This invention is concerned with the moulding of fibrous products, particularly although not exclusively fibrous products which are building panels. BACKGROUND ART

Lightweight partition panels and the like made from brittle materials such as gypsum plaster require fibrous reinforcement to prevent cracking, and with a sufficiently high fibre content it is possible to transform the otherwise rigid plaster into a resilient "fibreboard" material with excellent sound insulation and impact resistance. Such high fibre content can be provided most economically by very low cost cellulose fibre derived from waste paper, and further major competitive advantage could be achieved if such cellulose gypsum plaster could be moulded intc hollow cored panels which can be rapidly assembled on site without the need for a supporting framework. However, despite the considerable commercial advantage of such panels, cellulose gypsum has only been produced in relatively low value flat sheets due to the seemingly intractable problems that have so far prevented the manufacture of higher value shapes using such materials.

Many of the problems stem from the entangled and woolly nature of highly fibrous mixtures such as are used for the manufacture of cellulose gypsum board. Such r.xxtures have

to be compressed to about l/5th of their volume to make viable products, which considerably complicates moulding operations for more complex shapes than flat panels and, in the course of mixing such materials with the water needed for binding and setting, the wet mixture usually forms into balls or pellets with little fibre bonding between them resulting in low product strength. Such pelletising from wet mixing is typically overcome when making ordinary flat panels by mixing the material in a completely dry state, laying the same on a horizontal, porous carrier belt and then spraying the dry material with setting liquid shortly before pressing to the required density. The water is driven into the dry material by the pressing action, which also expels entrapped air and excess water ir. the mixture through the porous carrier. Such excess water is necessary to ensure complete wetting and normally leaves a significant excess in the product over the amount needed for the chemical reaction of setting.

Excess water is substantially reduced in further developments of conventional methods by mixing part of the water with the dry ingredients before laying or. the conveyor belt, such partial wetting being below the level where pelletising can become critical to product strength. Nevertheless, significant pelletising occurs in such processes and high pressures are needed to ensure adequate consolidation of such pelletised material inr; products of

adequate strength. High pressures are also required in the aforementioned dry laying method in order to drive the water into the dry material within a commercially acceptable production time, and existing methods are characterised by the high cost of the hydraulic presses needed for production. Another characteristic of commercial flat bed processes is the short pressing times needed to achieve commercially acceptable outputs from such costly equipment, and this in turn requires tight control over chemical setting to ensure the right degree of setting is achieved within the short press time available. The short press times also usually require that most of the setting has to occur after pressing while the material is supported on a long carrier belt, and this lack of containment during setting leads to characteristic dimple marks on the surface of the panels which subsequently have to be machine sanded to achieve a commercially acceptable finish.

Extending such horizontal bed methods to making hollow panels would require retractable core void formers which, in order to compress all parts of the material, have to move relative to each other and relative to the platens forming the outer surface. Such complications would increase the already high cost of the press equipment and increase the process time due to the extra time needed fcr positioning the core formers and following this with a second layer of material and a sequence of vertical and lateral pressure

movements. Both the extra capital costs and the extended process time would adversely affect amortisation for commercial production. There are also numerous other problems, such as providing escape paths for entrapped air and excess water when manufacturing hollow partition panels, where both outer surfaces of the finished panel need to have a smooth finish free of pore marks from a porous belt or platen.

It would appear that some of the aforesaid multiple step moulding operations for making hollow cored products could be avoided by moulding such products vertically rather than on a horizontal belt, possibly using dry materials and wetting by removing the vertical core formers and spraying setting liquid into the dry formed hollov; cores, for example, as described in British Patent No. 2183200. Such methods have been developed commercially by the present inventors, but are generally only applicable to very small compression movements characteristic of particulate materials and relatively very low fibre contents and, due to process stability problems and internal fissures arising from the large compression movements of cellulose gypsum materials, such vertical dry forming methods whilst offering some advance, can fall short of requirements for commercial production. An alternative to forming the materials dry or partially dry would be to add a substantial excess of liquid

sufficient for the mixture to behave as a liquid with solids in suspension. This approach is commonly used in asbestos cement and fibreboard production but, although such wet methods avoid pelletising and might be extended to vertical casting, they have been confined to simple flat or folded shapes at least in part due to the practical difficulties of extracting the excess liquid when manufacturing more complex hollow forms.

