METHOD FOR POSITIONING A CORE IN A MOULD

Abstract

The invention relates to a method for determining the position of the cores in an injection mould, comprising the steps essentially consisting of: selecting a core R.sub.rep in a population of cores with the least difference from the mean of the measured differences between k cores and the theoretical three-dimensional spatial model, positioning this core R.sub.rep in space relative to at least one of the functional faces of a theoretical three-dimensional spatial model of the core, and repositioning core support points so that they can support the core R.sub.rep in the position corresponding to its repositioning in space performed in the previous step.

Claims

1.-11. (canceled)

12. A method for determining the position of the cores in an injection mould, comprising the steps of: a) Collecting k cores noted R.sub.1 . . . R.sub.i . . . R.sub.k in a population of cores all produced from the same theoretical three-dimensional core model, b) making a three-dimensional model of each of the cores, c) relocating each of the three-dimensional models in space relative to l support points T.sub.1 . . . T.sub.q . . . T.sub.1 of the core in the mould to obtain a relocated three-dimensional spatial model of each core V1, d) selecting the core noted R.sub.rep the three-dimensional spatial model V1 of which has the least difference with the theoretical three-dimensional spatial model, e) relocating the three-dimensional model of the core R.sub.rep with the theoretical three-dimensional spatial model by taking into account at least one functional face of the theoretical model of the core in order to obtain a relocated three-dimensional spatial model V2 of the core R.sub.rep, f) repositioning the support points T.sub.q so that it can support the core R.sub.rep in the spatial position corresponding to the relocated three-dimensional spatial model V2 of the core R.sub.rep.

13. A method according to claim 12, wherein each three-dimensional model is obtained from a three-dimensional survey of the outer surface of the core, for example obtained from a contactless measurement.

14. A method according to claim 12, wherein step d) comprises the following steps: selecting n points noted P.sub.1 . . . P.sub.j . . . P.sub.n on at least one of the functional faces of the theoretical model of the core, selecting the core R.sub.rep the n points of the relocated three-dimensional spatial model V1 of which have the least difference with the same points n of the spatial theoretical model.

15. A method according to claim 13, wherein step d) comprises the following steps: selecting n points noted P.sub.1 . . . P.sub.j . . . P.sub.n on at least one of the functional faces of the theoretical model of the core, selecting the core R.sub.rep the n points of the relocated three-dimensional spatial model V1 of which have the least difference with the same points n of the spatial theoretical model.

16. A method according to claim 14, wherein step d) comprises the following steps for each core R.sub.i: i. determining the difference E.sub.i,j.sup.1 between each point P.sub.j of the theoretical model and the model V1, ii. calculating the average $M_j\left(E_{i,j}^1\right)=\frac{1}{k}.Math.\underset{i=1}{\overset{k}{.Math.}}.Math.E_{i,j}^1$ iii. calculating .sub.i,j.sup.1=E.sub.i,j.sup.1M.sub.j(E.sub.i,j.sup.1) iv. calculating for each core R.sub.i, S.sub.i.sup.1=.sub.j=1.sup.n.sub.i,j.sup.1 2 v. considering the core R.sub.i which the lowest value is assigned S.sub.i.sup.1 to as the representative core R.sub.rep of the core population.

17. A method according to claim 16, comprising a checking step, between steps e) and f), consisting in verifying that the relocated spatial model V2 of the core R.sub.rep is better positioned than the relocated spatial model V1 of the core R.sub.rep.

18. A method according to claim 17, wherein the checking step includes the following steps: i. determining the difference E.sub.rep,j.sup.2 between each point P.sub.j of the theoretical three-dimensional spatial model and the relocated three-dimensional spatial model V2 of the core R.sub.rep relative to the functional faces, ii. calculating S.sub.rep.sup.2=.sub.j=1.sup.n E.sub.i,j.sup.2 2, iii. comparing S.sub.rep.sup.2 with S.sub.rep.sup.1 in order to verify that S.sub.rep.sup.2 is less than S.sub.rep.sup.1.

19. A method according to claim 16, wherein the difference and/or the difference E.sub.rep,j.sup.2 are determined according to the normal to the theoretical three-dimensional spatial model at the point P.sub.j.

20. A method according to claim 18, wherein the difference and/or the difference E.sub.rep,j.sup.2 are determined according to the normal to the theoretical three-dimensional spatial model at the point P.sub.j.

