Method for producing castings, molding sand and its use for carrying out said method

Abstract

The invention provides a method with which high-quality complex-shaped castings can be manufactured and with which, after the end of the casting process, the casting mould parts can be easily removed out of or from the casting without any risk. To achieve this the following steps are carried out:

Description

[0001] The invention relates to a method for the manufacture of castings from a molten metal, in particular molten light metal, such as molten aluminium.

[0002] In addition to this, the invention relates to a moulding material and its use for the manufacture of casting mould parts, which are used for the casting of molten metal, in particular molten light metal, such as molten aluminium. Such casting mould parts may, for example, be casting cores, through which cavities are formed in the interior of the castings to be produced. Likewise, the casting mould parts according to the invention can be structural elements from which a multi-part casting mould can be composed, by means of which the external shape of the casting to be manufactured is determined.

[0003] The manufacture of structural components made of metal by casting technology requires casting mould parts by means of which, on the one hand, the inner shape and, on the other, the outer shape of the workpieces which are to be cast are determined. Such casting mould parts can accordingly be casting cores, by means of which cavities are formed in the interior of the casting which is to be created, or casting mould elements from which a multi-part casting mould is assembled, which determines the outer shape of the casting which is to be created.

[0004] To manufacture casting mould parts, mould material systems are used, as a rule, which are composed of a basic mould material and a binding agent. These two components are mixed with each other, shaped, and processed in a suitable hardening process to form a compact body. As a basic mould material, quartz sand is usually used, which in the majority of applications is bonded with an organic binding agent.

[0005] The use of quartz sand as a basic material for the manufacture of casting mould parts has proved its value in many respects in particular in the sector of the casting of light metal materials. Thus, such quartz sand can be obtained economically and is characterised by easy processing qualities and good quality in the imaging of the mould elements of the casting which is to be produced in each case.

[0006] As a more environmentally compatible alternative to organic binding agents, the use of binding agents on a water glass base has been proposed. This water glass binding agent is mixed with the mould sand. The mixture obtained is then shot into the moulding box of a moulding machine, in which a cavity is formed which represents the shape of the mould part which is to be produced. Next, by supplying heat, water is drawn off from the mixture introduced into the mould. The supply of heat can in this situation be effected by an appropriate heating of the moulding box or by means of microwave heating taking effect directly on the mixture (WO-A-86/00033, EP 0 917 499 B1, DE 196 32 293 A1).

[0007] In order to guarantee an optimum operational result when casting the molten metal, the mould material used for the manufacture of the casting mould parts must have high strength and high dimensional consistency, which will still be maintained under the loads incurred during the manufacture of the casting mould and the casting of the melt. In addition to this, the moulding material should be easy to remove after the casting. This becomes particularly important if casting cores are used which form complex shaped interiors in the casting.

[0008] Finally, the mould materials should be capable of being regenerated after use in such a way that the highest possible quota of reuse can be achieved with the basic mould material. This can be achieved in a known manner by the use of inorganic binding agents, which release low emissions during the manufacture of the mould parts and, after the end of the casting process, can be incinerated almost free of any residue under the effect of sufficiently high temperatures.

[0009] In practical application it has been shown that the known mould material systems, regardless of whether they contain organic or inorganic binding agents, under normal conditions have the properties required for optimum operational result.

[0010] However, especially with thin-walled mould parts, such as are used, for example, in the casting of engine blocks or cylinder heads, as mould cores for oil channels, as a result of unavoidable thermal expansion the requirements for the dimensional stability of the casting can no longer be fulfilled.

[0011] A further problem with the casting of complexly shaped castings making use of casting mould parts manufactured by conventional means lies in the fact that the sand can only be removed from the casting with difficulty after cooling. To do this, the casting is usually subjected to shaking or knocking, which causes the disintegration of the casting cores on the inside of the casting and of the mould parts adhering to the outside of the casting, and is intended to promote the pouring out of the mould material particles obtained. These mechanical methods for the removal of the mould parts do, however, incur the risk of damage to the casting; for example, especially with filigree-shaped or thin-walled components, crack formation may occur.