Such difficulties should be avoided if mixtures have neither excess nor deficiency of liquid, but such mixtures would appear from the prior art to be in the range where pelletising would be at a critical level, resulting in unacceptably low product strength even when moulded at very high pressure. Such pressure implies costly hydraulically operated pressure moulds particularly in view of the complications arising from moulding hollow shapes, and a substantial number of such expensive moulds would be needed to achieve commercial rates of production. Costs would increase even further if process time was increased to achieve adequate handling strength and a smooth finish before demoulding and, in the face of such negative prior art, it would appear impossible to mould acceptable hollow fibrous panels except perhaps at unacceptably high capital cost. One object of the present invention is to provide a method of moulding fibrous products whereby :: is feasible

to mould acceptable cored construction products, such as hollow panels, in an effective and economic manner. DISCLOSURE OF THE INVENTION

According to one aspect of the present invention there is provided a method of moulding a cored construction product from a liquid-settable particulate material and fibrous material, said method comprising the steps of: mixing said particulate and fibrous materials with sufficient said liquid at least to set all of the particulate material but insufficient for the mixture to behave as a fluid, said setting liquid being distributed evenly into the particulate and fibrous material to dampen same; distributing the dampened mixture into a vertical mould cavity defined between at least one core former and at least one side wall, the (or each) side wall being movable towards the (or each) core former; moving the (or each) said movable side wall towards the (or each) core former to a position to compress the dampened mixture; maintaining the (or each) movable side wall in the said compression position for a period of time sufficient to affect at least partial setting of the mixture; and removing the compressed and at least partially set mixture from the mould.

The present invention permits the use of moulding

procedures, particularly as described in more detail hereinafter, which can include utilising a different regime of press time and pressure compared to the manufacture of conventional flat panels, and which are preferably such as to reduce or eliminate the tendency to pelletise during mixing, so as to provide conditions in which acceptable product strength can be achieved using relatively lightly constructed moulds that can be actuated e.g. by normal compressed air equipment at commercially acceptable cost. Preferably the mixture used in the method of the invention contains a liquid settable powder and a greater bulk volume of fibrous material of a substantially woolly nature such as cellulose fibre derived from waste paper.

Preferably multiple movable side walls are used particularly opposed movable side walls. Where the product is hollow cored, the mould preferably contains at least one substantially vertical core void former and has substantially vertical mould side walls, particularly two pairs of opposite side walls, existing between static mould corners, the mould side walls being movable between first and second positions respectively corresponding to open and closed conditions of the mould, said first position being remote from said at least one core former and said mould side walls being in said first position while the dampened mixture is being distributed into the mould; and said side walls being moved inwardly of the mould tc said second

position after filling the mould to compress the dampened mixture preferably by at least half its uncompressed volume.

In a further feature of the invention, the at least one core former may be expandible and said former is expanded either before or after filling by a relatively small amount compared to the movement of the movable side walls. In one embodiment of this feature the said expansion is effected by inflating an elastomeric sleeve covering the said former and in this embodiment said expansion occurs after filling the mould and with the movable side walls substantially at their inward positions, which positions are maintained while the said core former is expanded to impart the desired pressure on the material.

In a further feature, the aforesaid sleeve is fully contained against bursting or stretching tc a damaging extent during inflation in particular at the upper part of the former where containment parts may be attached thereto, such containment parts having thickness sufficient to contain the sleeve when the mould side walls are at their inward positions, but having insufficient thickness to block the flow of dampened material into the mould when the side walls are at their outward positions. Ir. a further embodiment of the aforesaid feature, the containment parts are angled to provide a gradual transition fcr the sleeve where the latter emerges from the containment parts for the purpose of reducing stretching of the sleeve.

In a further feature of the invention the setting liquid is distributed into the dry or partially predampened powder and fibrous mixture material in a manner which minimises or eliminates the formation of balls or pellets

5 during mixing by procedures, which include dispersing the liquid rapidly and evenly into the materials while the mixture is maintained in a mobile state and at a lower bulk density relative to its static state.

In a further feature, air entrapped within the dampened

- ^• - material in its relatively loose state before the movable sides move to their innermost positions is provided with an escape route during such movement through small gaps between the vertical edges of the movable side walls and the aforesaid static corners of the mould, such gaps being of

¹⁵ sufficiently small size substantially to prevent escape of material, while permitting the escape of air. Further air escape routes can additionally be provided through similarly small gaps in or between the expandible core formers and, in the case of core formers with elastomeric sleeves, any

- - material pressed partially into said gaps between adjacent formers may be compressed by the expansion of rhe aforesaid sleeves so that such material lodged in the said gaps becomes an integral part of the final product ar.d is removed with same during demoulding.

05 In a further feature in the case of rectilinear products with more than one core void, the core formers for

- in ¬ forming said voids may be aligned in a row, the axis of which lies parallel to and between opposing faces of one pair of movable side walls, and the material falling between adjacent formers in the said row may be compressed by moving said formers toward each other by the pressure exerted from the inward movement of the other pair of movable sides positioned at either end of the row of core formers, said formers being free to move under the influence of said pressure. In such cases, the aforesaid inward movement of the mould side walls at the either end of the row of formers is completed before the other pair of opposing movable side walls parallel to the axis of the row of formers are moved inwards.

