EP0051101A1 - Cement slab, and process and device for producing the same - Google Patents

Abstract

1. Slab-shaped cement-based structural element with, embedded close to the surfaces of the slab, grids in the form of intersecting bundles of glass fibres comprising endless fibres arranged in a longitudinal direction and possibly sheathed in plastics material, characterized in that the structural element (1) comprises, constituted by a highly flowable mortar, a matrix (2) which completely encloses the grids (4', 4") and which, in conjunction with the grids, provides a slab-shaped body, the flexural strength of which amounts to at least 18 MN/sq.m for a slab thickness of 1 cm.

Description

The invention relates to a cement board or the like with reinforcing inserts made of fibers located near one or near the two board surfaces, and to elements formed by such cement boards. The invention also relates to a method for producing cement slabs and to an apparatus for carrying out the method.

Normal thin cement slabs - for example with a thickness of 1 cm - are able to withstand high pressure loads, but they can only absorb relatively low tensile stresses. However, these always occur when the cement board is subjected to bending.

In order to increase the possible stress with regard to the bending tensile strength, the company has already started to produce fiber-reinforced cement boards. Two possible paths have been followed in this direction: firstly, the entire volume of the cement board can contain more or less statistically distributed short fibers, and secondly, the board can be provided with one or more mat-like reinforcing inserts, at least to a certain extent Degrees absorbs tensile forces.

Typical of the first category are the asbestos-cement boards, in which asbestos fibers are embedded in the cement mortar. Even though the fibers contribute to an increase in the bending tensile strength, the asbestos cement boards have a serious disadvantage in that their breaking strength and impact resistance are not sufficient, which makes these boards very sensitive to dynamic stress. Asbestos fibers are also known to be harmful to health, and inhalation of these fibers can lead to serious lung diseases to lead. The production of asbestos cement slabs is therefore extremely problematic.

The plate according to CH-PS 59 03 79 (in particular column 3, lines 40-50) is typical of the second category. Here reinforcement mats made of short glass fibers are used as reinforcement inserts for cement slabs. In this case, an area-covering silvering insert is provided near the two surfaces of the cement board. Here too, the individual short fibers are largely statistically distributed in the mat, being intertwined with one another in such a way that they form the mat.

The arrangement of the reinforcement mats close to the surfaces is of course advantageous in itself because the tensile stresses in bending stresses are particularly high here. Nevertheless, this mat-reinforced cement board does not meet all requirements in practice, and in particular it has been shown that the achievable values for the flexural tensile strength are often not sufficient. The fiber requirement for this plate is also disproportionately large. Since the fibers in question are special alkali-resistant and therefore relatively expensive products, this board is also economically unfavorable. Another disadvantage is that the known mat-reinforced plate can only be used to a limited extent in many respects. So it can be, for example, not nailed, which is why this type of fastening had to be eliminated.

With the invention, a cement board is now to be created, which is reinforced with reinforcement inserts made of glass fibers located near one or both of the plate surfaces and in which the reinforcement inserts are used and designed such that the plate has a desired high flexural strength with a significantly lower fiber content, one Compared to asbestos cement boards, it has significantly better impact resistance and can also be processed as required ten, for example, can be nailed.

The invention achieves this goal in that the reinforcing insert is a grid which is formed by intersecting glass fiber bundles, the glass fiber bundles consisting of continuous fibers arranged in the longitudinal direction.

The basic idea of the invention is thus to be seen in the novel design of the reinforcement insert used. In a complete departure from previous practice, a lattice structure made of endless fibers is used for the first time. In contrast to the reinforcement mats, the lattice structure is not full-surface, but has free spaces in the manner of a network. With the same panel thickness, this structure enables a significant reduction in the amount of fibers with an even higher bending tensile strength and a significant improvement in impact resistance compared to the asbestos cement panel. Finally, the use of a lattice fabric instead of a coherent mat also enables the use of the manufacturing process described below, as a result of which an economical production of fiber-reinforced panels with "infinite" bevels is only possible.

The invention makes use of the knowledge that the reinforcement insert used hitherto cannot be completely penetrated by the cement mortar or is not optimally embedded in the cement mortar because of its surface-covering mat-shaped structure formed by interweaving the statistically distributed short fibers. However, this means that the finished cement board - consisting of the cement mortar and the reinforcement inserts embedded near the surfaces - cannot be regarded as a coherent system if the cement board bends due to a load. As a result, the mat in the tensile zone is only able to absorb the existing tensile forces to a limited extent and ensure the stability of the plate under bending tensile stress. Because of the deviations of the "uncontrollable" short fiber in relation to the main torque directions, there is also a reduction in the structure when the known mat-reinforced cement board is stressed.

