FR2668094A1 - Mineral compositions for pressing, grading or casting moulds - Google Patents

Abstract

Mineral compositions for moulds intended for the shaping, by pressing, grading and/or casting, of ceramic objects or the like, based on at least one of the plasters of known type for this use, characterised in that, according to a novel application, they include, expressed in parts by weight: Plaster 80 to 97 Reactive agent of formula mCaO,nAl2O3 of 20 to 3 The compositions have the property of conferring upon the moulds excellent resistance to breakage, to mechanical erosion or chemical erosion and an exceptional capability of withstanding repeated thermal shocks.

Description

FIELD OF THE INVENTION
The invention relates to mineral compositions for molds intended for the formation by pressing, sizing and / or casting of ceramic or similar objects.

 The invention also relates to the molds produced by means of the compositions.

BACKGROUND OF THE INVENTION
The molds intended for the formation of ceramic objects, according to the pressing, sizing or casting processes, are most generally produced by mixing special plasters, called "molding plasters". The mineral materials constituting such molds are chosen for their capacity to create, during their implementation, capillary or microporous structures allowing the circulation of liquid and / or gaseous fluids, such as water coming from the ceramic paste used for the forming of the desired object, and the air injected into the mold to allow the separation of the molded object from the mold when it is produced, for example by pressing.

 However, the molds intended for the formation of ceramic objects are subjected, during their industrial exploitation to multiple constraints which are at the origin of rapid and significant deteriorations, leading to their often early reform.

 One of these constraints, which is substantially common to all types of molds (for pressing, sizing or casting), appears under the appearance of an erosion of the mold walls which causes changes in shape and thickness of the molds. walls of the molded object. Such erosion can not only change the aesthetic aspects of the molded object but also generate, during post-molding heat treatments, major incidents, such as cracks, making the object unfit for sale: the mold is therefore unusable.

 Another of these constraints, which relates more particularly to pressing molds, is caused by the application of high pressure to the plastic ceramic paste to be molded, by the male part of the mold and by its transmission to the female part. The frequent repetition of this constraint, in a mass production process, is at the origin of an often early rupture of the female part of the mold, rupture locating in the area at the limit of the side walls and the bottom. This rupture, which is manifested by the presence of cracks or local bursts, produces apparent defects on the molded objects making them unsuitable for sale: the mold is, therefore, reformed.

 Another of these constraints, which is induced by the mineral materials currently used in the production of molds, is the slow, but certain, dissolution of these materials in the aqueous phase of the ceramic paste used to achieve the desired object. This dissolution (associated with the aforementioned erosion) is at the origin of a progressive obturation of the capillary or microporous structures of the mold by effect of saturation and recrystallization "in situ" which requires their early reform.

 However, in the specialized literature, no particular mineral compositions appear for the production of molds (for shaping ceramic objects) which are oriented towards the elimination of the abovementioned constraints.

 On the contrary, the particular mineral compositions described are intended to eliminate other constraints than those set out in connection with the shaping molds mentioned.

 For example, French patent 730 358 describes a mineral composition, a mixture of 5% to 95% of calcium sulphate (of the anhydrite type) and 95% to 5% of a cement (lime, Portland cement or molten cement), intended for the production of shiny surfaces on supports having a great capacity of absorption of water or aspiration.

 Other examples are provided by French patents 1,261,887 and 1,338,701. The first relates to a process for the manufacture of an ultra-hard water-resistant powder product, the composition of which is a dry powder mixture of calcium sulphate semihydrate, a metallurgical cement (molten) containing silica and an aluminum cement this mixture, diluted with water, can be used in the open air or under water. The second relates to a process for the preparation of a synthetic plaster (phosphogypsum semihydrate for example) by mixing with 2% to 10% of a cement (aluminate and / or sili-cate) to eliminate its acid character and the phenomenon of well known bloom.

 Finally, a final example is given by Belgian patent 706,214 which relates to the preparation of molds intended to be used according to the casting process. The filling composition is special because it is formed from calcium hydrate and silicate: it no longer contains gypsum and therefore eliminates the well-known effects of the dissolution of the mold surface on contact with the aqueous phase of the ceramic paste. . The author of this document insists that a composition for the production of a mold comprising a mixture of plaster and cement makes this mold practically unusable because it is insufficiently waterproof, or even absence of porosity preventing its elimination. by aspiration.

