Microwave Heating Apparatus

Abstract

A microwave heating apparatus is disclosed having a system for support and radiating articles within an enclosure. Such system comprises a movable table adapted to move sequentially in orthogonal directions and provide a substantially continuous orbital movement. The microwave energy radiation is thereby distributed uniformly to heat a large number of supported articles without resorting to prior art distribution means such as cyclically varying mode stirring means.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to microwave heating apparatus having an improved system for the distribution of energy during heating.

2. Description of the Prior Art

The use of microwave energy as a source of heat has become widely accepted for a large number of domestic and industrial products due to the rapid heating times provided by such energy sources. Heating with microwaves provides the so-called "dielectric heating" phenomenon by the absorption of energy and high frequency oscillatory motion of molecules within the article being heated by the combined electric and magnetic fields to cause rapid rise in temperature due to molecular friction. Various materials have differing dielectric loss characteristics and, therefore, the rate of heating is a variable factor. The energy is typically radiated within enclosures defining a confined area capable of supporting many free space wavelengths at the operating frequency and the articles are commonly disposed on a fixed table or transported through the enclosure by means of a conveyor. The energy is typically cyclically varied in multimode energy patterns by such means as mode stirrers.

Microwave energy is generated from such sources as the magnetron oscillator. The region operation is in the electromagnetic energy spectrum at frequencies of 915 .+-. 13 MHz or 2,450 .+-. 50 MHz in the industrial, scientific and medical band assigned for microwave heating apparatus by government regulatory agencies. For the purposes of the present description, the term "microwave" is defined as energy in the region of the spectrum having wavelengths in the order of 1 meter to 1 millimeter and frequencies in the order of 300 MHz to 300 GHz.

Microwave heating is particularly useful for poor thermally conductive materials, such as rubber, foods, paper, wood, leather, plaster, plastic and the like. In the foundry industry molds or cores used in casting of metal parts are typically fabricated from such materials as sand or plaster mixed with a binder which is cured by heat to render the mold or core self-supporting. The base material, such as sand or plaster, is inherently a poor or nonthermally conductive material and the curing times by prior art energy sources are extremely lengthy. U.S. Pat. No. 3,519,517, issued July 7, 1970 to E. C. Dench and assigned to the assignee of the present invention describes the utilization of electromagnetic microwave energy for curing green foundry molds or cores having a predominantly granular refractory composition by using lossy additives, as well as resin binders. The use of microwave energy significantly reduces the over-all time required for curing from several hours to times in the order of minutes. Numerous co-pending applications such as the applications of James M. Valentine, Ser. No. 253,204, filed May 15, 1972 and Ser. No. 253,205, also filed on May 15, 1972 also provide additional examples of microwave energy usage in the foundry industry.

In view of the fact that the heating enclosures typically utilized are of a conductive material, the energy from such sources as the magnetron is reflected from the walls in numerous field mode distribution patterns which can be varied by means of stirrers having vanes actuated by a motor to result in an even distribution of the energy throughout the enclosure. Additionally, prior art embodiments suggest the use of a rotating turntable on which an article is supported. Such means, however, require a substantial enclosure area for the circular path which limits the quantity of articles which can be supported on the turntable. Where large volume production is desired, therefore, tunnel-type energy applicators and transporting conveyors are utilized.

SUMMARY OF THE INVENTION

In accordance with the present invention a microwave heating apparatus is provided having a support and movement system including a table and attached bearing assembly means adapted for sequential linear movement in orthogonal directions to provide a substantially continuously moving orbital path. The system is substantially free of backlash and is capable of handling a large number of articles since all of the space provided by the table is utilized. The microwave energy is efficiently and uniformly radiated to heat all of the product supported and moved by the orbital system. The invention is particularly advantageous for use in the heating of articles having a considerable amount of dust, sand or other particulate matter which results during the processing. It is difficult to heat such products by rotation during curing, particularly where intricate surface detail and close dimensional tolerances are required. The invention also introduces a capability of utilizing groups of individual juxtapositioned microwave power modules with the total desired power being supplied by the combined output of the individual modules.

BRIEF DESCRIPTION OF THE DRAWINGS

Details of the illustrative embodiments of the invention will now be described with reference being directed to the accompanying drawings, wherein:

FIG. 1 is an isometric view of the embodiment of the invention;

FIG. 2 is an exploded isometric view of a linear bearing assembly of the invention;

FIG. 3 is a cross-sectional view of a portion assembly shown in FIG. 2;

FIGS. 4 and 5 are diagrammatic views of the center and lower ball race members of the bearing assembly shown in FIGS. 2 and 3 at the outermost excursion points;

FIG. 6 is an isometric view of the bottom of the assembled orbital support and movement system embodying the invention;

FIG. 7 is an exploded view of a drive mechanism for the system shown in FIG. 6;

FIG. 8 is a diagrammatic representation of a typical orbital excursion path of an article handled by the disclosed system;

FIG. 9 is a schematic view of a portion of an alternative bearing assembly;

FIG. 10 is a diagrammatic representation of an orbital support and movement system embodying the assembly shown in FIG. 9; and

FIG. 11 is an isometric view of an alternative component of an eccentric drive mechanism of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1 the microwave heating apparatus 10 embodying the invention comprises a plurality of parallel conductive walls 12 defining an enclosure 14 having a predetermined confined area. An access opening 16 is provided in the front wall of the apparatus. Door 18 encloses the access opening 16 and is controlled by arms 20 provided with locking means 22 engaging slots 24 to support the door in an open position for loading. The door 18 is provided with a viewing window 26 having a plurality of perforations dimensioned to prevent the escape of energy at the operating frequency. Door 18 is supported by hinges 28 affixed to the front peripheral wall 30 surrounding the access opening 16.

