Note: Descriptions are shown in the official language in which they were submitted.
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WO 98/46401 PCTIGB98I01042
METHOD AND PUNCH FOR PERFORATING CEMENTITIOUS TILES AND SO OBTAINED
CEMENTITIOUS TILES
The present invention relates to a cementitious tile
having good acoustic properties, to a method of making such
a tile and to a die assembly for use in the method.
Board made from gypsum plaster is generically termed
plasterboard. Conventional paper faced plasterboard is used
as a cladding for building interiors, either to give, or to
provide a base for, the desired decorative finish.
Plasterboard has been successfully used in other
applications, such as ceiling tiles, but has not generally
been very successful in applications where good acoustic
absorption properties are required. GB-A-2 203 772
discloses a plasterboard having relatively good acoustic
absorption properties. The board is perforated by holes or
slits which are covered on one face of the board by cloth
bonded to the board. w0-A-87/00116 discloses a plasterboard
for use as an acoustic tile perforated with regular slots.
It has been desired to improve the acoustic absorption
properties of plasterboard tiles; it has also been desired
to achieve this in a tile of aesthetically pleasing
appearance.
In known perforated~plasterboard tiles, the edges of
the perforations tend to be somewhat indistinct, perhaps
because fibres from the lining material extend into them.
According to the present invention, there is provided
a sound absorbent tile comprising cementitious material
having perforations which extend into the cementitious
material, characterised in that at least some of the
perforations have unusually sharp, well defined, edges.
Preferably, at least some of the perforations are fissure-
like perforations. Also preferably some of the perforations
are circular in section.
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By "fissure-like" is meant generally elongate
perforations having irregular edges, preferably with an
aspect ratio tthe ratio of the length of the fissure to its
maximum width) of at most 6:1, preferably no more than 4:1.
An aspect ratio of at least 2:1 is preferred.
Preferably, the tile is lined, for example with a
paper liner and the lined surface has an array of
indentations which extend through the liner and terminate in
the cementitious material. The liner of the plasterboard is
ruptured, giving rise to a product of distinctive
appearance. The liner is forced into the indentations
during their formation giving rise to a level of contrast in
between the two extremes produced by the machining
operations described previously.
Preferably, the openings of the through perforations
on the side of the board opposite the lined surface (if any)
are covered. In an especially preferred embodiment, these
openings are covered with a sound absorbent material,
preferably in sheet form such as acoustic paper or felt.
Also according to the invention there is provided a
punch of elongate section for perforating a tile comprising
cementitious material, the leading face of the punch having
a saw tooth profile along its length.
Preferably, the punch is shaped to form fissure-like
perforations.
Preferably, the side faces of the punch are
corrugated along the length of the punch, and the end faces
are rounded.
Also preferably, the pitch of the saw tooth profile
is substantially the same as the pitch of the corrugations.
Also according to the invention, there is provided a
punch for perforating a tile comprising cementitious
material, the punch having a leading face with a saw tooth
profile, the leading-face having rounded edges and a tooth
depth no greater than 2 mm.
2
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Preferably, the punch embodying the invention has a
tooth depth no greater than 2mm.
Preferably, the included angle between teeth is
' between 90° and 150°. Preferably, the profile is at a peak
at each end of the leading face of the punch.
' A punch embodying this aspect of the invention may be
circular.
There is further provided a sound absorbent tile
comprising cementitious material having a plurality of
punched perforations which extend into the cementitious
material, the perforations having a cut quality index
greater than 0.98 wherein the cut quality index is defined
as the ratio of pixels occupied by the perimeter of a
digitised image of a perforation after a smoothing operation
to the number.of pixels occupied by the digitised image of
the perimeter before the smoothing operation, the smoothing
operation comprising four levels of binary erosion followed
by binary dilation.
Also according to the invention there is provided a
die assembly for use in perforating cementitious board,
comprising a punch plate and punches according to the
invention arranged in an array on the surface of the punch
plate, the punches preferably each having~a fissure-like
profile to form fissure-like perforations in a board.
Preferably, the punches of the die assembly include short
indentor punches and long through punches, the indentor
punches being arranged on the surface of the punch plate and
extending a smaller distance from the said surface than do
the long punches. Particularly preferably, the punch plate
also carries circular punches for producing circular
perforations in the board.
