Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1~;7624
(PCT12730) - 1 -
(2.1.81)
MOULDING OF ARTICLES
TECHNICAL FIELD OF THE INVENTION
-
This invention relates to the moulding of articles
and in particular to the moulding of construction
5. products, such as partition panels, roof decking and
pipes, from liquid setting particulate materials.
STATE OF THE ART
It has been customary hitherto to mould such
articles as aforesaid by mixing the constituent
10. materials, applying a sufficient quantity of setting
liquid to the mix, introducing the moistened mix to
the mould and allowing the mix to set before removal
of the set article from the mould. This process is
time-consuming and for quantity production of such
15. articles, since the setting of the article occurs in
the mould, a large number of moulds is required.
It has been proposed for example in British Patents
Nos. 528,657, 1,067,671, 1,346,767, 1,417,001 and
1,466,663 that a dry mixture of constituent materials
20. be introduced into the mould and compacted therein.
The mould is then immersed in a setting liquid or the
liquid is allowed to permeate the mix by capillary
action. Of these Patents, only in the case of 1,346,767
is the liquid applied to a vertical surface which is
25. unsupported by a part of the mould apparatus, and in
that case the mould is immersed in the water so that
the buoyancy effect thus created offsets the tendency
r~
11~7624
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of such unsupported walls to collapse due to the
inc~e~se in weight of the mix.
~ -It has also been proposed in U.S. Patent 1,427,103
tha~ for producing very small moulded articles, for
5. example buttons, the dry constituent materials be
pressed into the mould, removed therefrom and then
sprayed with setting liquid. How~ver, this process
is restricted to use for the production of very small
articles and has not been used for the production of
lO. relatively very large articles, such as construction
products, since such articles would be expected to
collapse under their own weight on demoulding and may
also shrink and crack during the spraying operation.
In consequence it has been considered that if there
15. is to be any vertical surface of mix which is unsupp-
orted by a part of the mould apparatus during the
wetting process by seepage rather than by total
immersion then it ls essential to incorporate into the
mixture of constituent materials some reinforcing
20. means from which the moulded article can derive support
during the spraying and setting stages of the process.
The reinforcing means may be fibres, and examples of
processes incorporating the use of such fibrous
reinforcing material for the supporting of the moulded
25. article whilst unsupported at least in part by the
mould are described in German Patent 1,683,829, British
Patent 1,346,767 and our British Patent No. 2,045,150.
DISCLOSU~E OF THE INVENTION
30. The invention provides a method for producing
moulded construction products from a liquid setting
mixture of fine and coarse particulate materials
comprising the steps of mixing the dry constituenet
materials, said materials including a proportion of
35. fine particles sufficient to substantially surround
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(PCT12730) ~ 3 ~
~2.1.81)
all coarse particles but not including fibrous rein-
forcing materials, introducing said mixture into a
mould, compacting said mixture in said mould to an
e-xtent that said fine particles substantially fill the
5. interstices between said coarse particles, removing
at least a part of the mould from contact with the
thus moulded product, spraying the product at a surface
unsupported by said mould with a predetermined quantity
of a setting liquid, being a quantity sufficient to wet
10. all of the compacted constituents but insufficient
completely to saturate the same, and allowing said
product to set.
Surprisingly, it has now been found that provided
that there is sufficient compaction and a sufficient
15. proportion of fine particles in the mixture of con-
stituents no fibres or other reinforcement are required
and a satisfactory moulded article may be obtained which,
without collapse, can be demoulded before the onset of
chemical curing and which does not shrink or crack
20. during the spraying and setting process. Because of
this the method of the invention can be used for the
manufacture of high quality precast concrete products
having no fibrous reinforcing therein, and in respect
of which removal of the article from the mould after
25. compaction and prior to spraying can be used to
economic advantage by reducing the number of moulds
needed for quantity production of such articles.
In addition to immediate demoulding, concrete
products produced by the new method have an unusually
30. high quality finish, high immediate demoulding strength
and can be moulded to intricate shapes, without the
application of high pressure or heavy ramming or tamping.
This combination of features is unique in concrete
making.