[0012] It has therefore been proposed that, instead of mechanical measures taking effect on the casting, the casting should be heated sufficiently for the binding agent to be combusted until the basic mould material alone remain, and can then be easily removed out of and from the casting as pourable material. The expenditure required in terms of apparatus for this is considerable. In addition to this, the temperatures required for the combustion of the binding agent are so high that the heating will inevitably also cause a change in the properties of the metallic castings.

[0013] The object of the invention was to provide a method with which high-quality complex-shaped castings can be manufactured, and with which, after the end of the casting process, the casting mould parts can also be removed out of and from the casting in a simple manner and without any risk. In addition to this, a mould material is also intended to be provided with which mould parts can be manufactured which are suitable for the production of high-quality complex-shaped castings, and which, after the termination of the casting process, can be removed out of and from the casting in a simple manner and without any risk.

[0014] This object is solved in respect of the method according to the invention by a method for the manufacture of castings from molten metal, in particular molten light metal, in which the following steps are carried out:

[0015] Manufacture of a casting mould part from the mould material, which is mixed from a pourable basic mould material which is inert in relation to the molten metal and from a binding agent, wherein the thermal expansion behaviour of basic mould material and binding agent are adjusted to one another in such a way that the coefficient of thermal expansion of the molten metal lies above the coefficient of thermal expansion of the casting mould part manufactured from the mould material,

[0016] Assembling a casting mould making use of the casting mould part,

[0017] Pouring of the molten metal into the casting mould to form a casting,

[0018] Cooling of the casting over a solidification and cooling period in which the casting mould part self actively disintegrates into fragments,

[0019] Removal of the fragments of the casting mould part from or out of the casting,

[0020] Reprocessing of the fragments of the mould material to form pourable basic mould material.

[0021] On the other hand, the object described above is also solved by a mould material for the manufacture of casting mould parts for the casting of molten metal, in particular of molten light metal, consisting of a pourable mixture of basic mould material, inert in relation to the molten metal, and a binding agent mixed with the basic mould material, wherein the thermal expansion behaviour of the basic mould material and binding agent are adjusted to one another in such a way that the coefficient of thermal expansion of the molten metal lies above the coefficient of thermal expansion of the casting mould part manufactured from the mould material.

[0022] The invention is based on the understanding that, by the selection of a suitable mould material, casting mould parts can be produced which combine in an optimum manner the properties required for the simple, reliable, and environmentally-friendly manufacture of high-quality and dimensionally-stable castings.

[0023] Mould material according to the invention provides an optimum combination of the properties which are a precondition for the manufacture of a high-quality casting with, at the same time, a simple way of manufacture. To this purpose, the mould material according to the invention contains a basic material which is present in a grain form, or comparable particle form, and which as such is pourable, which, during the heating which inevitably arises during the casting, shows considerably less thermal expansion in relation to conventionally used quartz sand.

[0024] The basic mould material accordingly guarantees, even with low material thicknesses, high dimensional stability in the manufacture of complex shaped castings. The basic material, which is pourable in the unbound state, is mixed with a binding agent which has a different expansion behaviour on heating from the basic material. Because of the different thermal expansion between the basic mould material and the binding agent, the application of heat from the casting heat leads to the release of the binding agent from the grains of the basic mould material. As a result, the binding agent accordingly bursts the mould part, when it expands more strongly than the basic material, in such a way that the mould part loses its solid shape and can be easily removed out of and from the casting. Conversely, the expansion behaviour of the basic mould material can be established in such a way that the change of volume which is inherent in the heating process breaks the binding with the binding agent and the basic material is again pourable. An essential factor is that, inherently with the heating, the core or mould component breaks up, so that, after the casting has cooled, it has disintegrated into loose individual parts which are easily removable.

