MANUFACTURE OF BUILDING BOARD

Abstract: Apparatus for producing layered gypsum fibre board having a conveyor (7) for carrying material through processing stations (4, 5, 6, 8, 10) including a degassing station (8) having lateral barriers (23; 27; 30; 31; 60; 40; 41) to restrict spread of material.

Description: MANUFACTURE OF BUILDING BOARD The present invention is concerned with the manufacture of building board from cementitious material. The invention is particularly useful in the manufacture of gypsum fibre board. Cementitious fibre board is made by mixing a predetermined amount of fibrous material (for example cellulose fibres or wood chippings) , water and dry cementitious material (for example dry calcined gypsum) , forming the mixture into a mat and compressing the mat and then allowing the cementitious material to set and form a board of bonded fibres and cementitious material. The fibrous material may be reinforcing fibres. Additives may be added to the mixture for acceleration of the binding process, or to improve board properties. The amount and type of additive is dependent on the desired properties of the final board. A three-layered cementitious fibre board comprising a mixture of fibres, water, cementitious material, lightweight aggregate such as perlite and any desired additives may be formed using the process described in International patent application No. W093/01932 of Carl Schenk for a method of production of gypsum fibre board. An uncompressed mat comprising a mixture of fibres, water, cementitious material and any desired additives is formed by pouring metered quantities of the mixture onto a moving conveyor belt. The mixture is spread out on the belt forming a three layer mat of significant height. The mixture making up this uncompressed mat contains about 70- 80° air by volume. This is known as the "forming stage". The levelled layer of mixture then passes into a continuous roller press having three main sections; a pre- compressor or pre-press (also known as a degassing press), a main press and a calibrator section. The moving conveyor belt carries the levelled mixture through the press sections and their associated pressing operations. The pre-press compresses the mat to remove air contained in the mixture, and to reduce the mat height in preparation for entering the main press. The pre-press is typically made up of a top belt which gradually converges with the mat carried on the moving conveyor. One or both of the belts of the pre-press are permeable so as to allow the air pressed out of the mat to escape. As the mat is moved into the pre-press, the top belt comes into contact with the top of the mat. The pre-press typically reduces the mat height by about 70% ( e.g. for a final board thickness of 10mm, from about 70mm to about 15-20mm) . The mat leaving the pre-press is about 120% - 150% of the desired thickness of the finished board. Most of the gypsum at the exit of the pre-press has not yet started setting and the compacted mixture is a semi-dry crumbly mat when leaving the pre-press. Sufficient water to complete hydration of the dry calcined gypsum is sprayed onto the compacted mat after it leaves the pre-press. The main press subjects the mat to a high load and presses it to the final board thickness. The main press typically consists of a frame supporting two continuous belt conveyors, one running above and one below the mat. These belt conveyors are supported by closely spaced rolls which permit the conveyors to exert a significant squeezing action on the mat. At the exit of the main press, the gypsum is partially set. The calibrator section is very similar to the main press and can also exert a significant load on the mat. The calibrator is designed to hold the mat at a set caliper until the mat cures to a solid state at the exit of the calibrator. The final setting of the gypsum takes place in the calibrator section or calibration press, such that the mat leaving the calibrator is a set board. In certain circumstances the board leaving the calibration press may not be finally set. Hydration of the mat may not be complete at the exit of the calibration press; a small further amount of setting may then take place after the board leaves the calibration press. In a typical gypsum fibre board plant of the type described above and in WO 9301932 the pressed set board is trimmed and cut before then being dried. The edges of the untrimmed board leaving the calibration press will be soft due to lateral spread of the mat mixture as it is spread and pressed. These soft lateral edges of the pressed board are unsuitable for inclusion in the final manufactured board as they reduce the strength of the board and would, if left on the finished board, result in a non-uniform board. The soft edges must therefore be removed. This is done by trimming the board using a water knife. It has been found that for gypsum fibre board plant of the type described above and forming a pressed mat having a width of about 2600 mm, between 100 and 200 mm must be trimmed from each lateral edge. This means that between about 8% and 16% of the pressed material has to be either discarded or recycled; both involving considerable waste of energy. A further disadvantage of this waste is that the width of board which can be produced by a given plant must be significantly less than the width of the plant belts; in some cases the maximum possible board width is about 400mm less than the width of the plant belts. There is a further problem associated with the manufacture of layered cementitious (eg gypsum) fibre board made up of layers of different mixtures. Gypsum fibre board may be produced in three layers; strong upper and lower surface layers and a less strong and less dense middle layer or core. The layered structure is advantageous