Wood fiber grinding disc

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Wood fiber grinding disks (often also MDF grinding disks or English: MDF plates, refiner segments) are used in the wood-based materials industry for the production of wood fibers in refiners .

The wood fibers obtained with their help are primarily processed into MDF, hard fiber and wood fiber insulation boards . Due to the large grinding surfaces in refiners used in industry , the grinding disks for these areas of application are divided into individual, still portable segments.

Working method

The wood chips are usually conveyed directly from a digester into the grinding space, i.e. between the rotor and stator . There they are picked up by the distributor blades on the rotor and conveyed outwards between the actual grinding plates with the help of centrifugal force .

Here, the ground material is crushed due to the interaction of the webs and grooves . So that the material cannot escape through the grooves without being crushed, so-called dams are placed between the webs, which force the ground material into the grinding gap. These dams can have the same height as the webs, or they can also be lower. The latter are called subsurface dams .

Directions of rotation

In the past, mainly bi-directional plates were used, the direction of rotation of which could be reversed after a certain time, in order to increase the service life of the grinding disks. The big advantage is that this means that the edges of the grinding plates can be worn evenly on both sides. In some cases, this can even result in re-sharpening of those who were not involved in the grinding process by wearing off the height of the web. However, since only half of a ring can be aligned with the working direction with these plates, the use of such plates is extremely energy-intensive and also limits the possible throughput of the machine.

Due to the steadily rising energy prices in most countries, the proportion of such panels is getting smaller. B. are rarely used in Central and Western Europe.

Concepts

Due to the required reversibility, the two halves of a grinding segment of bidirectional plates must always be constructed symmetrically to one another. Since this requirement does not exist with unidirectional grinding plates, there is much greater freedom when designing the grinding surfaces, especially with regard to the web angles used, which ultimately form the cutting angle. For the unidirectional grinding plates, the following design concepts have been established over the last few decades, which are used by almost all manufacturers in this way or in a slightly modified form.

Parallel bars (parallel bar design)

In this design, the webs are parallel to one another, but there is a very wide range of varying cutting angles in the grinding process, since the angle between the web and the radius is different on all webs. In addition, the webs have to be adapted to the circular shape using so-called correction angles , which results in further deviations in the cutting angle .

Constant bar design

In order to minimize these angular deviations that occur when the rotor and stator interact, it helps to arrange the webs at the same angle to the radius along a circular arc. This means that there are still different angles, since the desired angle is only constant at the point of the arch, but these are significantly smaller and also the same at all webs (see graphics). The disadvantage of this web arrangement is that the grooves become wider and wider towards the outside. If there is an increased number of webs or if the web length is too great, there may be a risk of clogging of the inlet area. However, this can be compensated for by different zones with different numbers of bars.

Spiral webs (spiral design)

In order to be able to achieve the same cutting angle at every position on the grinding plate, grinding plates have been on the market for a few years whose webs are curved in the form of a logarithmic spiral . Due to the constant cutting angle, it should be possible to produce more uniform fiber dimensions. In addition, the constant cutting conditions should have a positive effect on the amount of energy required. Since the grooves narrow towards the center, the risk of clogging with an increased number of webs can also be prevented here by forming different zones.

See also

Web links