Print

Web Guiding



A variety of technologies are available today to make sure that the web stays where it should.



Published July 20, 2005
Related Searches: Label substrates
Post a comment
Nothing is perfect, of course. Sometimes it seems that the printing and converting process is simply an exercise in correcting imperfections in every step of the process, on every machine and part. As perfect as they might appear, label substrates are not 100 percent uniform in thickness and weight. Imperfections are among the causes of web drift, the unwanted movement of the web from side to side on press.

That’s why web guides exist. They come in many forms, and everyone uses them. They range from the simple to the complex, and they have evolved to be extremely sensitive and precise in keeping high speed webs where they belong.

“A web is a very dynamic creature,” says Jeffrey Damour, sales manager for Converter Accessory Corp., Wind Gap, PA. “Just because the guide gets it into position doesn’t mean it’s not going to move again.”

“Any material that is made is not straight,” says Randy Adams, national sales manager at Fife Corp., Oklahoma City, OK. “Plastic and paper tend to be thicker on one side than on the other, and longer edges on one side than the other. Because of those inherent properties in webs, they don’t track straight on rollers.”

“A roll of stock that appears to be perfect might actually have a lot of cross-web tension that you can’t see,” notes Paul Henke, sales manager for Accuweb Inc., Madison, WI. “As soon as the web gets into a relaxed state (after unwinding from the roll) it will try to go back to the way it was, and move around on the machine.”

Web guides use a variety of technologies to achieve their end. Some utilize air, others infrared sensors or ultrasonic sensors. Some make use of hydraulic web adjustment machinery, others use electromechanical equipment.

Fife Corporation manufactures a broad range of sensors, including infrared and ultrasonic, with electromechanical guiding systems. A majority of narrow web converters use infrared systems, notes Adams.

An infrared system works this way: The sensor is placed on press directly over the edge of the web, where its light beam can “see” the edge as well as be picked up by the photo cell that is positioned to receive its signal. If the web wanders in either direction, the resultant change in the infrared beam will be picked up by the photo cell, which is hooked up to a controller, which will send a signal to an actuator, the unit that will make the necessary moves to get the web back to where it belongs. “As the web moves past the sensor, the controller sends a signal to the actuator motor to shift that web,” Adams says.

The other type of sensor is ultrasonic, which uses sound waves rather than light waves to detect the edge of the web. Henke says that the ultrasonic systems are most popular in the broader printing and converting community.

Both infrared and ultrasonic have their drawbacks, however, when it comes to certain types of webs. “The only thing ultrasonic doesn’t handle well are very light nonwovens, such as tissues,” says Henke. That’s because the “sound” used to sense the edge of the web can go right through a highly porous material, thus rendering it invisible to the sensor.

Infrared sensors have trouble with some clear films. “Ultrasonic is good if you have extremely clear material that a light based sensor couldn’t pick up,” says Damour. “They each have their place.”


Actuators
“In simple terms,” says Damour, “the actuator is a motor, a gearbox and a ball screw. That unit positions the web where it is supposed to be.”

The actuator is the unit that makes the physical adjustment to the web on a prompt by the sensor. It can come in a couple of configurations. Converter Accessory Corp. makes the Gydabase, which mounts under the unwind or rewind stand, and moves the entire stand back and forth to adjust the web as it flows from the roll at the front of the press. “There are intermediate guides also,” Damour adds. “They are a set of rollers that pivot the web, so that instead of moving the whole roll you are moving a single layer as it’s moving through the machine.”

Wide web presses can have many sensors and guides placed along the web, though in narrow web shops the customary number is two, maybe three. “The first one is placed before the first print cylinder,” says Henke, “and then one downstream.

The guiding systems are extremely sensitive, and can make adjustments to moving webs of thousandths of an inch, constantly.

Another system on the market today is the All-Air Automatic Web Guiding system from Coast Controls Inc., of Sarasota, FL. The system uses an air-flow sensor to detect the edge of the web, operating entirely on plant air. Because it doesn’t utilize infrared light or ultrasonic waves, the sensor is not affected by clear films or nonwoven materials.