as it allows one to reduce the weight of the finished board without sacrificing strength at the critical areas, that is, the surfaces. The less strong and less dense core layer is typically made using a gypsum mixture including a lightweight aggregate such as perlite, vermiculite or a similar filler. The core layer typically has a lower fibre content than the surface layers. The layered fibre board is made by having plant which includes means for the supply of surface and middle layer gypsum mixtures. A layer of surface mixture is spread onto the moving conveyor, a layer of the core mixture is then spread onto this first surface layer and an upper layer of surface mixture is then spread onto the core low density layer. The layered mat is then pressed, trimmed, cut and dried as discussed above. With layered cementitious fibre board, the lateral spreading of the mat as it passes through the forming stage, pre-press and main press causes a problem in addition to those discussed above; the lateral spread of the mixtures and the adhesion of the surface layers to the press belts mean that the low density, weak core layer flows to the bottom of the mat at its edges. This spreading effect results in a board having weak edge portions extending a significant distance into the finished board as illustrated in figures 3(a) to 3(f). Having a finished board with this low density and low strength material at the surface seriously affects the strength of the board. The lateral edges of the board must therefore be sufficiently trimmed that all parts of the board' s surface into which the low density mixture has been spread, and all parts of the board from which the low density mixture has been squeezed out from are discarded. The present invention seeks to reduce the wastage associated with cementitious fibre board plant in which an unset cementitious material is significantly compressed or compacted before it sets. The present invention provides process and apparatus for manufacturing building board as defined in the independent and omnibus claims to which reference should now be made. Preferred features of the invention are defined in the dependent claims. Preferred embodiments of the present invention allow one to significantly reduce the wastage associated with the known plant of the type described above and in International Patent Application No. WO93/01932, without reducing the strength of the finished board. It has been proposed that the problems associated with the edges of the board could be reduced by providing a fixed lateral barrier to the spread of the mixture in the forming station before the mixture enters the press. However, such barriers only partially relieve the edge problem. The inventors of the present invention are the first to appreciate that a significant proportion of the lateral spread occurs in the pre-press. It has not previously been appreciated that although the mat leaving the pre-press is not set the subsequent higher pressure pressing in the main and calibrator presses does not significantly further spread the mat lateral edges . The present invention derives from the inventors' realisation that the critical and main source of lateral mat spread occurs during the pre-press even though the mat leaving the pre-press is not set and is yet to undergo processing steps in which significant pressures are applied to the mat before it sets . A significant improvement in the edge characteristics of the press and board can be achieved by providing a lateral barrier within the pre-press. Although it has previously been proposed to provide lateral barriers up to the pre-press, these previous systems, did not effectively deal with the problem of lateral spread as most of this occurs during the mat pressing in the pre-press. The inventors are the first to appreciate this. Consequently, cementitious fibre board produced in a process of the type described above including pressing and significant neight reduction of a mat during the pressing and n which there is a lateral barrier in the forming station only will have significant soft edges. As discussed above the inventors of the present invention are the first to appreciate that the problem of soft edges can be significantly reduced by constraining lateral movement (i.e. spread) of the mat in the degassing or pre-press. The known systems having fixed lateral barriers m the forming stage are also highly inefficient as the mixture including cementitious material tends to stick to the barrier and stationary material builds up on the barriers . The inventors have appreciated that very significant improvements are achieved by restraining lateral spread in the pre-press. Although not set, the mat leaving the prepress is sufficiently compacted for the lateral spread in a main press section without lateral barriers to be less significant than that in a pre-press section without lateral barriers, even though the main press exerts a greater pressure, and exerts a pressure for longer. Embodiments of the invention will now be described with reference to the following drawings in which :- Figure 1 is an illustration of a method and corresponding plant for the manufacture of layered cementitious building board; Figure 2 is an illustration of the cross-section of a layered gypsum fibre mat on the forming belt of figure 1, Figures 3(a) to 3(g) are an illustration of the cross-section at different points in the manufacture of half of a layered gypsum fibre mat made according to the known method using the method and plant of Figure 1, Figures 4(a) to 4(f) are an illustration of the cross-sections of half of a layered gypsum fibre mat made according to an embodiment of the present invention; Figure 5 is an illustration of an embodiment of a pre-press lateral barrier; Figure 6 illustrate