The invention relates to a dental furnace in accordance with the preamble of claim 1 as well as a method for controlling the position of a closing plate at a dental furnace according to claim 10.
Artificial dentures of ceramic are often manufactured on frames in layer technology or as tooth caps. The frame is mainly made from metal alloys, on which the various layers of ceramic material are applied, dried and then fired. For example, for drying and/or firing multi-function dental furnaces are used, which process programmes defined for the coated layer. For instance, the programmes cover a drying, a firing and a cooling phase. The different phases are all processed in the same firing chamber, wherein their durations are at best tuned to the thickness of the coated layer. For instance, the drying phase requires a predetermined drying time. The employed apparatuses consist at least of a firing chamber and a closure plate, which are movable relative to each other. The firing chamber is maintained at 400-600° C. (standby temperature), in dependence of the selected material and firing procedure, and radiates the generated heat in the opened state to the closure plate and/or the material positioned on it. For a drying phase (closing time) a temperature of approx. 120-160° C. is used as optimum. The furnace will be gradually closed, usually within a period of 6 min. Experience shows that a sufficient drying process and slow heating of the material is achieved after this time. When the furnace is completely closed, the firing chamber will be evacuated and the actual heating phase is started.
Although various furnaces are equipped with functions, which are to make the firing process as smooth as possible and convenient, opaquers and ceramics are often strongly damaged on pre-drying. For example, known furnaces are provided with a lifting system (lift) for closing the firing chamber by lowering it onto the closing plate (firing plate) and or by driving the closing plate towards the firing chamber. Almost all ceramic furnaces control the pre-drying phase, as the lift travels a defined distance (insertion into the firing chamber) in a certain time (closing time). When the dental object moves into the opened firing chamber of 400-600° C. with uniform movement during this closing time, the object, which homogeneously dries at a temperature between 120 and 160° C., is outside of the necessary temperature range most of the closing time. At the start of the closing operation with a completely opened firing chamber the object lies in a too cold region, while the temperature drastically increases in the upward direction. The liquid portions of the surface of such inadequately pre-dried objects evaporate under pressure at high temperature. In more favourable controls of some furnaces a particular position of the firing plate is set for drying process, at which drying occurs in the suitable temperature range, before the lift continues the closing process. However, in such ceramics furnaces, the target position for this firing plate position is associated with the adjusted standby temperature. As long as this temperature is not reached, no start of the closing process can take place.
Furthermore, apparatuses with pivoting firing chambers or with swinging firing plates are available. The variety of the present apparatuses causes various disadvantages for the drying process. Thus, a non-uniform course of the drying phase results from such designs. From this again it results that a comparison of the drying parameters between the different apparatuses is only partially possible. Thus, it is almost impossible to derive clear and standardized instructions for use, in order to reach reproducible results with each dental material employed.
The mentioned drying faults may cause material stress, which can lead to fissures and bubble formation and thus quality losses of the surfaces of each layer. Further, bonding of the ceramic layers to each other might be poor. In addition, the process includes waiting periods, as the drying or closing of the furnace only begins when the standby temperature of the firing chamber is reached.
Thus, the object of the current invention is to achieve a uniform drying process of a material, in particular artificial dentures, with optimum temperature and to shorten the drying and closing time to a minimum.
This is achieved a dental furnace according to claim 1 and by a method for controlling the position of a closing plate at a dental furnace in accordance with claim 10. Other favourable embodiments of the invention are subject matter of the dependent claims.
The dental furnace according to the invention has a hood-shaped firing chamber, in whose interior at least a heating element is arranged. The opening of the firing chamber is directed to a closure plate, which is relatively movable to the firing chamber. The closure plate (firing plate) serves as the receptacle of dental material, in particular an artificial dentures object. For instance, the artificial dentures object can be a complete object made of ceramics or a frame made of metal, on which further layers of dental materials are burned or sintered in the manufacturing process. The dental furnace is characterised by a sensor for determining the firing chamber temperature. Further, the dental furnace has a measuring device to detect the actual value of the closing plate position relative to the firing chamber as well as a computing unit for determining the offset by comparison of the detected actual value with a defined desired value and to derive a drive variable from it.
Preferably, the dental furnace is provided with an electric, hydraulic and/or pneumatic drive. With a stationary mounted firing chamber the closure plate is moved into the firing chamber up to its closure by this drive (lift). Beyond that it is however also possible that with embodiments having a stationary firing plate that the firing chamber is moved towards the firing plate. In another embodiment of the dental furnace both closing plate as well as firing chamber are movable mounted and can thus moved towards each other or away from each other, respectively.
According to the invention, the actual position value of the closing plate in relation to the firing chamber is detected by a measuring device. In a preferred embodiment of the dental furnace the actual value is derived from the gap distance between the closure plate (firing plate) and the opening of the firing chamber.
To achieve a reproducible drying result of the various materials or artificial dentures objects, it is preferred to define a specific desired value of the closing plate position for each one of the materials or layer materials depending on the firing chamber temperature. The target value can be further dependent on the temperature value at the firing plate and/or at the material and/or on the ambient temperature.
The desired value thus results for example from a function, depending on the optimum drying parameters for the respective material, on the temperature of the firing chamber, on the temperature at the closing plate (firing plate) and/or at the material as well as the ambient temperature as computation parameters. The specific values for the closing plate position are stored in the memory unit, which the computer unit can access during the entire drying phase, in order to derive from the comparison of target/actual values the correcting variable.
