CN1298456C - Method for producing castings, molding sand and its use for carrying out said method - Google Patents

Abstract

The invention relates to a method with which high-quality castings of complex shapes can be produced and by which mold parts can be safely removed from or from the casting after the casting process has been completed in a simple manner. For this purpose, the following steps are carried out: A mold material is produced by mixing a loose mold material which is inert compared with the metal melt and a binder, which expands less than quartz sand when heated and is viscous. The expansion of the mixture when heated is different from that of the mold material; a mold part is made from the mold material; a mold is combined using the mold parts; a metal melt is poured into the mold to make a casting; during the solidification and cooling time, the casting is cooled , during which parts of the mold are automatically broken into pieces; pieces are removed from or from the casting; pieces of mold material are processed into loose mold stock.

Description

Method of making castings, molding sand and its use for carrying out this method

technical field

The invention relates to a method for producing castings from metal melts, especially light metal melts, such as aluminum melts.

Furthermore, the invention relates to a mold material and its use for producing mold parts for casting metal melts, in particular light metal melts, such as aluminum melts. Such casting mold parts can, for example, be casting cores, by means of which a cavity is formed inside the casting to be produced. Likewise, mold parts according to the invention can also be components which are combined to form a multi-part mold which determines the shape of the casting to be produced.

Background technique

During the production of components from metal by casting, mold parts are required, by means of which the internal shape of the workpiece to be cast is determined on the one hand and its outer shape on the other hand. Such mold parts can, as already mentioned, be casting cores, with which the cavity is formed inside the casting to be produced, or mold parts, which are combined to form a multi-part casting mold, which defines the shape of the casting to be produced.

Molding material systems are usually used for producing molded parts, which consist of a mold material and a binder. These two components are mixed with each other, shaped and processed into a compact body in the right hardening process. Here, quartz sand is usually used as casting material, which is combined with an organic binder in most applications.

The use of quartz sand as raw material for the production of mold parts has proven to be expedient in many respects, especially when casting light metal materials. For example, such quartz sand is available inexpensively and is characterized by ease of processing and good quality when forming the mold components of the individual mold parts to be produced.

As an environmentally compatible alternative to organic adhesives, water glass based adhesives are proposed. This water glass binder is mixed with molding sand. The resulting mixture is then injected into the mold box of the molding machine, where it forms a cavity that reflects the shape of the mold part to be produced. Next, the mixture in the mold is dehydrated by supplying heat. Here, the heat supply can take place via a correspondingly heated type box or via microwave heating acting directly on the mixture (WO-A-86/00033, EP 0 917 499 B1, DE 196 32 293 A1).

In order to ensure the best processing results when casting metal melts, the mold materials used to manufacture the mold parts must have high strength and dimensional consistency. Keep. In addition, the mold material should be easily removable after casting. This has proven to be particularly important in particular when casting cores are used, which form complex-shaped cavities in the casting.

Finally, the mold material should be able to regenerate after use, so that the mold raw material can achieve the highest possible reuse rate. This can be achieved in a known manner by using an inorganic binder which releases a small amount of emissions during the manufacture of the mold parts and which can be produced almost residue-free by the action of a sufficiently high temperature after the casting process has ended. combustion.

Practical use has shown that the known molding material systems, regardless of organic or inorganic binders, have under normal conditions the properties required for optimal processing results.

However, especially with thin-walled mold parts, such as they are used, for example, for casting cores for engine blocks or cylinder heads as oil passages, the required dimensional accuracy of the casting may no longer be met due to unavoidable thermal expansion.

When casting complex-shaped castings, there is a further problem with conventionally produced mold parts that the molding sand can only be removed with difficulty from the casting after cooling. The casting is usually subjected to vibrations or shocks for this purpose, which should cause the casting core located inside the casting and the parts of the mold adhered to the outside of the casting to break apart and should cause particles of the mold material to flow out. However, this mechanical method for removing parts of the mold carries with it the risk of damaging the casting. Especially for thin or thin-walled components, cracks may occur.

