Troubleshooting web lateral instability
Lateral instability of the web may lead to several issues and as a consequence to rejects and claims from customers. An empirical study was completed to evaluate the impact of this complex problem on printing presses and converting equipment and how to solve them.
This article was written by Frederic Parent
Lateral instability of the web may lead to quality issues such as print mis-registration, wrinkles or tracking difficulties. The problem may come from the equipment, the control of the equipment or the web itself. This often leads to rejects and claims from customers.
There is limited information available to web producers about chronic web wandering during printing and converting. Web wandering was characterised on the actual equipment using portable positioning edge sensors to quantify the amplitude and frequency of sideways movements. Web and roll samples were also tested with laboratory equipment to assess variability of web properties that could explain the fluctutation measured on printing and converting equipment. In this article, we present typical case studies to illustrate how web producers can work on reducing web non-uniformity to improve lateral stability.
Web lateral stability on printing and converting equipment is critical to ensure good runnability and good product quality. Problems such as web weaving or web shifting may lead to performance issues such as wrinkle formation, mis-registration, web breaks and folder issues [1, 2, 3, 4]. Some equipment has the capabilities to control the web lateral position. But lateral web movements are sometimes quick or so severe that equipment adjustments cannot compensate for them [1, 3].
Troubleshooting web lateral instability is not an easy task — it is a complex issue. The problem could come from the equipment itself [2, 4], the roll build [5, 6], the web [2, 4, 5], or an entire combination of these factors.
Printing and converting equipment such as unwind stands, rollers, print units, etc. are generally aligned to prevent web lateral instability. But the subsequent rewetting and drying processes can deteriorate web uniformity, thus producing an unstable web. The tension control of the equipment is another potential cause for web weaving, if the tune-up of the drives is not adequate . Roll winding and roll out-of-roundness are other possible causes [5, 6]. Out-of-roundness is another well-known problem. It is easier to reject rolls that are out-of-round (a defect clearly visible), than risk running into imbalance issues.
Ultimately, one critical yet more subtle factor for lateral instability is the web itself. The non-uniformity of web properties in CD and MD has been shown to generate web lateral instability [5, 7, 8]. Basic web properties include basis weight, caliper, moisture (important for paper web material) and CD tension profile that was found to be a key property to be considered for all type of web materials.
To help understand the correlation between web lateral instability and web properties, an empirical study was completed to determine the impact of web variability on web lateral stability. Participants in this study were mostly web producers (paper, board, flexible packaging and plastic films) confronted with thie problem and claims from printers and converters. The objective of the study was to identify what properties impact the web lateral stability the most. The goal was also to help web producers identify specific elements in their processes that were causing these issues. By bringing improvements into the right area, they should be able to improve the web stability and ultimately reduce rejects and claims.
Field trials – Measuring web lateral movement
Trials were conducted at different pressrooms and converters in North America. Measurements were conducted with an in-house device built specifically for this purpose, as seen in figure 1. It consists of two portable laser beam sensors installed at the edges of the web to measure its lateral displacement. The web weaving is calculated as the lateral movement of the median position, or the difference between the front and the back sensors divided by two.
Using the median position eliminates the impact of variations of the web width (due to web shrinkage for instance). For all cases, web weaving was measured at the end of the printing/converting process (before slitter, or folder or reeling) on problematic and good running materials. Only the measurements at constant equipment speeds were analysed. The pasting and equipment stops/slow-downs were excluded from the calculation.
The web weaving was calculated as the standard deviation of the median position of the web. For transparent plastic films (clear films with no print), a specialised technique had to be used as the laser beams were not properly detecting the edges.
Laboratory Trials – Measuring web properties and their variations
Parallel to data collected in the field, the different web properties were tested with laboratory equipment installed at FPInnovations research centre in Pointe-Claire, QC, Canada. Correlations were established between the two sets of data. Evaluation of the web properties was conducted with:
- The Roll Testing Facility was used to quantify CD tension variations as well as web weaving of rolls.
- The Tapio was used to measure basis weight and caliper at high resolution, in both CD and MD.
- The Lorentzen & Wettre TSI-TSO instrument was used to measure fibre orientation (for paper and board only). Based on the literature [1, 3, 4, 5, 7, 8] and on our experience with webs on printing and converting equipment, it was believed that these properties would be the leading contributors to web lateral instability.
The Roll Testing Facility (RTF)
Rolls of material were processed through the Roll Testing Facility (Figure 2). This unique piece of equipment was developed by FPInnovations to evaluate rolls for two specific features: web uniformity and roll structure.
- Measurement of bagginess or slackness of the web (cross direction tension profile) is obtained with a 1,270 mm wide tension beam, equipped with 25 mm width Teflon pads individually assembled on load cells. A Measurex scanner allows measuring web properties as the roll unwinds: basis weight, moisture and caliper.
- Measurements for roll structure include wound-out-tension (WOT), roll density, out-of-roundness (OOR), web width variations and wandering (or lateral instability). For the purpose of this study, only web lateral stability data are presented. The web weaving is calculated as the same way as the field measurements, using laser beam sensors (see above).
Tapio is an instrument that measures basis weight, ash content, caliper, gloss and opacity. It allows high resolution measurements in either CD (resolution of 0.8 mm) or MD (resolution of 12.8 mm). Precise time profiles as well as FFT analyses are generated. Wavelengths and frequencies can be correlated to the dimensions of machine component or time loops, thus allowing identification of specific elements causing these periodic variations.
Lorentzen & Wettre TSI-TSO instrument
For paper and board grades more specifically, we used a Lorentzen & Wettre TSI-TSO instrument to quantify fibre orientation. The L&W TSO Tester is an ultrasonic instrument for measuring Tensile Stiffness Index (TSI) and Tensile Stiffness Orientation (TSO) on web. When required, this equipment was used with a resolution of 0.5 meter in MD and 0.1 meter in CD.