a drive and/or guiding system for the barrier of figure 5; Figures 7 (a) to 7 (b) and Figures 8 (a) and 8 (d) are alternative pre-press lateral barrier constructions. Figure 9 is another alternative pre-press barrier; Figures 10(a) to 10(c) illustrate the compression of the barrier of figure 9 as it passes through the prepress; Figure 11 illustrates an alternative pre-press barrier; Figure 12 illustrates the barrier of Figure 11 as it is compressed; Figures 13 and 14 are further alternative pre-press lateral barriers; and Figure 15 illustrates a possible guiding mechanism for the lateral barriers of figures 8 to 11. The described embodiments and associated drawings are for illustration purposes and it can readily be appreciated that there are further possible embodiments of the invention. In particular, the described embodiments are all concerned with the manufacture of three layered gypsum having a thickness of about 10mm. It will readily be appreciated that the invention is applicable to other board configurations and thicknesses. In the following examples, layered gypsum fibre board is made by spreading three layers (1, 2, 3) through three separate mixing heads (4,5,6) onto a forming belt 7, subsequent compression to expel air from the mat (typically a reduction of about 70% in a pre-press (8) defined by a top cover belt (9) which converges towards the forming belt (7) and reduction to a final board thickness in main and calibrating presses (10, 20) defined by a press top cover belt (21) and the forming belt (7). The method and plant of the described embodiments are modifications to the known plant described in WO 93/01932 and reference is therefore made to this publication for details of the plant and method which are not included in the following description. In the process shown in Figure 1, wet recycled paper fibres for a bottom surface layer (1) are spread on a first preforming belt (not shown) , followed by a layer of hemihydrate plaster or gypsum together with additives. The belt carries the materials to a mixing head (4), which deposits the resulting mix on a main forming belt (7) . Water is then sprayed on the mix; this water starts hydration of the gypsum in the bottom surface layer. Wetted aggregate (e.g. perlite or a similar lightweight material) and fibres for the core mix are supplied to a blender from where the core mix is spread on a second pre-forming belt (not shown) , passing through a mixing head (5) to be spread on top of the bottom surface layer (1) on the main forming belt (7) . At the same time fibres and plaster for a top surface layer (3) are supplied to a third pre-forming belt (not shown) , are mixed in a mixing head (6) and spread on top of the core layer (2) on the main forming belt (7) . The layers of gypsum mixture form a mat. Carried by the belt (7), the mat (1,2, 3) passes into a pre-press or degassing press (8) which compacts the mat to a fraction of its initial height; in the particular embodiments of the invention which will be described in this document to about 30% of its initial height. This pre- or degassing press expels most of the excess air from the mat. The compacted mat is then carried by the belt through a spraying station where sufficient water for hydration of all the gypsum in the mat is sprayed onto the compacted mat and then through main (10) and calibration (20) presses successively. These may exert significant compressive pressures and compact the mat a further 10% to 20% or so. In a preferred embodiment producing finished board 10.2 mm thick (± 0.2mm), the mat has a height of about 70 mm when entering the pre-press (8), about 20 mm on leaving the pre-press and 10.2 mm on leaving the calibration press (20). During the passage through the main and calibration presses, the gypsum hemihydrate hydrates and sets (to gypsum dihydrate) to form a structure in which the core and surface layers (1, 2, 3) and the fibres therein are bonded together by interpenetrative gypsum dihydrate crystal growth. The paper fibres reinforce the board. Fibres of alternative materials may be used to affect different board properties. For example, polymer fibres may be used to improve the impact absorbence of a finished board. As shown in Figures 2 and 3 and discussed above, the known gypsum fibre board plant of the type shown in Figure 1 but not including the present invention and having the successive pouring of the bottom surface, core and top surface gypsum mixtures produces an uncompacted layered mat (2) having a cross-section as shown in figures 2 and 3(c) . This uncompacted mat then passes into the pre-press (8). The pressure applied in the pre-press and the tendency for the top and bottom surfaces of the mat to adhere to the belts (7, 9) with which they are in contact both lead to further lateral or sidewise spreading of the mat (see Figures 3(d) and (e)) and of the core layer relative to the bottom and/or top surface (s) of the mat (see Figure 3(e)). The presence of core layer material containing aggregate such as perlite on the board surface would significantly weaken the board and it is therefore necessary to trim the edges (27) of the board as shown in Figure 3 (g) so as to produce a sufficiently strong board having a uniform structure across its width. When the setting of the gypsum is substantially complete, the mat leaves the calibration press (20) and is trimmed to remove the non-uniform portions of the board in order to produce a uniform structure which is then dried. In the example of the invention shown in Figures 4(a) to (f), 5, 6 and 7, vertical edge or side belts (23) run alongside the pre-press belts and define a barrier to the lateral spread of the mat as it is compacted in the prepress. The vertical edge or side belts (23) move at the same speed as the conveyor or forming