 Thus, no solution seems to have been found to eliminate the constraints to which the pressing, sizing and casting molds are subjected, as these constraints were previously stated.

 This is why a first object of the invention aims to produce mineral compositions for pressing, sizing and / or casting molds having the property of resisting the erosion effect of the walls of the molds.

 Another object of the invention is to produce mineral compositions for pressing, sizing and / or casting molds having the property of substantially improving the resistance of said molds to the repeated effect of mechanical stress, such as the pressure in the case for example of pressing molds.

 Another object of the invention is to produce mineral compositions for pressing, sizing, and / or casting molds having the property of providing the molds with a capillary or microporous structure allowing the easy circulation of liquid and / or gaseous fluids, such as water from the ceramic paste and the air injected into the mold to facilitate takeoff of the molded part.

 A final object of the invention aims to create molds with a frankly improved lifetime, performing in industrial production at least twice as many pressing operations as with molds of known art.

SUMMARY OF THE INVENTION
Aware of the aforementioned drawbacks, the applicant has continued, through its research, to develop mineral compositions for molds intended for the formation by pressing, sizing and / or casting, of ceramic or similar objects meeting the objectives which it has assi gned.

According to the invention, the mineral compositions for molds, based on at least one of plasters of the type known for this use, are characterized in that they comprise, expressed in part by weight, according to a new application,
Plaster 80 to 97
Reactive agent of formula
mCaO, nAl203, pZ 20 to 3.

DETAILED DESCRIPTION OF THE INVENTION
By comparison with the prior art, the invention manifests itself as a new application of known means, in particular with regard to Belgian patent 706,214 which states that a composition for producing a mold, in particular for casting , comprising a mixture of plaster and cement is unusable because it makes said mold impermeable to water and air due to an absence of porosity.

Preferably, the mineral compositions for molds according to the invention comprise, expressed in parts by weight
Plaster 88 to 96
Reactive agent of formula
mCaO, nAl203, pZ 12 to 4.

The plaster for mold can be chosen from the products marketed for this purpose, such as for example,
PRESTIAFORM marketed by PRESTIA LAFARCE, MOLDA
PRESSING marketed by LAMBERT, KERADUR marketed by BOERGARDTS, ASDUR marketed by GIULINI, or others.

The reactive agent of formula mCaO, na203, pZ in which
z Z represents S03 and / or NO3 and,
mm can take the values of the interval 1 to 12,
limits included.

nn can take the values of the interval 1 to 7, li-
moths included, and pp can take the values 0 to 3, limits included.

can be introduced into the compositions according to the invention, in the form, for example of calcium aluminates, of calcium sulfoaluminates, of calcium nitroaluminates, of mixtures of sodium aluminate and of lime, used alone or in combination. However, this reactive agent can also be used in more commercial forms, such as for example in the form of an aluminous cement such as FONDU and SECAR 51 from LAFARGE FONDU
INTERNATIONAL, LUMNITE from UNIVERSAL ATLAS, CA 14 from ALCOA, or others.

 In the production of the mold, the implementation of the aforementioned mineral composition is carried out starting from a dispersion of an appropriate weight amount of the mineral phase of said composition represented by (P) expressed in parts by weight in a volume unit of water represented by (E) also expressed in parts by weight leading to the definition of a P / E ratio.

 Thus, in the case of the pressing mold, if it appears desirable to create an identical porosity in the male and female parts of the mold, the P / E ratio is chosen in the range 2.5 / 1 to 3.2 / 1.

But, in the case where it appears beneficial to create different porosities in the male or female parts of the molds, the P / E ratio is chosen
in the range of 2.7 / 1 to 3.2 / 1 for
the female part,
in the range of 2.5 / 1 to 3.0 / 1 for
the male part,
it being understood that the choice made in the abovementioned intervals is in relation, on the one hand with the type of plastic ceramic paste used during pressing and on the other hand with the shape of the part to be produced.

 In the case of the calibration mold, the P / E ratio is chosen in the range 1.4 / 1 to 1.8 / 1 and preferably in the range 1.5 / 1 to 1.6 / 1.

 In the case of the casting mold, the P / E ratio is chosen in the range 1.2 / 1 to 1.6 / 1 and preferably in the range 1.4 / 1 to 1.5 / 1.

