Fiber-cement articles are obtained from an aqueous suspension of a mixture of hydraulic binder, reinforcing fibers and optional additives, said suspension being dewatered and allowed to set. Before curing, the said articles are appropriately shaped, for example in the form of flat or corrugated sheets.

Fiber-cement products for roofing and cladding are subjected to continuous and sometimes intense weathering conditions. Such products must therefore exhibit adequate mechanical resistance and durability.

The most commonly used fabrication technique is the "Hatschek" process, the technology of which is well known in the art. Other known methods are for example the Magnani, Flow-on, extrusion and injection processes.

There are essentially two types of fiber-cement products, namely air-cured products, where the hardening step takes place under atmospheric conditions (air curing) and steam-cured products where the hardening step occurs under specific conditions of pressure, temperature and humidity in an autoclave (hydrothermal or steam curing).

Air-cured products reinforced with synthetic fibers often exhibit low fiber- cement adhesion. This affects the mechanical properties of the fiber-cement product and precludes a number of applications.

Mixtures used in autoclaved products give lower fiber-cement densities leading to fiber-cement products that are mechanically less resistant and in particular are not always sufficiently waterproof.

A generally applicable method for improving the fiber-matrix bond in a fiber-cement sheet is to densify the sheet by compressing the fresh product after forming and shaping but before curing and hardening. This so-called post-compression step improves the fiber-matrix bond in the fiber-cement product in addition to increasing its density and decreasing its porosity. Post-compression can be applied regardless of the fiber-cement composition.

One can thus distinguish between pressed and non-pressed shaped fiber-cement articles. Pressed fiber-cement articles have been subjected to a post- compression whereas non-pressed articles have not.

In the present context, the terms "fresh" or "green" mean the same and refer to the condition of a fiber-cement article during manufacturing, whereby the article

has reached a level of consistency so as no longer to be fluid but rather coherent, while still being readily deformable such as through corrugation on a corrugation bench.

There are two options for compressing fresh fiber-cement sheets: individual pressing in a single-sheet press and stack pressing in a stack press.

Specialised firms in the construction of industrial presses, such as Bell in Switzerland and Siempelkamp in Germany, have developed single-sheet presses especially designed for the post-compression of fiber-cement sheets, in particular corrugated fiber-cement sheets. Such a type of press is mentioned for instance in EP 0 068741.

Single-sheet presses are fitted with a rigid top die with a non adherent elastic membrane thereon and a rigid bottom die. Each die surface has the surface profile (curvature) which one desires to impart to the surface of the fiber-cement article the die comes into contact with during pressing.

Such single-sheet presses generally present a short stroke (i.e. an opening of a few decimeters) and operate with a short cycle during which a pressure of 20 to 250 kgf/cm2 (1.96 MPa to 24.52 MPa) is applied to the shaped article.

In view of the large sizes of some commercial fiber-cement articles, the pressures to be applied and the speed at which one must operate in order to keep pace with the fiber-cement production unit, single-sheet presses are extremely expensive and moreover require special peripheral equipment to control the hydraulic circuits, the pressures applied and the elimination of the excess water from the densified sheet within a cycle of a few seconds.

In addition, single-sheet presses consume much energy and, contrary to conventional slower running stack presses, their maintenance costs are very high.

The compression step of fiber-cement articles by means of a single-sheet press is also a limiting factor in the production cycle. Depending on the format of the sheets, a compression cycle can take between 10 and 30 seconds. One single press can therefore only handle about 2 to 6 sheets per minute. The currently available single-sheet presses can handle the output of only one fiber-cement production unit.Their operation must thus be carefully synchronised with upstream equipment.

The installation of a single-sheet press downstream of every fiber-cement production unit is extremely high in investment costs.

Like single-sheet presses, known stack presses are fitted with a rigid top die and a rigid bottom die. On top of the bottom die is piled a succession of fiber-cement sheets. separated by intermediate metal platens. The entire stack is compressed simultaneously at a maximum pressure of generally between 20 and 250 kgf/cm2 (1.96

MPa to 24.52 MPa) . After compression, the stack is disassembled and the platens are cleaned for reuse.

For the same production capacity, stack presses have lower investment costs and are less expensive to operate than are single-sheet presses.

As stack presses operate in batch (off-line), they can, in one go, compress sheets produced by several production units.

