Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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:Etubber sheet structure for use in the production of slabs of granulate or
fine particles of stone
material bonded with hardening resin.
*****
The present invention relates to the production of manufactured slabs composed
of a granulate
or fine particles of stone materical bonded with a hardening resin and, more
speciScally, to an
improvement to the production process therefor.
These slabs are produced using a method in which a mixture composed of
granulate matenal of
selected particle size and synthetic resin is deposited in metered quantities
on a conveyor belt
which is advanced to a forming station (in which it is subjected ta a vacuum
compaction
operation with simailtaneous application of a vibratory movement of
pr.edeteimined frequency)
and subsequently to a station for hardening of the resin (preferably by means
of the action of a
catalyst and/or heat).
Upstream of the forming station, the upper surface of the mixtm deposited on
the conveyor
belt is covered with a sheet or layer of protective materiai, wluch prevents
the pressing ram
from being soiled by the mixture.
In the past, this sheet of protective material was made of paper, the use of
which, however, was
accompanied by cmtain secondary problems subsequent to the actuai production
process, but
nonetheless of considerable importance.
An improvement was subsequently introduced (described and claiimed in Italian
patent
appfication No. TV96A!O00007 filed on 29 January 1996), according to which the
paper layer
or sheet is replaced by a sheet of resilient material, pre&rably rubber.
In this way, upon completion of the resinous binder hardening step, the rubber
sheet can be
removed, by means of tearing, from the surface of the finished slab and
subsequently reused.
In the preferred embodiment, the support on which the mixture is deposited
before being
transferred to the vacawm and vibration compacting station is also protected
by a simdar sheet
of resilient material, in particular rubber.
After the forming step, the mixfure deposited on the support and enclosed
between the two
rubber sheets is in the form of a slab, at the edges of which the two
protective rubber sheets are
joined together by overlapping their respective edges, substantially
completely so as to enclose
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the raw slab which has been formed but not yet subjected to the resinous
binder hardening step.
Since during compaction a portion of the mixture, albeit minimal, inevitably
forms a burr
interposed between the two edges, after the resinous binder hardening step
this burr forms
incrustations on the rubber sheets, which are difficult to remove.
Again in the previously know method, a separating liquid is applied to the two
edges which are
intended to mate, immediately before use on the production line, with the
purpose of
preventing this problem and also of reducing the chemical ageing of the
rubber.
In spite of these measures, however, operations to clean the two rubber
sheets, and in particular
the two mating edges, are stiil necessary.
U.S.Patent 6,773,641 in the name of the same Applicant describes
further improvements aimed at preventing the formation of incrustations on the
rubber sheets,
which require laborious cleaning operations, and involving modifications both
to the shaping of
the protective rubber sheets and to the operating process.
In the first place, the flat configur-ation of the lower sheet is replaced by
a shaped configuration,
comprising a flat base and a peripheral border projecting from said flat base
over a
predetermined height which is smaller by a predetermined amount than the
height of the final
slab to be produced, so that, after the compaction step, a gap of a
pr.edetermined thickness
remains between the peripheral edge of the upper sheet and the top of the
aforementioned
border.
The excess mixture material penetrates into this gap.of predetermined
thickness during the
vacuum vibrati'on compacting step and this material then remains attached to
the edge of the
slab during the hardening step.,
When, after the resinous binder hardening step, the final slab is released by
tearing off the two
rubber sheets, a hardened raw slab is obtained, said slab having periphezally
a border of
2.5 hardened material which can easily be removed during the normal finishing
operations for a slab
of stone material.
U.S. Patent 6,773,641 also describes and claims a specific structure of the
rubber sheet, in particular the lower sheet, to which the peripheral border is
attached, said
border defining the cavity for initial deposition of the metered quantity of
mixture.
