Method for producing a compressed band of wood fibers for the production of wood fiber boards

Abstract

Wood is ground into fibers and the fibers are partially dried so that the moisture percentage is about 18 to 20 percent. The fibers are then compressed by two screw presses in sequence and extruded through an annular nozzle to form a hollow cylindrical piece. The compressive action of the screw presses liberates the lignines in the wood fibers to provide a binding agent for the extruded piece. The extruded piece is cut open along the axis thereof and spread out flat to form a continuous band which may be subsequently dried and/or compressed and cut into individual pieces.

Parent Case Text

This is a continuation, of application Ser. No. 886,921, filed Dec. 22, 1969, now abandoned.

Description

This invention relates to the production of wood fiber boards.

Three kinds of methods are known for the manufacture of boards from agglomerated wood fibers impregnated with resins or similar binders, namely the said "wet" process, "semi-dry" process and "dry" process.

The semi-dry and dry methods begin in the same manner, that is by drying, wood fibers obtained from a disintegrating grinder to a relatively low moisture content. The wood fibers are then conveyed and dispersed in an air stream in order to be collected on a perforated band under which a negative pressure is provided.

The so-called "felting" operation provides a fiber mat which is compressed at a high temperature in order to obtain the polymerisation of the incorporated binder to produce a hard board.

In the said semi-dry process, the wood fibers are dried to about 6 percent moisture content before felting. After felting, the fiber mat is rehumidified at about 25 percent in order to permit compression and agglomeration with reduced addition of resin, binder or the like, and to obtain a smooth and hard skin. About 1.5 percent resin is used in the semi-dry process instead of 2.5 percent in the dry process. However, it is necessary to leave steam passages between the heating platens of the press. For this purpose, a wire mesh is used as a support of the mat and, consequently, one side of the board is scored.

In the said dry process, the wood fibers are also dried to about 6 percent before felting, but the percentage of resin incorporated into the fibers should be higher than in the case of the semi-dry process. Then, it is possible to compress the fiber mat between two smooth heating platens without using a wire mesh. However, extra-fine fibers should be dispersed on the mat in order to get one side with smooth skin. In this case, there is no scoring of one side of the board.

The main drawbacks of these known dry and semi-dry methods are as follows:

1. Both methods comprise a felting installation which is bulky and expensive. Such a felting installation requires a high power consumption due to the fans as well as cumbersome equipment due to the important air flow and to the necessity of eliminating dust particles before discharging this air to the ambient atmosphere. On the other hand, a felting installation entails a permanent risk of fire or explosion owing to the presence of the suspension of dust and dry fibers in the conveying and dispersing hot air stream. Homogeneous repartition of the fibers in the felter is a very difficult and sophisticated matter.

2. It is impossible to carry out alternately both dry and semi-dry methods on the same plant. It is necessary to use quite independent installations, one for the dry process and one for the semi-dry process.

One object of this invention is to avoid the above-mentioned drawbacks and to realize at a low cost, a new method for manufacturing compressed wood fiber bands for the production of wood fiber boards of the same kind as hitherto obtained, by the semi-dry process, without use of resins or similar binders.

Another object of the invention is to provide a new method for the production of compressed wood fiber bands for making wood fiber boards having improved mechanical properties, specially concerning their tensile strength.

Another object of the invention is to provide a new method for making compressed wood fiber bands for the production of wood fiber boards, either according to the semi-dry process by operating without resins, or according to the dry process by impregnating the wood fibers with resins and by using a mat dryer between the extrusion and pressing operations.

Still another object of the invention is to provide a method for suppressing the fire hazards inherent to the felting operation in the prior art installations.

The method according to the invention for making a wood fiber board is characterized by drying partially a mass of wood fibers to a residual moisture content sufficient for ensuring their flexibility, whereafter said mass of wood fibers is extruded through an annular nozzle in order to obtain a hollow cylindrical shaped product, which is slit along a generatrix and is opened and flattened on a planar surface, after which the compressed flat mat thus obtained is cut into successive portions which are treated in a conventional press to produce a wood fiber board.

Owing to the compression to which the slightly wet fibrous mass is submitted upstream of the extrusion nozzle, the individual fibers are bent and tangled with each other thereby ensuring that the panels obtained have the required physical qualities. The lignines contained in the wood fibers are acting as binders of entangled fibers, said lignines being squeezed out in the extruding nozzle. Any use of pneumatic systems with dry air is eliminated with all the disadvantages which result therefrom in the known processes.

According to another feature of the invention, the mass of wood fibers is extruded free of any additional resin or binder, whereafter the flat mat directly enters into a press supported by a wire mesh of the kind used in the known semi-dry process.

According to another feature of the invention, a percentage of binder, resin or the like is incorporated to the mass of wood fibers in the extruding machine, and, after opening of the hollow cylindrical product, the resulting flat mat is dried before entering into a press having two perfectly smooth platens of the kind used in the known dry process.

