Patent Publication Number: US-2015069042-A1

Title: Vacuum Oven

Description:
PRIORITY STATEMENT &amp; CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of co-pending U.S. patent application Ser. No. 13/796,320 entitled “Vacuum Oven” and filed on Mar. 12, 2013 in the names of Daniel F. Serrago and James D. Emmons, now U.S. Pat. No. 8,890,036, issued on Nov. 18, 2014; which is a continuation of U.S. patent application Ser. No. 12/949,145 entitled “Vacuum Oven” and filed on Nov. 18, 2010, in the names of Daniel F. Serrago and James D. Emmons, now U.S. Pat. No. 8,487,220, issued on Jul. 16, 2013; which claims the benefit of U.S. patent application Ser. No. 61/262,318, entitled “Vacuum Oven”, filed on Nov. 18, 2009, in the names of Daniel F. Serrago and James D. Emmons; all of which are hereby incorporated by reference for all purposes. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     This invention relates, in general, to temperature distribution and regulation and, in particular, to a vacuum oven adapted for heat treating a work piece positioned therein. 
     BACKGROUND OF THE INVENTION 
     One of the problems that has arisen in connection with vacuum ovens or furnaces is that of heat distribution in the oven. That is, all of the work area doesn&#39;t see a similar radiation field. Inconsistent and irregular radiation fields can result in hard spots or residual stress in metals, different surface finishes and color variations in ceramics and porcelains, and a myriad of other issues in more exotic materials. These inconsistent and irregular radiation fields necessitate new vacuum ovens that have more uniform radiation fields. 
     SUMMARY OF THE INVENTION 
     It would be advantageous to achieve a vacuum oven adapted for heat treating a work piece. It would also be desirable to enable consistent and regular radiation fields when applying heat treatment to a work piece. To better address one or more of these concerns, in one embodiment, a bottom loading vacuum oven or vacuum furnace is disclosed having an energy distribution sleeve that conforms to the shape of an interior heating chamber. The energy distribution sleeve may be of generally annular shape, like a ring, and located in a substantially regularly spaced and offset relationship from a heating element located within walls adjacent the interior heating chamber. The energy distribution sleeve includes a thermal conductive material which absorbs and re-radiates radiant energy emitted from the heating element, thereby providing more consistent and regular radiation fields for heat treating a work piece that is loaded on a work holding tray and, upon the bottom loading vacuum oven being in an operation position, the work piece is located proximate to the furnace chamber. The teachings disclosed herein while relating to vacuum furnaces are particularly applicable to small vacuum furnaces of the type used in the dental industry for firing crowns, implants and any type of porcelain fixture. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the features and advantages of the present invention, reference is now made to the detailed description of the invention along with the accompanying figures in which corresponding numerals in the different figures refer to corresponding parts and in which: 
         FIG. 1  is a front perspective view of one embodiment of a vacuum oven for heat treating a work piece and having an energy distribution apparatus constructed according to the teachings presented herein; 
         FIG. 2  is a front perspective view, with a partial cutaway, of the vacuum oven illustrated in  FIG. 1  depicted in a closed or operational position for loading and unloading a work piece; 
         FIG. 3  is a front perspective view of one embodiment of a vacuum chamber assembly of the vacuum oven illustrated in  FIG. 1 ; 
         FIG. 4  is an exploded front perspective view of the vacuum chamber assembly illustrated in  FIG. 3 ; 
         FIG. 5  is a bottom plan view of the vacuum chamber assembly illustrated in  FIG. 3 ; 
         FIG. 6  is a cross-sectional front plan view of the vacuum chamber assembly illustrated in  FIG. 3 ; and 
         FIG. 7  is also a cross-sectional front plan view of the vacuum chamber assembly illustrated in  FIG. 3 , wherein a work piece is being fired. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     While the making and using of various embodiments of the present invention are discussed in detail below, it should be appreciated that the present invention provides many applicable inventive concepts which can be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative of specific ways to make and use the invention, and do not delimit the scope of the present invention. 
     Referring to  FIGS. 1-6 , therein is depicted a vacuum oven that is schematically illustrated and generally designated  10 . A body  12 , which includes panels  15  (cutaway or removed in  FIG. 2 ), supports a vacuum chamber assembly  14 , which is depicted as a two-part, bottom loading vacuum chamber assembly. A control panel  16  with display and various supporting electronics  18  are mounted to a base  20  of the body  12  and, by way of internal communication through the body  12 , located in electronic communication with the vacuum chamber assembly  14 . The vacuum chamber assembly  14  is secured to the vacuum oven  10  and includes a vacuum chamber subassembly  22 , and a lower chamber cover  24 , among other components. 
