EP3194094B1 - Lost pattern intended for use in casting - Google Patents

Description

The invention is in the field of foundry casting using a lost model. It relates to the lost models used for the manufacture of a casting, and to the method of assembling parts constituting lost models.

In the field of foundry, it is known to manufacture a piece of metal material from the use of a lost model. The latter has a conformation and dimensions identical to the desired part. The lost model is formed of a sublimable material, such as for example polystyrene, polyurethane or polyethylene. The lost model is placed inside a matrix made of sand that receives the molten metal material. In contact with the molten metal material, the sublimable material is sublimated to make room for the metallic material. After cooling of the latter, the metal part obtained is of shape and dimensions identical to those of the lost model.

One use of this technology of interest to the present invention is the casting of foundry metal parts, especially engine parts such as cylinder heads. The lost model used in this type of technology is usually expanded polystyrene.

As is, for example, described in the patent EP-2,052,799 in the patent EP-2,357,049 in the patent EP-2,397,245 , or in the patent FR-2 959 145 Polystyrene molded parts, in this case called strata, may be, after molding, assembled to form foundry models called "non-permanent models" or "lost models", since the models will be destroyed and replaced by molten metal when molding metal parts. This assembly is necessary especially when the castings to be obtained are of complex shape, which is the case of the cylinder heads.

The assembling of the layers is usually done by gluing, for example using a hot melt glue. An example of sizing technique by spray of a polyurethane-based glue in aqueous emulsion is given in the patent FR-2 768 068 .

In addition, publications JP-01202338 and WO00 / 15370 disclose casting methods using lost models, the models comprising closed cavities to reduce the volume of gas produced during casting of the metal.

Take the example of a molten aluminum casting on a lost model of expanded polystyrene: during casting, polystyrene vaporizes under the effect of heat, and liquid aluminum takes the place of polystyrene before solidifying . During this casting, there is a gas evolution due to the vaporization of the polystyrene (and the adhesive bonding the layers together), which can cause the appearance of what are called "blowholes" during solidification of the aluminum part. These "blisters" are defects manifested by porosity in the metal part / roughness on its surface. The risk of occurrence of these defects is even higher in areas where the piece has a significant thickness.

The object of the invention is therefore to remedy this drawback. In particular, it aims to reduce or even eliminate the occurrence of blow-type defects in foundry metal parts with lost models.

The subject of the invention is a lost model intended to be used in a foundry for obtaining metal parts and comprising a stack of layers assembled to each other by gluing, as defined by claim 1, as well as a method of performing a lost model as defined by claim 9.

The terms "lower" and "upper" and / or "upper" and "lower" faces will be used throughout the present text because the method of assembling the strata to constitute the lost model is generally done by stacking strata vertically, where one begins stacking with a stratum, whose lower face is glued and then placed on the upper face of another layer, the set of both layers being then glued on a new so-called lower stratum etc. ... These spatial terms are therefore indicated for the sake of brevity and clarity, but the invention must be understood as also encompassing any assembly technique where the stacking of layers would be different, for example according to a horizontal axis (strata arranged in vertical planes) or vertically but in opposite direction (we add a layer above the stack being formed, instead of adding the stack being formed to a layer disposed below this one).

It is also understood by "closed" recess that this recess does not open once the model is constituted. Thus, in particular, when one comes to dispose the model in the foundry sand, this sand can not penetrate them.

It is also included in the definition of the invention that the closed recess can be defined jointly not by two successive layers, but three or more.

It is also understood in the spirit of the invention that these recesses are not functional, in that they do not participate in the shape of the final metal part, since, during casting, the molten metal will fill these cavities / recesses, just as the metal would replace the polystyrene if these cavities did not exist.

It has been found that these recesses surprisingly make it possible to limit the gaseous emissions at the origin of the blowholes mentioned above, or, at the very least, to better channel the evacuation: the mechanical parts obtained with lost models with at least one of these closed recesses, and especially when they are located in the "thickest" areas of the model, are of better quality, with much less blistering and therefore with a much lower scrap rate. This has notably been verified during the production of metal parts to make heat engines, including engine cylinder heads. Indeed, the cylinder heads are made from lost molds consisting of many strata, usually at least four or five, all of complex shapes that fit into each other, and have some fairly "thick" areas, including proximity of the bearing zones of the valves, conducive to the appearance of blowholes. It is for this type of piece with complex shapes that the invention is the most advantageous.

Preferably, the lost model according to the invention comprises several closed recesses. For the example of the cylinder head mentioned above, it will be particularly in all areas corresponding to the valve support areas (intake and / or exhaust).

