Note: Descriptions are shown in the official language in which they were submitted.
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"Pressing of concrete bodies and a filter
fabric for use therein".
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Concrete bodies such as paving stones or slabs
are produced by inserting a wet concrete mix, which
may or may not contain an aggregate, in a mould, and
then compressing the concrete mix in the mould to
compact the mix. Typically the rnoulding machine
comprises a ram movable into and out of the upper
portion of the mould, so that concrete mix is pressed
between the base of the mould and the underside of
the ram. During com?ression, the concrete mix is
compacted and formed into the shape corresponding
to the internal shape of the mould, but also, much
of the water in the wet mix is expressed from the mix,
and it is necessary to provide for the release of this
water from the mould.
In order to facilitate removal of the compacted
mix from the mould, it is conventional to place a sheet
of filter material (e.g. filter paper) between the base
of the mould and the mix and between the underside of
the ram and the mix. The filter material acts as a
parting medium between the mix on the one hand and the
mould and ram on the other hand, but because of its
porous nature, it also allows the expression of the
water from the mix during moulding. It is necessary
to provide foraminous surfaces on~the base of the
mould and the underside of the ram to allow the water
to escape after it has passed through the filter
material~ and for this purpose, perforated metal
plates may be attached respectively to the base of
the mould and the underside of the ram.
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Traditionally, the filter material has been a
paper, but a paper filter sheet has to be discarded
after it has been used to produce a single paving
stone. In the Specification of United Kingdom Patent
No: 1 575 000 there is described apparatus for
producing concrete bodies in which the filter material
is secured to the ram or the base of the mould as the
case may be, so that it remains in the mould when the
compacted concrete body is removed. That specification
also describes the use of filter material made of
thermo-plastic fibres such as polypropylene.
The use of filter material secured to the ram
and/or base of the mould, particularly if it is made
according to the preferred features described in
Specification 1 575 000, enables the filter material
to be used repeatedly in the manufacture of a number
of concrete bodiesg and in same instances, such a
filter material has been used to produce some
hundreds of concrete bodies before it was necessary to
change it. However, with certain ~ypes of concrete
mix, good results have not been achieved and in extreme
cases, the filter material has become ineffective after
only a small number of conrete bodies has been produced.
The present invention provides an improvement
useable in all instances where the filter material
is adhered to the ram and/or base of the mould,
irrespective of the constitution of the filter material
itself~, though good, and in some instances exceptionally
good,`~results have been obtained when the inYention has
been used ~ith the preferred filter material described
in Specification 1 575 000.
~ he invention i5 based on the discovery that
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the method of adhering the filter material to thesupporting structure (i.e. ram or mould base or
perforated plate) apparently plays a significant part
in determining the performance of the filter material,
especially with regard to the number of times that
the filter material can be used before it has to be
changed. The precise theory of operation is not
understood, but it is believed that the effect of
the adhesive on the flow of water through the filter
material when water is being expressed from the
concrete is of considerable importance.
According to a first aspect of this invention
a method of producing concrete bodies comprises
pressing wet concrete between a ram and a mould to
compact the concrete and express water from it, there
being a layer of filter material secured to at least
one of the ram and the base of the mould by a
discontinuous coat of adhesive applied over a suficient
area of the filter material to ensure the adhesion of
the filter material to the ram and/or base of the mould
under normal working conditions. References throughout
this specification to the filter material being adhered
to the ram or base of the mould, are to be-taken to
include adhesion of the filter material to a foraminous
element such as a perforated plate which is itself
secured to the underside of the ram or lying on the
base o the mould.
The use of a discontinuous coating o adhesive
ensures that there are significant areas of the filter
material through which water can flow substantially
unimpeded by adhesive. Prior to the present invention,
the adhesive was applied as a continuous coat over
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the complete reverse face of filter material and
therefore the adhesive has to be of such a nature that
whilst providing the necessary adhesion, it also allowed
some penetration by water. The water pressure during
compacting ~ay be of the order of 1200 to 1500 pounds per
square inch, and during the pressing of the first slab,
after a filter has been fitted, the adhesive coat (which
may be on a plastics or paper carrier) was ruptured by
the water over each hole in the perforated place, so that
the coating itsel~ then had a series of holes formed
through it. This did not provide any selectable control
over the resistance to water flow.
Another way of stating this aspect of theinvention
is tha~ in the production of concrete bodies by pressing
wet concrete between a ram and a mould to compact the
concrete and express water from it, and in which a filter
material is secured to the face of at least one of the
ram and the base of the mould, the rate of flow of water
from the compacted concrete is regulated by preselecting
the density of adhesive applied as a discontinuous coat to
the side of the filter materia`l which is in engagement
with the ram and/or base of the mould.
Preferably the discontinuous coating adhesive is
applied as a transfer to the filter material from a
substrate on which the discontinuous coating of adhesive
is formed. The actual method of transferring the coating
to the filter material is conventional: the substrate
is cut to the same size as the sheet of filter material;
the~substrate brought with its coated face into
engagement with the reverse face of the filter material
and the filter material and substrate pressed together.
