TECHNICAL FIELD The present invention relates to a method of the kind set forth in the preamble of claim 1.

BACKGROUND ART

In methods of this kind it is known to form and mould so-called BMC material (Brittle-Matrix-Composite material) from a suspension of the particle system concerned in water or other fluent medium.

The particle system may consist of a powder with a certain particle-size distribution, but in many cases it will also comprise fibres, in the end product intended to act as a reinforcement to improve the properties of the finished composite material, especially with regard to strength, toughness and durability.

The BMC material may be a material based on clay, after mixing and moulding to be dried and fired (tiles, bricks, drainpipes and the like) , but it may also be a cementitious material (cement, fibre cement, concrete or fibre concrete) , which after being mixed and cast in a mould sets and solidifies (within some 2-8 hours) , so that it may now be removed from the mould without being deformed, after which it hardens through the chemical reaction between the cement and part of the water in the pores (hydratization) .

The material may also consist of hydrated lime and silica mixed with water (Ca(OH) ₂ + Si0 ₂ + H ₂ 0) , after casting

being autoclave-hardened (temperature 150-220°C) , calcium silicate being formed in this process. Finally, the material may consist of gypsum, after casting in the mould setting in the normal manner by taking up water of crystallization.

A common feature of all this materials is that their starting material consists of an inorganic particle system, normally in the form of a relatively fine-grained powder, although in certain cases - such as concerning concrete and fibre concrete - they may also contain coarser particles. In order to bring these materials in a condition, in which they may be shaped and cast, they are mixed with a certain quantity of liquid, normally water, so as to form a paste-like suspension or a slurry of the particle system with the admixed liquid occupying practically all interspaces between the particles. By attuning the amount of liquid relative to the amount of solid particles, it is possible to adjust the viscosity or flowability of the suspension, so as to make it suitable for mixing and casting in a fully homogeneous state to fill out the mould completely. If the proportion of liquid is too small, the flowability of the suspension will be insufficient, so that air pockets may be formed or not all the nooks and crannies of the moulding space will be filled. On the other hand, the proportion of liquid must not be too high, as this will cause the end product to become too porous and hence both weak and brittle.

Further, there is one more complication: in order to achieve the best possible mixing of the starting ingredients with practically 100% homogenization of the solid components (fine particles, fillers and additives) ,

essential for optimum material proporties in the end product, it is necessary to work with a relatively high ratio of liquid to solid matter, preferably considerably higher than required to achieve good casting proporties. This applies especially when the material contains a suitable quantity of long and thin fibres, because the amount of liquid necessary for the complete splitting-up and dispersion of fibres in such a particle system is considerably greater than for similar mixes without fibres depending primarily on the volume concentration of bribre.

On the other hand, it is of decisive importance that the liquid content in the material having been shaped is as low as at all possible, since the end product in that case will have such a low porosity and high strength and toughness as possible.

These two conflicting views with regard to the choice of the magnitude of the liquid proportion will especially be in high mutual contrast in the case of producing fibre- -reinforced BMC materials, since it is necessary to use a considerable amount of surplus liquid to achieve a total distribution of the fibres, but on the other hand, the fibres will be of little or no use in the end product, because the bond between the fibres and the highly porous matrix material will be practically equal to zero.

In the production or so-called FRC material (Fibre-Rein- forced Cementitious Material)-, e.g. in the form of asbestos cement or cellulose cement, the problems refer¬ red to above are normally solved by first mixing cement and fibres with a great surplus of water (typically 2-10 times as much water as the weight of the cement powder

and fibres) , until all fibres are dispersed and the cement particles are evenly distributed on the surfaces of all fibres as a thin coating. During a subsequent dewatering, e.g. on a filter web, completed by removing part of the surplus water by vacuum treatment, sheet-formed and plate- formed objects are formed, and this may be done without disturbing the effective homogenization achieved prior to the dewatering, because the extremely fine cement particles possess a natural affinity to the surface of the thin asbestos or cellulose fibres, so that no "de- homogenization" occurs when the surplus water is removed from the mix.

