What's My Line?

Experts debate the role of electronic line shafts in register control.

By Susan Friedman

Earth-shattering advances, break-through innovations? Well, not exactly. Developments on the register control front are a little more low-key at the moment.

"Not a lot has happened [recently in register controls], other than improvements in human-machine interface to make life easier," notes Jack Woolley, president of PC Industries. "Refinements will be subtle, small, and ongoing."

However—if the focus is expanded to include progressions in press motion control systems, specifically the electronic line shaft and its ultimate implications for register control, the discussion becomes a little more animated.

Some suppliers believe electronic line shafts can eliminate, or at least minimize, the need for separate register control systems; others say opting for such state-of-the-art press designs won't improve or change register control requirements, and in fact may just confuse the issue by adding an overload of electronic complexity.

State-of-the-art realities

In an exploration of the true advantages of electronic line shaft designs, several suppliers emphasize flexibility and efficiency gains.

"The replacement of mechanical line shafts with individual servo motors allows the decoupling of individual printing units, thus reducing the time necessary to initially phase multiple cylinders of a variable repeat press," explains Bud Lewis, president of CC1. "Another advantage is the capability of running true variable repeat on a flexographic press with the elimination of finite pitch gears." Lewis cautions these advantages must be weighed against the higher cost, complexity, and increased maintenance of sophisticated power electronics.

Electronic line shafts do provide a little more versatility in that stations can be run at different speeds, agrees Chris Popp, sales manager for Diequa Corp. He estimates, however, that 90 percent of the time printers stick with running stations at the same speed.

Moreover, Popp believes mechanical line shafts are still preferable in many cases, particularly to narrow-web package printers, because they are more predictable in terms of performance and maintenance.

Suppliers who support the possibility of an electronic line shaft functioning as a stand-alone register control device do so on a case-specific basis.

Dan Jahn, director of sales/marketing at Hurletron, states the electronic line shaft can play the role of register control, "if the information gathering and control functions include accurate feedback from the web."

"Position-matching two rotating cylinders will not, in itself, provide accurate register-control due to the dynamics of the substrate," Jahn elaborates. "Web stretch, web thickness variations and tension upsets are the dynamic variables encountered during the printing process and each of these can cause misregister. If the electronic line shaft is not capable of analyzing substrate and tension variables, and responding to the changes, a separate automatic register control will be required to maintain registration."

Richard McDonald, product business manager for Bobst Registron Equipment, cites sources of misregister beyond substrate inconsistencies, such as caliper changes, drying, mechanical drag from rollers, tension upsets due to splices, and long web lengths. "Electronic line shaft technology does not address any of these issues," he notes. "With the tendency toward more complex materials and press layouts that include inline laminations, coatings, cold seals, etc., register controls will play a greater role in not only controlling precise and accurate register, but also in reducing material waste during press startups, restarts, splices, acceleration, and deceleration."

PC Industries' Woolley believes register controls are still a necessity for electronic line shaft systems, and that automatic register controls can easily be added.

Woolley sees registration control of the electronic line shaft system as an effective way to match cylinder circumference to the image on the web. Registration of the station to the web can be accomplished using advance or retard keys, which add or subtract pulses to the motor drive, and by adjusting the base rate multiplier, either during makeready or run mode.

For automatic registration control, two color registration scanners can be added—one sensing the position of the image on the web and the other sensing the position of the cylinder.

At Candy Controls, Sales Manager Chris Duval describes a similar registration approach for electronic line shafts, in which the company's Web Master™ motion controller is used in conjunction with a DC motor system, an encoder, a sensor on the tooling, and a sensor on the web.

Duval says the decision to go with an electronic line shaft does not hinge on register control alone, but may include productivity issues such as changeover time and job run expansion.

Similarities and trade-offs

Industry experts who declare register controls to be requisite for both mechanical and electronic line shafts reference performance similarities—and trade-offs—as the basis for their views.

"Electronic line shafts are equivalent to their mechanical counterparts in performance," Lewis asserts. "That is, they can synchronize any number of printing cylinders and maintain their relative position with great accuracy. Similar to their mechanical counterparts, however, electronic line shafts do not improve, correct, or have any additional effect on image registration."

Regardless of press design, affirms Popp, printers still need register controls to compare where the impression is to where it needs to be, and adjust if necessary. He contends printers can achieve decent register results without dealing with electronic line shaft complexities that may actually sabotage intended corrections.

"Servo drives change speed and position based on electronic feedback," he explains.

This feedback may travel quickly, but it takes precious time to accelerate or decelerate loads.

"At slower press speeds, electronic line shafts may react quickly enough to deliver the quality they promise," he adds. "However, at higher press speeds more material goes by before changes can be made. The servo is constantly chasing the correct position, but is always behind."

Doug Tewes, general manager at Eltromat, believes that only the algorithms in register controls need adapt to accommodate electronic line shafts. "We believe electronic line shafts are the future of presses, but the principle of register has not changed," he states.

Register control systems offered by Bobst Group reduce waste through automatic control of web elongation, and feature a dual-head scanner, no error accumulation, optimized pre-setting, and statistical process control.

EXECUTIVE SUMMARY

Electronic Line Shaft Considerations

On the plus side...

+ Decouples individual printing units, enabling reduced initial phase time for multiple cylinders in a variable repeat press

+ Enables print stations to be run at different speeds

+ Permits true variable repeat on a flexo press

+ Versatility for printing or diecutting of preprinted webs

Possible drawbacks...

— Higher cost, complexity, and increased maintenance

— Servo drives change speed/position based on electronic feedback, which may not reach them fast enough at higher press speeds