In addition, the longer compression cycle of a stack press allows the surplus water to be expelled more gradually from the fresh sheets which is beneficial to the mechanical properties of the pressed fiber-cement products.

For the compression of fiber-cement sheets to produce the desired strength-enhancing effect, the entire sheet must have been compressed. This requires: I) that during compression, both surfaces of-the fiber-cement sheet are in full contact with the platen or die and that ii) every part of the fiber-cement sheet is subjected to a, preferably homogeneous, compression force perpendicular to the surfaces of the sheet.

Wlth a singsleeet press, both these conditions can easily be met for flat as well as for corrugated sheets through an appropriate choice of the shape of the top and bottom dies.

The stack pressing of flat fiber-cement sheets also presents no problems in this respect.

The situation is more problematic when one envisages stack-pressing corrugated fiber-cement sheets.

Contrary to the situation with flat fiber-cement sheets, corrugated fiber- clement sheets present a topsurface curvature which may be different from their bottom- surface curvature. In a stack of alternating corrugated fiber-cement sheets and metal platens having surface curvature corresponding for example to the curvature of the upper surface of the sheets, the metal platens will not be in full contact with the lower surface of the corrugated fiber-cement sheet immediately above. Gaps will appear adjacent the furrow and ridge area of the said fiber-cement sheet. A similar phenomenon is observed with metal platens having curvatures corresponding to the bottom-surface curvature of the fiber-cement sheets.

A possible solution to the above problem is suggested in DE-OS-1584 395. In DE-OS-1584 395, it is proposed to use metal platens, having a higher thickness in the furrow and ridge area as compared to in the slope areas of the platen. This can be achieved by covering the upper surface of the metal platen with a polymer layer which is

thicker in the furrow and ridge areas than in the slope areas, by glueing strips of such polymeric material into the furrow and ridge areas of the platen or through a platen consisting of two corrugated outer metal sheets which are more spaced apart in their furrow and ridge areas than in their slope areas, and whereby the space between the two sheets is filled with a rigid, self-hardening polymer. In this way, a compact stack of alternating corrugated fiber-cement sheets and metal platens can be realized, i.e. a stack substantially without voids between subsequent fiber-cement sheets and platens.

However, with the platens according to DE-OS-1584 395, the problem of inhomogeneous compression during stack pressing persists.

During stack pressing, the stack is subjected to a global vertically downward compression force. In view of the geometry of the metal platens and of the corrugated sheets, the descent of the upper die during stack compression will lead to different degrees of compression and thus different mechanical properties in the different areas (furrow, ridge and slope) of the pressed corrugated sheets. Attempts at compressing corrugated fiber-cement sheets in stacks using state-of-the-art platens have produced corrugated sheets of insufficient quality.

For the above reasons, stack presses have thus far only proven industrially successful in the case of flat fiber-cement sheets. In industry, corrugated fiber- cement sheets are as a rule compressed by means of a single-sheet press.

It is an object of the invention to provide a simple process for the simultaneous compression of a multitude of non planar fiber-cement products on a stack press, whereby said process does not present the disadvantages of the state-of-the-art processes.

It has now surprisingly been found that this object can be reached when platens different from state-of-the-art metal platens are used.

It has furthermore surprisingly been found that the process according to the invention enables the production of corrugated fiber-cement sheets of a far better quality than when state-of-the-art metal platens are used.

It was found that these of platens which can resiliently deform during the pressing process provides for a more homogeneous distribution of the compression forces than do state-of-the-art platens.

In fact, the process according to the invention enables the production by means of stack-pressing of corrugated fiber-cement sheets suitable for use as cladding and/or roofing.

Brief description of the invention

The invention concerns a process using undulated platens with special mechanical characteristics and design wherein said platens undergo a controlled elastic, i.e. reversible, deformation during stack pressing.

The present invention consists in a process for producing compressed corrugated fiber-cement sheets.

According to said process, corrugated fiber-cement sheets having a bottom surface with a given bottom-surface curvature and a top surface with a given top- surface curvature can be obtained by pressing a compact stack built up of an aligned vertical arrangement of alternating undulated platens and fresh fiber-cement sheets.

The said platens are made of an elastomeric material which deforms resiliently during pressing.

As mentioned above, a compact stack is a stack with substantially no voids between subsequent sheets and platens.

The lowermost and the uppermost elements of the stack both are platens.

