Patent Publication Number: US-8109761-B1

Title: Dental furnace with cooling system

Description:
This application claims priority from U.S. Provisional Application Ser. No. 60/766,813, filed on Feb. 13, 2006, which is hereby incorporated herein by reference. 
    
    
     BACKGROUND 
     The present invention relates to dental porcelain furnaces. In a typical dental porcelain firing cycle, the porcelain work piece is placed on a firing table and then is slowly moved into a furnace chamber that is preheated to a starting temperature. The starting temperature, or entry temperature, of the furnace chamber is typically around 500 degrees Celsius. Once the work piece is in the chamber, air is evacuated from the chamber via an external vacuum pump. Then the temperature is raised, typically to approximately 750 to 1050 degrees Celsius, and the work piece is held at that temperature for a specified amount of time. At the end of the firing cycle, the work piece is moved out of the furnace chamber and is removed from the firing table. Before another work piece is run through the firing cycle, the operator waits until the furnace chamber cools down to the starting temperature. 
     SUMMARY 
     In the present example, air or some other gas is directed into the furnace chamber to accelerate cooling of the furnace chamber. This decreases the amount of time between firing cycles. In one embodiment, the vacuum pump assists in providing the cooling gas. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of a typical dental furnace found in the prior art; 
         FIG. 2  is a schematic view of an example of a dental furnace having a cooling system made in accordance with the present invention; 
         FIG. 3  is a front perspective view of the furnace of  FIG. 2 ; 
         FIG. 4  is a rear perspective view of the dental furnace of  FIG. 2 ; 
         FIG. 5  is a schematic view of another example of a dental furnace having a cooling system made in accordance with the present invention; 
         FIG. 6  is a schematic view of yet another example of a dental furnace having a cooling system made in accordance with the present invention; 
         FIG. 7  is a front perspective view of the furnace of  FIG. 6 ; and 
         FIG. 8  is a rear perspective view of the dental furnace of  FIG. 6 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a schematic view of a typical prior art dental porcelain furnace, including a furnace chamber  10  and a vacuum pump  20 . The bottom of the furnace chamber  10  defines an opening  60  through which a dental work piece  30  may be inserted and removed. A firing table  50  supports the dental work piece  30 , and it is movable between a lowered position and a raised position. In the lowered position, the work piece  30  may be loaded or removed from the firing table. In the raised position (shown in phantom in  FIG. 1 ), the work piece  30  is raised into the furnace chamber  10  and the firing table  50  seals off the opening  60  in the bottom of the furnace chamber  10  so it can hold a vacuum. The vacuum pump  20  is used to draw a vacuum in the furnace chamber as the furnace temperature is raised to heat the dental work piece  30 . The vacuum pump  20  has an inlet port  22 , which communicates with the furnace chamber  10  through an inlet hose  23 , and an exhaust port  24 , which dispels air to the atmosphere. 
     In use, a dental work piece  30  is placed on the firing table  50 , the furnace chamber  10  is brought to a starting or entry temperature, and the firing table  50  is raised vertically to lift the dental work piece  30  into the furnace chamber  10  and seal off the opening  60 . Then, the vacuum pump  20  is activated, and it pumps air out of the furnace (discharging it to the atmosphere). As the air is being pumped out of the furnace chamber  10 , the temperature in the furnace is increased until it reaches a final temperature, and the work piece  30  is held in the furnace at the desired temperature for the desired time. After the firing cycle is complete, the firing table  50  is lowered, and the furnace chamber  10  is allowed to cool. Once the furnace chamber has cooled down to the desired entry temperature, another work piece can be inserted, and another firing cycle can begin. The time required for cooling down the furnace chamber may be substantial, which limits the productivity of the furnace. 
       FIG. 2  shows a schematic of a furnace including a furnace chamber  110  and vacuum pump  120  and having a cooling system made in accordance with the present invention. Like the vacuum pump of  FIG. 1 , this vacuum pump  120  has an inlet port  122  and inlet hose  123 , with the inlet hose  123  providing gas communication between the vacuum pump  120  and the furnace chamber  110 . The vacuum pump  120  also has an exhaust port  124 . However, in this case, the exhaust port  124  is connected to an exhaust hose  125 , which in turn, is connected to a nozzle or other gas outlet  170  that is directed toward the opening  160  in the furnace chamber  110 . 
     With this configuration, the vacuum pump  120  not only serves its primary function of pumping air out of the furnace chamber  110  during the firing cycle, but it also helps cool down the furnace chamber  110  between firing cycles. When the firing table  150  is raised and the opening  160  is closed, the nozzle  170  simply discharges air to atmosphere. However, when the firing table  150  is lowered and the opening  160  is open, the nozzle  170 , which is directed toward the opening  160 , pulls additional outside air into the furnace  110  and creates an air circulation path through the furnace chamber  110  to cool the chamber  110 . Thus, the pump  120  may be used after one firing cycle is completed to quickly cool the furnace chamber down to the specified entry temperature for the next firing cycle. 
