Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to an expansible mold for the manufacture of
plaster parts or pieces.
Usually the molding of plaster pieces is performed by molds which
can be disassembled, or have parts which are movable at the time of removal
and of extraction of the piece, enabling the effects of friction on the mold
walls to be avoided.
Nevertheless, the disadvantage of these molds consists principally
of the fact that assembly of the mold movable parts cannot be obtained with
sufficient accuracy, thereby giving rise to dimensional variations of the
finished product.
More particularly, this inaccuracy is due to the mechanical clear-
ances between the different movable parts and to the eventual retention within
the clearance space of plaster particles which oppose mold closure.
A second process, more particularly practiced for the molding of
prismatic pieces, consists in using molds equipped with fixed side walls, re-
movable parts, for instance for the upper sides, and sliding parts for the
lower sides.
After disengagement of the upper molding part, the removal of pieces
can be carried out by displacing the mold bottom into the interior of the mold,
for instance with the aid of hydraulic c~linders~ in order to eject the plaster
through the upper part of the mold.
To enable such an operation, which requires very large stresses, it
is necessary that the internal sides of the mold be quite flat, smooth, and
have very great surface hardness in order to avoid any possibility of tearing
up of the plaster surface and of abrasion of the mold surface.
This process, which requires molds of very high accuracy, is
extremely expensive.
In order to reduce these disadvantages, this invention has as an
object an expansible mold which, although it employs relative moving parts,
essentially differs from known molds which can be disassembled or have movable
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parts, in order to allow the free removal of the plaster product; at no time
of its use is there a clearance between these parts enabling the inclusion :
of plaster particles that interfere with closure or impairs the accuracy of
the mold. :-
The invention provides a mold for the production of plaster parts
~-~ comprising at least two mold walls relatively movable between a closed ,J,
position wherein ~hR aefine the mold space, and an expanded position
wherein the spacing between the walls is greater than in the closed position,
at least two deformable components positioned between outer ends of the said
walls and in sealing ergagement therewith ini~both expanded and closed posi-
tions, a feed opening communicating with the mold space for the introduction
of plaster material to be molded, rigid spacers between said outer ends of the
walls immediately outwardly of said deformable components having a width
corresponding to the spacing between said walls when in closed position and
positioned to be engaged by said walls in the closed position, a pressure
means operative on at least one of said walls for displacement under pressure
to be closed position whereby the deformable components are deformed in the
direction of said pressure means and expanded into the space between the
said mold walls to define the end walls of the mold space therebetween, said
deformable components in response to release of pressure being returned to
normal position between said walls by expanding to displace the mold walls
away from each other to the expanded position while retracting lengthwise
between the mold walls to release the article molded in the mold space.
In the case where the articles to be molded are parallelepiped and
involve profiles on their side faces, both walls have parallel flat surfaces, :
and the two deformable components have two prismatic surfaces reciprocally
opposite, the limit of deformation of the deformable components being deter-
mined by interposing two metal blades between the walls outside the deformable
components, and having a width corresponding to the desired thickness of the
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pieces to be molded.
Embodiments of the invention will be described hereinafter by way
of non-restrictive example with reference to the accompanying drawings in
which -
Figures 1 and 2 are two horizontal sectional views of a mold usedfor the manufacture of plaster squares, respectively in positions of stripping
(Figure 1) or of molding (Figure 2);
Figure 3 is a transversal elevational view of a set of molds iden-
tical to that represented in Figures 1 and 2;
10Figure 4 is a horizontal section of a mold according to an alternate
method, represented at the time of removal~ and
Figure 5 is a vertical section of the mold of Figure 4 in the same
phase of the manufacture showing another alternate method.
With reference to Figures 1 and 2, a mold 1 has two vertical and
parallel walls 2 made of a material properly surface finished and without
particular hardness, and two deformable substantially incompressible component
parts 3 made of an elastic material having two reciprocally opposite prismatic -
moulding surfaces 4. More particularly these component parts 3 can be made
of elastomer, for instance of neoprene or of silicone, or of any other material
having similar mechanical properties. The parts 3 are substantially inco~-
pressible in the sense that when they are deformed under a force applied in
one direction they expand in a transverse direction so that there is no
significant reduction in volume.
This elastomer has the property of being able to collapse in a
direction (arrow 5) under the effect of a force applied in the same direction.
Owing to the fact that this elastomer is incompressible, its volume
remains constant and the dimensional loss resulting from this collapse in the
direction of arrow 5 produces a directional increasing along the arrow 6.
The invention makes use of this property of the elastomer to enable
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the removal of plaster pieces, the elastomer being subjected to a pressure at
the time of casting the plaster and then relieved from mechanical stress at
the time of removal. -.