In a further feature of the invention, the formation of the dampened material into pellets during mixing is avoided by impinging a stream of air or gas containing finely dispersed setting liquid into or onto the dry iixed material while the latter is falling freely or otherwise not in contact with containing surfaces, such impinging action both dampening the material and breaking up any agglomerates of dry material that may otherwise give rise to uneven dampening and resulting in a dampened material which is substantially free of pellets and of relatively low density compared to material containing pellets. In a further embodiment of this feature the said pellet free material is partially consolidated and then mechanically broken or

shredded into pellet free agglomerations of increased density which agglomerations can flow into moulds with less risk of blockage and reduce the amount of mould side movement needed to compress the material in the moulds. In accordance with a second aspect of the present invention there is provided a method of moulding a product ^' from a mixture containing fibrous material and a liquid wherein the fibrous material is dampened with the liquid by the steps of: causing the fibrous material to move through a zone; and exposing the said material to a dispersed spray of said liquid during said movement and whilst the material is free of contact with any surface bounding said zone. This method may include all or any of the features of the method of the first aspect of the invention as described above and also later herein particularly with reference to the accompanying drawings, as appropriate.

A further feature of the invention is the feasibility of manufacturing products of acceptable strength at low or moderate moulding pressures, which pressures may be below 20 bar e.g. 4 to 8 bar for the aforesaid pellet free dampened material or 7 to below 20 bar for dampened material containing pellets. A further feature of the invention is a lev; cost method of actuating the movable mould side walls, particularly at

the aforesaid low or moderate pressures, by inflating flexible preferably fabric- or fibre-reinforced tubes which are flattened and sandwiched between the backs of the movable side walls and bearing surfaces of a fixed outer reaction frame containing said movable side walls. In a further embodiment of this feature, the backs of the movable side walls may be extended to provide a larger area than that of the movable faces in contact with the material, in order to accommodate flattened tubes of larger area, thereby increasing the force exerted on the material from a given inflation pressure of said tubes. In a further embodiment the ends of the tubes are sealed by clamp plates and the tube material between the said plates and the said backs of the movable side walls are supported by hinged plates connected to the said backs and said clamp plares .

Moulding pressure may be maintained for a period of longer than 1 minute, such time being sufficient for the material to gain sufficient strength for demoulding. Preferably the pressure is maintained for a sufficient time, preferably more than 1 minute, to provide ^~ commercially acceptable smooth exterior surface finish fcr the product without requiring sanding.

In accordance with a third aspect of -he invention there is provided apparatus for moulding cored construction products from a liquid settable particulate material and fibrous material said apparatus comprising a rrculd having a

substantially vertical cavity defined between at least one core former and mould side walls between mould corners, feed means disposed above an open top of the mould for delivering a mixture of said particulate and fibrous material in even distribution into the mould cavity, and displacement means operable on the mould to effect compression of the mixture in said mould, characterised in that the mould corners are static and the mould side walls are movable relative to said static mould corners between first and second positions respectively corresponding to open and closed conditions of the mould by operation of the said displacement means, and said displacement means including a reaction means.

The apparatus is preferably used in performing the method as described above and in this case will incorporate the requisite constructional features as set cut for use in connection with the method.

The invention also includes apparatus for moulding cored construction products from a mixture containing liquid setting powder and fibrous material of a substantially woolly nature, the apparatus comprising a vertical mould, at least one core former loc table in the mould in substantially vertical disposition therein, feed means disposed above the open top of t e mould for delivering a mixture of powder/fibrous material in even distribution into the mould and displacement neans operable on the mould to effect compression of a powder/fibrous

material mix present in said mould, characterised in that the mould includes static mould corners and substantially vertically disposed mould side walls existing between said static mould corners, the said mould side walls being movable relative to said static mould corners between first and second positions respectively corresponding to open and closed conditions of the mould by operation of the said displacement means, and said displacement means including a reaction means . Preferably the displacement means including the reaction means comprises an inflatable flexible tube.

In accordance with a further aspect of the present invention there is provided apparatus for moulding a product from a mouldable mixture comprising a mould having a cavity bounded by at least one movable side wall, and displacement means for moving said side wall to compress said mouldable mixture within the mould, characterised in that the displacement means comprises at least one inflatable tube formed from flexible material, and inflation means operable to effect inflation of the tube to bear against and apply pressure to the said side wall. This apparatus may include all or any of the features of the apparatus of the third aspect of the inventior. as described above and also later herein particularly with reference to the accompanying drawings, as appropriate.

