Naturally sensitive to moisture, wood shrinks when moisture is lost, which can result in warping and twisting. The amount of shrinkage depends on the wood species and also on the pattern of the year rings or growth rings.
Most shrinkage occurs in the direction of the year rings, while minimal shrinkage occurs perpendicular to the year rings. The different shrinkage factors within a board cause warping by pulling the edges and sides of a board in different directions. This can lead to cupping, crowning, bowing or twisting — or a combination of all four.
What can be done to minimize warping and twisting? To start, when buying lumber, specify the expected moisture range. Different moisture levels are expected in furniture grade lumber, mouldings or lumber. Upon delivery, use a meter to check the moisture content and notify the supplier promptly if it is not right.
Not all wood species shrink the same; know the shrinkage factors of the woods you are working with. When mixing different wood species in the same project, select wood with the same shrinkage factors. One rule applies to all wood: The more parallel the grain runs to the edges of the board, the less cupping, crowning or warping can be expected. Another option for manufacturing is to place boards with reverse growth rings adjacent to each other to counteract their shrinking tendency.
(Also used in manufacturing, engineered wood products can be very stable and not as sensitive to moisture changes. The performance depends on the shrinking tendencies of the top layer, while the multi-layered backing is dimensionally stable. Whenever the top layer shrinks too much and the backing does not follow, problems can occur.)
Along with the drying process, proper storage is also critical. Dry lumber should be stored in a closed-in area with a controlled climate, otherwise it will pick up excess moisture from the air if stored in a damp place. Check the relative humidity and temperature of the warehouse or other storage areas and compare it to the moisture content of the wood
If the EMC for relative humidity and temperature is the same as the moisture content of the wood, no changes will occur. (Consult an EMC table.)
Wood products, such as floors, trim, ceiling panels, cabinets and other woodwork should also be allowed to acclimate before being installed. Proper acclimation only takes place when the room conditions are the same as afterward, when the room is in use. Ambient conditions inside of homes or offices are considered acceptable at a temperature between 15-25C and at a relative humidity of 30-50%. Therefore most wood products are made with wood between 6-9%.
Use of a meter to measure the moisture and relative humidity of the wood and environment will help ensure quality control.
Source: Lignomat USA. For information call 800-227-2105 or visit Lignomat.com.
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