Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~O 94/25236 ~ ~ ~ ~ ~ PCTIGB94/00905
FILTER FABRIC
The invention relates to a filter fabric, and a filter assembly for use
in a press mould for moulding of cementitious products.
S
Press moulds are used for forming cementitious products such as
paving slabs by compressing a mixture of water, sand and cement whilst
simultaneously removing excess water usually assisted by vacuum. The
mixture may contain other constituents such as aggregate, fibres (textile
or glass etc), wood, straw etc.
The mixture is placed into a chamber of the press mould and
compressed between the surfaces of a static table and piston operated
platen.
Normal practice is to place a filter paper between the contacting
surfaces of the mixture and the table and platen. This serves to prevent
particulate material from entering the press mould ducts along which
extracted water is conducted and also prevents cementitious product
sticlang to the surfaces of the table or platen on removal of the product.
A major limitation of paper when used as a filter medium is the
necessity for a fresh piece of paper to be used on each surface during each
product cycle. In addition, use of paper involves subsequent removal of
the paper each time the product is removed from the mould.
Repeatedly removing and destroying the paper filter after use is a
labour intensive, time-consuming and expensive task.
It is therefore desirable to provide a reusable filter that would allow
water to filter out from a compressed mixture of water, sand and cement
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whilst preventing adhesion of the contacting surfaces. It is also desirable to
enable the filter to
be mounted for retention by the table/platen for repeated moulding of
cementitious products.
According to one aspect of the present invention there is provided a concrete
press
mould filter fabric, the fabric being knit or woven, at least in part, from a
hydrophobic yarn
and having a fabric density which permits the passage of water therethrough
but prohibits the
passage of particulate material. The fabric is woven or knitted so as to have
a ground
construction which provides integrity for the fabric and a plurality of
surface yarns which are
secured in the ground construction so as to define at least one face of the
fabric which in use
contacts the cementitious mixture, said surface yarns being arranged to define
closely spaced
side by side floats on one surface of the ground construction to collectively
define a
substantially continuous flat surface for said face.
Preferably, surface loops anchored in the ground construction collectively
defining the
face of the fabric for contacting concrete to be pressed, may lie flat against
the ground
construction and be of a length which enables the loops to peel away from the
surface of the
pressed concrete.
Preferably the fabric is knitted on a warp knitting machine and the surface
yarns are
incorporated into the ground construction using a fall plate.
Preferably the desired spacing between the side-by-side floats is achieved by
adopting
a suitable weight count for said surface yarns and/or adopting a suitable
needle gauge.
Preferably at least the surface yarns are hydrophobic and are preferably made
of
polyvinylchloride or polypropylene.
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A filter assembly comprising a fabric, as defined above, can be secured about
its
periphery to a perforate plate adapted for detachable securance to a table or
platen of a press
mould.
Preferably the periphery of the fabric is secured to the plate by adhesion.
Preferably the peripheral edge of the fabric is defined by a fold, the fold
being secured
in face to face contact with the plate.
Preferably the folded edging is a continuous smooth surfaced band located at
the
periphery of the fabric, the integral band being preferably formed by fusing
the knit or weave
structure along and adjacent to the periphery of the fabric.
Preferably a porous sponge layer is provided between the plate and the fabric.
Preferably the ground construction is constructed to be flexible to enable it
to peel
away from the surface of the pressed concrete.
A filter may have heat meltable yarns and be bounded around its periphery with
at
least one continuous imperforate band integrally formed with the fabric by
fusion of said heat
meltable yarns.
Various aspects of the present invention will hereinafter be described in
greater detail,
by way of example, with reference to the accompanying drawings, in which:-
Figure 1 is a side view of a press mould according to a first embodiment of
the present
invention;
Figure 2 is a plan view of Figure 1;
Figure 3 is a plan view of a filter fabric according to the first embodiment;
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Figure 4 is a section through III-III of Figure 3;
Figure 5 is a lapping diagram for producing a warp knitted filter fabric
according to
the present invention;
Figure 6 is a perspective view of a filter according to a second embodiment of
the
present invention; and
Figure 7 is a side view of Figure 6.
Referring to Figures 1 to 5, there is shown a filter 19 for use in a concrete
press mould
according to a first embodiment. The filter 19 comprises a fabric which is
knit or woven
10 from a hydrophobic yarn 30 to produce a fabric having a density which
permits the passage of
water 18
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therethrough but prohibits the passage of particulate material such as sand
or cement 17.
In the present example, the press mould is intended to produce
rectilinear paving slabs. Therefore the filter fabric 19 shown in the first
embodiment is generally square shaped allowing it to fit within a chamber
40 of the press mould 10. However, it must be appreciated that any
alternative shape of filter 19 may be used in adapting it to fit a particular
cross-section of chamber for moulding alternative shapes of slabs or other
products.
