FR3071754A1 - PROCESS FOR THE PREPARATION OF CERAMIC CORES OF FOUNDRY - Google Patents

Abstract

The present invention relates to a method of manufacturing a ceramic core comprising the following successive steps: -a) casting or low-pressure injection into a sacrificial or permanent 3D printed mold of a suspension comprising a ceramic powder, a solvent, a dispersant and a gelling system consisting of either a monomer / crosslinking / catalyst mixture or a gelable polymer; b) gelation of the suspension of step a); c) drying the ceramic gel obtained in step b); -d) demolding or removing the mold so as to obtain a compact green core; -e) debinding the compact green core obtained in step d); -f) sintering of the part obtained in step e) -g) recovery of the ceramic core. It further relates to a suspension for the manufacture of a ceramic core by low-pressure casting or injection, comprising a ceramic powder, a solvent, a dispersant and a gelling system consisting of either a monomer / crosslinking / catalyst mixture or a gelling polymer.

Description

The present invention relates to the field of ceramic foundry cores.

The ceramic cores are generally produced by the ceramic injection molding process (CIM process). This process consists in introducing a masterbatch (feedstock) of ceramic powder and binding agents (thermoplastic polymers or others) in a mold by applying pressure.

However, this technique has certain drawbacks such as the relatively high percentage of polymers in the masterbatch (15-20% by weight) and the proportionately large debinding time inducing significant rejections. The combustion products of most of the binders used (aldehydes, ethers, glycol ethers, ethylene oxide, organic acids, 1,4 dioxane, PAH (polycyclic aromatic hydrocarbons) and BTEX (benzene, toluene, ethyl benzene, xylenes )) present significant risks for the environment and human health.

This large proportion of binders is however necessary for shaping by injection. It generates significant shrinkage and therefore a difficulty in controlling the size of the cores.

In addition, the high development and manufacturing cost of the tools used in the CIM process makes it impossible to manufacture small series of parts competitively.

It is therefore necessary to find a new ecological and economical process for making foundry cores.

The inventors surprisingly realized that it was possible to combine the gel casting process (Gelcasting) and additive manufacturing to manufacture foundry cores of complex geometries while satisfying the objectives:

• reduction of time and deadlines;

• the cost of manufacturing such cores (tool cost, etc.);

• respect for the environment and human health.

Indeed thanks to this new process it is possible to:

• reduce the percentages of binders used and thus rejections;

• better control withdrawal;

• use ecological binder formulations;

• reduce costs and lead times for small series production;

• reduce costs and lead times for tool manufacturing;

• Obtain complex parts to manufacture without the problems associated with demouldability.

The present invention therefore relates to a method for manufacturing a ceramic core, in particular for foundry, comprising the following successive steps:

-a) low pressure casting or injection into a 3D printed sacrificial or permanent mold of a suspension comprising (advantageously consisting essentially of, in particular consisting of) a ceramic powder, a solvent, optionally a dispersant and a gelling system consisting of either a monomer / crosslinking / initiator mixture, ie a gelable polymer;

-b) gelation of the suspension from step a), advantageously by thermal or chemical means;

-c) optional drying of the ceramic gel obtained in step b);

-d) demoulding or elimination of the mold so as to obtain a compact raw core;

e) drying the raw core obtained in step d) if step c) has not been implemented;

-f) debinding of the compact raw core obtained in step d) or e);

-g) sintering of the part obtained in step f)

-h) recovery of the ceramic core.

Advantageously, the pressure of step a) is between 2 × 10 ⁵ and 7 × 10 ⁵ Pa.

The suspension of step a) is therefore prepared from a ceramic powder, a solvent, a dispersant and a gelling system consisting of either a monomer / crosslinking / initiator mixture or a gelling polymer.

The suspension from step a) advantageously has a high content of solid filler, that is to say ceramic particles, in particular between 25 and 70% vol, advantageously between 45 and 70% by volume, relative to the total volume of the suspension, which allows the shrinkage properties of the part to be controlled during drying, in particular for cores with very complex geometries. It also advantageously has a low viscosity, in particular less than 2

Pa.s, more particularly between 0.2 and 0.8 Pa.s, measured at 20 ° C with a shear rate of 100s' ¹ by the viscometer MCR 301 from the company Anton Paar (The acquisition software used is "Rheoplus / 32 V3.40") which makes it easier to fill the molds when injecting the suspension.

