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1. WO1994007126 - PROCEDE ET INSTALLATION PERMETTANT DE DETECTER LA NON-HOMOGENEITE DE LA STRUCTURE DU PAPIER

Note: Texte fondé sur des processus automatiques de reconnaissance optique de caractères. Seule la version PDF a une valeur juridique

[ EN ]

A METHOD AND EQUIPMENT FOR DISCERNING INHOMOGENEITY IN THE STRUCTURE OF PAPER

The object of the invention is a method and equipment for discerning inhomogeneity, in which method a sheet of paper is examined on the basis of radiation directed towards it.

The structure of the base of paper is formed in the wire section of the paper machine. There are always various unevennesses in the structure due to the operation of the wire section. These are measured by means of, among other methods, formation (small- scale base weight variation) and orientation (the average direction of the fibres of the paper) . Inhomogeneities in the structure of the paper appear as operational properties of the paper, causing problems in the further finishing of the paper and in its final use.

When paper dries, unevennesses in the structure cause different places to shrink in different ways, and due to this the faults appear in the dry paper as, for example, wrinkling or curving. The moisture content of paper is often measured by means of the IR technique, in which case the diameter of the point being measured is about 20 mm. Smaller-scale moisture variation, which may be a partial cause of various kinds of faults, cannot be easily measured using present techniques.

Variation in the base weight of paper has been investigated by means of beta-radiation. Variation in the moisture content is then also present. Using present techniques, porosity and moisture must be measured as averages over large areas. Optical transmission gives a visual picture of the precise distribution of light penetrability. This does not necessarily correlate with beta-formation, especially in papers with additives.

The aim of this invention is to create a new kind of method for discerning inhomogeneities in paper and the possibility of classifying examined paper samples on the basis of a new criteria. The characteristic features of the method in accordance with the invention are presented in the accompanying Patent Claim 1 and the characteristic features of corresponding equipment are presented in Patent Claim 4.

The invention is to a large extent based on the observation that the inhomogeneities of paper appear indirectly. In order to promote the drying of paper a vacuum is directed onto it to accelerate evaporation, which emphasizes possible temperature differences on the surface of the paper. The temperature distribution first of all indicates the distribution of the air penetrability of the paper, which further indicates the quantities actually under investigation such as, for example, local differences in base weight, porosity, and orientation. Even though these cannot be distinguished from one another by means of this method, it is in any event possible to classify the samples examined in a new way, which can form one part of the quality evaluation of paper.

In what follows the invention is illustrated by reference to the accompanying figures, which shows a simple test arrangement.

The sheet of paper 3 is placed on top of the vacuum box l, in which a vacuum of 3 - 20 kPa is created. At the commencement of the test the dry substance content of the paper is 40 - 50 %. In the production process the paper sample can be taken directly from the press. If the paper sample is dry, it is dampened to equilibrium moisture content e.g. between damp press-dry sheets of paper.

Due to the vacuum, the sheet of paper begins to dry. Drying takes from 1 - 3 minutes, depending on the vacuum.

The rate of drying of the paper at different places varies according to the local structure of the paper. Local unevennesses in drying are influenced by the structure of the paper, lumps of fibre, variations in porosity, unevenness in the distribution of fillers and fine substances, etc.. Due to these variations, the amount of water contained by the paper may vary locally. There may also be local variations in the ratio of free to bound water. Due to the aforementioned factors the rate of drying of the paper varies locally.

Due to different rates of drying, the surface temperature of the paper settles to a certain equilibrium temperature depending on the moisture content of the place in question. The development of temperature differences is now monitored during the drying process with the aid of thermograph 2. In tests carried out on newsprint temperature differences in the surface began to appear when the dry-substance content reached 70 % and were apparent until the paper was nearly dry. By means of the technique described it is possible to investigate inhomogenities in the structure of paper and local variations due to the them in the drying process of paper.