-set color and if the printed color density exceeds the tolerance error for the tolerance value, the system sounds an alarm. It then automatically supplies either solvent or ink to correct the difference. This system includes an automatic scale and an automatic supply system to supply the required amount, as well as handles ink blending, viscosity, and temperature control.
If these systems can be perfected, it is assumed that closedloop systems will allow for the development of operatorless printing machines.
Printing Machine Group Control
In Japan’s printing industry, there is a significant number of problems, including an absolute shortage of operators, a shortage of skilled employees, and a reduction of employees per machine to ensure profitability. One measure that has been implemented to resolve these problems is the development and usage of various peripheral equipment, but
in general three employees are still required for automated printing machines. To further reduce the manual labor requirements, group management of the printing machines is necessary.
Moreover, effective installation and rational placement of the various peripheral equipment are essential for a small number of operators to run the machines. In particular, logistics concerns are critical, while a smoother flow will improve productivity as well as reduce the burden on the operator.
Logistics includes the repetitive heavy labor of transporting unprinted rolls, rewound products, printing cylinders, and ink, which were handled by the operator. Logistics is the movement of objects, and the fundamental printing operations of color matching and registration are only one aspect of the work. In the past, such logistics were not often considered, whereas the installment of quality related peripheral equipment was prioritized. Today, however, given the shortage of personnel and labor reduction for operators, there is an increasing focus on and installment of peripheral equipment related to logistics, and a lot of time has been focused on quality assurance. Even with printing machines that include automated options, three operators are still required, so it is assumed that further reductions in personnel are difficult.
The goal of group management is to reduce the workload on individual operators and increase productivity. In general, here we can envision group management that considers
two gravure printing machines as one unit. In the future, as printing plants and the individual printing machine units shift towards full automation, several printing machines will be operated as one unit under group management.
1 Group Management Layout
There are two possible layouts. The first is to align two printing machines in parallel so that they operate in the same direction and the operator sides are opposite. The second is to face the operator sides across from each other and operate the two printing machines in opposite directions (mirror image). In the latter case, the work space is wide and the two printing machines can be operated, and even if the other machine of which the operators are not in charge has a problem, they can respond quickly.
In general, when the two machines face each other and operate in mirror image, it is best to align two of the same machines. Moreover, with group management, it is best if the operators are not assigned to a specific machine, but if three to five operators efficiently operate the two machines. When the machines are parallel in the forward or reverse positioning, group management is generally difficult. In addition, it is necessary to consider space for transport of the unprinted rolls, rewound product, printing cylinders, and ink.
2 Operations Room
Here, an operations room is installed between the two printing machines, in which the printing machine operations panels, monitoring equipment for the various peripheral
equipment, and management systems for control are located to allow for centralized control.
To handle the above mentioned group management, it is necessary to consider both printing management systems and logistics management systems.
(1) Printing Management Systems
These systems observe the printing status of two printing machines from one room, and display both the progress control of the work as well as trouble spots during emergencies. The design should allow the operator to deal with malfunctions in advance.
1 Pre-set: printing conditions including web width, tension, hot air drying temperature, printing pressure, doctor position, printing speed, register
2 Monitoring: pre-set items, printing register, printing quality inspection, energy (electricity, steam, etc.)
3 Abnormality Diagnosis: machine maintenance inspection (requires the machines to be stopped), operational status (check necessary operational conditions during start-up of printing machines)
(2) Logistics Management Systems
It is necessary to supply the necessary items at the required time to each of the printing machines to realize smooth production, so devices are required that can transport the
unwinding roll, the rewound product, the printing cylinders (exchange of new and old cylinders), and ink without overwhelming the operator. As such, in the future it will be necessary to install automated transport control systems and improve the production management systems.
The unprinted roll prepared for automated splicing will automatically be transported from the storage area and be automatically mounted on the printing machine unwinding unit. The empty core will automatically be recovered when the new roll is mounted.
As with unwinding of the unprinted roll, a rewinding completion signal will automatically trigger the removal of the rewound roll and an automated transport cart will stack the
roll at the desired location. The roll can also be transported to the next process.
3 Printing Cylinder
If the printing cylinder for the next job is set on a cart outside of the printing machine, the next process can be operated automatically. After printing, the ink pan will be lowered from the mounted printing cylinder automatically, the used ink recovered, solvent applied to the pan, and the ink left on the printing cylinder surface washed off without an operator having to do much work. The cleaned printing cylinder will be removed and the printing cylinder for the next job that is waiting on the printing cylinder cart will be mounted. Once set in the fixed position (using an automated system up to the control range of the automated register system), the previous printing cylinder will be transported to the desired storage position.
The above chapter 8 is the closing part of this Technical Article. Reprinted by permission from the March-April 2015 issue of Convertech International, published by the Converting
Technical Institute, based in Tokyo, Japan. www.ctiweb.co.jp/. Convertech is the monthly technical journal that covers all the converting technologies for webs and sheets, including
films (simple, functional and multilayer), metal foils, paper, paperboard, functional paper, nonwoven fabric, synthetic paper, textile, metal plates, carbon fiber complex sheets, thin
film glass, ceramic sheets, and foamed sheets.