FURNACE AND METHOD FOR OPERATING A FURNACE

Abstract

A furnace (10) having a pressing punch (36), a pressure, distance and/or speed sensor and a control device for controlling a pressing process based on the output signal of the sensor. The sensor detects at least a pressure, position and/or motion parameter of the pressing punch (36). The pressing punch (36) acts on the bulk material of glass particles (32)possibly via an interposed ram (28), said glass particles being guided and crystallizable in a press channel (30). The trigger criterion for the process control is a change of at least a motion parameter of the pressing punch (36) upon softening of the bulk material of glass particles (32) which change is detected by the sensor.

Claims

1. A furnace for heating a bulk material of glass particles, comprising: a pressing punch for acting on the bulk material of glass particles; a sensor for detecting a pressure, a position, a speed and/or an acceleration of the pressing punch and generating an output signal; a process control device configured for controlling a pressing process based on the output signal of the sensor and for starting the pressing process when the output signal of the sensor indicates a softening of the bulk material of glass particles.

2. The furnace according to claim 1, wherein the output signal of the sensor indicates a softening of the bulk material of glass particles when the position of the pressing punch changes, the speed of the pressing punch increases and/or the acceleration of the pressing punch is above a predetermined value.

3. The furnace according to claim 1, wherein the output signal of the sensor indicates a softening of the bulk material of glass particles when a movement of the pressing punch is detected while the pressure is constant.

4. The furnace according to claim 1, wherein the process control device is configured for detecting a backward motion of the pressing punch in correspondence with a thermal expansion of the bulk material of glass particles within the output signal and for starting the pressing process at an end of the backward motion.

5. The furnace according to claim 1, wherein the process control device is configured to end the pressing process when the position of the pressing punch remains constant within a predefined time.

6. The furnace according to claim 1, wherein the process control device is configured to end the pressing process when the speed of the pressing punch decreases.

7. The furnace according to claim 1, wherein the process control device is configured to control the start of the pressing process, in which start a furnace temperature, a pressure and/or a press force of the pressing punch is increased by the process control device; and/or the process control device is configured to control the end of the pressing process, in which end the press force of the pressing punch and/or the furnace temperature is decreased by the process control device.

8. The furnace according to claim 1, wherein the process control device is configured to detect a termination of micro pulses, a number of micro pulses and/or a temporal distance between the micro pulses of the position, speed and/or acceleration of the pressing punch.

9. The furnace according to claim 1, wherein the process control device is configured for starting the pressing process based on a termination, number and/or temporal distance of micro pulses.

10. The furnace according to claim 1, wherein the process control device is configured for identifying the material of the bulk material of glass particles and for controlling the pressing process based on parameters associated with the identified material.

11. A method for operating a furnace for heating a bulk material of glass particles; comprising the steps: acting (S201) on the bulk material of glass particles by a pressing punch; detecting (S102) a pressure, position, speed and/or acceleration of the pressing punch and generating an output signal; controlling (S103) a pressing process based on the output signal of the sensor; and starting (S104) the pressing process when the output signal of the sensor indicates a softening of the bulk material of glass particles.

12. The method according to claim 11, wherein the pressing punch is moved to a predefined extent during a heating of the bulk material of glass particles.

13. The method according to claim 11, wherein after the pressing process a crystallization program is started in which the pressing punch does not exert pressure anymore.

14. The method according to claim 13, wherein after the pressing process and/or after the crystallization program, a cooling process and/or a demolding process for the bulk material of glass particles is started.

15. The method according to claim 11, wherein the bulk material of glass particles is heated above the transformation point Tg of the bulk material of glass particles.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0072] Further advantages, details and features may be taken from the following description of an exemplary embodiment of the invention in conjunction with the drawings in which:

[0073] FIG. 1 shows a section through the firing chamber of an inventive furnace in one embodiment;

[0074] FIG. 2 shows a diagram illustrating the speed of the pressing punch and further physical quantities, plotted over time; and

[0075] FIG. 3 shows a schematically illustrated diagram of the speed of nucleation, compression and crystal growth, plotted over temperature.

DETAILED DESCRIPTION

[0076] FIG. 1 illustrates an inventive furnace 10, which is in particular suitable for the production of dental restoration parts, showing a portion which is relevant for the invention.

