How does a typical printing machine work?
Today's most common printing machines use a flexographic process. Flexography is a direct rotary printing method that uses a raised image surface to print on a variety of substrates. The printed image is created on rubber or photopolymer plates by removing and lowering nonprinting areas. The patterned plates are then attached to rotating cylinders of various diameters to produce images.
Ink is transferred to the plate surface from a cell-structured ink-metering roll called an anilox roll. An ink fountain working in conjunction with a doctoring blade is typically used to supply the anilox roll with fluid ink. One plate/cylinder/roll system exists for every color that is printed. The average machine today has about ten of these printing stations, with webs ranging in width from 6 to 136 in.
What are the primary types of motion in printing applications?
There are a wide variety of printing applications, from simple one-color printing to complex multi-color printing with registration. Most printing setups use a rotary print head that contains the image to be printed; typically, the print surface is a web of material. The web is often linear and can be any of a wide variety of materials including paper, plastic or poly film, corrugated, and so on. Ink is applied to the print head, and when it contacts the web, the image transfers to the target area.
The primary motion setup in printing applications is master-slave, where the web of material to be printed on is the master, and the print head is the slave. Traditional printing machines have a mechanical link between the web and print head, but most newer machines use servos employing electronic cam-shaft profiling to drive individual axes. Compared to mechanical linkages, digitally controlled servos make printing machines infinitely more flexible and especially suitable for short production runs and quick changeovers.
In multi-color printing applications where there are multiple print heads (one for each color) slave axes must not only maintain registration with the master axis, but with other slaves as well. It is not uncommon for such printing applications to also include other types of motion; for example, setup axes might move print heads in and out during machine maintenance and product changeover. In short, with high-performance digital motion controllers there's really no limit on the number and function of motion axes.
Other motion components found in a typical printing application (besides servomotors and controllers) include gearboxes, bearings, couplings, servoamplifiers, and feedback devices such as encoders or resolvers. This same set of components, similarly configured, is viable in many other converting applications where a rotary slave axis contacts a linear web — applications including die-cutting, embossing, perforating, and rotary knives.
What are the main challenges in implementing motion in a printing process?
One challenge is regulating the speed of the print head and web. In particular, the speed of the print head must be precisely controlled when it's in contact with the web. Any unintentional speed difference (faster or slower) degrades printing quality and can even damage the web material.
In some cases, the circumference of the print head is the same as the product length. Here, the motion relationship between the head and web is a gear ratio synchronized with position. In other words, the speed of the motors driving the head and web is a constant ratio, but the print head (slave) locks on at a particular web (master) position to place the ink in the right spot.
If the circumference of the print head is different than the product length, the speed match occurs only while the print head is in contact with the web. For the remainder of the product length, the print head either speeds up or slows down to contact the web at the appropriate point for the next product.
In printing, motion profiles must be implemented so they're not only synchronized by speed, but also position. This ensures that the master-slave relationship is valid at all speeds, from stop to full speed, which in turn minimizes scrap. Speed depends largely on the substrate material. Labels, for example, typically print at speeds from 300 to 1,000 fpm. Paper, on the other hand, can be printed at speeds in excess of 3,000 fpm. Plastic is somewhere in the middle, from 1,000 to 2,000 fpm, assuming the use of CI flexo (central impression flexographic) printing, where the web wraps around a large central impression drum and the print heads are positioned at various locations around the drum.
Naturally, there are exceptions. Sometimes the web and plate are not speed matched. For example, the web may intentionally “overspeed” or “underspeed” the plate roll at 2% of the repeat length. This can save on plate cylinder and sleeve inventory, as well as material.
Most printing applications also require registration, aligning multiple patterns. The best registration is within 0.0005 in. (With CI flexo, it's within 0.002 in., while for inline printing, it's within 0.003 in.) To meet such tight tolerances, motion profiles must be adjusted on the fly to compensate for small variations in the distance between individual registration marks on the web.
Another concern when implementing motion in a printing application is the length of the product. Motor/amplifier combinations, in particular, must be sized for the entire range of product lengths. Note however, that the smallest and longest products may not represent the worst case situation for sizing. We recommend that the motor-amplifier be sized for several different product lengths — say, five or ten. Then all servodrives can work well over the entire product range.
Where can motion technology make the biggest difference in printing?
One area is high-speed product registration. This takes advantage of high-speed position latching inputs, either on the drive or motion controller, to capture the exact master-axis position coinciding with the registration mark. The difference between this position and the one previously captured is used to calculate the actual distance between registration marks. This distance is compared to the theoretical distance between marks, generating a correction factor that's applied to the slave axis' motion profile.
The elapsed time from when the registration mark is detected to when the correction is applied is critical to product quality. It depends solely on the motion controller, although it may be affected by the technology used. Some digital motion-control networks, for example, have a transport lag that may be intolerable or, at the very least, require some sort of compensation.
Modern motion control is also making printing machines more flexible. A typical ten-color press with a base configuration may incorporate over 65 axes of closed-loop motion control; bigger lines easily reach 100 axes. In the past, each axis was mechanically geared, which limited flexibility as well as registration. But now with updated control systems, registration is half of what it used to be. In addition, printing machines are running more and more unique jobs and are providing infinite variable repeat.
These improvements (along with flexographic technology) also raise the bar on productivity, bumping up press speeds from 1,200 to 2,000 fpm. At the same time, transitioning a machine from one job to the next has dropped from an average of several hours to 30 min, and in some cases even less. Robotics and deck positioning systems, both of which employ motion control, also contribute to faster changeover times. The same can be said for the availability of diagnostic information.
For more information on motion in printing contact the author, Sue Dorscheid, at (920) 906-7804 or via e-mail at [email protected].