Each platen of the stack has a top surface, the curvature of which corresponds substantially to the curvature of the bottom surface of the fiber-cement sheets to be produced.

Similarly, each platen of the stack has a bottom surface, the curvature of which corresponds substantially to the curvature of the top surface of the corrugated fiber-cement sheets to be produced.

The said stack is pressed in a vertical press having rigid bottom die and a rigid top die.

The curvature of the top surface of the bottom die corresponds substantially to the curvature of the top surface of the corrugated fiber-cement sheets to be produced.

Similarly, the curvature of the bottom surface of the top die corresponds substantially to the curvature of the bottom surface of the corrugated fiber-cement sheets to be produced.

For each fiber-cement sheet profile type, a corresponding set of platens is therefore required.

Similarly, for each fibercement profile type a corresponding set of bottom and top dies is required.

The shape of the platens enables the piling of a compact stack before pressing, i.e. one presenting few or no voids. The shape of the top and bottom dies permits maximum contact between the dies and the stack during pressing. The stack is positioned between the bottom and top die in aligned relationship with same. During

pressing, pressure is applied and released in a controlled manner.

During pressing, the platens resiliently deform as a direct consequence of the applied pressure. When the pressure is released, the platens regain their initial shape.

It has surprisingly been observed that using these elastomeric platens in the process according to the invention enables the production of corrugated fiber-cement sheets of sufficiently homogeneous quality.

Preferably, the minimum distance between the top surface and the bottom surface of each platen is at least once or more preferably at least twice the thickness of the corrugated fiber-cement sheets to be produced.

For example, when one wishes to obtain a sheet presenting a N" 7 profile with a thickness of 4.5 mm, the thickness of the corresponding platen should preferably be at least 4.5 or even at least 9mm.

It is equally preferred that the Shore A hardness of the platens is comprised between 50° and 100°, or more specifically between 75" and 85".

Advantageously, the platens present a tensile stress at 300 % elongation of between 30 and 90 kg/cm2, whereby 300 % elongation refers to the condition of the test sample three times its original length, the said tensile strength being measured according to ASTM Do12).

Said platens consist substantially of a material selected from the group comprising silicone, polybutadiene, polyurethane, synthetic rubber, nitrile rubber and EPDM (Ethylene Propylene Diene Modified) rubber.

In the process according to the invention the surfaces of the platens which come into contact with the green fiber-cement sheets may be coated with a platen oil before piling the stack, so as to keep the sheets from adhering to the platens.

In practice, the thickness of the platens, and the material out of which they are made will depend on the process conditions, such as the composition of the hydraulically setting mixture and the pressure applied.

The pressure applied to the fresh fiber-cement sheets in the stack can be between 20 kgf/cm2 (1.96 MPa) and 100 kgf/cm2 (9.81 MPa) and is preferably between 30 kgf/cm2 (2.94 MPa) and 80 kgf/cm2 (7.85 MPa). The pressure to be applied will generally depend on process conditions such as the height of the stack, the desired degree of compaction (density of the sheets) etc.

According to an advantageous embodiment of the invention, the fresh fiber-cement sheets are corrugated to substantially the desired corrugated end-shape before being placed in the stack.

If the bottom die is removably connected to the press, the stack may be piled onto the bottom die away from the press before pressing. The stack, resting on the bottom die in aligned relationship therewith, is then placed in the press beneath and in aligned relationship with the top die.

In that case the stack may be removed from the press together with the bottom die after pressing and before depiling.

After pressing, the compressed fresh fiber-cement can be handled as are conventionally pressed fresh fiber-cement sheets.

Thus, when the stack is depiled after pressing the compressed fresh fiber-cement sheets are ideally placed onto rigid forms having an upper surface, the curvature of which corresponds to the desired curvature of the bottom surface of the corrugated fiber-cement sheets.

The compressed fresh fiber-cement sheets may be steam-cured or air- cured depending on the type of composition used.

Advantageously the platens can be moulded between a first and a second steel mould member, the first mould member having a surface curvature corresponding substantially to the curvature of the top surface of the corrugated fiber-cement sheets to be produced, the second mould member having a surface curvature corresponding substantially to the curvature of the bottom surface of said corrugated fiber-cement sheets.

In that case the first mould member may be used as the bottom die, the second mould member being suitable for use as the top die.

