NO994953L - Method and punching for perforation of cementitious tiles and thus produced cementitious tiles - Google Patents

Description

The present invention relates to a cementitious tile with good acoustic properties, as well as a method for producing such a tile and a mold assembly for use with the method.

Boards made from gypsum are mainly referred to as plaster boards. Ordinary plasterboard with a paper surface is used as a covering inside buildings, either to provide, or to produce, a basis for the desired decorative appearance.

Plasterboards have been used with great success for other applications, such as ceiling tiles, but have mainly not been very successful in applications where good acoustic absorption properties are required. GB-A-2 203 772 describes a plasterboard with relatively good acoustic absorption properties. The plate is perforated with holes or slits which are covered on one surface of the plate with cloth tied to the plate. DW-A-87/00116 describes a plasterboard for use as an acoustic tile perforated with regular slots. It has become desirable to improve the acoustic absorption properties of plasterboard tiles; it has also become desirable to achieve this in a tile with an aesthetically pleasing appearance.

In the case of known perforated plasterboard tiles, the edges of the perforations tend to become somewhat unclear, possibly because the fibers from the covering material extend down into them.

In accordance with the present invention, a sound-absorbing tile comprising cementitious material with perforations extending into the cementitious material has been produced, characterized by the fact that at least some of the perforations have unusually sharp, well-defined edges. At least some of the perforations are preferably slit-like. Some of the perforations are also preferably circular in cross-section.

By "slit-like" is meant mainly elongated perforations with irregular edges, preferably with an aspect ratio (ratio between the length of the slit and its maximum width) of a maximum of 6:1, preferably not more than 4:1. An aspect ratio of at least 2:1 is preferred.

The tile is preferably covered, for example with a paper cover and the covered surface has an array of compressions which extend through the cover and terminate in the cementitious material. The cover of the plasterboard is broken, which gives rise to a product with a specific appearance. The tire is forced into the compressions during their formation and gives rise to a level of contrast between the two outer edges produced by the processing operations as previously described.

The openings of the through perforations on the side of the plate opposite the (optionally) covered surface are preferably covered. In a particularly preferred embodiment, these openings are covered with a sound-absorbing material, preferably in plate form such as an acoustic paper or felt.

In accordance with the invention, an elongated punch has also been placed to perforate a tile containing cementitious material, where the leading surface of the punch has a sawtooth profile along its length.

The punch is preferably shaped to form slit-like perforations.

The side surfaces of the punch are preferably corrugated along the length of the punch and the end surfaces are rounded.

The pitch of the sawtooth profile is also mainly the same as the pitch of the corrugations.

A punch has also been produced in accordance with the invention for perforating a tile containing cementitious material, which punch has a conductive surface with a sawtooth profile, which conductive surface has rounded edges and a tooth depth that is not greater than 2 mm.

The punch that constitutes the invention preferably has a tooth depth that is no deeper than 2 mm.

The enclosed angle between the teeth is preferably between 90° and 150°. The profile is preferably at maximum height at each end of the leading face of the punch.

A punch which exhibits this aspect of the invention may be circular.

A sound-absorbing tile comprising cementitious material with a plurality of punched perforations extending into the cementitious material has also been produced, which perforations have a shear quality index higher than 0.98, where the shear quality index is defined as the ratio between points taken at the outer edges of a digitized image of a perforation after a smoothing operation and the number of points occupied by the digitized image of the outer edge before the smoothing operation, where the smoothing operation includes four levels of binary erosion followed by binary expansion.

According to the invention, a mold assembly for use in perforating a cementitious plate has also been produced, including a punch plate and punches in accordance with the invention placed in an arrangement on the surface of the punch plate, which punches preferably each have a slot-like profile to form slot-like perforations in a plate . The punches in the mold assembly preferably include impression punches and long continuous punches, where the impression punches are arranged on the surface of the punch plate and extend a shorter distance from said surface than the long punches. The punch plate also carries, in a particularly preferred form, circular punches to produce circular perforations in the plate.

It is preferred that the long punches extend past the impression punches by an amount at least as great as the thickness of the cementitious plate so that the long punches will have passed through the plate before the impression punches abut the plate, thereby producing the through perforations before the impressions . By producing the perforations prior to the indentations, the amount of pressure required to push the mold assembly into operation will be kept to a minimum.

