The present invention relates to the precision extrusion of high-quality BMC materials (Brittle Matrix Composites) .

It is known to form and produce so-called BMC materials by extrusion through a nozzle or die with a cross-section which is equal to that of the desired profile of the material.

The BMC material can be argilliferous material which after extrusion must be dried and thereafter fired (tiles, drain¬ pipes and the like), but it can also be a cement-based material (cement, fibre-cement, concrete or fibre-concrete) which after forming through the extruder debinds and becomes firm and thereafter hardens by the chemical reac¬ tion between the cement and a part of the water in the pores.

The material can also consist of hydrate calcium and sili¬ con (Ca(OH) ₂ + Si0 ₂ ) which, after extrusion, is autoclave hardened (T <-^ 150-220°C) during the formation of calcium silicate. Finally, the material can consist of gypsum which, after extrusion, hardens in the normal way by ab- sorption of crystallization water.

It is common to all of these materials that as basic material they consist of an inorganic particle system - normally in the form of relatively fine-grained powder, although in some cases, such as with concrete or fibre- concrete, also containing coarser particles. To make these materials workable and moldable, they are mixed with a certain amount of liquid, normally with water, so that there is formed a paste-like or porridge-like suspension of the particle system, with the mixing water filling out practically speaking all of the spaces between the par-

tides. The viscosity of the suspension is adjusted by adjusting the amount of liquid in relation to the amount of solid particles, so that it is suitable for molding and extrusion in continuous lengths with the desired profile, i.e. not too inert or too viscous through the addition of too little water, so that air pockets are formed during the extrusion or the profile is not filled out completely in all corners and channels, but then neither with too much liquid in the mixture, in that this can make it impossible to maintain the extruded profile after the paste strings have left the extrusion nozzle.

Moreover, here there is an added complication in that in order to obtain as good a mixture of the basic materials as possible, with practically speaking 100% homogenization of the solid components (fine particles, fillers, fibres, additives, aggregates), which is important for achieving optimum material characteristics in the end product, it is necessary to work with relativly high additions of water, and even with relatively higher additions than those pre¬ scribed with regard to the above-mentioned form-stability after the extrusion.

On the other hand, it is preferred that the finish-formed and extruded material has the lowest possible liquid content, in that the end product is hereby of the lowest possible porosity, and it is a known feature of all of these micro-porous, inorganic materials that their mech¬ anical characteristics decline drastically with increasing porosity.

The invention now comes to grips with this key problem, in that it enables the precision molding of such BMC materials by extrusion of completely form-stable profiles of the material, while at the same time achieving the lowest possible porosity in the end product. In the present con-

nection, the expression "form-stability" means that the product, when it leaves the extruder, is sufficiently firm to ensure that it will not be subject to any plastic deformation as a consequence of its own weight in the un- hardened state, in that even at this point, i.e. before possible firing or hardening, the product has such a firm consistency that it has no visco-flexibility. In other words, the product is purely elastic, which is due to the fact that all "unnecessary" liquid, i.e. surplus liquid, which during the first part of the product's manufacture serves to separate the particles from one another and enables a uniform distribution of the particles, has been drained off. Extrusion is thus carried out with a randomly- selected surplus of liquid in the basic mixture, this being so great that all of the particles in the suspension are removed sufficiently from one another so that they can move quite freely during the mixing without any damage, e.g. of mixed-in fibres, until the desired high degree of homogen¬ ization has been achieved.

This suspension of a suitably low viscosity is now pressed or pumped forward towards the extruder nozzle where it meets a counterpressure, in that in the nozzle there is formed a firm plug of the material, where the particles throughout the material are in contact with one another, in that under pressure (e.g. 20-400 bar, typically 50-200 bar, more typically 50-100 bar) all of the surplus liquid is pressed out of the particle system through the fine pores or slots in the wall or parts of the walls of the extrusion nozzle, these pores or slots being so fine (less than approx. 0.5 mm, typically less than approx. 0.1 mm, more typically less than approx. 0.01 mm, e.g. approx. 0.001- 0.01 mm) that the surplus liquid can pass through and be led away, while the densified particle system, with a homo- geneity and firmness quite like material produced by so- called powder pressing, can now be pressed out of the

nozzle ready for the hardening.

Under the most favourable circumstances, the feeding forward of the profile hereby molded and compacted will be able to be controlled at a desired speed by regulation of the pressure in the suspension in the extrusion chamber, and before liquid is pressed out through the above- mentioned pores or slots, in that this pressure will then be so great that the total force on the end surface of the compacted plug in the chamber will be equal to or slightly greater than the whole of the frictional force along the periphery of the plug.

