One example of a roll winding defect is illustrated by a “starred” roll. In the image to the left this defect is illustrated. A roll that becomes starred was produced with low wound in tension when the roll was started on the winder or soon after the roll was started. When rolls are produced with low roll density or low wound in tension followed by rings or bands of higher roll density, or higher wound in tension, there is a risk of the higher density bands collapsing the lower density interior of the rolls resulting in the “starring effect.” This type of defect is more common on older winders that do not have rider rolls to help provide a uniform, hard, dense start to the roll.
In one example of a starred roll, as the roll diameter became larger, more weight (from the roll itself) and the resulting higher wound in tension on the outside wraps of the roll caused a larger diameter roll being rewound (without a rider roll) for an envelope converter to have higher density (higher wound in tension) on the outside wraps of the roll. While the rolls looked good coming off the winder, pressure from clamp trucks picking up the rolls caused enough stress to collapse the softer wound (lower density) interior of the roll.
In the winder of a modern paper machine, crews have tension controls, brakes, and rider roll pressure to help control the roll density. Uniform roll density translates into uniform roll hardness. This image illustrates roll hardness, indicated by the deceleration measurements from the ACA RoQ, from the core to the outside diameter of the roll. By cutting slabs off the roll and retesting roll hardness at descending diameters, a graph sometimes known as a plot of “wound-in tension,” can be created. Understanding roll density and the WIT characteristics of a roll becomes a powerful diagnostic tool for evaluating winder performance.
By using a WIT (Wound-In Tension) analysis with a ACA RoQ, the winder crew was able to modify tension and braking (even without a rider roll) to deliver rolls with more uniform hard starts and more uniform roll density. Along with subtle winder modifications, the clamping pressure on clamp trucks was limited, eliminating two factors that resulted in starred rolls and eliminating the defect.
These images are intended to show one of the more advanced applications for a roll hardness test, or roll quality (ACA RoQ). Please visit the Technidyne website or the ACA RoQ website for more information.