Abstract:
In a permanent casting die with an at least local ceramic lining, the ceramic lining at least partially forms surface contours of a die cavity. The ceramic lining is applied in a form-fitting manner to a metallic core and the ceramic lining with the core is anchored firmly or movably in the die cavity.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
         [0001]    The present application claims priority to Application No. 102 07 989.7, filed in the Federal Republic of Germany on Feb. 25, 2002, which is expressly incorporated herein in its entirety by reference thereto.  
         FIELD OF THE INVENTION  
         [0002]    The present invention relates to a permanent casting die and to a method for producing a ceramic lining for a permanent casting die.  
         BACKGROUND INFORMATION  
         [0003]    Permanent casting dies, both in gravity casting and especially in pressure die casting, are exposed to very high thermal and mechanical loads, which ultimately lead to steady wear. In series production, this leads to continuous overhauling and renewal of the casting dies. This gives rise to very high costs, which in turn have a sustained effect on component costs.  
           [0004]    European Published Patent Application No. 0 065 996 describes a metallic casting mold which has ceramic elements as a lining. The lining can be removed easily and is suitable especially for small individual components. However, the lining in the configuration described cannot be used for complex large components subject to high loads and is not capable of withstanding high mechanical stresses in continuous operation.  
           [0005]    It is an object of the present invention to configure a permanent casting die such that wear may be reduced and that remachining cycles may be increased.  
         SUMMARY  
         [0006]    The above and other beneficial objects of the present invention are achieved by providing a permanent casting die and a method as described herein.  
           [0007]    In an example embodiment of a permanent casting die according to the present invention, the permanent casting die has a ceramic lining which is firmly connected to a metallic core. For simplicity, the ceramic lining with the core is referred to below as a composite. The composite is inserted in a fixed or movable manner into a die cavity of the permanent casting die, the ceramic lining forming parts of the surface contour of the die cavity. In certain cases, it is possible for partial areas of the core to lie at the surface.  
           [0008]    The effect of this arrangement is that the comparatively brittle ceramic has a ductile support and, on the other hand, the ceramic surface is extremely wear-resistant.  
           [0009]    The service life of the composite may be increased even further if the coefficient of thermal expansion of the core and the lining is approximately the same. The coefficient of expansion of the core and the lining may differ by no more than 4×10 −6  K −1 , e.g., by no more than 2.7×10 −6  K −1 , absolute.  
           [0010]    This may be achieved by pairing materials in various manners. Thus it is possible to use any industrial ceramic, e.g., aluminum oxide, aluminum nitride, silicon or titanium carbide, hard metals and silicon nitride, etc., for the ceramic lining. Ceramic which exhibits almost no wetting, if any, with respect to a casting metal, which may be an aluminum alloy, may be suitable. This applies, e.g., to silicon nitride and aluminum nitride.  
           [0011]    All high-melting metals or metal alloys may be suitable as core materials. A suitable pair of materials may be silicon nitride and a molybdenum alloy. The difference between the coefficients of expansion of these materials may be small. The coefficients of expansion may be additionally aligned by alloying additives and/or additives to ceramic raw materials. It may be provided that the metal of the core has a high thermal conductivity to ensure as rapid as possible heat dissipation. This applies to molybdenum as a core material inasmuch as its thermal conductivity is about 140 W/mK. In this context, it may be provided that the core accounts for the highest proportion of the volume of the composite since the core may provide better heat dissipation than the ceramic lining.  
           [0012]    The ceramic lining may be only as thick as is required by the geometrical conditions. In the case of a self-supporting ceramic lining, the wall thickness of the lining is between 2 mm and 50 mm, e.g., between 3 mm and 15 mm. Ceramic linings applied in layers or by sintering have wall thicknesses of between 0.05 mm and 20 mm, e.g., between 0.5 mm and 5 mm.  
           [0013]    In an example embodiment of the present invention, the composite constitutes a largely cylindrical mandrel. In this example embodiment, the core may be cylindrical in shape, while the ceramic lining may be matched in a form-fitting manner to the cylindrical core.  
           [0014]    Certain advantages of the present invention may be achieved when the permanent casting die is a pressure die-casting die (pressure die-casting tool). The stresses on the permanent casting die are highest and the wear most severe in the pressure die-casting method (all casting methods in which casting is carried out at increased pressure).  
           [0015]    Cylinder/crankcases are components which require a particularly complex casting tool and are subject to high wear. The application of the present invention to a crankcase-casting tool may be advantageous, e.g., in its application to one or more mandrels reproducing the cylinder bores of the cylinder/crankcase. The mandrels may be subject to wear. The mandrels are mounted in the casting tool such that they may move and, after the casting of the crankcase, they move out of the tool for the removal of the crankcase from the die. As they move out, the mandrels are once again subjected to increased wear.  
           [0016]    The form-fitting connection between the ceramic lining and the core may be achieved through a large number of measures. An undulating (sinusoidal) core cross-section may be suitable, for example. The surface area of the core and hence the area of contact between the lining and the core may be increased by this refinement, resulting in better heat transfer. The enlargement of the areas of contact may be provided for the removal of the surface heat.  
           [0017]    For better matching of the ceramic lining and of the core, the core may be provided with a cone. The ceramic lining has a mating cone, leading to a firm, form-fitting connection.  
