Process for producing a ceramic casting core

Abstract

A production method for a ceramic casting core (20) in which one manufactures the core (20) by machining by mechanical removal of material from a fired ceramic material block (1), the machining operation comprising at least a first machining step to realize a first machined surface (6, 7) in the material block (1), and a second machining step to realize a second machined surface (9) in the material block (1), substantially opposite to the first machined surface (6). Prior to the second machining step, applying a reinforcement layer (8), made of a stiffening solution to protect the material block (1) from breaking during the second machining step, on at least part or the entire first machined surface (6, 7).

Claims

1. A process for producing a ceramic casting core (20) for the manufacture of a hollow part with a complex cavity by lost-wax casting, and the core (20) being an image of the complex cavity of the hollow part to be produced, the method comprising: manufacturing the core (20) by machining a fired ceramic material block (1) with the machining being performed by mechanical removal of material via a cutting tool, and the machining comprises at least a first machining step to realize a first machined surface (6, 7) in the material block (1) and a second machining step to realize a second machined surface (9) in the material block (1), substantially opposite to the first machined surface (6, 7), prior to the second machining step, applying a reinforcement layer (8, 11), made of a stiffening solution to protect the material block (1) from breaking during the second machining step, on at least part of the first machined surface (6, 7), and waiting for solidification of the reinforcement layer (8, 11) before carrying out the second machining step, and after the machining of the core (20), removing the reinforcement layer(s) (8, 11).

2. The production process according to claim 1, further comprising using several machining steps during machining and repeating application of the reinforcement layer (8, 11) before every new machining step on at least part of a surface of the material block (1) substantially opposite to the new surface to be machined.

3. The production process according to claim 1, further comprising cleaning and degreasing the material block (1), prior to the application of the reinforcement layer (8, 11), to improve adhesion of the stiffening solution thereto.

4. The production process according to claim 1, further comprising using a liquid or a semi-liquid machining glue having machinable and dissolvable properties as the stiffening solution.

5. The production process according to claim 4, further comprising applying the reinforcement layer (8, 11) during one or more applications of the stiffening solution.

6. The production process according to claim 4, further comprising applying the stiffening solution on the material block (1) with a brush.

7. The production process according to claim 4, further comprising applying the stiffening solution on the material block (1) by pouring and gravity.

8. The production process according to claim 1, further comprising using a numerically controlled multi-axis machining center to machine the core (20) in the material block (1).

9. The production process according to claim 1, further comprising using diamond cutting tools for machining the core (20) in the material block (1).

10. The production process according to claim 1, further comprising, to machine the core (20) in the material block (1), using a material block (1) comprising at least two parallel opposite sides arranged to form two clamping faces (4) on which jaws (2) of a clamping vise (3) of a machining equipment are applied.

11. The production process according to claim 1, further comprising, prior to the machining of the core (20), machining at least one reference surface (5) in the material block (1) that will allow removing and putting the material block (1) back in place on a machining equipment while respecting parallelism deviation lower than 0.00196 inches (0.05 mm).

12. The production process according to claim 1, further comprising dipping the core (20) in a solvent bath in order to remove the reinforcement layer(s) (8, 11).

13. The production process according to claim 1, further comprising, to remove the reinforcement layer(s) (8, 11), subjecting the core (20) to a temperature rise up to at least a melting temperature of the stiffening solution.

14. The production process according to claim 13, further comprising suspending the core (20) on a bracket to allow draining of the stiffening solution by gravity.

15. The production process according to claim 13, further comprising producing one of a rotor, a stator for a gas turbine, an aircraft engine, a reactor, a combustion chamber or the like as the hollow part with the complex cavity.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention and its advantages will be better revealed in the following description of an embodiment given as a non limiting example, in reference to the drawings in appendix, in which FIGS. 1 to 4 represent schematically front views of several steps of a production process of a ceramic core according to the invention, in which FIG. 1 illustrates the mounting of a ceramic block between two clamping jaws of a machining equipment to machine a first side of a blank of said core, FIG. 2 illustrates the application of a stiffening solution on the first machined side of the blank, FIG. 3 illustrates the machining of a second side of the blank of said core, located opposite to the first machined and stiffened side, and FIG. 4 illustrates the removal of the stiffening solution after machining the second side of the blank.

ILLUSTRATIONS OF THE INVENTION AND BEST WAY OF REALIZING IT

(2) The method for producing a ceramic core 10 out of a ceramic material of the like according to the invention takes place by mechanical machining of said core directly in the mass of a machinable technical ceramic block intended for investment casting, machining being performed by material removal using one or several cutting tools on a traditional machining equipment. This machining equipment can be for example a numerically controlled multi-axis machining center that allows realizing a plurality of simple up to very complex shapes. Of course, any mechanical machining equipment can be suitable. In the embodiment example described below, one used a five-axis milling center which allows machining complex shapes, which are very current in ceramic cores. There are of course machining centers specifically equipped for machining ceramics and which allow increasing productivity, but their cost cannot always be amortized.

