Casting mould for casting a cast part and use of such a casting mould

Abstract

The present invention relates to a casting mould for casting a casting, in particular an engine block for a combustion engine, as well as to the use of such a casting mould, comprising mould parts 1, 2, 3 and casting cores 4, 5, 6, 7 produced from moulding sand, and at least one casting core 4, 5, 6, 7 for the formation of a space in the casting. The casting mould according to the invention makes it possible, in a simple manner, for castings to be produced in which at least one locally closely confined section is formed with a microstructure which differs from the remainder of the casting. This is achieved in that a section of the casting core 4, 5, 6, 7 is formed by a chill 9 which is manufactured from a material of which the thermal conductivity is many times higher than the thermal conductivity of the other section 8 of the casting core 4, 5, 6, 7, consisting of moulding sand.

Description

The invention relates to a casting mould for casting a casting, comprising mould parts produced from moulding sand and casting cores and having at least one casting core for the formation of a space in the casting.

Casting moulds of this type are used in particular for the casting of engine blocks for combustion engines. In this context, the casting core in the engine block casting forms the individual combustion chamber, while the other mould part forms the head surface of the engine block, on which, during assembly, the cylinder head of the individual combustion engine is placed and secured, as necessary, as an add-on part.

In practical operation, it is precisely in the area between the space formed in the casting by the casting core and the surface on which the add-on component is mounted that substantial loads may be incurred by the effect of heat or mechanical loading. The stresses resulting from this in the casting material surrounding the space in the area affected can be so great that crack formation and fractures may occur. This problem is particularly critical with engine blocks for modern combustion engines which are cast from light metal or light metal alloys, such as aluminum or an aluminum casting alloy. Because of the ever increasing demands being made on the one hand on the performance and, on the other, on the minimization of the weight of such engines, it is specifically in the area of the opening, allocated to the cylinder head, of the cylinder chamber or combustion chamber respectively formed that extreme loads are incurred which can lead to local material failure in the area of the inner surfaces of the combustion chamber. With a multi-cylinder engine block, with combustion chambers arranged closely adjacent to one another, this damage affects in particular the webs by means of which the cylinders are separated from one another on the cylinder head side. With modern engine designs, it is precisely in this area that the accumulation of casting material is reduced to a minimum, in order, by arranging the combustion chambers as close to one another as possible, to attain with optimum energy utilization the shortest possible structural length of the engine block.

Attempts have been made to improve the mechanical and thermal stress resistance of the casting material in the area of the sections of castings which in practical operation are particularly subject to stress by the sections affected being specifically cooled in the course of solidification, such that they form at that point a microstructure which is favorable in respect of the particular demands imposed.

For this purpose, for example, according to the method described in DE 195 33 529 C2 for the casting of an engine block made of aluminum in a sand mould, the cylinder cavities of the engine block are formed by dies inserted into the sand mould, these consisting of a brass material. The aluminum solidifies on the surface of the brass die as a consequence of the higher thermal conductivity of the metal of the die more rapidly than on the surface of the sand mould. As a result, a microstructure is produced there to an adequate depth, such that it possesses a higher resistance to the loads incurred in practical operation on the running surfaces of the cylinders.

Practical experience with the known method described above has shown that it is necessary for the brass dies to be pre-heated before the melt is poured into the individual casting mould. Otherwise the risk arises that the casting metal coming in contact with the metal of the die will solidify too rapidly and cracks or metal fractures will appear. The need for pre-heating the dies not only leads to higher manufacturing effort and expenditure, but also brings with it the problem that it is only possible with great difficulty to create, in specific closely confined areas of the combustion or cylinder chambers of combustion engines, a specific microstructure which will resist the conditions which prevail in practical operation.

Against this background, the object of the invention was to provide a casting mould which makes it possible, in a simple manner, for castings to be produced in which at least one locally closely confined section is formed with a microstructure which differs from the remainder of the casting. In addition to this, it should also specify an advantageous use of such a casting mould.

With regard to the casting mould, this object is resolved according to the invention by the subject matter of Claim 1. Advantageous embodiments of this solution are provided in the claims referring back to Claim 1.

With regard to the use, the object referred to above is resolved according to the invention by the subject matter of Claim 11. Advantageous embodiments of this use are specified in the claims referring back to Claim 11.