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The said structure consists of two or more layers, usually four layers, such
as:
a first layer of rubber (VRF, EPM, PU)
TM
a first cloth or fabric of non-deformable material (Kevlar, polyester, nylon,
aranvde)
a second layer of rubber (EPM, NBR (Nitrile Butadiene Rubber))
a second cloth or fabric of non-deformable material.
Preferably this cloth or fabric is subjected to preliminary treatments, such
as a pretensioning
treatment, to avoid subsequent hysteresis phenomena, a theimal stabilization,
etc.
However, certain requirements remain in connection with the rubber sheets and
must be
satisfied in order to obtain optimum results.
In sbort, these requirements correspond to the following characteristics which
the rubber sheet
nnist have:
a resistance to beat and chemical agents so as to withstand operating
temperatures which
during the resi*i hardening step are in the region of 150 C;
a good resistance to abrasion and tearing;
(c) retain substantially unaltered properties over time;
(d) possess optimal mechanical properties so as to contain the mixture without
undergoing
excessive deformations;
(e) undergo minimal shrinkage after forming so as to prevent the formation of
surface
irregularities in the final product; and
(f) a good heat conductivity in order to favour the hardening of the slab.
- As regards the nature of the rubber, the most suitable are of a polymeric
type resistant to
temperature and to chemical agents, usually not for sulphur vulcanization, in
particular
synthetic rubbers known as EPM (ethylene and propylene copolymers), VKF
(fluorocarbon
rubbers) and PU (polyurethane rubbers).
Even the use of the previously mentioned composite structure, consisting of a
layer of cloth or
fabric interposed in the rubber sheet and then embedded or inset therein, does
not completely
solve the problem
In fact, even if the rubber sheet is in t6is way able to contain the mixhure
during tbe forming
step without undergoing excessive deformation, the final product has surFace
corrugations, as a
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resuh of the shrinkage of the material during the cooling phase and the -
albeit minimal -
residual elasticity of the rubber.
It has now been discovered that all these problems can be completely and
satisfactorily solved
with a rubber sh8et structure of the type comprising two layers of rubber
between which a layer
of non-deformable cloth or fabric is interposed, said structure being
characterized m that the
base of the composite sheet is completed with a second layer of cloth or
fabric consisting of
non-deformable material, which thereby constitutes the outwardly directed
siirface. The second
layer of cloth or fabric also constitutes an outwardly directed surface which
is suitable for being
in contact with the oven surface.
In one aspect, the invention provides a composite structure for use as a
supporting sheet
in a process for production of slabs where a mixture composed of granulate
material of
selected particle size and synthetic resin and deposited in metered quantities
on a
supporting sheet is subjected to a vacuum compaction step with simultaneous
application
of a vibratory motion and to a subsequent hardening step, wherein the
composite
structure consists of a first layer of vulcanized rubber forming a first
outwardly directed
surface, a second layer of vulcanized rubber, a first layer of cloth or fabric
of non-
deformable material interposed between the first and second layer of
vulcanized rubber,
and a second layer of cloth or fabric of non-deformable material provided on
the second
layer of vulcanized rubber and forming another outwardly directed surface
which is the
base of the composite structure, and the composite structure is subjected to a
step of
dimensional stabilization which consists of heating to a temperature in the
region of
160 C.
In one aspect, the invention provides a shaped sheet for the production of
slabs from a
material mixture of granulated stone material and/or sand and by a binding
resin, the
shaped sheet comprising a flat base and a peripheral frame projecting from the
base for a
predetermined height so as to define a seat of dimensions corresponding in
plan to those
of the slab to be produced, wherein the structure of the flat base comprises a
first rubber
layer of previously cured rubber forming a first surface adapted to be in
contact with the
material mixture and a second rubber layer of previously cured rubber, the
first and
second rubber layers having the same structure as regards the homogeneity of
heat
transfer, a first cloth made of one of a non-deformable cloth and a fabric
interposed
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between the first and second rubber layers, and a second cloth made of one of
a non-
deformable cloth and a fabric provided on the second rubber layer, wherein the
second
cloth comprises a second outwardly directed surface of the shaped sheet.
