Induction drying arrangements and methods

Abstract

An induction heating apparatus includes a support frame disposed within a housing and defining a workpiece conveying passage having a cross-sectional perimeter, and a magnetic induction heating coil disposed within the housing and surrounding the support frame. The support frame includes workpiece-engaging rail elements defining the cross-sectional perimeter. At least one flux inhibiting element is secured to at least one of the rail elements to define a shielded portion of the cross-sectional perimeter. No flux inhibiting elements are secured to at least a portion of the cross-sectional perimeter of the workpiece conveying passage to define a non-shielded portion of the cross-sectional perimeter, such that when a workpiece is conveyed through the workpiece conveying passage, a first outer peripheral portion of the workpiece facing the shielded portion is exposed to a smaller heat flux than a second outer peripheral portion of the workpiece facing the non-shielded portion.

Description

INDUCTION DRYING ARRANGEMENTS AND METHODSCROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims priority to and all benefit of U.S. Provisional Patent Application Serial No. 63 / 573,697, filed on April 3, 2024 and entitled INDUCTION DRYING ARRANGEMENTS AND METHODS, the entire disclosure of which is incorporated by reference herein.TECHNICAL FIELD

[0002] The present invention relates to apparatus and methods for heating and, thereby, drying, a plurality of plate-like metal objects such as metal can lids, also known as “closures” or “ends.”BACKGROUND

[0003] Closures or lids for metal containers are usually made of aluminum or steel and include a lip or curl that is used in attaching the closure to a can body through a seaming operation. To aid the integrity of the seal that is formed between the can body and the closure, it is a common practice to apply a bead of sealant or adhesive ("compound") within the curl of the can end during manufacture of the closure. Different types of coatings are also selectively or generally applied to can closures and can bodies for various other purposes as well, for example, to repair damaged coatings. For the purposes of the present description, coatings, sealants and adhesives are all considered to be "liquids" applied to a workpiece.

[0004] These manufacturing operations commonly include arrangements for curing or drying these applied liquids. Known drying arrangements include infrared radiation, convection heating, or induction heating. An induction dryer, for example, typically includes a cabinet that supports a tube extending generally horizontally across the cabinet from one end to the other. The tube is larger in diameter than the can ends. An induction coil is wrapped around the tube. The ends move through the tube in a stacked relationship, that is, with abutting face-to-face contact with each other ("in-stick"). When a suitable electric current is passed through the coil, the metal can ends are inductively heated. The heat is transferred to the compound on the can ends by conduction from the heated metal. The compound is heated and water is driven off from the compound into the surrounding air. SUMMARY

[0005] According to an exemplary implementation of the present disclosure, an induction heating apparatus for heating plate-shaped workpieces includes a housing, a support frame disposed within the housing and defining a workpiece conveying passagehaving a cross-sectional perimeter, and a magnetic induction heating coil disposed within the housing and surrounding the support frame to generate a magnetic field for heating the workpiece conveying passage. The support frame includes a first workpiece-engaging rail element defining an upper portion of the cross-sectional perimeter, a second workpieceengaging rail element defining a lower portion of the cross-sectional perimeter, and third and fourth workpiece-engaging rail elements defining opposed side portions of the cross-sectional perimeter. At least one flux inhibiting element is secured to at least one of the first, second, third and fourth workpiece-engaging rail elements to define a shielded portion of the cross- sectional perimeter of the workpiece conveying passage, and no flux inhibiting elements are secured to at least a portion of the cross-sectional perimeter of the workpiece conveying passage to define a non-shielded portion of the cross-sectional perimeter of the workpiece conveying passage. When a workpiece is conveyed through the workpiece conveying passage, a first outer peripheral portion of the workpiece facing the shielded portion of the cross-sectional perimeter of the workpiece conveying passage is exposed to a smaller heat flux than a second outer peripheral portion of the workpiece facing the non-shielded portion of the cross-sectional perimeter of the workpiece conveying passage.

