The wood in living trees has a high moisture content because of the transport of sugars and nutrients in the sap. Essentially, water enters the tree via the root system and departs through the leaves by the process of transpiration.

The bark plays an important role in this process as it is impervious and thus prevents the tree from drying out and rain from washing away the sugars. The anatomy, chemistry and physical properties of bark are quite different from that of the wood within. In addition, the bark accumulates stones and grit during felling, transport and storage. Consequently, debarking the log is an obligatory step for the manufacture of all wood based panels except particleboards.

Ideally, however, bark should not be included in a particleboard furnish as it reduces board strength properties. This is partially due to the nature of bark but also because it is reduced to a fine dust during wood chipping and subsequent drying and sorting operations. It is thought that the adhesive is absorbed by the dust, because of its high surface area to volume ratio, thus lowering the amount of adhesive available for inter-particle bonding. Including bark also tends to increase panel thickness swelling and linear expansion.

Some people consider that the darker board colour which results from including bark in a panel reduces its marketability. Despite these disadvantages, many manufacturers do not debark their logs before further processing for cost reasons. Even without debarking, a significant proportion of the bark is removed from the wood furnish during primary chipping, where large pieces of bark fall off and are removed, and after drying where bark dust makes up a high proportion of the dust collected at the air-cleaning cyclones.

Three basic types of debarking machines are used by the panel industries: Rosser head, ring and drum.

Drum debarkers are so named because they consist of a large rotating drum in which the logs tumble, if they are shorter than the drum diameter, or rotate if they are longer than the diameter. Either way the bark is removed by friction and collisions between logs. The bark drops through slots in the outer casing of the drum and is taken to silos by conveyor.

Drum debarkers are popular in the paper and panel industries because they have high throughputs of up to 350m³/hr of debarked logs. They generate rather a lot of noise and so are often installed within a building to minimise noise pollution to the surrounding areas. The fact that the log ends tend to be slightly damaged by the impacts with other logs is of minimal importance to these two sectors as the logs are subsequently chipped.

Damaging log ends is to be avoided in plywood manufacture, however, because the logs are rotated in the peeling lathe, gripped by their ends. Consequently, the plywood industry prefers to use the Rosser head and ring debarkers. These remove the bark efficiently, can process a wide range of diameters (especially the Rosser head as can been seen by the differences between the two logs in the bottom corner of Fig 1) and, most importantly, they do not damage the log ends.

Sometimes, the bark is sold as a byproduct of panel manufacture to the horticultural and landscaping sectors. Otherwise the bark can be burnt in the factory’s boilers as it has a high calorific value. The wood based panel sector makes very efficient use of the trees it buys, as it converts the vast majority of the wood into finished products and burns the rest for process energy. Some of the CO2 given off is reabsorbed by trees to start a new production cycle.