       FIGS. 3 and 4  are perspective views of a furnace  140  using the cooling system outlined in  FIG. 2 . The furnace  140  is programmable and includes an external vacuum pump  120 . An upper section  140 A of the furnace  140  houses the furnace chamber  110 , and a lower section  140 B includes circuitry, buttons and a display for programming the furnace  140 . The circuitry of the lower section  140 B and the heating arrangement for the furnace chamber  110  are not described here, as the basic structure and operation of dental furnaces is well known in the art. In addition, the furnace chamber  110  is shown in phantom lines to generally indicate its location in the upper section  140 A of the furnace  140 . It is not intended to illustrate the furnace chamber  110  in detail. 
     The upper section  140 A and lower section  140 B of the furnace  140  are separated by a middle section  140 C, and the middle section  140 C includes a vertical wall defining a track  145 , which guides an arm  146  connected to the firing table  150 , as is known in the art. The arm  146  and firing table  150  are raised and lowered by a motor (not shown), as is also commonly known in the art. When the firing table  150  is fully raised, it closes off the opening  160  to the furnace chamber  110 . When the firing table  150  is lowered, the opening  160  is exposed. 
     Departing from the typical configuration of the prior art, this furnace  140  includes a nozzle or other gas outlet  170  mounted on its middle section  140 C, and the nozzle  170  is directed toward the opening  160  of the furnace chamber  110 . The nozzle  170  extends through the vertical wall in the middle section  140 C of the furnace  140  and has a barbed fitting  172  projecting out the back of the wall (shown in  FIG. 4 ). The barbed fitting  172  receives one end of an exhaust hose  125 , and the other end of the exhaust hose  125  is connected to the exhaust port  124  of the vacuum pump  120 . Thus, the nozzle  170  directs air discharged from the vacuum pump  120  toward the opening  160  of the furnace chamber  110 . As depicted in  FIG. 3 , the nozzle  170  preferably is configured to spray air as an expanding stream, such that it pulls outside air into the opening  160  of the furnace chamber  110 . When the firing table  150  is raised to seal the opening  160 , air expelled from the vacuum pump  120  simply strikes the bottom of the firing table  150  and dissipates into the atmosphere. 
     The inlet port  122  of the vacuum pump  120  is connected to the inlet hose  123 , which is connected to a vacuum port  126  on the back of the furnace  140 , as is common in the art. In this embodiment, the inlet hose  123 , as well as the exhaust hose  125 , are made of clear plastic. Of course, other types of hoses or tubing could alternatively be used. The vacuum port  126  is connected to the furnace chamber  110  via an internal inlet hose portion  123 A inside the furnace, as is also common in the art. The internal inlet hose portion  123 A shown in  FIG. 4  is simplified for illustrative purposes, and the actual routing of the internal hose  123 A may be more complex. However, the inlet hose  123  and internal inlet hose portion  123 A together provide gas communication between the furnace chamber  110  and the vacuum pump  120 . When the pump  120  is activated, air in the furnace chamber  110  is pumped through the hoses  123 ,  123 A into the vacuum pump  120 , then out through the outlet hose  125  to the nozzle  170 . 
     When the firing table  150  is in the lowered position, a dental work piece  130  may be placed on the firing table  150 , and the furnace  140  is ready to be programmed for a particular firing cycle. For each programmed firing cycle, a desired starting or entry temperature is specified, typically around 500 degrees Celsius. The starting or entry temperature is the temperature at which the furnace chamber  110  should be when the work piece  130  enters the furnace chamber  110 . A temperature measuring instrument (not shown), such as a thermometer or thermocouple, measures the temperature in the furnace chamber  110 , and the measured temperature in the furnace chamber  110  should match the desired starting or entry temperature before each firing cycle begins. If the furnace  140  has not been used for a while, then the temperature in the furnace chamber  110  would be raised before inserting the work piece. However, if the temperature in the furnace chamber  110  is higher than the desired starting or entry temperature (e.g. a firing cycle has just completed), then the furnace chamber  110  is cooled down to the desired starting or entry temperature before inserting the work piece. 