     The vacuum chamber subassembly  22  includes ends  26 ,  28 . As shown, the vacuum chamber subassembly  22  is coupled or suspended from the body  12 , by taps  35  having openings  37  therein. A top chamber cover  30  is fastened to the end  26  and secured to the body  12  by fasteners, such as fastener  32  that are secured by mounting bores, such as bores  33 . The vacuum chamber subassembly  22  is generally cylindrical with an opening  34  formed at the end  28  to provide access to an interior vacuum chamber  36 . A muffle is fastened to the top chamber cover  30 , by fasteners and mounting bores, such as fastener  40  and bore  41 , and suspended therefrom within the interior vacuum chamber  36 . The muffle  38  may be generally cylindrical and may include an opening  42  providing access to an interior heating chamber  44 . An annulus  44  is formed within the interior vacuum chamber  36  between the muffle  38  and the vacuum chamber subassembly  22  or there may be a friction fit between the muffle  38  and the vacuum chamber subassembly  22 . It should be appreciated that the shape of the vacuum chamber subassembly  22  and the muffle  38  may vary with application and furnace. 
     Heating element  46  is under regulatable power and located within the muffle  38  proximate to the interior heating chamber  44 . The heating element  46  may be a wire wound element or helical wound wire, for example. In one implementation, the heating element  46  includes a conic helix defined by a spiral traversing the muffle such that the pitch of the conic helix spans the interior heating chamber  44 . In one embodiment, the heating element  46  is configured to provide radiant heat in a range from about 700° C. (1292° F.) to about 1200° C. (2192° F.). Radiant heat is provided as the operation of the vacuum minimizes or eliminates convection heat. It should be appreciated that multiple heating elements or heating element arrangements may also be used and are within the teachings presented herein to provide one resistive circuit/loop or multiple resistive circuits/loops. 
     A heat distribution sleeve, which more generally is an energy distribution sleeve  48 , conforms to the shape of the interior heating chamber  44 . As depicted, the energy distribution sleeve  48  is located in a substantially regularly spaced and offset relationship from the heating element  46 . A thermal conductive material  50  of the energy distribution sleeve  48  absorbs and re-radiates radiant heat energy emitted from the heating element  46 . That is, the energy distribution sleeve re-radiates heat at particular temperatures and frequencies, thereby re-radiating particular bands of energy. A furnace chamber  52  is formed within the energy distribution sleeve  48 . In one implementation, hanging rods  54 ,  56 ,  58  suspend the energy distribution sleeve  48  from the vacuum chamber subassembly through the muffle  38 . It should be appreciated, however, that any type of offset or suspension technique may be utilized. As a result of the performance requirements of the heating element  44 , the energy distribution sleeve  48  is configured to absorb and re-radiate radiant energy, including heat, in the range from about 700° C. (1292° F.) to about 1200° C. (2192° F.) 
     As mentioned, the energy distribution sleeve  48  matches the shape of the interior heating chamber  44  and as such inner chambers are often circular, the energy distribution sleeve  48  may be an annular shape, a ring, or similar circular shape in many embodiments. It should be further appreciated that although a particular design and structure for the energy distribution sleeve  48  is presented, the shape, spacing, and off-set of the heat distribution sleeve  48  may vary and include other shapes, including faceted shapes, irregular angles, and varied spacing, for example. The energy distribution sleeve  48  may comprise a material of high thermal conductivity, such as a metal, ceramic, or other material that will not melt or distort when repeatedly fired under the furnace conditions of the vacuum oven. 
     It should be understood that other mounting and installation techniques for the energy distribution sleeve  48 , including side mounting and mounting from beneath the energy distribution sleeve  48 , are within the teachings presented herein. In one embodiment, the energy distribution sleeve  48  has a length and dimensions that cover the heating element  46  having exposure to the interior heating chamber  44 . It should be understood, however, that the dimensions including the thickness may vary so as to appropriately compliment the timing cycle of the vacuum oven. As depicted, the energy distribution sleeve  48  is of a cylindrical shape or normalizing ring having no top or bottom. In another embodiment, the energy distribution sleeve  48  conforms more completely or totally to the shape of the cavity defined by the interior heating chamber  44 . In this embodiment, the energy distribution sleeve  48  has a form approximating a five or six sided chamber or its cylindrical equivalent. 
     In one embodiment, the lower chamber cover  24  is moveably secured to the body  12  and actuatable between an open or loading position ( FIG. 1 ) where the lower chamber cover is located in a spaced relationship below the vacuum chamber subassembly  22  and a closed or operational position ( FIG. 2 ) where the lower chamber cover  24  engages the vacuum chamber subassembly  22  at the opening  34 . As shown, a vertical track  60  is mounted to body  12  behind the vacuum oven assembly  14 . An arm is slidably secured to the vertical track  60  in order to support the lower chamber cover  24  and provide mobility, as described, thereto. 
     It should be appreciated that alternative embodiments to the bottom loaded vacuum oven described in the previous paragraph are applicable, wherein, upon the lower chamber cover and vacuum chamber subassembly being in the closed position, the work piece is located within the furnace chamber. That is, the lower chamber cover may be stationary and the vacuum chamber is moveably coupled to the body or, as previously discussed, the lower chamber cover is moveably coupled to the body and the vacuum chamber subassembly is stationary. Moreover, the heat distribution sleeve  66  may be utilized with a front loading vacuum oven. 
