Abstract:
A muffle furnace for producing dental prosthesis has a muffle into which dental material is placed and a drive to which a piston is connected. The piston is moveable by the drive into the muffle to apply pressure onto the dental material. A pressure sensing device for measuring pressure applied to the dental material is provided. The pressure sensing device has a pressure sensor and a deformation member having a first side and a second side. The first side of the deformation member is subjected to a counter force of the piston and the second side of the deformation member is fixedly attached to the muffle furnace.

Description:
This application claims benefit of U.S. Provisional application Ser. No. 60/108,098, filed Nov. 12, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a furnace for producing dental prostheses with a muffle, whereby the furnace comprises a piston driven by a drive which can be introduced into the muffle for exerting pressure onto the dental material, whereby a pressure sensor for detecting the pressure exerted by the piston is provided. 
     Furnaces for producing dental prostheses or dental replacement parts comprised of dental materials, especially dental ceramics, have been known for a long period of time. A plunger-type piston applies pressure onto the dental material positioned in the muffle whereby the muffle together with at least the lower portion of the piston is heated in the furnace. The furnace heats the muffle and thus the dental material for such a length of time until the dental material, under pressure applied by the piston, has completely filled the voids present in the muffle for producing the dental prostheses. 
     From German Patent 664 133 it is known that the inclusion formation of bubbles can be avoided when pressure is applied for an extended period of time. 
     The drive of the piston can be provided either by weight application or pneumatically or electrically with corresponding drive devices. For providing a cycle time as short as possible, while preventing inclusion of bubbles, it is favorable when the advancing speed of the piston is controllable. In this respect, pneumatically or electrically driven drive devices have been successfully used. 
     Different types of such furnaces are known. It has also been suggested already to provide an electric pressure cylinder design for driving the piston. For detecting the working pressure of the piston, the current uptake of the drive motor for the corresponding drive spindle is employed whereby, in addition, a travel/time measurement is carried out. 
     It is an object of the present invention to improve a muffle furnace of the aforementioned kind such that, while preventing bubble inclusion, an improved product quality of the aforementioned dental prostheses is ensured. 
     SUMMARY OF THE INVENTION 
     This object is inventively solved in that the pressure sensing device comprises a deformation member that is loaded at one side, especially at the rear end of the piston with respect to the muffle, by a counter force of the piston and is supported with its other side at the muffle furnace. 
     The inventive measures, i.e., loading one side of the pressure sensing device with a counter force of the piston, take into account especially the elasticity of the piston, which, when employing travel measurement, remains unaccounted for, for the control function, respectively, the desired control parameters. Surprisingly, a very precise observation of the operating perimeters of the furnace and of the piston can be ensured. The inventive solution detects exactly the pressure forces acting within the piston whereby it is essential that the entire load of the piston is taken into consideration for computation. 
     In a preferred embodiment of the inventive furnace, a deformation member, which is especially made of metal, is positioned between the rear end of the drive and a support provided at the muffle furnace. This embodiment allows a realization of the pressure sensing device that is not prone to fatigue or aging. 
     In a further preferred embodiment of the invention, the pressure sensing device is arranged between the end of the piston, which is remote from the pressure-applying end of the piston, or the drive provided thereat and a counter plate. The drive is provided between the piston and the counter plate and is preferably a step motor. The entire force which is produced by the drive is then received by the counter plate and introduced into the pressure sensing device which is secured by pull elements that are connected to the bottom plate of the muffle. 
     Thus, a closed force circuit is provided. It is understood that the pressure force of the piston is distributed onto all employed pull elements. Inventively, it is especially favorable when the pressure sensing device is arranged coaxially so that angular deviations are compensated or arranged. 
     In another preferred embodiment, a homogenous rubber plate is provided as the deformation member. It supports one of the sensor elements while the other sensor element is supported at the sensor plate. 
     In this embodiment, a defined portion of the force produced by the drive can be detected by the pressure sensor whereby this portion corresponds to the surface area occupied by the pressure sensor at the surface area of the deformation element. 
     When, for example, a maximum force of 300 N is produced by the drive, a precision pressure sensor with a measuring range of 30 N can be employed which takes up a surface area of {fraction (1/10)} of the total surface area of the deformation member. 
