A. Equilibrium moisture content.
MDF is normally manufactured at a moisture content of 8 ±3% but the moisture content at the time of delivery to the user may have been affected by any adverse transportation and storage conditions. In particular, some absorption of moisture is inevitable during long periods in damp storage conditions on building sites, and conversely, a loss of moisture is likely in dry conditions. These changes will initially affect the exposed edges and the top layers of stacked boards but eventually extend to all boards in a stack.
Individual boards or panels of MDF freely exposed to the atmosphere will reach an equilibrium moisture content with the surroundings in a few days. The centre of boards in a stack of MDF will take many weeks to reach this equilibrium moisture content. A curve can be drawn relating the equilibrium moisture content of MDF to the relative humidity of the surrounding air.
B. Dimensional stability of MDF
All wood and wood based sheet materials shrink or swell with changes in moisture content. In relative terms, however, MDF is a highly stable material compared with solid wood. It moves by approximately 0.05% in the plane of the board for each 1% change in moisture content compared with the corresponding transverse movement of solid wood of up to 0.5% in the tangential direction and up to 0.2% in the radial direction. A 1% change in moisture content will result in a 0.35% thickness change in MDF.
As an example of the high stability of MDF in use, a 600 mm wide door panel cut from 15 mm thickness MDF will swell by about 1.5 mm in width and 0.3 mm in thickness for a 5% increase in moisture content equivalent to a change in conditions from 35% rh to 85% rh. Where higher stability is required, a moisture resistant or exterior grade MDF should be used.
Problems resulting from any changes in the dimensions of MDF due to changes in moisture content in use can be minimised by machining and assembling MDF panels and components when their moisture contents are close to the seasonal average moisture content expected in the place of use. As far as countries in Northern Europe are concerned, an MDF moisture content of 9 ±2% is typical. A lower equilibrium moisture content would be expected in Southern Europe. Small variations in moisture content outside these limits are acceptable for most applications because of the high stability of MDF.