The microwave energy is fed by the conventional probe antenna means from individual packaged power modules 32 including all electrical circuits. A cable 34 provides for the coupling of the electrical energy from a source to the modules. Each of the microwave energy modules 32 has, illustratively, an output of approximately 750 watts at the allocated frequency. In this embodiment ten such sources are provided with a total power capability of 7,500 watts. Each module is separately controlled so that the power levels can be set in increments, if desired. The total number of modules is dependent on the required power for heating the articles within the enclosure. The microwave heating apparatus is supported by frame members 36. The system of the invention comprises a continuously movable table member 38 disposed adjacent to the bottom enclosure wall which is actuated by eccentric drive mechanism 40 supported on plate 42.

The details of the support member 38 adapted to move in an orbital path by sequential translation of linear movement in orthogonal directions will now be described with reference being directed to FIGS. 2-6, inclusive. The linear movement translation means comprise bearing assemblies 44 disposed at the four corners of the movable table 38. Each of the bearing assemblies 44 comprise a plurality of superimposed members including a lower ball race member 46 which is attached to the enclosure bottom wall and defines a closed end race groove 50 on its upper surface 48. Balls 52, 53 and 54 are located within the groove 50. It will be noted that outer balls 52 and 54 are of the larger diameter and bear the load weight while the intervening ball 53 is somewhat smaller and acts as a spacer to prevent undue wear and chafing. The balls may be of any durable material such as metal or plastic.

A movable center ball race member 56 has on its lower surface an open-ended channel 58 which contacts the balls 52, 53 and 54 disposed within the groove 50. With arrangement the linear motion of the race members is not rigidly limited by tolerances of the race grooves and channels. The upper surface of member 56 has a closed end race groove 62 substantially similar to groove 50 in lower ball race member 46. The respective grooves 50 and 62, however, are orthogonally oriented. A similar set of balls 64, 65 and 66 are disposed within the groove 62.

A fixed upper race member 68, substantially similar to lower ball race member 46, has an open-ended channel 70, substantially similar to channel 58 in center ball race member 56. The balls 64, 65 and 66 contact the walls of the channel 70. The upper surface of the race member 68 is secured to the table member 38.

The linear movement in the orthogonal directions is controlled by the movable center ball race members 56. Coplanar rod members 72 interconnect opposite center ball race members along the longer sides of table member 38. Rod members 74 interconnect the oppositely disposed center ball race members along the shorter sidewalls. In the final assembly, as shown in FIG. 6, a rigid plate 76 having an aperture 78 is attached by means of clamps 80 to the coplanar rods to keep the movable center race members at the corners of the arrangement in the desired rectangular orientation to prevent twisting or rotary motion with the sequential orbital movement of the plate member 38. An eccentric mechanism, to be described, provides for the continuous orbital movement in the directions indicated by the arrows 81-84 inclusive.

Referring again to FIG. 3, a cross section of assembled lower and center ball race assemblies 46 and 56, respectively, is shown. The orientation of the outer balls 52 and 54 riding on the channel 58 wall surfaces is illustrated with ball 54 being visible. The different elevations of the coplanar rod members 72 and 74 will also be noted. The interrelationship of the race members having orthogonally disposed grooves at the different levels permits the orthogonal linear movement.

In FIGS. 4 and 5 another feature of the invention will be noted. In these views the lower ball race assemblies 46 are shown in the outermost excursion points. In FIG. 4 the ball 52 contacts one wall of the groove 50 as the center member 56 moves to the left. In FIG. 5 the oppositely disposed larger diameter ball 54 contacts the opposing wall of the closed end groove 50 to thereby limit the travel of the center ball race member 56 in the reverse direction. Channel 58 provided in the underside of the center ball race member 56 is open-ended which reduces the need for critical tolerances. It will be noted that the outermost balls 52 and 54 carry the entire load weight.

In FIG. 7 the eccentric drive mechanism 40 for actuation of the table member 38 in the orbital path is illustrated including the motor and gearing arrangement coupled to vertical shaft 86. An eccentric 88 is keyed to the vertical shaft 86 and carries a cam follower bearing 90 on its upper surface. Drive socket 92 is rotatably supported on cam follower 90 and extends through aperture 78 in plate 76.