It is preferred that the long punches extend beyond
the indentor punches by an amount at least as great as the
' thickness of the cementitious board so that the long punches
will have passed through the board 'before the indentor
punches impinge on the board, thus making the through
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perforations before the indentations. By making the
perforations before the indentations, the size of the press
required to put the die assembly into operation is kept to
a minimum.
It is also preferred that the die assembly includes
a stripper plate and a die plate between which a tile is
sandwiched to be perforated. The stripper plate has holes
therethrough to allow the punches to pass through the plate
and into the tile, and the die plate has holes therethrough
for the passage of the punches after they have perforated
the tile.
Also according to the invention there is provided a
method of manufacturing a sound absorbent tile of
cementitious board comprising:
contacting a planar surface of a cernentitious board
with the profiled surface of a punch plate having punches
according to the invention thereon, the shapes of at least
some of the punches preferably being such as to form
fissure-like perforations;
perforating the board applying pressure between the
board and the die such that the punches pass through the
board; and
thereafter separating the punch plate from the board.
Preferably, the punches include short indentor
punches and long through punches, the method comprising
embedding all the punches in the board so that the indentor
punches penetrate but do not pass through the board and the
long punches pass through the board.
If the board is lined, it is preferred that the punch
plate impinges on the lined surface.
In a preferred method, the planar surface of the
board is painted after the board has been punched and
indented. In this .way, any liner forced into the
indentations can be~ left unpainted, particularly if the
paint is applied with a roller for example. Painting
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provides a way of varying the degree of contrast between the
indentations and the rest of the board. .
In an especially preferred method, the surface of the
board is spiked using a roller having radially extending
spikes. Spiking can be used to produce fine pinholes in the
surface of the board which gives a particularly pleasing
appearance in combination with fissure-like perforations and
indentations.
Also according to the invention there is provided a
suspended ceiling comprising the tiles of the invention.
Such a ceiling can have non-uniform acoustic properties and
a substantially uniform appearance by using a mixture of
tiles according to the invention and tiles of similar
appearance having no through perforations but only fissure-
like indentations. Ceilings can thus be made having desired
overall acoustic properties; for example a ceiling can be
made 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 way of example, with reference to the
accompanying drawings in which:
Figure 1 shows a perspective view of a punch
according to the invention;
Figure 2 shows a cross-section through the punch of
Figure 1;
Figure 3 shows a side elevation of the punch of
Figures 1 and 2;
Figure 4 shows schematically a plan view of a punch
plate for use in a die assembly according to the invention,
with the punches absent;
Figure 5 is a schematic cross sectional view of a die
- assembly according to the invention in use to make a tile
according to the invention; and
Figure 6 shows part of a tile according to the
invention.
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The punch 10 shown in Figures 1, 2 and 3 is solid and
made of high carbon steel. It is of elongate section with
a change of direction part way along the section, shaped to
form a fissure-like perforation in a plasterboard tile. The
side faces 12 of punch are corrugated, and the end faces 14
are rounded. In another embodiment, the end faces taper to
a ridge; the perforations formed in the tile by punches of
this embodiment have more pointed ends than those formed by
the punches of Figures 1, 2 and 3. The leading face 16, that
is, the face which in use impinges first on the tile being
perforated, has a saw tooth profile along its length. The
pitch of the saw tooth profile is the same as that of the
corrugations, 3 mm. The preferred included angle a is 120°;
this has been found to give perforations with sharp, clean
edges. As is best seen in Figure 3, the saw tooth profile
of the leading face 16 is such that there is a peak 18 at
each end of the face. -
A punch intended to be used both to perforate and
indent tiles may have teeth with an included angle of
between 115° to 150°. A preferred range is 120° to
130°.
The tooth depth, that is the distance between the tooth tip
and base is preferably up to 1.5 mm with a preferred depth
of 1.0 mm.
where the punch is intended to be used only to
perforate, the included angle may be between 90° to 150° with
a preferred range of 110° to 130° and a still preferred angle
of 120°. The tooth depth may 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 punch are less critical. Where a tile is
being indented, the depth of the hole is important. The
correct depth will product a shadow that is very similar to
that produced by a fulllperforation. As will be explained,
this is useful as it ~allaws perforated and indented tiles to
be mixed without loss of aesthetic effect. If the tooth
angle is too great, or the tooth depth too large, the tips
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of the teeth will penetrate the bottom paper of the tile
while still only indenting the tile. This has an unwanted
aesthetic effect and may vary the acoustic properties.