21. A method according to claim 12, wherein step f) comprises the following steps for each of the support points T.sub.q: projecting a point T.sub.q as normal to the theoretical three-dimensional spatial model passing through the contact point of the support point T.sub.q with the theoretical three-dimensional spatial model, on the relocated three-dimensional spatial model V2, in order to obtain a point T.sub.q, modifying the support points in the mould so that they are brought to the level of the points T.sub.q.

22. A method according to claim 12, wherein k is greater than or equal to five and/or l is greater than or equal to six.

23. A method according to claim 14, wherein n is greater than or equal to three.

24. A method according to claim 12, wherein the injection mould is a wax injection mould.

25. A method according to claim 12, comprising a checking step, between steps e) and f), consisting in verifying that the relocated spatial model V2 of the core R.sub.rep is better positioned than the relocated spatial model V1 of the core R.sub.rep.

Description

[0011] The purpose of the invention is in particular to provide a simple, effective and economical solution to the problems of the prior art described above.

[0012] To this end, it proposes a method for determining the position of the cores in an injection mould, including the steps of: [0013] a) Collecting k cores noted R.sub.1 . . . R.sub.i . . . R.sub.k in a population of cores all based on the same theoretical three-dimensional core model, [0014] b) making a three-dimensional model of each of the cores, [0015] c) relocating each of the three-dimensional models in space relative to a plurality of support points T.sub.1 . . . T.sub.q . . . T.sub.1 of the core in the mould to obtain a relocated three-dimensional spatial model of each core V1, [0016] d) selecting the core noted R.sub.rep the three-dimensional spatial model V1 of which has the least difference with the theoretical three-dimensional spatial model, [0017] e) perform a repositioning of the three-dimensional model of the core R.sub.rep with the theoretical three-dimensional spatial model by taking into account at least one functional face of the theoretical model of the core in order to obtain an updated three-dimensional spatial model V2 of the core R.sub.rep, [0018] f) repositioning the support points T.sub.q so that it can support the core R.sub.rep in the position corresponding to the relocated three-dimensional spatial model V2 of the core R.sub.rep.

[0019] According to the invention, a defect in the geometry of the cores is compensated by a repositioning of a representative core relative to the functional faces of the theoretical model. All cores are then positioned in an injection mould in the same way as the representative core is positioned in a mould. The method is therefore particularly interesting when the geometry defect (or defects) of the cores corresponds to a deviation of one dimension from a nominal value. The collection of k cores is carried out randomly.

[0020] The term functional face of the core refers to a face of the core intended to form, before assembling the part, a face with the final geometry of the part. Such a functional face is an outer face of the core that enables the shaping of the inner or outer faces of the metal part and has an impact on the aerodynamics and thermal properties of the part in operation. In the case of a turbine blade, a functional face can refer to an outer face of the core forming an inner face of a core wall, such as a front side or back side wall for example. The internal cavity of the blade can be a cavity for cooling the blade.

[0021] The term three-dimensional model in reference to a core should be interpreted as a set of digital data allowing a three-dimensional digital reconstruction of the core, for example using a geometric mesh.

[0022] The term spatial refers to a three-dimensional model positioned in space.

[0023] The term relocated refers to a three-dimensional spatial model that has been positioned or repositioned in space.

[0024] According to another characteristic, each three-dimensional model can be obtained from a three-dimensional survey of the outer surface of the core, for example from a contactless measurement that can be performed by optical triangulation. In such a configuration, a central projector illuminates a room with a network of fringes that are observed by two CCD cameras. A polygonal mesh of the outer surface of each of the cores is deduced from this.

[0025] In one particular embodiment of the invention, step d) may include the following steps: [0026] selecting n points noted P.sub.1 . . . P.sub.j . . . P.sub.n on at least one of the functional faces of the theoretical model of the core, [0027] selecting the core R.sub.rep the n points of the relocated three-dimensional spatial model V1 of which have the least difference with the same n points of the spatial theoretical model.

[0028] In this alternative embodiment, the determination of the representative core is thus carried out by measuring the differences on a functional side after relocation on the support points. It is indeed interesting to measure the differences relative to at least one functional face since it is a face that has a direct impact on a corresponding face of the final part.

[0029] Also, step d) may include the following steps for each core R.sub.i: [0030] i. determining the difference E.sub.i,j.sup.1 between each point P.sub.j of the theoretical model and the model V1,

[0031] ii. calculating the average

[00001]$M_j\left(E_{i,j}^1\right)=\frac{1}{k}.Math.\underset{i=1}{\overset{k}{.Math.}}.Math.E_{i,j}^1$ [0032] iii. calculating .sub.i,j.sup.1=E.sub.i,j.sup.1M.sub.j(E.sub.i,j.sup.1) [0033] iv. calculating, for each core R.sub.1, S.sub.i.sup.1=.sub.j=1.sup.n.sub.i,j.sup.1 2 [0034] v. considering the core R.sub.i which the lowest value is assigned S.sub.i.sup.1 to as the representative core R.sub.rep of the core population.

