Apparatus For Compressing Chipboards

Abstract

Improved apparatus for the production of chipboard wherein the gap width between two heated pressing members at the commencement of the pressing operation is smaller than the thickness of the chipboard to be manufactured.

Description

BACKGROUND OF THE INVENTION

The invention relates to an apparatus for producing chipboard, in which a layer or bed of wood chips, to which binders are added, is scattered onto a steel band and the chips moved into a gap between two heated pressing components which compress the wood chip layer to produce the chipboard.

In the prior known apparatus for compressing chipboards, wood chips with added binders are scattered on a steel band, which is moved into a gap between two heated pressure plates. The pressure plates are then moved towards one another and compress the wood chips until a mutual spacing is produced. This spacing is determined by spacer elements between the pressure plates. Since the layer of wood chips settles during the pressing operation, its resistance to the pressing dies is reduced, and as a result, the compressing pressure can be reduced as the pressing time increases. With these known apparatus for compressing wood chipboards, at no instant is there affected a compression of the layer of wood chips to a dimension which is smaller than the thickness of the chipboard which is to be manufactured. Since the spacer element always guarantee a constant spacing, the layer or bed of the wood chips is held during the pressing operation at a constant thickness, until the curing of the binder has occurred. In this way, there is no springing back, or at most, only a very slight springing back, depending on the elastic properties of the manufactured and cured chipboard.

In order to produce smooth surfaces which, as far as possible, are free from pores, it is known to first spread or scatter a layer of fine wood chips, which are to form the lower external surface. A usually thicker layer, which is to form the middle layer of the chipboard to be manufactured is then added to be followed by a top layer of fine chips which forms a smooth upper surface layer. The use of very fine chips did, however, present difficulties in connection with the mixing of the fine chips with binders, which requires a device for scattering that is substantially more complicated. Furthermore, fine wood chips do not contribute to or increase in strength in the board, particularly with respect to bending strength.

SUMMARY OF THE INVENTION

An object of this invention is to provide a simple arrangement for compressing chipboards, whereby it is possible to produce a smoother surface than with the previously known chipboards and, as far as possible, to have a greater strength.

This object which forms the basis of this invention, is achieved by maintaining the width of the gap between the two pressing components at the commencement of the pressing operation at a distance smaller than the thickness of the chipboard to be manufactured.

It was found that by an initial, brief, increased compression of the chip layer to a dimension smaller than the intended final thickness of the chipboard to be manufactured, an extremely strong compression of the chips is produced. This leads to a very rapid penetration of the heat from the pressing dies or pressing cylinders into the outer zones of the compressed layer or bed of chips, particularly when using a continuously operating arrangement. As a consequence, the outside layer is cured particularly quickly and is given the strength required of it. Moreover, by the high degree of compression, a substantially greater freedom from pores is produced. This leads to a better surface quality.

Since the increased compression to a smaller or undersized dimension takes place for only a short period of time with the apparatus of this invention, the inner portion of the chipboard will spring outwardly again, since curing in this portion has still not occurred. The extent the inner portion springs out is determined by the continuing downstream compressive pressure or the width of the following pressure gap, when using a continuously operating arrangement, and by the thickness of the heaped layer of chips and the material properties of the chips. The outward springing produces a smaller chip density in the central region of the board, where, as is known, the chips only contribute to a small degree to the strength of the chipboard. By means of the simple construction of this invention, especially a continuously operating arrangement for the manufacture of chipboards, it is possible to produce a board having high surface quality, high strength, and a low consumption of material which results in a lower weight.

With an apparatus for continuously manufacturing chipboards, the teaching of this invention can be achieved in a quite simple manner by adjusting the gap between two pressure cylinders to a distance smaller than the thickness of the chipboard which is to be manufactured. Following the pressing gap, the pressure cylinder is wrapped or embraced by a steel band under tension, at a spaced apart distance corresponding to the thickness of the chipboard to be manufactured.

The pressure cylinders can be mounted for movement relative to one another and can be pressed against one another in such a way that the required narrowing or undersize of the pressure gap is produced at the start of the pressing operation. By pretensioning the pressure cylinders relative to one another at a certain force, heat expansions of the heated pressure cylinders scarcely have any effect.