' The punches 10 are, in use, mounted in a punch plate
20, shown in Figure 4. The punch plate has punch holes 22
' corresponding to various shapes of punch 10, as well as
circular punch holes 24 for circular punches. The circular
punches also have a leading face having a saw tooth profile
and have preferred and acceptable included angles between
teeth and tooth depths as the elongate punch illustrated in
Figure 3. Moreover, it has been found that the circular
punches having the preferred profile can produce cuts as
clean as those produced by the elongate punch. By providing
a punch plate 20 having a large number of punch holes 22,24,
a variety of punch patterns can be made without changing the
punch plate, by rearranging or removing some of the punches.
The punch plate 20 can be provided with long and
short punches, the long punches for making through
perforations in the plasterboard tile and the short punches
for making perforations or indentations extending into but
not passing 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 enables the punches to fit the holes very accurately,
to a total tolerance of about 5 ~m (that is, for a circular
hole, 2.5 um around the circumference). This enables exact
reproduction of punch patterns to be achieved.
Figure 5 illustrates part of a die assembly 40 which
includes long punches 42 and short indentor punches 44
attached to 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 die assembly 40 also includes a top, stripper,
plate 46 having apertures 48,4 8 corresponding to and large
enough to accommodate the long punches 42 and short indentor
punches 44, and a bottom, die, plate 50 having apertures 52
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corresponding to the long punches 42. In use, the die
assembly 40 is mounted in a press and a lined plasterboard
tile 54 is sandwiched between the rigidly mounted stripper
46 and die 56 plates. As the punch plate 20 is moved towards
the tile 54, the punches and then the indentors exert a
pressure of about 1.5 MN/mz on the tile. The long punches 42
pass through the apertures 48 in the stripper plate 46 and
press into the plasterboard. The long punches 42 force
plugs of plasterboard through the openings 52 in the die
plate 50. In this way, the perforations are formed in the
tile 54 before the short indentor punches 44 engage the
tile. As the punch plate 18 continues to advance towards
the tile 54, the short indentor punches 44 pass through. the
holes 48' in the stripper plate 46 and are embedded in the
tile. Once the paper liner 56 of the tile 54 has been
ruptured by the short indentor.punches 44, the operation is
complete and the punch plate I8 is withdrawn.
The clearances between the long punches 42 and the
corresponding holes 52 in the die plate 50 should be chosen
to ensure that the paper backing, if any, of the
plasterboard tile 54 is cut cleanly away where the punches
exit the plasterboard, while allowing the punches to be
withdrawn from the die plate. If the top face of the tile
is lined, for example with paper, the appearance of the top
surface of the tile can be determined by the clearance
between the punches 42,44 and the holes 48,48' through the
stripper plate 46. A very small clearance will give
perforations and indentations having sharply defined edges
while a greater clearance will give perforations and
indentations with less well defined edges, where the fibres
of the liner material are visible at those edges.
The holes 52 in. the die plate 46 is preferably made
by wire erosion, as are the punches 42,44 and the punch
mounting holes in the punch plate 20. This enables a very
close fit to be achieved between the punches and the holes
8
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in the die plate, giving a very clean edge to the exit
holes.
After being punched, the fissure (or fissure and
circular hole) pattern on the tile can be supplemented by a
pinhole pattern imposed by spiking. the surface of the
plasterboard using a roller having spikes mounted radially
on its periphery. The spikes in contact with the tile at
any given time have a much smaller cross sectional area than
the punches 42,44 so the force on the roller required to
drive the spikes into the plasterboard is significantly less
than the force required on the punch plate 18 to produce the
fissure indentations.
A tile 60 produced by use of the die assembly 10 is
shown in Figure 6. The tile has fissure-like indentations
A and circular perforations B. Preferably the ratio of
fissure-like to 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 60 of the total area of the
principal faces of the tile has perforations. An
aesthetically pleasing effect is achieved when additionally
about 6% of the total area of the front face of the tile has
indentations which do not pass through the tile.