According to a preferred feature, the displacement

means includes an outer reaction frame disposed outwardly of the mould, a series of platens, one for each mould side, disposed intermediate the reaction frame and mould, and fluid actuated expansion members positioned between the platens and associated mould sides and adapted, upon expansion, to effect movement of the mould sides between the first and second positions thereof. BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described further by way of example only and with reference to the accompanying drawings in which:

Fig. 1 is a cross section through a partition panel which can be made by the method of the invention; Figs. 2 & 3 are diagrammatic sectional elevations of apparatus for wetting materials used in the method, respectively comprising a rotary batch mixer and a continuous or intermittent spray wetting apparatus; Figs. 4 to 7 are diagrammatic sectional elevations of one form of moulding apparatus according to the invention shown in successive operational stages;

Figs. 8 to 11 are diagrammatic horizontal sections through the moulding apparatus at successive operational stages;

Figs. 12a,b&c show an enlarged view of core void formers of the mould apparatus in expanded and retracted states;

Figs. 13 & 14 are horizontal sections showing open and closed positions respectively of the compressed air (or other fluid) displacement equipment for movable side walls of the moulding apparatus;

Figs. 15 & 16 are sectional elevations of part of the displacement equipment showing open and closed positions of a top clamped end of a pressure tube of the equipment on one side of the moulding apparatus; Fig. 17 is a horizontal section through the clamped end of the pressure tube; and

Fig. 18 is a graph showing the relationship between strength and density of finiεned material after compression and drying using different methods of dampening the fibrous mixture. DESCRIPTION OF PREFERRED EMBODIMENTS

The panel cross-section shown in Fig. 1 nay typically have outside dimensions of 75mm X 600mr, with the thicknesses of sides 1, ends 2 and internal web 3, being typically around 11mm. The panel length is sufficient to span 2.5m between floor and ceiling of a typical building, and the panels are strong enough to be easily r.anhandled on

site. The tongue and groove details of ends 2 are accurately formed to facilitate rapid assembly into a flush faced wall, which accurate tongue and groove details considerably reduce the degree of joint finishing normally required for conventional plasterboard construction; and have the smooth moulded external faces on both sides of the panel which do not require sanding.

The moulding materials for such a panel typically comprise 1 part by weight of dry fiberised waste paper and 4 parts of dry gypsum hemi-hydrate plaster, to which is added a small proportion of additives such as set accelerators and retarders described in more detail later. The waste paper is fiberised conventionally, for example, by means of a hammer mill and mixed with the plaster powder and additives by, for example, a conventional plough share mixer fitted with high shear cutting blades to ensure the powder is impregnated thoroughly into the interstices cf the fibrous material .

This dry mixture is dampened in the case of the method shown in Fig. 2 by a vertical axis mixer, with blades 4 with a tip speed of typically of at least around 25m/sec and the blades angled sufficiently to lift and aerate the material in a rapid swirling motion past fine atomising sprays 5 positioned around or near the perimeter cf cylindrical container 6. The sprays deliver sufficient water within typically 15 seconds to provide typically 2 parts of water

to 5 parts of dry mix, whereafter the blade speed is maintained typically for up to a further 10 seconds after which gate 7 is withdrawn, permitting the dampened mixture to pass through exit chute 8, while blades 4 preferably continue to rotate without loss of speed in order to minimise pelletising which would otherwise occur to a much greater degree if the speed is reduced. An alternative method of dampening the mix is shown in Fig. 3, which is described in more detail later. The aforesaid dampened mixture is manufactured into panels by the apparatus shown in Fig. 4 where the said mixture is distributed evenly into mould filling head 9 containing core void former 10 fitted with elastomeric sleeve 11, which sleeve may be held at the ends of the core former by plugs 12, which plugs may be cf elastomeric material which, on being compressed and contained by the metal parts of the former, provide an airtight seal. The former may be located at the base by pins 13 in suitable slotted holes in movable base plates 14, and filling occurs while movable mould sides 15 are in their outward positions as shown both in Fig. 4 and in sectional plan in Fig. 8, which latter sectional plan also shows the outward positions of the core formers 17 and mould ends 16.

Mould filling may typically take as little as 25 seconds, after which mould ends 16 are moved inwards as shown in Fig. 9, thus compressing the dampened material

which in turn presses against core formers 10 and moves same with inner core formers 17 to compress the material between formers 17 in web zone 18 to form the web of the final product. The movement of the core formers is facilitated by the latter being located at the top by a suitable sliding track or other means and typically at the bottom by slotted holes in the base plates 14 which locate base pins 13.

When mould ends 16 have reached their inward positions, which may typically be with their moulding faces flush with the inner corners of static corner posts 19, mould sides 15 are moved inwards as shown in Figs. 5 and 10, to their inward positions, which positions may also typically be flush with the inner corners of the corner posts 19. Such positions define the outer dimensions of the final product and are maintained by suitable fixed stops which prevent further movement as, for example, shown in Figs. 13 and 14. The distance moved by mould sides 15 for the particular mixture and mixing method cited for this example is around 40mm, with a greater distance moved by ends 16 to accommodate the extra movement for compressing the web zone 18. During such compression the large volume of air within the dampened mixture escapes through small gaps 20 provided between corner posts 19 and said sides and ends as shown in Fig. 10, and with further air escape provided upwards along gap 21 between core formers 11 a d 17 as shown in Figs. 12a and 9.