It is therefore proposed, instead of mechanical measures acting on the casting, to heat the casting so intensely that the binder burns off and only the mold material remains, which can be easily removed as loose material from the casting and from the casting. The equipment required for this is expensive. Furthermore, the temperatures required to burn the binder are so high that the heating inevitably also causes changes in the properties of the metal casting.

Contents of the invention

It is an object of the present invention to provide a method with which high-quality cast parts of complex shapes can be produced and in which way the mold parts can be safely removed from or from the cast part in a simple manner after the casting process has been completed. Furthermore, a mold material is to be provided with which mold parts can be produced which are suitable for the production of high-quality castings with complex shapes and which can be safely removed from or produced in a simple manner after the casting process Casting removed.

This method-related object is achieved according to the invention by a method for producing a casting from a metal melt, in particular from a light metal melt, the method implementing the following steps:

- manufacture of mold parts from a mold material consisting of a mixture of a loose mold stock which is inert compared to the metal melt and a binder, wherein the thermal expansion characteristics of the mold stock and of the binder In coordination with each other, the thermal expansion coefficient of the metal melt is greater than the thermal expansion coefficient of the mold part made of the mold material,

- Using mold parts combined into one mold,

- casting a metal melt in a mold to make a casting,

- cooling of the casting during the solidification and cooling time, during which time the mold parts are automatically broken into pieces,

- fragments of molded parts removed from or from castings,

- Processing of pieces of mold material into loose mold stock.

The object of the above-mentioned further aspect is achieved by a casting mold material for the production of casting mold parts for casting metal melts, especially light metal melts, which consists of a mixture comprising a , loose mold stock and a binder mixed with the mold stock, wherein the thermal expansion properties of the mold stock and the binder are coordinated to each other such that the coefficient of thermal expansion of the metal melt is always greater than that of the mold part made of the mold material Thermal expansion coefficient.

The invention is based on the realization that by selecting a suitable mold material it is possible to produce mold parts which optimally combine the properties required for simple, safe and environmentally friendly production of high-quality, dimensionally accurate castings combined with each other.

The casting mold material according to the invention combines in an optimal manner those properties which are prerequisites for producing high-quality castings while being simple to manufacture. For this purpose, the molding material according to the invention contains a loose raw material in granular or granular-like form, that is, when it is heated unavoidably during casting, it exhibits a Sand has a much smaller amount of thermal expansion than sand. This casting material ensures high dimensional accuracy in the production of castings with complex shapes, even with small material thicknesses.

The raw material, which is loose in its unbonded state, is mixed with a binder which has different expansion characteristics when heated than the raw material. Due to the different thermal expansions of the mold stock and the binder, heating by the casting heat causes the binder to separate from the mold stock particles. Thus, as a result, when the binder expands more strongly than the raw material it will swell the mold part, causing it to lose its fixed shape and be easily removed from or from the casting. Conversely, the expansion behavior of the casting mold stock can be brought about in such a way that the volume change associated with heating breaks the bond with the binder and brings the stock back into a loose state. The key is to cause the core or mold member to swell with heating so that it disintegrates into easily removable and loose individuals after the casting cools.

By adapting the thermal expansion behavior of the molding material according to the invention to the thermal expansion behavior of the metal melt to be cast and, at the same time, producing the molding material on the basis of loose raw materials, it is achieved After cooling, the parts of the casting mold which are at least partially surrounded by the casting or which adjoin the casting break into loose individual parts due to the forces occurring during cooling, which can be easily removed. Here, the rupture of the mold part is caused by the forces produced by the different expansions of the cast metal and the mold material.

The invention has particularly advantageous effects when casting components from aluminum melts. Aluminum has a relatively large coefficient of thermal expansion, so during the casting and solidification of the melt, a large force is exerted on the part of the mold that is in contact with the cast component, thereby reliably breaking the part of the mold involved into smaller individuals . It has proven to be particularly advantageous if the mold part is a core.

Another advantageous feature of the molding material obtained and used according to the invention is that the binder and the molding material are coordinated in such a way that the particles of the material are thermally stabilized and non-toxic by the binder in the mold part produced from the molding material. elastically held together. Mold parts produced from such mold stock obtained exhibit brittleness in the entire temperature range experienced during casting of the melt, thereby promoting the desired fracturing of the mold parts according to the invention.

