Density of Particleboard and Fiberboard

Density of Particleboard and Fiberboard

The density of particleboard and fiberboard generally refers to the average density, and the density of a test piece is usually measured directly. It refers to the ratio of mass to volume of the test piece. The following formula is used to calculate:

Rho =m/v

In the formula, the density of p-particleboard and fiberboard specimens (g/cm3);

M-test sample quality (g);

V-test specimen volume (cm3).

According to the national standards, the density of fiberboard and particleboard is a required item.

The density of particleboard and fiberboard refers to the arithmetic average value of all density samples tested. According to the national standards, the density deviation of particleboard and fiberboard should be calculated while the density of particleboard and fiberboard is measured. The density deviation of particleboard reflects the equilibrium of density of particleboard everywhere. The calculation formula is as follows:

In the formula: p-density deviation (%);

P max – maximum density (g/p);

P min – minimum density (g / p);

P CP – mean density (g / p).

The density deviation of MDF reflects the difference between the average density and the nominal density. The calculation formula is as follows:

In formula: p’- nominal density (g / cm3).

The density of particleboard and fiberboard is a very important performance index, which affects almost all physical and mechanical properties of particleboard and fiberboard. Generally, the greater the density of particleboard and fiberboard, the higher its strength, the greater the nail holding force, but the more difficult it is to machine. Conversely, the lower the density, the better the sound absorption and heat insulation, easy to process, but the lower the strength. The density of particleboard and fiberboard also directly affects the relationship between product and water. See Table 1-8.

There are many factors affecting the density of particleboard and fiberboard, but the main factors are wood species, sizing amount and hot pressing pressure.

Softer wood has greater plasticity. When hot-pressed, it can make the bonded materials contact more closely and form better bonding force. Soft wood has good compressibility and can be compacted more compactly during hot pressing. Therefore, under the same hot pressing conditions, the board made of coniferous wood is more compact than that made of broad-leaved wood.

Table 1-8 Relationship between Fiberboard Density and Water Absorption and Expansion Rate

The Relation between Fiberboard Density and Water Absorption and Expansion Rate

The density of adhesives is higher than that of wood, and with the increase of sizing amount, the bonding point between the adhesives increases and the porosity in the board decreases. At the same time, the moisture content of the slab increases with the increase of sizing amount, and the plasticity of wood increases, which makes it easy to compact. Therefore, with the increase of sizing amount, the density will increase.

Hot-pressing pressure also affects the density of wood-based panels. The higher the pressure is, the closer the contact between the adhesives will be, and the higher the density of the panels will be.

It should be noted that in the same particleboard or fiberboard, the density of each part is different, especially in particleboard and medium density fiberboard. In the direction of thickness, the density is not only different, but also regularly distributed. From the plate to the central layer, the density becomes smaller and smaller, forming a density gradient (Fig. 1-7). The formation of density gradient is due to the increase of plasticity and compression of wood materials under the action of water and heat during hot pressing. In the hot pressing process, the temperature distribution along the thickness direction of the board is different, the surface temperature is the highest and the central layer temperature is the lowest, so the compression degree of wood material is different, forming a density gradient. The actual density gradient of particleboard and MDF is shown in Fig. 1-8. It can be seen that the peak density is not in the surface layer, but in a layer not far from the surface layer, which is due to the pre-curing of the adhesive in the uncompressive state of the surface layer directly contacting the hot-pressed plate.

There are many factors affecting the density gradient of particleboard and MDF: the faster the press closes, the greater the density gradient of the board; the higher the surface moisture content of the slab and the bigger the difference between surface and core moisture content, the more obvious the density difference of the board; the bigger the thickness of the board, the greater the density difference of the board. The greater the density gradient is. The density gradient of particleboard and MDF can be controlled and adjusted in the production process.

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