[0077] A firing chamber 12 is surrounded by a heating device 14 which is illustrated schematically and comprises a helically extending heating coil which is shielded by a quartz glass 16 as a protective device. Large-volume thermal insulation elements 18 surround the firing chamber 12 on all sides, that is to say also towards the bottom, even if this is not apparent from FIG. 1.

[0078] The bottom of the firing chamber 12 is formed by a firing tray 20 which comprises recesses 22 for receiving the muffle 24, in fact graduated by different sizes, two sizes in the exemplary embodiment illustrated.

[0079] The firing chamber 12 comprises a roof cone 26 which expands the firing chamber centrally towards the top. In this area, a part of a ram 28 is received which is inserted into a press channel 30, in contact with bulk material of glass particles 32 made of lithium silicate which is received in the muffle 24 in the press channel 30 completely and which forms color and translucency gradients in the embodiment illustrated.

[0080] The ram may consist of any suitable material, for instance of Al.sub.2O.sub.3, boron nitride, graphite and/or of investment material itself.

[0081] In a pressing punch channel a pressing punch 36 is guided which is driven by a drive unit and which can exert pressure onto the ram 28 and thus indirectly onto the bulk material of glass particles 32.

[0082] In order to operate the furnace, the furnace is initially opened. A muffle which has been pre-heated, for instance, to 850 C. in a furnace referred to as a pre-heating furnace and into which cold bulk material of glass particles 32 and a cold ram 28 have already been inserted is positioned centrally on the firing tray 20. The dimensions of the recess 22 exactly match the associated muffle 24 such that the muffle 24 is positioned exactly in the center. The furnace hood is closed and a negative pressure source draws air present in the firing chamber 12 and in the thermal insulation elements until negative pressure is produced.

[0083] Seals are provided between a furnace base unit and the firing hood of the furnace which are compressed as the negative pressure becomes stronger. The negative pressure is built up within one to two minutes and is kept constant during the entire following press cycle, for instance by regulating the pressure of the negative pressure source or by continuing to run the corresponding suction pump.

[0084] As soon as the bulk material of glass particles 32 has been inserted into the muffle 24 the heating of the glass particles 32 starts which have a considerably smaller mass than the muffle 24. Because of this, the bulk material of glass particles expands wherein the pre-heating temperature is considerably below the softening temperature. The coefficient of thermal expansion of the muffle 24, that is to say of the investment material which is used for forming the muffle, is considerably smaller than the coefficient of thermal expansion of the bulk material of glass particles and of the ram and the pressing punch, wherein the pressing punch can for instance also consist of Al.sub.2O.sub.3 or for instance of steel. A sensor 29 can detect the thermal expansion of the ram of the pressing punch and added thermal expansion of the bulk material of glass particles.

[0085] In the exemplary embodiment illustrated herein, the coefficient of thermal expansion of the muffle is 310.sup.6/K and of the respective glass particles is approximately 1010.sup.6/K, and that of the pressing punch is 810.sup.6/K.

[0086] In the thermal expansion in axial direction considered herein, the thermal expansions L.sub.0 sum up during the heating. The total thermal expansion L.sub.0 tot is:

L.sub.0 tot=L.sub.0 IVM+L.sub.0 bulk material of glass particles+L.sub.0 Alox ram+L.sub.0 pressing punch

[0087] In this connection, the pressing punch 36 only needs to be taken into account in as much as it is heated, that is to say in the region of the thermal insulation elements 18 adjacent to the firing chamber.

[0088] The opposite end of the pressing punch 36 which is connected to the drive unit is considerably less hot, for instance under 100 C.

[0089] The temperature gradient of the pressing punch 36 is high in particular if a pressing punch for instance made of Al.sub.2O.sub.3 is used; if a metallic pressing punch is used, an additional thermal insulation cylinder may be integrated into the pressing punch.

[0090] L.sub.0 IVM is the axial length of the muffle or investment material below the press channel 30.

[0091] In the illustrated embodiment of the furnace the firing tray 20 is only warm on its surface and already comparatively cool in the lower region. However, this does not hold true if a heating referred to as a base heating of a firing chamber is realized, that is to say if a further heating is provided below the firing tray 20. In this embodiment the additional thermal expansion in the region thereat needs to be added to the above-mentioned total thermal expansion.