After this first dewatering on the filter web, the sheet material having been formed is normally sufficiently coherent to be removed from the substrate and placed onto plane (oil-lubricated) steel plates for setting and hardening. The FRC sheets are, however, - as long as the cement is not yet completely hardened - still fully plastic, so that within the next hour or so, they could still be shaped into corrugated sheets or into bodies of even more complicated shape (in the asbestos-cement industry, this stage in the process is referred to as one, in which plastic shaping is possible) .

Before the cement begins to set, it is also possible to improve the density and other properties of the end product by means of a "post-pressing" (between steel platens) causing additional removal of part of the surplus water along the free edges of .the FRC sheets (pressure 100-200 bar, typical water/cement ratio approximately 0.25 in the sheet material just having been dewatered by vacuum treatment, reduced to approximately 0,15-0.20 after "post-pressing"). Of course, such post-pressing

should, however, be carried out with a gentle raise in pressure (from zero to maximum within 30 minutes or so) in order to prevent that the surplus water now being squeezed out along the plane of the sheets towards its free edges will burst and destroy the shaped sheet, and it will be understood that such a process will under all circumstances result in an FRC product with "woolly" and indistinct edges.

In the case of other materials than asbestos cement or cellulose cement, such as several materials with other types of reinforcement fibres (steel, glass, carbon, polypropylene, polyethylene or other types of synthetic fibres) , it should be noted that these types of fibre do not possess the natural affinity to ("buoyancy" for) the cement grains referred to above, for which reason a certain "de-mixing" with fibre surfaces being laid bare during the dewatering process is practically unavoidable with the traditional methods of production and dewatering.

Disclosure of the invention

It is the object of the present invention to provide a method of the kind referred to initially, with which it is possible to avoid the disadvantages referred to above, and this object is achieved with such a method, according to the present invention being characterized by the features set forth in the characterizing clause of claim 1. When proceeding in this manner, the solid components are mixed with as much water as necessary to achieve a total dispersion of all components in the mixture.

If the squeezing-out of the liquid occurs at the same time over the whole surface of the mould, there is a risk

that dewatered and un-dewatered material moves about uncontrollably in the moulding space with the result that the end product does not become fully homogeneous. This disadvantage may be avoided by proceeding as set forth in claim 2.

When proceeding in this manner, the final part of the pressing process, when no further water may be squeezed out, can be characterized as powder pressing.

Thus, the process as such commences in the form of high- -pressure slurry pumping in one end of the mould and terminates as a powder-pressing process steadily progres¬ sing from the other end of the mould. It will be under- stood that in this case, the low-viscosity suspension will have no difficulty in flowing out into all nooks and crannies of the mould, and any air having been trapped during the filling-up of the mould will leave the mould cavity through its perforations. The finished press- -moulded object will constitute an accurate replica of the internal surfaces of the mould, and since the composite material already has set and solidified in the mould in the same moment as all surplus water has been squeezed out and mutual contact between the solid-matter particles has been achieved, it is now possible to remove the moulded object from the mould immediately - just as with any other powder-pressing method - since this object is now fully rigid and self-supporting and requires no more than being allowed to harden completely in a suitable manner.

Similar results with regard to making the dewatering and consolidation process progress steadily from one end or side of the mould to the other may be achieved by

proceeding as set forth in claim 3 or claim 4.

The perforations holes in the walls of the moulds should, of course, be exstremely fine, so that the water, but not the solid-matter particles may escape from the mould, but since water molecules are extremely small

(approximately 20 A) , this should not be a problem.

The en product made by proceeding according to one of the embodiments of the method according to the invention is characterized by being exceptionally dense and with an absolute minimum of porosity and highly homogeneous, and by in the fully-hardened condition to possess valuable physical properties comprising an optimum combination of strength and toughness.

Since, as described above, the mixing process is carried out with an arbitrary surplus of liquid, and the con¬ centration of the material subsequently during the casting or moulding process is increased without "de-mixing" taking place, until no more liquid can be squeezed out from the confined material, it is possible in this case to achieve a considerably higher concentration of fibres in the end product than by using any other known moulding or casting principle, still with the fibres lying fully dispersed and well distributed and oriented throughout the product.