The filter fabric 19 is preferably knitted on a raschel warp-knitting
machine and is preferably a fall plate type fabric. A suitable fabric may
be produced on a raschel warp-knitting machine having a 12 gauge needle
bar and in which 3 guide bars are fully threaded with a hydrophobic yarn
such as polypropylene yarn and arranged to undergo the following lapping
motions, viz
Guide bar 1 1-0/0-1 repeat
Guide bar 2 1-0/2-3 repeat
Guide bar 3 2-3/1-0 repeat
This is illustrated in Figure 5.
A fall plate is located behind guide bar 3 and in front of guide bars
1 and 2 so that the yarns guided by bars 1 and 2 are knitted to create a
ground construction and the yarns guided by guide bar 3 are not knitted
but are connected to the ground construction by passing under the
underlaps of the ground construction at the extremity of their movement
so as to provide flat lying floats which lie on the top of the technical back
of the fabric between the points of connection with the ground
construction.
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When knitted on a 12 gauge machine, the hydrophobic yarn for
bars 1 and 2 preferably collectively have a yarn count of about 350-650
dtex, most preferably about 500-600 dtex. In addition, the courses per
inch are preferably about 28-31; more preferably about 29-30.
S
Typically the yarn count for bar 1 is about 170 dtex and that for bar
2 is about 420 dtex. The yarn count for bar 3 is preferably in the region
of 350 to 550 dtex, and is typically about 420 dtex.
It is envisaged that the lapping motions for guide bars may be
varied from those given above.
The ground construction is produced by guide bars 1 and 2 and the
lapping motions may be altered to provide different densities of fabric
construction by for example altering the number of courses per inch which
are knitted. This can also alter the flexibility of the fabric.
It is envisaged that guide bar 2 may undergo the following
alternative lapping motions, e.g. 1-0/1-2 repeat or 1-0/3-4 repeat.
It is also envisaged that this lapping motion of bar 3 may be varied
in order to provide a different Ioop length. For example, shorter loops
may be produced by a 2-1/1-0 lapping motion, or longer loops may be
produced by a 3-4/1-0 or even a 4-5l1-0 lapping motion.
After knitting the fabric is preferably heat set, so that it retains its
integrity when exposed to use in the press mould.
The above fabric defines a closely knit ground construction having
closely spaced side-by-side floats defining a substantially continuous and
_ ~1
~O 94125236 , ~ ,;: ~~ , ,.U,1 ~~~ PCT/GB94/00905
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flat face for the fabric. The closely spaced floats enables water to pass
through but which due to the closeness of the yarns acts to restrain
passage of cement/sand particles therethrough. Since the yarn is
hydrophobic it is not wetted by the water and so on release of the mould
S parts the yarn is able to leave the surface of the compacted cement/sand
mixture without wetted particles sticking thereto.
On separation the floats tend to be pulled from the surface of the
fabric, and since the floats are secured at both ends and not in their middle
the centre of the floats tends to lift and thereby enables separation from
the surface of the compacted cement/sand mixture by a peeling action.
The length of the floats is chosen to provide the required peeling
effect whilst maintaining the desired filtration effect. Preferably the loop
length is in the region of 2 to 4mm, more preferably about 3mm. In
order to assist release of the loops from the pressed concrete, the yarn
forming the loops is preferably smooth walled e.g. a flat yarn.
The ground construction is preferably knit in such a manner as to
provide a relatively flexible material which on separation yields in the
centre of the fabric and thereby provides a macro peeling effect across the
face of the fabric.
Preferably the hydrophobic yarn used in producing the fabric is a
non-textured yarn.
The filter fabric 19 of the first embodiment is adapted for mounting
on a metal perforate plate 14, adapted for detachable securance to the table
12 and platen 11.
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The fabric 19 is bounded about its periphery with an integral band
32 which, as shown, has been folded to overlie the remainder of the
fabric. The corners of the folded over band 32 are preferably cut to
define mitre joints. The band 32 is of a width 'W' and is formed by
S fusing the heat meltable yarn of the fabric 19 to form a smooth
_,
imperforate surface. A band of an adhesive in the form of a double-sided
adhesive tape 22 is applied to the band 32. A strong bond is possible due
to the smooth surface of band 32 and also the adhesive is shielded from
the concrete due to the imperforate nature of the band 32.