[0092] In order to identify the axial displacement of the pressing punch 36 as a consequence of the thermal expansion, the thermal expansion of the furnace itself upon heating needs to be deducted from L.sub.0 tot. However, in this connection it must be taken into account that the sealing and the closing force of the furnace are typically realized far to the outside and that the thermal expansion thereat is typically limited to a range between room temperature and a temperature below 100 C., for instance 60 C.; the thermal insulation elements 18 that become very hot at least on the inside do not exert any axial forces but are supported easily in the furnace.

[0093] Here, axial refers to the axis 40 of the pressing punch 36 which runs centrally through both the ram and the bulk material of glass particles 32 as well as through the muffle 24 and the firing tray 20; all elements are configured circular symmetrically in the exemplary embodiment illustrated.

[0094] In FIG. 2 a plurality of curves are plotted in the same diagram and the axis of abscissas is time. Here, the following values are concerned:

[0095] The temperature curve 51 shows the regulated temperature in the firing chamber of the furnace.

[0096] The pressure curve 52 shows the pressure in the interior of the furnace.

[0097] The speed curve 53 shows the speed of the pressing punch 36 or of the associated drive unit.

[0098] Here, the neutral point of the ordinate is vertically offset and the speed values in m/min are recorded on the right hand side.

[0099] The applied press force is recorded in the press force curve 54.

[0100] As can be seen from FIG. 2, the furnace 10 has already been pre-heated to approximately 550 C. at the considered beginning. The heating continues working. At the point in time of approximately 70 seconds from the start heating is carried out, wherein the target temperature is at approximately 580 C. in the exemplary embodiment illustrated. After 10 seconds, at approximately 80 seconds, the peak of the speed thereat is used to insert the pressing punch 36 rapidly into the press channel 30, until pressing punch, ram and the bulk material of glass particles are in contact with one another. The pressing punch 36 exerts a constant pressure of approximately 100N onto the bulk material of glass particles 32.

[0101] As can be seen from the pressure curve 52, the pressure in the furnace drops rapidly within the first 40 seconds and amounts to 100 mbar at 70 seconds. At 240 seconds at the latest, the final pressure of 50 mbar is reached. This pressure is kept constant during the entire press cycle.

[0102] From 70 seconds to approximately 115 seconds the speed of the pressing punch 36 amounts to 0, as can be seen from FIG. 2. At slightly more than 115 seconds an inventive micro pulse 60 at a negative speed is produced. This micro pulse 60 corresponds to a thermal expansion L0 tot, as has been explained with regard to FIG. 1. The micro pulse has a height of approximately 500 m/min and a duration of less than one second, depending on the quality of the press force regulationeven if this is not clearly evident from FIG. 2, based on the embodiment illustrated herein.

[0103] The upper end of the bulk material of glass particles 32 automatically slides upwards slightly, contrary to the force of the press power according to the press curve 54, and drops again within a short period of time due to the softening of the glass particles. At the point in time of approximately 135 seconds, the next micro pulse 62 is produced. Further micro pulses 64, 66, 68 and 70 follow at 150, 170, 195 and 225 seconds, while the following period of time until 250 seconds remains free of micro pulses.

[0104] In the exemplary embodiment illustrated, the pressing process is triggered inventively through detection of a number of micro pulses predefined by experience when the number of micro pulses according to FIG. 2 reaches six.

[0105] According to the invention, this means that the glass particles 32 have become micro-plastic or viscous; the further thermal expansion occurs without pulses. According to the invention, this fact is judged as an indication that the bulk material of glass particles has reached the actual desired temperature for preparing the pressing process.

[0106] For detecting the micro pulses it is basically possible to carry out a speed detection, a distance detection or a press force detection, depending on the resolution of the available sensors and depending on the elasticity of the drive. In tests in connection with the exemplary embodiment discussed herein, a speed detection has proven to be the most advantageous detection, whereas a distance detection is basically also possible.

[0107] Although the entire heating time of the bulk material of glass particles until the beginning of the compression is not apparent from FIG. 2, it has to be noted that a considerably faster heating process is carried out inventively compared to the prior art. As a result, a controlled, small amount of nuclei is produced.

[0108] The compression process starts at 250 seconds until approximately 660 seconds. Meanwhile, the compression reaches its maximum speed at approximately 470 seconds.

[0109] In a preferred embodiment it is provided that a cooling program is started immediately after the compression process to avoid the crystal growth which would take place otherwise at a higher temperature.