It is also preferred that the mold assembly includes a groove plate and a mold plate, between which a tile is placed to be perforated. The groove plate has holes through it to allow the punches to pass through the plate and into the tile, and the form plate has holes through it for the passage of the punches after they have perforated the tile.

There is also, in accordance with the invention, a method for producing a sound-absorbing tile of cementitious material including: bringing the profiled surface of a punched board with punches into contact with a flat surface of a cementitious board in accordance with the invention, where the shape of i the smallest individual of the punches is preferably such that they form slit-like perforations;

perforating the plate by applying pressure to the plate and the mold so that the punches

passes through the plate;

then separate the punch plate from the plate.

The punches preferably include short indentation punches and long through punches, and the method includes using all the punches in the plate so that the indentation punches penetrate but do not pass through the plate and the long punches pass through the plate.

If the plate is covered, it is preferred that the punching plate presses against the covered surface.

In a preferred method, the flat surface of the plate is painted after the plate has been perforated and pressed down. In this way, the tire which is forced into the recesses can be left unpainted, especially if the paint is applied, for example, with a roller. Painting provides a way to vary the degree of contrast between the impressions and the rest of the plate.

In a particularly preferred method, the surface of the plate is pierced with needles using a roller with needles projecting radially outwards. Needle punching can be used to produce fine pinholes in the surface of the plate, which gives a particularly appealing appearance in combination with slit-like perforations and indentations.

A suspended ceiling including tiles according to the invention has also been produced in accordance with the invention. Such a ceiling can have uneven acoustic properties and a substantially uniform appearance by using a mixture of tiles in accordance with the invention and tiles of ordinary appearance without continuous perforations with only slit-like indentations. Ceilings can thereby be produced with the desired total acoustic properties; for example, a ceiling can be produced, which is particularly suitable for an auditorium where speech must be clearly audible throughout.

An embodiment of the invention will now be described in detail, by means of an example, with reference to the attached drawings where: Figure 1 in perspective shows a punch in accordance with the invention; Figure 2 shows a cross-section through the punch in Figure 1; Figure 3 shows a plan view from the side of the punch in figures 1 and 2; Figure 4 schematically shows a plan view of a punch plate for use in a mold assembly in accordance with the invention, where the punches are absent; Figure 5 schematically shows a cross-section of a mold assembly according to the invention in use to produce a tile in accordance with the invention; and Figure 6 shows part of a tile in accordance with the invention.

The punch 10 shown in figures 1, 2 and 3 is a solid body made of high-quality carbon steel. It has an elongated shape with a change of direction some distance along the shape, shaped to form a slit-like perforation in a plasterboard tile. The side surfaces 12 of the punch are corrugated, and the end surfaces 14 are rounded. In a further embodiment, the end surfaces are sloped to a high point; the perforations formed in the tile using this embodiment have sharper ends than those formed by the punches in Figures 1, 2 and 3. The leading surface 16, i.e., the surface which in use first meets the tile to be perforated, has a sawtooth profile along its length. The pitch of the sawtooth profile is the same as that of the corrugations, 3mm. The preferred included angle α is 120°; and this has been shown to produce perforations with sharp, clean edges. As is best shown in figure 3, the sawtooth profile of the leading edge 16 is such that there is an elevation 18 at each end of the surface.

A punch which is intended to be used both for perforating and indenting tiles can have teeth with an enclosed angle between 115° and 150°. A preferred range is 120° to 130°. The tooth depth, i.e. the distance between the tip of the tooth and the base is preferably up to 1.5 mm with a preferred depth of 1 mm.

Where the punch is intended to be used only for perforating, the included angle can be between 90° and 150° with a preferred range from 110° to 130° and a further preferred angle of 120°. The tooth depth can be up to 2 mm with a preferred depth of 1 mm.