     It is especially preferred to provide a drive comprising a step motor and to preassemble the drive as a complete drive unit. When needed, the preassembled unit is attached by an adapter to already existing furnaces for improving the manufacturing precision. The preassembled drive unit allows to adjust the initial pressure exerted on the deformation member to such a low level that it remains, for example, under half the measuring precision of the pressure sensor. When the measuring precision of the pressure sensor is, for example, 0.2%, and when the maximum force to be applied by the piston is 300 N, the initial securing force which acts on the deformation member can be adjusted such that it does not surpass 3 N. In this manner, on the one hand, a safe securing action is ensured and, on the other hand, it is ensured that no measurable false readings are produced by mounting the drive unit. 
     Even though, in principle, a controlled dc motor for providing the drive force can be used, it is preferred to employ a step motor. It comprises preferably a threaded spindle which is a unitary part of the drive axle and supports a threaded sleeve. The threaded sleeve transforms the rotational movement into a linear movement in accordance with the pitch of the thread. It is understood that a rotational stop is provided which prevents rotation of the threaded sleeve. Such a rotational stop can be, for example, positioned with minimal play at one of the pull elements and can be embodied as a stop that acts in both rotational directions. Such a stop, in a modified embodiment, can also serve as a base for providing a travel sensor. For this purpose, the stop can either be coupled to a potentiometer slide or can provide an optical means that indicates the exact position of the piston by a binary code. 
     Surprisingly, the inventive coaxially arranged drive exhibits a substantially improved driving precision, especially in comparison to a pneumatic drive or a drive with a motor that acts on a toothed piston rod. The axial force application direction eliminates angular errors and the resulting frictional losses, respectively, reduces frictional losses to a neglectable magnitude. 
     It is especially advantageous that for a complete pressing of the material into the muffle voids, no air buffers are present so that the drive system has a very minimal elasticity coefficient. When employing a step motor as the drive, each individual step of the motor provides a substantial increase of the drive force. The travel/force characteristic line of the inventive furnace is thus advantageously suddenly very steep so that an instant detection of the end of the pressing step can be realized. 
     In this context it is especially advantageous that, inventively, the end of firing within the furnace can be coupled to an exact point in time which is defined by the end of the pressing step. After completion of a programed, advantageously fixedly adjusted post-pressing period, in an advantageous embodiment the electrically driven pivot mechanism of the furnace cover can be pivoted so that the muffle can be automatically and quickly cooled by opening automatically the furnace chamber. 
     This embodiment of the furnace is especially advantageous for firing lithium disilicate glass ceramics which exhibit a very strong reaction with the embedding material in the hot state of the embedding material. The intensity of the reaction directly depends on the exposure time and is accordingly substantially reduced when the end of the firing process can be detected and a cooling process can be automatically started. 
     This solution provides an especially advantageous progress with respect to conventional solutions in which a time control was used and the end of the pressing step could not be detected within a time period of less than three minutes. The inventive solution thus allows to reduce the processing time by up to three minutes. 
     According to a further especially advantageous aspect of the invention, the inventive solution may eliminate a subsequent etching with acid such as HF so that the respective manufacturing time period is no longer needed and a more precise fitting of the dental prostheses can be achieved. 
     Inventively, it is further advantageous to adapt the advancing speed of the piston with respect to the speed as well as the force so that an optimized adaptation to the specified pressing and firing task can be performed. In a preferred embodiment a constant force, for example, 250 N is first applied and a constant speed results in an increase of the force whereby reaching of the maximum set-point driving force coincides with the end of the pressing step. 
     With a programable adaptation to different viscosities but also to different reactivity in regard to the embedding material, the different types of glass ceramic materials can be taken into account. It is also possible by providing free programing to adapt the inventive furnace to currently unknown materials of the future so that respective pressing steps can be provided and optimized for such materials. 
     It is especially advantageous that the preferred embodiment of embodying the drive as a step motor, in combination with the inventive pressure sensing device, prevents overloading of the step motor and thus the loss of steps. The pressure sensing device controls the step motor such that no overload can occur so that the advancing travel for the pressing step always corresponds to the preset values and the step motor operates within safe limits without requiring additional travel sensors. 
     It is especially advantageous that the inventive furnace allows to considerably shorten the exposure time of the ceramic to be pressed in the embedding mass. The switch off criteria can be precisely determined and it is also possible to employ ceramics with fine channels having inherently a higher flow resistance. By shortening the exposure time, the reaction between the ceramic to be pressed and the embedding material can be greatly reduced or prevented so that new high-quality materials which are comparatively reactive can be employed as a ceramic to be pressed. The invention can also be safely used for fine dental bridge parts due to the slow force build-up during pressing. 