[0035] Preferably, the method includes a checking step, between steps e) and f), consisting in verifying that the relocated spatial model V2 of the core R.sub.rep is better positioned than the relocated spatial model V1 of the core R.sub.rep.

[0036] If the relocation V2 is worse than the relocation V1, then the relocation V2 should be repeated on a smaller number of functional faces than the number of functional faces previously used.

[0037] The checking step includes the following steps: [0038] i. determining the difference E.sub.rep,j.sup.2 between each point P.sub.j of the theoretical three-dimensional spatial model and the relocated three-dimensional spatial model V2 of the core R.sub.rep relative to the functional faces, [0039] ii. calculating S.sub.rep.sup.2=.sub.j=1.sup.n.sub.i,j.sup.2 2, [0040] iii. comparing S.sub.rep.sup.2 with S.sub.rep.sup.1 in order to verify that S.sub.rep.sup.2 is less than S.sub.rep.sup.1.

[0041] The difference E.sub.i,j.sup.1 and/or the difference E.sub.rep,j.sup.2 can be determined along the normal to the theoretical three-dimensional spatial model at the point P.sub.j.

[0042] The repositioning of the support points in step f) can be done as follows, for each of the support points T.sub.q: [0043] projecting a point T.sub.q as normal to the theoretical three-dimensional spatial model passing through the contact point of the support point T.sub.q with the theoretical three-dimensional spatial model, on the relocated three-dimensional spatial model V2, in order to obtain a point T.sub.q, [0044] modifying the support points in the mould so that they are brought to the level of the points T.sub.q.

[0045] In a practical embodiment of the invention, k is greater than or equal to five and/or l is greater than or equal to six and/or n is greater than or equal to three. In practice, n is a function of the curvature and tolerance of the functional face considered. The lower the curvature, the smaller.sub.n. Thus, the minimum number of n is three, which corresponds to the minimum number of points required to position a plane isostatically in space.

[0046] In the manufacture of a turbomachine part, the injection mould is a wax injection mould. The core can be a turbine blade core for example.

[0047] The invention will be better understood and other details, advantages and characteristics of the invention will appear in the following non-exhaustive illustrative description, with reference to FIG. 1 representing the main steps of the method according to the invention.

[0048] In a first step a) of the method, k cores noted R.sub.1 . . . R.sub.i . . . R.sub.k are selected in a population of cores, all based on the same theoretical three-dimensional core model. The term population here refers to a set of cores, the number of which can be determined or undetermined.

[0049] In a second step (b) of the method, a three-dimensional measurement of the external surface of each of the cores is obtained from a contactless measurement that can be an optical measurement, for example by optical triangulation as mentioned above. Of course, other methods of measurement could be used without going beyond the scope of the invention. For example, another method may consist in using a more accurate but much slower sensing device or a three-dimensional measuring machine (TDMM). The three-dimensional survey makes it possible to establish a three-dimensional model of each of the cores, i.e. a digital model including a set of coordinates of points on the surface of a core, thus enabling a relative positioning of the points.

[0050] In a third step c), the method includes a step of spatially positioning each of the three-dimensional models relative to l support points T.sub.1 . . . T.sub.q . . . T.sub.1 of the core in the mould in order to obtain a three-dimensional spatial model V.sub.1 for each core. This positioning thus consists of a spatial relocation relative to thel support points.

[0051] In practice, this relocation can be achieved by minimizing the difference between the theoretical three-dimensional spatial model and the relocated three-dimensional spatial model V1 of each of the cores, at the level of points T.sub.q. Minimizing can be done using the least squares method.

[0052] The method then consists, in a fourth step, in selecting the core noted R.sub.rep, the three-dimensional spatial model V1 of which has the smallest deviation from the calculated mean deviations between the actual models and the theoretical three-dimensional spatial model. This step is executed on n points P.sub.j noted P.sub.1 . . . P.sub.j . . . P.sub.n belonging to at least one of the functional faces of the theoretical model of the theoretical core. Preferably, the n points are distributed over a maximum number of functional faces. Preferably, the n points are distributed over the selected functional faces and a number of points per face is selected according to the curvature and tolerance applied to the face considered.