belt (7) and the mat (8) . As illustrated in Figures 4(a) to 4(f) (which are not to scale), the presence of the side barrier in the prepress significantly reduces or eliminates the amount of finished board which must be trimmed off to ensure a uniform board with hard edges. As well as reducing wastage this allows one to produce a wider board on plant having belts or presses of a given width. As shown in Figure 6, the edge or side belts (23) are guided around rollers (24) which define a horizontal path (25) with a section (26) running parallel to the main forming belt (7) and forming a side barrier to mat spread in the pre-press or degassing press. The pre-press barriers of the present invention may be combined with barriers in other stages of the manufacturing process; e.g. in the forming stage, main press and/or calibration press . In an alternative embodiment of the invention (see figures 8(a) to 8(d) the barrier is formed by a hollow rubber tube (27) containing a fluid under pressure. In a preferred embodiment, the tube has a fabric or textile covering in a manner similar to a fire hose. The tube should be capable of being repeatedly compressed with little wear and return to its original state when it is no longer compressed. Suitable fluids include air or gas under pressure, oil or water. A pneumatic hollow tube (27) containing air under pressure (somewhat similar to a vehicle tyre inner tube) is believed to be particularly advantageous as it allows significant compression. The rubber hose-like tube 27 is guided around a horizontal path having a section running through the pre-press by guiding rollers (28) such that it passes through the pre-or degassing press (8) within the width of the press and forms a resiliently deformable barrier to spread of the mat as it is compacted in the pre-press. Although the illustrated hollow tube of figures 8 (a) to 8 (d) has a circular cross-section, other cross-sections (e.g. triangular) are suitable provided that they are able to deform resiliently as they pass through and emerge from the pre-press. The inventors believe that a tube having a cross-section with a flat vertical side facing the mat and a curved side facing the guiding rollers is particularly advantageous. The straight face helps to keep the lateral edge of the compressed mat straight and the curved surface can more easily cooperate with the guiding rollers which would have concave surfaces for contacting the tube. In a plant for providing finished GFB (gypsum fibre board) about 10 mm thick from an uncompacted mat about 70 mm thick, a tube having a diameter of about 80 mm is recommended. Figure 9 illustrates an alternative barrier construction comprising two continuous belts (30, 31) consisting of a series of elastic strips of foam forming two continuous blocks-like belts (30, 31). The lower and thicker block (30) of foam moves around a path similar to that described above for the edge belt of figure 6, and the upper thinner block moves around a vertical path (32) defined by rollers and having a section running through the pre-press parallel to and above the section of the belt path for the thicker lower belt (33) (see Figure 13 in which side barriers operate in forming and pre-press stages) . The foam belts each have co-operating and contiguous sloping faces (39,35) (30,33) which can slide across each other as shown in Figures 10(a) to 10(c) such that as the 40 mm high foam blocks enter the pre-press they overlap slightly (see Figure 10(a)) and together define a continuous barrier about 70mm high. (The height of the entry to the pre-press) . As the blocks pass through the pre-press they move relative to each other and progressively overlap more. When they overlap fully (see Figure 10(b)) they are compressed by the pre-press belts as they move further into the pre-press (8). The blocks may be made of a polyurethane foam having a density in the range of 50 to 400 kgm- 3 . A preferred range is 200 to 300 kgm "3 and a value of 300 kgm "3 is thought to be optimal. An advantage of this system is that it allows one to use materials which are sufficiently stiff to act as effective side barriers but which are not sufficiently elastic to be continuously compressed by about 70% as they pass through the pre-press and return to their original size as they emerge therefrom. The system of figures 9 and 15 allows one to overcome the conflicting requirements for a stiff side barrier which can effectively hold in the mat as it is compacted, and for an elastic barrier which can be repeatedly compressed by about 70% without degrading or losing its elasticity. In an alternative embodiment (see figures 11 and 12) the barrier is formed by a single foam or siegling transilon belt 60. The foam belt has a sloping face when it is uncompressed; this face takes up a vertical position during compression so as to help produce a mat with straight vertical edges. Figure 14 shows an alternative set of foam block profiles (40, 41) which can be used in a similar manner to those of figures 9 and 10. The lower foam belt (40) has a rectangular cross-section about 35 mm high and can slide within an upper channel-like foam belt (41) about 40 mm high. In an alternative embodiment (see figure 13) the twin foam belts are replaced by a single foam belt (50) which is guided around a path in a similar manner to the edge belt of figures 5 and 6. The foam (50) has a V-shaped profile so as to be sufficiently deformable to be able to be reversibly compacted or compressed by the requisite 70% necessary to pass repeatedly through the pre-press (8). The foregoing description includes a number of different embodiments of the invention. Features on one or more of the described embodiments may be used either alone or in combination with other features on the other embodiments .