     A further advantage of the inventively improved switch-off criteria is the shortening of the pressing step so that the productivity of the inventive furnace in comparison to those of the prior art is improved. Furthermore, the manufacturing precision is improved when etching of the dental prostheses is no longer needed and the surface of the dental prostheses is more smooth and more visually pleasing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The object and advantages of the present invention will appear more clearly from the following specification in conjunction with the accompanying drawings, in which: 
     FIG. 1 shows a view of a portion of the inventive furnace, showing partly in section the drive and the pressure sensing device; 
     FIG. 2 shows a bottom view in the direction of arrow II—II of FIG. 1 representing the unit receiving the drive and the pressure sensing device; 
     FIG. 3 shows an enlarged representation of the pressure sensing device of the embodiment according to FIG. 1; 
     FIG. 4 shows a representation of a further embodiment of a portion of the inventive furnace. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention will now be described in detail with the aid of several specific embodiments utilizing FIGS. 1 though  4 . 
     The embodiment of the inventive furnace  10  represented in FIG. 1 has a muffle  12  which is covered in a well-known manner by a hood. 
     A piston  14  and, in the shown embodiment, pull elements  16  and  18  extend through the hood whereby, according to a modified embodiment, it is suggested that the pull elements  16  and  18  extend external to the hood to a non-represented bottom plate on which the muffle with the dental material can be placed. 
     The furnace hood can be heated by known means and melting of the dental material allows movement of the piston  14  in the downward direction so that the dental material can fill the hollow spaces or voids of the muffle for forming the dental prostheses. 
     The piston  14  is connected axially to a drive  20  which comprises a step motor  22 . The step motor  22  is supported at the side facing away from the piston  14  on a sensor plate  24  of a pressure sensing device PSD. The sensor plate  24  supports a pressure sensor  26 . The pressure sensor  26  rests together with a sensor plate  24  at a deformation member  28  which is preferably a rubber or silicone plate. The deformation member  28  is supported in the upward direction across its entire surface area at a support plate  30  which is connected fixedly to the pull elements  16  and  18 . 
     The step motor  22  comprises a shaft  31  which is an integral part of a drive spindle  32 . A nut  34  is connected to the drive spindle  32  which is fixedly connected to a sleeve  36 . The sleeve  36  thus functions as a threaded sleeve. According to another embodiment, it is suggested to provide the sleeve itself with a corresponding inner thread and to turn the sleeve downward of the inner thread to produce a blind bore. 
     Both embodiments have in common that independent of the position of the sleeve  36  relative to the drive spindle  32  the engagement area, i.e., the axial length along which the threaded engagement between drive spindle  32  and sleeve is realized, is identical. Accordingly, the frictional drive forces are also identical so that the step motor substantially can be actuated with the same drive currents and thus no additional non-linearity is introduced. 
     The sleeve  36  receives in the receiving opening  38  the piston  14 . The piston  14  is received without play whereby optionally for facilitating insertion an insertion slant (not represented in FIG. 1) may be provided. It is especially preferred that the end face  40  of the piston  14  which is opposite the acting end (pressure-applying end) of the piston is supported over a large surface area in order to avoid deformation of the sleeve  36  and the piston  14 . 
     The piston  14  is preferably comprised of a ceramic material while the sleeve  36 , for example, is comprised of stainless steel. 
     In order to prevent rotation of the sleeve  36  upon actuating the step motor  22 , a rotational stop  42  is provided which, as can be seen in FIG. 2, surrounds the pull element  18  in a substantially U-shaped manner so that a rotation of the sleeve  36  is prevented. The pull element  18  is comprised preferably of polished steel and the stop  42  can glide substantially without play and thus with minimal friction thereat, whereby the stop  42  is securely attached to the sleeve  36 . 
     The drive  20  is received together with the pressure sensing device PSD in the pre-assembled unit  44 . The unit  44  extends from the counter plate  30  to the support plate  46  whereby the two plates  30  and  46  are supported at one another by two support rods two of which,  50  and  52 , are shown in FIG.  1 . 
     Preferably, the support rods  50  and  52  are embodied as threaded rods secured by lock nuts so that the spacing between the counter plate  30  and the support plate  46  can be adjusted. The support plate  46  has a central cutout  60  in which the projection  62  of the step motor  22  is received. A flange  64  of the step motor  22  surrounds the projection  62  and is supported at the support plate  46  so that upon loading by the counter force of the piston  14  the step motor  22  is slightly removed from the support plate  46  but is securely guided in the cutout  60  while the deformation element  28  is compressed. 