A plant for carrying out said new manufacturing method is comprised of wood fibers generator: an adjustable wood fiber dryer: a screw press provided with an adjustable binder inlet; an outlet nozzle provided on said screwpress for extruding a hollow cylinder of compressed wood fibers: a circular saw mounted downstream of said outlet nozzle on a fixed support for slitting said hollow cylinder along its upper generatrix; fixed deflectors for supporting and opening said slit hollow cylinder in order to form a flat mat of said precompressed wood fibers; and an adjustable dryer through which said flat mat is delivered to a press of known type having two heating platens.

In a preferred embodiment, the press has two screws mounted successively to increase compaction.

FIG. 1 is a general diagrammatical view of a plant for the carrying into practice of the process according to the invention.

FIG. 2 is a view in elevation with parts in section showing the extruding device of this plant.

FIG. 3 is a fragmental plan view corresponding to FIG. 2.

FIG. 4 is a section illustrating a modification of the extruding device.

FIGS. 5 to 7 are transverse sections respectively taken along lines V--V, VI--VI and VII--VII (FIG. 4).

Referring to FIG. 1 reference numeral 1 designates two disc grinders or disintegrators suitably fed with wood and which disintegrate this material in the known manner. The grinders 1 can be of the type operating under atmospheric pressure. The fibers from these grinders fall into a conduit 2 to which hot gases are supplied from a combustion chamber 3 into which extends a burner 4 preferably fed with gas from a suitable source. It is known that such an arrangement generates a hot mixture of burnt gases and of air with reduced oxygen content, which is quite suitable for the pneumatic transport of combustible materials, such as wood fibers. The transport of the fibers issuing from grinders 1 is ensured by a fan 5 which blows the said fibers into a dryer 6. The fibers issuing from dryer 6 are directed towards a cyclone separator 8 within which they are separated from the air stream in which they had been conveyed.

The assembly 1 to 8 forms a wood fibers generator 9, which is characterized in that the dryer 6 is adjusted for delivering at the outlet of the separator 8, a wood fiber mass still containing a relatively high percentage of residual moisture. This moisture percentage is for example about 18 to 20 percent, that is enough for ensuring the fiber mass the flexibility necessary for subsequent extrusion.

The seaparated fibers from the bottom of the cyclone 8 are introduced through a duct 10 into the body of a first screw press 14 having a driven feed screw 11. Said first screw press 14 opens into the body or conical casing of a second screw press 29 having a driven feed screw 30. This second screw press 29 is connected with the inlet of an extruding device designated by the general reference 15. This device, which will be described in detail below, yields in a continuous manner a hollow cylinder 16 which is slit along its upper generatrix by means of a circular saw 17 and which is thereafter opened, unrolled and flattened on an appropriate table (not shown) in the form of a band 18.

The use of two screw presses 14 and 29 mounted successively permits an increase in the fiber compaction. As a matter of fact, since the fibers in the inlet duct 10 have a bulk density of about 25 to 30 Kg./m.sup.3, it has been observed that said density is about 150 Kg./m.sup.3 at the outlet 12 of the first screw press, and about 750 Kg./m.sup.3 in band 18. Therefore, there is obtained a compression of the fibers, which results in the following consequences:

reduction of the power necessary for a conventional platen press (not shown) located after dryer 27;

the lignines contained within the wood fibers are squeezed out and they form a natural binder in band 18.

It has been experienced that the use of circular saw 17 is necessary because of the consistency of the pre-compressed mass of wood fibers in hollow cylinder 16. More particularly, it would be quite impossible to replace circular saw 17 by a fixed cutting blade.

On the other hand, the body 31 of the extruding nozzle is provided with a pair of conduits 46 and 47 each having an adjustable valve 48 and 49. Through conduits 46 and/or 47, it is possible to discharge into the body 31 an adjustable flow of water, steam, resin, or any other substance eventually used as a binder for the mass of fibers.

The band 18 passes through a dryer 27 equipped with heat-radiating panels 28 heated by gas burners, not illustrated. The gas used is preferably supplied by a gas generator operating with wood waste from the present apparatus, in such manner as to produce an inexpensive heat energy.

The burners of the heating panels 28 can be as desired either operated or switched off as will be further explained.

At the outlet of the dryer 27, the band 18 is cut into portions which are transferred to a press of known type (not shown) for finally obtaining rigid wood fiber boards.

FIG. 2 shows a possible embodiment of the extruding device 15. Reference 14 again designates the vertical feeding press of the device. This press opens into the conical casing 29 of the horizontal screw 30 of the said device, this screw being preferably provided with two threads in order to improve the distribution and the compression of the material. Screw 30 forces the material into a divergent body 31 the circular outlet of which is almost closed by a core 32 which only leaves an intermediate annular space adapted to form an annular circular nozzle. In the embodiment illustrated in FIG. 2, the end of the shaft 33 of screw 30 which is situated within body 31 is supported by a profiled bearing 34 secured to the body by arms 35 which are also profiled. Core 32 is itself supported within body 31 by a rod 36 secured to a transverse supporting beam 37. This beam carries an arm 38 on the upper end of which the circular saw 17 is mounted. The saw rotates in the vertical plane which contains the axis of body 31, in such a manner as to intersect the annular space which separates body 31 from core 32.

