Twisting and warping of plywood.Whilst we take care to store our plywood and other sheet materials flat, in some cases there will be unavoidable warping or twisting. Read on to find out the best way to look after it. To understand why plywood in particular is prone to twisting it is worth mentioning that during the manufacturing process each veneer is cut parallel to the log centre. This by definition leaves us with more warp than when timber is cut parallel to the bark, as we are cutting against the grain, and have to contend with what is known as the slope of the grain. Once all the veneers have been cut they are dried before gluing together. Because it is a natural product there may be some parts of the veneer which are drier than others. As the assembled piece finally reaches a uniform moisture content one corner or another may get pulled up due to the slight shrinkage in one side. Some woods are better than others, for example poplar plywood tends to have less twisting or warping than birch. This is because poplar is much less dense, and dried to a more even moisture content before gluing. Also thicker pieces tend to warp less than thinner pieces as each veneer is thicker it offers more stability when gluing up, and in some cases thicker sheets have more veneers. When plywood arrives with us we always make sure we lay it flat, and we keep it away from direct heat sources and sunlight (which may cause further drying out of one side over the other). Even with all of these precautions in place there are times when the plywood you receive will still have a noticeable warp or twist. There are a few things you can do to help before you work with it:
- If possible store it flat, but not directly on the floor.
- Store it with the concave surface down in sunlight. This will cause the moisture content of the other side to reduce, but keep your eye on it or you may end up with twist in the opposite direction.
- If the plywood is being used to cut out pieces from the full sheet then securing it before working can help. In the case of laser cutting a strong magnet strategically placed (make sure it is not in the cut path) can help, and for other jobs, securing it firmly to the workbench with clamps could be considered.
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