[0006] According to an exemplary implementation of the present disclosure, a method of heating plate-shaped workpieces is contemplated. In an exemplary method, the plate-shaped workpieces are conveyed through a workpiece conveying passage of an induction heating apparatus, the workpiece conveying passage having a cross-sectional perimeter including upper and lower portions spaced apart to engage first and second edges of the plate-shaped workpieces and opposed side portions spaced apart to engage third and fourth edges of the plate-shaped workpieces. A magnetic field is generated around the cross- sectional perimeter of the workpiece conveying passage for heating the workpiece conveying passage. Heat flux from the magnetic field is inhibited along at least a first portion of the cross-sectional perimeter of the workpiece conveying passage without inhibiting heat flux from the magnetic field along at least a second portion of the cross-sectional perimeter of the workpiece conveying passage, such that a first outer peripheral portion of the plate-shaped workpieces facing the first portion of the cross-sectional perimeter of the workpiece conveying passage is exposed to a smaller heat flux than a second outer peripheral portion of the workpieces facing the second portion of the cross-sectional perimeter of the workpiece conveying passage.BRIEF DESCRIPTION OF THE DRAWINGS

[0007] The foregoing and other features of the present invention will become apparent to one skilled in the art to which the present invention relates upon consideration of the following description of the invention with reference to the accompanying drawings, in which:

[0008] FIG. l is a perspective view of an exemplary inductive dryer apparatus;

[0009] FIG. 2 is a schematic, longitudinal sectional view of the dryer apparatus ofFIG. 1;

[0010] FIG. 3 is a side view of an exemplary inductive dryer apparatus, in accordance with an exemplary embodiment of the present disclosure;

[0011] FIG. 4 is a top view of the inductive dryer apparatus of FIG. 3;

[0012] FIG. 5 is an end view of the inductive dryer apparatus of FIG. 3;

[0013] FIG. 6 is a partial end view of the inductive dryer apparatus of FIG. 3;

[0014] FIG. 6A is a partial end view of the inductive dryer apparatus of FIG. 3, having a workpiece conveying passage with a different cross-sectional shape; and

[0015] FIG. 7 is a partial cross-sectional top view of the inductive dryer apparatus of FIG. 3.DETAILED DESCRIPTION

[0016] This Detailed Description merely describes exemplary embodiments and is not intended to limit the scope of the claims in any way. Indeed, the invention as claimed is broader than and unlimited by the described embodiments, and the terms used have their full ordinary meaning.

[0017] While various inventive aspects, concepts and features of the inventions may be described and illustrated herein as embodied in combination in the exemplary embodiments, these various aspects, concepts and features may be used in many alternative embodiments, either individually or in various combinations and sub-combinations thereof. Unless expressly excluded herein all such combinations and sub-combinations are intended to be within the scope of the present inventions. Still further, while various alternative embodiments as to the various aspects, concepts and features of the inventions— such as alternative materials, structures, configurations, methods, circuits, devices and components, software, hardware, control logic, alternatives as to form, fit and function, and so on— may be described herein, such descriptions are not intended to be a complete or exhaustive list of available alternative embodiments, whether presently known or later developed. Those skilled in the art may readily adopt one or more of the inventive aspects, concepts or featuresinto additional embodiments and uses within the scope of the present inventions even if such embodiments are not expressly disclosed herein. Additionally, even though some features, concepts or aspects of the inventions may be described herein as being a preferred arrangement or method, such description is not intended to suggest that such feature is required or necessary unless expressly so stated. Still further, exemplary or representative values and ranges may be included to assist in understanding the present disclosure, however, such values and ranges are not to be construed in a limiting sense and are intended to be critical values or ranges only if so expressly stated. Parameters identified as “approximate” or “about” a specified value are intended to include the specified value, values within 5% of the specified value, and values within 10% of the specified value, unless expressly stated otherwise. Further, it is to be understood that the drawings accompanying the present disclosure may, but need not, be to scale, and therefore may be understood as teaching various ratios and proportions evident in the drawings. Moreover, while various aspects, features and concepts may be expressly identified herein as being inventive or forming part of an invention, such identification is not intended to be exclusive, but rather there may be inventive aspects, concepts and features that are fully described herein without being expressly identified as such or as part of a specific invention, the inventions instead being set forth in the appended claims. Descriptions of exemplary methods or processes are not limited to inclusion of all steps as being required in all cases, nor is the order that the steps are presented to be construed as required or necessary unless expressly so stated.

