The best practices in manufacturing can help in producing the highest quality product possible, but all that can go for naught if the moisture content is incorrect.
The moisture in wood changes when the humidity of the surrounding air changes. If air is humid, wood absorbs moisture and expands. If air is very dry wood looses moisture and shrinks. To prevent defects, the wood moisture should be at 6-9%.
Inside heated buildings the relative humidity is usually between 30-50% at 60-800F. At these in-house conditions the stable wood moisture content is 6-9%. If wood moisture and relative humidity stay within these limits, expansion and contraction are limited. For example, if a piece of wood with a moisture content of 6.2% would be moved to an area with 60% relative humidity and 800F, it will absorb moisture and expand until 11% has been reached. Depending on the wood species, the 5% change in moisture can be accompanied by a substantial amount of expansion.
When boards loose moisture they shrink, and the amount and direction of shrinkage depends on the structure of the growth rings. Boards with similar growth ring orientation show similar shrinking and warping tendencies. Otherwise, every board moves differently and in most cases unpredictably.
The least amount of shrinkage occurs across the growth rings — the vertical grain. Most shrinkage occurs in the direction of the growth rings — flat grain. Boards usually have mixed grain and the different shrinkage factors within the same board cause warpage by pulling the board in different directions.
The force from shrinking can be strong enough to destroy the structure of wood. Then, moisture defects become irreversible.
Source: Lignomat USA. For information visit LignmatUSA.com or call (800) 227-2105.
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