Referring to FIG. 8 the translation of movement articles supported on the table member 38 to equalize the exposure to the microwave energy is illustrated. The combined movement of the rotating eccentric and linear bearing assemblies with interconnected coplanar rods results in continuous sequential orthogonal movement as shown by arrows 81-84, inclusive in FIG. 6 to define an orbital path. The diameter of the orbit is indicated by circular arrow 94 for each article. For the optimum dielectric heating the diameter of the orbit is desirably one-quarter of a wavelength of the operating frequency. At the allocated frequency of 2,450 MHz the approximate electrical wavelength would be 4.8 inches so that the required total movement of each object supported on the table would be one-quarter of this value or approximately 1.2 inches. The sequential movement of the orbital support system in each orthogonal direction is required to be only one-half of this value or one-eighth of a wavelength so that the movement of the support system in each direction is in the range of 1/2 to 3/4 of an inch. The invention, therefore, provides means for heating a much larger number of objects within the enclosure since the inscribed orbit required for movement is substantially smaller than the area required for a rotating turntable. In addition, where the processing requires the utilization of atmosphere exhausting means, such as an evacuated Bell jar connected to external pump means, it is much simpler to provide such an arrangement with the orbital moving and support system rather than a rotating turntable which requires an intricate rotary hose coupling. The exposure to the radiated microwave energy provided by the system of the invention results in the energy being uniformly distributed without the need for auxiliary mode stirring or other distribution arrangements of prior art embodiments.

Referring next to FIG. 9 an alternative bearing assembly 96 will be described. In this embodiment a first circular ball race member 98 is affixed to the table member 38 at the corners. A lower similar ball race member 100 is affixed to the bottom enclosure conductive wall 12. Each of the ball race members is provided with a circular groove 102 and 104 with wall sections 106 and 108 to effectively stop the limit of the movement of a single ball 110 disposed in contact with each of the grooves 102 and 104. Referring to FIG. 10 the overall movement of the alternative bearing assemblies is schematically illustrated. The assembly in each of the four corners is fixed to the respective components, hereinbefore enumerated, and an eccentric drive mechanism having a cam follower 90 similar to that illustrated in FIG. 7 provides for the continuous orbital movement of the plate member 38 as indicated by the arrow 112 and the dashed line path 114. The sequential movement, for example, in the orthogonal direction indicated by arrow 116 results in the movement of the table until the ball within each of the four bearing assemblies comes in contact with an opposing stop wall 106 and 108 as indicated in FIG. 9 The next step in the sequential operation of the orbital support system is in the orthogonal direction indicated by the arrrow 118. Rotation of the ball within the grooves 102 and 104 permits the movement of the support table member 38 in this direction. Similarly and sequentially the movement of the table member is provided as indicated by arrows 120 and 122 until the full orbital path has been traversed.

Referring to FIG. 11 an alternative eccentric 124 is shown. The cam follower bearing 126 is supported on a movable block 130 which is controlled by the movement of lead screw 128 to adjust the disposition of the block 130 relative to fixed block 132 having a surface 134 on which the movable block rides. The movement of the screw 128 thereby adjusts the eccentricity gap 136 to compensate for any discrepancies in mechanical tolerances existing between different components and to control, to a substantial degree, any backlash in the overall system.

Numerous other modifications, variations and alterations will be evident to those skilled in the art. The foregoing description of the preferred embodiments is, therefore, intended to be interpreted broadly and not in a limiting sense.

Claims

I claim:

1. Microwave heating apparatus comprising:

a plurality of conductive walls defining an enclosure;

a source of electromagnetic energy having a predetermined operating frequency;

means for radiating said energy within said enclosure;

a system for supporting and moving an article to be heated within said enclosure;

said system comprising a table member and means to move said table member sequentially in orthogonal linear directions to provide substantially continuous orbital movement.

2. The apparatus according to claim 1 wherein said system comprises a plurality of bearing assemblies including superimposed upper and lower fixed ball race members and a movable center ball race member with said upper and lower members secured, respectively, to said table member and an enclosure conductive wall at predetermined points with said center members being interconnected by coplanar rods.

3. The apparatus according to claim 2 wherein said bearing assemblies provide orthogonally disposed channel and race grooves together with a plurality of balls in each assembly adapted to move said center members orthogonally in two linear directions.

4. The apparatus according to claim 3 wherein at least three balls are provided in each race groove and at least two of said balls are of a larger diameter.

5. The apparatus according to claim 1 wherein said table member is provided with opposing parallel sides and said bearing assemblies are disposed adjacent to the corners of said table member.

6. The apparatus according to claim 2 wherein said coplanar rods are loosely coupled to a substantially rigid plate member.

7. The apparatus according to claim 1 wherein the total excursion distance of said continuous orbital movement in orthogonal directions is at least one-quarter of a wavelength at the frequency of the free space energy waves radiated within said enclosure.

8. The apparatus according to claim 1 wherein said bearing assemblies comprise superimposed upper and lower ball race members secured, respectively, to said table member and an enclosure conductive wall; said members defining circular oppositely disposed race grooves with a ball disposed within both grooves.