 Various adjuvants of known type can also be introduced into the abovementioned compositions, these various adjuvants being, for example, glass fibers, carbon fibers, synthetic fibers, such as polyolefin fibers, polyamides, polyesters, polyacrylics.

 The molds produced by means of the compositions according to the invention are made from aqueous suspensions of said compositions.

 These molds, according to the invention, for the formation of ceramic objects by pressing, sizing and / or casting, are particularly remarkable because of the exceptional qualities which they develop in comparison with the molds of the prior art.

 Among these exceptional qualities, may be more particularly mentioned an excellent resistance to rupture granting a rather exceptional lifetime (three times to five times that of the molds of the prior art).

 Among these qualities, a substantial improvement in the resistance to erosion can also be indicated, allowing the preservation of frank impressions and devoid of early defects.

 Finally, among the qualities, a very good resistance to repeated thermal shocks can also be specified.

 The invention will be better understood thanks to the illustrative description of the examples of implementation of molds produced according to the invention in comparison with a mold produced according to the prior art.

Example 1
- This example illustrates the production and the industrial implementation of a pressing mold from a mineral composition according to the invention.

This mold has two parts, one female (static) and the other male (mobile). Each part is produced according to a process known (Ram Process) to those skilled in the art, by means of an aqueous mineral filling suspension having the following formulation
For the female part, in parts by weight
Plaster 27.9
(PRESTIAFORM from PRESTIA LAGARGE)
Calcium aluminate 2.1
(in the form of cement "FONDU" marketed by
LAFARGE FONDU INTERNATIONAL)
Water 10
P / E 3.0
For the male part, partly by weight
Plaster 26.6
(PRESTIAFORM from PRESTIA LAFARGE)
Calcium aluminate 1.4
(in the form of cement "FONDU" marketed by
LAFARGE FONDU INTERNATIONAL)
Water 10
P / E 3
The two male and female parts are mounted on a VICENTINI hydraulic press with a power of 20 tonnes on the piston.

 The ceramic paste used for pressing the object is of the earthenware type (clay-based) having a water content of 20% by weight.

 The container produced is a hollow oval dish having the dimensions: 270 mm x 180 mm x 175 mm.

 During its pressing operation, the mold was able to produce pressed parts up to around four thousand parts without any defect.

 From this number, the male and female parts began to manifest an initiation of abrasion which increased over time. The mold was reformed to around 6000 pieces produced, due to the abrasion noted.

For comparison, an identical pressing mold was produced from an aqueous mineral dispersion belonging to the prior art and having the following formulation
For the female part, in parts by weight
Plaster 30
(PRESTIAFORM from PRESTIA LAFARGE)
Water 10
P / E 3
For the male part, in parts by weight
Plaster 28
(PRESTIAFORM from PRESTIA LAFARGE)
Water 10
The two male and female parts of the mold thus prepared were mounted on the same hydraulic press of the VICENTINI type, having a power of 20 tonnes at the piston.

 The ceramic paste used for pressing the object is the same as above, that is to say of the earthenware type (clay-based) having a water content of 20%.

 During the exploitation of this mold, the female and male parts revealed, from about the hundredth part, an erosion which became more marked over time. This defect caused the reform of the mold after approximately 1,500 parts produced.

EXAMPLE 2
- This example illustrates the production and industrial implementation of a sizing mold from a mineral composition according to the invention.

This mold is produced by means of an aqueous suspension having the following formulation, expressed in parts by weight
Plaster 13.8
(PRESTIAFORM from PRESTIA LAFARGE)
Calcium aluminate 1,2
(in the form of cement "FONDU" marketed by
LAFARGE FONDU INTERNATIONAL)
Water 10
P / E 1.5
The mold thus prepared was mounted on a machine
ELMETHERM PLANI ROLL with a power of 6 tonnes on the piston.

 The ceramic paste used to produce the ceramic object (which is a plate) is of the earthenware type (clay-based) having a water content of 20% by weight.

 During its operation, the mold was able to produce flawlessly 200 molds an hour with a drying time (in a Tunnel dryer at 40 OC) reduced to 2 hours.

