Edited by
Larry Berardinis

Sue Dorscheid,
Danaher Motion
Wood Dale, IL

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 inkmetering 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 onecolor printing to complex multicolor 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 masterslave, 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 camshaft 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?

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.

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.