35. In conventional concrete practice, immediate de-
2~
- 4 -
moulding can be achieved by vibrating or ramming so-
called "earth damp" mixes into moulds but the products
are generally characterised by a granular surface finish
as in "breeze" blocks. At present, smooth finishes
5. for immediately demoulding products can only be obtained
by using extremely high compacting forces, such as the
centrifugal forces, used in the "Packer-head" process
for pipe manufacture. Such processes, howeverr are only
suitable for simple shapes, compared to the intricate
lO. section, which can be produced by the new method. Alter-
natively, relatively smooth finishes can be obtained
by conventional wet casting but here the wet concrete
sticks to the moulds and can only be removed once the
material has 'set. Furthermore, although these surfaces
15. tend to be smo~ther than those made from "earth damp"
mixes, they are characterised by "pin holes" and other
blemishes, arising from bubbles within the liquid which
do not occur with the new method.
Another departure from the core spray method of
20. our British Patent No. 2,045,150
is the discovery that with adequate compaction and
suitable powder formulation, it is possible for com-
pletely dry mixes to stand intact with one or more of
the mould sides removed. If rigid steel bar or mesh
25. reinforcement is incorporated, it is sometimes possible
to remove all the sides of the mould (other than the
base) without collapse of the dry compacted mix. This
means that, whereas previously access for spraying could
only be via internal core holes, it is now possible to
30. spray onto free-standing external vertical surfaces.
This widens the range of shapes which can be handled.
Also, water penetration can be speeded (particularly
for thick sections) by, for example stabilizing the
core zones by an initial internal spray and then
35. removing both main sides of the mould for further
24
(PCT12730) - 5 -
(~.1.81)
spraying via the outer surface.
These developments are surprising when viewed in
relation to normal preconceptions in the industry
or in relation to the published prior art. So far,
5. it has been considered essential that some form of
support be provided to the dry vertical surfaces to
prevent collapse either prior to or during the appli-
cation of liquid, typical means of support being
either some form of external support (such as per-
10. forated plates or membranes) or more recently internalfibres. We have now found that if the correct pro-
cedures are followed, no support of the dry surfaces
being sprayed is needed at all. (Such support as may
be required for the mass as a whole can be provided
15. at the surfaces which do not need to be sprayed as
described later). Furthermore, it was previously
thought that at least some fibres were needed to
prevent erosion of the free-standing material in, for
example, the core holes, and it has now been found that
20. with sufficient compaction and fines content and
a sufficiently fine spray, remarkably smooth bores can
be obtained with no fibrous support.
Another very surprising feature is the unexpect-
edly high strength of the dampened, compacted material
25. immediately after demoulding. In slow setting Portland
cement-based formulations, this so-called "green"
strength occurs well before any strength can develop
from the chemical reaction with the water. Hence the
unusual stiffness and cohesiveness of the moulding at
30. this stage can.only be due to physical properties, such
as mechanical particle interlock and surface tension
effects.
It is possible for example to demould some pro-
ducts made by the new method by hand, without requiring
35. vacuum lifting or other special equipment designed to
1~'76~
minimise demoulding stresses.
In common with fibrous panels made by the afore-
ment~ned core spray method, it is worth noting the
lack of any adhesion to the mould sides after spraying,
5. despite the very strong adhesion between the particles
themselves. Provided the amount of water sprayed is
such as not to saturate the mass, mould sides come away
remarkably cleanly and sufficiently dry to be ready for
the next filling.
10. Broadly, the range of products and manufacturing
sequence for the present method follows the method of
fibrous core spraying, except that fibres are omitted
and spraying can be other than via the core holes.
Spraying is largely on vertical (or approximately
lS. vertical) surfaces, which generally comprise at least
half of the total vertical surfaces of the products.
In the case of spraying via the cores in panel
products, the spray area is significantly more than
half the total vertical area. Sprayed surfaces can
20. be ribbed or textured, particularly in the case of
exterior sprayed surfaces, where the moulds do not
have to be withdrawn by sliding parallel to the
surface, as is usually the case with core hole sur-
faces. Generally, ~n the case of rectangular products,
25. the dry compacted material needs at least two mould
sides to remain in place during spraying, so the
dry material can support itself by arch act on between
the remaining two mould surfaces. In the case of
annular shapes, generally at least the outer or inner
30. mould surface should remain in place during spraying
to provide support to the dry compacted mass.
The remaining distlnctions,b,etween the present
method and the method of British Patent No.
2045150 largely relate to the degree of dry com-
35. paction applied and the provision of adequate fines
1~
lif~762~
(PCT12730) ~ 7 ~
(2.1.81)
in the mix.
For example, in the science of soil mechanics,
particles are broadly categorised as clays, silts,
or sands. The particle sizes of clays are extremely
5. cohesive when in a damp, compressed state. Sands, on
the other hand, are not cohesive under any circumstances
and silts occupy an intermediate position. It is not
necessary with the present process to do down to clay-
like particle sizes and the process will not work
10. solely with sands (unless the sand is combined with
finer material).