[0025] By adjusting the thermal expansion behaviour of the inventive mould material according to the invention to the thermal expansion behaviour of the molten metal which is to be cast, and by, at the sometime manufacturing this moulding material on the basis of a pourable basic material it is attained that, after the cooling of the casting, the mould part which is at least in part surrounded by the casting or is adjoing by the casting, is broken into loose individual parts because of the forces which occur in the course of the cooling, which individual parts can then be easily removed. The breaking of the casting mould parts is in this case effected as a result of the forces, caused bay the different expansion of the cast metal and the mould material.

[0026] The invention has a particularly favourable effect with the casting of components made from molten aluminium. Aluminium exhibits a high coefficient of thermal expansion, with the result that, in the course of the casting and solidifying of the melt, sufficiently high forces are exerted on the mould parts in contact with the cast part that the mould part concerned will be certain to break into smaller fragments. This proves to be particularly favourable if the mould part is a core mould.

[0027] A further property which is favourable for the invention of the mould material constituted and used according to the invention is that the binding agent and the basic mould material are adjusted to one another in such a way that, with the casting mould parts produced from the mould material, the particles of the basic material are thermally stable and are not elastically bonded by the binding agent. The casting mould part which is produced from a basic mould material constituted in this manner behaves throughout the entire temperature interval run through during the casting of the molten metal in a brittle manner, as a result of which the disintegration of the mould parts wanted according to the invention is promoted.

[0028] The binding agent of the mould is for preference selected in such a way that it does not disintegrate due to the effect of heat. In this way the situation is avoided in which volumes are released in the core which could lead to a compliance of the mould concerned, which is undesirable according to the invention.

[0029] A further advantageous embodiment of the invention is that the particles of the basic mould material exhibit an essentially round, spherical shape. The spherical shape of the basic mould material and the prevalence of point contacts between the basic moulding material particles, associated with this promotes the self-acting disintegration of the mould parts as a consequence of the mechanical forces which arise during the casting and solidification of the melt. A basic mould material which fulfils this requirement particularly well is synthetically manufactured mullite. Accordingly, a further advantageous embodiment of the invention makes provision for the basic mould material to contain at least a part of aluminium oxide sand (mullite), for preference substituting for more than 50% or more than 70% of the quartz sand. Mullite exhibits a round grain shape and a density comparable to quartz sand. Accordingly, the mould materials manufactured from mullite are considerably easier to process than, for example, known ZrO2 sands. In addition to the advantages with regard to the mechanically-induced disintegration of the mould parts, which is striven for according to the invention, the round spherical shape of the mullite particles leads in practice to simplified processability of the mould materials manufactured from such basic mould materials, and therefore inherently to a reduced wear on the tools and machines used for the manufacture of the mould parts. In addition to this, because of its low thermal expansion a mould material with high mullite content has high dimensional stability during the manufacture of complex shaped castings even with low material thickness.

[0030] It has surprisingly been shown that the disintegration of the mould parts manufactured from the mould material constituted according to the invention occurs self-actingly with a time delay in relation to the casting of the molten metal to such an extent that it has no negative effect any more on the quality of the castings, which at this time are already sufficiently solidified.

[0031] Due to its particular properties, mould material composed according to the invention is particularly well-suited for the manufacture of casting cores. These can be removed after casting without the risk of damage to the finished castings.

[0032] Thermically a basic mould material composed of a mullite-quartz sand mixture and a mould material manufactured from this tends to have an insulating effect. Accordingly, these substances can be used in a targeted manner for such casting applications, in which there is indeed heating to above the critical temperature for quartz sand, of 573° C., at which the thermal conductivity of the mould parts produced from the materials concerned, however, plays a subordinate role or, respectively, the thermal conductivity is to be deliberately restricted.

