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 closedloop 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 sue.dorscheid@danahermotion.com.