The angles and depth of the punch for use with a fully perforated hole is less critical. Where a tile is impressed, the depth of the hole is important. The correct depth will produce a shade very similar to that produced by a full perforation. As will be described, this is advantageous in that it allows perforated and depressed tiles to be mixed without loss of aesthetic effect. If the tooth angle is too large, or the tooth depth is too deep, the tips of the teeth will penetrate the bottom paper of the tile while still only pressing down on the tile. This has an undesirable aesthetic effect and can vary the acoustic properties.

The punches 10 are, in use, placed in a punch plate 20, shown in Figure 4. The punch plate has holes 22 corresponding to various shapes of punches 10, as well as circular punch holes 24 for circular punches. The circular punches also have a conductive surface with a sawtooth profile and have preferred and acceptable included angles between teeth and tooth depths similar to the elongated punch illustrated in Figure 3. Furthermore, it has been found that the circular punches with the preferred profile can produce cutouts that are as clean as those produced with the elongated punches. By producing a punch plate with a high number of punch holes 22, 24, a plurality of punch patterns can be produced without changing the punch plate, by rearranging and removing some of the punches.

The punch plate 20 can be fitted with long and short punches, where the long punches are to produce perforations through the plasterboard tile and the short punches are to produce perforations or indentations that extend into but do not pass through the tile.

Both the punches 10 and the holes 22, 24 in the punch plate 20 are preferably formed by wire erosion cutting. This allows the punches to fit very precisely in the holes, with a total tolerance of about 5µm (that is, for a circular hole, 2.5µm around the circumference). This enables accurate reproduction of the punch patterns to be achieved.

Figure 5 illustrates part of a mold assembly 40 including long punches 42 and short impressioning punches 44 placed on a punch plate 20. The punches 42 are as shown in Figures 1, 2 and 3, and the punch plate 20 is as shown in Figure 4.

The mold assembly 40 also includes an upper groove plate 46 with openings 48, 48' corresponding to, and large enough to receive the long punches 42 and short impressing punches 44, and a lower, mold plate 50 with openings 52 corresponding to the long punches 42. application, the form plate 40 is fixed in a press and a set up plasterboard chip 54 is located between the rigidly placed corrugated plate 46 and the form plate 56. As the punch plate 20 is moved towards the chip 54, the punches, and then the indenters, exert a pressure of about 1.5 MN/m <2> on the tile. The long punches 42 pass through the openings 48 in the rifle plate 46 and are pushed into the plasterboard. The long punches 42 force plasterboard plugs through openings 52 in the form plate 50. In this way, the perforations are formed in the tile 54 before the short impressions punches 44 meet the tile. As the punch plate 18 continues to advance towards the tile 54, the short impressions punches 44 pass through the holes 48' in the rifle plate 46 and are immersed in the tile. When the paper cover 56 on the tile 54 is broken by the short impressions punches 44, the operation is finished and the punch plate 18 is withdrawn.

The clearance between the long punches 42 and the corresponding holes 52 in the form plate 50 should be chosen to ensure that any paper covering on the plasterboard tile 54 is cut away cleanly where the punches come out of the plasterboard, while the punches are allowed to be withdrawn from the formplate. If the upper surface of the tile is covered, for example with paper, the appearance of the upper surface of the tile can be determined by the clearance of the punches 42, 44 and the holes 48, 48' through the rifle plate 46. A very small clearance will produce perforations and indentations with sharply defined edges, while a larger clearance will give perforations and indentations with less defined edges, where the fibers will make the covering material visible at the edges.

The holes 52 in the form plate 46 are preferably made by wire erosion, like the punches 42, 44 and the punch attachment holes in the punch plate 20. This enables a very precise fit between the punches and the holes in the form plate, which gives a very clean edge at the exit of the holes.

After being punched out, the slot pattern (or slots and circular holes) in the tile can be supplemented with the pinhole pattern applied by pin pricking into the surface of the plasterboard using a roller with pins placed radially on its periphery. The needles which are in contact with the tile at all times have a much smaller cross-sectional area than the punches 42, 44 so that the force required on the roller to drive the needles into the plasterboard is considerably less than the force required on the punch plate 18 to produce the slit-shaped impressions.