The binder of the mold part is preferably chosen such that it does not decompose under the action of heat. In this way, it is avoided that volumes are released in the mandrel, which could lead to an undesired elasticity according to the invention of the mold part concerned.

Another advantageous configuration of the invention is that the particles of the molding material have an essentially round, spherical shape. The spherical shape of the mold stock and the associated point contacts which predominate between the particles of the mold stock promote the automatic splitting of the mold parts under the action of the mechanical forces which occur during the casting and solidification of the melt. A mold material that meets this requirement particularly well is synthetic mullite. A further advantageous refinement of the invention therefore provides that the casting mold raw material has at least a portion, preferably alumina sand (mullite) replacing more than 50% or more than 70% of the quartz sand. Mullite has a round particle shape and has a density similar to quartz sand. As a result, mold materials made from it are much simpler to process than, for example, known _ZrO2 sand. In addition to the advantages of the mechanical fragmentation of the mold parts achieved according to the invention, the spherical shape of the mullite particles leads in practice to simplify the processing of mold materials made of this mold stock and at the same time to reduce the Tooling and machine wear for molded parts. In addition, casting mold materials with a high mullite content, due to their low thermal expansion, enable high dimensional accuracy in the manufacture of castings with complex shapes, even with small material thicknesses.

Surprisingly, it has been found that the breakage of the mold parts produced with the mold material composed according to the invention is automatically delayed in time relative to the casting of the metal melt to such an extent that the breakage is no longer relevant to what has already been done at this moment. Fully solidified castings bring about any negative effects.

Due to its special properties, the molding material composed according to the invention is particularly suitable for the production of casting cores. They can be removed after casting without risk of damaging the finished casting.

The casting mold material consisting of the mullite-quartz sand mixture has rather a thermal insulation effect on the casting mold material produced therefrom. These materials can therefore be used purposefully for casting technology applications in which, despite the resulting heating above the critical temperature of quartz sand of 573° C. Thermal conductivity plays a secondary role or should consciously limit heat transfer.

Practical tests have shown that by adding a sufficient amount of mullite sand to the quartz sand, it is possible to prevent the spontaneous geometry change that would occur when using quartz sand alone for the manufacture of slender, slender castings. The key here is that the proportion of Al ₂ SiO ₅ sand is in any case sufficiently large to be able to compensate for the length change of the quartz that would otherwise occur in the quartz sand when heating exceeds a critical temperature.

In the molding material obtained according to the invention, the binder and the raw material are preferably coordinated to one another in such a way that the mold parts produced from the molding material have a low thermal conductivity. This characteristic leads to the fact that after casting the metal melt, the temperature difference between the casting material and the mold part remains large, so that the risk of premature thermal or chemical decomposition of the mold part is minimized.

In addition, the fragmentation of the casting core can be supported by measures, that is, the composition of the mold material is coordinated so that the mold material and the binder expand differently when heated. The connections are broken by the heating caused while the melt is being cast.

In particular, the invention can be implemented in such a way that a molding material is processed which consists of a mixture of loose raw material in granular or similar granular form and an inorganic binder.

The advantage of using inorganic binders is their better environmental compatibility and the fact that mold parts produced with such binders can be returned to the mold material cycle without problems. Molding materials which have proven to be particularly suitable in this respect are those which consist of a water glass-based binder mixed with a molding material composed according to the invention. The point here is that the expansion properties of the intermixed components differ sufficiently from one another.

It is particularly advantageous in this respect that the molding stock expands differently from the binder. In this case, heating by the casting heat leads to separation of the binder from the particles of the casting mold material. For example when the binder expands more strongly than the casting material the binder expands the mold part causing it to lose its fixed shape and break into pieces. These fragments can be easily detached from or from the casting without risk of mechanical damage. For this reason, it is important in this solution that the self-fracturing of the mold part, which is sought according to the invention, is based on the fact that the mold material and the binder expand differently and in such a way that under the action of the casting heat the binder Due to the thermal stress formed between the mold material and the binder, the particles of the mold material are detached or self-ruptured. Due to this brittle fracture behavior of the binder after the mold part has hardened, the bonds between the individual particles of the mold material are broken and the mold part is broken. The remaining loose mixture of mold stock fragments and binder fragments is loose and can be easily removed from or from the casting.

