EP1575745B1 - Method of producing a ceramic article by means of pressure casting - Google Patents

Description

The invention relates to the manufacture of ceramic articles.

• prise lente (formation des pièces),
• démoulage différé (nécessité d'attendre le raffermissement des pièces en moule avant démoulage),
• séchage nécessaire des moules après quelques utilisations,
• courte durée de vie des moules (moins de 150 cycles),
• encombrement important (stock de moules).

The casting under pressure (CSP) of a slip (aqueous suspension of different mineral materials constituting the "formula" of ceramics) is a widespread technique in the traditional ceramics sectors that are the arts of the table and the manufacture of sanitary products . The technique derives from the traditional casting in plaster mold which is the ancestral method used to produce pieces of complex shape. This method of manufacture nevertheless has a number of disadvantages that the casting under pressure partially solves:

• slow grip (forming parts),
• delayed release (need to wait for the firming of mold parts before demolding),
• necessary drying of the molds after a few uses,
• short life of the molds (less than 150 cycles),
• large size (mold stock).

Various documents have attempted to solve these disadvantages in the context of traditional casting.

The document CA 2124863 has a device for mixing two slips of distinct compositions before injection into the mold and a method of manufacturing a ceramic article comprising the step of casting a slip under pressure in a mold to form a deposit.

The document US 5948335 presents an injection process in which the composition of the slip is modified by adding polymers to the slip prior to injection.

Die Casting consists of shaping articles from a slip identical to that used in plaster mold casting. This time, the mold is porous resin and the slip is injected under a pressure ranging from 8 to 40.10 ⁵ Pa approximately. This deposit is made by pressure filtration through the mold of most of the water that was used to initially suspend the various components of the ceramic. Thus the formation of the part is accelerated and as soon as it is formed the mold can be opened to proceed with demolding. As soon as this operation is complete, the mold can be closed again for a new casting cycle. The mold does not require drying, its average life is 20,000 cycles and it is not no need to have more than one or two molds per piece type, which greatly reduces the bulk of the workshop.

The pressure casting cycles depend to a large extent on the rheological characteristics of the slip. These characteristics can be regulated by additives called deflocculants whose action can be purely electrostatic, purely steric or electro-steric. The characteristics of the slip must allow a casting cycle as fast as possible while preserving good mechanical behavior of the part after pouring. This means that the freshly formed part must be strong enough to undergo the various handling required by demolding and finishing. These constraints lead most of the time to adjust the slips identically for the CSP and for conventional casting, while the CSP would allow even better yields if the slips were under-deflocculated. Unfortunately, the use of such slip, if it allows a speed of forming parts faster, leads to a poor firming of the ceramic in the mold and an irreversible deformation of the parts during demolding.

It should be noted at this point that the structure and rate of formation of the deposit during die casting are the results of two types of mechanisms depending on the degree of deflocculation of the slurry in suspension.

In the deflocculated suspensions, the repulsive forces between the mineral particles are high and the particles can move independently of each other. They will therefore be able to settle individually and rearrange into a denser deposit (high relative density, low porosity), incompressible and homogeneous. However, because of the high degree of compactness of the deposit, casting speeds are low.

In the flocculated suspensions, the attraction forces are strong and the particles will move and settle by agglomerates. The deposit thus formed will be less dense (high porosity), compressible (rearrangement of particles under the action of pressure) and heterogeneous. On the other hand, the casting speeds will in this case be higher because of a higher porosity.

On the other hand, all the slips do not have the same casting behavior. The mineralogical nature of the constituents plays a very important role with respect to the rheological characteristics. For simplicity, the slips made from kaolin (such as porcelain or vitreous) "flow well" in the traditional casting sense, which means that their deflocculation is easy and that the setting speeds obtained are high. On the other hand, clay-based slips (such as earthenware or sandstone) do not flow well, which means that they are difficult to deflocculate and the resulting setting speeds are poor. This is why the vast majority of products made in the field of traditional die-casting ceramics are porcelains and vitreous. The producers of faience and sandstone can not generally access this technology due to the poor intrinsic rheological characteristics of their suspensions.

The objective of the user is to accelerate the speed of formation of the deposit in order to increase the profitability of the machine. However, this acceleration is limited by the capacity of the deposit formed to eliminate the residual water and thus allow the gripping of the article during the opening of the mold. This means that if the slip is "set" so that the rate of formation of the deposit is as fast as possible, the article can not be demolded without being deformed because it behaves like a thixotropic solid.

An object of the invention is to accelerate the manufacture by die casting without threatening the mechanical strength of the part from demoulding. Another object of the invention may be to allow the manufacture of traditional ceramic articles by die casting.

• couler une barbotine sous pression dans un moule pour former un dépôt ; et
• filtrer sur le dépôt une solution contenant un défloculant.

