Beating – the most important stage in stuff preparation
Stuff beating is an extremely important technological process that affects almost all the properties of the resulting paper product. Beating of the fibres is carried out by means of beating elements (knives), one part of which is rotating and the other part generally remains stationary, possibly rotating in the opposite direction. Rotary beating elements passing by very close to them with fixed knives exert various mechanical actions on the fibres, such as combing, smoothing, rubbing, shearing and expanding, crushing, splitting, striking or cutting. The beating effect depends on many factors, e.g. the type of fibres to be beaten, the design of the beating machine (peripheral speed of the knives and the type of positioning) and the technological conditions of the process. The direct result of these activities and the purpose of beating the fibres is:
giving the fibres the necessary flexibility and plasticity (internal fibrillation) and developing and hydrating their surface (external fibrillation)
giving the individual fibre fractions the right size, which results in paper with the required structure and lookthrough.
These phenomena, i.e. the internal and external fibrillation of the fibres on the one hand, and their cutting and shortening on the other, can be adjusted as necessary so that one of them prevails or they occur more or less equally. Both phenomena increase the slowness of the stuff in a slightly different way, i.e. decrease its drainage property under certain conditions. However, because the ability of the fibres to hold water increases only due to fibrillation and surface development, therefore, when there is a strong fibrillating effect of the fibres, a slow stuff is obtained, and when it is insignificant – a free stuff. However, when there is a strong shortening of the fibres due to the cutting action of the knives, the resulting stuff is called short-fibred, and when the cutting is performed to a limited extent – long-fibred. By appropriately conducting the beating process, stuffs with different degrees of slowness and fibre shortening can be obtained. The fibrillating action prevails if the beating elements spin not too close to the fixed elements. This is facilitated by a higher peripheral speed and width of the knives, higher stuff concentration and not too high beating pressure. This is called stuff refining. Splitting and cutting action prevails if the rotating elements form a narrow gap with fixed elements. The conditions for this are – narrow knives, rotating at low peripheral speed, low stuff concentration and high beating pressure. This is what we call the after-beating (cutting) of stuff. The beating process is usually carried out in conical or disc mills. If the design of the mill is conducive to stuff refining – it is called a refiner. Regardless of whether the beating elements are placed on the rotor of a conical mill and its casing or on the discs of a disk refiner, and regardless of the design of the beating machine, there are a number of theoretically derived dependencies and laws to select the right equipment and appropriate technological conditions. It is also extremely important to know and determine the primary and secondary beating effects and their dependence on the type of fibre intermediates and various stuff additives used.