[0018] Exemplary implementations of the present disclosure relate to arrangements and methods for magnetic induction drying of plate-like metal objects or components, such as metal can lids or “ends.” As described herein, “plate-shaped” may include workpieces having a height or thickness dimension that is substantially smaller than (e.g., less than about 10% of, or less than about 5% of) each of the length and width dimensions of the workpiece. In other implementations, the arrangements and methods described herein may be applied to other arrangement and methods for heating, drying, or otherwise treating other types of workpieces.

[0019] An exemplary induction dryer is described in co-owned U.S. Patent No. 7,432,480 (the “’480 Patent”), the entire disclosure of which is incorporated herein by reference. FIGS. 1 and 2, reproduced from the ‘480 Patent, illustrate an exemplary dryer 10 including a heating cabinet 12, through which workpieces such as can ends 14 pass to be heated and dried, and a power and control cabinet 16. The power and control cabinet 16 serves as a base for and supports the heating cabinet 12. The power and control cabinet 16includes power and control circuitry indicated schematically at 18 which may include, for example, one or more transformers. The heating cabinet 12 supports a nonconductive tube 20 around which an induction coil 22 extends. The induction coil 22 is electrically connected with the power and control circuitry 18 by wires 24. Operation of the power and control circuitry 18 generates an electric current that flows through the induction coil 22 to heat any conductive material located within the tube 20. Thus, steel or aluminum ends can be heated. The tube 20 defines a generally enclosed space or workpiece conveying passage 160 in the heating cabinet 12, through which can ends 14 travel as they move through the dryer 12.

[0020] Can ends 14 to be dried are conveyed into the inlet passage 68 of the inlet hub 60 (e.g., by a conveyor belt, workpiece gripping wheels, or other suitable transport device) and into the inlet end 76 of the tube 20. The can ends 14 as they move through the tube 20 are acted upon by an alternating magnetic field generated by the induction coil 22. The can ends 14 are heated as a result, and this heat is conducted into the compound on the can ends. As the compound is heated, water is driven out of the compound into the surrounding air within the enclosed space 160 of the tube 20. This water is removed from the tube 20 to enable more can ends 14 to be dried within the tube.

[0021] The induction coil 22 may have a circular coil form, for example to apply substantially uniform heat flux to the circular outer periphery of the can ends 14, which may be positioned substantially centrally within the induction coil. In other arrangements, the induction coil may have a different shape coil form (e.g., rectangular, square, triangular, etc., or more irregular shapes) for similar uniform spacing (and uniformly applied heat flux) between the induction coil and different shaped workpieces.

[0022] In some applications, a workpiece may have outer peripheral portions that are under material stress generated during part formation, such as, for example, comer portions of a rectangular metal can end or container lid. The elevated stress at these locations on the workpiece periphery cause these portions to heat at a slower rate than other, lower stress locations on the periphery of the workpiece. The resulting uneven heating of the workpiece periphery can cause insufficient heating / drying at the high stress locations, and / or excessive heating of the lower stress locations (e.g., causing blistering or other damage to the overheated portions).

[0023] According to an exemplary aspect of the present disclosure, an induction heating apparatus may be configured to inhibit heat flux from the magnetic field (applied, for example, by a magnetic induction heating coil) along at least a first portion of a cross- sectional perimeter of a workpiece conveying passage without inhibiting heat flux from themagnetic field along at least a second portion of the cross-sectional perimeter of the workpiece conveying passage, such that a first (e.g., high stress) outer peripheral portion of the workpieces facing the first portion of the cross-sectional perimeter of the workpiece conveying passage is exposed to a smaller heat flux than a second (e.g., lower stress) outer peripheral portion of the workpieces facing the second portion of the cross-sectional perimeter of the workpiece conveying passage. In such an arrangement, a greater heat flux applied to the high stress outer peripheral portions and a smaller heat flux applied to the lower stress outer peripheral portions results in more uniform heating around the outer periphery of the workpieces.

