Abstract:
The invention proposes a muffle with a pressing channel and at least one mold cavity that is coupled to the pressing channel via at least one connection channel. In this case, in order to increase the pressing pressure and to improve the intermixing of the blank material, it is proposed to arrange at least two blanks ( 18 ) side by side in the pressing channel ( 12 ).

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims foreign priority benefits under 35 U.S.C. §119(a)-(d) from Austrian patent application ser. no. A 1773/2007 filed Nov. 5, 2005. In addition, this application claims foreign priority benefits under 35 U.S.C. §119(a)-(d) from European patent application ser. no. 08 116 141.5 filed Oct. 8, 2008. 
     TECHNICAL FIELD 
     The invention relates to a muffle in particular for producing ceramic dental restoration components, with a pressing channel serving to receive a press blank and with at least one mold cavity that is coupled to the pressing channel via at least one connection channel, wherein the pressing channel ( 12 ) is capable of receiving at least two press blanks ( 18 ) side by side, and in particular abuting on one another. and a method for producing ceramic dental restoration components by using the aforementioned muffle. 
     BACKGROUND OF THE INVENTION 
     A muffle for producing ceramic dental restoration components, as known for example from DE 10 2004 013 668 A1, usually comprises a pressing channel and at least one mold cavity that is coupled to the pressing channel via at least one connection channel, whereby the material of a press blank inserted into the pressing channel fills the mold cavity if a pressing force is applied and the press blank is heated, and thus provides a fired ceramic dental restoration component. 
     In practice, however, again and again cracks and even cavities arise in ceramic dental restoration components that impair the strength and the quality of the ceramic dental restoration component. 
     It has been proposed to form a press chamber with a particular cross-section in a muffle, whereby a blank material in the form of a bulk material can be filled into the press chamber and is pressed into a ceramic dental restoration component. In this case, however, air pockets can remain that cannot be completely eliminated by the pressure of the press stamp during the sintering process either. 
     The pressing force that is effective in the mold cavity according to the proposed solution, is comparatively low due to the shape of the pressing channel—probably due the the wall friction. Further, there is the danger that a press stamp guided in the pressing channel easily jams as a result of the particular cross-section. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     Accordingly, it is the object of the invention to provide a muffle as claimed in the preamble of claim  1  and a method for producing a ceramic dental restoration component as claimed in the preamble of claim  9 , with the aid of which the tendency to form cracks and cavities is considerably reduced when pressing and firing ceramic dental restoration parts in particular. 
     According to the invention it is provided to shape a pressing channel in such a manner that at least two press blanks can be received in the pressing channel side by side, in order to combine the large cross-section of the pressing channel with a comparatively small cross-section of the connection channel. 
     Apparently, the comparatively small connection channel prevents the blank material from entering the connection channel for the first moment, when the melting of the grain boundaries of the blank material begins and the blank material is still quite viscous. Only if the blank material is quite thin fluid due to further melting, the blank material overcomes the “barrier” of the connection channel inlet and in its thin fluid form is then also capable of penetrating into the fine branches of the mold cavity, in fact without the risk of air pockets. 
     Thus, it is preferred that the one or each connection channel has a diameter of 0.8 mm to 1 mm and in particular of 0.6 to 0.7 mm whereby it is appreciated that further connection channels can also just like that be provided in the muffle that do not change the inhibiting effect for viscous compounds. 
     Based on the fact that several press blanks are received side by side, a quite large cross-section of the pressing channel arises, whereby the transition between the pressing channel and the connection channel runs still more sudden. When pressing the molten blank material from the press blanks, the molten blank material is subjected to a shear stress as a result of the edges at the entrance or inlet to the connection channel. The shearing forces are the larger, the larger is the cross-sectional area of the pressing channel in comparison to the cross-sectional area of the connection channel. Large shearing forces lead to the fact that air pockets in the molten blank material due to compression can migrate along the grain boundaries of the material, and are thus squeezed out of the material. Thus, with the aid of a sudden transition between pressing channel and connection channel, air pockets and bubbles can be squeezed out of the the molten blank material, thus further improving the quality of the finished product. 
     Further, according to the invention, the molten material from the press blanks is better swirled due to the sudden transition between pressing channel and connection channel and is thus better intermixed. Based on a large cross-section of the pressing channel in combination with a small cross-section of the connection channel, a large shearing effect or stress can be generated at the transition that so to speak squeezes out air pockets. 
     The swirl or turbulence can be further increased by a specific surface treatment of the wall of the connection channel. Further, based on a substantially concave construction of the pressing channel when viewed from the inside that has no protruding corners and edges, the friction of the press stamp at the wall of the pressing channel is reduced. Thus, the press stamp in the pressing channel less easily jams, whereby the pressing channel not necessarily needs to have completely concave shape. 
     The inventive construction of the pressing channel whose cross-section matches the outer contour or external form of a multiple arrangement of flat blanks abutting on one another further provides more space in the pressing channel for receiving a large blank volume and simultaneously provides a compact construction of the muffle. 