According to the invention, the closed recess or recesses are located in one or more zones of the model having a thickness (including that of the recess) of at least 10 mm measured along the stacking axis of the strata.

According to one embodiment, the volume of the closed recess or at least one of them is distributed substantially equally between the two recess portions that define it. Thus, the two recess portions (when, in the most frequent case, the recess is defined jointly by only two strata), each define approximately half the volume of the complete closed recess. It can also be provided that the shape of these two recess portions is similar, and even that they have a symmetry with respect to their sharing plane corresponding to the interface area between the two layers.

According to another embodiment, the volume of the closed recess or at least one of them is distributed differently between the two recess portions that define it. In this case, one of the recess portions defines more than 50% of the total volume of the complete recess, for example 60, 70 or even 90% and more. One can thus have the complementary recess portion defining a very small volume share, and just just "close" the other portion.

Advantageously, the shape of the wall of the recess portion belonging to a stratum at least partially follows the shape of the opposite wall of said stratum. Thus, if we take the example of the engine cylinder head valve support zones, we can predict that the recess will be located below what will correspond to the bearing surface of the valve in the cylinder head , and that its upper part will be flat and parallel to the flat surface of the stratum which will correspond to the bearing surface of the valve.

Preferably, the volume of the closed recess or at least one of them is of the order of 1 cm ³ , in particular between 0.5 and 1.5 cm ³ .

Advantageously the thickness of the closed recess or at least one of them, measured along the stacking axis of the strata (that is to say its height when the stacking axis is vertical), is about 1 cm, and especially between 0.5 and 1.5 cm.

According to an example already mentioned, the invention also relates to such a lost model, which corresponds to the shape of a cylinder head or a portion of the engine cylinder head, and such that the closed recesses are located in the corresponding areas. to the supports of the valves intended to be associated with the cylinder head.

Especially in this case, it is preferable that the recesses have flat upper walls parallel to the upper walls of the upper stratum corresponding to the valve supports, also planes. This configuration allows in particular to have a maximum volume of recess.

The subject of the invention is also a process for producing a lost model intended to be used in a foundry for obtaining metal parts and comprising a stack of layers assembled to each other by gluing, where the lost model comprises at least a closed recess which is defined jointly by two recess portions respectively belonging to two successive strata, respectively called lower stratum and upper stratum in the stack, and in particular being as defined above.

The invention also relates to a device for implementing the method described above, which comprises the lost model in question.

The invention also relates to a foundry piece obtained from the lost model described above.

• figure 1, une section verticale d'un modèle perdu assemblé selon l'invention et comprenant deux évidements clos définis par deux strates successives du modèle ;
• figure 2, un agrandissement de la figure 1 dans la zone des deux évidements clos ;
• figure 3, une vue en perspective des deux strates successives définissant les deux évidements clos selon les figures précédentes.

Other features and advantages of the invention will appear on reading the following detailed description of an embodiment of the invention, given by way of example only and with reference to the drawings which show:

• figure 1 a vertical section of a lost model assembled according to the invention and comprising two closed recesses defined by two successive layers of the model;
• figure 2 , an enlargement of the figure 1 in the area of the two closed recesses;
• figure 3 , a perspective view of the two successive layers defining the two closed recesses according to the preceding figures.

The identical components from one figure to another have the same reference. The figures do not necessarily represent all the components at scale and remain schematic.

It is proposed in the following example a lost model for molding a metal part for example here based on aluminum, in this case a motor vehicle engine cylinder head. It will not be possible to refer to the patents cited in the preamble of the present application describing this technique.

The figure 1 represents in vertical section the lost model M in polystyrene, composed of 5 layers 1,2,3,4,5 assembled in known manner by gluing according to a stack along a vertical axis X. Two recesses 6,7 are represented, which are entirely closed in the complete lost model, and which are not "functional", in that they are not intended to define the shape of the metal part to be produced.

Enlargements of these two recesses are represented at figure 2 they are delimited by partitions, part of which belongs to the so-called lower stratum 3 and part to the so-called upper stratum 4. The recess 6 thus has a substantially trapezoidal section, with an upper partition belonging to the stratum 4 and defining a planar and oblique zone 61 facing an equally flat and oblique zone 41 belonging to the upper face of the stratum 4 (that is to say the face of the stratum 4 directed towards the next stratum, the stratum 5. This zone 41 corresponds, in the cylinder head once produced, to a zone of support of the valves which will be mounted in the breech. The lower partition 62 belonging to the lower layer 3 is curved, with a substantially flat bottom.

