1. Field of the Invention
This invention relates to apparatus for applying a coating to a web of material, and particularly to apparatus for applying a coating to a paper web.
2. Description of the Prior Art
Various methods of applying a coating to a moving web of material, for example a web of paper, are well known, including air-knife coating methods, blade coating methods, and forward or reverse roll coating methods, each of these methods requiring particular apparatus.
When it is required to apply a low weight coating to a moving web of material, for example a coating of less than 10 g/m.sup.2, a gravure coating method is often used, in which method the coating material to be applied is transferred onto the surface of the moving web of material from cells or depressions in the surface of a gravure roll over which the moving web of material passes and which is rotated in the direction of movement of the web of material.
A disadvantage of the simple gravure coating method mentioned above, in which the web of material to be coated is in direct contact with the gravure roll, is that it is difficult to control the weight of coating applied.
In order to overcome this disadvantage it is known to use so-called off-set gravure coating apparatus in which a second, applicator roll which is rotated by contact with the gravure roll in the opposite direction to the gravure roll, is interposed between the gravure roll and the web of material. With such apparatus the coating material to be applied is transferred from the gravure roll to the applicator roll and then from the applicator roll to the web of material which moves against the direction of rotation of the applicator roll. However, while such apparatus enables the weight of the coating applied to the web of material to be better controlled than is possible with simple gravure coating apparatus, nevertheless such apparatus has the disadvantage that it can be used to apply only a limited range of coating weights.
This is because both the weight of coating applied to the web of material and the quality of the applied coating, i.e. the evenness of the applied coating, are dependent upon the speed of rotation of the applicator roll. However, with regard to the weight of the applied coating, the possible speed of rotation of the applicator roll is determined by the normally predetermined speed of rotation of the gravure roll, whereas with regard to the quality of the applied coating, the desired speed of rotation of the applicator roll is determined by the ratio between the normally predetermined speed of the web of material over the applicator roll and the speed of rotation of the applicator roll. This latter relationship is of particular importance since at ratios outside certain limits the coating material is not evenly transferred to the web of material being coated.
Thus, the desired speed of rotation for the applicator roll for any coating operation is dependent upon two independent and possibly incompatible parameters, i.e. the speed of movement of the web of material being coated, and the speed of rotation of the gravure rolls, and thus it is not always possible to obtain an even coating at a desired coating weight.
This major disadvantage of known gravure coating apparatus is also a disadvantage of all other known coating apparatus in which the speed of rotation of an applicator roll cannot be altered, for example in order to obtain an even coating, independently of a change in the weight of the applied coating.