The composition of the ceramic powder of the suspension is advantageously a mixture of silica powder, such as fused or vitreous silica, zircon such as zircon flour, and others, such as cristobalite, alumina or zirconia. Examples of ceramic compositions can be found in US patent 5043014. In particular it is a mixture of silica, zircon and cristobalite, particularly in a proportion of 70-80 / 15-25 / 1-5 in mass% , even more particularly of 77/20/3. The silica powder can have different particle sizes, for example a volume average diameter (dv50) of between 10 and 20 μm measured by the laser granulometer of the brand HORIBA, LA-950.

Advantageously, the solvent of step a) is water.

The gelling system is advantageously prepared beforehand and constitutes a "premix" with the solvent, before adding the ceramic powder and the dispersant and optionally an initiator, in particular in the case where the gelling system consists of a monomer / crosslinker / initiator. The dispersant can be dolapix PC 75 (Zschimmer & Schwarz, Germany).

In a first embodiment, the gelling system consists of a monomer / crosslinking / initiator mixture. In this case, advantageously the monomer / crosslinking mixture of the gelling system is chosen from the group consisting of the methacrylamide / N mixture, N'méthylènebisacrylamide (MAM / MMFA) and the 2hydroxyethylmethacrylate mixture / N, N'-methylenebisacrylamide (HEMA / MBAM) , advantageously it is the methacrylamide / N mixture, N'méthylènebisacrylamide.

In the case of the MAM / MMFA mixture, a MAMiMBAM mass ratio ranging from [3: 1] to [10: 1] can be used, advantageously 6: 1. For the HEMA / MMFA mixture, the HEMA: MMFA mass ratio introduced into the premix can be between [20: 1] and [100: 1].

In all cases, water is used as solvent (advantageously in a content of 70 to 90% by weight, in particular 85% by weight, relative to the total weight of the premix). Once the premix has been added to the ceramic powder (advantageously in a content of 50 to 70% by volume of ceramic powder relative to the total volume of the suspension), the suspension is homogenized.

Advantageously, the initiator according to the invention is ammonium persulfate. Advantageously, the initiator is added to the suspension according to the invention shortly before the start of step a), that is to say shortly before the pouring or injection, in particular within 5 minutes preceding step a). Advantageously, the ammonium persulfate content in the suspension according to the invention is approximately 0.20% by weight relative to the total weight of the suspension. The initiator makes it possible to initiate the crosslinking of the monomer after the shaping.

Stage b) of gelation, in particular by thermal means, and the possible stage c) of drying are then implemented, more particularly by placing the mold containing the suspension in an oven which will therefore accelerate the kinetics of gelation then dry the piece.

The drying conditions applied must neither distort nor influence the size of the mold.

Advantageously, the gelling is carried out via the oven or in a water bath by immersion of the previously sealed molds, the temperature being in particular fixed between 50 ° C. and 80 ° C., advantageously 70 ° C. The duration of exposure is advantageously from 20 min to 2 hours in a water bath, more advantageously 1 hour, and from 1 hour to 5 hours in an oven, more advantageously 2 hours. For gelling in an oven, the humidity level is advantageously maximum, between 70 and 95% RH (relative humidity).

The drying of the parts of step c) can be done in the oven by a gradual reduction of the humidity in the climatic chamber: for example loss of 5% RH per slice of 6h to 24h (advantageously 8h), 95% RH to 40% RH, with a temperature set between 30 and 60 ° C, advantageously 45 ° C. The oven is in particular a model SH-642 from ESPEC Corporation.

The MAM / MMFA mixture is more advantageous than the HEMA / MMFA mixture because the oxygen contained in the air can have a detrimental effect on the polymerization reaction of the HEMA / MMFA mixture by inhibiting it, which decreases the quality of the gelation and therefore produce a poorer quality ceramic. In addition, the mechanical resistance of the parts (ceramic cores) manufactured from the MAM / MMFA mixture is better than for those obtained from the HEMA / MMFA mixture. In general, the parts (ceramic cores) manufactured (raw or sintered, that is to say after step d) or step g)) from MAM / MMFA have a smooth appearance, without macroporosity or other apparent defect with good mechanical strength.

In a second embodiment, the gelling system consists of a gelling polymer.