Immediately, just prior to, or after sides 15 reach their inward positions, sleeves 11 are inflated to a pressure insufficient to move the ends and sides outwards from their fixed stops, but sufficient to compress the dampened mixture, typically around 10 to 17 bar for the material mix cited in this example, said pressure also being sufficient to move the material a small distance compared to that of the side and end walls of typically around 0.5 to 4mm, resulting in a similar clearance between the sleeves 12 and the body of the core formers 10 and 17, all as shown in Figs. 6, 11 and 12b. During such inflation, the sleeves are fully contained against bursting by base plates 14 and angled containment parts 22 as shown in Fig. 6, which parts perform their containing function regardless of the level of the dampened material in the mould and provide a gradual transition for the elastomeric sleeve and reduce local stretching of same. Inflation of the sleeves also compresses any material that may have started to enter the air escape gap 21 so that such material becomes part of the finished product 23 and is removed with same on demoulding as shown in Fig. 12c.

The time to complete all the foregoing compression movements may take 30 seconds, after which the dampened material remains under pressure typically for a further 240 seconds, during which time setting is initiated to a stage where the material will substantially retain ts dimensions after removal of pressure, and provide, after further curing

and drying, the standard of surface finish, density and strength required for the final product. The material is also normally sufficiently set at this stage to be safely demoulded, which latter operation is achieved by deflating core former sleeves 11, retracting mould sides 15 and ends 16 and allowing the moulded product 23 to withdraw downwards from the mould after retraction of base plates 14 as shown in Fig. 7. The product is then ready for trimming the bottom and top ends and drying. The moderate pressure of around 10 to 12 bar in this example is within the range of compressed air produced at moderate cost by standard compressors and intensifiers, and such compressed air can be used to actuate the mould sides and ends by expanding flexible flattened tubes 24 and 25 as shown in Fig. 13. Suitable tubes for this purpose are typically manufactured for delivering water cr fuel under pressure and are designed to collapse and flatten when empty. They are generally made of flexible polymer or rubber with a high tensile fabric reinforcement which resists stretching and can withstand the required pressures, particularly when partially contained within substantially rigid plates as shown in Fig. 14.

The operational steps for this preferred method of pressurising the mould commence with the flexible tubes 24 or 25 sandwiched in their flattened position oetween outer reaction frame 26 and pressure transfer platens 27 or 28, as

shown in Figs. 13 and 15. The said platens may be of larger area than the mould sides 15 or mould ends 16 respectively, in order to compensate for the loss of effective contact area between the tubes and the platens when the tubes are expanded as shown in Fig. 14. This large platen area can also allow higher pressures to be exerted on the moulding material than provided by the pressure of the compressed air in the tubes, thus reducing the cost of equipment for providing compressed air. The platens may be guided ^' and supported during their movement by normal engineering means and returned to their outermost positions after pressurisation by suitable springs or other conventional means.

The ends of the flexible tubes are sealed by clamp plates 29 shown in Figs. 15, 16 and 17, which may be shaped to allow access for compressed air delivery pipe 30 and to provide location for hinged support plates 31 which contain the flexible tube 24 (or 25) over the area between the clamp plates 29 and the fixed reaction frame 26 on one side, and movable platens 27 (or 28) on the other side. The hinged connections may be provided by simple recesses as shown or by any other hinge detail. The same arrangement can be provided for the other end of the flexible tube, which need not have any provision for air access pipe 30 or alternatively, the air inlet may be at either or both ends of the flexible tube.

Products can be made using material dampened by various methods, including the improvement in mixing method shown in

Fig. 3, where dry mixed material 32 is fed at a controlled rate by gravimetric feeder 33 into feed chute 34 in a uniform manner into a substantially larger chamber 35 into which at least two and preferably three or more compressed air atomising sprays 36 are fitted so as to impinge onto the downward flowing material with a blast of air containing finely dispersed mist of setting liquid, causing the material to become sufficiently turbulent to break up agglomerations that may have been formed during mixing the dry material and to become impregnated with the required amount of setting liquid.

The delivery rate of water from the sprays can be controlled in relation to the flow rate of the dry material by conventional automatic means linking the spray metering system and material feeder 33 to ensure constant proportioning to maintain the required degree of dampening. Such controls can be adjusted so that the flow can be continuous or intermittent, with such intermittent operation typically being phased to correspond to the incidence of mould filling in a commercial production line. In order to further ensure uniformity of dampening, feed chute 34 may contain a rotating axial shaft fitted with snort vertical fins to concentrate the dry material by centrifugal force near the walls of chute 34 and emerge from sar.e to present

the sprays with an approximately uniform hollow cylinder of material rather than a more randomly dispersed solid column of material .

The sprays are angled inwards around the central flow of dry material to contain the latter in the central part of chamber 36, thereby minimising adherence of dampened material to the chamber walls. Such adherence is further minimised by drawing the dampened material downwards and away from the walls onto a perforated belt 37 by a vacuum box 38, whereafter the dampened material, free of air from the atomising sprays, can be fed directly into the feed chutes for mould filling. In a further embodiment of this method the material is partially compressed by pneumatic compression roller 39 or other conventional means and then broken into pellet free agglomerations by rotating chopper knives 40, which pellet free agglomerations are then fed into the feed chutes for mould filling as described earlier.