During the terminal part of the pressing process, during which the solid particles are stolidly wedged and pressed together, so that the material solidifies, the particles are also pressed firmly against all fibre surfaces - in certain cases even into the surfaces of the fibres- resulting in optimum bond between of the fibre and the

matrix material and hence optimum fibre effect in the end product.

In this process, fibres and matrix material "grow together" in a manner not being known from other casting or moulding processes, and after having fully hardened, the end product possesses unique physical proporties.

With uniaxial tension loading, which is the most problematic form of loading to such brittle-matrix materials (because it is difficult for the fibres to take over the whole tensional load when the matrix) , it is possible with a correctly reinforced BMC material produced according to the present invention to achieve a stress-strain curve more reminiscent of the stress-strain curve for a metal or for a plastic material than for an ordinary brittle matrix material normally exhibiting an ultimate elongation at rupture of only approximately 0.01-0.02 per cent (0.1-0.2 mm per m) .

After hardening, a correctly made BMC material produced according to the present invention will have a tensile stress-strain curve exhibiting so-called strain hardening, in which the tensile stress continues to in- crease - without any formation of visible or harmful cracks - even right up to a strain of 1-2% or more. Thus, the strainability (elasticity or flexibility if so pre¬ ferred) of the matrix material has, by extreme use of the admixed fibres, been increased by a factor of 100 or more - and this without .causing any damage to the composite material.

The cause of the dramatically increased strainability is that the internal rupturing of the matrix material

between the fibres due to tensile straining occurs in a different manner than in similar non-reinforced material, as on a microscopic level, an evenly distributed pattern of extremely fine and short microscopic cracks are formed, increasing in number with increased straining of the material; these microscopic cracks are, however, so small that they may be stopped or blocked by the surrounding fibres, and for this reason they cause no dramatic damage to the material as such.

This is in itself extremely valuable and applies in general to the high-quality BMC materials mentioned above as produced by the methods according to the invention. Further, experience has shown that for so-called FRC material produced with a normal Portland-cement matrix, the network of micro-cracks formed in the manner referred to above (with possible lengths of approximately 0.5-1 mm or less, width typically 10-50 μm) after being formed shows a marked tendency to self-healing, so that the material in the presence of moisture will again be dense, and so that the material when again being tension loaded achieves its original rigidity and strength and may be subjected to increased stresses in the same manner as during the first loading, also here exhibiting a smooth working curve and a convincing strain hardening with steadily increasing tensile stresses up to an ultimate straining capacity of 1-2% or more before the stresses begin to decrease.

The present invention also relates to an apparatus for carrying out the method of the invention. This apparatus is of the kind set forth in the preamble of claim 18, and according to the present invention, it also comprises the features set forth in the characterizing clause of

this claim 18.

Finally, the invention relates to a product, such as set forth in claim 30.

Advantageous embodiments of the method and the apparatus, the effects of which - beyond what is self-evident - are explained in the following detailed part of the present description, are set forth in claims 5-17 and 19-29, respectively.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following detailed portion of the present descrip¬ tion, the invention will be explained in more detail with reference to the drawings, in which Figure 1 is a diagrammatic longitudinal sectional view through the parts of an extruder relevant to the inven¬ tion,

Figure 2 shows an example of the formation of draining openings in the part of the extruder wall constituting the drainage section,

Figure 3 is a sectional view through a ring adapted to co-operate with a number of similar rings to form an extruder wall with draining slits, and Figure 4 shows a part of an extruder wall composed of a number of rings of the kind shown in Figure 3.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Figure 1 shows the parts of an extruder essential to the invention, specially designed for producing tubular prod- ucts, it being obvious that an extruder based on the same principles could also be used for extruding products with other cross-sectional shapes, such as flat or cor¬ rugated sheets or profiled stock of various cross-sec¬ tional shapes.

The parts of the extruder shown comprise an outer part 1, an inner part 2, a plurality of nozzles or slits 3 for draining-off liquid, as well as a pressure-regulating chamber 5.