For the purposes of the present invention, the term “gelable polymer” is understood to mean a polymer having an ability to gelate, that is to say to form a gel. Advantageously, the gelable polymer according to the invention is a biopolymer, in particular a biopolymer which can be used in the food industry, advantageously a biopolymer chosen from the group consisting of whole rice flour, starch (in particular rice, wheat or peas), chitosan, egg white and gelatin, advantageously it is chosen from the group consisting of whole rice flour, starch (in particular rice, wheat or peas) and chitosan, even more advantageously it is chosen from the group consisting of whole rice flour and starch (in particular rice, wheat or peas), particularly advantageously it is starch ( in particular rice, wheat or peas), in particular pea starch, more particularly marketed by the company Roquette (France).

Indeed, the compounds used in a gelling system which consists of a monomer / crosslinking / catalyst mixture (first embodiment according to the invention) are generally harmful to the operator or the environment. Although replaced for a few years by less harmful compounds, acrylamide (AM), toxic and reprotoxic molecule, was initially used as a monomer in this type of mixture and still refers today. It is therefore advantageous to be able to use gelling polymers and in particular biopolymers in place of these blends.

An example of a gelable biopolymer which can be used in the context of the present invention is therefore wholegrain rice flour, in particular glutinous rice flour (FRG), more particularly of the variety Oryza sativa, the starch of this rice is characterized by its content. negligible in amylose and in its high amylopectin content. While amylose is a linear polymer, amylopectin has many ramifications favorable to the formation of a gel). It is made up of 75% starch, 10% protein and a small amount of water and fat. This biopolymer is interesting because it has two methods of gelation in aqueous solution. In fact 98% of the starch in the FRG is amylopectin, which swells in hot water (step b) of gelation is therefore advantageously thermally). Thus, the amount of water available for the fluidity of the mixture gradually decreases because it takes part in the constitution of the amylopectin network, forcing the solid particles of ceramic powder according to the invention to agglomerate which results in consolidation of the part (ceramic core). Furthermore, FRG proteins also have a gelling power in hot water. The polypeptide chains can organize themselves to form a 3D network by the formation of hydrogen bonds. Thus, in the case of the use of FRG, the solvent for the suspension is water.

Another advantage of the FRG is that the presence of crosslinker, initiator or catalyst is not essential.

Advantageously, the amounts introduced into the suspension according to the invention are between 1 and 6% by weight of FRG relative to the total weight of the suspension. The FRG therefore allows the preparation of a natural and non-toxic gel.

In order to get rid of “parasitic” compounds, the nature and composition of which can be variable, which whole rice flour contains, it is also possible to use a purified starch, in particular of food origin (for example starch rice, wheat, peas ...). The different starch species do not have the same plant origin which slightly modifies their composition (the amylose / amylopectin ratio is thus higher for pea starch) and therefore their gelling property (lower theoretical gelling temperature for pea starch). The starch granules have a complex structure with alternating amorphous zones (amylose and branching points of the a (1-6) bonds of amylopectin chains) and semi-crystalline zones (mainly amylopectin chains). When the starch suspension is heated to between 60 ° C and 80 ° C, the granules absorb water and swell. The gelatinization of starch involves the solubilization of amylose molecules and the destruction of the crystal regions formed by amylopectin. Cooling the suspension to a temperature below 60 ° C, between 25 ° C and 50 ° C, results in the formation of starch gel. Starch gel results from a reorganization of the amylose and amylopectin chains into a three-dimensional network during cooling. These processes are irreversible. The amount of free water in the suspension gradually decreases, which allows the particles of ceramic powder present in the suspension according to the invention to stick together and, consequently, to form a solid part (ceramic core).

A dispersant, in particular anionic, such as an ammonium salt of polymethacrylic acid (for example Darvan CN sold by the company Vanderbilt) can be introduced into the suspension according to the invention to improve the homogeneity of the mixture containing the starch. . Water advantageously remains used as solvent for the suspension according to the invention. The suspension according to the invention thus obtained can be mixed for several hours (in particular from 2 to 48 hours), for example in a jar using grinding balls, before step b). Advantageously, the content of solid ceramic particle of the suspension according to the invention is in this case between 45% vol and 70% vol relative to the total volume of the suspension. The amount of starch introduced into the suspension according to the invention can vary between 1 to 5% by weight relative to the total weight of the suspension.

Advantageously step b) according to the present invention in the case where the gelling polymer is starch is a gelling by thermal route which consists in maintaining at a temperature between 60 ° C and 90 ° C, in particular during a duration between 1 h and 5 h under a humid atmosphere (for example 50% to 95% relative humidity) in the case of gelling in an oven or in a water bath with waterproof molds, the exposure then being advantageously from 20 min to 2h.