[0033] Practical experiments have shown that by admixing a sufficient quantity of mullite sand to a quartz sand, the problems of spontaneous geometry changes can be preempted, which occur with the use solely of quartz sand as a basic mould material for the manufacture of slim filigree castings. Essential in this respect is that the proportion of the Al2SiO5 sand is sufficiently high in each case for the change in length of the quartz to be compensated for, which otherwise occurs with quartz sand with heating above the critical temperature.

[0034] With mould material constituted according to the invention, binding agent and basic material are, in addition to this, for preference adjusted to one another in such a way that a casting mould part produced from the mould material has a low thermal conductivity. This property has the effect that, after the casting of the molten metal, the temperature difference between the cast material and the mould part remains large, so that the risk of premature disintegration of the mould part induced by thermal or chemical means is reduced to a minimum.

[0035] In addition to this, the disintegration of the mould cores is enhanced by the fact that the constituents of the mould material are matched to one another in such a way that basic mould material and binding agent expand differently when heated, with the result that the bindings between them break under the heating which occurs with the casting of the melt.

[0036] The invention can be realised in a manner particularly well-suited to practice in that a mould material is processed which is formed from a mixture of a basic substance which is present in a grain or comparable particle form, and as such is pourable, and an inorganic binding agent.

[0037] The advantage of the use of inorganic binding agents lies in their better environmental compatibility and the fact that the mould parts manufactured with such binding agents can be returned without any problem into the mould materials cycle. In this connection, mould materials have proved to be particularly well-suited which are mixed from a binding agent based on water glass and a basic mould material composed according to the invention. An essential consideration in this situation, however, is that the expansion behaviour of the components mixed with one another differs in a sufficient manner.

[0038] It is particularly advantageous in this connection if the basic mould material and binding agent expand differently. In this case, after the onset of the heat from the casting heat, the binding agent becomes detached from the grains of the basic mould material. This causes the binding agent, if it expands stronger than the basic substance, to burst the mould part in such a way that it loses its solid shape and disintegrates into fragments. These can be easily shaken out of or from the casting without the risk of mechanical damage. Essential for ensuring that with this variant the self-acting disintegration of the mould part takes place, striven for according to the invention, is consequently the different thermal expansion of the basic mould material and the binding agent, in such a way that, under the effect of the casting heat, the binding agent chips off from the basic mould material particles or inherently breaks up, as a result of the thermal stresses which occur between the basic mould material and the binding agent. As a result of this brittle break behaviour of the binding agent after the hardening of the mould part, the binding between the individual particles of the basic mould material is broken up, and the mould part disintegrates. The remaining loose mixture of basic mould material and binding agent fragments is pourable and can be easily removed out of or from the casting.

[0039] With the method according to the invention, the casting mould part is also manufactured by way of a mould material mixture composed in accordance with the invention being shot in a known manner into the core box of a core moulding machine. The mould material is then hardened, for example in the manner described in DE 196 32 293 A1, in that an low pressure is imposed on the hollow mould of the core box, heated to a temperature of 100° C. to 160° C., and the core mould is heated over a period from 20 to 30 seconds by the core box.

[0040] During this period the casting mould part becomes so solid that it can be removed from the core box and can be placed into a heating device, such as a microwave furnace, arranged outside the core moulding box. In this heating device it is heated at an adequate thermal output until a sufficient volume of water has been drawn from it for complete hardening.

[0041] As an alternative or supplement to a microwave heating arranged outside the core moulding box, the removal of water can also be effected by sufficient heating of the core box itself, or by hot air gassing. These measures can in each case be combined with heating applied outside the core box. It is likewise possible for the removal of water to be effected by microwave heating taking effect directly on the core mould when still located in the core box.

[0042] If the mould part is subjected to heating outside the core box for hardening, the respective mould part can be sprayed with binding fluid in order to increase the core surface hardness. The mould parts treated in this way exhibit increased stability with likewise increased abrasion resistance, so that they can be stored without any problem and meet the highest demands on dimensional stability. This proves to be particularly favourable with regard to optimised quality of the casting which is to be produced if a water glass binding agent is used.

