Method and apparatus for controlling thickness of a web in a calendering nip

Details

Inventors: Verkasalo, Matti;
Assignee: Valmet Oy (FI)
Primary Examiner: Paschall; M. H.
Assistant Examiner:
Attorney, Agent or Firm: Steinberg & Raskin

Method and apparatus for controlling the thickness profile of a web passing through a calendering nip formed by two rolls, in which a variable magnetic flux is directed onto an outer surface on one of the rolls from a magnetic shoe device comprising a plurality of cores situated externally of the roll and over substantially the entire axial length thereof. Each core is spaced a discrete distance from the roll surface, with the magnetic flux acting to induce eddy currents along the surface of the roll, generating heat thereon. The roll diameter is changed over the length of the roll in a manner corresponding to the axial temperature profile of the roll, with the changes in diameter of the roll in turn determining the pressure profile across the calendering nip, and thereby determining the thickness profile of the web passing therethrough.

DETAILED DESCRIPTION One of the objectives of the present invention is partly to reach the same goals as in U.
S.
patent application 560,394.
A further objective of this invention is to provide a method and a device by means of which the heating effect can be adjusted in a controlled way and rapidly in the axial direction of the calender roll for the purpose of controlling the thickness profile and/or the surface properties of the web to be calendered.
As is well known, changes in the temperature profile of the calender roll affect the web to be calendered in two ways.
Firstly, the temperature acts directly upon the surface properties of the web to be calendered, and secondly the diameter of the calender roll is changed to a certain extent as a function of the temperature, and these variations in the diameter, of course, act upon the pressure profile of the calendering nip and thereby upon the thickness profile of the web to be calendered.
A further objective of the invention is to provide such an inductive heating method of the sort concerned and such a method for adjustment of the temperature profile of the roll in which the transfer of power to the calender roll has an improved efficiency (overall efficiency).
A further objective of the invention is to provide a said heating method in connection with which it is possible to apply such closed systems of adjustment of the temperature profile in which the profile in which the problems of stability have been solved better than in prior art.
A further objective of the invention is to provide such a method for the adjustment of the temperature profile in which, instead of adjustment of the positions of adjoining cores or component cores of induction coils and instead of adjustment of the air gap, or together with these adjustments, it is possible to use an advantageous novel mode of controlling the heating power.
It is another object of the present invention to provide a method and an apparatus for controlling pressure profile in a calendering nip and thereby controlling the thickness profile of a web passing through the nip

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