[0110] In a further preferred embodiment it is provided that a crystallization program is started after the compression process such that a desired, homogenous and finer crystal structure is produced.

[0111] In a further preferred embodiment it is provided that a cooling program and/or a crystallization program and/or a demolding program and/or a cleaning program is carried out after the compression process. As a result, for instance, a block of lithium metasilicate glass ceramic or of lithium silicate glass with nucleating agents is produced.

[0112] In FIG. 3, three curves 72, 74, 76 are recorded in a diagram wherein the axis of abscissas is the temperature. The speed curves 72, 74, 76, each comprising a plurality of triangles, circles and squares to differentiate them from one another, show the speed of nucleation, of compression and of crystal growth of the glass particles made of lithium silicate under different temperatures.

[0113] Here, the neutral point of the axis of abscissasthat is to say of the temperatureis vertically offset and corresponds to the transformation range Tg of glasses made of lithium silicate which amounts to approximately 450 C. In the transformation range Tg the viscosity of the glass particles made of lithium silicate amounts to approximately 10exp13.2 Poise and the micro pulses have already ended as long as the thermal expansion of the bulk material of glass particles is compensated by micro-plastic deformation at the positions of contact of the glass particles and the viscosity reaches a height of 10exp14.5 Poise.

[0114] It is apparent from FIG. 3 that nucleation of glasses made of lithium silicate takes place in the temperature range between 460 and 530 C., that is to say between 10 C. above the transformation range Tg of the glass particles (Tg+10 C.) and 80 C. above the transformation range Tg of the glass particles (Tg+80 C.), compression thereof takes place in the temperature range between 500 and 580 C., that is to say between 50 C. above the transformation range Tg of the glass particles (Tg+50 C.) and 130 C. above the transformation range Tg of the glass particles (Tg+130 C.), crystal growth thereof takes place in the temperature range between 510 and 610 C., that is to say between approximately 60 C. above the transformation range Tg of the glass particles (Tg+60 C.) and 160 C. above the transformation range Tg of the glass particles (Tg+160 C.).

[0115] These three temperature ranges overlap one another, and in these temperature ranges there is a maximum speed of nucleation (at approximately 500 C.), a maximum speed of compression (at approximately 550 C.) and a maximum speed of crystal growth (at approximately 580 C.), respectively.

[0116] To form only few nuclei (to a desired extent) before and during the compression process, to ensure that complete compression is not prevented strongly by the nuclei, rapid heating is carried out inventively, for instance within three minutes, from 460 C. (Tg+10 C.) to 520 C. (Tg+70 C.) in accordance with the temperature point 78 according to FIG. 3.

[0117] At the temperature point 78, both nucleation and crystal growth have a relatively small speed, while the speed of compression is relatively large. Thus, this temperature point 78 is utilized according to the invention. If a rapid heating process to the temperature point 78 is carried out, as few nuclei and glass crystals as possible are formed before and during compression.

[0118] Except for the optimum temperature point 78, the end temperature for the rapid heating process may be another predefined temperature of between 500 C. (Tg+50 C.) and 530 C. (Tg+80 C.) which at least corresponds to the dilatometric softening of the glass particles and at which the viscosity of the glass particles ? amounts to at least 10exp11.5 Poise.

[0119] References to process control or control device can be understood to include one or more microprocessors that can communicate in a stand-alone and/or a distributed environment(s), and can thus be configured to communicate via wired or wireless communications with other processors, where such one or more processor can be configured to operate on one or more processor-controlled devices that can be similar or different devices.

[0120] The control device may be implemented as one or more central processing unit (CPU) chips, logic units, cores (e.g. as a multi-core processor), field-programmable gate arrays (FPGAs), application specific integrated circuits (ASICs), or digital signal processors (DSPs).

[0121] References to sensors can be understood to include, but are not limited to, any known sensor such as transducers, optical sensors such as ir, uv, photo detectors, photodiodes, phototransistors; motion, inertia and position measurement sensors such as accelerometers, gyroscopes, force, tilt, vibration, flow, float level sensors; capacitive touch/proximity sensors, ultrasonic receivers, transmitters, image sensors, cameras, magnetic sensors; pressure sensors such as absolute, sealed gauge, compound, switch, differential, vacuum and vented gauge sensors.