EP0328452A1 - Process for manufacturing ceramic foundry cores - Google Patents

Abstract

Process for the manufacture of foundry cores, in a first stage of production of a pasty mixture employs a ceramic charge based on molten silica, zircon and cristobalite and a binder based on polyethylene glycol of average molecular weight 1500 or 1550, and optional additives, in a second stage, injects the said paste at a temperature of 50 to 100 DEG C into a mould at room temperature and, in a third stage, carries out the heat treatment of the formed core according to a single baking cycle in four stages, namely: (a) temperature rise up to 300 DEG C at a rate of between 30 DEG C and 50 DEG C per hour; (b) rise from 300 DEG C to a maximum temperature of 1200 DEG C or 1250 DEG C at a rate of between 100 DEG C and 200 DEG C per hour; (c) holding at the said maximum temperature for 4 to 5 hours; (d) fast cooling using pulsed air.

Description

The present invention relates to a method for manufacturing ceramic cores for foundries from a thermoplastic paste.

The use of foundry cores of a so-called "ceramic" type is particularly known in certain applications which require obtaining a set of characteristics and strict quality criteria such as resistance to high temperatures, lack of reactivity , dimensional stability and good mechanical characteristics. Among these applications presenting such requirements, mention will be made in particular of aeronautical applications and, for example, the obtaining in the foundry of turbine blades for turbojet engines. The improvement of foundry processes, evolving from equiax foundry to foundry by directed solidification or monocrystalline, has further increased these requirements concerning cores whose use and complexity are imposed by the search for high performance for the parts to be obtained, as is the case for example for hollow blades with internal cooling.

Examples of known composition intended for the preparation of such cores are given by FR-A 2 371 257 and essentially comprise molten silica, zircon flour and cristobalite which is a form of crystallized silica, a silicone resin being used as a binder and additional elements in small quantities such as lubricant and catalyst being added. The preparation process is also described. According to FR-A-2,569,586, the addition of catalyst is avoided by taking advantage in the process of preparation of certain properties of the resin used.

• (a) - montée en température jusqu'à 300°C, à une vitesse comprise entre 30°C et 50°C par heure,
• (b) - montée de la température de 300°C jusqu'à la température maximale, à une vitesse comprise entre 100°C et 200°C par heure,
• (c) - maintien en palier à ladite température maximale, pendant une durée comprise entre 4 et 5 heures,
• (d) - refroidissement rapide par air pulsé,

de manière à assurer, à la fois, l'élimination du liant, une consolidation par frittage du matériau des noyaux et une stabilisation de leur structure par transformation de silice amorphe en cristobalite, la durée totale du cycle de cuisson étant comprise entre 24 et 36 heures.The known prior solutions have not however been entirely satisfactory in certain particular applications of directed or monocrystalline solidification foundry to turbine blades. Improvements have been sought in particular with regard to the surface conditions and a reduction in the roughness of the cores obtained, also with a view to facilitating implementation, by avoiding the presence of odors due to certain products as well as by allowing a calibration operation. cores before cooking and finally, by improving the process for preparing the cores, in particular by reducing the duration of the cooking cycles and simplifying them. The previous solutions have also left, for certain applications, problems of core fragility or insufficient dimensional stability. These problems are solved and improved results are obtained by means of a process for the manufacture of ceramic cores for foundries obtained from a thermoplastic paste consisting of a ceramic filler and at least one organic binder based on polyethylene- glycol characterized in that the polyethylene glycol used has a molar mass of between 1400 and 1600 and in that the paste is injected at a temperature between 50 and 100 ° C in a mold at room temperature and in that said process comprises a single cooking cycle in four sequences:

• (a) - temperature rise up to 300 ° C, at a speed between 30 ° C and 50 ° C per hour,
• (b) - temperature rise from 300 ° C to the maximum temperature, at a speed of between 100 ° C and 200 ° C per hour,
• (c) - leveling off at said maximum temperature, for a period of between 4 and 5 hours,
• (d) - rapid forced air cooling,

so as to ensure, at the same time, the elimination of the binder, a consolidation by sintering of the material of the cores and a stabilization of their structure by transformation of amorphous silica into cristobalite, the total duration of the baking cycle being between 24 and 36 hours.

Depending on the applications, the maximum temperature can be 1200 ° C or 1250 ° C.

Other characteristics and advantages of the invention will be better understood on reading the description which follows of examples of embodiments of the invention.