The pressing cylinders can also be fixedly mounted, however, it is expedient for at least one pressing cylinder to be made movable relative to the other cylinder and to be controlled by a control mechanism which explores the thickness of the chipboard to be manufactured. For example, if the thickness of the manufactured chipboard is too large, the control mechanism moves the movably mounted pressure cylinder towards the other cylinder and, in this way, reduces the width of the pressing gap to the necessary degree. Heat expansion of the pressure cylinders and also changes in the nature of materials can be compensated for in this way.

DETAILED DESCRIPTION OF THE INVENTION AND DRAWING

The invention is hereinafter to be more fully explained by reference to a constructional example shown in the drawing. Chips 2, to which a binder is added, are scattered by means of a scattering or broadcasting machine 1, on a band 3, which runs continuously in the direction of arrow 4 over a table 5. In this way, a constant layer or bed 6 of chips is formed.

This bed of chips travels on the band 3, which is preferably made of steel, into a gap 7 formed between a large heated pressure cylinder 8 and a small heated pressure cylinder 9. The band 3 wraps around the major part of the pressure cylinder 8 following the gap 7 and then runs around a guide roller 10. In the wrapping region of the pressure cylinder 8, a gap 11 is formed between the cylinder and the band 3. This gap is filled by the compressed wood chips or, strictly speaking, the already partially cured chipboard.

The width of the gap 7 between the pressure cylinders 8 and 9 is smaller than the thickness of the chipboard which is to be manufactured. For example, the width of gap 7 is 3.8 mm when the thickness of the chipboard to be manufactured is 4.2 mm. In turn, the width of the gap 11 is approximately the same as the thickness of the chipboard to be manufactured, i.e., about 4.2 mm.

By the excessive compression in the narrowed gap 7, a particularly intimate thermal contact is produced between the surface of band 3 which rests on the heated pressure cylinder 9, the adjacent region of the layer 6, the surface of the heated pressure cylinder 8 and the opposite surface region of layer 6. This produces a rapid curing of the binder in the chips. Preferably the curing in the region of the gap 7 occurs in at least the surface regions. This curing takes place with a high degree of compression in the gap 7, so that a high density is guaranteed in the surface region of the chipboard to be manufactured.

After leaving the gap 7, the partially cured chipboard springs out again in the middle region of the board. This is because no curing or only a slight curing has occurred, and as a result, the middle region of the chipboard is no longer as strongly compressed. As a result, in regards to strength, the chipboard has a lower density in the less significant central region. Hence, wood is saved and a lower weight is produced. The curing of the central region of the chipboard then occurs in the gap 11 between the band 3 and the pressing cylinder 8. To produce a uniform temperature transversely of the chipboard in the region of the gap 11, it may be expedient to additionally heat the band 3 in the wrapping region of the pressing cylinder 8.

In the example as shown, the pressing cylinder 9 is provided with bearings 12, which are displaceably mounted on a guide 13 substantially in the direction towards the pressing cylinder 8. In this way, the gap 7 between the pressure cylinders 8 and 9 is readily adjustable. The adjustment of gap 7 can be made by appropriate establishment of a certain pressure on the bearing 12 in the direction of the pressure cylinder 8. The width of the gap 7 also depends on the thickness of the layer 6. Layer 6, however, can very easily be kept constant, so that a constant width in the gap 7 is produced.

It is also possible to move the bearings 12 by a control mechanism, is not shown. This movement depends on the width of the gap 11 and thus on the thickness of the chipboard to be manufactured.

Claims

I claim as my invention:

1. In an apparatus for producing chipboard from a layer of wood chips and binders, said chipboard having a predetermined thickness, said apparatus including a first and second heated pressure member and a steel band, said steel band carrying said layer between said first and second heated pressure members, the improvement comprising means for producing high chip density outer regions and a low chip density intermediate region in said chipboard, said producing means including means for positioning said first heated pressure member relative to said second heated pressure member to initially compress said layer to a thickness less than said predetermined thickness.

2. An improvement as claimed in claim 1 wherein said first heated pressure member is a first pressure cylinder and said second heated pressure member is a second pressure cylinder, said first and second pressure cylinders defining a gap therebetween, said gap being less than said predetermined thickness.

3. An improvement as claimed in claim 2 wherein said steel band wraps around a portion of said first pressure cylinder to define a second gap between said steel band and said portion of said second pressure cylinder and downstream from said gap between said first and second pressure cylinders, said second gap being substantially equal to said predetermined thickness.