Common commercially available liquid setting
powders such as Portland cement or gypsum would
probably be classified (in terms of particle size) as
15. silts. It has been found that such powders work well
with the present process. Finer powders would give
more stable mouldings, but these are more difficult
to compact properly (unless the mix contains a pro
portion of coarse particles or compacting means other
20. than vibration alone are used). Broadly, it has
been found that to achieve adequate compaction, powder
feed rates have to be slower, e.g. up to half the
speed that has been used heretofore. If filling rates
are too fast (and/or vibration insufficient), some of
25. the interstices may not be completely filled before
subsequent layers of material compact into an effective
bridge above. If this happens no further downward
percolation is generally possible and the voids remain
only partly filled, even if subjected to prolonged
30. or even increased vibration.
Optimum filling rates depend very much on mix
proportions, particle size, etc. Generally for mixes
with near to the optimum economic proportions of coarse
aggregate, filling rates are generally slow - i.e. less
35. lOmm per sec. Compacting vibration must be more intense
11~7~24
(PCT12730) - 8 -
(2.1.81)
and of a higher frequency than has been usual hereto-
fore e.g. preferably at least 12,000 cycles per
minute. The more effective the compaction, the less
critical is the quantity of fines present, provided
5. at least sufficient fines are present to surround
the coarse particles. Mixes need to be as dry as
possible to obtain optimum compaction as even a small
degree of dampness can inhibit full compaction.
"Coarse" in this context means everything above
10. the ~silt" fraction discussed earlier i.e. it includes
the proportion of sand which is generally added to
concrete mixes. The ideal mix is one in which the
cement (for example) compacts into all the interstices
between the sand and the sand/cement mix in turn com-
15. pacts into all the interstices between the coarseaggregate.
From the processing point of view, there appears
to be no particular upper limit to the size of coarse
aggregate, provided that they fit readily into the
20. mould and are completely surrounded by compacted sand/
cement. Provided the aggregate component in the mix
is not too coarse, in some cases the proportion of
cement powder in the mix needed to generate adequate
final cured strength provides all the fines needed for
25. dry stability during manufacture. Whexe this is not
sufficient, additional fines are added, usually in
the form of pulverised fuel ash or some other suitable
cheap extender. Aggregates usually consist of a range
of larger particle sizes and include sand and light-
30. weight aggregates such as those manufactured from
expanded clay or sintered pulverised fuel ash. For
small sectioned products, such as sewerage pipes or
hollow concrete blocks, the maximum aggregate size is
generally around 5mm.
35. Although readily processible by the present method
ll~i 7624
(PCT12730) - 9 -
~2.1.81)
thin sectioned, large area panels are generally not
suitable as fibre reinforcement is usually required
in the end product for structural reasons. However,
the present method can be used for making products
5. containing non-fibrous reinforcement, for example,
such rigid reinforcement steel rods or bars as used
in conventional reinforced concrete.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a diagrammatic elevation of one form of
10. apparatus suitable for use in practising
the invention;
Fig. 2 is a plan view of the apparatus of Fiq. 1
with the core removed; and
Figs. 3 and 4 are cross-sectional elevations of typical
15. construction products manufactured in
accordance with the present invention.
BEST MODE OF CARRYING OUT THE INVENTION
One of the simplest types of equipment using the
new method is shown in Fig. 1. A vibrating tray 1
20. distributes the dry powder mix into a laterally oscill-
ating chute 2 so that two equal streams of material
pass either side of a bore former support 3 and are
guided by a hopper 4 into a mould 5, containing at least
one bore former 6 which is fitted at its base with a
25. vibrator 7. While filling the mould, the bore former 6
and/or the hopper and bore former support, are vibrated
to settle and thoroughly compact the mixture. After
filling the mould, the upper parts of the mixture which
are not compacted by a head of material above them, may
30. be further consolidated by precsing the bore former
support 3 (preferably together with the bore former 6)
onto the powder mix surface until the whole mass is
uniformly compacted. Vibration then ceases and the
bore former 6 and bore former support are withdrawn
35. from the mould, which then moves laterally to locate
1~;i762~
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(2.1.81)
over one or more spray tubes 8. Each tube 8 is fitted
at its end with a fine spray nozzle 9, which is
oscillated vertically in a bore until sufficient liquid
has been delivered to the bore surface(s) to just wet
5. the mixture throughout.