[0024] Many different arrangements may be utilized to inhibit or reduce heat flux applied to selected (e.g., lower stress) peripheral portions of a workpiece. According to an exemplary aspect of the present disclosure, an induction heating apparatus may be provided with a support frame, disposed within the apparatus housing and surrounded by the induction heating coil, with the support frame defining a longitudinally extending workpiece conveying passage having a cross-sectional perimeter. At least one flux inhibiting element is secured to the support frame along at least a first portion of the cross-sectional perimeter of the workpiece conveying passage, and no flux inhibiting elements are secured to at least a second portion of the cross-sectional perimeter of the workpiece conveying passage. In such an arrangement, when a workpiece is conveyed through the workpiece conveying passage, a first outer peripheral portion (e.g., lower stress portion) of the workpiece facing the first portion of the cross-sectional perimeter of the workpiece conveying passage is exposed to a smaller heat flux than a second outer peripheral portion (e.g., high stress portion) of the workpiece facing the second portion of the cross-sectional perimeter of the workpiece conveying passage.

[0025] FIGS. 3-7 illustrate various views of an exemplary induction heating apparatus 100 including a housing 110, a support frame 120 disposed within the housing and defining a longitudinally extending, generally rectangular workpiece conveying passage 101 having a generally rectangular cross-sectional perimeter 102, and a magnetic induction heating coil 130 disposed within the housing and surrounding the support frame to generate a magnetic field for heating the workpiece conveying passage. The magnetic induction heating coil 130 may have a circular coil form, as shown, or any other suitable coil form shape (e.g., rectangular, for example, of similar dimensional proportions to the passage cross-sectional perimeter). A workpiece transport device (e.g., conveyor belt, workpiece gripping wheels), shown schematically at 105, may be operated to push or propel the workpieces (e.g., arranged in a horizontal stack) into the housing 110. As shown, the transport device 105 may beexternal to the housing 110 and as such, may be disposed away from the passage 101 (e.g., so as not to affect the proximity of the workpieces and the flux inhibiting elements).

[0026] As shown in FIGS. 3 and 4, an induction drying system 50 may include multiple induction heating apparatuses or tandem units 100, 100a, which may, but need not, both include a housing 110, 110a, support frame 120, 120a (defining passage 101, 101a), and magnetic induction heating coil 130, 130a, as described herein. Such arrangements may, for example, provide for faster line speeds.

[0027] The support frame 120 may be positioned within the magnetic induction heating coil 130 by a longitudinally extending nonmagnetic induction tube 140 (e.g., fiberglass pipe such as Centricast RB-1520 fiberglass pipe) secured within the magnetic induction heating coil, within which the support frame is attached. In the illustrated example, the support frame 120 includes a plurality of longitudinally extending workpiece-engaging rail elements 121a-d secured to the inner surface of the induction tube 140, for example, by radially extending brackets 122a-d (which may be integrally formed with the rail elements) and fasteners 123 (e.g., screws), such that the support frame is radially spaced from the induction tube and the surrounding heating coil 130. The first, second, third and fourth rail elements 121a-d define corresponding upper, lower, and opposed side portions 102a-d of the cross-sectional perimeter 102. These rail elements 121a-d may be discretely attached to the housing (e.g., to the induction tube 140, as shown) as separate components. Alternatively, two or more of the rail elements may be integrally formed with each other.

[0028] As shown in FIG. 6, the support frame 120 may be positioned such that the workpiece conveying passage 101 is sized to closely receive a generally rectangular workpiece 14, such as, for example, a rectangular metal lid having straight side edges 14a-d and rounded comers 14e-h (e.g., ’A inch radius). The lid may have a variety of sizes and dimensions, such as, for example, a length to width ratio between about 1 : 1 and about 3 : 1 (e.g., a length of about 102 mm and a width of about 56 mm, as shown in FIG. 6, or a length of about 89 mm and a width of about 72 mm, as shown in FIG. 6A). FIG. 6A illustrates an end view of an alternative arrangement with a support frame 120' configured to define a different shaped workpiece conveying passage 101 ' sized to closely receive a different shaped generally rectangular workpiece 14'.

[0029] As shown, each rail element 121a-d, 121a-d' may include rib portions or other such projections 124a-d, 124a-d' extending from a base portion 125a-d, 125a-d' of the rail element to engage the corresponding edges of the workpiece 14, 14', for example, to maintain the workpiece in a desired position and orientation within the passage 101, 101 '.

