Manufacture of wood composite board and other building materials requires the evaluation of many variables. The size of the wood fibers and particles, the resin or adhesive content, the pressure, the pressing time, the temperature, and other factors must be determined and checked in order to produce a board of consistent quality.
An accepted measure of consistency is the density profile of the board. If the density profile of a composite board with the desired strength characteristics can be reproduced, then the strength characteristics will be maintained.
Several different methods for evaluating density profiles have been used in industry and in the laboratory. The gravimetric method, in which the specimen is planed, measured, and weighed in small increments, is the most direct. The density measurements, in this case, can be evaluated approximately every 0.030 inches (0.076 cm) of thickness.
Two methods involving radiation sources have also been evaluted. One such method uses X rays to produce a photographic image of the profile of a composite wood specimen; this method allows for virtually continuous density measurements across the cross-section of the board, with correlation made with known standards. A second such method uses a gamma ray source to scan the thickness of a specimen. The transmitted radiation is then correlated to the material density, and a continuous measurement is again possible.
Of the methods mentioned and discussed above, the conventional gravimetric method for density determination is the most time-consuming and the least resolute. Although the density calculations are the most direct (weight/volume), the measurements become more difficult to obtain as the specimen is planed, because the thickness decreases. A thicker specimen would be easier to handle, but would also require more time, due to the greater number of sections that would have to be removed.
The X ray and gamma ray methods for density measurement each provide continuous profiles of the specimen, but they also require radiation sources and a greater knowledge of equipment operation by the user. These methods are faster than the gravimetric method and provide information about densities at specific points rather than averaging the densities over small sections. Both such methods claim to be nondestructive: However, this would not be the case in practical applications in industry. The samples being evaluated must be no more than 3 inches (7.62 cm) in width; so some part of the manufactured board must be cut. The operation of these densitometers not only requires training: The densitometers also incur a substantial initial cost before consistent measurements can be made.