Servodriven technology long used for conventional offset presses now gets applied more widely in flexo printing.
Judging by the most recent print media exhibitions, shaftless printing is seeing wider adoption in the label-making industry. The drupa show in Dusseldorf, Germany, last month provided evidence that new flexographic printing presses increasingly employ servo technology to synchronize print axes.
The motivation for label makers is the same as for conventional offset-printing applications where servo technology has been applied for years. Shaftless drives incur less make-ready time and can print with higher quality than presses driven by traditional mechanical driveshafts and geartrains. The reason is that servodriven presses are electronically synchronized by a master motion controller. This eliminates torsional twist and gear backlash that get transmitted down a mechanical driveline when print stands derive power from a central drive shaft.
The limitations of multistand mechanical printing presses are becoming widely recognized. The line shaft, gears, gearboxes, and clutches that synchronize press stands all have a mechanical tolerance. Tolerances of components in the system are cumulative. The total deviation from one end of the driveshaft to the other can be appreciable. The accumulated tolerance manifests itself as colors that are out of registration by varying amounts.
Accumulated tolerance also makes it clear why a mechanical press experiences a high degree of position error during acceleration and deceleration. Stiffness is the issue. Gears and shafts deflect and spring back to their steady state as the machine gets up to continuous speed. Impression rolls are out of registration while the system stabilizes.
In contrast, shaftless drives employ ac servos synchronized to one common tolerance. An example of the tolerances possible comes from Bosch Rexroth Corp., a German manufacturer of servos with U.S. offices in Hoffman Estates, Ill. Bosch says its ac servomotors can maintain synchronization within 0.5 arc min, or 0.008°. All motors maintain synchronization regardless of their position within the web lead, even during acceleration or deceleration ramps.
Make-ready in such a system can be relatively easy because press operators can independently rotate each print unit, die cutter, or sheeter. Plate cylinder changing, wash-ups, and cutter setup can take place simultaneously rather than sequentially.
The shaftless systems transmit signals from a motion controller to each drive via some sort of network. Bosch controllers, for example, use a fiber-optic Sercos (for Serial Realtime Communication System) connection. Network connections also make it possible for operators to see drive diagnostics and make motor-specific adjustments such as register offsets.
Shaftless presses also can carry out what is called dynamic synchronization. This makes it possible to change plates while maintaining full production speed on presses that have an extra print unit. The idea is to change the plate cylinders on the extra unit while the main units are printing. This is only possible on electronically driven presses that can
each be positioned independently. The extra unit then gets synchronized to the speed and position of the running press. The main print unit is then taken off impression and the extra unit goes in. This changeover takes just a few seconds and produces only a few dozen copies of waste.
The same technology makes it possible to change full web leads. Press makers say an eight-color servocontrolled press can change from one four-color job to another in a matter of seconds.
Cutting cylinders on shaftless presses generally make use of electronic camming, the ability to speed the motor in a single revolution to simulate a cam lobe. Put another way, a motor/cylinder accelerates during a set part of the cylinder revolution. This lets a cutting cylinder cut various repeat lengths with no change in the cutting apparatus. There's no need to buy length-specific cutting cylinders.
Servos excel at narrow gauge printing
One recent example of servo technology in narrow-web printing equipment is the Variable size Offset Printing (VSOP) press from Drent Goebel Graphic Machines Eerbeck in the Netherlands. The VSOP is billed as combining the advantages of flexographic printing with the strengths of rotary offset printing. It uses inexpensive image-recording media and colors that are more environmentally friendly than is the case with rotogravure printing.
Servo technology lets the print towers run in synchronization. Each color is applied in its own print tower. To keep shifts between plate and paper to a minimum, the towers control the difference in pressure rollers amounts to no more than 0.01 mm all the way through the print stands.
Four ac servomotors from Bosch Rexroth drive print cylinders in each VSOP print tower. Two spindle drives set support arms which, in turn, support the various roller diameters. There are also pneumatic components to control the relevant strokes as well as color settings.
Rexroth supplies the fundamental drive components and systems. The Synax 200 servosystem uses a Sercos scheme for communication between servo components. Communication with a Scada system is via ArcNet.
FLEXO FOR DUMBOS
Flexographic printing differs from other kinds of offset printing technologies through the use of a special roller for applying ink. The anilox roll has an unusual textured surface that determines how much ink will deposit on the printing plate. The cellular engravings on the surface of the roll are analogous to a honeycomb. They are typically created by a laser. The roll material is generally ceramic.
The special texture on the anilox roll provides a high degree of control when dispensing ink. This sort of control is necessary for handling substrates with a wide variety of imaging qualities, as is the case when printing packaging material or labels.
Thus in flexography, ink transfers to the patterned anilox roll and then to the printing plate. The plate is then ready to put an image on a label, box, or other container. This procedure contrasts with offset and letterpress processes where smooth rollers transfer the ink to the plate. And in gravure printing, the image cylinder makes direct contact with an ink bath before imaging the media directly.
A Model 8518 Sevoflex flexographic printing press from NilPeter USA in Cincinnati is installed at LTi Printing in Sturgis, Mich. The presses are built around an electronic line shaft system from Bosch Rexroth. LTi uses the press to handle substrates ranging from 0.001-in. film to 0.02-in. paperboard carton stock.
Electronic gearbox functions let the servomotor at each driven axis run at a given ratio with respect to the main press speed. This sort of electronic gearboxing lets servos drive various cylinder sizes (such as plate, impression, and tension cylinders) within presses such as those from NilPeter. If the gear ratio is 3:1, the motor will run three times faster to maintain the same surface speed and proper cylinder position. Similarly, servos operating in velocity and torque mode provide the ability to control web tension. The draw roller speed can be adjusted to increments of 0.01% on Bosch systems. For closed-loop tension applications, draw roller speed can be adjusted based on the motor torque or feedback from a tension load cell.