The mineral filler used in the present invention consists, as known, of a mixture with suitable particle sizes, molten silica (or vitreous), zircon and cristobalite. Good results are obtained by using a filler comprising, for 100 parts by weight, from 60% to 85% by weight of a fused silica itself composed, for 15 to 80% of the weight of the filler, of a silica fused granulometry 0 to 63 micrometers and, for 0 to 60% of the weight of the load, of fused silica granulometry from 0 to 100 micrometers, from 15 to 35% by weight of zircon with granulometry 0 to 50 micrometers and 1% to 5% by weight of cristobalite in the form of a flour which is a fine powder material having a particle size less than 50 micrometers. Preferably, cristobalite is used in the form of fine flour with a particle size of less than 20 micrometers.

The presence of cristobalite, and preferably in very fine particle size, was retained in the compositions according to the invention. It is indeed known that materials containing amorphous (or molten) silica have a low creep resistance. Obtaining foundry cores which can be used at high temperatures consequently requires a transformation of amorphous silica into cristobalite which is the only stable phase of silica between 1470 ° C and 1710 ° C and also the phase which has the best creep resistance , property sought in the use of foundry cores. In the compositions described above in accordance with the invention, the cristobalite originally present acts as an accelerator for devitrification of the silica fused to cristobalite during a rise in temperature. Another remarkable result and important advantage obtained, is that the foundry cores after baking do not undergo any significant dimensional variation when they are brought to operating temperatures of the order of 1500 ° C.

This mineral filler is incorporated, usually in two or three times in a mixer with a molten product constituted by the organic binder which comprises, per 100 parts by weight of mineral filler, from 15 to 20 parts by weight of a polyethylene glycol, the polymer being remarkably and in accordance with the invention, in a form with an average molar mass of between 1400 and 1600, and with a release agent in a proportion of 0.2 to 0.5 parts by weight, preferably consisting calcium stearate. This realization of the mixture constitutes the first step, known per se, of the process for manufacturing ceramic cores for foundries according to the invention.
After mixing, a thermoplastic paste is thus obtained, which can be crushed or ground in order to continue the following stages, of principle known per se, of the preparation of foundry cores.

By way of nonlimiting examples, the composition of thermoplastic pastes used in the process for manufacturing cores according to the invention is given below.

EXAMPLE 1

Thermoplastic paste, for 100 parts by weight of mineral filler composed of:
- 77% of fused silica, with a particle size from 0 to 63 micrometers,
- 20% zircon, grain size 0 to 50 micrometers,
- 3% cristobalite, with a particle size of 2 to 5 micrometers
includes a release agent consisting of:
- 0.5 parts by weight of calcium stearate
and an organic binder consisting of:
- 18 parts by weight of polyethylene glycol with a molecular weight of 1550
- 4.5 parts by weight of ethyl alcohol.

EXAMPLE 2

For 100 parts by weight of mineral filler of the same composition as in Example 1 described above, the paste thermoplastic contains the same amounts of calcium stearate and cethyl alcohol and 20 parts by weight of polyethylene glycol of molar mass 1550.

EXAMPLE 3

The same constituents are kept in the same proportions as in the previous examples 1 and 2, except for the polyethylene glycol of molar mass 1550 which is used in 17 parts by weight and the particle size of the molten silica used is chosen from 0 to 50 micrometers .

EXAMPLE 4

The constituents of the thermoplastic paste differ from those of the previous example 3 only by the molten silica which in this case is provided in two forms:
- 17% of grain size 0 to 50 micrometers
- and 60% of grain size 0 to 100 micrometers.

From these thermoplastic pastes according to the invention, the shaping of the foundry cores calls for known methods, such as thermoplastic injection molding in the press. This injection of the mixture into a mold constitutes the second step in the process for manufacturing cores. Remarkably, according to the invention, the mixture is injected in this case between 50 ° C and 100 ° C in a mold at room temperature, where it solidifies.
The invention also relates to the third step of the improved process for manufacturing foundry cores. Indeed, during this third step, as it is known in principle, a foundry core after shaping must be subjected, before use for casting parts, to a heat treatment.

For this operation, the core can either be placed in a preformed mold or, and it is the preferred mode applied by the present invention, placed in a bed of alumina sand which drowns the core. It may also be desirable to coat the surface of the core with a release agent such as a PTFE type product before introduction into the sand. Note that the cooking method chosen, "in sand", also saves manufacturing time by allowing the charging of a higher number of cores. In all cases, the sand used has properties of good absorbency, vis-à-vis the decomposition products of binders and PTFE.

• - (a) une montée en température jusqu'à 300°C, à une vitesse comprise entre 30°C à 50°C par heure ;
• - (b) une montée en température de 300°C jusqu'à la température maximale, à une vitesse comprise entre 100°C et 200°C par heure ;
• - (c) un maintien en palier à ladite température maximale, pendant une durée comprise entre 4 et 5 heures ;
• - (d) un refroidissement rapide par air pulsé.