[0030] Flux inhibiting elements 150a-d, 150a-d' are secured to the rail elements 121a- d, 121a-d' at side portions 102a-d, 102a-d' of the rectangular cross-sectional perimeter 102, 102' to position the flux inhibiting elements adjacent to or facing the lower stress side edge portions 14a-d, 14a-d' of the workpiece 14, 14' when the workpiece is transported through the workpiece conveying passage 101, 101 ', thereby providing shielded portions of the cross- sectional perimeter that face or are aligned with the lower stress side edge portions of the workpieces. The flux inhibiting elements 150a-d, 150a-d' are sized so they do not extend over the corner portions 102e-h, 102e-h' of the rectangular cross-sectional perimeter 102, 102', such that no flux inhibiting elements are positioned adjacent to or facing the high stress comer portions 14e-h, 14e-h' of the workpiece 14, 14' (i.e., providing a non-shielded portion of the cross-sectional perimeter) when the workpiece is transported through the workpiece conveying passage 101, 101 '. As shown, the flux inhibiting elements 150a-d, 150a-d' may be secured to the base portions 125a-d, 125a-d' of the rail elements 121a-d, 121a-d', and may be sized such that the flux inhibiting elements are recessed from the rail element projections 124a-d, 124a-d', and therefore recessed from the passage 101, 101 ', for example, to prevent contact between the flux inhibiting elements and the workpieces. A gap or airspace between the flux inhibiting elements 150a-d, 150a-d' and the workpiece 14, 14' / passage 101, 101 ' may be minimal (e.g., less than about 1 / 8 inch, or between about 0.030 inches and about 0.060 inches), for example, to maximize shielding of the facing edge portion of the workpiece.

[0031] Depending on the workpiece dimensions, different sizes or numbers of flux inhibiting elements 150a-d, 150a-d' may be used. For example, in the embodiment of FIG. 6, the shielded portions of the cross-sectional perimeter 102 include two side-by-side flux inhibiting elements 150a, 150b along the larger upper and lower portions of the cross- sectional perimeter, and a single flux inhibit element 150c, 150d along the smaller opposed side portions of the cross-sectional perimeter. In the embodiment of FIG. 6A, the shielded portions of the cross-sectional perimeter 102' include two side-by-side flux inhibiting elements 150a-d' along all four portions of the cross-sectional perimeter.

[0032] As shown in the top cross-sectional view of FIG. 7, the flux inhibiting elements 150a-d may be positioned at discrete first longitudinal or flux inhibiting regions 103 of the workpiece conveying passage 101 and not positioned at discrete second longitudinal or non-flux inhibiting regions 104 of the workpiece conveying passage, such that when the workpieces are conveyed through the workpiece conveying passage, the first outer peripheral (lower stress) portions of the workpieces are exposed to a smaller heat flux when theworkpieces are in a flux inhibiting region of the workpiece conveying passage and a greater heat flux when the workpiece is in a non-flux inhibiting region of the workpiece conveying passage. The flux inhibiting elements 150a-d may be selectively sized and longitudinally positioned to control the reduction and timing of heat flux applied to the lower stress portions of the workpieces, for example, with alternating flux inhibiting and non-flux inhibiting regions along the length of the passage.

[0033] The flux inhibiting elements 150a-d may be provided in a variety of suitable materials, including, for example, ferrite and ferromagnetic materials. One such exemplary material is a soft ferrite material, not initially magnetized and not a permanent magnet (such as a hard ferrite), but that shows the effect of magnetic properties when positioned such that the element is near a magnetic field. The ferrite material may be selected to have high values of relative magnetic permeability, saturation flux density, electrical resistivity, and thermal conductivity. Other suitable materials may additionally or alternatively be used, including, for example, laminations made of grain oriented magnetic alloys, such as nickel-iron alloys and silicon-iron alloys (ferrosilicon), and magneto-dielectric materials, including electrolytic iron-based materials and carbonyl iron-based materials.

[0034] The flux inhibiting elements 150a-d may be provided in a variety of suitable shapes or forms, including, for example, strips, bars, blocks or plates of any suitable length, width, and thickness.