These problems do not occur with the invention. Because of the grid structure, a secure and complete embedding of the reinforcement insert in the cement mortar is guaranteed. The individual glass fiber bundles are completely covered by the cement mortar, so that a good bond is ensured. The cement board with the reinforcement inserts is therefore actually to be regarded as a single composite. It therefore acts as an isotropic plate with constant plate rigidity in the entire plate area, which means excellent load-bearing capacity. The cement board according to the invention bends under stress as a uniform system. The embedded reinforcement mats are not to be regarded as independent elements, rather they are in a secure bond with the cement mortar. Therefore, the desired effect of the reinforcement inserts - namely the absorption of tensile forces - is retained in any case. The deformations occurring in the state of use are also small in the plate according to the invention according to the elasticity theory, assuming homogeneous and isotropic cross sections.

Another important aspect of the invention is the fact that short fibers are not used. If the known cement board based on short fibers breaks, it can be observed that the fibers are "pulled out" of the cement mortar. So there is no tearing of the short fibers, ie their strength remains largely unused. In contrast, in the invention, continuous fibers are used, each of which forms a bundle of glass fibers. With a plate constructed in this way, the fibers cannot be pulled out of the matrix and the plate does not break until the tensile strength of the fibers is overcome and the fibers tear sen. This means that the tensile strength of the fibers is fully exploited here. In addition, the lattice structure with the intersecting endless glass fiber bundles has the advantage that the tensile strength is increased in all directions of the plate plane, which is not the case, for example, with the asbestos-cement plates mentioned at the beginning. Another advantage of using infinite fibers is the fact that this gives the panel optimum elasticity and increases its impact resistance.

The glass fiber bundles used in the invention from continuous fibers arranged in the longitudinal direction or the grids formed therefrom can be produced in a simple manner and are therefore available as a mass product. They meet the requirement that their modulus of elasticity be greater than the modulus of elasticity of the cement matrix, and they allow this fact to be used optimally. The glass fiber bundles can be normal glass, but then, as is known per se, they must be protected against corrosion due to the high pH value of the cement stone by coating the bundles with synthetic resin, in particular polyester resin . Likewise, the glass fiber bundles can also consist of a special glass that is resistant to cement. In this case, sheathing of the bundles can be omitted, which has the further advantage that the glass fiber bundles can also be biased in the direction of one or both axes of the grid. This is not possible with coated bundles, because the glass can hardly be preloaded into the cement matrix due to the coating. In the case of bundles that are not covered, however, the prestressing of the glass can be transferred almost completely to the cement matrix, which results in a considerable additional increase in strength (analogous to prestressed concrete).

Finally, the cement board according to the invention also has the advantageous property that it can be nailed. It assembly is possible and the cut panels can easily be attached to ceilings, walls, etc. with nails.

Using the new types of cement plates, spatial elements can also be constructed in an advantageous manner. According to an expedient embodiment of the invention, a plurality of ribbed plates are built up vertically on a base plate formed by the new cement plate. This makes it possible to produce a spatial element, for example in the form of a multi-wall sheet, the bending tensile strength of which, based on the weight per unit area, is significantly higher than that of the flat sheet. The rib plates also have at least one grid, which extends not only near the two parallel surfaces, but also near the surface of the upper edge facing away from the base plate.

Advantageously, the element can also be built up so that a second base plate is used. Between the two parallel base plates are the rib plates, through which the base plates are connected to a unit. This construction of a double-walled plate is characterized by a particularly high stability.

• a) reine Zementpaste ohne Zuschlag
• b) Zementmörtel mit Normalzuschlag (z.B. Quarzsand)
• c) Zementmörtel mit Leichtzuschlag (z.B. expandierter Vermikulit)
• d) Gemische von b) und c)

The following can be used as a matrix for the plate or elements:

• a) pure cement paste without surcharge
• b) cement mortar with normal aggregate (e.g. quartz sand)
• c) cement mortar with light aggregate (e.g. expanded vermiculite)
• d) mixtures of b) and c)

The use of a light aggregate makes sense if a low weight of the plate is required.