For this purpose, there is provided according to the invention a method of manufacturing a ceramic article, comprising the steps of:

• casting a slip under pressure in a mold to form a deposit; and
• filter on the deposit a solution containing a deflocculant.

Thus, the filtering step makes it possible to compact the relatively sparse deposit formed in the previous step. This post-treatment consists in passing through the deposit a solution containing the deflocculant. It can be assumed that during this post-filtration process, the deflocculant molecules will be able to adsorb to the surface of the particles and thus increase the repulsion forces. The particles will then be able to "move" and rearrange into a denser deposit with the help of pressure (higher mechanical strength of the green part). The piece then has mechanical characteristics suitable for its demolding and finishing.

• la barbotine est floculée,
• la barbotine comprend du kaolin,
• la barbotine comprend de l'argile,
• la barbotine comprend du quartz,
• le défloculant représente au maximum 3% en masse de l'article,
• le défloculant représente au maximum 5% en masse de la solution,
• le défloculant représente entre 0,20% et 3% en masse de la solution .

The method according to the invention may furthermore have at least one of the following characteristics:

• the slip is flocculated,
• the slip includes kaolin,
• the slip includes clay,
• the slip includes quartz,
• the deflocculant represents a maximum of 3 % by mass of the article,
• the deflocculant represents a maximum of 5% by weight of the solution,
• the deflocculant represents between 0.20% and 3% by weight of the solution.

• la figure 1 est une vue schématique d'un mode de réalisation du dispositif de l'invention,
• les figures 2 et 3 sont deux vues schématiques de la structure de l'article à l'échelle microscopique à l'issue respectivement de la première étape et de la deuxième étape du procédé de l'invention,
• la figure 4 présente des courbes illustrant pour différentes compositions de la solution de filtrage l'évolution de la masse du filtrat en fonction du temps,
• la figure 5 est une courbe illustrant la résistance spécifique du produit intermédiaire en fonction de sa concentration en défloculant,
• la figure 6 est une vue en coupe de l'article obtenu au moyen de l'invention, et
• la figure 7 est une vue en coupe plus précise du moule de la figure 1.

Other characteristics and advantages of the invention will emerge during the following description, in particular presenting a preferred embodiment by way of non-limiting example. In the accompanying drawings:

• the figure 1 is a schematic view of an embodiment of the device of the invention,
• the Figures 2 and 3 are two schematic views of the structure of the article at the microscopic scale at the end of the first step and the second step respectively of the method of the invention,
• the figure 4 presents curves illustrating, for different compositions of the filtering solution, the evolution of the mass of the filtrate as a function of time,
• the figure 5 is a curve illustrating the specific resistance of the intermediate product as a function of its deflocculant concentration,
• the figure 6 is a sectional view of the article obtained by means of the invention, and
• the figure 7 is a more precise sectional view of the mold of the figure 1 .

The manufacturing device 2 is schematically illustrated in FIG. figure 1 .

It comprises two tanks 4 and 6. The tank 4 is able to receive a slip 8 while the tank 6 is able to receive a filtration solution containing a deflocculant.

The device comprises a mold 12 of die casting of a conventional type which may have a horizontal or vertical joint plane. It also comprises means 14 capable of injecting under pressure into the mold 12 in turn the slip 8 and the solution 10. These means may be formed by two independent injectors respectively assigned to the injection of the slip 8 and the solution 10, following two separate circuits upstream.

The device comprises means 16 for purging or cleaning the downstream circuit connecting the injection means of the solution to the mold 12.

We have illustrated more precisely at the figure 7 the mold 12 of the figure 1 . The mold 12 comprises two high and low end portions 13 and 13. Each of the upper and lower parts has an internal chamber in which a supply duct coming from the outside of the mold opens out and forms the injection means 14. upper part 13 has a cavity 33 and the lower part has an advance 25 able to enter the cavity 33 when the two mold parts are assembled in a male-female connection. In this position illustrated in figure 7 the step 25 occupies only a part of the cavity 33 so that the remainder of the cavity 33 forms the molding chamber for the formation of the workpiece 30 to be molded.

The portions of the upper and lower parts contiguous to the enclosure are made of porous materials. Several ducts 29 are formed in each of the upper and lower parts. The conduits 29 are rectilinear, parallel to each other and separated from each other by identical intervals. They extend in the direction 37 in which the two upper and lower parts are movable relative to each other to allow the mold to extract the formed part. In both upper and lower parts, the ducts 29 extend to the right of the cavity 33, without however reaching it so that they are blind. The ducts of the lower part 15 penetrate into the projection 25. In each of the upper and lower parts, the ducts 29 connect the main supply duct to the core of the porous material. The mold 12 further comprises a lateral duct 39 extending in one of the two upper and lower parts, for example the upper part 13, from the outside of the latter opening directly into the chamber 33.

