Patent Grant
7145117
The present invention concerns an apparatus for heating a board product containing glued wood, primarily for affecting the hardening reactions of the glue, said apparatus applying the heating energy to the board product by means of an alternating electrical field at microwave frequency. Before the heating step, the board product has been manufactured to be continuous, and it is conveyed through a stationary heating apparatus. The board product generally comprises wood layers arranged parallel to the board, ply layers, the spaces between them being glued with glue to be hardened by means of heat. A typical product is the so-called LVL balk (Laminated Veneer Lumber). The invention is also applicable to other types of wood based board products, in which the glued wood component is bound to a solid board construction by hardening the glue. Before being transported to heating, the board product is usually exposed to pressure in order to get the glued wood components into a close contact and to remove air spaces disturbing the alternating electrical field in the board construction.
In an apparatus to be used for heating of this kind of a wood product, the alternating electrical energy is exerted to the board product to be heated perpendicular to the board plane, and the electrical field affecting the board has been formed by the device exerting the alternating electrical energy to the board so that the field affects in the plane of the board, in cross direction to the proceeding direction of the board. An alternating electrical field arranged in this way affects the material to be heated depending on its dielectric properties. For example the dielectric factor of the wood material of light and dry wood (moisture content 0%) is approximately 1.5 and respectively the loss tangent is approximately 0.02. With dense and damp wood (moisture content 100%) the dielectric constant is approximately 20 and respectively the loss tangent is about 0.25. On the average, with the sorts of wood to be used for board products, the respective values of dielectric constant range in the average humidity conditions from 2 to 4 and of loss tangent from 0.05 to 0.4. These values change to some extent, when the wood gets warm during the heating. With the glues used for board products these values are higher, for example dielectric values from 28 to 12 can be measured for phenol formaldehyde glue, whereby the higher value is received when the glue is in the room temperature and liquid. Thus, the heating effect of the alternating electric field is more efficient for the part of the glue. This effect appears advantageously especially with board products, in which the glue is applied as layers parallel with the board (parallel with the alternating electrical field).
The heating effect caused by the electric field is distributed to the area of the board coherently (taken into account the differences between the wood material and the glue), in case that the edges of the board are in contact with the walls of the device, the device being dimensioned according to the used frequency and the nominal width of the board. In praxis, however, there must be left a running clearance between the edges of the board parallel with the proceeding direction and the walls of the device. The air gap formed in this way, however, disturbs the formation of the alternating electrical field at its extreme ends, and as a result of that, the field at the edge areas of the board is formed narrower in the traveling direction of the board, and consequently the edges of the board get less of heating power, and the temperature remains lower than in the center areas of the board.
The problem mentioned above has been solved in accordance with the present invention by providing each of the gaps between the edges of the board parallel with the proceeding direction and the walls of the heating apparatus with a piece positioned to fill substantially clearance-free the actual gap, said slide piece being made of material having a small loss tangent and a low dielectric factor.
By means of the arrangement described above, the disturbance caused by the air gap in the formation of the electric field in the board product can be eliminated.
For the part of the dielectric properties of the material to be used for the slide piece, it must be taken into account, that in the alternating electric field it is practically free of losses, in other words, the loss tangent of the material is small and its dielectric constant has a very low value with frequencies of the alternating electric fields to be used. Standard frequency of the alternating electrical field in the heating apparatus in question is 915 MHz. Alternatively also a frequency of 2450 MHz can be considered.
A suitable value for the dielectric constant ranges from 1 to 10 and the loss tangent is about 0.02 or below this value. In addition, the sliding properties and/or abrasion strength of the material must be taken into account.
Suitable materials for the slides are different plastic materials and also ceramic materials having the above mentioned dielectric properties. Plastic materials include polytetrafluoroethylene (PTFE), polycarbonate (PC), polyphenylene sulfide (PPS), polyvinylidene fluoride (PVDF). Usable material and the dimensioning thereof as a slide can be chosen by modeling the form of the alternating electrical field to be implemented with it. Other applicable materials have been listed in Table 1 (appearing at the end of the specification), together with the loss tangent values of the materials.
One problem for implementing the apparatus is caused by the heat dissipating from the board to be heated to the slides, as well as the heat formed eventually in the slide material itself by the electric field, eventually making the temperature of the slide material to rise in use. The rise of the temperature can have influence on the durability of the material and also on its behavior in the alternating electrical field. These problems can appear pronouncedly in equipment configurations, where the slides are incorporated in the heating apparatus. When the slides are incorporated in the heating apparatus, the loss tangent of the material should not exceed the value 0.02.
The apparatus can alternatively be implemented so that the slide or a part thereof is formed as a band in a state of motion, following the board product to be heated pressed against its edge and sliding against the wall of the heating apparatus or a fixed part of the slide. In this embodiment the band-like slide or a part thereof can be advantageously implemented as a loop circulating along an endless path, the returning path thereof being located outside the heating apparatus. There the required cooling can be arranged for the slide band, said arrangement increasing the selection of suitable slide materials.
One embodiment of the apparatus in accordance with the present invention is shown schematically in
In
By filling the clearance between the board 1 and the walls of the microwave chamber 3 with pieces 2, 2′ made of a material having the right electrical properties, the prevailing microwave pattern in the board 1 will be formed to a pattern 4′ corresponding the dimensioning of chamber 3, said pattern 4′ shown in
| Number | Date | Country | Kind |
|---|---|---|---|
| 20031680 | Nov 2003 | FI | national |
| Number | Name | Date | Kind |
|---|---|---|---|
| 4456498 | Churchland | Jun 1984 | A |
| 5552583 | Berteaud et al. | Sep 1996 | A |
| 6242726 | Harris et al. | Jun 2001 | B1 |
| 6287410 | Klemarewski | Sep 2001 | B1 |
| 6744025 | Vilo | Jun 2004 | B1 |
| 20040065982 | Churchland et al. | Apr 2004 | A1 |
| Number | Date | Country |
|---|---|---|
| 112026 | Aug 2003 | FI |
| Number | Date | Country | |
|---|---|---|---|
| 20050127067 A1 | Jun 2005 | US |