A tile 60 produced using the mold assembly 10 is shown in Figure 6. The tile has slot-like indentations A and circular perforations B. The ratio between slot-like and circular perforations is preferably within the range 2:1 to 1:2. It has been found that satisfactory acoustic properties are achieved, without significant loss of strength, when about 6% of the total area of the main surface of the tile has perforations. An aesthetically pleasing tile is achieved when a further 6% of the total area of the front surface of the tile has indentations that do not pass through the tile.

By varying the part of the surface area of the tile that is occupied by perforations, the acoustic properties of the tiles can be varied. The appearance of the tile can be kept constant by producing indentations instead of perforations, which indentations do not have a significant effect on the acoustic properties of the tile.

Tiles according to the invention, which can be produced using punches according to the invention, have very sharp, clean holes on both surfaces. This gives the tiles an attractive design, which cannot be achieved with previously known techniques.

One application of tiles in accordance with the invention is in the construction of suspended ceilings. It may be desirable to produce an acoustically absorbent suspended ceiling with different acoustic properties in different parts. Tiles of similar appearance to those in the invention can be produced without perforations but only with slot-like indentations; such tiles can be used with tiles in accordance with the invention to produce a suspended ceiling of uniform appearance but with acoustic properties that vary over the extent of the ceiling.

As discussed above, a punch which exhibits the invention can be used to produce a perforated or impressed cementitious tile which has exceptionally clean edges in the perforations or indentations. These edges are unusually sharp, well-defined edges.

The cleanliness or sharpness of the edges can be measured and a quantitative measure of edge quality derived.

It has been observed that a rough hole will have a larger circumference for a particular shape compared to an ideal minimum, due to the broken edge of a rough hole. Thus, the ideal outer edge of a circular hole in a simple example is 2nD. For a very rough hole, the actual outer edge may be of size 2.57iD where D is the diameter of the hole.

As the edge values will vary with hole shape, frequency and size, we have developed a technique to derive an absolute measure of hole cleanliness using edge smoothing techniques.

A digital image of the hole is formed and the outer edge is measured, for example by counting the number of points occupied by the perimeter following smoothing operations. Measurements can be performed using an Optimas 5.2 image analysis system with a Cohu high-performance CCD camera on a kaiser copier. A 50mm Cosmicar TV lens at f/5.6 was used with a 10mm extension tube and 2 intermediate rings. The height was 42.2 mm with the camera mounted on the front tripod mount giving a resulting magnification of a 10 mm thick tile of x5.325. Correct lighting is important to achieve satisfactory results. This can be achieved by positioning four 75 watt Tungsten lamps with the lowest angle, widest separation and greatest possible separation. Satisfactory illumination is achieved with two pairs of lamps on either side of the sample at a distance of 56 cm from the optical axis, with 30 cm distance between the lamps and at a height of 12.5 cm above the tile surface. As the outer edge is smoothed, the perimeter length and the number of points included in the perimeter are reduced. The cutting quality is defined as the ratio between the measured outer edge after smoothing operations against the original. It should be noted that a high number indicates a smooth edge, as the smoothing operation has not reduced the outer edge considerably.

The value will vary according to the number of smoothing operations performed. For the present purposes, the measurement is defined as the ratio between points occupied in the outer edge after four smoothing operations using the standard smoothing algorithm.

To obtain reproducible results, the final cut quality index is an average value of a number of measurements, for example 16, with the sample rotated 45° per set of measurements to eliminate any orientation effect.

On this basis, an index of 1 would indicate an ideal outer edge and therefore

the lower the index, the worse the edge quality. We have found that a hole punched with the preferred punch in Figures 1-3 with an included angle of 120° and a tooth depth of 1mm has an index of 0.99, while a prototype punch has an index of 0.90. We have further established that, in order for a reef to be visually acceptable, it must have a purity index greater than or equal to 0.98.

The smoothing algorithm used is a standard binary erosion followed by a binary expansion. The first level includes an erosion and then an extension. In binary morphology, erosion is an operation where foreground points that are 8-connected (referring to surrounding points left, higher, above, below, and on the diagonals) of a background point are eliminated. A dilation is the opposite of an erosion. After segmenting a grayscale image into a binary image, the dilation operation identifies the background points that are 8-connected to a foreground point and changes these to foreground. Finally, the extended bitmap is copied to the image grabber.