Thus, a preferable embodiment of the dental furnace additionally has a detector for the ambient temperature beside the sensor for the temperature in the firing chamber. Such a sensor is particularly recommendable, if the dental technology furnace is for example operated at various locations with strongly different temperatures, or within laboratory areas and/or locations of the furnace, at which high temperature variations occur, e.g. due to different sun exposure. As the drying process starts with an incompletely closed firing chamber, the dental material is affected by the ambient temperature, as well. Thus, additional detection of this parameter is in the interest of an optimum control of the drying phase.
In another favourable embodiment of the dental furnace according to the invention determining of temperature values is made at the firing plate and/or at the material by a measuring system, arranged thereon. The detection of these temperature values is particularly favourable in dual respect. To the one, the temperature at the firing plate and/or material can be used as parameters in the calculation of the desired value of the firing plate position; on the other hand this parameter, detected in the development phase of new materials or apparatuses, can be used as basis for manuals or operating instructions.
The measuring system can be made of one or several sensors or detectors. These are arranged either in the vicinity of the closing plate and/or the material or are directly mounted thereon. It is also possible to arrange the sensor(-)s on, at or in the supports (firing bases), used for the dental materials. A preferred position of the measuring system corresponds to the layer structure of the material. In a preferable embodiment, the material has a multi-layer structure,
It is positive, when the dental furnace has a memory unit, as all or selected parts of the detected values can be stored in such a unit. First, this storage proves to be favourable for the above definition of the drying parameters for the development of new materials and apparatuses; on the other hand, such storage permits complete evaluation of past drying operations. This again is helpful for error tracing and, for example, also in tracing working steps in a certification procedure.
In addition, the current invention provides a method for controlling the position of a closure plate at a dental furnace. This method includes the subsequent steps:
First, a determination of a current firing chamber temperature is made. Additionally or in the alternative, a temperature value can be detected at the closing plate and/or at the material to be dried. In another step the actual value of the closing plate position relative to the firing chamber is received. The computing unit compares the actual value with a desired value of the closing plate position depending on the firing chamber temperature and/or the temperature value at the closing plate and/or at the material and derives a correcting variable. This manipulation variable is passed on to the control to balance the deviation and the position of the closing plate and/or the firing chamber by operation of an electric, a pneumatic and/or a hydraulic drive correspondingly adjusted. The determination of the above values and the derivation of the manipulation variables are continuously made during the entire drying phase. The desired values for the respective dental materials are stored in the memory unit, which the computer unit accesses during the target/actual value comparison. For the drying phase, which is processed with constant temperature by controlling the closing plate position as described above, a duration of approx. 3 minutes is sufficient, in order to bring that material subsequently in 1-2 minutes gradually and/or slow into the hot firing chamber. Before closing, another step of interrogation of the current firing chamber temperature is made and from it the assignment of the appropriate closing time results from the programme. In addition, a continuous target/actual value comparison prevents complete closing or insertion into the firing chamber, as long as the temperature within the firing chamber lies above the permissible standby temperature.
It is particularly favourable when the gap distance between the closure plate and the opening of the firing chamber directed thereto is detected as actual value by a corresponding measuring device.
In order to reach particularly reproducible results, it is favourable to make a determination of the ambient temperature since the desired value for the closing plate position in a preferable embodiment of the invention process depends on the firing chamber temperature and in a smaller extent also on the ambient temperature. Further, the target value can also include the respective temperature value at the closing plate and/or at the material additionally.
Other advantages, features and features of the invention will result from the subsequent description of preferred, but not limiting embodiments of the invention on the basis the schematic drawing. It shows:
Inside the firing chamber 3 heating elements 12 (cf.
In the embodiment of
A firing base 23 made of insulating material and a material carrier 24 for the dental material 5 are centrally disposed on the closure plate 4. In the embodiment of the
By the previously mentioned position measuring system in the guide 7 the actual value of the position of the closure plate 4 relative the firing chamber 3 is continuously detected. Thus, the gap between the opening 13 (here downwardly orientated) of the firing chamber 3 and the surface of the firing base 23, directed to the firing chamber 3 is measured.
In the embodiment of
In the embodiment of
The manipulation variable is passed on to a controller, which regulates the distance between closing plate 4 and firing chamber 3 by the drive 8, i.e. these two components are moved closer or more distanced to each other. By the continuous determination of this manipulation variable and the change of the distance between closing plate 4 and firing chamber 3, material 5 can be introduced already in the pre-heating or cooling phase of the firing chamber 3 of the dental furnace 1 and the drying process can start independently. Thus, a significant saving of time results in the entire manufacturing process of artificial dentures objects, as the furnace does not require a fixed starting temperature. Thus, the waiting period for cooling-down is avoided, in particular with subsequent firing processes. Further, energy for heating of the dental furnace 1 can be used more efficiently. By the automatic correction of the closing time, a drying process can even be completed within the cooling time of the firing chamber. Due to the continuous temperature monitoring, as mentioned above, the drying process is always in the optimum temperature range. By the invention process also the closing occurs at a uniform temperature rise until the moment of the complete closure (cf. illustration in
Number | Date | Country | Kind |
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10 2006032 655.5 | Jul 2006 | DE | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/EP2007/006223 | 7/13/2007 | WO | 00 | 10/6/2009 |