The dampened material ^' produced by such equipment as is shown in Fig. 3, has a distinctly different appearance and product strength in relation to density from material wetted by rotary mixers shown in Fig. 2. These different strengths and densities are shown graphically in Fig. 18, in which graph A is for unagglomerated, pellet free material dampened by the foregoing method in Fig. 3, and shows a substantially higher bending strength than the partially pelletised material represented by graph B dampened by the vertical

axis rotary mixer method shown in Fig. 2. Graph C is for a material dampened by a standard horizontal axis mixer with a combination of slow turning horizontal paddles interspersed with faster turning cutting blades, which blades help to break up pellets formed by the slow turning paddles and to disperse material which may be locally overwetted when passing the spray nozzles.

As can be seen from Fig. 18, the strength of the compressed dry material dampened by any of the aforesaid methods depends on the density of the material, which in turn depends on the pressure applied to the dampened material during moulding. For such applied pressure to be effective in compressing the material, such pressure should be applied before the stiffening effects of setting become significant, as such stiffening tends to resist compression and results in reduced product density and strength. Strength and density can, however, be brought up to the required levels by applying additional pressure to overcome the resistance from such stiffening. Loss of density, and hence of strength, can also occur if there is insufficient setting while the material is under pressure to hold the material from expanding slightly on release of pressure. Although such expansion can be small, it can be seen from Fig. 18 that even only a sιr.all change in thickness of material of under 10%, giving a drop in density from say 1.1 to 1.0, can result in a drop in strength to

substantially below a typical commercially acceptable target of around 5N/mm ² . In such circumstances, commercially acceptable strength can be achieved for a given material mix by increasing the applied pressure so that the density after said slight expansion is at a level needed to achieve the required strength. Alternatively, the required strength can be achieved by providing a longer time under which the material is held under pressure, so it can gain sufficient strength to resist expansion after pressure release. In the case of conventional cellulose gypsum flat board production, economic viability usually requires that the time during which the material is held under pressure is as little as between 1/2 to 1 minute in order to obtain sufficient throughput to achieve commercially acceptable amortisation of the costly press equipment. At these short times the setting reaction for the dampened material has to be very rapid to avoid loss of strength from expansion after pressure release, and such rapid reactions make it difficult to avoid loss of strength from stiffening before the pressure is applied. Strength loss from either of these causes can be overcome by increasing pressure to around 20 bar, which is around the level commonly used in conventional horizontal flat board processes. This gives = narrow band to operate within and, because in such horizontal processes the material can be fully supported after pressing on a continuous belt, the tendency in these processes is to

release pressure early which results in characteristic surface markings which require sanding.

In the case of hollow panels, there is no effective way of supporting all faces of the product until the material gains sufficient handling strength other than in the mould. This tends to exacerbate the amortisation problem, which is further accentuated by the extra time needed to mix all the setting liquid before pressing. These seemingly intractable problems are overcome in the present invention by going in the opposite direction to conventional thinking and, rather than shortening the process time to improve amortisation, the approach is to lengthen process time, which lengthening permits the use of lower moulding pressures and allows a sufficient reduction in mould costs to overcome the amortisation difficulties by this means rather than solely by reducing press time.

The pressure and the time during which pressure is maintained in the present invention depend on numerous factors and, for products made with Type E material, a commercially acceptable product strength in flexure of around 5N/mm ² can typically be obtained with a moulding pressure of as little as 7 bar for a press tine of around 8 minutes. This press time can be reduced to around 3 minutes if. pressure is increased to around 12 bar aε described in the example, which pressure is still within the range of commercially available compressed air equipment and the low

cost mould actuation techniques such as described in Figs. 13 to 17. Such equipment may be upgraded for higher pressures and shorter press times until reaching a stage where mould costs increase to the point where they are not economically viable, such stage generally being reached before 20 bar. If pellet free Type A material is used, commercially acceptable strength can be obtained using lower pressures than for Type B and C materials and in effect there is a range of different pressure/time relationships corresponding to each type of material and degree of pelletising.

The mixtures used for all such moulding pressures and times contain a blend of set control additives which delay setting until the pressure is applied and then accelerate setting rapidly in order to gain as much strength as possible while pressure is being maintained. Such blends of additives are well known in the plaster industry, and may typically include sodium citrate to delay the set initially and calcium sulphate to accelerate the set after an initial quiescent period, which acceleration may be further boosted by more powerful accelerators such as potassium sulphate. The proportions of such additives depend on the characteristics of the gypsum hemi-hydrate plaster being used, which plaster normally contains a proportion of uncalcined gypsum which acts as an accelerator. Such proportions are usually optimised by practical testing and

total additive content is generally around 1% to 2% or even to 3% by weight of dry plaster, most of which is calcium sulphate in the form of pulverised gypsum rock or recycled waste panel material. The type and content of cellulose or other woolly fibre will also affect strength, and the fibre:plaster ratio cited in the example giving a fibre content of around 17% of finished product weight, is typical for the cellulose gypsum flat board industry. Such a fibre content usually provides a satisfactory balance of strength, density, surface hardness and sound insulation properties, but the process can operate at much higher fibre contents where there may even be insufficient plaster to bind the fibre effectively. The lower limit is where the mixture loses its woolly, high bulking character to the extent that the operating procedures of this invention are not required. Such lower limit depends on the type of fibre and degree to which fibre length is lost during fiberising and mixing but, in general, the process is unlikely to be used commercially with less fibre than a 1:1 ratio of fibrerplaster by bulk volume.