The erosion operation is performed by an erosion filter that performs grayscale to binary (white on black only) conversion and then performs a binary erosion operation. The bitmap is then copied back. The filter uses the following arguments:

ArgO: NULL use the current region of interest (ROI)

Arg2: NULL use existing lower foreground value(s)

Arg3: NULL use existing upper foreground value(s)

Arg4: NULL use the presumptive 3x3 square

structuring element

Arg6: NULL every point is in the Arg4 x Arg5 rectangular

structuring element

Arg7: The ZERO starting point in the X direction is located at the width

divided by 2

Arg8: The ZERO starting point in the Y direction is located by the height divided

on 2.

The dilation operation is performed by applying a dilation filter that enlarges foreground areas of an image by performing grayscale to binary (white on black only) conversion and then a binary dilation operation. The expanded bitmap is then copied back. The arguments used are as follows:

ArgO: NULL use the current region of interest (ROI)

Arg2: NULL use existing lower foreground value(s)

Arg3: NULL use existing upper foreground value(s)

Arg4: NULL use the presumptive 3x3 square

structuring element

Arg6: NULL each point is in the Arg4 x Arg5 rectangular structuring element

Arg7: The ZERO starting point in the X direction is located at the width

divided by 2

Arg8: The ZERO starting point in the Y direction is located by the height divided

on 2.

An outline filter is also used to form white outer edges around foreground areas. This function first brings a grayscale image into the foreground and background components. Foreground points in the resulting binary image that are 4-connected to a background point remain unchanged. All other points change to background. This gives an 8-connected foreground outline. The outline bitmap is then copied back to the image grabber. This filter uses the following arguments:

ArgO: NULL use the current region of interest (ROI)

Argl: NULL use existing lower foreground value(s)

Arg2: NULL use the existing upper foreground value(s).

The Optimas macro is shown below from which it can be seen that there are four iterations in the smoothing algorithm. The first level has been described. The second level uses two erosions and then two expansions. The third level uses three erosions and then three expansions, etc.

We have determined that an acceptable shear quality is achieved where the perforations have an index of 0.98 or higher measured as an average of 16 sets of measurements each involving four iterations of the smoothing algorithm discussed above at a magnification of 5.325. The index is preferably 0.99 or higher.

We have also decided that unacceptable shear quality falls below the index of 0.98. The following result was obtained:

Image analysis sequence - Optima's smooth algorithm.

GrayToBinary (,0.0,67.0);

OutlineFilter();

Histogram(NULL); MacroMessage(ArRO|HistogramStats[0] /255); Undo();

BINB_ilterations=l;

ErodeFilter(,BINB_ilterations) ; DilateFilter(,BINB_ilterations) ; BINB_ilterations=l;

OutlineFilter (). ;

Histogram(NULL);

MacroMessage(ArRO|HistogramStats[0] /255) ; Undo();

BINB_ilterations=2;

ErodeFilter(,BINB_ilterations) ; DilateFilter(,BINB_ilterations) / BINB_ilterations=l;

OutlineFilter'0 ;

Histogram(NULL);

MacroMessage(ArRO|HistogramStats[0] /255) ; Undo()/

BINB_ilterations=3;

ErodeFilter(,BINB_ilterations) ; DilateFilter(,BINB_ilterations) ; BINB_ilterations=l;

OutlineFilter();

Histogram(NULL);

MacroMessage(ArROIHistogramStats[0] /255) ; Undo();

BINB_ilterations=4;

ErodeFilter(,BINB_ilterations); DilateFilter (,BINB_ilterations) ; BINB_ilterations=l;

OutlineFilter();

Histogram(NULL); MacroMessage(ArRO|HistogramStats[0] /255) ; Undo();

Claims (35)