By way of comparison, an identical calibration mold was produced from an aqueous mineral dispersion belonging to the prior art and having the following formulation in parts by weight
Plaster 15
(PRESTIAFORM from PRESTIA LAFARGE)
Water 10
P / E 1.5
The mold thus prepared was mounted on the same calibrating machine and the object produced (plate) was assembled from the same ceramic paste.

 During its operation, this mold was able to produce flawlessly 200 molds an hour, but the drying time (in the Tunnel dryer at 40 OC) was at least 4 hours.

EXAMPLE 3
- This example illustrates the production and the industrial implementation of a casting mold from a mineral composition according to the invention.

This mold is produced by means of an aqueous suspension having the following formulation, in parts by weight
Plaster 14.25
(PRESTIAFORM from PRESTIA LAFARGE)
Calcium aluminate 0.75
(in the form of cement "FONDU" marketed by
LAGARGE FONDU INTERNATIONAL)
Water 10
P / E 1.5
The object to be produced was a terrine of dimensions 150 mm x 110 mm x 70 mm.

 The mold thus prepared was fed by means of a slip of the earthenware type.

During its operation, the mold was able to produce around 200 objects in 50 days.
At the end of this production, the mold showed some erosion and therefore was reformed
For comparison, an identical casting molding was carried out from an aqueous mineral dispersion belonging to the prior art and having the following formulation expressed in parts by weight
Plaster 15
(PRESTIAFORM from PRESTIA LAFARGE)
Water 10
P / E 1.5
The mold thus prepared was put into operation and was fed by means of the same slip as above.

 During its operation for 50 days, the mold could only produce around 100 objects and had to be reformed due to the strong erosion it manifested.

Claims (10)

 1 / Mineral compositions for molds intended for the formation by pressing, sizing and / or casting of ceramic or similar objects, based on at least one of plasters of type known for this use, characterized in that, according to an application new, they contain, expressed in parts by weight  Plaster 80 to 97  Reactive agent of formula  mCaO, nA1203, pZ 20 to 3  2 / Compositions according to claim 1, characterized in that they preferably comprise, expressed in parts by weight  Plaster 88 to 96  Reactive agent of formula  MCaO, nA1203, pZ 12 to 4  3 / Compositions according to any one of claims 1 or 2 characterized in that the reactive agent of formula mCaO, nAl203, pZ is such that Z represents S03 and / or N03, m takes a value from the interval 1 to 12 (limits included), n a value in the interval 1 to 7 (limits included) and p takes a value in the interval 0 to 3 (limits included).  4 / Compositions according to any one of claims 1 to 3 characterized in that the reactive agent of formula m CaO, nA1203, pZ is chosen from the group consisting of calcium aluminates, calcium sulfoaluminates, calcium nitroaluminates , mixtures of sodium aluminate and lime and aluminous cements used alone or in combination.  5 / Compositions according to any one of claims 1 to 4 characterized in that, in the case of the pressing mold, they are used in the form of a dispersion of an appropriate weight amount (P) of the phase mineral, expressed in part by weight, in a unit volume of water (E) according to a weight ratio (P / E) chosen in the range 2.5 / 1 to 3.2 / 1.  6 / Compositions according to claim 5, characterized in that the P / E weight ratio is preferably chosen  - for the female part of the molds in the range 2.7 / 1 and 3.2 / 1  - for the male part of the molds in the range 2.5 / 1 to 3.0 / 1.  7 / Compositions according to any one of claims 1 to 4 characterized in that, in the case of the calibration mold, they are used in the form of a dispersion of an appropriate weight amount (P) of the phase mineral expressed in part by weight, in a unit volume of water (E) according to a P / E weight ratio chosen in the range 1.4 / 1 to 1.8 / 1 and preferably chosen in the range 1.5 / 1 to 1.6 / 1.  8 / Compositions according to any one of claims 1 to 4 characterized in that, in the case of the casting mold, they are used in the form of a dispersion of an appropriate weight amount (P) expressed in part by weight, in a unit volume of water (E), according to the P / E weight ratio chosen in the range 1.2 / 1 to 1.6 / 1 and preferably in the range 1.4 / 1 to 1 , 5/1.  9 / Compositions according to any one of claims 1 to 8 characterized in that they contain adjuvants chosen from the group consisting of glass fibers, carbon fibers and synthetic fibers.  10 / Pressing, sizing and pouring molds produced using the compositions according to claims 1 to 9.