Presses – With additional treatment of material – Heating – cooling – or drying
Patent
1996-12-09
1998-11-17
Gerrity, Stephen F.
Presses
With additional treatment of material
Heating, cooling, or drying
100153, 100155R, 1623584, 1623585, D21G 100, B30B 304
Patent
active
058362428
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to a system for calendering a web of paper, paperboard or the like. More specifically, the invention relates to a calendering system of the type using an endless, compressible and elastic calender belt passing, together with the web, through a press nip.
2. Description of the Prior Art
Paper or paperboard is calendered during manufacture with a view to imparting to it increased surfaces smoothness and gloss. In many printing papers, calendering is necessary to provide a sufficiently high printing quality. Calendering is carried out both on coated and non-coated paper or paperboard.
Calendering can be performed on-line in a papermaking or board machine immediately after the drying section thereof. In some configurations, the web is calendered at the end of the drying section. In on-line calendering, use is traditionally made of a machine calender comprising at least one press nip between two hard rolls.
Calendering can also be performed off-line, i.e. substantially separate from the papermaking or board machine, in which case use is traditionally made of a so-called supercalender, which is made up of a relatively large number of rolls placed one upon the other in a vertical stack. Normally, every other roll in the supercalender is hard and every other is of a softer material, the side of the web running on the hard roll receiving increased gloss. A more uniform treatment of the web can be achieved if the relative positions of the hard and soft rolls are changed at the centre of the supercalender.
Also in on-line calendering, calenders with elastic rolls ("soft calendering") have been developed. The soft calender, which can thus be arranged on-line after the papermaking or board machine or a coating unit, normally has a relatively small number of rolls. In soft calendering, each nip is formed between a heated steel roll and an associated elastic roll, for example a polymer-coated roll. Heating, which makes the web soften in the nip, is necessary for the paper to become sufficiently smooth and glossy despite the small number of rolls. The elasticity of the roll in a soft calender entails that the press nip becomes extended, this in turn resulting in a flatter pressure pulse in the soft calender, whereby the pressure force can advantageously be limited as compared with a machine calender.
It is generally known, for example as described in EP-A1-0 361 402 with reference to FIGS. 1 and 2 therein, that there is an essential difference in the calendering result achieved with a machine calender using hard rolls only, on the one hand, and a soft calender using one hard, heated roll and one elastic roll, on the other. A machine calender with hard rolls calenders to a constant web thickness, however with an undesired density variation in the web as a result because of the high, localised pressure pulse giving a comparatively stronger compression of the thicker portions of the web. A soft calender, on the other hand, calenders to a more constant web density, but instead yields a web which suffers from remaining unevenness, i.e. non-constant thickness, and poorer gloss.
EP-A1-0 361 402 proposes in a soft calender to provide the elastic side of the press nip by means of a separate, relatively long calender belt which passes in an endless path around this roll and spaced from the periphery thereof outside the nip. Thus, the paper or paperboard web is located in the nip between the elastic, endless belt and the hard roll. By such a design, the calender belt, which is heated in the press nip by heat from the heated, hard roll, can be cooled during its return travel in the closed loop.
DE 36 32 692 discloses the use of an elastic calender belt which, together with the paper or paperboard web to be calendered, passes through a press nip, e.g. in a supercalender, in an endless path around a hard roll and an additional roll parallel thereto.
To further extend the press nip in soft calenders with a view to further reducing the maximum
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Albany Nordiskafilt AB
Gerrity Stephen F.
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