As shown, the extruder is divided into four consecutive sections, i.e.

- an inlet section A for the supply of flowable suspension to be compacted, and - a flow section B, in which the suspension having been supplied flows towards

- a drainage and consolidation section C leading into

- a solid-friction section D.

Further, Figure 1 shows a further section, designated the exit section E, in which the extruded product leaves the extruder.

For ease of understanding, Figure 1 shows the above-men- tioned sections as quite distinct from each other, but in practice, two or more sections may overlap to a greater or lesser degree. Thus, the nozzles 3, shown in Figure 1 as solely being present in the drainage and consolidation section C, may well also extend along at least a part of the solid-friction section D.

In the inlet section A, a flowable suspension containing the requisite amounts of powder, liquid (normally water) and possibly further components flows into the flow sec- tion B. The suspension supplied.to the extruder comprises a surplus of water or other liquid, making it possible to achieve a good and homogeneous intermixing of the components of the suspension, that may have a consistency ranging from a thin slurry to a thick paste.

The mixing process may be carried out in a manner known per se, i.e. by using a high-performance mixer producing a paste-like particle suspension with the desired flow¬ ability, prior to supplying the latter to the inlet sec- tion A of the extruder by means of a high-pressure pump of a type capable of pumping material of this kind.

From the inlet section A, the suspension flows in the forward direction through the flow section B. The cross- sectional shape of the shaped product in this section B and the subsequent drainage and consolidation section C is determined by the internal shape of the outer part 1 and the external shape of the inner part 2. In the drain¬ age and consolidation section C, surplus liquid is drained off, and the suspension is consolidated to form a solid material with direct contact between the individual par¬ ticles throughout the product, as substantially all sur¬ plus liquid, i.e. substantially all liquid not remaining to occupy the interspaces between the closely packed particles in direct mutual contact, is removed. This draining-off function is caused by the pressure differen¬ tial across the outer part 1 in the drainage and con¬ solidation section C being applied to the nozzles or slits 3. The pressure differential constitutes the dif- ference between on the one hand the hydrostatic pressure in the suspension in the flow section B and part of the drainage and consolidation section C, which may lie in the range of 20-400 bar, and on the other hand the pres¬ sure within the pressure-regulating chamber 5, that may be atmospheric pressure or somewhat higher or lower, as will be explained below.

Obviously, the high hydrostatic pressure reigning in the flow section B and at least the adjacent part of the

drainage and consolidation section C can only be main¬ tained, if the part of the extruder downstream of the drainage and consolidation section C comprises some means of obstructing flow. In the method according to the pres- ent invention, these means are provided by the non-flow¬ able extruded product resulting from the drainage and consolidation described above, being present in the solid- friction section D. In this section D, the friction be¬ tween the product 4 and the walls of the outer part 1 and the inner part 2 in contact with it is sufficient to provide a reaction force of substantially the same mag¬ nitude as the oppositely acting hydraulic force resulting from the hydraulic pressure upstream of the solid-friction section D. In operation, the supply pressure and the pressure in the pressure-regulating chamber 5 are attuned to each other and to the friction referred to in the solid-friction section D so as to allow the product 4 to advance at a suitable speed.

When the product 4 leaves the extruder in the exit section E, its porosity is extremely low and it contains substan¬ tially no more liquid than that occupying the interspaces between the closely packed particles, so that the product 4 has a sufficient dimensional stability to withstand handling during the subsequent processing without being deformed due to its own weight. Such subsequent processing may i.e. be firing in the case of a product containing clay, or hardening in the case of a product based on cement.

When starting-up the process, it is necessary to provide the reaction force referred to above by separate means, as the non-flowable product part has not yet been formed in the solid-friction section D. This may suitably be

achieved by inserting a reaction-force plug (not shown) into the downstream end of the interspace between the outer part 1 and the inner part 2 so as to effect a tem¬ porary closure.

As soon as the non-flowable "plug" of consolidated mate¬ rial has been formed in the solid-friction section D, it will normally provide a sufficient reaction force, but will on the other hand, of course, require a considerable force to act upon it to overcome the friction against the extruder walls and move it forward.