In the present embodiment of the invention, the slip 8 is injected under pressure into the mold 12 to form a deposit 20, and then the solution 10 is injected into the mold.

et une aire spécifique BET telle que a_BET = 6,9 m²/g. La phase solide représente en masse 70% de la barbotine. La barbotine a une densité de 1,77. La solution acqueuse comprend en très petite quantité le défloculant commercialisé par la firme Zschimmer et Schwartz sous l'appellation PC 67, de sorte que la suspension est considérée comme sous-défloculée. En l'espèce, le floculant représente en masse 0,06% de la barbotine.In the first step, the casting is carried out under a pressure of 20 × 10 ⁵ Pa. The slip 8 comprises a powder suspended in an aqueous solution. The powder consists here of 50% of kaolin and 50% of quartz. It has a median grain diameter such that ${\mathrm{d}}_{\mathrm{BOY\; WUT}}^{\mathrm{50}}\mathrm{=}\underset{\mathrm{}}{\mathrm{7}}\mathrm{microns}$

and a BET specific surface area such that _BET = 6.9 m ² / g. The solid phase represents in mass 70% of the slip. The slip has a density of 1.77. The aqueous solution comprises in very small quantities the deflocculant marketed by the firm Zschimmer and Schwartz under the name PC 67, so that the suspension is considered as under-deflocculated. In this case, the flocculant represents 0.06% of the mass of the slip.

The slip is injected in this case via the lateral duct 39, the water evacuating through the porous material then the ducts 29.

The injection of this slip makes it possible to obtain a relatively low density deposit after the evacuation of a portion of the water through the wall of the mold.

In the second step, the injection is also carried out under a pressure of 20 × 10 ⁵ Pa. Solution 10 is an aqueous solution of deflocculant PC 67 representing by mass between 0.10 and 4.70% of the solution (for example up to 1% by weight of the final article 30). The solution 10 is injected here from the lateral channel 39. During this second step, the solution passes through the deposit 20 and the water escapes through the wall of the mold and then the vertical ducts 29.

At the end of a suitable period, the mold is opened and water and compressed air are injected to take off the part made with respect to the two mold parts. This injection takes place via the conduits 29. The intermediate product 20 is removed for finishing in a manner known per se (cooking, etc.) to obtain the article 30 of the figure 6 .

Different mass concentrations (mass of deflocculant / total mass of the solution 10) were tested, ranging between 0 and 4.70% (ie 0 to 1% relative to the mass of solid). For each test, the kinetics of filtration were characterized (measurement of the mass of filtrate collected over time, calculation of the specific resistance, that is to say of the resistance to the passage of water) as well as the structure the deposit obtained (porosity, pore diameter, mechanical strength).

The figure 4 presents the kinetics of filtration of the deflocculant solutions whose concentration varies from 0 to 4.70% through the deposition 20. Two behaviors can be observed.

In the absence of deflocculant, the filtrate passes through the deposit very quickly. There is no time-out before the start of the filtrate flow.

In the presence of deflocculant, the flow of the filtrate through the entire deposit is only effective after 114, 169, 222 and 128 s for mass deflocculant concentrations of 0.10, 0.45, 0, 65 and 4.70% respectively. This dead time therefore increases with the concentration of deflocculant except for the highest concentration.

The variation in the flow rate of the filtrate during the filtration of deflocculant solutions whose concentration varies from 0 to 4.70% through the deposition was also studied. For times longer than the dead time, the flow rate of the filtrate is time independent but varies slightly with the deflocculant concentration.

The figure 5 presents the specific resistance of the deposits after and before treatment. It clearly appears that the resistance of the deposits after treatment (curve in solid line) is 2 times higher than that obtained by simple casting (curve in dashed line). This result shows that the porous structure of the deposit has been modified (rearrangement of the particles into a denser structure).

Flexural strength tests were also carried out on the deposits after treatment and drying. The results are shown in the table below. C _D indicates the deflocculant concentration as a percentage by mass in the solution and σ is the 3-point bending stress in MPa. Sample 1 2 3 C _D (% m) 0 0.65 4.70 σ (Mpa) 0.6 0.7 1.3

It appears that the deposit obtained after filtration of a solution containing 4.70% deflocculant has a mechanical strength 2 times higher than that obtained for the other deposits. This significant variation in mechanical strength shows that the structure of the deposit has been modified.

