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PT Gas Content
Effects of free and dissolved gas on sheet uniformity
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Causes formation variability and visible streaks
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Destabilizes basis weight and moisture profiles
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Increases pinholes, breaks and strength variability
Sheet formation is highly sensitive to disturbances in the approach flow system. Gas is a leading contributor to formation variability across paper grades
Gas variability affects sheet properties
Entrained and dissolved gas can influence stock flow stability, sheet formation, and retention of fines and chemicals within the approach flow system and headbox. When gas levels become excessive or unstable, the resulting hydraulic disturbances may contribute to poor formation, basis weight variation, and inconsistent performance on other important quality metrics.
Gas can produce bubbles in the forming zone, causing thin areas, pinholes, and weak spots. Lighter basis weight grades are particularly sensitive to these effects because there is little opportunity to mask sheet non-uniformities. Holes and weak spots originating at the headbox may lead to downstream problems such as sheet breaks on the machine or in converting lines, higher reject rates in quality control, or even customer complaints.
Gas can affect the distribution and retention of fines, short fibers, and wet-end additives. Because these components play an important role in sheet strength, surface properties, and print performance, changes in their retention may contribute to variability in sheet quality and overuse of chemical additives.
Variable gas content can affect stock compressibility and may contribute to pressure and flow instability in the approach flow system. These disturbances can appear as profile variation, localized heavy and light spots, and reduced formation uniformity. For premium grades, even small changes in sheet structure may influence appearance and surface quality. The variation in formation and basis weight may also contribute to moisture variability entering the press section. Since downstream converting and finishing processes tend to amplify existing sheet non-uniformities, variations originating upstream can become more visible in the finished product.
For many paper machines, the principal benefit of effective deaeration is improved process stability and sheet uniformity — supporting more consistent product quality, better machine runnability, and better converting performance.
Typical mill symptoms when sheet uniformity is affected
- Formation index fluctuations
- Visible cloudiness in the sheet
- CD basis weight profile instability
- Strength variability
- Visible cloudiness in the sheet
- CD basis weight profile instability
- CD basis weight profile instability
- Strength variability
- Surface and print quality defects
- Increased dusting
- More frequent sheet breaks
- Higher reject rates
Sheet properties most sensitive to gas variability
- Formation
- Basis weight uniformity
- Strength consistency
- Surface and print quality
- Runnability
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