[0043] The invention is explained hereinafter in greater detail, on the basis of a drawing showing an embodiment. The only figure shows in diagrammatic form a camshaft core 1 of a casting mould, not further represented, for the casting of a cylinder head from an aluminium cast alloy.

[0044] Two recesses 3, 4, are formed in the underside 2 of the camshaft core 1, spaced at a distance from one another in the longitudinal direction, by means of which in each case the shape of the pillow blocks of the cylinder head which is to be manufactured, intended for the bearing of the camshaft is determined. Extending into the recesses 3,4 respectively is a branch 5,6 of an oil channel core 8, which with its main section 7 extends parallel to and at a distance from the camshaft core 1. The length A of the branches 5,6 is many times larger than its diameter B.

[0045] Likewise, the length C of the main section 7 of the oil channel core 8 is many times larger than its diameter D.

[0046] The oil channel core 8 has been manufactured in an inherently known manner in a conventional shoot-moulding machine from a mould material according to the invention, manufactured by mixing a basic mould material consisting of a mullite sand and quartz sand with a water glass binder. Due to the proportion of mullite sand, it is guaranteed that the oil channel core 8 expands uniformly and consequently in a manner which can be distinctly predetermined, also under the heat rising in the course of the casting of the cylinder head to be manufactured, up to more than 573° C.

[0047] Fractures in the area of the branches 5,6, extensions of the main section 7 in the area between the branches 5,6, and curvatures in the area of the free ends of the main section 7, such as are encountered in the conventional manner based on oil channel cores manufactured from basic mould materials containing pure quartz sand, are in this way reliably avoided. By means of the use of a mould material composed in the manner according to the invention, cylinder heads and comparable castings, which exhibit thin channels extending over considerable lengths, can thus be reliably manufactured with high precision in large numbers in light metal casting.

[0048] During the pouring of the molten metal, which for preference is an aluminium melt or another light metal melt, and during the time in which the metal of the casting is still capable of flowing, the casting cores 1,8, are only insignificantly deformed because of the features of the basic mould material and the binding agents being adjusted to one another in accordance with the invention. The low thermal expansion of the basic mould material thus supports the reliable process attainment of the dimensional requirements of the casting.

[0049] After a solidification and cooling period, during which the casting attains a solidity sufficient for further processing, the fragments into which the respective casting core 1,8, self-actingly disintegrates, as a result of the effect of the casting heat and due to the different thermal expansion behaviour of the basic mould material and the binding agent, are emptied out of the casting and reprocessed. In the course of the solidification process and in the cooling phase after the complete solidification of the metal, the respective casting core 1,8, is subjected to high mechanical stresses due to the substantially higher solid body contraction of the cast metal in comparison with the casting cores 1,8. Because of the brittle non-elastic consistency of the casting cores 1,8, this leads to the casting cores 1,8 bursting apart into nodule-like fragments. The volume and hardness of these is so low that the cast parts can be cleared of sand solely by the application of vibration energy, since all the old core sand is present already released from the casting. Hammer blows applied by a compressed air hammer, such as are still required with the prior art, are not required for the sand removal.

[0050] The reprocessing of the fragments of the casting core can include gentle breaking into grainy particles. The grainy particles thus obtained can then be subjected to a metal separation process and dust extraction, in order to produce the state required for their reuse. The casting mould parts recycled to grainy material are then again used as basic material for mould material composed in accordance with the invention.

[0051] If mould materials are used in the manner according to the invention, consisting of basic mould material such as synthetic mullite, mixed with water glass binding agent, then no emissions worth mentioning are incurred during the manufacture of the casting mould parts. As a result, it is possible to avoid the casting defects extensive precautionary measures for the suction extraction of gases and elaborate tool cleaning processes required repeatedly with conventional procedures as a result of gas formation. Environmental burdens and stress on operating personnel are thereby reduced to a minimum.

