Jun 18, 2019
Quality is key to profitability in today’s competitive paper industry. There are many factors that determine the quality of a finished paper roll, but among the most important is consistent roll structure. Some roll defects, such as telescoping, off-color paper and offsets, are easily identified by visual inspection. Defects caused by poor roll structure, however, are often invisible until they manifest themselves through press breaks or crepe wrinkles in subsequent processes, particularly printing. Maintaining adequate and consistent wound-in tension is the most important component to roll structure. To ensure the highest quality, mills need a precise, cost-effective method for measuring wound-in tension.
A roll of paper is a mechanical system. It consists of many layers of paper wound on top of each other with some degree of tension wound into each layer. This tension creates a compressive force, which, when combined with the coefficient of friction between layers of paper, forms the mechanism that holds the roll together. The distribution of tensile and compressive stresses from the core to the outside of the roll is the roll structure. Wound-in tension controls the magnitude of these forces. It is the most important component for proper roll structure, therefore it is vital for mills to track and control wound-in tension.
There are a variety of methods for studying roll structure. Most methods measure hardness, layer-to-layer compression or density – all of which are created by wound-in tension, but they are not wound-in tension. At the present time, there are only two accurate methods to measure wound-in tension: the WIT-WOT and the Gap Test.
WIT-WOT stands for Wound-In Tension and Wound-Off Tension and is the most accurate method to measure wound-in tension. Using a specially designed test-winder, a sophisticated computer analysis system measures the tension wound into the roll as its being unwound. By collecting thousands of data points, even the most minute problems can be detected. This test is limiting and can be expensive, however, because rolls must be shipped to a research facility. For an accurate and reliable method that can be performed on the plant floor, the Gap Test is the answer.
A simple procedure to learn and perform, the Gap Test remains the foremost method for in-mill analysis that provides quick results. It can be used for quality control, winder calibration and tune-up, or as an analytical troubleshooting tool. The procedure for the test is as follows:
At each test interval in the Gap Test, a section of paper is sliced from the roll and falls on the floor. Repeat the procedure for each marked interval. Allow at least two hours to Gap Test the entire roll.
As roll diameter and nip intensity continue to decrease and the nip area increases, slippage can occur, impairing the results (see above figure). To help retain wound-in tension and ensure accurate and relevant gap measurements, an alternate method for staging the Gap Test involves a simple clamping device that stabilizes the roll.
The roll is elevated on a sawhorse-type stand when it has been reduced to approximately 30 inches in diameter. A core-sized aluminum tube is inserted through the roll’s core. A C-clamp fastened to the aluminum tube and the sawhorse secures the roll in place and prevents slippage. While some grades are more prone to slippage than others, this clamping method helps ensure Gap Test accuracy on all types of paper.
The final step in the Gap Test is to analyze the data:
The curve on the graph should show decreasing values from the core outward. Tests that show inconsistent roll structure (i.e., an increasing tension at any point of the roll) are a signal to begin troubleshooting the process to alleviate problems before they escalate. Calculating and graphing the strain alone can often reveal problems, but for the most accurate analysis, actual tension should also be measured and examined. To track winder performance, the test should be conducted periodically and the data from each test compared. The equipment manufacturer can pinpoint problems and offer solutions for alleviating them.
Because wound-in tension plays such a key role in maintaining roll quality, it is vital for mills to be able to track and manage their operation’s ability to maintain adequate levels of tension in finished rolls. The Gap Test offers a quick, easy and cost-effective solution.
For more information on measuring wound roll quality, contact your Valmet representative.