WO1997040968A1

WO1997040968A1 - Method for the preparation of pressed wood

Info

Publication number: WO1997040968A1
Application number: PCT/FI1997/000249
Authority: WO
Inventors: Ari Hottinen; Pekka Hottinen
Original assignee: PURISTEPUU HOTTINEN Oy
Current assignee: PURISTEPUU HOTTINEN Oy
Priority date: 1996-04-25
Filing date: 1997-04-25
Publication date: 1997-11-06
Anticipated expiration: 1998-10-25
Also published as: AU2390097A; FI97961C; FI961768A7; FI97961B; FI961768A0

Abstract

The subject of this invention is a method for the preparation of pressed wood, in which method damp or new wood (1) is heated and pressed during the compression stage, mainly with a constant pressure between permeable pressing surfaces (2) which permit gases to pass through them. In the method according to the invention, during the pre-heating time, and at least during the early stages of the pressing, moisture is added to the air surrounding the wood continually, or at suitable intervals, to even out the dampness of the wood.

Description

METHOD FOR THE PREPARATION OF PRESSED WOOD

The subject of this invention is a method for the preparation of pressed wood, in which method, damp or new wood is heated and pressed during the compression stage, mainly with a constant pressure between permeable pressing surfaces which permit gases to pass through them.

When a customary dryer is used to dry wood it takes about 2-3 weeks. In order to reduce the drying time various methods of pressing the wood are exploited with different temperatures and various pressures. The aim is to achieve dry, strong, straight and machineable wood products. The consolidation and drying of wood by pressing has, however, been successful primarily in the research environment. Earlier methods have not produced wood in sufficient quantities for practical requirements. An additional problem is the uneven drying of various parts of the wood; the surface layer dries quicker than the internal layer. When drying and pressing wood products quickly, i.e. when trying to achieve a total handling time of less than 10 hours, the formation of splits in the surface and internal parts is likely.

The purpose of this invention is to introduce a method which eliminates the drawbacks in the present methods. In particular, it is the purpose of this invention to introduce a method which, when used, will produce strong, straight wood products. In addition, the purpose of the invention is to introduce a method which, when used, will facilitate the preparation of this kind of wood products in large batch quantities on an industrial scale. A further purpose of the invention is to introduce a method which, when used, will form the fewest possible splits in the wood products.

The purpose of the invention is achieved with a method in which, during the pre-heating time and at least during the early stages of the pressing, moisture is added to the air surrounding the wood continually, or at suitable intervals, to even out the dampness of the wood. When moisture is added during the warming and pressing stages, the surface layers of the wood product do not essentially dry quicker than the internal parts, so that the wood dries evenly and does not cause splits to form. In the advantageous arrangement of the invention, the pressing pressure of the wood product is 5-30 kg/cm² and the wood product is pressed slowly into the desired form taking about 7-15 hours. This slow pressing results in the wood drying evenly.

In the additional advantageous arrangement of the invention, the wood product is pressed in stages to the desired form, during which the pressing is halted at determined intervals. When the pressing is halted at determined intervals, the moisture between the pressing tool and the wood material can evaporate better.

The wood product is generally cooled after the drying and pressing either by allowing it to cool or by cooling it by mechanically. In an advantageous arrangement of the method according to the invention the pressing pressure is maintained during the cooling period at the same level as during the drying time. In this way the wood cannot twist while drying.

According to the invention, new wood which has been felled no more than 10 days before use, is advantageously used as the wood product. New wood is still naturally damp neither have its outer layers essentially managed to dry compared with the inner layers so that the wood has not generally split. This also depends on the environmental conditions such that it may be possible to advantageously use such wood products which have been felled 7 days at the most prior to use.

In the following, the invention is defined in more detail with reference to the attached illustrations, in which:

Figure 1 shows, diagrammatically, a part of the equipment used in an arrangement of the method and viewed from the side, and figure 2 shows the part of the equipment on a larger scale.

