Paper making and fiber liberation – Processes and products – Multi-layer waterlaid webs or sheets
Reexamination Certificate
2000-04-13
2002-01-29
Silverman, Stanley S. (Department: 1731)
Paper making and fiber liberation
Processes and products
Multi-layer waterlaid webs or sheets
C162S300000, C162S133000, C162S299000, C162S352000, C162S303000, C162S304000, C162S306000, C162S317000, C162S351000, C162S123000, C162S203000, C162S212000, C162S213000, C162S214000
Reexamination Certificate
active
06342125
ABSTRACT:
This invention concerns a method for high speed forming of multi-ply paper or board, a forming apparatus utilizing the method and a paper or board product formed by the method and/or the apparatus according to the invention.
BACKGROUND OF THE INVENTION
Multi-ply forming enables the cost performance relationship of the product to be optimized by using different furnishes in different plies. For many paper products multi-ply technology makes it possible to increase the content of recycled paper and high yield pulps which are interesting both for cost reducing reasons and for environmental reasons. Existing technology fails however to accomplish the task of high speed forming of multi-ply paper or board with superior mechanical properties as well as favourable ply coverage characteristics.
The need for a forming technology applicable to high production rates is stressed by recent developments in wet pressing technology. Shoe presses giving high press impulses and high pressing efficiency are now being installed in the production of most major paper and board grades.
Good ply coverage characteristics, i.e. good formation and purity of the individual plies, is an obvious requirement in order to fully utilize the potential of a multi-ply product. The need for a forming technology yielding superior mechanical properties is stressed by the growing interest to use raw materials with a relatively low strength potential such as recycled fibres and high yield pulps.
Multi-ply forming technology may be grouped into three main categories:
1. Forming each ply in a separate forming unit before couching the plies together.
2. Simultaneous forming of all plies in one forming unit using a multi-layer headbox.
3. Forming the web plies on top of each other in a sequential mode, i.e. forming the second ply on top of the first ply and the third ply on top of the second ply etc. The present invention belongs to this category.
Separate forming is commonly carried out with a multi-fourdrinier machine. Hybrid forming or twin-wire forming (cf. e.g. DE 44 02 273 C2) may also be applied. The increase in dewatering capacity given by two or more separate forming units may be utilized for increased production rates and/or lowered forming consistency for improved sheet properties. All variants of separate forming have one problem in common, however, viz. the ply bonding which generally limits the Z-direction strength of the multi-ply product. Often starch or some other bonding agent has to be sprayed on the plies before couching them together.
While twin-wire forming would be preferable for speeds above 1000 m/min, avoiding free surface instabilities and providing higher dewatering capacity the problem of ply-bonding then becomes worse. This is because a twin-wire-formed sheet ply has two wire sides with poor ply-bonding ability, in contrast to a fourdrinier ply which has one wire side and one top side with a better ply-bonding ability.
Simultaneous forming of a multi-ply product with a multi-layer headbox may also be employed. Examples of multi-layer headboxes are found in EP 0 681 057 A2 and in GB 2 019 465. With this method, however, the dewatering capacity is limited to that given by a single dewatering unit. Hence, this principle is not suitable for high speed forming, of moderate to high grammages at low forming consistency. Hitherto, it has moreover proved difficult to accomplish acceptable ply coverage characteristics with simultaneous forming.
Multi-ply forming in a sequential mode has traditionally been applied in the forming of two-ply linerboard using a secondary headbox placed some distance downstream a fourdrinier wire with dewatering of the top ply through the base ply formed upstream of the secondary headbox. The problem of ply-bonding is essentially avoided by depositing a fibre suspension onto the pre-formed web. This means that the Z-direction strength of the multi-ply product is often determined by the Z-direction strength of the individual plies rather than by the ply-bonding. Forming a top ply onto a base ply on a fourdrinier wire involves several disadvantages, however. It suffices to mention the drawback regarding dewatering capacity and the severe grammage variations due to the free surface occurring especially above 1000 m/min.
Dewatering in a twin-wire zone created by the wire carrying the base ply and an additional, web-free wire through which the top ply is essentially dewatered has been applied shortly after the secondary headbox in units relying extensively on vacuum generated dewatering (see e.g. Attwood (1991) “Multi-ply forming”, Pulp and Paper manufacture Vol. 7 Paper Machine Operations, TAPPI & CPPA; p.250-251). Dewatering through the previously formed web is thus essentially avoided whereby an improved dewatering efficiency is achieved. Dewatering of the top ply through a web-free wire is moreover advantageous with regard to purity and formation of the top ply, because a separate handling of the white-water of the top ply is possible and because any influence of the base ply structure on the top ply is substantially avoided. The capacity of this kind of units is still limited, however, and they are typically used on multi-ply board machines running slower than 600 m/min.
In U.S. Pat. No. 3,543,834 there is disclosed a multi-ply web former utilizing cylinders or rolls. Successive web plies are formed in a forming area between the foraminous belts wrapping a forming cylinder where one of the belts are used in the preceding forming area. According to U.S. Pat. No. 3,543,834 dewatering is accomplished by “centrifugal force and pressure of the foraminous belt against the web”.
There is no indication of conditions involving the impinging headbox jet to deflect the outer wire and penetrate into the twin-wire nip. Rather, it can therefore be inferred that the wire geometry is fixed. This means that the principal forming phase is not accomplished over the roll periphery at an essentially constant dewatering pressure. It is then not possible with this arrangement to achieve favourable mechanical sheet properties, since a substantially constant dewatering pressure is a prerequisite for good mechanical properties. The dewatering capacity is moreover unsatisfactory with this arrangement.
U.S. Pat. No. 3,625,814 discloses a multi-ply web former of a similar kind. Dewatering of the pulp stock is said to take place “as the belts come together on the impervious forming roll”, which indicates that the geometry of the outer wire is fixed.
The same applies to the multi-ply web former disclosed in U.S. Pat. No. 3,821,073. The fibre suspension is dewatered “in that the water is forced through the two wires as these run together along a portion of the cylindrical surface of the forming roll”.
In DE 44 02 273 A1 there is disclosed a two-ply forming unit utilizing twin-wire roll-blade forming for both the base ply and the top ply formed onto the base ply. Roll-blade forming employs only an initial roll dewatering phase followed by blade dewatering, During roll forming, which was introduced in its basic sense some four decades ago (U.S. Pat. No. 3,056,719) and is well known from the field of high speed production of (single-ply) printing paper, the two wires containing the fibre suspension runs on the periphery of the rotating forming roll. The dewatering pressure is determined by the outer wire tension divided by the instantaneous radius of curvature, and during roll dewatering the pressure rises steeply during an initial phase after which it levels off to a plateau. During blade dewatering the wires are deflected over stationary blades resulting in a pulsating dewatering pressure.
Although the employment of roll-blade dewatering means a significantly better machine speed potential than the previously described methods for forming a top ply onto a base ply, it has still drawbacks, in particular with regard to the mechanical sheet properties. The blade dewatering can have a strong adverse effect on the Z-direction strength of the individual plies, meaning that the Z-direction strength of the multi-
Halpern M.
Silverman Stanley S.
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