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Swelling and shrinking of the wood

Wood movement always happens to wood and wood products. It may the most challenging part in the woodworking world.  Wood is a hygroscopic material, he will absorb or release his moisture content according to his environment condition. In the open condition the wood moisture content will change to reach the equilibrium with its environment air humidity. You can see the graph of the wood m.c. vs the humidity in : moisture content of the wood

 sunken joint caused by wood movement

When the wood pieces release his moisture the wood will shrink, while when he absorbs water from the air, and his moisture content is rising then he will swell. The swelling and shrinkage of the wood can cause problems in the wood products such as: crack, warp, uneven surfaces and other construction problems. 
The shrinkage and swelling of timber is more complicated since it varies from one wood to another. The wood behavior, is dependent on the structure of molecules and cells in the wood. While the cell structure depends on the type of wood and how the wood grew. Therefore the shrinkage and the swelling is depend on the type of wood, the origin of wood and also the grain direction. The amount of shrinkage and swelling changes with a given moisture content is about twice as great as the flat-grain surface surface the edge grain. A wet piece of wood will of wood shrinks more on one direction the another to distort itself.

 wood swelling and shrinking varies depend to the grain direction

This different in shrinkage tendencies mean when lumber of different moisture content, species or different grain direction is glued together side by side, some pieces has risk to swell or shrink more then the other. This phenomenon is resulting on some wood construction problems such as: crack, swell, open joints, warping, defects, etc.. In the bare core for the plywood, the wood pieces with uneven volume will result on the "sunken cores" or "wash board" appearance that all the make uneven surface for the plywood veneer.

How to deal with wood movement

Eliminating the wood movement is impossible, because it is a basic character. But with proper handling you can minimize the risk of problems arising. There are things you can do to minimize the risk of wood movement
  • Choosing the Right Wood
Choosing the right type of wood is a very important. Some types of wood have less movement than others. For example, hardwoods like oak and maple tend to be more stable than softwoods like pine. Make sure you choose the right wood according to the need. The wood which is hard, strong and stable is more suitable for construction, while the wood with beautiful grain and color is more suitable for furniture and decorative product. 
  • Properly processing 
Before use, wood must be cut and drying. The drying process  is very important in the woodworking. The wood must follow the standard moisture content when it processed. The wet wood will keep release his water and shrinkage until it reach his balance with his environment. When wood dry, the finishing can help to make it stable
  • .Design and construction 
No matter how good you process, the wood will still has risk to move. The every woodworker must accommodate this phenomenon in the design and construction. Use joinery techniques that allow the wood to expand and contract. For example, the use of 'tongue and groove', 'dado', or 'mortise and tenon' is flexible to the natural movement of wood. Avoid using glue or nails that are too tight which can hinder the natural movement of the wood.
  • Use of appropriate gluing process
Choose the proper glue that has good elasticity and can withstand the pressure of wood movement. Choose the glue according to the product requirement. The wood joint for outdoor product must be strong enough to withstand the outdoor weather, while for indoor product, the glue may need to fulfill its aesthetic need, instead of durability . Make sure the gluing process is done correctly to ensure the wood tight well. 
  • Finishing process
Finishing is help to make wood more stable. The finishing layer will cover the wood pores and grain and minimize the water absorption. You need to choose the proper finishing according to the product need. 

The data number of wood shrinkage and swelling

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