The spray needs to be fine and of modest velocity
to avoid surface pitting and should generally deliver
liquid at an average rate which does not exceed the rate
at which the liquid can be absorbed into the powder
10. by capillary action. This prevents the surface from
becoming saturated and causing drip marks or local
collapse. Spraying is usually terminated before full
wetting occurs, so that wetting of the still dry thicker
parts of the moulding is completed by capillary action,
15. drawing li~uid from the adjacent wet parts. This allows
the minimum quantity of liquid to be applied for full
wetting, thus avoiding the risk of over-wetting which
can cause the mixture to stick to the mould sides and
reduce demoulding strengths. When the damp areas have
20. spread throughout the mass, the mould is opened and the
uncured product is removed therefrom (by vacuum lifting
methods, for example) and allowed to cure.
Fig. 2 illustrates the method described above as
applied to the manufacture of paving flags or the like,
25. two such flags 10 being formed simultaneously in mould
12. The process is described in greater detail in
Example 1 below.
Figs. 3 and 4 illustrate other construction products
which may be manufactured by the present process as
30. described in Examples 2 and 3 below.
Example 1
Simple paving flags and the like can be produced,
without core holes, as shown in Fig. 2. In this case
the "core former" 6 in Fig. 1 is two complete mould sides,
35. which on withdrawal, expose the compacted particulate
1~'7624
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material for spraying (items 10 in Fig. 2). The dry
material is held up by arch action between mould sides
11. Sides 12 restrain buckling in one direction but
not the other, so they can also be removed before
5. spraying. This allows both faces of material 10 to
be sprayed, which is an advantage with relatively thick
products like paving flags (typically measuring 50mm
thick X 600mm X 600mm.).
It the material is correctly formulated and
10. compacted, it can freely span the 600mm without any
support other than at the base and at the sides 11.
Product thickness for this span can be as little as
15mm, which is surprisingly slender bearing in mind
there is no binding material at all between the particles.
15. To be competitive, paving flags require a high
coarse aggregate fraction of sufficient size to minimise
the surface area and hence the amount of relatively
expensive cement needed to bind the aggregate together.
A typical mix which gives a satisfactory product strength
20. for this application and can be processed satisfactorily
consists of 1:0.3:1.2:4 parts by weight of ordinary
Portland cement, pulverised fuel ash (as commonly used
for concrete manufacture) standard fine grade "sharp"
concreting sand and granite aggregate chippings passing
25. a 12mm mesh and retained on a 6mm mesh.
The dry mixture is poured evenly into the vibrating
mould, so that the level rises at approximately 500mm
per minute, while vibration frequency is maintained at
12,000 cycles per minute. Amplitude is adjusted so
30. that the coarse aggregate on the surface is just mobile
but the layers below are locked into position with the
fines flowing and compacting around them. After fllling,
the top layer can be compacted by plunger 3 Fia. 1 but
generally with the specified mix this is not very effective
35. (due to the almost point-to-point contact of the coarse
i'7624
(PCT12730) - 12 -
(2.1.81)
aggregate preventing movement). On removal of the
core former/mould sides 6, the free surfaces are
lightly sprayed until the material is just dampened
throughout and the mouldings then removed by vacuum
5. lifter to the curing zone.
Example 2
Pulverised fuel ash (PFA) is a silicious waste
material from coal fired power stations and is one of
the cheapest fillers available. If the mix is auto-
10. claved after dampening, the silica reacts with the freelime in the cement, resulting in a strong chemical bond
between filler and binder. In these respects therefore
it is advantageous to increase the PFA content and
adjust the production procedures and mix proportions
150 to overcome the fine powder compaction problems
mentioned in Example 1.
With high PFA concentrations it has been found
almost impossible to achieve the required compaction
by vibration alone and a preferred method is to rely
20. largely on direct externally applied pressure. For
compression compaction to be effective, the proportion
of coarse aggregate in any case has to be limited, as
point-to-point contact of the latter tends to cause a
series of "bridges" which shield the loose powder in
25. the interstices from externally applied pressure. It
is also preferable to limit the size of coarse aggregate
to sand rather than gravel, as the former is generally
easier to compact by direct pressure.
