Paper making and fiber liberation – Processes and products – With coating after drying
Reexamination Certificate
2000-11-30
2002-10-22
Nguyen, Dean T. (Department: 1731)
Paper making and fiber liberation
Processes and products
With coating after drying
C162S172000, C427S384000, C106S002000
Reexamination Certificate
active
06468394
ABSTRACT:
TECHNICAL FIELD
This invention relates to a press roll used for a paper machine and, more particularly, to a method of, in a press roll used for a paper machine, preventing paper from over-adhering (i.e., stubbornly clinging) onto the surface of the press roll.
BACKGROUND ART
In a paper machine, a sheet of wet paper is made from raw materials, and, after removing water from the wet paper, it is made into finished goods.
In order to remove water, dehydration/drying is essential.
A dehydrating process, i.e., a so-called “pressing part” is supremely important because the extent to which the wet paper can be dehydrated exerts a great influence upon a load allocated to a subsequent drying process, in other words, upon steam consumption (energy cost) during a “dry part”.
In the pressing part of the paper machine, there are about 2 to 4 places where paper (wet paper) is dehydrated while being pressed firmly by a pair of press rolls and a felt.
The wet paper and the felt are placed on each other and are sandwiched between the pair of press rolls, and thereafter nip pressure is applied between the rolls, so that water in the paper moves to the felt side and is removed.
The press rolls make a pair by two kinds of rolls one of which comes in contact with the wet paper and the other one comes in contact with the felt.
On the wet paper side of the two sides, a hard-rubber-made roll that can easily conform to paper and has smoothness, or a natural stone roll, or an artificial stone roll (so-called stone roll) is used since great importance is attached to the releasability of paper.
Especially in recent years, as paper machines speed up, a ceramic-made artificial stone roll that is cheaper than the aforementioned stone roll and utilizes the characteristics of the stone roll has come to be increasingly used.
(Over-adhesion Phenomenon)
By the way, paper is supplied to the press roll by driving the paper machine. In the press roll in the state where paper is being supplied, a considerable problem has resided in that, if press rolls like the aforementioned ones are used, the paper causes over-adhesion onto its surface. (Herein, the term “over-adhesion” denotes the state of clinging stubbornly.)
More specifically, after passing over a nip point of the paper-side press roll of the pair of press rolls, normally, the paper is swiftly released from its surface. However, there occurs a phenomenon in which the paper rotates in a dragged state while adhering to the surface of the roll even if the paper has gone beyond the releasing point to a considerable extent.
FIG. 7
shows positions X
1
and X
2
where the phenomenon of over-adhesion of the paper onto the surfaces of press rolls occurs in the pressing part.
FIG. 8
is an enlarged view that shows the over-adhesion phenomenon.
Paper
2
adheres onto the surface of the press roll and is not released therefrom in spite of having gone past a normal ideal releasing point Q. A paper releasing point P appears past the ideal releasing point Q.
Therefore, the paper
2
is pulled by the following roll
1
A when the paper
2
that has gone out of the press roll
1
is detached from the surface thereof, whereby tension (what is called “draw”) is generated.
It is preferable that this “draw” be smaller. However, the draw has a tendency to become great as the paper releasing point P becomes high (i.e., a releasing angle a becomes wide) as shown in FIG.
8
.
The reason is that the draw maintains a balance at the position of the paper releasing point P.
If the draw becomes great, sheet break may occur, and, as a result, productivity will decrease.
Even in a case where such sheet break does not occur, disadvantages in quality, such as growth of curl or reduction in paper width, will increase.
In a recent paper machine that is faster, wet paper moves at a higher speed, and therefore the draw is required to be raised for this condition.
Thus, it is a present objective to lower the draw, in other words, to prevent over-adhesion as much as possible.
(Sticking of Deposits)
On the other hand, paper contains various contents, such as pitch, tar, and fine fibers that are included in the raw pulp material itself, and sticky materials that are produced from recycled waste paper, added agents and loading materials that are included in various paper, and so forth.
These contents reach a pressurized, heated state when the paper is pressed by high nip pressure against the surface of the press roll during dehydration, and thus they are apt to firmly adhere onto the surface thereof and be contaminated.
The firmly stuck contents have the action of stripping off fibers on the surface of the paper, thereby causing the fluffing of the paper, and, if the fibers that have been stripped off increase greatly, dents will be made in the wet paper.
(Wear)
Normally, a method of scraping contaminants or deposits off by the blade of a doctor that is an attachment of the press roll is dominant, in order to remove deposits such as those mentioned above, that have been firmly stuck to the surface of the press roll.
However, the roll surface wears down and becomes rougher because of the pressed contact between the doctor blade and the surface of the press roll.
Accordingly, the deposits enter the rugged parts and undergo pressure, thus sticking firmly there.
As a result, they must be scraped off again with the doctor blade, which means the repetition of similar work, and a vicious cycle occurs.
From this fact, an attempt has been made to resolve the above defect as much as possible. For example, there is a case in which use is made of a press roll in which contamination prevention processing has been applied to its surface beforehand.
However, the contamination prevention effect diminishes as the paper machine is run, and therefore durability becomes inadequate. There is also a method of, for example, making the pressure of the doctor blade higher. However, although the deposits stuck firmly to the roll surface can be scraped off more easily according to this method, the doctor blade whose pressure has been heightened furthers the wear of the roll surface, and the doctor blade itself is also worn down thoroughly.
Therefore, the durability of the press roll and the doctor blade is lowered, and, as a result, the exchange cycle of them is also shortened (especially in a recent high-speed paper machine, the doctor blade must be exchanged once every three to seven days, depending partly on the material of the doctor blade).
Additionally, reparatory work for the exchange accumulates a time loss, thus decreasing production efficiency. The over-adhesion phenomenon cannot be greatly decreased (prevented) according to the method of heightening the pressure of the doctor blade.
On the other hand, there is a method of, for example, forming a water pond at the blade edge of the doctor of the roll and forming a water film on the roll surface by the use of water that has passed through the pond so that sticky materials, such as pitch, or fine fibers do not gather on the roll surface.
However, the blade pressure of the doctor weakens unavoidably as a result of securing the passing of water, and the scraping ability cannot be fully exercised.
In other words, deposits that have already been stuck firmly to the roll surface are not removed easily.
Further, a case occurs in which, when deposits make inroads into the blade edge of the doctor, it is temporarily lifted, and, as a result, much water overflows from there so as to dampen the paper. This is contrary to the role of the pressing part that is to carry out dehydration primarily.
Furthermore, if paper does not have a uniform water distribution in the direction of an onward movement, it will become vastly inferior in quality.
If the recycling of waste paper advances and sticky materials or fine fibers in a raw material increase in the future, the method does not necessarily serve as a satisfactory countermeasure.
As mentioned above, the conventional countermeasures have both merits and demerits, and a method of lowering the over-adhesion of paper onto the surface of
Sekiya Hiroshi
Sekiya Kunio
Maintech Co., Ltd.
Nguyen Dean T.
LandOfFree
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