1. A punch of elongated cross-section for perforating a tile containing cementitious material, the leading face of the punch having a sawtooth profile along its length. 2. A punch according to claim 1, characterized in that the profile is at a peak at each end of the leading surface of the punch. 3. A punch according to claim 1 or 2, characterized in that the enclosed angle of the sawtooth profile is in the range 90° to 150°. 4. A punch according to claim 3, characterized in that the enclosed angle of the sawtooth profile is in the range 115° to 150°. 5. A punch according to claim 5, characterized in that the enclosed angle in the sawtooth profile is around 120°. 6. A punch according to any of the preceding claims, characterized in that the sawtooth depth is less than or equal to 2mm. 7. A punch according to claim 6, characterized by the sawtooth depth being less than or equal to 1.5mm. 8. A punch according to claim 7, characterized by the sawtooth depth being less than or equal to lmm. 9. A punch according to each of the preceding claims, characterized in that the side surface is corrugated along the length of the punch. 10. A punch according to claim 9, characterized in that the pitch of the sawtooth profile is mainly the same as the pitch of the corrugations. 11. A punch according to claim 10, characterized by the pitch of the sawtooth profile and the corrugations being around 3mm. 12. A punch according to any one of the preceding claims, characterized in that the end surfaces are rounded. 13. A punch according to any one of the preceding claims, characterized in that the punch is shaped to form slit-like perforations. 14. A punch according to any of the preceding claims, characterized in that the punch is massive. 15. A punch for perforating a tile comprising cementitious material which punch has a leading face with a sawtooth profile, which leading face has rounded edges and a tooth depth not greater than 2mm. 16. A die assembly for use in perforating cementitious sheets comprising a punch plate and punches arranged on the surface of the punch plate, which punches are in accordance with any of claims 1-15. 17. A mold assembly according to claim 16, characterized by the punches being lined up in the same direction. 18. A mold assembly according to claims 16 or 17, characterized in that at least some of the punches are shorter punches for penetrating a cementitious plate, which shorter punches are arranged on the surface of the punched plate and extend a smaller distance from the said plate than the longer the stops. 19. A mold assembly according to claim 18, characterized in that the long punches extend past the short punches by an amount at least as large as the thickness of a cementitious plate to be penetrated so that the long punches will have passed through the plate before the short punches press into the plate. 20. A method for producing a sound-absorbing tile from a cementitious board characterized by : bringing a planar surface of a cementitious plate into contact with the profiled surface of a punch plate having punches therein, at least some of the punches being according to any one of claims 1-8; perforating the plate by applying pressure between the plate and the mold so that the punches pass through the plate; and then separate the punch plate from the plate. 21. Method according to claim 20, characterized in that the plate is plasterboard. 22. Method according to claim 21 characterized in that the plate is perforated with mainly circular perforations. 23. Method according to claims 19, 20 or 21, characterized in that the punches include short punches and long punches, where all the punches are immersed in the plate so that the short punches penetrate the plate but do not pass through it and the long punches pass through the plate. 24. Method according to any one of claims 19-23, characterized in that the plate is covered. 25. Method according to claim 24, characterized in that the punch plate is supplied to the covered surface of the plate. 26. Method according to any one of claims 19-25, characterized by painting a flat surface of the plate after separating the punch plate from the plate. 27. Method according to any one of claims 19-26, characterized by needling the surface of the plate using a roller with radially extending needles. 28. Method according to any one of claims 19-27, characterized by covering the openings of the perforations on one side of the plate. 29. Method according to claim 28 characterized in that the openings are covered with a sound-absorbing material. 30. Method according to any one of claims 19-29, characterized in that the through perforations are formed before the impression punches are immersed in the plate. 31. A sound absorbing tile comprising cementitious material with a plurality of punched perforations extending into the cementitious material, said perforations having a shear quality index greater than 0.98 where the shear quality index is defined as the ratio of points at the outer edge of a digitized image of a perforation after a smoothing operation and the number of points in the digitized image of the perimeter before the smoothing operation, which smoothing operation consists of four levels of binary eration followed by binary expansion. 32. A cementitious tile according to claim 31 characterized in that the shear quality index is at least 0.99. 33. A cementitious tile according to claims 31 or 32, characterized in that the shear quality index is derived from an average of a plurality of shear quality index measurements. 34. A cementitious tile according to claim 33 characterized by the fact that the average value is derived from 16 sets of measurements, the cutting quality index. 35. A cementitious tile according to any one of claims 31-34, characterized in that the points are derived with a digital camera that operates with a magnification of x5.325.