Monitoring Repeat Length“On the Fly”
Author: Frank Burgos, FlexoExchange
Have you ever had the problem of repeat length varying through a printed roll? You measure the repeat when you stop the press, and it’s correct, but when the roll is being converted, unacceptable variations or shifts in repeat length are encountered throughout the roll. Perhaps the repeat measures 12 ¾” near the core, but measures 13” towards the outside of the roll. Sound familiar? I suspect that this is not an uncommon problem.
Repeat length can vary when printing on substrates that are “stretchy”, or have properties that push the limits of our equipment’s ability to control web tension or web speed relative to plate surface speed. With a stretchy or elastic substrate, we often print on the material while it’s slightly stretched by excessive tension and end up with a short repeat when the substrate contracts after printing. With a rigid or stiff substrate, we may rewind so tightly that we pull the material past the plate at a speed slightly faster than that of the plate’s surface and end up with a repeat length that’s too long. Or, we may not be able to pull the material through the press fast enough and end up with a short repeat length. The tension control demands we face when working with touchy substrates can pose problems with respect to repeat length.
Variation in repeat length is often associated with poor tension control. However, even under the best of tension control conditions, the simultaneously and inversely changing diameters of the unwinding and rewinding rolls almost certainly guarantee tension variation throughout the printed roll. With many substrates, this is not a major problem. But printing on a light gauge low-density polyethylene web with old, cranky equipment, or pulling heavy gauge material through our presses, for example, can test the limits of our ability to control repeat length.
Most of us lack the technology to monitor repeat length while the press is running. However, imagine being able to tell while the press is running that your repeat might be long or short, simply by analyzing the print with the unaided eye or an average video inspection device. It could mean a reduction in difficulties and waste associated with repeat variation. What follows is a way to do just that.
One of the trademark properties of flexographic printing is the appearance of a halo around the printed image. This halo is mainly caused by slight over-impression of the plate to the substrate. Keeping this halo to a minimum is a basic goal of quality flexographic printing, but often we cannot avoid at least some degree of visible halo.
Let’s define the leading edge of a printed image as the edge closest to the rewinder, and the trailing edge as the edge furthest away from it. At times the halo is more pronounced at either the leading or trailing edge of the printed image. This can result from a difference in plate surface speed and substrate speed, and can be present whether or not our plate is over-impressed. Fortunately, it’s sometimes possible to interpret the relationship of the substrate speed to the plate surface speed by close observation of this type of halo, and judge from its asymmetry the likelihood of a discrepancy in repeat length.
I’ve found that if the halo is more pronounced at the leading edge of the
image, the substrate is traveling faster than the plate surface and the repeat length is longer than what it should be. If the halo is more pronounced at the trailing edge of the image, the substrate is traveling slower that the plate surface and the repeat is short. Therefore, our goal is to try to avoid halos or keep them equally distributed between the leading and trailing edges of the images, by making the appropriate adjustments in tension.
I hope that you find this information helpful. If you have any questions, please don't hesitate to contact me.
Author: Frank Burgos
Web Site: http://www.flexoexchange.com/
© 1999; Frank Burgos; All Rights Reserved