     A firebrick base  62  is mounted to the lower chamber cover  24  to support a work holding tray  64  configured to hold one or more work pieces  66 . The work holding tray  64  provides a work area that is located within the furnace chamber and superposed or above the firebrick base for providing a raised or elevated space above the firebrick base  62  onto which the work piece or pieces  66  may be accepted, positioned, or set, for example. The work area may use pins, pegs, and variety of surfaces, for example, to provide for the securing of the work piece  66 . It should be appreciated that a variety of techniques may be utilized to secure the work piece  66  and a work holding tray is but one embodiment. The portion of the furnace chamber  52  that extends above the work holding tray  64  defines an inner zone of maximal radiant energy within the furnace chamber  52  and within the energy distribution sleeve  48 . That is, in operation, upon the lower chamber cover  24  being in the closed position, the work holding tray  64  is located proximate to or within the furnace chamber  52 , in this location. 
     A thermocouple  68  extends through the vacuum chamber subassembly  22  and the muffle  38  by way of mounting holes  70 ,  72  to accurately measure the temperature in the furnace chamber  52  proximate to the work holding tray and work pieces. The mounting holes  70 ,  72  for the thermocouple  68  may provide for a threadable engagement. Power conduits  74 ,  76  are configured to provide electrical communication between the heating element  46  and a power source. A fan  78  is secured to the body  12  and oriented to circulate air across the opening  34  of the vacuum chamber subassembly  22 . As previously alluded, the teachings disclosed herein while relating to vacuum furnaces are particularly applicable to dental vacuum ovens and furnaces of the type used in the dental industry for firing crowns, implants and any type of porcelain fixture. 
     Referring to  FIG. 7 , the working area provided by the work holding tray  64  may be loaded with work pieces or parts  66  that may be made of many materials including steel, ceramics, porcelain, clays, composites, or other materials. The characteristics of the work piece are important to the vacuum oven  10  operation. In particular, the heating cycle of the vacuum oven  10  is proportional to the thickness of the work piece  66 , as well as the material of the work piece  66 . As illustrated, a porcelain work piece  66  is positioned on the work holding tray  64  for heat treatment. In operation, the vacuum oven  10  is held at a vacuum, with the parts being fired determining the required quality of the vacuum. As previously discussed, the energy distribution sleeve  48  includes a thermal conductive material  50  which absorbs heat  80  emitted from the heating element  46  and re-radiates the heat  82  emitted from the heating element  46  as infrared and visible radiant energy, for example. 
     In particular, the energy distribution sleeve  48  absorbs the heat, becomes hot and then re-radiates the heat. The energy distribution sleeve  48  therefore functions like a normalizing device or heat capacitance device, which mitigates unwanted variations in the radiant heat energy provided by the heating element  46 . Due to the vacuum inside, the main heat transfer that occurs is a result of radiation from the coils or panels functioning as the heating element  46 . As radiant heat transfer is a line of sight type transfer, any difference in exposure can cause different temperatures on the parts within the working area. The heat distribution sleeve  48  is positioned between or interposed between the interior heating chamber  44  having the heating element  46  therein and the work pieces  66  to reduce temperature variation and create a more balanced distribution of radiation. The heat distribution sleeve  48  lowers the temperature variations within the work area compared to vacuum ovens or furnaces without the device. 
     As previously alluded, the inconsistent and irregular radiation fields may cause problems when heat treating a work piece. This is especially true with substances having low heat transfer coefficients. In this respect, the energy distribution sleeve  48  provides a device which may be inserted, e.g., an after-market solution, or built into the furnace to reduce spatial temperature variations within the work area. 
     Moreover, with respect to the energy distribution sleeve  48  and the inconsistent and irregular radiation fields that may cause problems when heat treating a work piece, the work piece may be dental porcelain and the energy distribution sleeve must emit the frequency of infrared radiation that will allow resonant modes of vibration to occur in the work piece, e.g., dental porcelain. In one embodiment, the energy distribution sleeve  48  includes a thermal conductive material that is operable at vacuum oven temperatures to absorb and re-radiate radiant heat energy emitted from the heating element. In particular, the emissivity curve of the heat distribution sleeve is always greater than about 0.6 from about 20 microns to about 1 micron, wherein the emissivity of the surface of a material is its effectiveness in emitting energy as thermal radiation. 
     In one implementation, the energy distribution sleeve  48  may be a high temperature superalloy such as a nickel-chromium-based superalloy, such as Inconel (or occasionally “Inco” or “Iconel”), including Inconel 625. The emissivity curve of Inconel 625, for example, is a relatively flat 0.85 to 0.90 from about 15 microns to about 1 micron. Quartz (silicon dioxide), on the other hand, is not a high temperature superalloy and is virtually opaque (i.e., 0 emissivity curve) to wavelengths longer than 4 microns, including from about 20 microns to about 1 micron. 
     The order of execution or performance of the methods and techniques illustrated and described herein is not essential, unless otherwise specified. That is, elements of the methods and techniques may be performed in any order, unless otherwise specified, and that the methods may include more or less elements than those disclosed herein. For example, it is contemplated that executing or performing a particular element before, contemporaneously with, or after another element are all possible sequences of execution. 
     While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is, therefore, intended that the appended claims encompass any such modifications or embodiments.