Without the recess 6, it can be seen that the zone under this surface 41 would be relatively "thick" (at least 10 mm), with, consequently, locally, a gas evolution due to the sublimation of the larger polystyrene, and therefore a risk blowholes in the metal part. With the recess 6, one comes to "dig" in this massive part, to limit the gas release. The metal will fill during casting, and the shape of the final piece remains unchanged, whether the recess 6 is present or not in the lost model. By cons, with the recess 6, the scrap rate on the cylinder heads could drop significantly.

It is the same for the recess 7: its upper partition define an oblique planar zone 71 which "follows" also the outer face 42 of the stratum 4 facing said wall and also corresponding to a valve bearing face in the breech. Its lower partition 72 belonging to the lower stratum 3 is substantially entirely flat and in continuity with the outer face of the stratum 3.

There is thus a large volume for the recess, and the strata in question remain demoldable during their manufacture.

It is advantageous to provide as many recesses as "massive / thick" zones in the model, or, at least, to provide recesses in those of these "thick" zones which are most likely to generate blow-type defects in the foundry parts obtained.

To give an order of magnitude, the recesses 6 and 7 define a volume of about 0.5 to 1.5 cm ³ , of the order of cm ^3, therefore. It can be seen that, in the case of the recess 6, approximately one-third of the volume is defined by the profile of the lower stratum 3 and the remainder by that of the upper stratum 4. In the case of the recess 7, it appears that it is the upper layer 4 which really defines the volume of the cavity, whereas the stratum 3 only comes to close it without any very significant modification of its profile: one sees that any form of recess is possible, and any distribution of the volume between the two strata is so too.

The number and geometry of the recesses thus adapt to the constitution of each lost model. The creation of these recesses does not complicate the manufacture of the lost model, and tends to lighten it by using less polystyrene, which is very advantageous.

The figure 3 is a representation of strata 3 and 4 complete before assembly, the arrows f indicating how the assembly is done. It can thus be seen that each closed recess 6 consists of a recess portion 6a of the stratum 3 and of a recess portion 6b complementary to the stratum 4. It can be seen that the recess 7 is defined by a portion of recess only in one of the strata 4, the other stratum 3 being unmodified and simply closing the cavity 7.

Claims (9)

1. Lost pattern (M) intended for use in casting to obtain metal parts and comprising a stack of layers (1, 2, 3, 4, 5) joined to each other by adhesive bonding, characterized in that said lost pattern includes at least one enclosed cavity (6, 7) which is defined jointly by two cavity sections which belong respectively to two successive layers (3, 4), referred to respectively as lower layer (3) and upper layer (4), in the stack, the one or more enclosed cavities (6,7) being located in one or more of the zones of the pattern having a thickness of at least 10 mm measured along the axis (X) of the stack of layers (1, 2, 3, 4, 5).
2. Lost pattern (M) according to the preceding claim, characterized in that it includes a plurality of enclosed cavities (6, 7).
3. Lost pattern (M) according to either of the preceding claims, characterized in that the volume of the enclosed cavity (6, 7) or of at least one of such cavities is divided substantially equally between the two cavity portions that define it.
4. Lost pattern (M) according to either of Claims 1 or 2, characterized in that the volume of the enclosed cavity (6, 7) or of at least one of such cavities is divided differently between the two cavity portions that define it.
5. Lost pattern (M) according to any one of the preceding claims, characterized in that the shape of the wall of the cavity portion (61, 71) belonging to one layer at least partially follows the shape of the opposite wall (41, 42) in said layer.
6. Lost pattern (M) according to any one of the preceding claims, characterized in that the volume of the enclosed cavity (6, 7) or of at least one of said cavities is in the order of 1 cm³, and particularly in a range from 0.5 to 1.5 cm³.
7. Lost pattern (M) according to any one of the preceding claims, characterized in that it corresponds to the shape of a cylinder head or part of a cylinder head of a combustion engine, and in that the enclosed cavities (6, 7) are located in the zones corresponding to the valve seatings that are intended to be associated with the cylinder head.
8. Lost pattern (M) according to the preceding claim, characterized in that the enclosed cavities (6, 7) have flat upper walls (61, 71) parallel to the upper walls (41, 42) of the upper layer (4) corresponding to the valve seatings, which are also flat.
9. Method for producing a lost pattern (M) for use in casting to obtain metal parts and comprising a stack of layers (1, 2, 3, 4, 5) joined to each other by adhesive bonding, characterized in that the lost pattern comprises at least one enclosed cavity (6, 7) which is defined jointly by two cavity portions belonging respectively to two successive layers (3, 4), called respectively lower layer (3) and upper layer (4) in the stack, the one or more enclosed cavities (6, 7) being located in one or more of the zones of the pattern having a thickness of at least 10 mm measured along the axis (X) of the stack of layers (1, 2, 3, 4, 5).

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