Step c) of drying can take place in an oven controlled in hygrometry, for example carried out at 70 ° C., 70% RH for 1 h to 5 h.

Chitosan is another biopolymer which can be used as a gelable polymer according to the invention in the suspension according to the invention. Indeed, its solubility in an acid medium can be useful for avoiding the use of toxic organic solvents during the process according to the invention. In addition, this type of gel has the advantage of forming at room temperature and can be used for example with molds which have good chemical resistance and, on the other hand, limited temperature resistance.

The amount of chitosan introduced into the premix is advantageously between 1 and 6% by weight for 94 to 99% by weight of acidified water (for example with acetic acid in particular at 1% vol) relative to the total weight of the acidified chitosan-water premix, which corresponds to 0.5 to

3% by weight of chitosan in the suspension according to the invention containing the ceramic powder. The acidified water is therefore the solvent for the suspension according to the invention. Polysorbate 80 (Tween80) can be used as a dispersant in the suspension according to the invention (advantageously in a content <0.5% by weight relative to the total weight of the suspension). It is also advantageous to provide for a degassing step after the addition of polysorbate 80 to the suspension because its presence tends to generate bubbles in the suspension. The content of solid ceramic particles in the suspension according to the invention can be between 25% vol and 60% vol relative to the total volume of the suspension.

Step b) of gelation takes place chemically and then consists of exposure to ammonia-containing vapors at room temperature in order to make the suspension basic. Once gelled, step c) of drying the green part obtained in step b) advantageously takes place at a temperature of 50 ° C. under vacuum to remove the water (the solvent for the suspension according to the invention) .

To obtain the suspension in step a) according to the invention, the mixing can be ensured by the use of a roller mixer, a jar and grinding balls (ball made of zirconium oxide stabilized with yttrium for example). The main advantage of this mixing technique is to ensure good homogeneity of the suspension according to the invention. Working in a closed environment makes it possible to avoid external contamination or evaporation of the solvent, but also to control the temperature.

To ensure proper filling of the mold in its interstices, an air withdrawal system to generate a vacuum in the mold can be developed, as well as a suspension injection system. A machining step is sometimes necessary in order to be able to eliminate these filling devices.

The mold in which the suspension according to the invention is cast or injected is a 3D printed mold, that is to say obtained by additive manufacturing (FA). It can be permanent or sacrificial.

Several additive methods can be used depending on the complexity of the geometry of the core and the desired mold release. The material and minimum thickness of the mold are chosen according to:

• the geometry of the core to be manufactured;

• the additive manufacturing process used;

• temperatures set for gelation and drying of the suspension (the mold must not be deformed during gelation / drying, the Tg of the polymer must be higher than the gelation temperature);

• the release method used.

The mold can be produced by polymerization of a resin under the action of a laser or UV lamp, by projection of drops of material, by projection of a binder on a bed of powder, by solidification of powder under the action an energy source (laser or electron beam), by fusion of wire through a heating nozzle, ... In order to guarantee a correct surface condition, advantageously during additive manufacturing, the thickness strata <0.10 mm.

For each additive process, several mold materials can be used. It may be advantageous to use bio-based materials, if the mold must be removed after gelation (sacrificial mold), for example by chemical, mechanical or thermal means, insofar as the materials available on the market allow the geometry to be developed of the desired mold. Examples of bio-based polymers available on the market are: poly- (lactic acid) PLA, polycarbonate, polycaprolactone, polyamide (PAU, PA12, PPA, ...). Thus, in a particular embodiment, in the case where the mold is sacrificial, the mold is made of biobased polymer, advantageously chosen from poly (lactic acid) (PLA), polycarbonate, polycaprolactone and polyamide.

Advantageously, the choice of FA materials is oriented towards:

• those which have a higher solubility than the others in the solvents not or little harmful for the environment and / or the operator, when the mold is a sacrificial mold and that stage d) consists in the elimination of the mold by way chemical, for example acetonitrile for PLA molds and dimethylsulfoxide (DMSO) for ABS molds;

• those with fragile mechanical properties (brittle) when the mold is a sacrificial mold and that step d) consists in removing the mold mechanically, such as for example an acrylonitrile-butadiene-styrene (ABS) polymer such as the PLASTCure Cast 200 marketed by the company Prodways which allows the printing of molds of small thicknesses which can easily be removed by mechanical means;

• those which are translucent such as acrylic polymers such as VeroClear-RGD810 sold by the company Stratasys (polyjet process which consists in injecting layers of photopolymer liquid curable on a manufacturing tray) which can be fragmented using DMSO (solvent) : to visually see the filling faults in the molds.