The mold parts can also be produced according to the method according to the invention by injecting the mold material mixture according to the invention into the core box of the core molding machine in a known manner. Subsequently, the mold material is hardened, for example, as described in DE 196 32 293 A1, for which a negative pressure is applied in the cavity heated to a temperature of 100°C to 160°C in the core box, and the core blank is cored The oven heating duration is 20 to 30 seconds.

During this time, the casting mold is partially cured enough that it can be removed from the core box and placed in a heating device, such as a microwave oven, provided outside the core box. In the heating device it is heated with sufficient thermal power until a sufficient quantity of water has been drained from the mold part for complete hardening.

As an alternative or in addition to microwave heating provided outside the core box, drainage can also be achieved by sufficiently heating the core box itself or by blowing hot air. These measures can each be combined with heating outside the core box. Drainage can likewise be caused by microwave heating directly on the core blank while it is still inside the core box.

If the heating of the mold parts takes place outside the core box for hardening, the relevant mold parts can be sprayed with a binder liquid in order to increase the surface strength of the core. The casting mold parts treated in this way have a higher stability with likewise increased wear resistance, so that they can be stored without problems and meet the highest demands on their dimensional accuracy. The use of water glass binders has proven to be particularly advantageous with regard to the optimization of the quality of the castings to be produced.

Description of drawings

The invention is explained in greater detail below with the aid of figures showing exemplary embodiments. The single figure schematically shows a sectional view of a camshaft core for a casting mold for a cast aluminum alloy cylinder head, not further shown.

Detailed ways

The underside of the camshaft core is formed at a distance from one another in the longitudinal direction with two recesses, which respectively determine the shape of the bearing seat provided for supporting the camshaft on the cylinder head to be produced. A branch of an oil passage core extending with its main section parallel to the camshaft core and at a certain distance therefrom is respectively placed in the grooves. Here, the length of the branch is many times greater than its diameter. Likewise, the length of the main section of the oil passage core is many times greater than its diameter.

The oil circuit core is produced in a known manner in a conventional sand-shot molding machine with the molding material according to the invention, by mixing a molding material consisting of mullite sand and quartz sand with a water glass Adhesive made. Due to the proportion of mullite sand, it is ensured that the oil duct core expands uniformly and thus uniquely predeterminable even if it is heated to more than 573° C. during casting of the cylinder head to be produced.

In this way, it is reliably avoided, such as fractures in the branches, elongation of the main section in the region between the branches, which occur in the conventional production of oil channel cores on the basis of casting mold materials containing pure quartz sand , and bends in the region of the free end of the main section. Cylinder heads and similar cast parts with fine channels extending over a large length can thus be produced reliably with high precision during the casting of large quantities of light metal by using the casting mold material composed in the manner according to the invention.

During the casting of the metal melt, preferably aluminum or other light metal melts, and while the metal of the casting is still flowable, the casting core deformation is slight. This slight thermal expansion of the mold stock helps the process reliably achieve the dimensional requirements of the casting.

After a solidification and cooling time, during which the casting acquires sufficient strength for further processing and the individual casting cores are automatically broken into pieces due to the action of the casting heat and due to the different thermal expansion properties of the casting material and the binder, They are evacuated and reworked from castings. During solidification and during the cooling phase after complete solidification of the metal, the casting cores are subjected to high mechanical stresses due to the much greater solid shrinkage of the cast metal compared to the casting cores. Due to the brittle, inelastic nature of the casting core, this mechanical stress causes the casting core to break into massive fragments. They are so small and strong that they can be dislodged from the casting simply by adding vibrational energy, since all the core sand is now separated from the casting. For screenout it is not necessary to apply hammering blows by pneumatic hammers, as is also required in the prior art.