When the air-flow sensor detects a misalignment of the web, it sends a low pressure air signal to a servo controller to make the necessary correction. The controller converts the low pressure signal into a higher actuator pressure.

Coast Controls’ air cylinders provide the force that moves the guiding assembly to align the web. Cylinder diameters range from three to eight inches; the smaller diameter units are typically used for intermediate guiding, and the larger diameter cylinders are employed to shift an unwind or a rewind stand.

Edge detectors from Accuweb

Care and handling
Web guiding systems have a long life span, but will require recalibration from time to time.

“A variety of things cause sensors to fall out of calibration,” says Henke, “such as temperature changes and humidity changes. They can cause the sensor to drift.” Depending on the product, recalibration can involve pushing buttons and blocking sensors, or opening cabinets and changing parameters.

Accuweb has for years marketed its compensated ultrasonic sensor, which tunes itself and does not require recalibration. The company has applied for a patent on a similar product using infrared.

“Recalibration, it’s something that an operator needs to think about, but our goal is to take the operator out of the equation,” Henke says.

Maintenance is also a factor to consider. “You need to maintain web guides,” says Damour. “One of the biggest headaches, and hard to spot, is that older web guides might have a certain amount of mechanical play in them. It’s similar to a car that gets older and develops a bunch of rattles. It’s all mechanical play.

“That all adds into how accurate a web guide is. It’s easy to spot if you know where to look, but 99 percent of people think that it’s an electronic problem. And it’s not.”


Web drift

The imperfections in webs are but one cause of web movement. Quite a few others have been identified by industry practitioners:

Rollers that are skewed, and not perfectly aligned to the other rollers.
Rollers that are warped or damaged. “If the roll’s not straight, the web’s not straight,” says Adams.
Rollers that do not have the same diameter all across the face. “Even one ten-thousandth of an inch difference in a roll face will cause a web to walk,” observes Damour.
Air entrainment: In high speed applications, the web can actually float atop the roller on an air bearing, and that alone will cause the web to move.
Tension and alignment variations throughout the machine.
Moisture: “In a really dry application your web is flowing fine,” says Jeffrey Damour. “In humidity you will have a different story.”
Static electricity.
Elements of the web itself, such as coatings that are not uniform in their application.

10 General Rules of Web Guiding
These rules, generated by Fife Corporation, can be viewed with detailed graphics at the company’s web site:

http://www.fife.com/fife_products_tenrules1.asp

1. There are no perfect webs or process machines.
2. A web seeks to align itself perpendicular to the axis of a roller it is approaching. This can also be stated as: A web seeks normal entry to each roller it is approaching.
3. Always place the sensor immediately after the exit guide roller, or at least in the upper third of the exiting span.
4. The guide should always be placed immediately upstream, or just prior to where accuracy is required. If the sensor is not located in the upper third of the exit span, the control loop will have excessive time phase lag, which will create unsatisfactory guiding results.
5. The web must not slip on the guide rollers, if the guide is to accurately position the web. Tension/hold-back force at the beginning of the process is an absolute must; without a hold-back force accurate web guiding is almost impossible to achieve.
6. Rewind guiding is not lateral control of the web, but it is chasing the web.
7. The entry and exit span for an offset pivot guide is a minimum of a one-half web width to a maximum of three web widths.
8. The entry span for a steering guide is a minimum of two web widths to a maximum of 50 web widths.
9. The best guiding results using an offset pivot guide are obtained when the entry and exit spans are perpendicular to the guide’s plane of motion. (Entry and exit spans can deviate ± 5° from the perpendicularity (90°) of the guide’s plane of motion.
10. The higher the system gain, the better the dynamic response/accuracy of the control system.


blog comments powered by Disqus
Top Searches
L&NW ENewsletter
Sign up now to receive the free weekly newsletter

Enter your email address:
Top Articles
Follow L&NW On