The geometry of the flakes (in the case of particleboard) and strands (in the case of OSB boards) plays a major role in the properties of the later board. In order to achieve high quality, the flakes and strands should have only slight variations in thickness, length, width and fines content.
In the usual two-stage process, in which wood chips are produced first, they are reduced to flakes in the second stage by a knife ring flaker. For this purpose, the chips are fed into a ring equipped with knives via a vibration chute and a heavy material separator. The chips are fed into the spinning rotor via a distribution disc. They are accelerated in their flight path and hit a stationary or counter-rotating ring of knives, which cuts them. Alternatively, in direct flaking, round wood can be flaked in a discontinuous process without the intermediate step using wood chips.
The production of the strands requires special care and attention. The strand length for a commercially available OSB board is 100 to 150 mm, the optimum strand width is 10 to 15 mm, the strand thickness is 0.6 to 0.8 mm.
Knife ring flaker
A knife ring flaker contains dozens of individual knives. These must be sharpened at regular intervals. Special knife grinding machines are available for this purpose. For OSB production, direct flakers, so-called stranders, are mainly used. The debarked round wood material is directly flaked parallel to the fibre. Large knife ring flakers have a capacity up to 45 t/h (atro). Quality criteria include a defined and above all constant geometry of the flakes and strands as well as an efficient knife changing procedure.
Flaker knives are available in different designs: Manufacturing tolerances, surface treatment and hardening (partially hardened / through hardened) play a role.