     As can be seen in FIG. 1, the unit  44  is fastened with threaded bolts  70 ,  72  to the pull elements  16  and  18  in the form of pull rods. This arrangement together with the embodiment of the sleeve  36  placed on the piston  14  allows a subsequent assembly of the inventive drive unit  44  which can be correspondingly pre-assembled and adjusted before it is mounted on the furnace. Preferably, the spacing between the support plate  46  and the counter plate  30  is such that the step motor  22  presses slightly onto the deformation member  28  without compressing it to a noticeable extent. This position corresponds to the zero loading of the pressure sensing device PSD which is electrically connected to a non-represented a control circuit for the step motor and the furnace. 
     FIG. 2 shows that instead of the two pull elements  16  and  18  shown in FIG. 1 a total of three such pull elements  16 ,  18 ,  74  arranged on the corners of a triangle can be realized. Identical reference numerals referred to same parts in the Figures so that no additional explanation is required for the parts. The support plate  46  in the shown embodiment is substantially triangular whereby it is understood that any other suitable guide can be employed without leaving the gist of the invention. 
     FIG. 2 shows also the arrangement of a travel sensor  76  which operates based on a potentiometer or by optical encoding and is fastened to the sleeve  36 . 
     FIG. 3 shows the design of the pressure sensing device PSD. The counter plate  30  is secured by the deformation member  28  at a spacing from the sensor plate  24 . The sensor plate  24  comprises a central cutout  80  having a surface area corresponding to {fraction (1/10)} of the surface area of the sensor plate  24  and corresponding in its dimensions to the actual pressure sensor  26  received therein. The pressure sensor  26  has two spaced apart plates as sensor elements  82 ,  84 . The change in distance between the sensor elements is then transformed into electric signals as is known in the prior art. An example for such a pressure sensor  26  is a piezoelement or a capacitive pressure sensor. 
     Upon compression of the sensor plate  24  and of the counter plate  30 , the deformation member  28  is compressed. Due to the elastic properties of the deformation member  28  it thus substantially uniformly applies pressure across its entire surface area and thus also onto the pressure sensor  26 . 
     Since the greater portion of the surface area of the deformation member  28  rests at the sensor plate  24  and not at the pressure sensor  26 , the supporting action is thus distributed over substantially large portions directly between the sensor plate  24  and the deformation member  28 . A proportional smaller force is thus received by the pressure sensor  26  so that for an increasing pressure it will emit a correspondingly greater output signal. 
     It is understood that the pressure sensing device PSD comprised of sensor plate  24 , counter plate  30 , deformation member  28 , and pressure sensor  26  is already calibrated. For this purpose, it is possible to apply for a short period of time an increasing and known force, to plot this force as a function of the usually non-linear output signal of the piezoelement and save the results so that the corresponding measured values can be entered directly into the electronic control circuit for the step motor. 
     It is understood that instead of the piezoelement any other suitable force uptake, for example, strain gauge elements can be used. It should be noted that even for a strain gauge element a systematic non-linearity can be compensated by performing a calibration step and saving the calibration results. 
     While the inventive support of the sensor element  84  at the muffle furnace is preferably ensured by supporting the sensor element  84  at the counter plate  30 , which is connected fixedly by the pull elements to the bottom plate for the muffle, it is understood that an indirect support at the muffle furnace is also possible in which the pull elements are connected to the pivotable furnace hood which, during firing, is in a defined position relative to the muffle. 
     The embodiment according to FIG. 4 shows a modified design of the inventive furnace which is especially preferred. In this embodiment the pressure sensing device PSD comprises a deformation member  28  which is embodied as a bending bar having on one surface thereof a strain gauge  86 . The bending bar  28  is securely fastened to a support  88  which is itself fixedly connected to the muffle furnace. 
     The bending bar is comprised in the shown embodiment of aluminum and has a central cutout  90  which separates a pull leg  92  and a pressure leg  94  from one another. The cutout  90  and the corresponding legs  92 ,  94  are shown in dashed lines in FIG. 4 because a cover  96  covers this sensitive area including the strain gauge  86 . 
     The drive  20  is connected to the deformation member  28  at the end adjacent to the pull leg  92  and supported on the support  88 . For centering, a mandrel  98  is provided at the deformation member  28  which is supported in the shown embodiment at the sensor plate  24  which is a pressure plate. 
     Despite this one-sided support action, the represented embodiment allows for an especially precise detection of the advancing force without being subject to fatigue. This embodiment is especially suitable for low viscosity ceramics to be pressed with short residence time of the ceramic in the embedding material. 
     The present invention is, of course, in no way restricted to the specific disclosure of the specification and drawings, but also encompasses any modifications within the scope of the appended claims.