The materials that can be used in the irethod include any liquid setting powder, including cement (particularly the quick setting varieties) and many types cf filler such as perlite and pulverised fuel ash. A wide variety of woolly fibres, such as mineral wool and various vegetable and polymer fibres, can be used provided tney are of a

sufficiently fine and woolly nature so that, when mixed intimately with powder, they will entrap the latter so that the powder does not separate significantly from the fibre during subsequent handling. It is also necessary with brittle fibres to provide mixing methods which do not damage the fibres.

Regarding liquid content, a typical proportion by weight of water:total dry ingredients for the typical cellulose plaster mix cited in the example, is 2:5, or in terms of water:plaster the ratio is 1:2. This latter ratio has a higher water content than required for the chemical reaction between the water and plaster but, due to the propensity of cellulose fibre to absorb and lock water within the fibre structure, an excess of water is required in order to ensure there is sufficient free water available to react with the plaster and promote optimum crystal growth. The optimum water:plaster ratio, allowing for such absorption, is determined by practical tests in which progressively less water is used until samples of product made from such test mixtures start showing loss of strength. This establishes water content at a level which safely maintains product strength during production, while minimising excess water which increases the propensity for pelletising and has subsequently to be removed by energy consuming drying operations.

The preferred method of introducing water into the

material is first to mix the fibrous and powder materials dry or substantially dry, which has the advantage that this enables mixing to be continued until there is complete impregnation of powder into the fibre material. This avoids premature setting or increased pelletising due to the longer wet mixing time that would otherwise be needed if all the liquid was added earlier. It is feasible to mix part of the required liquid with the fibrous material and part with the powder and then mix the two pre-dampenec ingredients together, but this is not a preferred procedure as the earlier onset of chemical setting still limits the time during which the powder can impregnate the fibre. Intimate mixing is essential in order to avoid small patches of unimpregnated cellulose fibre, which can cause noticeable surface blemishes in the finished product.

Regarding mixers, a blade tip speed of 25 metres per second for vertical axis rotary mixers has been found effective for breaking up local overwetted parts of the mix, although speeds nearer to 40m/s are preferred. Generally, either of the aforesaid speeds are sufficient to maintain the material in a suspended mobile state, but there is advantage in the faster speed in carrying the material past the liquid delivery points at a rate which thinly distributes the liquid. Local overwetting is further minimised by widely distributing the liquid through a multiplicity of fine atomising sprays, such wide

distribution also facilitating rapid absorption of liquid so that the time needed for the material to be maintained in the mixer is minimised, thus minimising the opportunity for the growth of pellets. Under such conditions the time from first contact of liquid with dry material to when the dampened material leaves the mixer can be reduced to around 10 seconds, after which pelletising is further minimised by maintaining the suspended mobile state by continual blade rotation until all the material exits from the mixer. The optimum blade speed and mix time to minimise pelletising can vary widely depending on the type and scale of mixer and is best determined by practical trials and successive adjustments and modifications following the principles and objectives described herein. After dampening, the material can be distributed into the mould by a variety of means, preferably in such a way that the material is deposited in even layers to give a more regular product by, for example, reciprocating a filling chute over the mould opening. Agitation or vibration of the mould or core formers is not necessary for the method, but such agitation can be of assistance for pellet free mixtures which have a tendency to "bridge" in the mould. Some vibration, for example of the core formers, can also be used for settling the material to a constant level in the mould, but such vibration should be kept to a low level as downward packing resulting from vibration tends

to bias the orientation of the fibres horizontally, which tends to reduce longitudinal strength of the product.

During filling, the mould sides and ends are spaced away from the core formers, by aperture widths of between 100% and 1000% of the thickness of the final product depending on the type of mix and the moulding pressure used. For the cellulose gypsum mixtures cited in the example, typical apertures were around 500% for partially pelletised mixtures such as produced in the aforesaid vertical axis rotary mixer, and around 800% for pellet free unagglomerated material produced in the aforesaid vertical spray method. In the case of large mould side movements it may be advantageous to have more than one layer of flattened tube 24 or 25 sandwiched between outer frame 26 and platens 27 or 28 to reduce the amount each tube has to expand, thereby reducing the loss of contact area with said platens and thus the applied force when the tubes are expanded.