In the modification of FIG. 4 the shaft 33 of the screw passes through core 32 and is supported by a bearing 40 secured to the supporting beam 37. Core 32 is then connected with beam 37 by means of appropriate arms, such as 41, disposed each side of shaft 33. Except for this arrangement the modification of FIG. 4 is identical with the embodiment of FIG. 2 and the operation remains the same.

As a further modification it is possible to rigidly mount core 32 on shaft 33. In this case the core rotates with the shaft while thus smoothing the inner side of the extruded cylinder, i.e., the side which is to become the upper side of band 18. This of course, implies a considerable reaction on a shaft 33 which should therefore be equipped with sufficiently strong thrust bearings.

The body 31 of the extruding device has an extension 42 made of a sheet-iron member adapted to support the slit cylindrical extruded product and which is shaped in such manner as to progressively open the said product until it is brought to the flat form of band 18. FIG. 5 clearly shows the outlet of the extruding nozzle defined by body 31 and core 32. This nozzle ensures the continuous production of the hollow cylindrical product 16 the upper generatrix of which is continuously slit by saw 17. In FIG. 5 the sheet-iron extension has already partially opened or flattened cylinder 16. In order to improve this flattening and to maintain the band 18 applied on extension 42, two inner deflectors 44 are suitably secured to the supporting beam 37. In FIG. 7 the sheet-iron extension is almost flat. It is easily understood that somewhat ahead of plane VII--VII this member is entirely horizontal. The extruded product then becomes the band 18 of FIG. 1. This band 18 is conveyed by a roller conveyor 45 towards the dryer 19.

It is possible, when necessary, to inject a binder such as resin, water or steam into the fibrous mass which flows through the extruding device. In FIG. 4 there is shown for example, a pair of conduits 46 and 47 through which said binder may be introduced into body 31 in order to humidify and/or to bind the fibers when required.

The invention thus permits the production of fiber boards by a process which does not comprise the dispersion of the fibers in a liquid medium, that is to say, by a process of the class of the dry and semi-dry types, but which, in contradistinction to the known processes of the kind in question, does not require the previous formation of a fiber mat distributed by an air stream. Furthermore, and as above-mentioned, it is possible to use this plant in two different ways.

According to a first possibility, the valves 48, 49, on conduits 46, 47 are closed whilst the dryer 27, is switched off. Therefore, the band 18 is free of any binder, resin or the like and contains a percentage of moisture about 18 to 20 percent. Said moisture percentage is not modified when band 18 passes through the not-operated dryer 27, whereafter said band 18 can be processed as in the known semi-dry process, that is compressed between heating platens (not shown), when supported by a wire mesh.

According to a second possibility, the valves 48, 49 are opened for delivering a resin, binder or the like into the mass of wood fibers. The dryer 27 is operated for reducing the moisture content of band 18 from about 18 - 20 percent to about 6 percent. Therefore, after leaving the dryer 27, the band 18 can be processed as in the known dry process, that is, compressed between heating platens which are both perfectly smooth.

Claims

What is claimed is:

1. In a method for producing a compressed flat band for use in the manufacture of wood fiber boards, the improvement comprising:

a. feeding wood to a grinding means to form wood fibers;

b. drying the wood fibers to a first predetermined moisture content sufficient to give the fibers the necessary flexibility for a subsequent extrusion and cutting operation;

c. conveying the partially dried wood fibers to a first screw press means;

d. compressing the partially dried wood fibers into a second screw press means by said first screw press means;

e. extruding said compressed wood fibers through an annular nozzle by means of said second screw press means to obtain a hollow cylindrically shaped continuous piece, whereby the compressive action of said first and second screw press means squeezes out the lignines in the wood fibers to provide a binding agent for the extruded piece;

f. continuously cutting the extruded hollow cylindrically shaped piece along its longitudinal length with a circular saw; and

g. flattening the cut extrusion on a planar surface to form a compressed flat band which can then be cut into individual boards.

2. In a method as recited in claim 1, further comprising the step of drying the cut extrusion to a second predetermined moisture content after said flattening.

3. In a method as recited in claim 1, wherein step (b) includes

conveying the wood fibers to a dryer and

supplying heated gases to said dryer to dry the wood fibers to the first predetermined moisture content.

4. In a method as recited in claim 1, wherein said first predetermined moisture content is approximately 18 to 20 percent.

5. In a method as recited in claim 2, wherein said second predetermined moisture content is approximately 6 percent.