• L'écoulement de la solution 10 contenant le défloculant n'est effectif qu'au bout de 100 à 200 s contrairement à l'eau pour laquelle l'écoulement se fait sans temps mort. Ce résultat montre qu'en présence de défloculant, le passage de la solution entraîne des modifications de la structure poreuse avant qu'elle puisse s'écouler à travers toute l'épaisseur du dépôt. Avec l'eau seule, l'écoulement à travers l'épaisseur du dépôt ne provoque pas de modification de la structure puisque son écoulement se fait sans temps mort.

The post-filtration tests highlight the following points:

• The flow of the solution 10 containing the deflocculant is effective only after 100 to 200 s unlike the water for which the flow is done without a dead time. This result shows that in the presence of deflocculant, the passage of the solution causes modifications of the porous structure before it can flow through the entire thickness of the deposit. With the water alone, the flow through the thickness of the deposit does not cause a modification of the structure since its flow is without a dead time.

After treatment, the specific resistance of the deposit is increased as well as its mechanical strength. This increase shows that the flow of a deflocculant solution through a low density deposit allows the rearrangement of the particles into a more compact structure (more mechanically resistant and more homogeneous).

This post-filtration method therefore makes it possible to obtain a denser deposit by rearranging the particles during this process.

The compaction mechanism of the deposit may be the following. The figure 2 illustrates the deposition after pouring of the flocculated suspension 8. The large quartz particles 22 whose zero charge point is towards a pH of 2 are negatively charged in the suspension where the pH is around 7-8. These particles will therefore repel by electrostatic repulsion. On the other hand, the small particles of kaolin 24 have a point of zero load towards 8-9. These uncharged particles in the suspension will therefore agglomerate together and around the quartz particles 22 of the makes attractive forces of Van der Walls and electrostatic respectively. The deposit obtained from the casting of this suspension will therefore be formed of blocks of quartz particles surrounded by kaolin with a large porosity allowing rapid flow of the liquid phase.

The figure 3 illustrates the deposition after injection of the solution 10. The deflocculant contained in the solution will be able to be absorbed on the kaolin particles 24 and charge negatively. The deflocculant is negatively charged (COO group ^- in the case of a polyacrylate). These particles will then be able to repel by repulsive forces (electro-steric) and thus individually rearrange in a denser deposit (lower porosity and higher mechanical resistance).

In these two figures, the arrows 26 represent the flow of the filtrate.

Pouring a slip, preferably flocculated, followed by a post-treatment to reorganize the deposition of particles to make it suitable for the rest of the manufacturing process opens up some prospects for CSP.

Indeed, it improves the production yield of products made with slip "that flow well" (porcelain, vitréous). It is thus possible to significantly reduce the duration of the pouring cycle by injecting a flocculated slip and then carrying out deflocculation in situ by post-treatment.

A second application of the invention is to apply the principle to slip "that does not flow well" such as earthenware and sandstone or other slip containing a high proportion of clay. The method described here makes it possible to use such slips in CSP whereas this was not the case previously in the design of the process and the machines.

• Le système 14 de pompage et de distribution sous pression devra de préférence être capable de véhiculer des suspensions de viscosité supérieure aux viscosités couramment utilisées dans l'art antérieur,

The application of the invention to prior die casting machines is relatively simple:

• The system 14 for pumping and dispensing under pressure should preferably be capable of conveying suspensions of viscosity higher than the viscosities commonly used in the prior art,

The purge of the system carrying the deflocculant solution must be complete before the new injection of slip so as not to produce an involuntary defloculation of the latter. Thus, after injection of the solution 10 in post-treatment, the purge means 16 is activated to clean the portion of circuit to be borrowed by the solution 8 during the next cycle.

Of course, we can bring to the invention many changes without departing from the scope thereof.

The invention is applicable to any type of ceramic. It will thus be applicable to traditional clay ceramics used for the arts of the table or sanitary. It will also be applicable to technical ceramics (such as those based on silicon nitride or silicon carbide) for example for the manufacture of electronic component supports or refractory materials.

Claims (8)

1. Method of producing a ceramic article (30), comprising the steps consisting in:

   - casting a slip (8) under pressure into a mold (12) in order to form a deposit (20); and

   - filtering a solution (10) containing a deflocculant through the deposit (20).
2. Method according to Claim 1, characterized in that the slip (10) is flocculated.
3. Method according to either of Claims 1 and 2, characterized in that the slip (10) comprises kaolin.
4. Method according to any one of Claims 1 to 3, characterized in that the slip comprises clay.
5. Method according to any one of Claims 1 to 4, characterized in that the slip (10) comprises quartz.
6. Method according to any one of Claims 1 to 5, characterized in that the deflocculant represents at most 3% by weight of the article (30).
7. Method according to any one of Claims 1 to 6, characterized in that the deflocculant represents at most 5% by weight of the solution (10).
8. Method according to any one of Claims 1 to 7, characterized in that the deflocculant represents between 0.20% and 3% by weight of the solution (10).

Images