By varying the proportion of the surface area of the
tile taken up by perforations, the acoustic properties of
the tiles can be varied. The appearance of the tiles can be
kept constant by providing indentations instead of
perforations; the indentations have no significant effect
on the acoustic properties of the tile.
Tiles according to the invention, which can be made
using punches according to the invention, have very sharp,
clean holes on both surfaces. This provides an attractive
finish to the tiles, which cannot be achieved with prior art
techniques.
One use of tiles according to the invention is in the
constructions of suspended ceilings. It may be desired to
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provide an acoustically absorbent suspended ceiling having
different acoustic properties in different parts. Tiles of
similar appearance to those of the invention can be
manufactured having no perforations but only fissure-like
indentations; such tiles can be used with tiles according
to the invention having to provide a suspended ceiling of
uniform appearance but with acoustic properties which vary
over the ceiling.
As discussed above, a punch embodying the invention
may be used to produce a perforated or indented cementitious
tile that has exceptionally clean edges to the perforations
or indentations. These edges are unusually sharp, well
defined, edges.
The cleanliness or sharpness of the edge may be
measured and a quantative measure of edge quality derived.
It has been observed that a rough hole will have a
larger perimeter for a given shape compared to an ideal
minimum, due to the jagged edge of a rough hole. Thus, for
a simple example of a circular hole the ideal perimeter is
2nD. For a very jagged edge, the actual perimeter may be in
the order of 2.5TZD where D is the diameter of the hole.
As perimeter values will vary with hole shape,
frequency and size, we have developed a technique for
deriving an absolute measure of hole cleanliness using
perimeter smoothing techniques.
A digital image of the hole is formed and the
periphery is measured, for example by counting the number of
pixels occupied by the perimeter successive smoothing
operations. Measurements may be made using an Optimas 5.2
image analysis system with Cohu high performance CCD camera
on a Kaiser copy stand. A 50 mm Cosmicar TV lens at f/5.6
was used with a 10 mm extension tube and 2 shims. The
column height was 42.2 mm with the camera mounted on the
front tripod socket giving a resultant magnification on a
mm thick tile of x5.325. Correct illumination is
important to achieve satisfactory results. This may be
.. ..,..... T ~ ~ ~ .. .,..
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achieved by positioning four 75 watt tungsten lamps at the
lowest angle, widest spacing and greatest separation
possible. Adequate illumination is achieved with two pairs
of lamps either side of the specimen at a distance of 56 cm
from the optical axis, with 30 cm spacing between lamps and
' at a height of 12.5 cm above the tile surface. As the
perimeter is smoothed, so the perimeter length and the
number of pixels occupied by the perimeter decreases. The
cut quality is then defined as the ratio of measured
perimeter after smoothing operations to that of the
original. It will be appreciated that a high figure
indicates a smooth cut as the smoothing operations have not
reduced the perimeter greatly.
The value will vary according to the number of
smoothing operations performed. For the present purposes,
the measurement is defined as the ratio of pixels occupied
by the perimeter after four smoothing operations performed
using a standard smoothing algorithm.
To achieve reproducible results, the final cut
quality index is a mean value of a number of measurements,
for example 16, with the samples rotated 45° per set of
measurements to eliminate any orientation effects.
On this basis, an index of 1 would indicate an ideal
perimeter and the lower the index the worse the perimeter
quality. We have found that a hole punched with the
preferred punch of Figures 1 to 3 having an included angle
of 120° and a tooth depth of 1 mm has an index of 0.99,
whereas a prototype punch has an index of 0.90. We have
further established that a cut, to be visually acceptable
must have a cleanliness index greater or equal to 0.98.
The smoothing algorithm used is a standard binary
erosion followed by a binary dilation. The first level
comprises one erosion and then one dilation. In binary
morphology an erosion is an operation where foreground
pixels that are 8-connected (referring to neighbouring
pixels left, right, above, below and on the diagonals) to a
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background pixel are eliminated. A dilation is the opposite
of an erosion. After segmenting a grey scale image into a
binary image, the dilate operation identifies background
pixels that are 8-connected to a foreground pixel and
changes them to foreground. Finally, the dilated bit-map is
copied to the frame grabber.
