Preparing for Use
Forming fabrics have properties quite different from metal wires. Consequently, when converting from metal wires to forming fabrics, the following items should be addressed.
Machine Centering
Since forming fabrics are pliable and sensitive to the guides, leveling and alignment of all drainage equipment and rolls should be checked. Special care must be taken in centering the fabric on the machine.
Roll Surface
All rolls must have good surface conditions and rotate smoothly. Care must be taken to ensure uniformity of the shell holes in the suction rolls. Under some conditions, copper alloy shells may be affected by electrolytic corrosion. (Since forming fabrics have excellent corrosion resistance, electrolytic corrosion will concentrate on certain parts of the machine.) Electrolytic corrosion does not occur suddenly. In addition to periodic checks, one should consider spraying anticorrosive agents, or changing to stainless steel, as necessary.
Materials of Fixed Equipment
The materials of fixed equipment including forming boards, foils, and suction boxes and others, have a great effect on fabric life. Poor condition of fixed equipment can definitely have a negative affect no fabric life. Typical covering materials presently used are listed below with their properties:
Polyurethane
Since it has a comparatively high kinematic coefficient of friction, polyurethane is usually unsuitable for forming fabrics with low durability. However, there are many cases in which it can be used without problems.
Silicon carbide
Silicon carbide is quite superior mechanically, but very expensive.
High-molecular-weight polyethylene
Since the abrasion of high-molecular-weight polyethylene is comparatively great, it is necessary to take a good care in maintenance. It is widely used because it is easy to polish and its kinematic coefficient of friction is low.
Hard ceramics
Hard ceramics are more expensive than high-molecular-weight polyethylene, but easier to maintain accuracy and requires less frequent repolishings. Hard ceramics’ kinematic coefficients of friction are low, thus suitable for plastics.
Foil and Table Arrangement
Compared with stainless steel wires, forming fabrics are more flexible and apt to be bent by the table rolls and the vacuum of the suction boxes, causing faster drainage, stock jump, poor formation, increased drag load, and increased power load. Foils are effective in remedying the bending of the forming fabric and the drainage, as well as reducing the suction box vacuum. Generally, foils become more necessary as speed increases. The arrangement of the forming table is also extremely important. The table should be arranged so that the forming fabric bends adequately in order to get good formation and drainage.
That is:
1. The economical introduction of foils is based on the desired drainage line diagram (that we can measure and draw) to determine the proper number and dwell length of blades as well as angles and spacing.
2. Grooved or plain rolls should be removed if there is too much turbulence.
3. Table arrangement should be kept flat.
4. The use of wet suction boxes or vacuum foils, which helps to reduce vacuum load or the number of suction boxes, should be considered.
Power Distribution and Usage
When forming fabrics are used, power consumption is sometimes increased, depending on the paper-making conditions. Generally, the load may be 5－10% greater in case of kraft paper, and 20% greater in the case of board grades. Consequently, when surplus power is scarce, it is safer to increase the power from the beginning. Moreover, the power distribution between the suction roll and drive roll should be approximately 40% to the former and 60% to the latter; distributing more than 50% to the helper drive should be avoided. This is important for preventing vertical lines between the rolls. Moreover, because it is important that the power be transmitted reliably to the fabrics to avoid slippage and reduce the amount of creep, proper fabric tension must be maintained and rubber covers (P&J 12-15, SHORE A95) should be placed on the drive rolls. Using the return roll with helper efficiently is a good method.
Determination of Stretch and Fabric Length
Generally, one must allow for about 1%（max) stretch in forming fabrics. Forming fabrics have:
Initial Strength at start-up: about 0.2%
Structural Strength: about 0.5－0.9% (7kN/m)
[Example]
![[Example]](images/pr_image_prep.jpg)
Determining fabric length is very important. It should be determined by actual measurement. Fabrics change, normally shrink during storage while awaiting installation. Amount of shrinkage is highly influenced by warehouse conditions. Fabrics should be kept in a place without exposure to heat and humidity.
High-pressure Shower
Forming fabrics need a proper countermeasure for contamination. Mechanical and chemical methods are available, and among them, the most effective and economical is a high-pressure shower. The following must be considered to help keep them clean:
a) affinity of synthetic fabrics to pitch, asphalt, latex or similar substances while in stock,
b) wire mark, formation, two-sidedness must be considered based on paper grades.
When installing high pressure showers, the following is recommended.
| Pressure: | 2－3MPa (20－30kg/cm2) When there is severe contamination, 3－4MPa (30－40kg/cm). |
| Nozzle diameter: | 1mm(0.04") dia. (needle) |
| Nozzle interval: | 75－150mm (4－6") |
| Stroke: | 170－320mm (3－6") |
| Cycle: | Oscillator 2－4/min |
| Distance from fabric: | 150－200mm (6－8") |
| Spray angle: | Adjusted so that dirt is ejected. |
| Position: | Within 300mm (12") from outlet of outside rolls and facing from the sheet side to the machine side of the fabrics. However, there are cases in which it is more effective to use showers from both sides with multi-layer machines, especially high-speed paper- making machines. Furthermore, in order to obtain a good cleaning effect, pure water should be used; continuous or intermittent use should be chosen according to the conditions. |
Caution: When the forming fabric is struck in a concentrated way with a high-pressure shower of 30kg/cm2 (430psi) or more, fibrilization
(yarns are scattered into the fiber state) may occur, and as a result, they may fall out, creating holes. (This should be attended to more carefully the shorter the fabric length and the more rapid the rotation.) Consequently, care should be taken concerning the following points:
1. Do not use the high-pressure shower when the fabric is stopped. When the machine stops suddenly, the high-pressure shower should be stopped simultaneously by means of an interlock.
2. Take care that the friction does not cause oscillation to stop at the return point of the stroke (in some cases, the cycles may need to be increased).
3. It is preferable to use the pump in such a way that there is surplus force, but do not use more pressure than necessary.
Trim Squirts
In the case of multiple-layer weaves, the trim squirt water is difficult to drain; hence there are cases in which trimming cannot be performed smoothly. The use of a double spray nozzle, with a diameter of 0.3－0.4mm (0.012－0.015") can solve this problem.
Filler Testing
The life of the fabric is determined by abrasion. If a low-quality filler is used (especially when calcium carbonate is used), abrasion progresses rapidly. It is no exaggeration that the life of the forming fabric depends on the quality and amount of the filler that is used. Consequently, it is safer to test the filler quality beforehand. Our company can perform comparison testing of fillers and provide rough judgments; please consult with us (A 1.5kg or 3lbs. sample of each kind of filler is required for such tests.)
Edge Showers
The nature of forming fabric is such that abrasion generally proceeds 50－100mm (2－4") from the edges. Since this part tends to become dry, and abrasion progresses due to leakage of fillers, etc., the installation of mist showers to moisten this part, prior to passing the various suction boxes, can be an effective countermeasure. Moreover, it is said that water deckles are also effective.