When the filter material is to be used, the substrate is
peel:ed off it, leaving the coating of adhesive on the
reverse face of the filter material, ready for that
face to be presented to the face of the ram or base of
the mould.
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Accoring to a second aspect of the invention a
filter for use in the proauction of concrete bodies
comprises a sheet of water-permeable material having
a discontinuous adhesive coating on one face thereof.
Preferably the adhesive coating is carried on a
substrate applied to the face of the filter material
and releaseable from the adhesive to leave the coating
on the filter material when the substrate is removed.
The following preferred features are applicable
to both aspects of the invention.
It is preferred that the adhesive is applied to
the filter medium in the form of discrete particles,
and although the particles will not cover the entire
surface of the filter material, it is preferred that
the pattern of particles covers substantially the entire
area of the face of the filter material which will
contact the face of the ram or base of the mould.
According to a preferred feature of the invention
the adhesive is present over between 2% and 15% of the
total surface area of the filter material. The precise
density of the adhesive coating will be preselected
according to the type of concrete being moulded and in
some instances according to other operating
characteristics, but very useful results have been
obtained with discontinuous adhesive substrates
manufactured by Coated Specialities Limited of the
fol~owing grades:-
(i) 1370 11% coated area:22 grammes adhesive
per square metre.
(ii) 1536 8% coated area:l6 to 18 grammes
adhesive per square metre.
(iii) 1407 3% coated area: 4 to 6 grammes
adhesive per square metre.
The grade 1536 has been found to give particularly good
`~ results over a wide spectrum of concrete mixes.
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One form of apparatus and its method of use in
accordance with the invention will now be described by
way of example only, with reference to the accompanying
drawings, in which:-
Figure 1 is a cross-section through a mould,
Figure 2 shows part of the reverse face of
a filter medium for use in the apparatus shown in
Figure 1, illustrating one form of discontinuous
adhesive coating, and
Figures 3 and 4 are views similar to Figure 2,
but showing alternative ~orms of adhesive coatings.
The apparatus which is illustrated in Figure 1,
is intended to be used for forming concrete paving
stones, and it generally comprises a mould 10 having
side walls 12 which define a substantially rectangular
cavity open at the top and bottom. The bottom surface
of the mould 10 is closed by a lower press plate 14
which is adapted to be moved upwardly through the mould
cavity by an injection mechanism (not shown). The lower
press plate 14 is also provided with a series of drain
~holes and connecting channels 16 to allow liquid to
drain therethrough from the mould cavity.
A ram mechanism indicated generally at 18 which
includes a die-head is mounted above the mould 10 and
is~adapted to be moved downwardly towards the mould
lO,~ and a compression plate 20~is mounted on the die-
head of the ram mechanism 18. The~compression plate 20
` is~arranged to be a close fit in the mould cavity and
~when the ram mechanism 18 moves downwardly towards the
mould 10, the plate ~0 slides into the open top of the
mould cavi~y. The compression plate 20 is provided
with a series of drain holes and connecting channels 21
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through which liquid can be drawn from the mould cavity.
A perforated metal plate 22 is mounted on the
lower surface of the compression plate 20 and is
connected thereto by countersunk stetscrews 24. The
perforated metal plate 22 is arranged to conform
closely in shape to the shape of the mould cavity and
the edges thereof are ground to give a sharp arrice
to the concrete body.
A similar perforated metal plate 26 is carried
on the upper surface of the lower press plate 14. The
perforated metal plate 16 provides an effective
foraminous base for the mould, and the perfGrated
metal plate 22 provides a foraminous bottom surface
on the compression plate 20, which can be regarded
as a ram.
Two layers of filter material 30 and 32 are
provided which both closely conform to the shape of
the mould cavity, and each layer of filter material
30,32 is formed of thermo-plastic fibres such as
polypropylene fibres. In this particular arrangement,
the upper and lower surfaces of the layers of filter
material 30 and 32 are subjected to a hot calendering
process to produce a surface of a smooth non-fibrous
nature which is porous to water. The hot calendering
process consists of, for example, passing the layer
of filter material over a heated roller. The layers
of filter material are cut to size by die stamping
on a cutting press.
The layers of filter material 30 and 32 are
secured to their associated metal plates 22 and 26 by
means of a hot melt adhesive. The manner in which this
adhesive is applied to the layers of filter material
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forms a significant feature of the present invention
and will be described in some detail.
The adhesive is a hot melt adhesive and is
provided in the form of a discontinuous coating on one
face of a substrate. The substrate is in the form of a
roll, but its two faces have a different release efect
with respect to the adhesive, so that as the substrate
is unrolled, all the adhesive is on one face. The
d;scontinuous adhesive coating is applied to the
substrate in the form of discrete particles 50 (see
Figure 2~. These particles 50 may themselves be
constituted by a plurality of very small individual
particles grouped together in clusters, and it will
be observed from Figure 2, that the particles 50 are
applied to the substrate in a pattern of ranks And files,
which pattern covers the entire surface of the substrate.
