The invention relates to a method for determining the thickness of layers of adhesive immediately after the adhesive is applied upon inner book blocks during the conveyance of the book blocks which are guided in a guiding device in the adhesive portion of a beading press through a device for applying adhesive.
Known machines of this kind do not contain any means for determining the thickness of the layers of adhesive applied to the backings of inner book blocks. Control occurs by visual observation, executed by service personnel at predetermined intervals or executed by measurements on sections through the layer of adhesive after the end of the adhesive process and after sufficient drying of the glue. The application of the adhesive device is usually completed previously and correction is impossible when too large a deviation from desired values is found. The quality of gumming, though, is related directly to the thickness of the glue layer, so that subjective estimates cause differences in the strength of the block, and a corresponding increase of spoilage.
A publication of the company Jagenberg in the program pamphlet of a conference about "Paper and Cardboard Packages in Packaging Systems" discloses that control of glue application may be performed by photoelectric and by capacitive methods. In the first method, the glue is colored. A photocell measures the intensity of the glue layer. The disadvantage of this method is, though, that the color of the board background influences the measurement and thereby falsifies it and the coloring also influences the outer appearance due to its contrast with the usually white paper. Furthermore, this method only determines whether or not glue is present. No quantitative results are possible.
In the capacitive method, measurement is performed on the amount of moisture in the applied strip of glue. Here, the range of tolerance of the glue layer may be fixed by setting an upper and a lower limit in two potentiometers. These methods, though, have the disadvantage that the measuring result does not determine with sufficient precision thickness of a glue layer between 0.5 and 1.5 mm with permissible tolerances of a few tenths of a millimeter because determined values are influenced by disrupting variables, such as differing board colors, differences in the coloring of the glue, or variations of glue composition.
The object of the invention is to obtain a uniformly high quality of adhesion and to prevent waste by determining the thhickness of the adhesive.