Claims: Claims : 1. Apparatus for producing board from a cementitious material, comprising: a conveyor for carrying material through a number of processing stations including a forming station for spreading said cementitious material on said conveyor to form a mat, a degassing station for compressing said mat to expel air therefrom, said degassing station including a barrier to restrict spread of said mat in a direction orthogonal to the direction in which the mat is compressed, as it is compressed; and a pressing station for applying pressure to said mat as the cementitious material sets. 2. Apparatus according to claim 1 wherein the conveyor and barrier travel at the same speed. 3. Apparatus according to claim 1 or claim 2 further including a mixing station for mixing said cementitious material with fibrous material before said cementitious material is spread onto said conveyor. . Apparatus according to claim 3 wherein said cementitious material is dry calcined gypsum and is mixed with dampened paper fibres. 5. Apparatus according to any of claims 1 to 4 wherein the height of the mat after processing in said degassing station is less than 50% of the height of the mat before processing in said degassing station. 6. Apparatus according to claim 5 wherein the height of the mat after processing in said degassing station is between 25 and 35% of the height of the mat before processing in said degassing station. 7. Apparatus according to claim 5 wherein the height of the mat after processing in said degassing station is about 30% of the height of the mat before processing in said degassing station. 8. Apparatus according to any preceding claim wherein said barrier is resiliently compressible. 9. Apparatus according to claim 8 wherein said barrier is a hollow resiliently compressible tube. 10. Apparatus according to claim 9 wherein said tube has a substantially circular cross-section. 11. Apparatus according to claim 9 wherein said tube has a substantially triangular cross-section. 12. Apparatus according to claims 9 or 11 wherein said tube has a substantially flat vertical face for restricting the spread of a mat as it is compressed. 13. Apparatus according to claim 9, or claim 12 when dependent on claim 8, wherein said tube has a substantially convex vertical face for cooperation with a guiding roller having a corresponding substantially concave face . 14. Apparatus according to any of claims 1 to 8 wherein said barrier is formed by discrete relatively moveable contiguous resilient blocks. 15. Apparatus according to claim 14 wherein said contiguous blocks have contiguous sloping faces. 16. Apparatus according to claim 14 wherein said barrier comprises two blocks, a first channel-shaped block and a second block having a profile matching the inside of the channel of said first block. 17. Apparatus according to any of claims 14 to 16 wherein said barrier comprises two blocks, each block having a different stiffness. 18. Apparatus according to any of claims 14 to 17 wherein said barrier comprises an upper and a lower block, one of the blocks being part of a continuous belt which moves around a path in a substantially horizontal plane and the other block being part of a continuous belt which moves around a path in a substantially vertical plane, the two paths running substantially parallel to each other through the degassing station. 19. Apparatus substantially as described hereinbefore with reference to one or more of figures 2 to 15. 20. A method for producing board from a cementitious material, including: forming cementitious material into a mat, compressing said mat in a vertical direction to expel air therefrom, whilst restricting horizontal spread of said mat, applying pressure to said mat as the cementitious material sets . 21. Method according to claim 20 including mixing said cementitious material with a fibrous material before forming said mat, 22. Method according to claim 21 which said cementitious material is dry calcined gypsum and is mixed with dampened paper fibres. 23. Method according to any of claims 20 to 22 wherein the height of the mat after being compressed is less than 50% of its height before being compressed. 24. Method according to claim 23 wherein the height of the mat after being compressed is between 25 and 30% of its height before being compressed. 25. Method according to claim 23 wherein the height of the mat after being compressed is about 30% of its height before being compressed. 26. Method according to any of claims 20 to 25 including providing a resiliently compressible barrier to restrict said horizontal spread. 27. Method substantially as hereinbefore described with reference to one or more of Figures 2 to 15.

Inventors: DAVIES DAVID LAURENCE, EMILIUS JUERGEN, WICHT ANDREAS, DAVIES, DAVID, LAURENCE, EMILIUS, JUERGEN, WICHT, ANDREAS
Applicants: BPB PLC
Application year: 1999
Main classification: B28B19
IPC codes: B28B1, B28B5