In addition to the novel cement board itself, the invention is also intended to provide a method for its production create, which is based on the procedural steps that first a first (prestressed or not prestressed) mesh is placed on a base, then poured onto this mesh or on the base of highly flowable grout or cement paste, and then the base is set in shaking movements .

With the invention, therefore, a completely new method is presented, with which it is possible to embed the lattice-like reinforcement insert in a simple manner in the matrix and at the same time to place it correctly spatially (ie near the surface), and to give it a sufficient cement matrix covering . Because of the shaking movements, the grid lying on the base prior to pouring the cement mortar is "raised" somewhat, so that it takes up the desired position near the surface, but already in the matrix, i.e. in the finished cement slab. In addition, the shaking movements mean that any trapped air can escape, so that the finished cement board has a smooth and closed surface.

In order to be able to use the manufacturing process mentioned above, the mortar used is given an appropriate consistency. On the one hand, the mortar must be free-flowing, on the other hand, it should not separate when flowing. This can be achieved by choosing an appropriate grain size distribution for the aggregate, by adding liquefiers (e.g. a sulfonated melamine-formaldehyde resin) and by adjusting the water solids value. Standard sand as well as light aggregates or a mixture of both can be used as aggregates.

In order to obtain boards of the desired size, a frame is attached to the base, the dimensions of which are adapted to the size of the desired cement board and which is advantageously designed as a plug-in frame, the four frame sides of which open in openings on the flat base are pluggable. In order to enable easy removal of the cement slab from the base, this is provided with a hydrophobic layer, for example in the form of a PVC plate or coating.

Even before the grout has solidified in the frame, a second grid can be inserted from above. You can push this grid in a little by hand or using an appropriate tool (e.g. a rubber-coated roller). However, it is also possible to cause the penetration of this grid by the shaking movements which "lift" the first grid. When the cement mortar has solidified, the two grids have their desired position near the two surfaces of the cement board, with sufficient coverage.

The use of reinforcing inserts with a lattice structure thus not only enables particularly good values for the bending tensile strength, impact resistance of the cement board, but also the problem-free and rapid production of the cement boards according to the invention is also characterized by great simplicity.

The length of time during which the underlay is exposed to the shaking movements depends, of course, within certain limits on the thickness of the cement slab to be produced and the consistency of the mortar. It has been shown that with thin cement slabs, the thickness of which is between 0.5 and 1 cm, a short period of time of only about 30 seconds is completely sufficient. The actual manufacturing process of a cement board therefore takes surprisingly little time.

The elements already mentioned using cement plates can also be produced easily and simply by inserting the ribbed plates into the not yet solidified grout, to a depth just before the neighboring grid of the cement plate, which forms the base plate.

In order to produce the ribbed plates themselves, a shape that is closed on all sides except for the upper surface is used, the dimensions of which correspond to the size of the desired ribbed plate. A grid is inserted into this form, the ends of the grid protruding from the form by a finite amount at the top. To ensure the desired position of the grid near the surfaces, at least one U-shaped spacer can be inserted into the mold before the cement mortar is poured into the mold.

A ribbed plate produced in the manner described thus has a continuous reinforcement insert in the form of the grid near three surfaces. The latter protrudes somewhat with its two ends from the upper edge of the ribbed plate. With this edge, the ribbed plates are inserted into the not yet solidified grout after the free ends of the grid are bent sideways, so that in the finished state they run approximately parallel to the surface of the base plate.

A vibrating table can also be used in an advantageous manner in the production of the ribbed plates as described, so that air can escape from the mold and smooth surfaces result. As with the cement board, a favorable positioning of the grid is also achieved here by the shaking movement, and here too this movement is preferably directed upwards and downwards.

To the preparation of _R type in plates to rationalize, several shapes can be arranged in parallel at a distance next to one another. A common grid can then be used, which is inserted in all forms.

• Fig. 1 eine Zementplatte in Seitenansicht,
• Fig. 2 ein als Verstärkungseinlage verwendetes Gitternetz,
• Fig. 3 eine Querschnittsansicht eines Glasfaserbündels,
• Fig. 4 einen Rütteltisch in schematischer Darstellung,
• Fig. 5 ein Element mit Rippenplatten,
• Fig. 6 eine Seitenansicht einer doppelwandigen Zementplatte mit Rippenplatten,
• Fig. 7 eine mit einem Gitternetz ausgelegte Form für eine Rippenplatte,
• Fig. 8 eine teilweise Querschnittsansicht eines Elements gemäß Fig. 5,
• Fig. 9 einen Abstandshalter, der in die Form für die Rippenplatten eingebracht wird,
• Fig. 10 mehrere Formen für die Rippenplatten mit einem gemeinsamen Gitternetz, und
• Fig. 11 eine detailliertere Darstellung eines Elements gemäß Fig. 6.