The present invention equally covers an assembly for producing compressed corrugated fiber-cement sheets as described above, said assembly comprising a vertical press and at least 3 undulated platens.

The platens are made of a resiliently deforming elastomeric material.

The vertical press has a rigid bottom die and a rigid top die. As explained above, the top surface of the bottom die has a curvature corresponding substantially to the curvature of the top surface of the corrugated fiber-cement sheets to be produced.

The bottom surface of the top die has a curvature corresponding substantially to the curvature of the bottom surface of the corrugated fiber-cement sheets to be produced.

The curvature of the bottom surface of each platen corresponds substantially to the curvature of the top surface of the corrugated fiber-cement sheets to be produced.

The curvature of the top surface of each platen corresponds substantially to the curvature of the bottom surface of said sheets.

The Shore A hardness of the platens can be comprised between 50° and 100°, preferably between 75" and 85".

The tensile stress at 300 % elongation of the platens (measured according to ASTM D412) is preferably between 30 and 90 kg/cm2.

The platens consist substantially of a material selected from the group comprising silicone, polybutadiene, polyurethane, synthetic rubber, nitrile rubber and EPDM (Ethylene Propylene Diene Modified) rubber.

The bottom die is preferably removably attached to the press.

The invention also covers the abovedescribed platens for use in the production of compressed corrugated fiber-cement sheets by means of stack-pressing.

Brief description of the drawings Details of the invention are described with reference to the accompanying figures 1 to 3.

Figure 1 represents a partial cross section of a corrugated fiber-cement sheet to be produced.

Figure 2 is a schematic cross sectional view of a stack with alternating platens and cement sheets.

Figure 3 is a schematic representation in elevation view of a stack press having a stack of platens and sheets between its bottom and top die.

For ease of understanding, the following abbreviations have systematically been used: S: corrugated fiber-cement sheet to be produced St: top surface of sheet S Sb: bottom surface of sheet S CSt: curvature of the top surface of sheet S CSb : curvature of the bottom surface of sheet S Sd: thickness of sheet S F: fresh fiber-cement sheet P: platen Pt: top surface of platen P Pb: bottom surface of platen P CPt: curvature of the top surface of platen P CPb: curvature of the bottom surface of platen P E: free border or edge of platen P

H: press head Pd: minimum thickness of platent P D: die BD: bottom die BDt: top surface of bottom die BD CBDt: curvature of the top surface of bottom die BD W: Wagon onto which bottom die BD is mounted TD: top die TDb: bottom surface of top die TD CTDb: curvature of the bottom surface of top die TD For the process according to the present invention, one first has to select the profile and the thickness of the fiber-cement sheet one intends to produce.

Figure 1 shows a partial cross-section of a possible corrugated sheet- having a constant thickness Sd.

Each type of corrugated fiber-cement sheet has a particular top surface (St) curvature CSt and a specific bottom surface (Sb) curvature CSb.

Consequently, once one has determined the shape of the corrugated fiber-cement sheet to be produced, the curvatures CPb and CPt of the bottom surface Pb and top surface Pt surface of the platens P is fixed according to the following relationships: CPb = CSt and CPt = CSb.

A partial cross-section of a compact, vertically aligned stack of 6 platens P and 5 fresh fiber-cement sheets F is represented in figure 2.

The platens P are made of a resilient elastomeric material such as polyurethane. The minimum thickness Pd of the platens is in excess of twice the thickness Sd of the corrugated sheets S to be produced.

The surface area of the platens P is greater than the surface area of the fresh sheets F. The stack is assembled so that when the fresh sheet F is placed onto platen P, there remains all around the border of the fresh sheet F, an edge E of platen P which is not covered by fresh sheet F.

As can be seen in figure 3, inside the press, the stack is placed between rigid bottom die BD and rigid top die TD in aligned relationship therewith. The bottom surface TDb of top die TD has a curvature CTDb which corresponds substantially to the curvature CSb of the bottom surface Sb of the corrugated fiber-cement sheet S which

one wishes to produce.

The top surface BDt of bottom die BD has a curvature CBDt which corresponds substantially to the curvature CSt of the top surface St of corrugated fiber- cement sheet S to be produced.

During pressing, the press is closed by means of a hydraulic control system (not shown) so as to apply the required compression force to the stack. After pressing is completed, the press is opened by said hydraulic system, so as to allow the stack to be removed from the press.