However, it is not always certain that this balance between the pressure on the end surface of the compacted material and the frictional forces along the nozzle walls which oppose the pressing out of the material can be achieved. The frictional forces which can be built up can be so high, particularly with thin-webbed profiles of very small thickness at right-angles to the direction of extrusion and the greater the pressure in the extrusion chamber the greater are these forces - that the system becomes self- blocking, and the plug of material lodges fast.

In this situation, according to the invention there are several courses which can be followed in order for the extrusion process to be effected at a suitable speed:

One method can be to set the extruder nozzle or parts of the nozzle under vibration (e.g. 10-400 Hz, typically 20- 200 Hz, more typically 50-150 Hz). Even at a very modest amplitude, such a vibration will be able to reduce the friction along the surface of the plug to a considerable degree, and hereby enable the speed of extrusion to be controlle .

Another method can be to work with varying pressures on the paste suspension in the chamber behind the drained and compacted material plug. If the work is thus carried out with a relatively moderate paste pressure for a certain period (e.g. 50-75 bar for 10 sees.), where the material in the section opposite the extruder nozzle's perforations or slots is given time to be drained of all surplus liquid up until full particle contact, but such that only relatively moderate frictional forces are built up corresponding to the moderate paste pressure with which the material is compacted, then it is possible thereafter to press the formed plug a distance forward by quite briefly increasing the paste pressure behind the plug by a factor of 2-4 (e.g. 200-400 bar for half a second). Hereafter, the pressure is again reduced to the low level, which is just great enough for all the surplus liquid to be pressed out of the paste plug which now lies opposite the perforation, and when the pressure is thereafter increased for a suitably brief period of time, the next pressing-out occurs, and so on.

Another method can also be to have a special compressed-air chamber around the extruder nozzle at that section where the nozzle is provided with pores or slots as described above. By placing this chamber under vacuum, the draining and the compacting of the material in the extruder can be effected at lower paste pressure, whereby there is a lower build-up of frictional forces, which thereafter can again be overcome by brief pulses of strongly increased paste pressure.

One possibility here, however, is also to maintain a constant, relatively high paste pressure for full draining of the material inside the perforated section of the nozzle, and in this case to utilize an overpressure in the outer chamber to reduce the friction just so much that the compacted plug begins to move at the desired speed. (Also

here it can naturally be envisaged to operate with pulsat¬ ing air pressure on the surface of the plug in order to achieve well-defined periods in which the material is fully compacted, and others in which the string of material is pressed a distance forward).

Finally, mention must be made of a principle which is probably the most simple and clear, in that the two part- processes, i.e. draining and compaction on the one hand and the feeding of the material on the other, are completely separate .

This consists of dividing the extruder nozzle into two almost equally large surfaces - for closed profiles at two split-lines arranged parallel with the direction of extrusion - and moreover by configuring the extruder in such a manner that one of these surfaces is a stationary part of the construction, while the second part can be made to move forwards and backwards parallel with the direction of extrusion (for closed profiles the extruder nozzle is divided at two split-lines parallel with the direction of movement, while for tubular profiles, where the extruder nozzle consists of an outer surface and an inner surface, and the material is built up in the space between these surfaces, it will be natural to let the outer surface be fixed and to move the inner part forwards and backwards in the axial direction) .

Here, regardless of how great the paste pressure being worked with, the fully drained and compacted material will hereby move out of the nozzle in step with the reciprocat¬ ing movement of the movable surface of the nozzle, in that the friction between the movable surface of the nozzle and the material string, when the nozzle moves outwards, is oppositely-directed and almost of the same dimension as the friction from the fixed nozzle surface against the string

of material. Consequently, the resulting force on the compacted plug of material stems more or less exclusively from the paste pressure behind the plug, and it will thus follow the movable nozzle surface for as long as this moves outwards. When the movable surface of the nozzle in the next instance moves the stroke length back again, the material string is stationary in relation to the extruder, locked by the outwardly-directed paste pressure and the inwardly-directed outer surface friction, and so on.

The invention is illustrated in more detail with reference to the drawing, where

fig. 1 shows an example of the extruder according to the invention,

fig. 2 shows an example of the perforation of the extruder wall at the draining section,

fig. 3 shows a single ring, in that a number of such rings are used to form a perforated extruder wall, and

fig. 4 shows a part of an extruder wall which is com- posed of a number of rings of the type shown in fig. 3.

Fig. 1 shows an extruder according to the invention especially for the production of tubular items, in that it will be obvious that an extruder produced on the same principle will also be able to be used for the extrusion of items with other cross-sections, e.g. flat or corrugated plates or edged profiles. The extruder comprises an outer part 1, an inner part 2, a number of pores or slots 3 for the draining of liquid, and a pressure chamber 5. As shown, the extruder is divided into four sections, where 1.1 is

for the introduction of the paste, 1.2 is for the feeding of the paste, 1.3 is for the draining and compaction of the paste to a firm material, and 1.4 is for the feeding after compaction and for the build-up of a counterpressure. The drawing also shows a further section 1.5 where the finished product leaves the extruder. It is obvious that the func¬ tionality offered by the section 1.3, i.e. perforations which provide the possibility of draining, if desired can exist all the way forward to the mouth of the extruder, and such that provided with a larger or smaller concentration of perforations it can constitute the friction section.