The invention is explained in more detail below on the basis of the exemplary embodiments illustrated in the drawing. Show it:

• 1 is a side view of a cement board,
• 2 shows a grid used as a reinforcing insert,
• 3 shows a cross-sectional view of a glass fiber bundle,
• 4 shows a vibrating table in a schematic representation,
• 5 an element with rib plates,
• 6 is a side view of a double-walled cement board with ribbed boards,
• 7 shows a form for a ribbed plate designed with a grid,
• 8 is a partial cross-sectional view of an element according to FIG. 5,
• 9 shows a spacer which is introduced into the mold for the rib plates,
• Fig. 10 several shapes for the rib plates with a common grid, and
• 11 shows a more detailed illustration of an element according to FIG. 6.

The cement plate 1 shown in partial side view consists of cement mortar 2, preferably highly flowable casting mortar, in which a grid 4 ¹ and 4 "is placed near the two surfaces of the cement plate 1. The structure of a grid 4 can be seen in FIG. 2 of intersecting glass fibers 6, each glass fiber bundle being made up of ordered continuous glass fibers 8. In order to prevent a corrosion attack due to the alkaline reaction of the cement, the glass fiber bundles 6 - as can be seen in FIG. 3 - are made of synthetic resin 7, Because of the net-like structure of the grid 4, this can absorb tensile forces not only in one direction, but in all directions.

4 shows a schematic representation of a vibrating table 16 for the production of the cement slabs 1. The vibrating movement, which takes place in the vertical direction, is indicated by the two arrows A and B. A laterally directed movement can also be superimposed on this movement. Conventional vibration devices can be used for the drive.

The vibrating table 16 comprises a flat table surface 10 on which a hydrophobic base in the form of a PVC plate 14 is applied. The closed frame 12 defines the outer shape of the cement board 1 to be produced, and different sizes can be realized in a simple manner by replacing the frame 12. For this purpose, the frame 12 is designed as a plug-in frame, the four frame sides of which are provided at the bottom with pins which are inserted into openings (not shown) in the table surface 10. According to the arrangement of the openings mentioned, different dimensions can now be realized in a simple manner.

The side walls of the plug-in frame are also made hydrophobic on their inwardly facing surfaces, so that no formwork oil is required.

For the production of a cement plate 1, the lower grid 4 ¹ is first placed on the hydrophobic PVC plate fourteenth Next, the highly flowable cement mortar is poured into the space formed by the frame 12. There are two options for the following process steps. You can now start up the vibrating table 16, and the grid 4 ¹ is raised somewhat as a result of the vibrations. In addition, the air escapes from below, so that there is a smooth surface on the side facing the PVC plate. With a plate thickness of approximately 0.5 to 1 cm, it is sufficient to leave the vibrating table 16 switched on for about 30 seconds. The other grid 4 "is then embedded in the cement mortar 2 from above, which can optionally be done by hand.

However, it is also possible to place the other grating 4 "immediately on the cement mortar and only then to generate the shaking movement. While the lower grating 4 ¹ is then raised again somewhat, the upper grating 4" is embedded in the cement mortar 2 In any case, the favorable positions of the two grids 4 ¹ and 4 ″ near the two surfaces of the cement plate 1 shown in FIG. 1 can be achieved in a simple manner.

The highly flowable grout 2 is expediently light aggregates, e.g. Expanded pearlite, added when it comes to reducing the bulk density of the plate.

5, a base plate 20 formed by a cement plate according to the invention is provided with vertically arranged rib plates, as a result of which the bending tensile strength can be increased to a significant extent if one considers the same cross sections. In the double-walled element 18 ¹ according to FIG. 6, two base plates 20 and 28 are provided, between which the rib plates 22 extend. This element 18 ¹ is characterized by an even better stability.