• those which expand the least during debinding and sintering of the part such as acrylic polymers such as VeroClear-RGD810 sold by the company Stratasys: chemical and / or mechanical detachment can be followed or supplemented by thermal detachment to eliminate parts and internal residues difficult to access by the solvent (example of the material: the PMMA of the voxeljet).

For the geometries of ceramic cores requiring a significant thickness of the mold to ensure the manufacturability and the rigidity of the structure, a honeycomb type structure between the two internal and external walls of the mold can be produced.

The molds designed by FA must have a surface condition which ensures good wettability with the suspension during step a). They must not interact chemically during all the stages of the process according to the invention (casting (stage a), gelation (stage b), drying (stage c or e), demolding (stage d)) with the suspension according to invention (ie ceramic powder, solvent, dispersant, gelling system). The addition of a wetting agent (for example silicone oil) on the mold before casting is also a solution to facilitate possible demolding.

Step b) according to the invention consists in gelling the suspension of step a), advantageously by thermal or chemical means, in particular by thermal means.

Thanks to this step, the particles of the ceramic powder are immobilized during the crosslinking of the suspension according to the invention in the gelled network.

Advantageously, step b) of gelation takes place thermally, advantageously at a temperature between 50 and 90 ° C., advantageously for a period of between 1 and 10 hours (gelation in an oven) or between 20 min. and 2 hours (gelling in a bain-marie in airtight molds) and in an atmosphere having a humidity rate advantageously between 50 and 95%, in particular in the case where the gelling system is starch.

Step c) of drying makes it possible to obtain the raw core still in the mold. Advantageously, it takes place in an oven controlled in temperature and hygrometry or under vacuum, for example the drying is done in an oven by a progressive reduction of the humidity in the climatic enclosure: such as loss of 5% RH per slice of 6h to 24h (preferably 8h), from 95% RH to 40% RH, with a temperature set between 30 and 60 ° C, advantageously 45 ° C. Drying can also be carried out at low temperature under vacuum by lyophilization, advantageously preceded by a deep-freezing step, for example by immersion in liquid nitrogen, in particular in the case where the mold is a sacrificial mold. The advantage of this method is that step c) of drying and step d) of mold elimination are carried out in a single step by lyophilization in the case of sacrificial molds. Indeed, the mold will become brittle and crack at low temperature which will facilitate after its elimination during step d). The gelling and drying conditions must not deform the mold and therefore cause a certain drift in the size of the core.

Step d) consists of demolding or eliminating the mold so as to obtain a compact raw core.

In a first embodiment, in particular for the complex geometries of the ceramic core, the mold of step a) is a sacrificial mold and step d) consists in eliminating the mold by mechanical, chemical or thermal means.

If the elimination of the mold is carried out chemically in a solvent (preferably for ceramic cores of complex geometries), the choice of the solvent will be directed towards the least toxic solvents. For example, to dissolve a PLA mold, it is possible to use acetonitrile. It is preferable that the thickness of the mold is as small as possible so as to accelerate the kinetics of elimination from the mold while taking account of the issues of manufacturability linked to the additive process chosen and the injection or casting process. Other solvents can be used such as acetone, acetonitrile, ethyl acetate, DMSO (dimethyl sulfoxide), NMP (N-Methyl-2-pyrrolidone), THF (tetrahydrofuran), chloroform, dichloromethane, DMF (dimethylformamide).

Mold making by additive manufacturing uses different processes and materials with mechanical, chemical and thermal properties specific to the printing technique. Dissolution tests of several materials (RGD525, VeroClear, TPU, PA12, Cast200, ABS2800T, ABS3650) available on the market have been carried out in different solvents (Acetone, Acetonitrile, Ethylacetate, DMSO, NMP, THF, Chloroform, DMF, Dichloromethane ). About 100 mg of material are immersed in the solvents with stirring (300 revolutions per min) for 24 h. The percentage of dissolved matter and the external appearance of the part were characterized (Table 1).

