Formation improvement

1. Reduce headbox consistency if drainage capacity of forming table permits.

2. If other aspects of paper quality allow it:
(a) change furnish to one of shorter fibers.
— for coating rawstock or offset bond use more hardwood.
— for groundwood papers, lower the groundwood freeness and/or increase amount of groundwood.
(b) where appropriate, refine more.
— controlled cutting may be more beneficial than excessive hydration.

3. Run with greatest practical wire tension.
— do not let forming fabric dip over drainage elements or loop down excessively at breast roll.
— do not let fabric change direction sharply at leading edge of forming board where table rolls are in use.

4. Increase drainage capacity of forming table by:
(a) moving flatboxes towards couch and putting in more drainage elements.
(b) adding foils or better wet suction boxes.
(c) installing low coefficient of friction flatbox tops.
— this enables forming fabric to be run tighter at the breast roll, and allows the earlier part of the forming fabric to be used to maximum capacity, apart from additional drainage at boxes.
(d) increase foil angles if stability of stock on forming fabric allows it, or increase the vacuum in wet forming boxes.
(e) reduce initial drainage to prevent sealing.

5. Improve setup of forming table by:
(a) setting deflectors correct distance from preceding roll so that roll-to-deflector space is not flooded.
(b) setting deflectors up accurately to forming fabric so that no water passes over deflector to create strings.
(c) ensuring that drainage elements are accurately aligned.
— with foils, ensure that they are all correctly touching the forming fabric at the leading edge.
— in practice, this means that all foils should be ground at one time.
(d) ensuring that all rolls turn freely.

6. At all times, maintain the optimum basis weight distribution that is possible with existing headbox.

7. Ensure that slice discharge is as homogeneous and uniform in distribution of fibers as possible.
— above all, preclude flocculation.

8 . Keep jet-to-wire speed ratio accurately set at the optimum value.
— in practice this means that instrumentation must be installed to read out these values.
— in advanced practices, the ratio is controlled automatically.

9. Maintain correct:
— top lip.
— bottom lip.
— forming board
— breast roll relationship.
— the lower part of the slice jet should just doctor off at the impact point of the forming board.
— avoid back flow on the forming board.
— avoid a breast roll discharge except in special circumstances.

10. Under special circumstances, where a severe wake effect is observed, a breast roll discharge may be advantageous, since first 15 to 20 per cent of the sheet is then laid down evenly and thus large light spots are avoided.
— when the headbox profile is not good, letting the sheet set further down the table may be beneficial.

11. Avoid stock jump at forming board.
— forming board should be slightly lower on leading edge.
— a working rule is 5 mm per 1000 m/min., which enables the forming fabric to ride up the board without a sharp upward acceleration.

12. The jet impingement on the wire should not be disruptive in character.

13. Check perforated rolls to:
(a) ensure that the design of the perforated roll at the slice is such that fibers do not staple across land areas and cause flocs.
(b) avoid “scooping” of fibers by slice perforated roll.
(c) modify perforated roll in slice position as follows:
— maintain an open area of 48% with the smallest holes the furnish permits.
— if formation problem is due to holes too small for fiber type, then larger holes (by increasing land thickness) will give an improvement.
— it may be necessary for long fibers and average consistencies (0.4 to 0.6%) to change to an open area of 43 – 44% to obtain a wide enough land area with hole size dictated by fiber type.
— ensure that the end design and journals of perforated rolls do not disturb edges of sheet on the forming fabric.
— check that the overlap of the solid parts of the perforated roll is not making lines in sheet.

14. Ensure sufficient small-scale turbulence on the wire, especially where the sheet is being formed.
— this ensures adequate deflocculation and better formation.

15. Delaying drainage, by removing one or more dewatering elements, usually deteriorates formation.

16. A compromise is always needed between the rate of drainage and formation.

17. Run a dandy.
— overrun wire speed by 3 – 5 m.
— for higher speeds a special construction is needed.– keep dandy turning when not in use.
— always have a spare dandy ready to replace running dandy at once if trouble develops.

18. If shake is in use and speed is low enough for shake to be effective, then:
(a) increase the energy of the shake.
— the effectiveness of the shake is roughly proportional to the frequency squared times the amplitude.
— mechanical limitations of the forming section may prevent this.
(b) if construction of the forming section permits it for low speed machines, new rails to carry the shake further down wire can effect improvement.

19. Use a formation tester to check formation quantitatively.
— ensure that there is not excessive formation variability across the machine.

20. Use velocity forming (slight drag) with first foil bank at reduced angles to improve first-pass retention and decrease the drainage at that point.
— if sheet is setting too quickly, may need to go to higher foil angles to increase activity on the wire.

21. Investigate use of formation showers to improve turbulence on early drainage.

22. If wet-end chemistry is causing flocculation, more activity or stock jump on the wire is necessary.
— better yet, correct the wet-end chemistry problem.

For a more descriptive narrative, see Formation notes.