[0052] If mullite or a comparabl inert refractory material is used as a basic substance for the mould material system according to the invention, then a further advantage of the invention is realised in the chemical resistance of the basic mould material in relation to binding agents and melt. This property ensures that, with the method according to the invention, a casting is obtained of which the surface is entirely free of residual sand adherence after the emptying of the fragments of the mould cores and mould parts, without additional cleaning measures.

REFERENCE FIGURES

[0053] 1 Camshaft core

[0054] 2 Underside of the camshaft core 1

[0055] 3,4 Recesses

[0056] 5,6 Branches of the oil channel core 8

[0057] 7 Main section 7 of the oil channel core 8

[0058] 8 Oil channel core

[0059] A Length A of the branches 5,6

[0060] B Diameter B of the branches 5,6

[0061] C Length of the main section 7

[0062] D Diameter of the main section 7

Claims

1-17 (canceled).

18. A method for the manufacture of castings from a molten metal, in particular a molten light metal, comprising the following steps:

Manufacture of a casting mould part from the mould material, which is mixed from a pourable basic mould material which is inert in relation to the molten metal, exhibiting a proportion of synthetic mullite, which consists of particles of which the shape is essentially spherical, and an inorganic binding agent, wherein the thermal expansion behaviour of basic mould material and binding agent are adjusted to one another in such a way that the coefficient of thermal expansion of the molten metal lies above the coefficient of thermal expansion of the casting mould part manufactured from the mould material,

Assembling a casting mould making use of the casting mould part,

Pouring of the molten metal into the casting mould to form a casting,

Cooling of the casting over a solidification and cooling period, in which the casting mould part self-actingly disintegrates into fragments,

Removal of the fragments of the casting mould part from or out of the casting,

Reprocessing of the fragments of the mould material to form a pourable basic mould material.

19. The method according to claim 18, wherein the manufacture of the casting mould part comprises the shooting of the mould material into a hollow mould formed in a core box, the preliminary hardening of the mould material shot into the core box to form the casting mould part by the application of heat, and the hardening of the casting mould part in a heating device arranged outside the core box.

20. The method according to claim 19, wherein the heating device comprises a microwave heating.

21. The method according to claim 19, wherein the casting mould part is sprayed with binding agent before the hardening.

22. The method according to claim 18, wherein the reprocessing of the fragments of the mould material comprises the breaking of the fragments, the metal separation, the individualisation of grains, and/or dust extraction.

23. A mould material for the manufacture of casting mould parts for the casting of molten metal, in particular of molten light metal, consisting of a mixture of a pourable basic mould material, inert in relation to the molten metal exhibiting a proportion of synthetic mullite, which consists of particles of which the shape is essentially spherical, and an inorganic binding agent mixed with the basic mould material, wherein the thermal expansion behaviour of the basic mould material and binding agent are adjusted to one another in such a way that the coefficient of thermal expansion of the molten metal in each case lies above the coefficient of thermal expansion of the casting mould part manufactured from the mould material.

24. The mould material according to claim 23, wherein the binding agent expands differently when heated in comparison with the basic mould material.

25. The mould material according to claim 23, wherein the binding agent is stable under the effect of the casting heat.

26. The mould material according to claim 23, wherein the binding agent is a water glass binding agent.

27. The mould material according to claim 23, wherein the basic mould material consists entirely of mullite.

28. The mould material according to claim 23, wherein the molten metal is an aluminium melt.

29. The mould material according to claim 24, wherein its thermal conductivity is lower than that of the metal which is to be cast.

30. The method according to claim 18 performed with a basic mould material formed in accordance with claim 24.

31. The method according to claim 30, wherein the casting mould part is manufactured from the mould material is a casting core.

32. The method according to claim 31, wherein the length (A,C) of the casting mould part is many times larger than its diameter (B,D).