The equipment according to the figures is a multi-layer press 3, and it can be formed from one or several chambers which can be closed airtight, or the equivalent. In the chambers are located the surfaces 2. which move relative to each other and on which the wood product 1 is placed. The number of surfaces, i.e. the press surfaces, can vary with different equipment and there can be, for example. 15-50 surfaces and advantageously, 25-50. The equipment includes hydraulic presses 5 which can be either side of the surfaces 2 or only on one side. The surfaces according to figure 2 are made from two perforated surfaces 6 and the support parts 7 between them. There are air ducts 4 between the perforated surfaces.

When using the method according to the invention the operations are as follows;

New wood is used which is no more than 7 days old and which has not been allowed to dry on the surface. The wood product 1 is placed on the surfaces 2 and initially pre-heated. During the pre-heating the temperature is raised using air and steam to around 100-120°C. Air and steam are blown through the air ducts 4 during the pre-heating period. The duration of the pre-heating depends on the thickness of the wood material and is about 2-5 hours. The relative humidity of the air should be over 100%.

After pre-heating, the press pressure is applied to the raw-material, which is about 5-30 kg/cm². The press pressure is achieved by moving the presses 5 relative to the surfaces and the surfaces 2 relative to each other so that they press against each other. The press pressure is maintained at the same level throughout the pressing stage. The press pressure depends on the wood material being used as well as the thickness of the wood material. When thin material is being used a lower press pressure must be used than when thicker material is being pressed.

The normal temperature used during the drying and pressure heating time is around 120- 160°C. The press pressure can be divided into stages, i.e. the press hydraulic cylinder and the surfaces are opened when required at regular intervals, for example, around 10-60 minute intervals. This time depends on the wood material and the progress of the process. The air humidity is monitored constantly and maintained at the desired level since the humidity must not fall too quickly in different wood layers. If this happens, increase the moisture via the air ducts. This should be carried out especially during the early stages of the drying process. Press stops are used in the equipment which prevent the hydraulic cylinders and surfaces from pressing together beyond a determined distance. The press stops are measured such that the surface is against the stop when the wood is sufficiently dry and movement ceases.

When staining the wood, the drying and pressing temperature during the final stages is raised to around 150-200°C. In this way it is easy and simple to stain the wood. The drying time is about 7- 15 hours.

Once the desired size has been achieved the wood material is cooled. During the cooling time the press pressure is maintained at the same level as during the drying time. Cooling continues until the temperature of the wood material has fallen below 40°C. Cooling generally lasts around 1-4 hours and the total processing time is about 10-24 hours.

The invention is not limited to the advantageous arrangements shown but can vary within the framework of the invention concept formed by the patent claims. In some arrangements of the invention various work stages have their own sections or the equivalent.

Claims

1. A method for the preparation of pressed wood, in which method, damp or new wood (1) is heated and pressed during the compression stage mainly with a constant pressure between permeable pressing surfaces (2) which permit gases to pass through them, characterised in that during the pre-heating time, and at least during the early stages of the pressing, moisture is added to the air surrounding the wood continually, or at suitable intervals, to even out the dampness of the wood.

2. The method according to the patent claim ^characterised in that the wood pressing pressure is 5-30 kg/cm² and the wood product is pressed slowly to the desired shape for a duration of 7-15 hours.

3. A method according to the patent claim 1 or 2, characterised in that the wood product is pressed in stages to the desired shape in which the pressing ceases at determined intervals.

4. A method according to any of the patent claims 1-3 in which method the wood material is cooled after drying, characterised in that the press pressure is maintained during the cooling time at the same level as during the drying time.

5. A method according to any of the patent claims 1-4. characterised in that the wood product is pre-heated before pressing and that during the pre¬ heating a temperature of 100-120°C is used.

6. A method according to any of the patent claims 1-5 in which the wood product is pre-heated before pressing, characterised in that the normal drying and pressing heating time a temperature of around 120- 160 °C is used.

7. A method according to any of the patent claims 1-6. characterised in that the wood product (1) is pressed in a multi-layer press (3) in which there are air ducts (4) and perforated surfaces (6) between the pressing surfaces (2) to direct the gases.

8. A method according to any of the patent claims 1-7, characterised in that new wood is used for the wood product ( 1 ) which was felled not more than 10 days before use.

9. A method according to any of the patent claims 1-8, characterised in that to stain the wood the drying and pressing temperature during the final stages of the drying is raised to a temperature of around 150-200°C.