Form Changes

During the tree's first year of life are formed what is called juvenile that encloses the pith in the center of the tree. For some species causes juvenile wood shape changes mainly in the longitudinal direction. Mature wood is changed a bit in the longitudinal direction (< 0.1 percent) between wet and dry conditions, while a juvenile can change up to 2.5 percent. When juvenile wood dries, the wood shrinks considerably more, which can result in cracks and deformations.

Properties of reaction wood (compression wood), juvenile wood and plant turning unit also affect the timber straightness. Tree trunks having a greater plant pivot unit than 3 °, that is, into which the fibers are inclined more than 3 ° relative to the marrow, are most likely to provide a timber for unacceptable warping.

Compression wood is perhaps the most common cause of edge hook and flat bend. The tree forms compression wood in response to an unfavorable loading to maintain the vertical growth direction. With this change the structure of compression wood cell to shrinkage / swelling movements in the cell's longitudinal direction will be up to 10 times greater than in normal wood. This leads to high internal stresses when the two wood types mixed and often results in large formal defect.

The annual rings and sawing planks

How the shape changes when the wood dries, from 30% moisture and below, is best understood if one realizes that the curved growth rings always want to straighten itself out, i.e. become straighter and shorter. The smallest form changes get boards and planks to be cut out so that they have "standing annual rings." The annual rings are where the board perpendicular to the flat side. Such annual rings are obtained at block sawing and often used by cabinetmakers. Biggest cupping get boards of sawn in the stock cross-section periphery with "lying rings." The annual rings which have the same direction as the flat side deck. The crooked and most cracked timber may planks sawn near the pith.

Cupping is due to the shrinkage is different on the two flat sides of the plank. The tendency to cupping depends on density, moisture content, width, thickness, and fiber orientation.

Moisture and wood form

Wood's natural moisture movement gives rise to various changes in shape of the timber. To a plank to get the right size after drying, it must therefore be cut out of the log with some excess. Planks and boards being cut with the same nominal dimensions, but dried to different moisture content, will therefore have different dimensions after drying. These measurements are handled by sawmills and planing mills, and thus does not affect the consumer, if the wood is used in the environment corresponding to supply moisture. However, it should be noted that the wood bought with a delivery moisture content of 18%, which is then built into the walls and ceilings, where the equilibrium moisture content is 6 to 10%, will shrink by about 3 percentage points across the grain.

Moisture migration

Above the fiber saturation point does not shrink or swell as wood moisture content changes. During the fiber saturation point, however wood shrinks when moisture decreases and swells when moisture content increases. The movement is proportional to the change of moisture. Moisture movements are, however, different sized in different directions. Spruce and pine have roughly equal moisture migration, but for other types of wood moisture movements are in different sizes.

Timber and wood products should always be designed with respect to the wood moisture movements. The understanding of these movements and the forces they can achieve is fundamental to good wood technology solutions.

Moisture Movements of pine and spruce
DirectionPercentage of fiber saturation point and absolutely dry wood
Tangential about 8
Radially about 4
In fiber direction 0.4
Volume Change about 12

Different types of deformations of timber
Different types of deformations of timber
Wood pieces deform in drying depending on where in the trunk is sawed
Wood pieces deform in drying depending on where in the trunk is sawed

European Wood (in China)
C412, Beijing Lufthansa Center
50 Liangmaqiao Road, Chaoyang District
Beijing, P.R. China 100125
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Sino-European Wood Center
Room 202, Engineering Department,
Taoliyuan Hou, Xuhui Campus of Jiaotong University,
No.655 Panyu Road, Xu Hui District, Shanghai