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As represented in Figure 2, the mold 1 is in molding position and
the two internal faces of the walls 2, onto which the compressive forces of
the elastomer are applied, are held in spaced relation at the thickness of
the plaster square 7 through the medium of two metal parts 8 arranged on
edge between the walls.
The component parts 3, compressed between the walls 2 of mold 1, in
this state of deformation, have profiled molding surfaces 4, enabling molding
of the sides of plaster square 7 to be effected.
Removal of the plaster square is obtained by releasing the forces
on walls 2 and consequently on the component parts 3.
This release simultaneously opens a gap between the walls 2 and the
plaster and, in consequence of the retracting of elastomer component parts 3,
a gap at the surfaces 4 delineating the square thickness 7 (Figure 1).
Then square 7 can then be removed freely without stress and without
friction, upwards or downwards, without any draw (draft angle formed in the
mold walls to faciliate ejection), and without gaps occurring which would
enable the inclusion of particles which might between the elastomer parts 3
and the walls 2 be detrimental to subsequent molding operations.
With reference to Figure 3, representing partially a set of molds 1
mounted in parallel, in which the walls 2 internal to the set are common to
two adjacent molds, molding of the upper parts of squares 7 is carried out by
molding component parts 9 fixed on the side edges of walls 2, reserving open-
ings 10 for the casting, the surface of squares corresponding to the openings
10 which can be obtained by leveling. Molding of the lower side of square 7
is effected by a bottom 11 having profiles 12 for each of the molds 1 and on
which the walls rest, this bottom being integral with a fixed base or even
with the ground.
This arrangement of molds 1 makes evident an important advantage of
the invention relating to the handling of plaster squares 7 immediately after
removal
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According to the currentl~ employed processes, after removal it is
known that it is necessary to convey the squares to a withdrawal station,
storage station, drying station or a further storing station. This transport
is particularly difficult owing to the fact that at the time of remsval,
setting of the plaster has not been completed and the squares possess little
mechanical strength. Consequently, it often happens that the plaster squares
are damaged during such conveying.
This disadvantage is particularly aggravated in the case where non-
homogeneous plasters whose setting time can vary considerably from one square
to the other, are used. Effectively for the same molding time, frequently the
squares from the same casting are not all transportable.
This invention reduces this disadvantage. It avoids the use of any
gripping means on the squares, such as handling tongs or the like, and enables
ejection of squares having very fair mechanical strength.
Thus it lends itself to automation and permits very high molding
shot capacities to be obtained, even by using non-homogeneous plasters, more
particularly of phosphogypsum.
For this purpose there is carried out transport of the whole molds
themselves in which the squares are partially hermetically sealed, freeing of
the lower side of the squares occurring at the time that the set of molds is
raised in the first phase of conveying, owing to the fact that the bottom 11
is fixed.
When the set of molds 1 has been brought to the withdrawal station,
removal occurs by removing the forces which keep the component parts 3 com-
pressed, and by lifting the molds from the loosened squares 7, before return-
ing the molds to the molding area. During this operation, ejection of squares
occurs through the lower part of the mold which remains open in the absence
of bottom 11.
Of course, in the case where the set of molds 2 comprises a plurality
of parallel molds, as represented in Figure 2, compression of component elem~nts
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3 can be achieved by exerting a pressure on both outside walls of the set of
molds, thus effecting deformation of said component parts 3.
It is advisable to specify that the invention is not restricted to
the use of component parts 3 made of elastomers or of materials which are
deformable, incompressible and elastic.
These component parts 3 can be replaced by devices enabling the
same results to be obtained.
By way of example, these component parts can involve a sealed
jacket made of plastic having an interior filled with a non-compressible
fluid or some more with small-sized hard walls.
The invention also includes a component part 3 in which both spacing - -
operations (spacing of its own lding surfaces and spacing of both walls 2)
are independent. Such a component part 3 can be provided by two sealed jackets,
each coupled with a hydraulic or pneumatic device, one of these jackets bear-
ing the molding surface and the other jacket used only to fill the gap between
walls 2 and the plaster piece. The operating of the hydraulic or pneumatic
device is such that:
- for molding, the jacket bearing the molding surface is set under pres-
sure whereas the second jacket is under low pressure;
- for removal, the second jacket is subjected to a pressure whereas the
first jacket is under low pressure.
It is noticeable that this system allows spacings of molding sur-
faces greater than previously obtained.
Lastly, the component part 3 can be composed of a mechanical device
enabling simultaneously spacing of the molding surfaces 4 and of the walls 2
under the effect of pressure, for instance of a hydraulic or pneumatic cylinder.However, at the time of removal, more particularly owing to the fact
of the pressure of a water skin between the flat surfaces respectively opposite
the mold and the molded piece, it happens that this latter separates only on
one side from the mold and clings on the other side onto one wall of this
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latter. Then it is difficult to loosen the piece from this wall of the mold
without damaging the plaster and such that the elastic component part of
which a projection is engaged into the groove or slot it has formedl then
exerts an assymetrical pressure on both flanks or edges of this hollow profile
which, having a reduced thickness, consequently splits or breaks, as observed
on a number of squares.
