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FLEXO Magazine : August 2009
TECHNOLOGIES & TECHNIQUES PRESS BUYER’S GUIDE Direct Drive Technology Improving Flexo Printing Quality and Throughput By Tom England TABLE 1. BILL OF MATERIALS LIST Geared Press Motor Precision Gears Lubrication System Oil Bath Filter Pump Sealed housing Shafts Bearings Frame members and supports Mounting hardware F lexographic printing technology has become increasingly able to maintain print quality and reduce startup time and waste. Most operators concern themselves with registration and color accuracy and consistency—as they should. But many don’t realize the importance that accurate synchronization of the anilox roller and plate cylinder used in each print deck module plays on print quality. Traditionally, this has been accomplished either by gearing the anilox roller and plate cylinder together and driving both with a single AC induction motor or by using separate servo motors to drive each axis through gearboxes. As press speed and printing quality requirements have increased, the inevitable inaccuracies in the gearing system have become a limiting factor on press print quality and speed. Advancements make it possible to synchronize the anilox roller and plate cylinder to a much higher level of precision without mechanical transmissions by using closed loop control technology and driving both directly with independent, direct-drive rotary (DDR) servo motors. The elimination • Speed control and phasing between the anilox and plate cylinder can be closely controlled in the absence of gear backlash. • The latest generation of servo controllers provide resolution feedback up to 27 bits with 64-bit positioning resolution. • DDR systems offer accuracy of about +/-25arc seconds--up to 20 times higher than conventional geared systems. of the mechanical transmission enables servo loop gain to be increased, and therefore, bandwidth of the servo loop. Speed control and phasing between the anilox and plate cylinder can now be closely controlled in the absence of the gear backlash, thus providing for higher speeds and accuracies for improved print quality. Throughput is also increased because the higher control loop gain enables faster machine operation. This article will examine the trend toward DDR systems in fl exo presses and consider alternative implementation methods. IMPORTANCE OF SYNCHRONIZING In the fl exo printing process, the anilox roller contacts the plate cylinder that carries the plate with a dot pattern that forms the printed image. The dots on the plate act as suction cups and lift the ink out off the anilox roller. Providing constant ink coverage is obviously critical to printing quality and this depends on maintaining consistent motion between the surfaces of the anilox roller and plate cylinder. Whenever the print cylinder moves faster than the anilox roller, less ink is transferred to the print cylinder, resulting in a light section in the printed piece. Whenever the anilox roller moves faster than the print cylinder, more than the normal amount of ink is transferred, resulting in a dark section. The traditional approach to synchronizing the anilox roller and plate cylinders is to use a bull gear attached to the central impression drum to drive them both as shown in Figure 1. The problem with this approach is that backlash is inevitable in any mechanical transmission system. Even when a geared system is tuned very tightly, within a short period of time the gears will wear and backlash will begin to occur. Backlash www.flexography.org AUGUST 2009 FLEXO 49 Gearless Press Servo motors Gearboxes Mounting brackets for gear motors Shaft couplings Direct-drive Press Direct-drive rotary motor