Various methods of mould pressure actuation can be used, including conventional hydraulic or pneumatic cylinders, although these latter methods usually involve substantially higher costs than the preferred method shown in Figs. 15 to 17. The escape of air from the mix during pressurisation can be effected by alternative neans to those described in the example, such as perforations in the mould sides, although such perforations generally can have difficulty with clogging during production and adversely

affect the surface finish of the product. Air escape can also be solely through the gap between the mould sides and the fixed corners or solely through the gaps between core formers, which formers and gaps may be more numerous or less numerous than shown in the example. With reasonably narrow panels (e.g. 600mm) , gaps between core formers are not essential and air escape may be solely along the outer corners of the mould.

Regarding core void formers, instead of the expandible part being an elastomeric sleeve, such part may alternatively be made of rigid materials such as aluminium or steel and expanded by internally located diaphragms or flexible tubes on similar principles to the actuation of the mould sides. In this case it is feasible to provide a mechanical stop within the former to limit the amount of expansion of the core former, so that the former can be expanded to such a stop before or during filling instead of after filling and compression by the mould sides as described earlier for the aforesaid elastomeric sleeve. in the case of rigid expandible formers the pressure exerted by the closure of the mould sides and ends or. such already expanded core former can preferably cause the former to at least partially retract, thereby ensuring that the sides and ends are able to move to their fully closed positions. The prior expansion of the core formers is preferably sufficient to allow for such retractions, plus an additional margin for

further retraction so as to provide a clearance to the moulded product on full retraction to facilitate demoulding.

Provided core formers are free to find their own expanded positions within the fixed positions of the mould sides, expandible core formers of almost any type can compress the material to a consistent pressure independently of inconsistencies in the compressibility of the material. This has the advantage that consistent strength can be maintained within the fixed outside dimensions of the finished product, which dimensions need to be consistent for the panels to fit together to form a flush faced wall. In such as arrangement, inconsistencies in feedstock material result in minor variations in core void dimensions, which latter dimensions have relatively little effect on the practical functioning of the product. In the case of non-expandible core formers, which are also within the scope of the invention, their use results in variable pressure if external dimensions are held constant, or variable external dimensions if compression is held constant. Neither of these options are as satisfactory as having bcth consistent compression and consistent external dimensions as obtained using expandible core formers.

Regarding mould design, several products can be formed simultaneously from one mould if the latter has a multiplicity of moulding apertures within a common outer reaction frame. Production can be further increased by

moving a train of moulds in a continuous circuit past a filling head, so that while one mould is being filled, other moulds are undergoing pressurisation, setting, demoulding and mould cleaning. Any method of mould transfer can be used, including carousels and various tracked systems as in normal engineering practice. Various methods of demoulding can be used including downward or upward exit of the product, in the latter case usually accompanied by upward withdrawal of the core formers. The finished product made by the method can have a wide variety of shapes, which may include patterned or textured surfaces, hollow cores with a multiplicity of webs or no webs at all. In the latter case, core formers do not need to move under the influence of pressure from the mould ends 16. The products made by the method are characterised by a much higher degree of dimensional accuracy and fibrous content than can be achieved by extrusion, which is the principal alternative method of manufacturing fibrous hollow products, and can produce shapes which are unobtainable by conventional methods of manufacturing cellulose gypsum boards. Because of the lack of any method such as the present invention, hollow products have hitherto never been produced which combine the level of fibre content in conventional cellulose gypsum board with the degree of dimensional accuracy and quality of surface finish obtainable so far only in cast plaster products . In the

latter products such accuracy and surface finish is only obtainable with little or no fibrous material and the particular combination of properties and characteristics resulting from the present invention is unique. As described herein, the invention is of particular application in the formation of a hollow cored product. Fig. 1 shows one such product which is a building panel with side walls 1, end walls 2, and an intermediate web 3. Two internal spaces are defined in the core of the panel, bounded by the walls 1, 2 and web on all vertical sides.

The invention may, however, also be applied to the formation of products having open-sided voids rather than enclosed internal hollows or spaces. This may be achieved by splitting a hollow cored product, or by forming the product with side void type core formers.

In the former respect, by way of example, the building panel shown in Fig. 1 can be split along the centre of the walls 2 and the web 3, parallel to the walls 1, to give two panels each having one of the walls 1, two end flanges defined by half the walls 2, and one central rib defined by half the web 3. Open sided voids are defined between each end flange and the central rib.

In the latter respect, by way of example, a building panel with open side voids, like that described above which _ca n be obtained by splitting the panel of F g. 1, can be formed directly by using essentially half the moulding

apparatus of Figs. 4-14 i.e. by using moulding apparatus with one vertical cavity defined between the core former and one movable side wall.

Accordingly, as used herein, the term cored product is intended to embrace products with open side voids as well as internal hollows or spaces, and the term core former is correspondingly intended to embrace formers for use in forming voids as well as hollows or spaces.