With a casting mould according to the invention, the casting core forming the space in the casting in each case is divided into at least two sections. The first section in this situation is formed by a chill, while the other section usually consists of mould sand placed in the area of the sand casting. Such mould sand is formed in a known manner from mixtures of basic mould material, as a rule free-flowing, and a binder. This mould sand mixture is shaped in a core mould to the individual mould part or casting core and then secured by a suitable mechanical, chemical, and/or thermal treatment in such a way that the part or core obtained has adequate shape stability for the casting process.

The chill used according to the invention as a part of the casting core forming the space, has according to the invention a thermal conductivity many times higher than the mould sand from which the other section of the casting core is produced. Accordingly, in the area of the casting mould in which the melt poured into the mould comes in contact with the die locally delimited accelerated cooling takes place. By the selection of the material and the volume of the chill it is possible, in this situation, for the speed at which the locally delimited cooling takes place to be directly influenced, as well as the quantity of heat drawn off the casting metal in each case.

In this situation, by shaping the casting chill, the extent of the area over which the specifically accelerated drawing off of heat should take place can be adjusted in a simple manner by the overall design of the chill. If it is intended, for example, when casting an engine block, that the casting material surrounding the cylinder space to be formed in the engine block should be specifically and massively cooled for the purpose of forming a particularly stress-resistant casting microstructure over a specific part length of the cylinder space, then for this purpose the casting core forming the individual cylinder space is provided with a ring-shaped chill, the height of which corresponds to the specifically cooled part length of the cylinder chamber, taking account of the heat migration.

As well as the possibility created by the invention of carrying out locally closely confined specific cooling in the area of the inner surfaces of a space to be formed in a casting, a further advantage of the invention which is of importance in actual practice lies in the fact that the chill used according to the invention does not need to be subjected before its use to either a special surface treatment or be pre-heated. It has been shown, for example, that with a suitable choice of material and shaping, the chill can be removed in a simple manner from the finished solidified casting. In addition to this, a chill used according to the invention produces, in the area of the casting material coming in contact with it, such a good surface quality that the chill can be inserted into the casting mould without a coating. Accordingly, with a casting mould according to the invention, the casting material comes in direct contact with the chill, such that a particularly rapid heat removal takes place, not impeded by any intermediate layer functioning as an insulator.

As a result, the invention in this way makes the economical manufacture of castings possible in which, in the area of a space to be formed in the castings, locally closely confined zones are present with a casting microstructure created by accelerated cooling.

The properties achieved by the embodiment according to the invention make casting moulds according to the invention particularly well-suited for the casting in large-scale series of engine blocks for combustion engines, wherein their advantages come to the fore in particular in the casting of such engine components made of light metal or light metal alloys, in particular of aluminum or aluminum alloys. It is particularly with the casting of engine blocks that the possibility created by the invention, of specific and massive cooling in a closely confined part area of the individual cylinder chamber has a particularly favorable effect. This becomes particularly noticeable if the zone of the engine block which is to be specifically cooled is the area at which the individual cylinder chamber merges into the assembly surface on which the cylinder head of the engine is mounted. With the aid of the invention, it is possible at that point specifically for the casting material to be cooled in a specific manner in the course of its solidification such that material properties which are always in keeping with the requirements arising in practical operation are present to optimum effect.

One variant of the invention which is particularly advantageous with regard to mass production makes provision for the chill to be manufactured from cast iron Chills consisting of cast iron can be economically manufactured and have a thermal conductivity which is favorable with regard to the processing aim being striven for in this situation.

For the reasons already mentioned, a casting mould according to the invention is particularly well-suited for the manufacture of such castings in which a microstructure is to be created in the area of a transition between the space to be created in the casting and an assembly surface, which is capable of supporting the loads occurring during operation. One embodiment of the invention which is of importance in practice therefore makes provision for at least one of the mould parts of a casting mould according to the invention to form an assembly surface on the casting, on which an add-on part can be mounted on the casting after its solidification, and for the mould core, in particular with the chill, to border on this mould part. In order to implement the desired locally delimited accelerated cooling in a reliable manner, in this situation the thermal conductivity of the chill should likewise be many times higher than the thermal conductivity of the mould part forming the assembly surface.

