Raw density profile
The determination of the raw density profile is an important part of the quality assurance of wood-based materials and serves as an assessment criterion for z. B. Chipboard . The routinely tested standard properties ( flexural strength , transverse tensile strength, and thickness swelling) are not sufficient to assess the suitability of industrially produced chipboard with regard to certain applications.
These test methods do not provide any further indications of the further development of process engineering and must be supplemented by further tests. This is where the raw density profile comes into play, since almost all chipboard properties are directly or indirectly dependent on the raw density profile. As a result, the raw density profile is to be regarded as a central property and has proven to be an informative, easily measurable property.
However, it is not enough to include the mean bulk density as a variable; its distribution in the direction of the surface normals must be determined, the so-called bulk density profile.
Raw density profile determination
Various methods are available for determining the raw density profile. The so-called online process is used in industry. Mention should be made of the stenographer presented by Dueholm and the Greecon DA Online raw density profile measuring device by Warnecke. In the laboratory, a basic distinction is made between destructive and non-destructive methods for determining the raw density profile.
The online processes in the industry are also among the non-destructive methods and also offer the possibility to carry out further destructive test methods (flexural strength, transverse tensile strength and thickness swelling) on the same test specimen, whereby correlations between the raw density profile and the mechanical properties can be checked. In all laboratory procedures, test specimens according to DIN EN 319 with the dimensions 5cm * 5cm * board thickness are used.
Measuring method for raw profile acquisition
Destructive process
Planing or grinding process
In this process, the test specimen is removed in layers and the remaining mass and layer thickness are determined after each layer. The density of the individual layers can be determined from this. (The achievable resolution depends on the layer thickness of the removed layers. A resolution of a few tenths of a millimeter is possible)
Sawing process
Thin slices of about 2 mm are cut from the test specimen with a circular saw. The bulk density of the pane can be determined directly with the aid of a laboratory balance. (The achievable resolution is about 2 mm. One problem is the gaps created by the width of the saw cut.)
Drilling resistance method
Drilling resistance method Conclusion about the density when drilling vertically through the wood-based panel based on the measured torque or power consumption of the motor by the drilling device. The achievable resolution is around 0.5 millimeters.
Non-destructive process
Acoustic emission
With the acoustic emission method, the sound propagation is measured while a tool is moved along the narrow surface perpendicular to the plane of the plate. The correlation between signal level and sound propagation can be used to determine the density.
Gamma rays
Step-by-step irradiation of the test body through permanently radiating isotopes. Detection takes place through the weakening of the radiation by the test body. Due to the different densities within the test body, the radiation is weakened differently.
X-rays
Step-by-step irradiation of the test specimen through an X-ray source ( Braun's tube ). Here, too, detection takes place through the weakening of the rays by the test body.
Computed Tomography
When computed tomography gamma or X-rays are used. With this method, too, the sample is irradiated step by step (rotation of radiation source and detector) and the absorption coefficient for the penetrating radiation is determined depending on the location. This gives a three-dimensional image of the interior of the test body. The destructive processes are rarely used in laboratories because the resolutions are too low and the test specimens cannot be used for further investigations.
Relationship between raw density profile and other mechanical properties
The outer layers are largely responsible for the mechanical properties of chipboard. The bending properties in particular are dependent on the cover layers, since the forces occur in these areas. On the upper side, where the sample is loaded, there is a pressure load, on the underside there is a tensile load. By increasing the density in this area z. B. by using finer wood particles, this property can be improved, but the process parameters of the press also have an influence on this property. A direct connection between the raw density profile and the swelling could also be determined for the thickness swelling.
With a lower density of the chipboard, a lower thickness swelling was found. The reason for this is that there are fewer wood particles to swell in the same room. The transverse tensile strength, at which the internal strength of the chipboard is checked, is also related to the raw density profile. The use of the adhesive, the pressing parameters, the type of chips used and the moisture all have an influence on the transverse tensile strength and the density of the middle layer of the chipboard.
Quantitative evaluation of the raw density profiles
Once the bulk density profile has been measured using one of the above methods, the panel cross-section is divided into defined zones:
- Outer zone above - transition zone above - inner zone - transition zone below - outer zone below
- This division into plate zones is also used to consider the symmetry and the gradient of the bulk density.
Criticism of the X-ray measurement method
The measuring accuracy of the X-ray measuring devices was sometimes overestimated. The currently used customary raw density profile measuring devices do not show a uniform result in comparison. In addition, the measurement results deviate from the raw density profiles assumed to be actual, which were determined using a gravimetric reference method.
The differences are greatest in the area of the top layers, which are particularly important for several board properties. The cause of these differences are the physical radiation connections when irradiating inhomogeneous materials with polychromatic ionizing radiation. This was not taken into account in the previous calibration of the measuring devices. An improvement in the measurement accuracy can be achieved through better calibration and optimization of the X-ray measuring devices.
literature
- HA. May: European Journal of Wood and Wood Products . No. 41 , May 1983, pp. 189-192 .
Individual evidence
- ↑ Jan T. Benthien, Martin Ohlmeyer: influence of face-to-core layer ratio and core layer resin content on the properties of density-decreased particle boards . In: Eur. Wood Prod . Springer, Hamburg May 5, 2016.
- ^ Ee Ding Wong: Effects of Density Profile on the Mechanical Properties of Particleboard and Fiberboard . May 1999.
- ↑ U. Jensen, E. Kehr: Quantitive evaluation of density profiles of particle boards and MDF . No. 53 . Springer, 1995.
- ↑ Konrad Solbrig, Matthias Fuchs, Katja Frühwald, Jörg B. Ressel: Accuracy of the radiometric determination of raw density gradients on wood-based materials . 55th edition. No. 6 . Dresden November 2014, p. 27 .