An alternate method applied to the mold consists in that the mold
walls assembled by means of two deformable component parts are composed of
plates which are flattened under the pressure effect exerted on said walls,
enabling the mold to take its molding position, and which assume a non-flat
shape when the mold is no longer subjected to the effect of these pressures
at the time of removal.
To this latter alternate method of this invention, there is appended
preferably a second means cooperating with the first and enabling avoidance of
an injurious defect of the edges of the side faces having a middle recess of
the plaster molded piece, formed by the deformable component parts when, at
the time of removal, a non-symmetrical pressure is exerted at the time of
relaxation of the deformable component part on edges having a reduced thickness
of the recesses of the side faces of said molded piece. Effectively it is
beneficial to insert a rigid profile piece into the deformable component part
contributing to the forming of side faces having a central recess, without
substantial elasticity the part, which projecting out of the deformable com-
ponent part, has the profile of the recess desired.
An embodiment of this invention comprising these alternate methods
is described hereinafter with the only purpose to illustrate the same. This
description, performed with reference to the accompanying drawings, cannot be
considered as a limitation of the scope of the invention.
In Figure 4 two substantially vertical walls 2' rest on a horizontal
bottom 11, fixed on a framework (not shown) or the floor of a work-shop~ and
equipped wlth desired profiles 12 during the molding operation, are represented
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in their equilibrium state without external stress at the time of stripping.
The curvature of these walls is exaggerated, being in practice hardly detect-
able with the naked eye. Metal parts 9, mounted on the upper edge of walls
2', give the desired shape to the upper dihedrons of the squares; a casting
hole 10 is provided between these parts for enabling the casting of the
tempered plaster which spreads into the squares 7.
In Figure 5 can be seen the walls 2', the molded piece 7, the de-
formable component parts 3 and 3' of which one 3 has a recess whereas in the ;-
other 3' is inserted a rigid piece 13, (for instance made of polished metal
13) whose profile corresponds to that of the groove it is to form; metal
blades 8 are used as thrust for component parts 3 and 3' and maintain the
interval between the walls 2 at the time of removal.
Of course, the separate molds represented can be coupled in series
as has been heretofore described, each wall being common to two adjoining
molds, except those arranged at the ends of the set. The whole of the alter-
nate methods or a part of the same described previously can be embodied in
the improved mold of the present invention.
For performing the plaster molding, the molds are placed on their
bottom 11, and on at least one of the walls 2' there is exerted a pressure
sufficient to bring the initially bent walls 2' to a fairly planar form, their
spacing and their positon being maintained by the bottom 11 and the different
parts 9 and 8. This pressure can be exerted in a fairly homogeneous manner
by a hydraulic or pneumatic transmission on a rigid flat plate (not represented)
having a surface fairly equal to these of walls 3. These walls 2', such as
represented in the figures, are bent. The sagitta corresponding to the great-
est distance between a point of the concavity of the bend and the position of
the same point when the planar shape is obtained, is in the range of 1/1000
of the major dîmension, that is to say, m general, of the flat faces of the
piece to be molded, or in other words for squares having usual sizes from
3/10 to 8/10 of a millimeter. T~ese walls can ~e performed b~ d~spo~ing on an
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iron framework, two sheet iron plates having at least a smooth surface and
separated one from the other by a fillîng having a constant thickness. At
least two of the sides opposite of the framework are bent. By becoming flat
under the effect of the pressure, these walls squash the deformable pieces
and make them spread in the only direction which is not restricted, that is
to say towards the interior of the mold. Then the plaster is cast, and after-
wards when a degree of setting sufficient to make it non-deformable under the
effect of its own weight has occurred, for instance the mold or the set of
molds is carried to the area where the squares have to be dried, and the
pressure is released. The deformable component parts, by returning to their
initial shape, push aside the walls which at the same time bow allowing air
to penetrate between them and the plaster. The rigid inserted part 3 follows
the general shrinkage direction in a translation in the axial plane of the
groove without exerting an assymetrical pressure on either side of this groove
even if one of the vertical walls returns to its equilibrium shape more
rapidly than the other or if a local adherence occurs.
Preferably the thickness of the walls is selected in such a way that
there is sufficient spacing between the squares to effect drying by air circu-
lation propelled by an impeller, this being made without difficulty by a
single vertical displacement of the mold. For instance this spacing is in the
range from 20 to 30 mm for squares of 30 mm in thickness.