Bottom die BD is mounted on a wagon W which can be roiled from under the press.

With the unit represented in figure 3, it is possible to assemble the stack on top of bottom die BD away from the press and to roll the assembled stack with the bottom die BD underneath and in aligned relationship with top die TD for pressing by merely moving wagon W. This arrangement not only facilitates the piling and depiling of the stack, it also makes it possible to increase the production rate as one can, for example, prepare one stack for pressing on top of a first bottom die BD mounted on a first wagon W, while simultaneously pressing a second stack sitting on top of a second bottom die BD mounted on a second wagon W inside the press and even while simultaneously depiling a third stack, which has already been compressed, and which rests on top of a third bottom die BD mounted on a third wagon W at the other side of the press.

The number of sheets that can be pressed simultaneously depends on the material used for the platens and the thickness of the sheets. Fifteen to thirty sheets have been stacked and pressed with good results.

The method can be applied to press any kind of conventional fiber-cement corrugated sheet regardless its profile and thickness.

When fiber-cement sheets are made with a Hatschek machine, the process according to the invention may be as follows: 1) placing a platen P on top of and in aligned relationship with a bottom die BD, as the first element of stack; 2) forming a fresh fiber-cement sheet in the sheet forming machine; 3) cutting said fresh sheet to the desired dimensions; 4) transferring said fresh sheet to a corrugation bench and imparting to the sheet the desired corrugated shape; 5) transferring the corrugated fresh sheet F onto the stack on top of and in aligned relationship with the platen P which is the uppermost element of the stack; 6) placing a platen P onto the stack on top of and in aligned relationship with the fresh

fiber-cement sheet F which is the uppermost element of the stack; 7) repeating steps 2) to 6) until the desired number of fresh sheets F have been stacked; 8) transferring the mixed stack thus obtained with the bottom die BD to the press directly beneath and in aligned relationship with the top die TD; 9) pressing the stack in the press to eliminate excess water, decrease the thickness of the fresh sheets and increase their density in a substantially homogeneous manner; 10) removing the stack from the press with the bottom die BD and depiling the compressed fresh sheets F and platens P in alternate sequence thereby liberating the platens P and the bottom die BD for cleaning and reuse in pressing a new stack; 13) piling the pressed fresh fiber-cement sheets F alternately between conventional rigids forms, and 14) curing the pressed fiber-cement sheets F in a usual manner.

It is to be noted that if sheets are made with a Magnani machine, the above steps 2) to 4) are performed during the Magnani fabrication stage. The pressing process can proceed directly from step 2) to step 5) and following, whereby the fresh sheets F are the corrugated fresh sheets leaving the Magnani forming Machine.

The pressing process of the invention may be effected with a conventional hydraulic press or with a press having more than one opening for introducing a corresponding number of mixed stacks to be pressed.

Examples Fiber-cement sheets having a profile generally known as profile no. 7 were pressed using the method according to the present invention.

The fiber-cement sheets having profile n" 7 short had the following dimensions: Length (along the ridges and furrows): 1080 mm, Width (along the ridges and furrows): 1050 mm.

The fiber-cement sheets having profile n" 7 long had the following dimensions: Length (accross the ridges and furrows): 2400 mm, Width (accross the ridges and furrows): 1050 mm.

The nominal thickness of the sheets was 4.5 mm before pressing and 4.0 mm after pressing.

The deformable platens used were about 20 cm longer and wider than

the sheets to be pressed. The thickness of the platens varied between 4 and 8 mm. The platens were made of polyurethane having a Shore A hardness of 80°, a modulus at 300% elongation according to ASTM D412 of 50 kgf/cm2 (4.90 MPa) and an elongation at break according to ASTM D412 of 760 %.

The platens had been manufactured in a mould (not represented in the figures). The mould was made of a pair of two steel pieces, each piece having one side mechanically machined to obtain a curvature corresponding to the curvature of respectively the top and bottom surface of the profile 7 fibre-cement sheets.

Said steel pieces were spaced apart in parallel position in order to form the lateral walls of the mould into which the resin was poured to produce the deformable platen. This operation was repeated to make the required number of platens.

The hydraullically setting fiber-cement mixture used for the manufacture of the sheets had the following composition (expressed in percentage by weight on dry weight basis): wt% Quartz sand 52 Cellulose 3 Aluminium hydroxyde 4 Bentonite 1.5 Cement 39.5 Table 1 Before being stacked, the platens were coated with a a silicone lubricant as a demoulding oil.

