Multiple-Axis Presses Step Up the Motion in Pad-Printing
The versatility and efficiency of pad-printing equipment has become even greater with the introduction of machines that employ stepper-motor-driven indexing systems
Technology tends to advance in fits and starts, and pad-printing equipment is no exception. New machinery is developed by manufacturers either as a means to overcome specific technical hurdles or in response to competitive pressures. On the following pages, we'll look at how the old technology of pad printing has learned new tricks when confronted with both of these issues. And we'll explore how improved equipment designs are opening doors for the pad-printing process in industrial-imaging applications. In particular, we'll consider large-format applications involving consumer appliances, where new pad-printing machines are making the technology viable as an alternative to other decorating methods.
To understand how pad-printing presses have evolved for these applications, it's best to start by examining trends in consumer-product design, particularly in Europe (where the vast majority of pad-printing equipment is also manufactured). One of the most influential trends has been the dramatic change in the design of control panels used on new European appliances, such as washing machines, dryers, stoves, dishwashers, etc.
In the past, appliance panels were typically flat. But in recent years, designers have gone to great lengths to incorporate more attractive curved surfaces into their panels with the aim of differentiating their products from those of competitors (Figure 1). To make matters more interesting, new panels often feature an assortment of multicolor graphics across their surfaces, and the panels may be as wide as the appliances on which they are mounted.
This design trend is only beginning to edge into the US marketplace. But the pad-printing equipment developed to address it has implications beyond just the home-appliance market. Large-format, multicolor pad-printing capabilities hold promise for a broad range of applications that have yet to be explored.
Evolving equipment for industrial apps
When curved and contoured panels emerged in Europe, the only available options for decorating them were to use in-mold decorating technology or turn to conventional pad-printing equipment. Neither option is ideal, especially considering that the same panel might be used on products sold in a dozen different countries with a dozen different language requirements. For the decorator, this means the order isn't really a single project, but a collection of short-run jobs.
Adding to the difficulty is the fact that many appliance OEMs demand just-in-time manufacturing on their orders. Under these circumstances, the make-ready required for in-mold decorating and the frequent press changeovers necessary with conventional pad-printing equipment simply aren't practical.
But with the latest generation of pad-printing machinery, it is no longer necessary to perform multiple setups, use multiple machines, or rely on inaccurate conveyors to move large-format parts through a multicolor pad-printing line. Today, pad presses are available that can print the entire length of large panels with as many as ten colors and deliver more than 60 im-pressions/cycle to achieve production rates of several hundred parts per hour. The secret behind this new level of performance is found in the stepper motor and the digital systems that drive it.
Stepper-motor technology in pad printing
Stepper motors have revolutionized the pad-printing industry in Europe. In basic terms, a stepper motor is a digitally controlled electric motor that doesn't rotate continuously, but moves in small increments or steps of a predetermined distance (measured in degrees). Stepper motors are controlled by computer programs or similar digital control systems that tell the motor how many steps to move and in which direction. Using this strategy, the motor can be used to repeatedly and precisely control the motion and position of any mechanical apparatus to which it is attached.
A stepper motor can be programmed to travel specific distances at specific speeds. When applied to pad printing, stepper-motor technology allows the machines to precisely control the position (linear or rotational) and motion (speed, direction, etc.) of parts along multiple independent axes (Figure 2).
This programmability means better process control, even for someone producing a simple single-color application in which parts are manually loaded and unloaded from the press. And in multicolor industrial applications, pad presses driven by stepper motors can provide a faster and less expensive alternative to in-mold decorating or direct screen printing.
Before steppers, pad presses were either driven with simple pneumatic cylinders or electromechanical devices that could only travel a non-adjustable distance. Sure, a press might employ a mechanical stop or a limit switch, but we couldn't move that stop or switch during a print run to vary the distance the machine moved in any particular direction. With a stepper motor, however, we can select the exact distances we need the printhead and part to travel in each axis and vary these distances over the course of the print run if necessary. Figures 3A-3G illustrate one scheme for using a stepper-motor-driven pad press and part conveying accessories. This sample application represents a six-color appliance panel job in which the panels are conveyed along two axes of motion during the printing cycle.
In Figure 3A, the part is in the "home" position, nested securely in a fixture. The fixture is mounted to a linear indexer that travels along the Y-axis. That linear indexer is, in turn, mounted to a second linear indexer that travels along the X- axis. The "home" position on the press does not have to be the same as the first printing position on the part; the part could easily be indexed into the first printing position at the start of the cycle. This allows the machine to incorporate proper safety guards without affecting the efficiency of machine operation The complete printing cycle on such a system is illustrated in Figures 3B-3G.
This sequence demonstrates six-color (or six-image) printing following a single-step program. But this isn't our only option on a press driven by stepper motors. We could also reprogram the press to employ a two-step program and place a pair of images on each cliche, effectively doubling the number of graphics we can apply during the print cycle.
