Q: What is the proper moisture content for manufactured wood products?
A:The question of determining the correct or proper moisture content for manufactured wood products centers on two issues:
1. What MC is required for efficient, quality manufacturing?
2. What MC is required for quality performance of the product while it's in use?
There is no MC standard within the wood products manufacturing industry for the correct MC during manufacturing, the correct MC at the time of installation or sale, or the expected average MC when a wood product is in use.
In manufacturing, it is important to have the MC of the wood close to the equilibrium moisture content (EMC) of the air. For example, if the plant is at 38 percent relative humidity, the wood exposed to this air will come to 7 percent MC; in this case, we would say that the air is at 7 percent EMC. If the MC of the wood and the EMC of the air are not very close, then significant shrinking and swelling will occur as the wood loses or gains moisture, trying to achieve an MC that is equal to the EMC.
As a rule of thumb, each 4 percent MC change results in a 1 percent size change. The size change is only in width and thickness; wood does not shrink appreciably along the grain. Shrinking and swelling during manufacturing can lead to warp, open glue joints, and other manufacturing problems. The EMC in manufacturing plants in most of the U.S. averages 7 percent to 9 percent EMC.
Most hardwoods - especially oak - glue and machine best between 6.5 percent MC to 8.0 percent MC; for softwoods, the numbers are 8.0 percent to 10.0 percent. If the lumber is wetter than 10.0 percent MC, then the glue will take longer to cure and may also form a weaker glue bond.
Wetter lumber also has more fuzzing when machining and requires sharper knives. Wood drier than 6.5 percent MC (8.0 percent for softwoods) develops more chipped and torn grain, splitting, and other machining defects. Dry wood, less than 6.0 percent MC, also is very difficult to glue successfully.
In use, the EMC conditions that a wood product is subjected to vary considerably. In a coastal environment (New Orleans, most of Florida, and Seattle, for example), the interior EMC may be in excess of 10 percent.
Finishing the wood with varnish or wax (these are vapor barriers) will reduce the moisture gain or loss when briefly exposed to high or low humidity. Water vapor barriers will reduce the moisture gain over a short time period. However, over a long exposure to high or low humidity, the wood will eventually pick up or lose substantial moisture.
On the opposite end of the spectrum, in a dry climate (Denver or Phoenix, for example), or in the Central or Northern States during the heating season, the EMC inside an office or home will be less than 6 percent.
Certainly, the first step in achieving a quality wood product is to design the product so that it can accommodate small changes in MC easily. The importance of good design cannot be overstated. In other words, a good design will allow the wood to shrink a little without creating a defect. Further, a good design must accommodate a little swelling without having a serious problem.
The second step is to ensure that the wood will not change size, meaning not change MC, between the time of manufacturing and the time of installation.
If the size were to change, there could be some serious installation problems due to incorrect sizes; there also could be warp or cracking.
It is probably impossible to design a wood product that can go from long exposure to a very dry condition to long exposure at a very wet condition or vice versa without some swelling or shrinking problems. However, we can manufacture and design a product that can tolerate short-term variations by using the coatings previously mentioned. These coatings buffer the extreme short-term exposure conditions.
Generally, for most products, shrinking will be more of a problem than swelling. A crack from shrinking is more serious than high pressure created by a little swelling. With swelling, the wood can actually compress a little, absorbing some of the swelling pressures. However, shrinking almost always causes the opening of a joint or check, or formation of a crack.
We can minimize these shrinking problems by manufacturing the wood product initially to a low MC - 6.5 percent to 7.5 percent for hardwoods and 1 percent higher for softwoods. Then any size change will be swelling. We wouldn't want the product to be manufactured with less than 6.5 percent MC because then there could be excessive swelling problems later on, plus there would be various manufacturing problems, as already mentioned.
With all things considered, I suggest that the correct manufacturing and installation/sale MC for a quality hardwood product should be 6.5 percent to 7.5 percent MC. Softwoods should be at 7.5 percent to 8.5 percent MC.
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