Said heat treatment, remarkably in accordance with the invention, consists of a single cooking cycle which comprises four sequences:

• - (a) a temperature rise up to 300 ° C, at a speed between 30 ° C to 50 ° C per hour;
• - (b) a temperature rise from 300 ° C to the maximum temperature, at a speed of between 100 ° C and 200 ° C per hour;
• - (c) leveling off at said maximum temperature, for a period of between 4 and 5 hours;
• - (d) rapid cooling by forced air.

This process makes it possible to ensure uniform evacuation of the binders and sufficient consolidation of the core by sintering, and to obtain good dimensional reproducibility of the cores.
While ensuring the good quality of the results, the baking cycle of foundry cores thus defined has a duration total significantly reduced compared to previously known solutions. The choice of organic binder consisting of polyethylene glycol seems to be a particularly determining factor for obtaining these results. In certain particular applications, using cores of complex shape and for which, taking into account the applications, in particular to turbine blades for high performance turbomachines, strict quality criteria are imposed, the temperature rise, at step (b) of the cooking cycle, for a maximum temperature of 1200 ° C. or 1250 ° C., was thus carried out in 9 hours and the cooling, in stage (d) of the cooking cycle, was carried out in 12 hours, which leads to a total duration of the cooking cycle of 36 hours.

Another remarkable result which has a direct repercussion on the costs of the process by reduction of the times is that the cooking cycle which has just been described is the only cooking applied to said cores. Indeed, this single cycle ensures at the same time the elimination of binders, the consolidation of the material of the cores by sintering and the stabilization of the structure, thanks to the presence of cristobalite.

The cores obtained have interesting properties which have been demonstrated following tests, in particular on test pieces and among which we can note:
- an operating temperature up to 1550 ° C;
- a breaking modulus of 110 kg / cm² at 1100 ° C after 5 minutes and 95 kg / cm² at 1500 ° C after 15 minutes;
- an apparent density of 1.72 and an actual density of 2.4;
- a porosity of 28%;
- thermal expansion at 1000 ° C from 0.13% to 0.16%.

A possible correction of the cores after injection is possible by straightening in a size thanks to the malleability of the thermoplastic pastes according to the invention. This advantage as well as the absence of deformation of the cores during the operations following the shaping seem to be due to the influence of the organic binder consisting of polyethylene glycol. Indeed, this component exhibits progressive solidification properties, without sudden rupture of its viscosity properties between 50 ° C and 100 ° C, unlike the number of binders used previously. The dimensional stability and the absence of creep thus constitute significant advantages of the foundry cores obtained from the thermoplastic pastes used in a manufacturing process according to the invention.

Claims (6)

1. Method for manufacturing ceramic cores for foundries from a thermoplastic paste consisting of a ceramic filler, composed of fused silica, zircon and cristobalite and an organic binder based on a polyethylene glycol and d '' optional additives, comprising, in a first step, producing the mixture to obtain said paste, in a second step, injecting said mixture into a mold and in a third step, heat treatment of the core which has been shaped characterized in that the polyethylene glycol used has a molar mass of between 1400 and 1600 and in that, during the second step, the paste is injected at a temperature between 50 ° C and 100 ° C in a temperature mold ambient, and during the third step, said heat treatment comprises a single cooking cycle in four sequences, namely:
(a) - a rise in temperature up to 300 ° C, at a speed between 30 ° C and 50 ° C per hour;
(b) - a rise in temperature from 300 ° C to the maximum temperature at a speed of between 100 ° C and 200 ° C per hour;
(c) - leveling off at said maximum temperature, for 4 to 5 hours;
(d) - rapid forced air cooling,
so as to ensure both the elimination of the binder, a consolidation by sintering the material of the cores and a stabilization of their structure by transformation of amorphous silica into cristobalite, the total duration of the baking cycle being between 24 and 36 hours. 2. A method of manufacturing foundry cores according to claim 1 wherein the polyethylene glycol used has a molar mass of 1500. 3. A method of manufacturing foundry cores according to claim 1 wherein the polyethylene glycol used has a molar mass of 1550. 4. Method for manufacturing foundry cores according to one of claims 2 or 3 in which the rise time from 300 ° C to the maximum temperature, in sequence (b), of the third step, is 9 hours and the duration of the cooling, in sequence (d) of the third step is 12 hours, the total duration of a cycle being 36 hours. 5. A method of manufacturing foundry cores according to any one of claims 2 to 4 wherein the maximum temperature reached at said sequence (b) and maintained at said sequence (c) of the third step is 1200 ° C. 6. A method of manufacturing foundry cores according to any one of claims 2 to 4 wherein the maximum temperature reached at said sequence (b) and maintained at said sequence (c) of the third step is 1250 ° C.