The arrangement of the chill and of the other section of the casting core forming the space in the casting in the correct position can be assured by a simple procedure, in that elements are formed on the chill and the other section of the casting core, by means of which the chill and the other section of the casting core are connected to one another in positive fit.

If the chill is used in such a way that it cools a zone of the casting which is to be produced at the transition into the assembly surface, it is advantageous if at least one element is formed on the chill which engages in positive fit into a correspondingly shaped cut-out aperture of the mould part.

In this situation, this element is preferably formed as a projection engaging into the mould part. The chill which engages in this way into the mould part concerned extends in this case over the assembly surface to be produced on the casting, such that the occurrence of burrs or comparable mould defects in the area of the aperture of the individual cylinder chamber is reliably prevented. Inasmuch as the inner surface of the projection is inclined at an oblique angle in relation to the longitudinal axis of the chill, in such a way that the diameter of the aperture of the casting core widens in the direction of the mould part, in this situation the removal of the chill from the finished casting can be made additionally easier.

With the large-scale technical use of a casting mould according to the invention in particular, it is advantageous if the casting core with the chill is held by a ram extending into the casting core. This ram can, on the one hand, be used for the precisely positioned holding of the casting core. In addition, such a ram also allows, when coupled to an appropriate adjustment device, automatic installing of the casting core itself.

Due to the high wear loads which arise in the area of the running surfaces of the cylinder chambers of a combustion engine, it is frequently necessary for what are referred to as “liners” to be cast into the engine block. These liners are, for example, prefabricated tubular structural elements made of grey cast iron, of which the inner diameter corresponds to the inner diameter of the cylinder chamber to be formed in the engine block and the inner surfaces of which form the running surfaces in the finished engine block, along which, in operation, the piston of the combustion engine moves. With a casting mould according to the invention, liners can also be cast into the casting which is to be produced, with the simultaneous exploitation of the advantages of the invention, if the casting core carries on its outer surface a liner which is to be cast into the casting.

The invention is described hereinafter in greater detail on the basis of a drawing representing an embodiment.

The single FIGURE shows in diagrammatic form a casting mould G for casting an engine block for a straight-4 combustion engine.

The casting mould 1 is composed of different mould parts 1, 2, 3 and casting cores 4, 5, 6, 7 produced in each case from moulding sand, which are formed in each case from a section 8 prefabricated from moulding sand and a chill 9.

The mould part 1 represented at the top in the FIGURE is what is referred to as a “bottom core”, which forms the assembly surface 10 on the engine block which is to be cast, also referred to as the “top-deck surface”, on which, in the course of the assembly of the combustion engine a cylinder head is secured, not represented here. The mould part 2, arranged opposite the mould part 1, represents what is referred to as the “crank chamber core”, which forms the crank chamber in the engine block to be cast.

The casting cores 4, 5, 6, 7 are in each case retained by a ram 11, 12, 13, 14. The rams 11-14 are coupled to an actuator device, not shown here, which moves them out of a mounting position, in which the casting cores 4, 5, 6, 7 are mounted on them, into the position represented in the FIGURE. The front section 15 of the rams 11-14 projecting into the individual casting core 4-7 is in this case formed such as to taper conically over the entire height of the casting cores 4-7 in the direction of the free end of the ram, in order to be able to draw the rams 11-14 unimpeded out of the casting cores 4-7 after the conclusion of the casting process.

The section 8 of the casting cores 4-7, made of moulding sand, has a beaker shape, wherein the interior which it surrounds is adapted to the shape of the front section 15 of the rams 11-14 in such a way that the section 15 concerned is located in positive fit in the interior. In this situation, a projection 16 is formed at the front free end of the section 15, in each case arranged centrically on the face side of the casting cores 47, which engages into a correspondingly shaped cut-out aperture 17 of the mould part 2 and is likewise held in this in positive fit.

On its opposite edge, facing the chill 9 which is produced as a monolithic block made of grey cast iron, the section 8 of the casting cores 4-7 has in each case a circumferential shoulder 18, into which a ring-shaped circumferential projection 19 engages, formed at the face-side edge of the chill 9 facing the section 8. In this way, the chills 9 of the casting cores 4-7 are linked in positive fit to the section 8, made of moulding sand, of the individual casting core 4-7.