15 Corrugated green fiber-cement sheets were stack-pressed in one batch. The stack thus also comprised 16 platens.

The stack was pressed in a standard press equipped for pressing flat sheets, but provided with the upper and lower dies according to the invention. The mould which had been used for moulding the platens were more used as the two dies. The respective corrugated die surfaces were parallelly adjusted by electronic means.

The maximum pressure applied to the sheets was 25 kg/cm2 (2.45 MPa).

Pressure was applied during a cycle of 3 minutes. The holding time at maximum pressure was 2 minutes.

A lateral deformation of the stack under load was observed of no more than 1% of the width of the sheets. Said deformation was entirely reversible.

The compressed sheets were steam-cured in a conventional manner during 10 bar (0.1 MPa) steam pressure.

The properties of the pressed sheets when air dry and when water saturation as determined by tests according to ISO 393/1 part 1 - "corrugated sheets and fittings for roofing and claddings" are given in table 2.

As a comparative example, a set of fiber-cement sheets was made with the same no. 7 profiles and the same fiber-cement composition, but without being pressed. The properties of these unpressed state-of-the-art sheets are equally represented in table 2. Unpressed sheets Sheets pressed according to the invention Thickness (mm) air dry/water satured 4.6/4.6 4.04/4.26 Density (g/cm3) air dry/ water satured 1.20 / 1.20 1.28 / 1.28 Modulus of rupture (kgf/cm2) 142/110 1581120 air dry/ water satured (13.93/10.79 MPa) (15.49/11.77 MPa) Impermeability ISO 393/1 Achieved Achieved Table 2 In a further set of examples, the same materials and conditions were used to illustrate the influence of pressing times on the properties of the corrugated fiber- cement sheets obtained through the process according to the present invention.

The maximum pressure P"M", applied to the fresh sheets and the length TCde of the cycle during which pressure was applied are given in table 3.

The properties of the sheets thus obtained are shown in table 4, both regarding dry fiber-cement sheets and regarding water-saturated fiber-cement sheets. Example no. I P (kgf/cm2) Tcycle min 1 25 (2.45 MPa) 1 3 2 25 (2.45 MPa) 5 3 25 (2.45 MPa) 8

Example no. Pmax(kgf/cm2) Tcycle(min) II 4 25 (2.45MPa) 11 5 25 (2.45 MPa) 14 Table 3 Ex.no. Breaking load Modulus of Rupture thickness Sd Density (kN/m) (MOR) (mm) (g/cm3) dry/saturated (kgf/cm2) dry/saturated dry/saturated d /saturated 1 335/225 150/105 3.8/3.9 1.3/1.3 (329/2.21 kN/m) (14.71/10.30 MPa) 2 375/284 164/125 3.8/4.0 1.31/1.31 (3.68/2.79 kN/m) (16.08/12.26 MPa) 3 375/291 163/122 4.1/4.1 1.33/1.33 (3.68/2.85 kN/m) (15.98/11.96 MPa) 4 418/313 171/127 4.1/4.1 1.33/1.33 (4.10/3.07 kNim) (16.77/12.45 MPa) 5 422/304 177/130 4.0/3.9 1.34/1.34 4.14/2.98 kN/m) (17.36/12.75MPa) Table 4 The present invention presents numerous advantages.

The main advantage of the invention is, that is now possible to stack- press corrugated fiber-cement sheets and, in doing so, to obtain products of sufficient quality.

The process according to the invention can advantageously be applied to any type of hydraulically setting fiber-cement mixture. Known additives may be used in the process according to the invention provided one chooses a platen material which is compatible with those additives at the occuring temperatures and pressures.

With different platens and dies, the whole range of currently used corrugated sheet profiles may be obtained. Through varying the post-compression pressure, different degrees of densification can be obtained.

The process can be performed with a whole range of known stack- presses, once they have been provided with the correctly profiled top die and bottom die.

Consequently, the process according to the present invention can be realized with small or rudimentary stack-presses, provided they can exert the minimum pressure required for post-compression.

This means that the present invention can advantageously be exploited without advanced technological installations and can even advantageously be used on a small scale.

Yet, the process according to the present invention can equally be incorporated in advanced installations, such as fully automated fabrication units.