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Ideal moisture content in plywood

By Abhijit Phadke

An important parameter for the determining the quality of plywood is its moisture content. If the plywood is very dry (having low moisture content) then the plywood strength gets adversely affected and it becomes weak. On the other hand if the plywood is too moist, then it is likely to decay and rot in the future.

A plywood sheet displaying ISI mark, photographed at a carpenters shop in Navi Mumbai, India.

Hence it is essential that the moisture content of the plywood be within acceptable limits, As per the Indian standards specification IS:303 (governed by BIS) the ideal content of moisture in plywood should be in the range of 5% to 15%. For marine grade plywood also, the moisture content specification is the same. At any time of the year the plywood moisture content (depending on which part of the country the testing is done), should be between 5 and 15 percent.

So how is the moisture reduced

Plywood is a very strong engineered wood that is manufactured by pressing several layers of veneers (thin slices of wood) over one another. During the process of manufacturing the plywood, the veneers after being rotary-cut are dried using mechanical dryers so as to reduce the moisture content present in the wood. This is essential because proper glueing of the layers is not possible if the veneers have moisture greater than 10%.

For the purpose of drying the veneers, they are passed through the mechanical dryers (hot ovens) on belts moved by rollers, and at the same time fans circulate the hot air generated by the heating coils of the oven all over the veneer faces, so as to completely dry them. After the veneers are sufficiently dry, they are stacked upon one another and bonded together using hot press machines (which glue the veneers together under high pressure and temperature) to make the plywood sheets.

How is the moisture content determined

Simple hand-held devices such as moisture meters are used for this purpose. These devices are available in both analog and digital versions and work in the same way that a resistance meter works. The moisture meter has two probes (pins) which are inserted into the plywood at a fixed distance apart, and the battery powered meter then passes a voltage through the wood.

Because water (which is present in the form of moisture in the plywood) is a good conductor of electricity an electrical path gets established, and the meter then calculates the resistance to flow of current, and displays it in terms of moisture content percentage.

Having said this, it is essential to note that this content will not remain the same. For example, after manufacturing if the plywood is transported to an area that has a very humid climate the moisture content is very likely to increase. Ambient heat and humidity affects the plywood and it will try to achieve an equilibrium with its new surroundings.