[0035] It will be appreciated that while the exemplary embodiment is described as providing a support frame defining a generally rectangular workpiece conveying passage sized to closely receive a generally rectangular workpiece, in other embodiments the support frame can be configured to define any other suitable shape (e.g., triangular, hexagonal, octagonal, oval, etc.) sized to closely receive a suitable workpiece having a corresponding shape, and the flux inhibiting elements may be selectively sized and longitudinally positioned to control the reduction and timing of heat flux applied to the lower stress portions of the workpieces.

[0036] The inventive aspects have been described with reference to the exemplary embodiments. Modification and alterations will occur to others upon a reading and understanding of this specification. It is intended to include all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.

Claims

We claim:

1. An induction heating apparatus for heating plate-shaped workpieces, the apparatus comprising: a housing; a support frame disposed within the housing and defining a longitudinally extending workpiece conveying passage having a cross-sectional perimeter, the support frame including a first longitudinally extending workpiece-engaging rail element defining an upper portion of the cross-sectional perimeter, a second longitudinally extending workpiece-engaging rail element defining a lower portion of the cross-sectional perimeter, and third and fourth longitudinally extending workpiece-engaging rail elements defining opposed side portions of the cross-sectional perimeter; a magnetic induction heating coil disposed within the housing and surrounding the support frame to generate a magnetic field for heating the workpiece conveying passage; and at least one flux inhibiting element secured to at least one of the first, second, third and fourth workpiece-engaging rail elements to define a shielded portion of the cross- sectional perimeter of the workpiece conveying passage; wherein no flux inhibiting elements are secured to at least a portion of the cross- sectional perimeter of the workpiece conveying passage to define a non-shielded portion of the cross-sectional perimeter of the workpiece conveying passage, such that when a workpiece is conveyed through the workpiece conveying passage, a first outer peripheral portion of the workpiece facing the shielded portion of the cross-sectional perimeter of the workpiece conveying passage is exposed to a smaller heat flux than a second outer peripheral portion of the workpiece facing the non-shielded portion of the cross-sectional perimeter of the workpiece conveying passage.

2. The induction heating apparatus of claim 1, wherein the at least one flux inhibiting element comprises a ferrite material.

3. The induction heating apparatus of claim 2, wherein the ferrite material comprises a soft ferrite.

4. The induction heating apparatus of claim 1, wherein the at least one flux inhibiting element comprises at least one of a grain oriented magnetic alloy and a magneto-dielectric materials.

5. The induction heating apparatus of claim 4, wherein the grain oriented magnetic alloy comprises at least one of a nickel-iron alloy and a silicone-iron alloy (ferrosilicon).

6. The induction heating apparatus of claim 4, wherein the magneto-dielectric material comprises at least one of an electrolytic iron-based material and a carbonyl iron-based material.

7. The induction heating apparatus of any of claims 1-6, wherein the shielded portion of the cross-sectional perimeter comprises a straight portion of the cross-sectional perimeter, and the non-shielded portion of the cross-sectional perimeter comprises a corner portion of the cross-sectional perimeter.

8. The induction heating apparatus of any of claims 1-7, wherein the cross-sectional perimeter of the workpiece conveying passage is substantially rectangular with the upper, lower, and opposed side portions separated by first, second, third and fourth comer portions, wherein the at least one flux inhibiting element comprises first, second, third, and fourth flux inhibiting elements secured to corresponding ones of the first, second, third, and fourth rail elements, and wherein no flux inhibiting elements are secured to the first, second, third and fourth corner portions.

9. The induction heating apparatus of any of claims 1-8, wherein the at least one flux inhibiting element is secured to the support frame at least at a first longitudinal region of the workpiece conveying passage, wherein no flux inhibiting elements are secured to the support frame along the shielded portion of the cross-sectional perimeter of the workpiece conveying passage at least at a second longitudinal region of the workpiece conveying passage, such that when the workpiece is conveyed through the workpiece conveying passage, the first outer peripheral portion of the workpiece is exposed to a smaller heat flux when the workpiece is in the first longitudinal region of the workpiece conveying passage and a greater heat flux when the workpiece is in the second longitudinal region of the workpiece conveying passage.

10. The induction heating apparatus of claim 9, wherein the first longitudinal region includes first and second portions separated by the second longitudinal region.