     Due to the large cross-sectional area of the pressing channel a large amount of blank material can be pressed by means of a low stroke carried out by the press stamp. 
     At the same time, the muffle according to the invention can be employed more flexibly, since both small ceramic dental restoration components and large ceramic dental restoration components can be easily produced by means of the muffle. The amount of dental restoration material required for production can be controlled by the number and size of the press blanks arranged in the pressing channel. 
     Incidentally, due to the larger difference in cross-section between pressing channel and connection channel, the lateral flow rate in the connection channel is increased. The result is a larger friction and thus a still better intermixing of the blank material. 
     The effect according to the invention can also be achieved with the aid of a single blank only that is employed in the large pressing channel according to the invention. Said blank fills the entire bottom surface of the large pressing channel according to the invention during the melting process, thus offering the above-mentioned advantages. 
     The intermixing of the blank material can be further improved in the case that the diameter of the connection channel leading from the pressing channel to the mold cavity at least partially increases, whereby an irregular surface of the connection channel can be used to further increase the turbulence. 
     In order to facilitate the flow of the blank material into the connection channel, said connection channel can be shaped as a funnel at its entrance. This can be achieved by a bezel, a chamfer or rounding off or any other technical measures. 
     In order to optimally make use of the muffle, the profile of the perimeter of the pressing channel should be adjusted to the arrangement of the blanks. The best results can be achieved if the blanks not only support each other in the pressing channel but also sit close to the wall. In the case of two round blanks this is achieved by providing an oval cross-section of the pressing channel for example. In such a pressing channel, preferably smaller gaps are provided so that a counter or back pressure that has an effect on the entrance of the connection channel, starts to build up relatively fast during the melting process. 
     In order to ensure an optimum stability of the muffle, the wall thickness of the muffle can amount to between half and 1.5-times the diameter of the pre-pressing chamber, in particular the single diameter. 
     Preferably, the pressing channel—if viewed in perimeter direction—can be free. from inward-facing edges. If the pressing channel in this case is bent edge-free in all peripheral directions, the risk that the press stamp in the pressing channel jams, is minimized. 
     In the muffle, the second blank can at least abut linearly on the first blank and on the wall of the pressing channel along lines that extend parallel to the axis of the pressing channel, and the second blank can two-dimensionally abut on the first blank and on the wall of the pressing channel at a surface that extends in a direction transverse to the axis of the pressing channel, the surface in particular extending in a bent manner. 
     The connection channel can be formed with sharp edges at its inlet from the pressing channel. The ratio between a diameter of the connection channel and a diameter of the pressing channel can be smaller than 10%, preferably smaller than 2%, and the wall thickness of the muffle can be in between 10% and 50% of its diameter, said muffle preferably having a wall thickness of approximately one third of its diameter, and the bottom thickness of the muffle can amount to between 25% and 75% of its total height, said muffle preferably having a bottom thickness of 50% of its total height. 
     The muffle can have a sloping face at the outside to its pressing channel side that extends over more than 10% of its height for example, in particular more than 20% of its height, and that renders the muffle more flexible at this location. 
     When processing press blanks, the sum of the volumes of the blanks can substantially be the same as the sum of the volumes of the connection channel and the mold cavity, and the blanks can have a cross-section that is substantially round, oval, elliptic and/or polygonal. 
     In a further refinement it is provided that the pressing channel is capable of receiving three press blanks. 
     In a further refinement it is provided that the press blanks abut linearly on one another. 
     In a further refinement it is provided that a cross-section of the pressing channel is substantially round, oval, elliptic or polygonal. 
     In a further refinement it is provided that all of the press blanks together substantially completely fill the pressing channel. 
     In a further refinement it is provided that a blank outer contour formed in the pressing channel by the press blanks arranged side by side, matches the cross-section of the pressing channel. 
     In a further refinement it is provided that the pressing channel is capable of receiving two levels or layers of at least two, in particular three press blanks each. 
     In a further refinement it is provided that the pressing channel is capable of receiving two levels or layers of blanks, said blanks being different in number and different in size. 
     In a further refinement it is provided that prior to pressing at least two press blanks are inserted into the pressing channel of the muffle side by side and preferably abutting on one another. 
     In a further refinement it is provided that prior to pressing three blanks are inserted into the pressing channel side by side and abutting on one another. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       Further advantages, details and features emerge from the following description of several exemplary embodiments based on the drawings, in which: 
         FIG. 1  shows a cross-section of a first exemplary embodiment of a muffle according to the invention with three blanks received therein and with one press stamp; 
         FIG. 2  shows a first to third exemplary embodiment of a connection channel within the muffle; 
         FIG. 3  shows a cross-section of a second exemplary embodiment of the muffle according to the invention; 
         FIG. 4  shows a partially broken perspective and selective view of a third exemplary embodiment of the muffle according to the invention with blanks received therein; 
         FIG. 5  shows a partially broken perspective and selective view of a fourth exemplary embodiment of the muffle according to the invention with blanks received therein; and 
         FIG. 6  shows a partially broken perspective and selective view of a fifth exemplary embodiment of the muffle according to the invention with blanks received therein. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , the inventive muffle  10  comprises a pressing channel  12 , several connection channels  14  and several mold cavities  16 . Three press blanks  18  are arranged side by side in the pressing channel  12  of the muffle  10 , said press blanks being pressurized with the aid of a press stamp  20 . 
