Basis weight (CD) variations

1. An off-machine basis weight profiler is the minimum tool that should be considered today.
(a) use profiler with composite and superimposed profiles.
— determine stability of variations shown as this indicates degree of correction that may be achieved.
(b) adjust the slice.
— always adjust using manufacturer’s restrictions on deviations between screws to avoid springing the slice.
2. A traversing on-machine profiler is the preferred tool. In use, one recorder should be near the headbox for ease of reference when making adjustments. Do not over adjust at any one spot. Watch the forming fabric, as well as profiles, as adjustments are made.
(a) on air-padded suction breast roll headboxes for tissue, CD weight variations are often corrected by shimming the apron lip with stainless steel shims at the high points.
(b) with basis weight profilers, Measurex, et al., you need to know the settings for the scan averages.
— if CD is unstable may be better to use a series of single scans to check stability or use off-line profiler.
3. Make sure there is minimum backlash in slice screws.
— modern headboxes have accurate screw position indicators.
4. Operate slice bleeds correctly.
— if they are functioning properly, the edge slice screws will have about the same position as the remainder.
5. Avoid trying to compensate for a sprung slice lip.
— if lip is damaged, change it.
6. Maintain an even flow from distribution system.
(a) regardless of what you have heard, ± 10% of design flow is all one can expect from a hydraulic box and still get an even CD profile.
7. Ensure that perforated rolls are seated with minimum practical gap that will mask distribution flow irregularities.
8. Ensure that excessive headbox showering is not disturbing the flow and causing basis weight variation.
9. Ensure that there are no deckle waves.
— do not let deckles stick into flow at slice.
10. Keep deckles just barely off forming fabric, so that they do not wear.
— try tilting the deckles slightly outward.
11. Keep sealing strip on deckles in good condition.
12. Use light deckles for easier adjustment, and maintain a spare set for installation at the first opportunity when any failure is noted.
13. Check the headbox for channeling, especially if machine has been rebuilt for higher speeds.
14. Ensure that machine speed-up has not exceeded original flow-through-the-box design.
15. Check stock impingement point onto forming board.
— should be about 0.5 – 1.5 cm before leading edge.
16. Check for macro-turbulence that extends beyond initial 3 – 5 m of forming area.
17. May result from varying fines and filler in white water loop that causes fluctuations in consistency and in the static head of fan pump.
18. Check for movement of the wet/dry line for potential flow variations.
— many times indicates a basic instability in the stock dilution loop.
— may also be due to air in the system that is affecting the flow.
19. Ensure that headbox stock and shower temperatures are the same.
— a 2.5°C differential may cause problems; a 10°C variation will definitely cause streaky areas in the web.
20. Basis weight variations also arise from:
— mixed stock consistencies varying.
— hydropulper consistencies too low so lose control at the machine.
— white water levels surging.
— loss of dilution white water at machine.
— headbox levels varying.
— fan pump drawing air through impeller shaft seals.
— couch pit overflowing excessively.
— pulsations in feed piping system to headbox.
— variable flow of retention aid.
See Basis weight, CD varying.
See also Basis weight, MD varying.