Electric heating – Heating devices – Combined with container – enclosure – or support for material...
Reexamination Certificate
1999-09-08
2001-01-30
Walberg, Teresa (Department: 3742)
Electric heating
Heating devices
Combined with container, enclosure, or support for material...
C219S393000, C433S025000, C433S032000
Reexamination Certificate
active
06180922
ABSTRACT:
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 disilicat
Lorunser Johannes
Rohner Gottfried
Ulbricht Horst
Fuqua Shawntina L.
Ivoclar A.G.
Korman Alan S.
Thompson John C.
Walberg Teresa
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