Regeneration of foundry core pieces may include careful breaking into granular particles. The granulated particles obtained can then be subjected to metal separation and dedusting in order to bring them to the necessary state for their reuse. The part of the mold that is recycled into granular material is then reused as raw material for the mold material composed according to the invention.

If molding materials are used in the manner according to the invention, which consist of molding raw materials, such as mullite, mixed with a waterglass binder, no appreciable emissions are produced during the production of the mold parts. This makes it possible to avoid the casting defects which often occur in conventional embodiments due to the formation of gases, the extensive precautions required to evacuate the gases, and the troublesome cleaning of the tool. The burden on the environment and the operator is thus reduced to a minimum.

If mullite or similar inert refractory materials are used as starting material for the mold material system according to the invention, a further advantage of the invention lies in the chemical stability of the mold material compared to the binder and the melt. This property guarantees that the surface of the cast parts obtained when carrying out the method according to the invention is completely free of adhering residual sand grains after the mold cores and mold parts have been emptied of fragments without additional cleaning measures.

Claims (15)

1. make the method for foundry goods with a kind of metal bath, comprise the following steps:

-make mould portion with molding material, molding material is in a ratio of inertia, loose, some synthetic mullite, that is made up of particle, that grain shape is essentially sphere mold raw material and a kind of inorganic bond by a kind of and metal bath and mixes, wherein, the thermal expansion character of mold raw material and adhesive is coordinated with each other to be, make the thermal coefficient of expansion of the thermal coefficient of expansion of metal bath greater than the mould portion of making by molding material

-use mould portion to be combined into a mold,

-metal bath is cast to makes a foundry goods in the mold,

-solidify with cool time in, make foundry goods cooling, at this moment between in the process, mould portion is fragmented into fragment automatically,

-from or remove the fragment of mould portion by foundry goods,

-fragment of molding material is processed as loose mold raw material.

2. it is characterized by in accordance with the method for claim 1: the manufacturing of mould portion comprise with molding material be injected in the hollow mould of a design in the core case, to make the molding material pre-hardening of injecting in the core case be mould portion and mould portion is located in the core case external heated device at one harden by heat supply.

3. it is characterized by in accordance with the method for claim 2: heater comprises a microwave applicator.

4. according to claim 2 or 3 described methods, it is characterized by: mould portion sprays with adhesive before sclerosis.

5. according to claim 2 or 3 described methods, it is characterized by: the processing of molding material fragment comprises pulverizes fragment, separating metal, particle classification and/or dedusting.

6. be used to make the molding material of the mould portion that casting metal melts uses, form by a kind of mixture, mixture comprise a kind of and metal bath be in a ratio of inertia, loose, some synthetic mullite, form by particle, its grain shape is essentially spherical mold raw material and a kind of inorganic bond that mixes with the mold raw material, wherein, the thermal expansion character of mold raw material and adhesive is coordinated with each other to be, the thermal coefficient of expansion that makes metal bath is always greater than the thermal coefficient of expansion of the mould portion of being made by molding material.

7. according to the described molding material of claim 6, it is characterized by: the expansion of adhesive is different from the mold raw material during heating.

8. according to claim 6 or 7 described molding materials, it is characterized by: adhesive is stable under the foundryman's fever effect.

9. according to claim 6 or 7 described molding materials, it is characterized by: adhesive is a kind of water glass binder.

10. according to claim 6 or 7 described molding materials, it is characterized by: the mold raw material is made up of mullite fully.

11. according to the described molding material of claim 7, it is characterized by: metal bath is a kind of aluminum melt.

12. according to the described molding material of claim 7, it is characterized by: the thermal conductivity of molding material is less than the thermal conductivity of metal to be cast.

13. the mold raw material that constitutes according to one of claim 7 to 12 is used to implement the application according to the method for one of claim 1 to 5 design.

14. according to the described application of claim 13, it is characterized by: the mould portion made from molding material is a casting core.

15. according to the described application of claim 14, it is characterized by: the length of mould portion is than the big manyfold of its diameter.