[0053] This step of selecting the core representative of the k cores is performed by executing the following steps: [0054] i. determining the difference E.sub.i,j.sup.1 between each point P.sub.j of the theoretical model and the model V1, along the normal to the theoretical model passing through the point P.sub.j, [0055] ii. calculating the average

[00002]$M_j\left(E_{i,j}^1\right)=\frac{1}{k}.Math.\underset{i=1}{\overset{k}{.Math.}}.Math.E_{i,j}^1$ [0056] iii. calculating .sub.i,j.sup.1=E.sub.i,j.sup.1M.sub.j(E.sub.i,j.sup.1) [0057] iv. calculating for each core R.sub.i, S.sub.i.sup.1=.sub.j=1.sup.n.sub.i,j.sup.1 2 [0058] v. considering the core R.sub.i which the lowest value is assigned to S.sub.i.sup.1 as the representative core R.sub.rep of the core population.

[0059] In order to be able to determine the desired new position of the representative core R.sub.rep in the mould, a second relocation of the three-dimensional model must then be performed, in a fifth step, by taking into account at least one functional face of the theoretical model of the core in order to obtain a relocated three-dimensional spatial model V2 of the core R.sub.rep.

[0060] Unlike the relocation V1 performed for each of the cores, the relocation of the representative core R.sub.rep is performed only on at least one of the functional faces and does not take into account the support points T.sub.q. The aim here is to enable a repositioning of the representative core R.sub.rep in order to minimize the shape differences between the part obtained from the representative core and a theoretical part from the theoretical core, with the constraint of the support points T.sub.q being eliminated.

[0061] Before proceeding to the sixth step, i.e. step f), a preliminary step of checking the relocation of the three-dimensional spatial model of the core (V2) R.sub.rep is performed. This checking step includes the following steps: [0062] i. determining the difference E.sub.rep,j.sup.2 between each point P.sub.j of the theoretical three-dimensional spatial model and the relocated three-dimensional spatial model V2 of the core R.sub.rep relative to the functional faces, with such difference being measured along the normal passing through the point P.sub.j of the theoretical three-dimensional spatial model, [0063] ii. calculating S.sub.rep.sup.2=.sub.j=1.sup.nE.sub.i,j.sup.2 2, [0064] iii. comparing S.sub.rep.sup.2 with S.sub.rep.sup.1 in order to verify that S.sub.rep.sup.2 is less than S.sub.rep.sup.1.

[0065] When S.sub.rep.sup.2 is greater than S.sub.rep.sup.1, several situations arise. If the relocation of the three-dimensional model of the core R.sub.rep has been performed on only one functional face, then it is necessary to establish that the three-dimensional spatial model V1 of the core R.sub.rep is preferable since it shows that the relocation of the cores on the functional faces does not make it possible to obtain a better positioning of the core. If the relocation of the three-dimensional model is performed on a plurality of functional faces, i.e. F functional faces, with F2, then the relocation of step e) is performed on F1 functional faces and then it is determined whether the new relocation V2 of the three-dimensional spatial model R.sub.rep is better than the relocation V1 of the core R.sub.rep by comparing S.sub.rep.sup.2 with S.sub.rep.sup.1.

[0066] In a complementary approach, it would be possible to classify functional faces into at least two groups, with a first group of primary functional faces and a second group of secondary functional faces. The main functional faces are faces for which the manufacturing tolerances are lower than for the secondary functional faces so that the relocation performed in step e) can be performed preferentially on the main functional faces. Thus, if the relocation of step e) has to be performed again, it is preferable to remove the constraint of the relocation relative to a secondary functional face. Eventually, it will be necessary to check that the differences in the secondary functional faces between the theoretical three-dimensional spatial model and the new three-dimensional model V2 do not exceed the permissible manufacturing tolerances.

[0067] The sixth step f) consists in repositioning the support points T.sub.q so that the core R.sub.rep can be supported in the position corresponding to the relocated three-dimensional spatial model V2 of the core R.sub.rep.

[0068] This repositioning is obtained by performing the following steps: [0069] projecting a point T.sub.q as normal to the theoretical three-dimensional spatial model passing through the contact point of the support point T.sub.q with the theoretical three-dimensional spatial model, on the relocated three-dimensional spatial model V2, in order to obtain a point T.sub.q, [0070] modifying the support points in the mould so that they are brought to the level of the points T.sub.q.

[0071] In practice, to carry out the second sub-step above, the distance between each pair of points T.sub.q and T.sub.q is determined, which gives us l distances. These distances correspond to the positioning corrections to be applied to the ends of the core support rods.