The particular pattern which is illustrated in Figure 2
is that of a discontinuous adhesive coated substrate
supplied by Coated Specialities Limited grade 1536,
which provldes adhesive over 8% of the total area of
the substrate to a density of 16 to 18 grammes of
adhesive per square metre. When the filter material
30~or 32 has been prepared it is placed in face-to-
face contact with the substrate which is then cut to the
same size as the filter material.~ The filter material
and the substrate are then press~ed together using a
hot rotary ironer. In effect, the discontinuous
~adhesive coated substrate is a transfer, but the
adhesi~e~particles have sufficient tackiness to enable
them~to secure the substrate to the face of the filter
material, particularly when subjected to the hot ironing
~q process. The filter~material with its substrate adhering
thereto can be stored ready for use.
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When a particular piece of filter material is
to be used, the substrate is peeled off, leaving the
pattern of adhesive particles 50 transferred on to
the reverse face of the filter material. The filter
material is then pressed with its reverse face against
the top face of the perforated plate 26 or the
underside of the perforated plate 22, to cause the
filter material to adhere to its respective perforated
plate.
The method of using the apparatus illustrated in
Figure 1 will now be described:-
The lower press plate is disposed in the bottomof the mould cavity as illustrated in Figure 1, with
the layer of fil~er material 32 secured thereto. A
quantity of wet concrete mix 34 is placed in the mould
10 to fill the mould cavity to the required depth,
dependant upon the desired thickness of the paving stone
which is to be produced.
The ram mechanism 18 is then operated to move the
pLate 20 downwardly into the mould 10 to the position
illustrated in Figure 1. During the movement, the
concrete mix 34 is compressed between the layers of
filter material 30 and 32 and the compaction of the
concrete mix 34 causes water to be expressed Erom the
concrete mix. This water passes through the porous
surfaces of the layers of filter material 30 and 32
which will allow the passage of water, but retain the
~cement and other fines content~in the concrete mix 34.
The water flows through the filter material, then
through the perforations of the metàl plates 27 and 26,
and thence out of~the mould cavity via the series of
holes and channels 16 and 21 formed in the lower press
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plate 14 and the compression plate 20. The concrete mix
34 takes up the shape of the concrete paving stone
defined by the mould cavity.
After compression of the concrete mix 34, the
compression plate 20 is withdrawn from the mould 10
by actuation of the ram mechanism 18, and the concrete
paving stone is ejected from the mould cavity by
actuating the ejection mechanism (not shown) which
moves the lower press plate 1~ upwardly through the
mould 10.
The concrete paving stone thus formed is removed
from the layer of filter material 32 which is attached
to the lower press plate 14 and the paving s~one is
then allowed to cure. The Layers of filter material
30 and 32 are retained on the compression plate 20
and the lower press plate 14 respectively and the
apparatus can then be used to repeat the above operation
and produce further concrete paving stones. The release
of the paving stone from the layers of filter material
30 and 32 can be assisted by the impregnation thereof
with a release agent such as, for example, a silicone.
It will be appreciated that the expression of
water from the concrete mix during the compression
thereof is a very important part of the method of
manufacture just described. The discontinuous adhesive
coating of the filter material plays an important part
in facilitating the expression of water through the
filter material and in the speciic instance which has
been described above, the water is able to flow
unimpeded across the interface between the filter
material and the perforated plate 22 or 26 over 92%
of the surface area of that interface. Hence, the
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presence of the adhesive has a minimal effect on the
expression of water from the mould, and it is thought
that this plays an important part in the success of
the invention in enabling the filter material applied
to the ram and/or the base of the mould, to be used
repeatedly for the production of a number of concrete
slabs. At the same time, the overall pattern effect
of the discrete particles of adhesive 50 has the effect
of providing an adequate bonding of the filter material
to its respective perforated plate.
It has been found that the compacted slab releases
cleanly from the filter material, that is to say without
signifi.cant attachment of the concrete to the face of
the filter (which would destroy the effectiveness of
the filter after a relatively few moulding operations
as well as having a deleterious effect on the slab).
Apparently the invention causes the concrete particles
to have a greater affinity for each other than for
the filter material and this causes them to remain in
the slab rather than attaching to the filter.
With certain types of concrete mix, it may be
permissible to utilise a denser discontinuous adhesive
coating such as the coatings iIlustrated in Figures 3
and 4. These coatings are still made up of discrete
particles 52 and 54 of adhesive, but it will be noted
that in Figure 3 for instance, the particles are
present in a much denser pattern than that illustrated
in Figure 2. In fact, Figure 3 is an illustration of
8rade 1370 supplied by Coated Specialities Limited
wherein 11% of the total surface area is covered by
adhesive particles, giving a density of 22 gra~mes
adhesive per square metre.
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It is to be understood that the invention can
be applied to any form of filter material, including
the traditional paper filter materials, though if paper
were used, it would probably be necessary to use thicker
papers than those which have been discarded after a
single operation.
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