by Fernando Martinez, Bosch Rexroth Corp.
A new generation printing press system integrates frameless motors, regenerative drives, an advanced controller platform and linear motion components that improve productivity and print quality.
Designing a new generation of printing presses from the ground up can be a daunting challenge. There are engineering challenges and customer expectations to consider, not to mention design-time and budget constraints. However, from the system designer’s perspective, the process can be simplified by working with motion-control suppliers and systems integrators who understand the nature of the problem and can streamline a solution.
Such was the case of Paper Converting Machine Company (PCMC) when they set out to develop a new generation of presses a few years ago. The company sought to satisfy several customer requirements, most of which focused on reducing the factors that affect printers‘ productivity and competitiveness. These included faster changeover times (which lead directly to more profitable printing time) and eliminating physical waste, such as film.
Customers also wanted to reduce the time it takes to set up registration and impressions to print good product. To achieve those goals, PCMC focused on using servo drive technology to make this step faster and easier. Two additional factors that needed to be addressed were reduced energy consumption and reduced system complexity, to keep costs down and make press maintenance easier.
The end result is the Fusion Flexographic printing press, an all servo-driven platform that includes up to 21 axes for a ten-color press, or 17 axes for an eight-color machine. There are also seven axes for web conveyance and tension control from the unwind axis to the rewind axis.
To develop the Fusion system, PCMC worked closely with Bosch Rexroth and local distributor and system integrator CMA/Flodyne/Hydradyne. The Rexroth drive and control systems powering the Fusion press include IndraDrive M servo drives with regenerative and DC bus sharing capabilities and IndraDyn T frameless torque motors, all controlled by the IndraMotion MLC motion logic controller. IndraMotion for Printing provides a full range of pre-engineered technology function blocks to solve the most complex printing and converting challenges. Implementation of these algorithms shortens engineering time and minimizes field support typical with implementation of new control algorithms. The simple configuration and the high diagnostic level of Sercos (the automation bus) also reduces engineering efforts during start-up and commissioning. The service computer can be connected to any free port of the Sercos communication system having access to all the Sercos nodes via the Unified Communication Channel (UCC) without affecting the real-time behavior of the bus.
Reducing setup time
The first area the development team focused on was changeovers. Reducing changeover time when a new job is put up on the press adds to profitable printing time.
Each Fusion system consists of a large central impression cylinder, eight to ten feet in diameter, radially surrounded by eight or ten printing decks with two cylinders, one containing the image and the other applying the ink. All of the cylinders are driven by Rexroth IndraDrive drives and motors. During changeover, once the color cylinders are exchanged and the press web rethreaded, setting the image registration and impressions is the next step. In the past, this involved time-consuming manual adjustment, deck by deck.
Using intelligent servo drives, PCMC developed a new setup feature called PrintSense. The image cylinder and anilox inking cylinder in each deck are brought together to reach the so-called “kiss point” where the image impression will be correct. Motor feedback data generated by each cylinder’s drive is captured and used by pre-established algorithms to calculate when the proper feedback setpoint is reached from both drives, eliminating the need for time-consuming initial print setup.
The added safety features and regenerative drive capabilities give the Fusion system a competitive edge.
Dave Wall, Engineering Value Stream Leader for Printing, Coating and Laminating at PCMC, said: “During setup, we can use the IndraDrive Safe Halt feature while we change the plate; normally we would have to build a brake or lockout mechanism to safely hold the axis in position, adding more parts to the system.”
As for the regenerative drive feature, it lets the Fusion system capture energy typically wasted when machine axes slow or stop, feeding it back into the system power bus to reduce energy demand.
Controller, motors and mechanical design challenges
Simplifying the user interface was another design goal, which helps improve system productivity. The focus here was on the printers themselves. Printers are more than machine operators; they’re skilled craftspeople tasked with ensuring the highest quality printing production. To help them exercise their craft, PCMC chose the Rexroth IndraMotion MLC motion logic controller to operate and control the tension and registration of the Fusion press.
The IndraMotion MLC system is suited for synchronizing multi-axis systems up to 64 axes. It features pre-engineered function blocks for winding, registration, tension, and other key settings, open system interfaces, and support for IEC 61131-3 function blocks and motion sequences.
The MLC helps to complete image registration faster, using an integrated end-of-line vision inspection system. The vision system shows the press operator the image registration at the operator controls. The operator can then use the controller to advance or retard a color, or move left or right, to get the registration correct.
System integrator CMA/Flodyne/Hydradyne used Rexroth IndraDyn T frameless motors to drive 11 of the Fusion’s axes. The torque motors are liquid-cooled kit motors consisting of a separate stator and rotor designed for maximum torque applications up to 13,800 Nm. The rotor is mounted directly to the extended journal of the cylinder, rather than coupling a traditional servomotor shaft to the axis. The central impression cylinder can be greater than 80 in. in diameter. In order to hold 0.001-in. accuracy on a 40-in. radius, the motors use a 32 million ppr sin/cos feedback device.
Doug Anderson, sales engineer with CMA/Flodyne/Hydradyne, worked with PCMC from the outset to select and size the IndraDyn motors. “We selected one torque motor size for the large impression drum, which needs to stop within 10 to 15 seconds, and then another size to handle the different widths of the image and color rolls,” said Anderson.
Using the IndraDyn torque motors let PCMC reduce the Fusion’s part count by nearly 60% compared to the previous generation machine. The frameless motors also allow the Fusion to accelerate six times faster than in the past.
Another crucial set of mechanical design challenges on a printing press are the decks mounting the image and inking rolls. These rolls range in width from 42- to 75-in. Each deck must move in and out for job changeovers when the press is re-webbed and the rolls exchanged. However, once printing starts, the decks have to be extremely rigid and hold the rolls in position in order to keep each color layer in perfect registration while the rolls spin up to maximum speed.
To attain this balance of needed flexibility and rigidity, each image cylinder and inking cylinder incorporate two Rexroth precision ball screws and four profiled ball rails. The ball screws and ball rails combine high rigidity and high load capacity in compact sizes. The mechanical elements of the press can’t move around when printing because that movement will show up in the print. So the mechanical elements must have the needed rigidity and resistance to backlash to ensure error-free prints.
“Working with Rexroth and CMA/Flodyne/Hydradyne we did a lot of complex engineering up front and chose the technology to help us make the system less complex,” said PCMC’s Dan Wall. “Since the Fusion was launched, we’ve had several customers tell us that the most compelling aspect of the design is its simplicity—and that’s helped to differentiate us from the competition.”
Paper Converting Machine Company (PCMC)