Table 1: Study of dissolution of 3D materials available on the 5 market

Solvent RGD525 VeroClear TPU PA12 cast 200 ABS2800T ABS3650U PLA Acetone 0 1 1 0 0 2 3 0 acetonitrile 1 0 1 0 0 2 2 2 Ethylacetate 0 0 1 0 0 2 3 0 DMSO 0 1 4 0 0 3 3 0 l-methyl-2- pyrrolidinone (NMP) 0 3 4 0 0 3 3 NC tetrahydrofuran (THF) 0 3 3 0 1 3 3 NC Chloroform 3 3 4 0 3 2 3 NC dichloromethane 1 4 4 0 4 2 3 NC N, N- dimethylformamide (DMF) 1 3 4 1 0 3 3 NC

: stable: Fragmentation + dissolution ("5% m): Fragmentation + dissolution (" 10% m): Significant degradation

4: Complete dissolution (“100%)

NC: Not known

In the light of these results, certain materials such as PA12 exhibit chemical resistance regardless of the solvent. RGD525 is also insoluble in the majority of non-toxic solvents, only chloroform seems to degrade it enough to envisage a chemical release from a mold of this composition. Cast200 seems to degrade only in chlorinated solvents (chloroform and dichloromethane). For its part, VeroClear is more or less sensitive to all of the solvents tested here. Unfortunately, the less "green" solvents seem to be the most effective. Dichloromethane even allows complete dissolution of the part. TPU seems to degrade and / or dissolve easily in most solvents. Finally, ABS2800T and ABS3650 materials seem to degrade simply regardless of the solvent.

In a second embodiment, in particular for ceramic cores of simple geometry, the mold is a permanent mold and step d) consists of demolding, by techniques well known to those skilled in the art, for example by adding a release agent and using ejectors that facilitate release.

Step e) of drying the raw core obtained in step d) is implemented if step c) has not been implemented. This drying can be carried out in an oven in a controlled atmosphere or by freeze-drying.

In the case of drying in an oven, it is advantageously implemented in a closed room with controlled temperature and humidity. It may for example be study SH-642 from ESPEC Corporation (Japan). The protocol advantageously consists in gradually reducing the humidity level in the enclosure in order to remove the water from the raw parts while avoiding generating cracks (for example from 95% RH to 45% RH, with a rate of descent of - 5% RH / 8h, at 45 ° C).

In the case of freeze-drying, it is possible to use the “FreeZone 4.5” model device from Labconco. The cold generated is 18

50 ° C with a vacuum of 0.018 mBar. In this case, the part is advantageously placed in the freeze dryer for 24 to 48 hours.

Steps f) debinding and g) sintering the part obtained in step

f) are carried out by methods well known to those skilled in the art. These debinding stages f) and sintering g) are sometimes preceded by a step el) of thermal release of the mold if the chemical or mechanical release has not made it possible to completely eliminate the parts enclosed inside the part. The debinding of ceramic is consecutive or contemporary, advantageously consecutive to the thermal release of the mold. Advantageously, the heat treatment is carried out in an oven (VECSTAR LTD, United Kingdom) before the sintering cycle. It consists, for example, of a slow rise in temperature (l ° C / min) and of several stages between 150 ° C and 5400 ° C for 1 to 3 hours each. The bearing temperatures are chosen as a function of the melting and degradation temperatures of the materials which constitute the molds, defined by thermogravimetric analyzes carried out upstream.

The sintering of the raw parts is advantageously carried out in the same oven as that used for thermal release. The sintering cycle advantageously consists of a temperature rise to 5 ° C / min, followed by a plateau at 1250 ° C for 2 h, then a fall to room temperature (10 ° C / min)

The ceramic core is thus recovered in step h).

The present invention further relates to a suspension for the manufacture of ceramic core, in particular foundry, by casting or low pressure injection, advantageously as described above, comprising a ceramic powder, a solvent, a dispersant and a gelling system consisting by either a monomer / crosslinking / initiator mixture or a gelable polymer, advantageously the suspension is as described above.

Whichever gelling system is used, the percentages of binders in the suspensions have been reduced compared to an injection masterbatch and therefore the percentages of rejections. In addition, this low percentage of binders allows better dimensional control during debinding and sintering of cores of very complex geometries.

The present invention will be better understood on reading the examples which are given without limitation.

2 powder mixtures were tested: (Silica particle size 1 (large particles: dv50 = 20 pm measured by the laser granulometer brand HORIBA, LA-950) / Silica particle size 2 (fine particle dv50 = 10 pm measured by the laser particle size analyzer HORIBA brand, LA-950) / Zircon (dv50 = 5 pm measured by the HORIBA brand laser particle size analyzer, LA20 950) / Cristobalite (dv50 = 3 pm measured by the HORIBA brand laser particle size analyzer, LA-950): 65/12 / 20/3 and 58/19/20/3 in percentage by mass These 2 mixtures give similar results.