Prior to mounting filter fabric 19 to plate 14 preferably a sponge
layer 41, for example, of a porous foam plastics material, is placed
between the fabric 19 and the plate 14. Preferably the size of the foam
layer 41 is such that it lies within the inner boundary of edges 45 (Figures
3,5) and is also of a thickness that, on compression, it has substantially the
same thickness as the thickness of band 32. In this way the face of the
fabric is substantially flat over its entire length and width defined between
the exterior fold 33 when under compression.
The sponge Layer 41 is compressible and is capable of absorbing
water. When the platen l I compresses the mixture 17 onto table 12 the
sponge layer 41 compresses flat and conveys water passed through the
fabric 19 to exit ports 16,16'. As the platen 11 decompresses the sponge
layers 4I expand and this helps to draw in surplus water that remains
resident at the contacting surface of the filter 19 and compressed mixture
17 and facilitates separation.
It is envisaged however that the sponge layer 41 may be omitted or
may extend inbetween the folded over band 32 and the remainder of the
fabric 19.
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When applied to plate 14, it will be appreciated that the edges of
the filter 19 defined by fold 33 can be substantially contiguous with the
edges of the plate 14 and thereby present a filter which extends across the
' full width and length of plate 14: Water is able to escape at the edges of
the filter 19 by passage through the fabric underlying the folded over band
32.
Referring to Figures 6 and 7 there is shown a filter 19' for use in
a concrete press mould 10 as with the first embodiment.
The filter 19' comprises a fabric 19", having a knitted or woven
structure substantially the same as that of the first embodiment, adapted
for mounting on a mount 2. Preferably the mount 2 is made of a
relatively thin plastics sheet such as polypropylene sheet having, inter alia,
flexibility and hydrophobic properties. The thickness of the mount 2
preferably varies in proportion to the area of sides 8, 9, for example a
range of area in the order of about 150mm'- to 400mm= may result in a
range of thickness in the order of about l.Smm to 3mm. A plastics
material such as polypropylene is substantially resistive to compressive
deformation which results in good recovery after compression in the press
mould 10.
The mount 2 has a pluraltiy of apertures 2' formed therein to allow
the release of water from the mixture 17 under compression in the press
mould 10. Preferably the apertures 2' have a range in diameter in the
order of about lmm to Smm and the density of apertures 2' also varies
,. .
according to their size, for example in the order of about 2/cm= to 5/cm2.
In the embodiment shown the sponge layer 41 is dispensed with and
the filter fabric 19" directly covers one side of the mount 2. The filter
WO 94/25236 PCT/GB94/00905
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to
fabric 19" is wrapped around one side 8 of the mount 2 and over the
edges 7 to leave overlapping marginal portions S which are folded over
and adhered to the opposite side 9.
S The filter fabric 19" is adehered to the opposite side 9 of the mount
2 preferably by means of an adhesive layer (not shown), for example
double-sided adhesive tape, between the adjacent overlapping marginal
portions 5 of the filter fabric 19" and the mount 2. Preferably the fabric
about the marginal portions 5 of the filter 19' is then heat fused to bond
the fabric to the mount 2 followed by heat fusing the edges 7 to accurately
size the mount 2 to a dimension providing a snug fit for the filter 19; in
the chamber 40 of the press mould 10 in order to prevent wear about the
edges 7 when the filter 19' is in use.
The filter 19' is adhered to the surface of the table 12 or platen 11
of the press mould 10 preferably by means of double-sided adhesive tape
22' applied between a compression surface of the table 12 or platen 11
and the opposite side 9 of the filter 19'. The double-sided adhesive tape
22' may be applied as continuous elongate bands on the opposite side 9 or
preferably as a plurality of individual blocks as shown in Figures 6 and 7.
As an alternative to using a fall plate warp knitting machine a
similar fabric to the fabric of the two embodiments discussed (above) may
be formed on a warp knitting machine not using a fall plate. Alternative
ground fabrics can be knitted in the following way:
a) Using one, or a combination of, yarns preferably of count 50
dtex to 600 dtex;
b) Using a machine preferably between 12 and 28 gauge.
Two examples for notation for the ground construction would be:
i) Guide bar 2 0-lll-0 repeat
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Guide bar 3 0-0/2-2 repeat; or
ii) Guide bar 2 0-1/1-0 repeat
Guide bar 3 0-1/3-2 repeat
' To produce the loops shown a hydrophobic yarn is laid on top of
either of these ground fabric constructions, or other similar constructions,
J
a comparable fabric can be formed. For example, using guide bar 1 or
a 12-guage machine, the following lapping motions may be possible
0-1/3-2 or 1-0/2-3 lapping motion on a 12 gauge machine. However, for
finer gauge machines, longer throws are required to form the preferred
loop length in the region of 2mm to 4mm, more preferably 3mm.