The chills 9 in this situation themselves have a ring-shaped design. The interior surrounded by them is in this situation adapted to the shape of the front section 15 of the rams 11-14 facing them in each case, such that the chills 9 are likewise retained in positive fit and essentially free of play on the rams 11-14 allocated to them.

On their upper edge, facing the shaped part 1, a circumferential projection 20 is likewise formed in each case on the chills 9, the outer circumferential surface of which, as with the projection 19, merges smoothly into the circumferential surface 21 of the main section of the individual chill 9.

The chills 9 in this situation have a slightly conical shape, tapering in the direction of the free end of their projection 19. For this purpose, the common circumferential surface 21 of the chills 9 is inclined by an angle of at least 2° in relation to their longitudinal axes L, congruent with the longitudinal axes of the rams 11-14. In the corresponding manner, the inner surfaces of the chills 9 are also inclined. The conical outer shape of the chills 9 facilitates the removal of the chills 9 from a liner 22, which is held in positive fit by the individual casting core 4-7 in each case, and, after solidification, remains in the engine block which is to be cast in the casting mould G. In the engine block which is to be cast, the liners 22 surround the cylinder chambers which are to be formed by the casting cores 4-7 in the engine block.

The height of the projections 20 of the chills 9 is dimensioned in such a way that in each case, with the casting cores 4-7 and mould parts 1-3 positioned ready in the casting mould G, they engage over the assembly surface 10 into a correspondingly shaped cut-out aperture 23 of the mould part 1. In this way, burr formation can be reliably prevented in the area of the opening of the cylinder chambers of the engine block which is to be cast.

In the casting mould G, in an inherently known manner, feeders and filling channels are formed, not shown here, by means of which the aluminum melt poured into the casting mould G flows into the mould cavity 24 surrounded by the casting mould G. Heat is drawn off in an accelerated fashion, via the liners 22 and the individual chill 9, from the melt which in this situation passes into the areas 25 of the mould cavity 24 adjacent to the chills 9, such that it solidifies more rapidly than the melt present in the other zones of the mould cavity 24. Accordingly, in the areas 25 of the engine block which is to be cast, an aligned finer-grain microstructure is formed, such that the section of the engine block adjacent to the assembly surface 10, which in this area is subjected to particular heavy loading, will reliably resist the thermal and mechanical loads occurring in practical operation.

Claims

1. Casting mould for casting a casting comprising mould parts produced from moulding sand and having at least one casting core for the formation of a space in the casting, wherein a section of the at least one casting core is formed by a chill which is manufactured from a material having a thermal conductivity that is many times higher than the thermal conductivity of another section of the at least one casting core, the another section consisting of casting sand.

2. Casting mould according to claim 1, wherein the chill is manufactured from cast iron.

3. Casting mould according to claim 1, wherein at least one mould part forms an assembly surface at the casting, at which an add-on part can be fitted to the casting after solidification, and in that the at least one casting core adjoins the at least one mould part.

4. Casting mould according to claim 3, wherein the thermal conductivity of the chill is many times higher than the thermal conductivity of the at least one mould part forming the assembly surface.

5. Casting mould according to claim 1, wherein at least one element is formed on the chill and the another section of the at least one casting core, by means of which the chill and the another section of the at least one casting core are connected to one another in positive fit.

6. Casting mould according to claim 1, wherein at least one element is formed on the chill, which engages in positive fit into a correspondingly shaped cut-out aperture of at least one mould part.

7. Casting mould according to claim 6, wherein the element is formed as a projection engaging in the at least one mould part.

8. Casting mould according to claim 7, wherein the inner surface of the projection is arranged at an oblique angle in relation to the longitudinal axis of the chill.

9. Casting mould according to claim 7, wherein the projection engages over an assembly surface into the at least one mould part.

10. Casting mould according to claim 1, wherein the at least one casting core is held with the chill by a ram extending into the at least one casting core.

11. Use of a casting mould formed according to claim 1, for the casting of an engine block for a combustion engine.

12. Use according to claim 11, wherein use is made as a casting material of a light metal or a light metal alloy.

13. Use according to claim 11, wherein the at least one casting core of the casting mould carries on an outer surface a liner, forming, in the finished casting, an inner wall of the space to be formed by the at least one casting core.

14. Use according to claim 11, wherein the at least one casting core of the casting mould forms a cylinder chamber of the engine block.

15. Use according to claim 11, wherein an add-on part is a cylinder head.