With an extruder constructed according to the principle shown in Figure 1, it may not always be possible to at- tune the pressures referred to above in such a manner, that the consolidated product in the solid-friction sec¬ tion D will be moved, as an increase in the supply pres¬ sure, i.e. an increase in the inlet section A and the flow section B, may cause the friction between the con- solidated product and the extruder walls to produce a reaction force that will always be too high. The effects of this high frictional force may be reduced in a number of different ways to be explained below.

A first method of reducing the effect of friction between the consolidated material and the walls of the extruder consists in subjecting the exit portion of the extruder or a part of same to mechanical vibrations. The frequence of these vibrations may lie in the interval 10-400 Hz, while the interval 20-200 Hz is preferred and the interval 50-150 Hz is more preferred. A frequency of 50 or 60 Hz or harmonics thereof are particularly advantageous, as they can be produced by connecting the vibrator concerned to an alternating-current mains supply.

Another method of reducing the effect of the high friction referred to above is to subject the flowable suspension upstream of the consolidated product to pressure varia¬ tions, so that periods with a first, lower pressure al- ternate with second, shorter periods with a second, higher pressure, said second pressure being approximately 1.5- 8, preferably 2-4 times greater than said first pressure.

A third method of reducing the effect of the high friction referred to above is to vary the pressure in the pressure- regulating chamber 5, so that the surface of the product in some periods is subjected to reduced pressure to sup¬ port the draining-off process, and in other periods being subjected to a high-pressure to reduce the friction be- tween the product and the extruder walls.

A fourth method of reducing the effect of the high fric¬ tion referred to above is based on using an extruder, in which a first part, i.e. the outer part 1 shown in Figure 1, is capable of being reciprocated in the longitudinal direction relative to another part of the extruder, e.g. the inner parts 2. With such relative movement, that may e.g. be effected by using a crank mechanism (not shown), the product 4 will be made to "walk" stepwise in the downstream direction.

Figure 2 shows one example of how the requisite permeabil¬ ity of the extruder wall in the drainage and consolidation section C may be achieved. Thus, in the outer part 1 a number of holes 6 have been drilled into the outer part 1 from the outside. As shown, the holes 6 only extend to within approx. 1 mm from the inside wall 7. In the latter, a plurality of extremely fine perforations 8 with trans¬ verse dimensions of the order of 0.001-0.01 mm extend

through the respective drilled holes 6. The perforations

8 may be produced by means of e.g. spark erosion or by using a laser beam. Figure 2 also shows the central axis

9 of the extruder.

Another way of providing the requisite openings in the drainage and consolidation section C is shown in Figures 3 and 4. Thus, Figure 3 shows a ring to be used for this purpose, and Figure 4 shows how a number of such rings are assembled to form a number of slits constituting said openings.

The ring 12 shown in Figure 3 comprises an inner periphery

10 and an outer periphery 11. The width b of the inner periphery 10 is a trifle, typically approximately 0.001-

0.01 mm, less than the width b ₂ of the outer periphery 11. Thus, when a number of rings 12 are clamped axially together in the extruder, slits 3 will be formed between them with a width of typically approximately 0.001-0.01 mm in the drainage and consolidation section C, through which the liquid to be drained off may escape.

Figure 4 shows a number of rings 12 of the kind shown in Figure 3 mounted in the axial direction in the outer part 1 of the extruder, so that the inner peripheries 10 of the rings are aligned with the inside surface of the outer part 1 of the extruder. Figure 4 shows the outer parts 1 and a plurality, in this case a total of six, individual rings 12 with the drainage slits 3 between the rings. The central axis 9 of the extruder will also be seen.

LIST OF PARTS

1 Outer part

2 Inner part 3 Nozzle/slit

4 Product

5 Pressure-regulating chamber

6 Hole

7 Inside wall 8 Perforation

9 Central axis

10 Inner periphery

11 Outer periphery

12 Ring

A Inlet section

B Flow section

C Drainage and consolidation section

D Solid-friction section E Exit section

bτ_ Width (of 10) b Width (of 11)