Stepper motors that drive the pad assembly can be programmed so that each pad picks up images from different locations of the cliches at each stage of the print cycle. By "stacking" images on the cliches from front to back, we could repeat the cycle we just illustrated using secondary image-pickup locations. Depending on the size of the cliches used, this approach could work with even a greater number of images per cliche, allowing dozens of images to be printed during the same cycle.
What if the press doesn't have enough space in front of or under the cliches to accommodate long parts? To solve this problem, instead of using two linear indexers, we replace one indexer with a stepper-motor-driven rotary table. By indexing along the X-axis and rotating the part, we can print longer items. For example, images 1-3 may be printed at one end of the panel, which is then rotated 180° to allow printing of images 4-6 on the other end.0
If our application requires that we index in both X and Y directions, as well as rotate, we can put the second linear indexer (for the Y-axis) back on the press and under the rotary table as illustrated in Figure 4. If we're working with more difficult three-dimensional parts, other handling system can also be employed. For example, Figure 5 depicts how a small rotary fixture (also stepper-motor driven) can be mounted vertically to a linear indexer. The part can be easily rotated to allow placement of multiple images around its entire circumference.
It is not uncommon for owners of stepper-motor-driven pad presses to have several different part-conveying accessories on the shelf to accommodate a range of different applications and movement schemes. With today's user-friendly digital operator interfaces, reprogramming the press for these accessories is simple and straightforward, allowing the machines to be easily reconfigured. Some European printing operations have developed routines that allow them to print as many as ten colors along six axes of motion, with over 60 impressions/cycle.
Figure 5 Indexing for 3-d parts
For three-dimensional parts that need to be printed around a circumference, a vertically configured rotating indexer can be used
Other benefits of stepper-driven pad presses
Besides supporting a broad range of part-conveyance accessories and motion sequences, pad presses driven with stepper motors also allow users to employ combinations of open inkwells, sealed ink cups, and varying cliche sizes in a single setup. The first image might be very small, requiring only a 60-mm (diameter) ink cup, whereas another image in the sequence may be 215 mm wide and require a 250-mm open inkwell. Even though all the cliches are doctored simultaneously on such a system, the use of stepper motors and specialized ink cups and/or doctor blade combinations allows the operator to use only the cliche sizes needed.
What about registration? Most stepper-motor pad-printing machines come with prepress equipment specifically designed to "preregister" films to cliches before they are exposed with the image. Typically, polymer cliches are used that are prepunched with holes that correspond to pin-registration systems. The films are then aligned and punched with registration holes so that they align precisely where they're needed on the cliches. The machine components driven by stepper motors generally maintain tolerances of ± 0.01 mm.
To reduce changeover times, some presses allow the entire cliche platform to be rotated or completely removed from the machine for cleaning or replacement. By having spare cliche platforms and accessories available, changeover can take as little as 15 min for applications that involve as many as ten colors.
Dozens of other applications are also good targets for the technology of stepper-driven pad presses. Bezels, for example, can benefit from these machines and their ability to support larger part sizes than conventional pad-printing presses. Just consider the variety of products that may require graphics on their bezels, such as computer monitors, home video products, and other electronic devices. Medical-device control panels are another large but untapped market that has potential for shops using multi-axis pad-printing equipment. And the list goes on with everything from automobile dashboard components to toys.
Labor reduction is a key justification for adopting stepper-driven pad presses. These labor savings are realized primarily by interfacing the machines with other elements of an automated production line. Because they are stepper-motor driven, the pad presses can be easily connected with the programmable logic controllers and computers that drive other systems, creating an integrated manufacturing system that be controlled through one operator interface.
Stepper-motor technology also allows machines and part-conveying accessories to be easily integrated with non-print-related automation. Picture a part-handling robot and a stepper pad-printing system configured with a plastic-molding machine. When the mold opens, the robot rapidly removes two bezels that were just formed, placing the parts, one at a time, on a rotary table. The table indexes to print the first bezel, then again to print the second. While the second is being printed, the first is being dried with a hot-air dryer that is also integrated with the system. As the parts leave the dryer, the robot picks them up and deposits them in a package, which is sealed and whisked off by a conveyor to the shipping department. By eliminating several material-handling steps, these machines working together can mold, convey, print, dry, and pack more bezels than a team of human workers using conventional production methods.
The future now
Orchestrated by the programmable logic system, the motions of a stepper-motor press are fantastic to see, and the speed at which complex printing jobs can be processed becomes instantly apparent. Our first exposure to this technology occurred at a trade show in Milan, Italy, where we watched in fascination as a stepper-driven pad press printed toy train cars on five sides using a combi-nation of open inkwells and sealed ink cups. The experience was like being a UFO geek with a day pass to Area 51.
The transition of pad printing from a labor-intensive specialty printing process to an automated industrial decoration method has been made possible by advances in many areas, from computer-based filmmaking and photopolymer cliches to self regulating inking systems and digital controls. The introduction of pad presses driven by stepper motors adds even more capability to a process already sought out for its versatility. With this technology, industrial printers can provide an efficient and accurate decorating method that supports both high-volume, high-speed production and short-run, just-in-time applications.