If a monomer / crosslinking / initiator mixture is used, the suspension is carried out as follows:

The monomers MAM (98%) and the crosslinker MBAM (99.5%) were purchased from Sigma-Aldrich. The mass ratios of monomer / crosslinker and the percentage of water in the premix were fixed as follows:

MAM / MMFA: 6/1

Gelling agent (MAM / MMFA) / Water: 15% / 85%

For the HEMA / MMFA mixture, the mass ratios of monomer / crosslinker and the percentage of water in the premix were fixed as follows:

HEMA / MMFA: 50/1

Gelling agent (HEMA / MMFA) / Water: 15% / 85%

The volume ratio of the suspension is [60 / 39.67 / 0.33] [Ceramic powder / Premix / dispersant], or in equivalent mass ratio of the suspension: [80.2 / 19.6 / 0.2] [ Ceramic powder / Premix / dispersant].

Dolapix PC 75 (Zschimmer & Schwarz, Germany) is used as a dispersant and introduced up to 2 mg / m ² of powder.

Once the premix has been added to the ceramic powder (one of the two mixtures indicated above), the suspension is homogenized using a screwdriver for several days. A “jar-jar-ball-ball” system was favored in the rest of the study for the homogenization of the suspension. It is a closed mixing system which allows the production of a homogeneous slip without evaporation or atmospheric contamination. The jar turner or “roller” used is “Roller 10 digital” from IKA with 10 rollers at variable speed (here set at 35 revolutions per minute) which allows the rotation of cylindrical container of volume 50 to 500 mL. Zirconium silicate beads stabilized with yttrium with a diameter of 5 mm or 7 mm (Procerox beads, EIP, France) were used as a mixing agent or "ball". They allow the suspension to be driven during the rotation of the jar. The composition and mechanical properties of these beads make it possible to limit contamination of the suspension. The quantity (by mass) of ball introduced into the jar is fixed at three times the mass of powder present in the slip.

In the five minutes preceding the pouring, the ammonium persulfate is added to the mixture (0.20% by mass of suspension) in order to be able to initiate the crosslinking after pouring or injection.

The impregnated molds are then placed in a hot environment (oven or water bath) in order to accelerate the kinetics of the gelation by immersion of the previously sealed molds, the temperature being set at 70 ° C. The duration of exposure is 1 hour in a water bath or 2 hours in an oven. For gelling in an oven, the humidity level is maximum, between 70 and 95% RH (relative humidity).

îo The drying of the parts of step c) is done in the oven by a gradual reduction of the humidity in the climatic chamber: loss of 5% RH per 8h period, from 95% RH to 40% RH, with a temperature set at 45 ° C. The oven is a SH-642 model from ESPEC Corporation.

In the case of the use of a gelable polymer, two rice flours, wheat starch, pea starch, chitosan were tested:

- “COMPLETE” (not glutinous) rice flour. The ratios are then 25.5% by mass of premix (13.2% by mass of FRG for 86.8% by mass of water) + 74.3% by mass of ceramic + 0.2% by mass of dolapix PC75 (dispersant) in the suspension. Or about 50 vol% of ceramic charge in suspension.

- “GLUANT” rice flour (Oryza sativa variety). The ratios are then 33% by mass of premix (17% by mass of FRG for 83% by mass of water) + 67% by mass of ceramic (without dispersant) in the suspension, or approximately 40 vol% of ceramic charge in suspension.

- wheat starch: The ratios are then 28.4% by mass of premix (16% by mass of wheat starch for 84% by mass of water) + 70.8% by mass of ceramic + 0.8% mass of dolapix PC75 (dispersant) in the suspension. Or about 47 vol% of ceramic charge in suspension.

- pea starch: The ratios are then 2.2% by mass of pea starch + 74.4% by mass of ceramic + 22.6 mass by water + 0.8% by mass of dolapix PC75 (dispersant) in the suspension, i.e. approximately 49.8 vol% of ceramic charge in suspension.

- chitosan: The ratios are then 26.3% by mass of premix (3% by mass of chitosan + 97% by mass of water acidified to 1% vol by acetic acid) + 73.6% by mass of ceramic + 0, 1% by mass of tween80 (dispersant) in the suspension, i.e. approximately 50 vol% of ceramic charge in suspension

The dispersion and homogenization protocols were the same as for the other gels.

The mixing system used is still: balls and jar.

For gelling polymers, apart from chitosan, gelling step b) is carried out thermally in the same way as for the MAM / MMFA mixture.