     In this embodiment, the cooling provided by the vacuum pump  120  is controlled by activating the vacuum pump  120  in response to a comparison between the measured temperature in the furnace chamber  110  and the desired starting or entry temperature, if the actual temperature is higher than the desired starting or entry temperature. To elaborate, a user activates a particular program by using the buttons on the lower section  140 B of the furnace  140 . The program defines a particular firing cycle, which has a particular entry temperature, heat rate, hold time, and so forth. When the program is activated, the furnace  140  measures the temperature currently in the furnace chamber  110  and compares it to the programmed entry temperature. If the programmed entry temperature is lower than the measured temperature, the furnace  140  switches on the vacuum pump  120  (through internal circuitry) to cool the furnace chamber  110 . The vacuum pump  120  withdraws hot air from the furnace chamber  110 , and the exhaust from the vacuum pump  120  is directed back toward the furnace chamber  110  through the nozzle  170 . The stream of air exiting the nozzle  170  pulls fresh air into the chamber, cooling the furnace chamber  110 . 
     Once the furnace chamber  110  has cooled to the programmed entry temperature, the furnace control system turns off the vacuum pump  120 , the work piece is placed on the work table  150 , the table is raised to close the furnace chamber  110 , and the firing cycle is initiated. The vacuum pump  120  then is switched back on as part of the firing cycle, as was described earlier. Of course, if the temperature measured in the furnace chamber at the time a program is activated is lower than the programmed entry temperature, then the vacuum pump is not turned on to cool the furnace chamber (as no cooling is necessary). Instead, the furnace chamber is heated to the desired entry temperature. 
     In this embodiment, the pump is switched on and off in response to the measured temperature in comparison to the desired entry temperature. However, other ways of activating the cooling system could alternatively be used. For example, the furnace may be programmed to switch the pump on automatically at the end of a firing cycle, the pump may be controlled by a switch that is activated in response to the vertical movement of the firing table, or the pump may be on from the time a button is pushed to initiate a cycle until the furnace is turned off. Alternatively, the pump could be switched on and off manually at the appropriate time, if desired. 
       FIG. 5  is a schematic of another example of a cooling arrangement for a furnace chamber  210 . In this embodiment, the cooling air injected into the furnace chamber  210  comes from a fan  280  instead of the vacuum pump, so the fan  280  blows air into the chamber  210 , while the pump pulls air out of the chamber  210 . The fan  280  may be a separate unit or it may be built directly into the furnace. The fan  280  directs air toward the furnace opening  260  for cooling. The fan  280  may be electronically linked to the furnace controls so that it can be programmed to switch on and off (similar to the previous embodiment). Alternatively, the fan  280  may be linked to a mechanical switch to activate the fan when the firing table is lowered and deactivate the fan when the firing table is raised. Various other controls alternatively could be used. For example, the fan could be turned on and off whenever the vacuum pump turns on and off, or it could simply remain on whenever the furnace is in operation. 
       FIG. 6  shows a schematic of yet another example of a cooling arrangement for a furnace chamber  310 . In this embodiment, the gas for cooling the furnace chamber  310  comes from a compressed gas supply  390 . The gas could be any gas that is stable at high temperature and is not chemically reactive with the furnace components. Examples of gases that may be used include air, carbon dioxide, nitrogen, and argon. 
       FIGS. 7 and 8  show more details of the system of  FIG. 6 . The compressed gas supply  390  is in fluid communication with the nozzle  370  via a hose  325 . The nozzle  370  is positioned on the middle section  340 C of the furnace  340 , and a control valve  341  (shown in  FIG. 6 ) controls the discharge of gas. The control valve  341  in this embodiment is inside the middle section  340 C of the furnace  340  and is not shown in  FIGS. 7 and 8 . 
     When the furnace chamber  310  is to be cooled, the valve  341  is opened and gas flows through the hose  325  and nozzle  370  toward the opening  360  in order to cool the furnace chamber  310  (shown in  FIG. 6 ) in the upper section  340 A of the furnace  340 . In this case, the control valve  341  is opened and closed via the programmable controls on the lower section  340 B of the furnace  340  similar to switching the pump on and off as in the first embodiment. Alternatively, the valve could be mechanically controlled, such as in response to the movement of the firing table  350 . At the same time that the compressed gas is injected into the furnace chamber  310 , the vacuum pump  320  is removing gas from the chamber  310  through the hose  323 . Although the compressed gas supply  390  shown here is a portable cylinder or canister, it also could be hooked up to a central gas supply system or the like, such as those available in many laboratory fume hoods or work stations.  FIG. 7  shows the work piece  330  resting on the firing table  350 . 
     It should be noted that existing dental furnaces, such as the furnace shown in  FIG. 1 , may be retrofitted by adding a hose from the discharge of the vacuum pump and directing the discharge toward the furnace chamber opening. 
     It will be obvious to those skilled in the art that modifications may be made to the embodiments described above without departing from the scope of the invention as claimed.