The reinforcement ribs 22 are produced in a manner similar to that already described with reference to the cement plates. 7, a grid 24 is first inserted into a mold 30 which is only open at the top. The grid 24 is formed in one piece and protrudes somewhat with its two ends 26. In order to ensure the desired position of the grid near the surfaces of the future ribbed plates, a spacer 32 (cf. FIG. 9) can be inserted into the mold 30 at intervals. Thereafter, the cement mortar 2 or the grout is poured into the mold 30 in a conventional manner. When the cement mortar has solidified, the mold 30 can be removed, whereby the ribbed plate 22 is completed. The ends 26 of the grid 24 mentioned protrude freely from the ribbed plate in front. This is done deliberately in order to improve the fastening of the rib plates 22 on the base plate 20.

As illustrated in FIG. 8, the rib plates 22 are pressed slightly into the not yet solidified cement mortar 2 of the base plate 20 with the free ends 26 of the grid 24 downward. The ends 26 of the grid 24 thus assume the shown approximately parallel position to the grid 4 ". This leads to a particularly secure hold of the rib plates 22 on the base plate 20. According to FIG. 10, a common grid 24 'can be used for several rib plates at the same time. to be used, which is placed in the juxtaposed molds 30. After the cement mortar poured into the molds 30 has solidified, the area of the grid 24 'between the individual molds can be separated again, which would result in the free ends 26 already described but also possible to maintain the common grid 24 ', which would then extend in the element 18 according to FIG. 8 between two adjacent ribs 22 parallel to the other grid 4 ".

The element 18 shown in Fig. 5 can also be manufactured in a single operation, e.g. after the cement mortar has been poured into the molds 30 according to FIG. 10, the grid network 4 "is placed on it, and then the cement mortar 2 of the base plate 20 is poured in - using a frame according to FIG. 4. Finally, the further grid network can then be added 4 'are embedded, and after the cement mortar has solidified, the element 18 according to FIG. 5 results.

With regard to the combination of the rib plates 22 with one or two base plates 20, 28, several combinations are conceivable, of which an expedient embodiment is shown in FIG. 11. What is decisive in any case is the important feature that reinforcement inserts with a lattice structure are provided near the surfaces, which lead to very favorable values for the breaking strength.

To illustrate the advantages of the cement board according to the invention, the following numerical example is given, which relates to a cement board with a thickness of 1 cm and with two grids 4 'and 4 "(mesh size 4 x 4 mm; basis weight of a grid: 140 g / m ² ):

In the case of a light board with a light aggregate and a bulk density of 1.2-1.8 g / cm ', the bending tensile strength is 5-12 MN / m ² .

Particularly high values can be achieved with a lightweight panel with reinforced reinforcement, in which two grids are provided near the surfaces. With such a plate with a total of four grids there is a bending tensile strength of 25-35 MN / m ^z .

In the exemplary embodiment described above it was assumed that grids consisted of glass fiber bundles made of normal, non-alkali-resistant glass and therefore had to be encased in a resin. When using grids whose glass fiber bundles consist of a special, sufficiently alkali-resistant glass, such a sheathing is no longer necessary. This makes it possible to embed the grids in the cement matrix with a direct cement-glass bond, and this in turn leads to the advantage that the glass fiber bundles can then be prestressed in one or both directions of the grille.

Prestressing the glass fiber bundle is only useful if there is sufficient cement-glass bonding. It increases the tensile strength of the cement in the respective clamping direction matrix again, and not insignificantly. It is generally sufficient to prestress only one of the grids 4 ¹ or 4 "provided in the cement slab 1, since a single slab is normally only subjected to bending in one direction after assembly and consequently only has to have increased tensile strength on one side thereof In contrast, the second grid has the primary function of stabilizing the plate during transport, where changing bending directions are unavoidable, and would otherwise be superfluous (apart from securing the "right" sides of the plate from being mixed up) 5 and 6 is sufficient one-sided bias of the grids located in the cement slabs.

In case of prestressing of the glass fiber bundles is suitably according to the first example, in the vibrating table 4 is inserted grid 4 ¹ subjected to the bias voltage, for which -, can be used of the plug frame 12 - with a suitable design.

