The mechanical properties of woodbased panels (WBP) generally increase with their density. This is especially so for densified panels like particleboards and fibreboards. Certainly, for a given panel type manufactured on a particular production line with the same raw materials, the relationship is fairly linear.

Manufacturers can use density to predict the properties of their panels and to control their processes. For example, if the quality control tests indicate that the mechanical properties are lower than expected, but the density is within the process limits then there must be a problem somewhere. It may be a problem with the resin dosing, or hardener mixing, a blocked spraying nozzle so that adhesive is not being evenly distributed – in fact a large number of possibilities that may take a little time to track down.

An option for densified panel manufacturers is to increase panel density so as to improve properties whilst the problem is identified and resolved. Of course, increasing density raises costs, but the inconvenience of additional costs is normally outweighed by the fact that the production line continues to run and the panels produced respect the market requirements. Once the problem is solved the density can be reduced to normal levels again.

There are density measuring devices that can provide continuous, online data.

The most common is to use a weigh belt scale, but there are novel systems like Grecon’s STENOGRAPH, which uses x-ray diffraction and which has the added advantage of giving an indication of the panel’s density profile – how its density varies across depth. Obtaining a precise measurement from a weigh belt scale can be difficult due to the continuous vibrations and the fact that the panel is relatively light compared to the weigh scale itself.

Currently, there is still a need to check panel density by cutting samples and physically measuring them. The protocols to follow vary depending on where you are in the World. In America you would use standard ASTM D1037, in Japan use JIS A 5908, in Europe use EN 323 and so on.

All follow the same basic steps of: cutting specimens of a specific size and position in a panel; conditioning the specimens in a specified relative humidity at a certain temperature; measuring the dimensions often to a precision of ±0.05mm for thickness and ±0.1mm for the other dimensions (see Figure 1); and weighing the specimen (Figure 2) to an accuracy of ±0.01 to ±0.1 g depending on the size to the specimen.

Conditioning can have a large effect on the observed density of the panel. Often in industry, however, there is no time for conditioning because the answer is needed now. A panel that has been recently hot pressed will have a relatively low moisture content, typically between 4 and 7%. Therefore the weight of most panels increases with conditioning due the adsorption of water. Since the density of water is 1000 kg/m3 the density of the panels tends to increase too. For example, I ask my students to make a particleboard with a conditioned density of 640kg/ m3 and the students are often surprised to find that their samples initially have densities of around 600-610. A week later, the samples are normally close to the target density.

The market currently wants low density products that have good mechanical properties. To make these requires fine control over all aspects of panel manufacture