A typical application for such mixes is the manu-
30. facture of sewerage and drainage pipes of approximatelylOOmm internal diameter and 15 to 20mm wall thickness
and a suitable mix would be 1:1:3 of ordinary Portland
cement, PFA and sand. This is poured fairly rapidly
into a moulding plant similar to that shown in Fig. 1
35. except that core former 6 is vibrating rather than the
i762~
{PCT12730) - 13 -
(2.1.81)
~nould. On filling, core former 6, together with
l:op plunger 3, move downwards to compress the powder/sand
rnix, while still vibrating. After full compaction
vibration ceases, core former 6 is completely
5. withdrawn downwards and plunger 3 withdrawn upwards,
before the mould moves to the spray station.
In this method, filling and top compression rates
are not critical, provided there is provision for the
escape of air te.g. between the mould side and top
10. plunger 3). Vibration is also not critical, provided
it is sufficient to disrupt dry resistance to compaction
by arch action in the material immediately below the
top plunger 3. With the apparatus shown in Fig. 1, the
core former acts as a poker vibrator, dislodging any
15. potential arching, so that the top pressure can be
fully effective throughout the product. Also, core
former 6 is one of the abutments against which the
material arches, so moving the core former relative to
the mould side 5 (forming the other abutment) also has
20. a powerful arch breaking effect during compaction.
Example 3
Insulating lightweight aggregate concrete blocks
can be manufactured by the new method, particularly
multi-slotted, thin-walled sections as shown in Fig. 4.
25. Although it has been known that such sections have
considerably greater thermal insulation than conventional
concrete blocks, the wet manufacturing methods for the
latter are not suitable for such extreme shapes.
By using dry methods and a specifically designed spray
30. system, it is possible to reduce slot dimensions to
lOmm and leaf thicknesses to under 5mm (using 4mm max
aggregate size). This is a surprisingly delicate
structure, considering that prior to spraying there is
no adhesion between the particles.
35. Manufacturing conditions and mix properties for
il ~7
- 14 -
thi~s product are intermediate between Examples 1 and 2.
A t~pical mix is 1:0.5:3 parts by weight of cement,
PFA and "Lytag" lightweight aggregate from 4mm down to
dust. The latter is made from sintered pulverised fuel
5. ash and is about half the density of the aggregates
in the previous Examples. This aggregate also contains
fines, so the mix properties are therefore not directly
comparable to those in earlier Examples.
The process described in our British Patent Number
lO. 2,045,150 relies on the fibres contained
in the constituent mix acting as tensile reinforcement,
preventing the dry compacted particles from cracking -
or, if cracks do form, by preventing these from
spreading to complete rupture. This is achieved by
15. fibres penetrating across a crack or potential crack
and holding the sections or clumps of compacted
material together. Fibre pull-out is prevented by
the frictional resistance of the particles bearing
on the length of fibre embedded on either side of the
20. crack.
In addition to these effects, the interlocking
network of fibres acts as a barrier or screen, resisting
the flow of particles between them. With such small
apertures between fibre barriers, relatively modest
25. compaction enables the particles to arch between the
fibre restraints and so prevent flow. Even modest
amounts of fibre have very marked affects on both dry
and wet stability. For example, the green strength
of the formulations in Examples 2 and 3 can be more than
30. doubled by adding under 1% of lOOmm glass fibre strands
to-the constituent mix.
In the process of the present invention there
are no such arch restraints, screen effects or tensile
reinforcement to stabilise the material. The dry
35. particulate mass has to be rendered stable enough for
* TRADEMARK
(PCT12730) - 15 -
(2.1.81)
subsequent processing by the frictional resistance
between particles and some slight mechanical inter-
locking with angular particles. This is why the fines
content and compaction requirements are so much more
5. critical with this method than with the aforementioned
mixes containing fibre reinforcement. In the present
process, the fine particles promote interlocking by
packing into all available spaces, while the applied
vibration and/or pressure ensures that the particles
10. penetrate between the coarse aggregate and pack firm
enough to generate the required frictional resistance.
The tensile strength generated by such frictional
effects is generally too small for the dry material
to stand entirely on its own and the structure stands
15. by arching between at least one pair of opposite mould
sides (or by ring compression, in the case of annular
structures like pipes). If suitable non-fibre reinforce-
ment is included in the product, it is possible to remove
all vertical support provided by the mould.
20. Stability of the mix is much enhanced by capillary
cohesion effects, when only just enough liauid is
added. In consequence local overwetting during liquid
application should be avoided, since this can cause
collapse of the upstanding surfaces. However, by means
25. of the process of the present invention i.e. providing
sufficient fines are present in the mix which is then
adequately compacted, the mix can possess adequate dry
and wet stability and a high enough green strength to
enable the mould to be removed completely after wetting
30. and before curing.