           [0018]    A thermally conductive paste, which is applied at least partially to the areas of contact, may help to promote heat dissipation from the surface to the core. The thermally conductive paste, which may be metal-based, e.g., zinc-based, titanium-based, etc., may provide the effect of balancing out differences in tolerances. Thus, the ceramic lining and/or the core may be produced with relatively wide tolerances, reducing machining costs.  
           [0019]    In an example embodiment of a method for the production of a ceramic lining of a permanent casting die according to the present invention, for the form-fitting matching and production of the ceramic lining, a raw ceramic compound is applied to a metallic core. This may be performed by the application of any technical skill, e.g., by pressing on, by slip application, by the application of a ceramic green body, by the application of preceramic polymers, etc.  
           [0020]    The raw ceramic compound with the core is subsequently solidified to form a ceramic. This is usually accomplished by a sintering process or, in the case of polymers, in the form of carbonization. The ceramic is then finish-machined, thereby generating the surface of the ceramic lining.  
           [0021]    In another example embodiment of a method for the production of a ceramic lining on a core according to the present invention, a ceramic layer is applied to the core, e.g., by a thermal spraying method. In this manner, very thin ceramic linings with wall thicknesses of between 0.05 and 5 mm, e.g., between 0.1 mm and 1.5 mm, may be achieved.  
           [0022]    Example embodiments of the present invention are explained in greater detail with reference to the following figures. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]    [0023]FIG. 1 illustrates a mandrel including a ceramic lining and a metallic core.  
         [0024]    [0024]FIG. 2 is a fragmentary view of a permanent casting die with a ceramic lining and a core.  
         [0025]    [0025]FIG. 3 a  is a cross-sectional view through a mandrel, the core having an undulating cross-section.  
         [0026]    [0026]FIG. 3 b  is a cross-sectional view through a mandrel, the core having grooves. 
     
    
     DETAILED DESCRIPTION  
       [0027]    The mandrel  2  illustrated in FIG. 1 is part of a pressure die-casting tool, which is used to produce a cylinder/crankcase. The mandrel  2  forms the aperture of the subsequent cylinder bore. It is mounted on a slide, by which it may be withdrawn after a casting operation. The mandrel  2  has a molybdenum core  6 , which is connected in a form-fitting manner to a ceramic lining  4  made of silicon nitride. Both the core  6  and the ceramic lining  4  have a cone  22 , by which a play-free composite may be ensured. The contact surfaces  5  between the core  6  and the ceramic lining  4  are coated with a zinc paste. The zinc paste serves for heat transmission from the ceramic  4  to the core  6 . The coefficient of thermal expansion of the molybdenum is approximately 5.6×10 −6  K −1 , while the coefficient of thermal expansion of the silicon nitride is about 3×10 −6  K −1 .  
         [0028]    The mandrel  2  may optionally have cooling channels passing through it in the core  6 . In the example embodiment illustrated in FIG. 1, the mandrel  2  has a free core surface  3 , which is used to form a surface contour. This free core surface may optionally be chosen with regard to the thermal and mechanical requirements in the respective permanent casting die. By virtue of the ceramic lining according to the present invention, the service life up to remachining of the mandrel may be tripled compared with conventional steel-based mandrels.  
         [0029]    In another example embodiment of the present invention illustrated in FIG. 2, one edge  11  of a permanent casting die  8  is reinforced locally by a ceramic lining  9  according to the present invention. The ceramic  9  is connected in a formfitting manner to a molybdenum core  7 . The composite is secured on the permanent casting die  8  on the rear side by a screw fastener  10 .  
         [0030]    Examples of core cross-sections are illustrated using a mandrel core ( 14 ,  18 ) as an example in FIGS. 3 a  and  3   b . The core  14  of the mandrel  12  has an undulating cross-section, which enlarges the area of contact with the ceramic lining  13 . This accelerates heat transfer from the core  14  to the ceramic  13 . The same effect is achieved by the measure illustrated in FIG. 3 d , in which the core  18  has grooves  20 , onto which the ceramic lining  17  is mounted. Both the grooves  20  and the wave shape of the core  14  serve to fix the ceramic  13 ,  17  on the core  14 ,  18 .  
         [0031]    The production of the composite according to the present invention may be achieved in, e.g., three manners. On the one hand, the core and the ceramic lining may be produced separately, finish-machined and then joined. Joining may be accomplished in a conventional manner. This may be performed by vacuum soldering, for example, leading to good heat transfer between the core and the lining, and any gaps may be eliminated.  
         [0032]    The second variant includes applying the ceramic as a raw ceramic compound to a prefabricated core. The ceramic is sintered on the core, it being possible for sintering to take place under pressure, e.g., by hot pressing. The complete composite is then finish-machined for the purpose of matching in the permanent casting die.  
         [0033]    By the second variant, it is possible to dispense with the finish-machining of the contact surfaces. The formfitting connection may be ensured by the sintering process. On the other hand, shrinkage of the raw ceramic compound occurs during the sintering process, and this may be taken into account beforehand. In the case of various geometric variants, gaps may thus occur between the core and the ceramic. This may be minimized by hot pressing for example. The particular production method for the composite according to the present invention may be determined from the geometric requirements of the composite.  
         [0034]    A third variant includes a ceramic layer being applied to the core. The layer may be applied by any conventional ceramic coating method. Thermal spraying methods, such as plasma spraying, flame spraying, wire arc spraying, etc., may be provided, for example.