Claims (30)

1. Cement plate or the like with reinforcement inserts made of fibers located near one or near both plate surfaces, characterized in that the reinforcement insert is a grid (4) which is formed by intersecting glass fiber bundles (6), the glass fiber bundle (6) from in There are continuous fibers (8) arranged in the longitudinal direction. 2. Cement plate according to claim 1, characterized in that the glass fiber bundles (6) are coated with synthetic resin (7). 3. Cement board according to claim 2, characterized in that the synthetic resin (7) is a polyester resin. 4. Cement plate according to claim 1, characterized in that the glass fiber bundles (6) consist of a glass resistant to cement. 5. Cement plate according to claim 4, characterized in that the glass fiber bundles (6) are biased in the direction of one or both axes of the grid. 6. Element formed from cement plates according to claims 1 to 5, characterized in that on a first base plate (20) a plurality of rib plates (22) are provided vertically, which are inserted into the base plate (20) and connected to it. 7. Element according to claim 6, characterized in that the rib plates (22) near their two parallel plate surfaces and near the surface of the base plate (20) facing away from the edge with a grid (24) are provided. 8. Element according to claim 7, characterized in that the grid (24) of the rib plates (22) is integrally formed and with its end ends (26) approximately parallel to that Grid (4 ¹ , 411) of the base plate (20) runs. 9. Element according to claim 8, characterized in that a plurality of rib plates (22) is assigned a common grid (24 ¹ ). 10. Element according to one of claims 6 to 9, characterized in that the rib plates (22) are arranged at equal intervals parallel to each other. 11. Element according to one of claims 6 to 10, characterized in that the rib plates (22) with their edges facing away from the first base plate (20) are connected to a second base plate (28) which runs parallel to the first base plate (20), and in which the rib plates (22) are inserted with their edges. 12. A method for producing a cement board according to one of claims 1-4, characterized in that first a first mesh (4 ') is placed on a base (10), then on this mesh or on the base of highly flowable grout (2) or cement paste is poured, and then the pad is shaken (A, B). 13. The method according to claim 12 for the production of a cement board according to claim 5, characterized in that the first grid (4 ¹ ) is placed in the prestressed state on the base (10). 14. The method according to claim 12 or 13, characterized in that a second grid (4 ") is embedded above in the not yet solidified grout (2) or in the cement paste. 15. The method according to claim 14, characterized in that the second grid (4 ") after completion of the shaking movements by hand or with a roller-like tool in the grout (2) or the cement paste is embedded. 16. The method according to claim 14, characterized in that the second grid (4 ") is placed on the grout (2) or on the cement paste, and that the base (10) is then set in shaking movements (A, B), then both grids are exposed. 17. The method according to claim 14; characterized in that first the first grating (4 ') and then the second grating (4 ") are subjected to the shaking movements (A, B) so that both grids (4', 4") are in their desired positions near the surfaces take in. 18. The method according to any one of claims 10 to 17, characterized in that the base (10) is exposed to the shaking movements (A, B) for about 30 seconds. 19. The method according to any one of claims 10 to 18 for the manufacture of an element according to claim 6, characterized in that the rib plates (22) in the not yet solidified grout (2) are used up to about the second grid (4 "). 20. A method for producing ribbed plates for an element according to claim 6, characterized in that a grid (24) is placed in a ribbed plate (22) associated with the open top (30), the ends (26) of the grid around protrude a finite amount from the mold (30), and that the mold (30) is filled with cement mortar (2). 21. The method according to claim 20, characterized in that the form filled with the cement mortar (2) is subjected to shaking movements before the cement mortar (2) has solidified. 22. The method according to claim 21, characterized in that the shaking movements in the direction of the two side surfaces of the rib plates (22) are directed upwards and downwards. 23. The method according to any one of claims 20 to 22, characterized in that in the mold (30) at least one U-shaped spacer (32) is used, by which the grid C24) is held in the desired position. 24. The method according to any one of claims 20 to 23, characterized in that a plurality of shapes (30) are arranged in parallel next to one another and are designed with a common grid (24 ¹ ). 25. Device for performing the method according to claim 12, characterized in that a vibrating table is provided with a table surface forming the base (10) on which a frame (12) is arranged which corresponds to the external dimensions of the cement board (1). 26. The apparatus according to claim 25 for performing the method according to claim 13, characterized in that the frame is designed as a biasing device for the grid (4 ¹ ). 27. The apparatus according to claim 25 or 26, characterized in that the table surface (10) is covered with a hydrophobic base (14). 28. The apparatus according to claim 27, characterized in that the hydrophobic base is a PVC plate (14). 29. Device according to one of claims 25 to 28, characterized in that the table surface (10) is flat. 30. Device according to one of claims 25 to 29, characterized in that the frame (12) is designed for the purpose of generating different dimensions as a plug-in frame, the side walls provided in on the table surface (10) or in the hydrophobic base (14) openings are releasably insertable.

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