(3) More specifically and referring to FIG. 1, the production process comprises a mounting step of a ceramic block 1 between two jaws 2 of a clamping vise 3 of a machining equipment (not represented) in the direction of arrows F. Ceramic block 1 is a machinable technical ceramic blank, that is to say a fired ceramic block having for example a hardness equivalent or comparable to that of fiberglass reinforced composite material. This ceramic block 1 can have a parallelepipedic shape as illustrated, or any other shape according to the general shape of core 20 to be machined, such as for example a polyhedron, a cylinder. The positioning and indexing of ceramic block 1 on the machining equipment are important to ensure the accuracy of the various machining steps, whatever the number of times said block is removed and put back in place. So, if ceramic block 1 is parallelepipedic, it must have two opposite and parallel clamping sides 4 with a parallelism deviation of for example no more than 0.1 mm. Clamping height h of the two jaws 2 on clamping sides 4 of ceramic block 1 must be minimal, but sufficient to ensure the immobilization of ceramic block 1 and for example equal to at least mm for a block height lower or equal to 30 mm and, beyond this height, equal to at least 10% of the height of said block. Height H of the two jaws 2 must be important and at least equal to 70 mm to facilitate the access of the machining tools to the different sides of ceramic block 1, and in particular to its lower side. The clamping of ceramic block 1 must be controlled to apply a low but sufficient clamping three, for example between 1 kN and 5 kN. One will use to that purpose a torque wrench to tighten the two jaws 2 according to arrows F. The values stated above are given as examples and have no limiting effect. Likewise, the way of mounting ceramic block 1 on a machining equipment can vary according to the shape of said block. For example, if it is cylindrical, one will use a cylindrical clamping chuck and the peripheral base of said block can be used as a reference surface.

(4) One starts machining ceramic block 1 by making a reference surface 5 that will allow removing and putting back in place ceramic block 1 with an accuracy of at the most 0.05 mm. In the illustrated example, one can choose at least the lower side and one of clamping sides 4 of ceramic block 1 as reference surface 5, which has the advantage of remaining accessible and available up to the last step of the machining process. One can then carry out a first machining step on a first part of ceramic block 1 to make a first machined surface 6 (see FIG. 2).

(5) Referring to FIG. 2, this first machined surface 6 has been made on the left side (on the figure) of ceramic block 1 by removing the corresponding angle of the block and in particular by creating cavities 7. After this first machining step and before carrying out the next machining step, one will stiffen machined surface 6 by applying a stiffening solution to form a reinforcement layer 8 and one will wait for the solidification of this reinforcement layer 8 before starting the second machining step. Prior to this application, ceramic block 1 must preferably be cleaned and degreased to free it from dust and machining oil and thus allow the adhesion of the stiffening solution on the surface of ceramic block 1. For this cleaning phase, one can use an automatic washing device adapted to avoid any damage to the ceramic. One then applies the stiffening solution at least on first machined surface 6, taking care to fill recesses 7. This stiffening solution, which is preferably a machining glue, can be applied by any suitable means in one or several applications. The thickness of reinforcement layer 8 obtained must be at least equal to 2 mm to achieve the expected stiffening effect. One can apply the stiffening solution when it is in liquid state by means of a brush or by gravity, pouring it from a determined, not too great height, in the order of some centimeters, from a container containing a sufficient quantity of solution. This technique for applying a stiffening solution in liquid state is the most suitable for filling recesses 7 with a depth exceeding 2 mm. Of course, any other application method may be suitable according to the geometry of machined surface 6 to be stiffened and according to the fluidity of the stiffening solution, it must be possible to clean the stiffening solution to allow removing it from ceramic block 1 after machining, if it does not have this property, its residues shall not make the use or the functions of the obtained ceramic core impossible. It must also keep its stiffening properties up to a temperature at least equal to 50 C., which corresponds to the temperature raise undergone by ceramic block 1 during machining, even with coolant.

(6) Suitable stiffening solutions are for example existing machining glues such as the adhesive pastes marketed under the names Araldite 2011 and Araldite 2012, the machining glue marketed under the name Rigidax by the Paramelt company, or any other stiffening solution in paste or semi-fluid form, adhesive or not, having the following specific characteristics: it must be machinable and dissolvable without causing the dissolution of the ceramic it is applied on. The existing solvents that allow dissolving these machining glues, adhesive pastes or any other stiffening solution can be for example a universal stripper marketed under the name Syntilor Chrono 10, a gelled aerosol stripper marketed under the reference 1310, a foaming stripper marketed under the name Sansil, etc. These examples are of course not limiting.