The erosion operation is performed by an erode filter
which performs grey scale to binary (white on black only)
conversion and then does a binary erosion operation. The
bit-map is then copied back. The filter uses the following
arguments:
ArgO: NULL use the current
region
of interest
(ROI)
Arg2: NULL use default lower foreground values)
Arg3: NULL use default upper foreground values)
Arg4: NULL use default 3by3 square structuring element
Arg6: NULL every is in the Arg4 by Arg5 rectangular
pixel
structuring
element.
Arg7: NULL the origin in the X direction is located at
width/2
ArgB: NULL the origin in the Y direction is located at
height/2
The dilation operation is performed using a dilation
filter which enlarges foreground regions of an image by
performing grey scale to binary (white on black only)
conversion and then a binary dilation operation. The
dilated bit-map is then copied back. The arguments used are
as follows:
ArgO: NULL use the current
region of
interest
(ROI)
Arg2: NULL use default lower foreground values)
Arg3: NULL use default upper foreground values)
Arg4: NULL use default 3by3 square structuring element
Arg6: NULL every pixel is the Arg4 by Arg5 rectangular
in
structuring
element.
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Arg7: NULL the origin in the X direction is located at
width/2
Arg8: NULL the origin in the Y direction is located at
height/2
An outline filter is also used to create white
boundaries around foreground regions. This function first
thresholds a grey scale image into foreground and background
components. Foreground pixels in the resulting binary image
which are 4-connected to a background pixel remain
unchanged. All other pixels change to background. This
produces an 8-connected foreground boundary. The outline
bit-map is then copied back to the frame grabber. This
filter uses the following arguments:
ArgO: NULL use the current region of interest (ROI)
Argl: NULL use default lower foreground values)
Arg2: NULL use default upper foreground values)
The Optimas macro is shown below from which it will
be seen that there are four iterations of the smoothing
algorithm. The first level has been described. The second
level uses two erosions and then two dilations, the third
level three erosions and then three dilations etc.
We have determined that an acceptable cut quality is
achieved where the perforations have an index of 0.98 or
greater measured as the average of 16 sets of measurements,
each involving four iterations of the smoothing algorithm as
discussed above at a magnification of x 5.325. Preferably,
the index is 0.99 or greater.
We have also determined that unacceptable cut
qualities fall below the index of 0.98. The following
results were obtained:
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Perimeter Standard
Sample Shape of PerforationMean Value after Deviation
I6 Fields !n-1)
R&D pilot
1/2 tool circular & elongate 0.90 0.024
Gyptone
Quattro square 0.97 0.010
20
Gyptone
Line No rectangular slots 0.97 0.020
4
Yoshino
Board circular 0.96 0.018
Production
tool circular & elongate 0.99 0.100
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Image Analysis Script - Optimas Smoothing Algorithm
GrayToBinary (,0.0,67.0)
OutlineFilter();
Histogram(NULL);
MacroMessage(ArRO~HistogramStats[0]/255);
Undo ( ) ;
BINB_ilterations=1;
ErodeFilter(,BINB_ilterations);
DilateFilter(,BINB alterations);
BINB alterations=1;
OutlineFilter(};
Histogram(NULL);
MacroMessage(ArRO~HistogramStats[0]/255);
Undo ( ) ;
BINB_ilterations=2;
ErodeFilter(,BINB_ilterations};
DilateFilter(,BINB alterations);
BINB alterations=1;
Out3ineFilter~( ) ;
Histogram(NULL);
MacroMessage(ArRO~HistogramStats[0]/255);
Undo ( ) ;
BINB_ilterations=3;
ErodeFilter(,BINB_ilterations};
DilateFilter(,BINB alterations);
BINB alterations=1;
OutlineFilter();
Histogram(NULL);
MacroMessage(ArRO~HistogramStats[0]/255);
Undo ( ) ;
BINB alterations=4;
ErodeFilter(,BINB_ilterations);
DilateFilter(,BINB alterations);
BINB_ilterations=1;
OutlineFilter();
Histogram(NULL);
MacroMessage(ArRO~HistogramStats[0]/255};
Undo ( ) ;