In the section 1.1, a paste mixture containing the desired amount of powder, liquid (normally water) and possibly other components, runs into that part of the extruder (1.2) where the feeding takes place. The paste mixture introduced into the extruder has a surplus of water or other liquid, so that a good and uniform mixing of the paste's component parts is achieved before it runs into the extruder. The mixing of the paste before it is fed into the forming part of the extruder can be effected in a known manner, e.g. by means of a high-efficiency mixing aggregate, to achieve a paste-like particle suspension with the desired viscosity. The paste is fed forward in the section 1.2, in that the extruder's outer part 1 and inner part 2 in this and the subsequent section 1.3 define the cross-section of the product. In the section 1.3, liquid is drained off and the paste is compacted to form firm material with direct par¬ ticle contact between the individual particles throughout the product, in that substantially all surplus liquid is removed, i.e. substantially all liquid which is not necess¬ ary for filling out the spaces between tightly-packed par¬ ticles in direct contact with one another. This is effected by pressing the liquid out through the openings, e.g. pores or slots 3, which are provided in the section 1.3 in the outer part 1. The pressure gradient that expels the liquid

from the extruder's section 1.3 is created by the resis¬ tance which the compacted part of the product 4 in the section 1.4 and the last part of the section 1.3 exerts due to the friction against the inner surface of the outer part 1 and the outer surface of the inner part 2. It is poss¬ ible, however, to increase the pressure gradient by apply¬ ing a vacuum to the pressure-regulating chamber. When the product 4 leaves the extruder (section 1.5), it has a very low porosity and is substantially liquid-free with the exception of that liquid which exists in the spaces between the tightly-packed particles. Consequently, it is suffi¬ ciently form-stable to be able to be handled during the further processing (e.g. firing in the case of argilli¬ ferous products, or hardening in the case of cement-based materials) without it becoming deformed as a consequence of its own weight.

At the beginning of the process, before the formation of a hard, compacted part of the product 4 in the section 1.4 and the last part of the section 1.3, the formation of such a compacted plug is promoted by the insertion of a mandrel in the nozzle, this mandrel having the same cross-section as the product for sealing against the extruder walls.

When the hard, compacted plug has been formed, it will normally extend a sufficient counterpressure, but on the other hand it will require influence from a considerable force for it to be able to overcome the friction against the extruder walls and be moved forward. The problem of ensuring a suitable feeding forward while maintaining a suitable counterpressure is solved according to the invention by one or more of the methods described earlier. The means which are used for this purpose, e.g. means for moving the inner part in the axial direction, respectively forwards for the feeding of the product 4 and backwards to "fetch" the next section, are not shown in the drawing, but

can for example be a sliding sleeve driven by a connecting- rod arrangement.

Fig. 2 shows an example of how the necessary perforation of the extruder wall can be effected in the draining section. The figure shows a longitudinal section, which here is perforated, with an outer side 1 and a number of holes 2 which are drilled into the wall from the outside. As shown, the holes 2 are only drilled in to a suitable distance (approx. 1 mm) from the inner wall 5. In the inner wall 5 there are a number of very fine perforations 3 (typically approx. 0.001-0.01 mm) which extend through to the re¬ spective drilled holes 2. The perforations 3 are produced, for example, by spark machining or with laser beams. The figure also shows the centre-axis 4 of the extruder.

Fig. 3 shows a specially configured, turned ring. In this case, a number of such rings are built into the outer extruder wall and are tightened against one another in the axial direction (as shown in fig. 4). The inside diameter of the ring is the same as the outside diameter of the extruded product.

As shown, the ring is configured with an inner surface 1 and an outer surface 2. The breadth of the inner surface b is insignificantly smaller (typically approx. 0.001-0.01) than the breadth of the outer surface b ₂ . Slots through which the surplus liquid can be led away will thus be formed between the rings when the rings are tightened together axially in the draining section in the extruder (the width of the slot is typically approx. 0.001-0.01 mm).

Fig. 4 shows a number of rings of the type shown in fig.

3, each mounted axially in the outer part of the extruder, so that the inner surface of the ring is flush with the inner surface 5 of the extruder. The figure shows the outer

side 1 of the extruder and a number of individual rings 2 (here a total of six such rings) with draining slots 3 between the individual rings 2. The central axis of the extruder is also shown.