(7) FIG. 3 illustrates ceramic block 1 remaining after the second machining step of the process, which has been carried out on the right side (on the figure) of the block and during which the corresponding angle of the block has been removed to create a second machining surface 9. This second machining surface 9 is substantially located opposite to or on the back of first machining surface 6. The terms opposite and back must not be construed in a restrictive sense. For example, the second machined surface can be the reverse side of the first machined surface forming the front of the core, or the inner side of the first machined surface forming the outer side of the core. So, during machining, the forces and vibrations induced in ceramic block 1 by the machining tool(s) (not represented) are directed towards first machined surface 6 and liable to lead to breakages in ceramic block 1. However, they will have no detrimental effect on first machined surface 6 or on cavities 7 as they have been protected and filled by reinforcement layer 8.

(8) At the end of this second machining step and before carrying out the next machining step, which consists in machining a third surface 10 to separate core 20 from the remaining ceramic block 1, one applies once more a stiffening solution to form a second reinforcement layer 11 on the back of third surface 10 to be machined. As explained previously, the remaining ceramic block 1 must be cleaned and degreased to free it from dust and machining oil and thus allow the adhesion of the stiffening solution on the surface of ceramic block 1. One then applies the stiffening solution in the angle formed between first machined surface 6 and the remaining part of ceramic block 1, opposite to third surface 10 to be machined. This second strengthening layer 11 thus allows holding core 20 obtained after relieving during a third machining step, namely after the separation of the obtained core 20 from the remaining part of ceramic block 1 commonly called a heel.

(9) FIG. 4 illustrates the last step of the production process according to the invention, which corresponds to the cleaning of core 20 obtained after the third machining step, which allows machining third surface 10 separating core 20 from ceramic block 1. In this example, the heel of ceramic block 1 is turned by a quarter turn and held vertically by a retainer clamp 12. A bracket 13 is located in front of retainer clamp 12, arranged to support core 20 by any suitable suspension means such as a tie 14 that can pass through the openings of core 20 to retain it after the stiffening solution has molten. The whole set is placed in a collecting vat 15 that resists at least to a temperature in the order of 200 C. The whole is placed in an oven, a stove or the like for at least 3 h at at least 120 C. to make stiffening solution 16 melt and flow by gravity from core 20 and from remaining ceramic block 1 into the bottom of collecting vat 15. One will position core 20 in such a way that the stiffening solution flows without contaminating the areas of core 20 that were not covered with it. Likewise, one will arrange tie 14 through core 20 so as not to damage it. The stiffening solution recovered in collecting vat 15 can be recycled one or several times, depending on its level of impurities. Of course, any other fixture and/or technical means allowing removing stiffening solution 16 from machined ceramic core 20 can be suitable. One can for example dip core 20 in a solvent bath.

(10) The above description of the production process according to the invention referring to the attached drawings is based on an implementation and realization example of a very simplified core, schematized to the extreme. The essential point of the invention lies in the fact of applying regularly, or even at every step of the machining process, a stiffening solution on the machined and therefore weakened areas of ceramic block 1 in order to avoid ceramic breakage.

(11) Other additional precautions can also be recommended. These include in particular machining the various surfaces of ceramic block 1 from top to bottom, which allows preserving the rigidity of said block, and using natural diamond cutting tools or super-abrasive cutting tools of the PCD or CBN type. On can perform the machining operations dry or with a soluble cutting oil or any other suitable coolant. The use of cutting oil allows reducing cutting tool wear, but it requires cleaning ceramic block 1 prior to every application of the stiffening solution. The cutting conditions must also be adapted to the rigidity of ceramic block 1 and of core 20 to be machined. If it is includes little hollowing, in the order of about 30% empty spaces, it is possible to use high machining conditions, for example exceeding 300 m/min up to the last machining step. If core 20 includes much hollowing, for example more than 30% empty spaces, the machining conditions must be divided at least by 2. It is also possible to complete the machining of ceramic block 1 with an ultrasonic spindle to machine the most fragile sections of core 20, such as for example the machining center Tongtai VU-5.

POSSIBILITIES FOR INDUSTRIAL APPLICATION

(12) This description shows clearly that the invention allows reaching the goals defined, that is to say produce a ceramic core only by mechanical machining and without going through a molding step, allowing to significantly shorten the lead times and to reduce production costs. The process according to the invention thus allows considering new, faster parts developments.

(13) The present invention is not restricted to the example of embodiment described, but extends to any modification and variant which is obvious to a person skilled in the art, in particular, the figures are only examples gained from the tests carried out to date to validate the process. They have no limiting effect on the scope of the invention.