11. The induction heating apparatus of any of claims 1-10, further comprising a nonmagnetic induction tube secured within the magnetic induction heating coil, wherein the support frame is secured to an inner surface of the induction tube.

12. The induction heating apparatus of claim 11, wherein the support frame is secured to the inner surface of the induction tube by radially extending brackets, such that the support frame is radially spaced from the induction tube.

13. The induction heating apparatus of any of claims 1-12, further comprising a transport device external to the housing and operable to move the workpieces through the workpiece conveying passage.

14. The induction heating apparatus of any of claims 1-13, wherein the heating coil has a circular coil form.

15. The induction heating apparatus of any of claims 1-14, wherein the heating coil has a rectangular coil form.

16. The induction heating apparatus of any of claims 1-15, wherein the at least one flux inhibiting element is secured to a base portion of the at least one of the first, second, third and fourth workpiece-engaging rail elements, wherein the at least one of the first, second, third and fourth workpiece-engaging rail elements includes a workpiece-engaging projection extending from the base portion beyond an end surface of the at least one flux inhibiting element to prevent contact between the workpiece and the at least one flux inhibiting element.

17. The induction heating apparatus of claim 16, wherein the workpiece-engaging projection extends between about 0.030 inches and about 0.060 inches beyond the end surface of the at least one flux inhibiting element.

18. A method of heating plate-shaped workpieces including first and second opposed edges extending in a length direction and third and fourth opposed edges extending in a width direction, the method comprising: conveying the plate-shaped workpieces through a workpiece conveying passage of an induction heating apparatus, the workpiece conveying passage having a cross-sectional perimeter including upper and lower portions spaced apart to engage the first and second edges of the plate-shaped workpieces and opposed side portions spaced apart to engage the third and fourth edges of the plate-shaped workpieces; generating a magnetic field around the cross-sectional perimeter of the workpiece conveying passage for heating the workpiece conveying passage; inhibiting heat flux from the magnetic field along at least a first portion of the cross- sectional perimeter of the workpiece conveying passage without inhibiting heat flux from the magnetic field along at least a second portion of the cross-sectional perimeter of the workpiece conveying passage, such that a first outer peripheral portion of the plate-shaped workpieces facing the first portion of the cross-sectional perimeter of the workpiece conveying passage is exposed to a smaller heat flux than a second outer peripheral portion of the workpieces facing the second portion of the cross-sectional perimeter of the workpiece conveying passage.

19. The method of claim 18, wherein the first outer peripheral portion of the plate-shaped workpieces comprises material at a lower stress and the second outer peripheral portion of the workpieces comprises material at a higher stress.

20. The method of any of claims 18-19, wherein the plate-shaped workpieces each have a generally rectangular outer periphery, wherein the first outer peripheral portion of the workpieces comprises a straight portion of the outer periphery and the second outer peripheral portion of the workpieces comprises a corner portion of the outer periphery.

21. The method of claim 20, wherein the first outer peripheral portion comprises at least a portion of the first, second, third and fourth edges of the plate-shaped workpieces and the second outer peripheral portion comprises first, second, third and fourth comer portions of the outer periphery.

22. The method of any of claims 18-21, wherein the workpieces comprise metal lids.

23. The method of any of claims 17-20, wherein inhibiting heat flux from the magnetic field along at least the first portion of the cross-sectional perimeter of the workpiece conveying passage comprises providing at least one flux inhibiting element aligned with the first portion of the cross-sectional perimeter of the workpiece conveying passage.

24. The method of claim 23, wherein the at least one flux inhibiting element is recessed from the workpiece conveying passage to prevent contact with the plate-shaped workpieces.

25. The method of any of claims 18-24, wherein inhibiting heat flux from the magnetic field along at least the first portion of the cross-sectional perimeter of the workpiece conveying passage comprises wherein inhibiting heat flux from the magnetic field along at least the first portion of the cross-sectional perimeter at a first longitudinal region of the workpiece conveying passage and not at a second longitudinal region of the workpiece conveying passage.

26. The method of claim 25, wherein the first longitudinal region includes first and second portions separated by the second longitudinal region.

27. The method of any of claims 18-26, wherein the induction heating apparatus comprises the induction heating apparatus of any of claims 1-17.

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