     As can be immediately seen from  FIG. 1 , the plurality of press blanks  18  arranged side by side, covers a large bottom surface  26  of the muffle  10 . In contrast to this, the inlet apertures  22  into the connection channels  14  are considerably smaller, for example by the factor 20 or 50. Thus, the press stamp  20  has to exert a high pressure on the press blanks  18  in the pressing channel  12 , in order to squeeze the molten blank material out of the press blanks  18  into the connection channels  14  and in order to overcome the resistance of the blank material to deformation. Due to the narrow inlet, the connection channels  14  act so to speak as a barrier against an insufficient free-flowing blank material that possibly still comprises air bubbles. If the material due to its fluidity or low viscosity, respectively, has been well intermixed and air bubbles have been squeezed out of the molten blank material, the material ist so thin fluid that it enters the connection channels  14 . As a result, the molten blank material entering the mold cavities  16 , is more homogeneous so that less cracks form upon curing the ceramic dental restoration component. 
     With the aid of a skillful arrangement of the connection channels  14 , the intermixing of the molten material from the blanks  18  can be further improved. Effective results can already be achieved with a cross-sectional ratio between the connection channel  14  and the pressing channel  12  of approximately 1:100; the best results can be achieved with a cross-sectional ratio of approximately 1:2500. 
       FIG. 2  shows measures how the turbulence of the blank material can be influenced by the inlet aperture  22 . 
     A sudden transition between the pressing channel  12  and the connection channel  14  can be achieved with the aid of sharp edges  32  at the inlet aperture  22 , thus effecting a more intensive shearing stress or effect. 
     Alternatively, it is possible that the inlet aperture  22  has rounded edges  28  or chamfered edges  30  between the connection channel  14  and the pressing channel  12 . In this case, the inlet apertures of the individual connection channels  14  act as funnels that further compress the molten blank material. Thus, it is intended to squeeze out air pockets in the material when it enters the connection channel  14 , if possible. 
       FIG. 3  shows a further exemplary embodiment of the inventive muffle  10 , however not to scale. Features that have already been discussed with reference to  FIG. 1 , have the same reference numerals and are not discussed once again. The muffle  10  comprises a bottom area  40  and a wall  42 . The thickness of the bottom area  40  is selected such that it approximately amounts to 0.5 to 1.5 times the depth H 1  of the pressing channel  12 . The wall  42  of the muffle  10  is also embodied in such a manner that it has a thickness of 0.5 to 1.5 times the width of the pressing channel  12 . Preferably, the thickness of the bottom area  40  is identical to the depth of the pressing channel. Similarly, the thickness of the wall  42  corresponds to the width of the pressing channel  12 . 
     In the present exemplary embodiment, the muffle  10  comprises a sloping edge  46  whose height HA corresponds to more than 10% and preferably more than 20% of the total height H of the muffle  10 , thus rendering the muffle  10  more flexible and enabling it to better absorb the stresses on the muffle material that result from pressing the press blanks  18 . 
     As it is illustrated in  FIG. 4 , the pressing channel  12  of the muffle  10  in principle can be constructed to have oval shape such that two press blanks  18  can be received side by side. With a muffle of this kind, the pressing channel  12  is capable of taking or holding comparatively large amounts of blank material. 
     Nevertheless, the stroke required for processing the blank material to be pressed, is comparatively small such that the inventive muffle  10  can be used in a standard press oven without any problems. 
     Since according to  FIG. 4  the pressing channel  12  of the muffle  10  is concave and is formed without points of discontinuity, it has a comparatively small periphery. This offers little working surface for the press stamp  20  such that the friction between the press stamp  20  and the wall of the pressing channel  12  ist small, and the press stamp  20  can exert a comparatively high pressure on the molten blank material. 
     In order to keep gaps  44  between the individual press blanks  18  as small as possible, press blanks  18  of various sizes can be used, as it is shown in  FIG. 5 . As a result, the processing time of the blank material in the muffle  10  is reduced and the quality of the dental restoration component to be produced is increased. The deformation of the press blank  18  by means of the press stamp  20  first leads to the filling of the gaps  44  between the press blanks  18 . These gaps  44  can have an extremely small volume if the press blanks  18  according to  FIG. 6  have a square cross-section, and if the pressing channel has a square construction, too. 
     While a preferred form of this invention has been described above and shown in the accompanying drawings, it should be understood that applicant does not intend to be limited to the particular details described above and illustrated in the accompanying drawings, but intends to be limited only to the scope of the invention as defined by the following claims. In this regard, the term “means for” as used in the claims is intended to include not only the designs illustrated in the drawings of this application and the equivalent designs discussed in the text, but it is also intended to cover other equivalents now known to those skilled in the art, or those equivalents which may become known to those skilled in the art in the future.