For chitosan, step b) of gelation is done chemically by exposure to ammonia vapors at room temperature to make the suspension basic. Once gelled, step c) of drying the green part obtained in step b) takes place at a temperature of 50 ° C. under vacuum to remove the water (the solvent of the suspension according to the invention).

PLA test molds are printed as follows: polylactic acid (PLA) is a fully biodegradable polymer used in food packaging or in the manufacture of a large number of injected, extruded or thermoformed objects. The printer used is a LeapFrog Creatr Dual Extruder (Netherlands), and the design software was that of the printer (Simplifÿ).

The printing parameters used are:

- 3600mm / min (extrusion speed)

- 0.2mm (layer by layer thickness)

- 400 pm (fineness / size of nozzle).

The parts are dried in the oven by a gradual reduction in the humidity in the climatic chamber: loss of 5% RH per 8h period, 95% RH to 40% RH, with a temperature set at 45 ° C.

The suspension obtained with the various gelling agents is poured into a PLA mold. After the gelling and drying phases, the mold containing the raw parts is placed in a container and then immersed in acetonitrile. The solvent is kept under magnetic stirring to accelerate the mold degradation process. The duration of exposure varies according to the size of the mold (from 10 min to 5 h). Once unchecked, the raw parts are collected and then dried.

The following debinding and sintering conditions are then applied:

Steps at 200 ° C / 2h + 400 ° C / 2h + 1250 ° C / 2h; rise ramps from 1 ° C / min to the 400 ° C level, then 5 ° C / min between 400 ° C and 1250 ° C; descent ramp -10 ° C / min.

The parts (ceramic cores) manufactured (raw or sintered, i.e. after step d) or step g)) from MAM / MMFA have a smooth appearance, without macroporosity or other apparent defect with good mechanical strength.

Claims (10)

1. Method for manufacturing a ceramic core comprising the following successive steps: a) casting or low pressure injection into a sacrificial or permanent mold 3D printed with a suspension comprising a ceramic powder, a solvent, optionally a dispersant and a gelling system consisting of either a monomer / crosslinking / initiator mixture or a gelling polymer; b) gelation of the suspension of step a), advantageously by thermal or chemical means; -c) optional drying of the ceramic gel obtained in step b); -d) demoulding or elimination of the mold so as to obtain a compact raw core; e) drying the raw core obtained in step d) if step c) has not been implemented; -f) debinding of the compact raw core obtained in step d) or e); -g) sintering of the part obtained in step f) -h) recovery of the ceramic core. 2. Method according to claim 1, characterized in that the solvent of step a) is water. 3. Method according to any one of claims 1 or 2, characterized in that the solid filler content of the suspension of step a) is between 45 and 70% by volume relative to the total volume of the suspension. 4. Method according to any one of claims 1 to 3, characterized in that the monomer / crosslinking mixture of the gelling system is chosen from the group consisting of the methacrylamide / N, N 'methylenebisacrylamide mixture and the 2-hydroxyethyl methacrylate / mixture N, N ' 5 methylenebisacrylamide, advantageously it is the mixture methacrylamide / N, N 'methylenebisacrylamide. 5. Method according to any one of claims 1 to 3, characterized in that the gelable polymer is a biopolymer, advantageously chosen îo in the group consisting of whole rice flour, starch, chitosan, white egg and gelatin, advantageously it is pea starch. 6. Method according to any one of claims 1 to 5, characterized in 15 that step b) of gelation takes place thermally at a temperature between 50 and 90 ° C, for a period between 1 and 10 hours and in an atmosphere having a humidity level between 50 and 95 % 20 7. Method according to any one of claims 1 to 6, characterized in that the mold of step a) is a sacrificial mold and in that step d) consists in removing the mold mechanically, chemical or thermal. 25 8. Method according to claim 7, characterized in that the mold is made of biobased polymer, advantageously chosen from poly- (lactic acid), polycarbonate, polycaprolactone and polyamide. 9. Method according to any one of claims 7 or 8, characterized in that step c) of drying and step d) of elimination of the mold are carried out in a single step by lyophilization. 5 10. Suspension for the manufacture of a ceramic core by casting or low pressure injection, advantageously according to any one of claims 1 to 9, comprising a ceramic powder, a solvent, a dispersant and a gelling system consisting of either a monomer mixture / crosslinking agent / initiator is a gelable polymer, ίο advantageously the suspension is as defined in any one of claims 2 to 5.