The specific aspects and advantages of the composite structure in accordance
with the present
invention wili appear more clearly from the foIIowing description, with
reference to the
drawings in which:
Figure 1 coincides with Fig. 3 of Italian application No. TV97A000148,
illustrating therefore
the structure of the rubber sheets which was used in the previously mentioned
method;
Figure 2 is a view, similar to that of Figure 1, of the composite structure in
accordance with the
present invention; and
Figure 3 is an enlarged cross-sectional view of the composite structure of the
present invention
corresponding to position A in Fig. 2.
It can be seen that Figure I corresponds to Figure 3 of Italian patent
application No.
TV97A000148, now U.S. Patent 6,773,641.
It can be seen from this figure that the starting m'vcture 130 is deposited in
a cavity defined by a
lower sheet 120 and by an upper or covering sheet 132.
A perimetral border 120A, the inner surface of which has a predetermined
inclination, is
attached to the lower sheet 120.
In Figure 1, the sheet 120 has a composite structure consisting of a lower
sheet of rubber 120
having a flat basis 120B and an inclined peripheral border 120A; a layer 120M
of substantially
inextensible cloth or fabric (such as Kevlar, polyester or nylon) is embedded
and an upper sheet
of nibber 132 which is spaced from the said border 120A through a gap 100 of a
predetermined
thickness along its peripheral edge 132A. The end portion 120E of the slab,
which is enclosed
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between the inclined muface 120C and a vertical cross-section defined by the
fine 120D, is the
scrap material to be removed at a subsequent step of the manufacturing
process. References X
-ind S designate the thickness of the said peripheral border 120A resp. of the
mixture 130.
'With reference now to the composite structure of the present invention, shown
in Figures 2 and
5 :3, using where possible the same references as in Figure 1 and maicing
particular reference to
Figure 3, the sheet 120 comprises two layers of rubber 220A and 220B, between
which the
llayer of inextensible doth or fabric 220M is interposed.
As previously mentioned, the rubber of the layers 220A and B is an EPM rubber
which is
vulcanised by meatis of a peroxide usirig a known method.
'The selected rubber preferably has the following characterisdcs:
:Density 1.05 g/cm3
:Hardness fi4 Shore (A)
Abrasion regstance 86 mm3
:Heat resistance 170 C
'The rubber is worked in such a way to aclueve a maximum controlled planarity
of 0.5 mm per
:linear metre, reaching the overall thickness of 4 mm.
In turn, the cloth or fabric of the layer 220M preferably is of the type
comprising a weft of
polyamide or nylon and a warp of polyester, and is pretensioned in both
directions (both weft
and waip).
As can be seen from Figure 3, a second layer of cloth or fabric 220N having
the same
characteristics as the layer 220M is added underneath the rubber layer 220B.
The composite sheet, thus formed, is preferably subjected to a stabilising
treatment prior to use,
consisting of heating for a few hours (generally from 1 to 3 hours) to a
temperature higher than
the m.aximwm operating temperature, so that, in the present method (in which
resin hardening
takes place at a temperature in the region of 150 C), the stabilising
temperature is in the region
of 160 C (for approximately 2 hours).
The desired results can be obtained with the sheet structure in accordance
with the present
invention.
In particular, it is desirable that both the lower sheet 220 and the upper
sheet should have the
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same struct re, so as to guarantee pf:rfect hobiogeneity of thermal exchange
with the mixture
contained in the "sheath" defined by the two - upper and lower - rubber
sheets.
Moreover, the lower cloth 220N also favours the handling of said sheath
containing the mixture
through the vorious stations of the slab production plant.
In particular, it is worth noting that with the stiucture according to the
present invention the
compressive strength or rather the resistance to extreme loads of the moulding
sheath is
increased, preventing the appearance of wrinides on the surfaces of the
product.
