CA1114458A

CA1114458A - Induction heating core and heating system for adhesive fasteners

Info

Publication number: CA1114458A
Application number: CA310,425A
Authority: CA
Inventors: Robert G. Kobetsky
Original assignee: Illinois Tool Works Inc
Current assignee: Illinois Tool Works Inc
Priority date: 1977-09-28
Filing date: 1978-08-31
Publication date: 1981-12-15
Also published as: IT1098915B; SE7809159L; FR2404986A1; IT7828075A0; AU3968278A; ES473779A1; DE2841448A1; JPS5457245A; BR7806235A; AU523901B2

Abstract

Abstract of the Disclosure A U-shaped core for association with a discrete ferromagnetic adhesive carrying article wherein heat is generated in the article through the completion of a flux circuit between the legs of the core and the article. The core and the article abut one another through protuberance means of limited area and height to reduce heat loss from the article back into the core.

Description

1 INDUCTIO~ HEATING CORE AND IIEATING
SYSTEM FOR ADHESIVE FASTENERS

This invention relates generally to an induction heating system hr use in heat activable adhesive fastening systems.

The invention more particularly relates to an improvemellt in a U-shaped induction core used to create heat in an adhesive carry-ing article through the completion of a magnetic flux circuit through the article.
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The in~7ention further relates to a hea-ting system ~ith a uniqlle core to heat irrcguldrly shaped adhesivc carrying f.lsteners so th~t a uniform heating pattern is applied to the adhesive layer.
., The use of a U-shaped core structure in an induction heating system is a known technique for heating predetermined areas of a .
ferromagnetic matcrial. Suci~ a technique re~ ircs th~t tl~e en~ -. ...
lS surfaces of the core directly contact the article to be heatcd to eliminate undesirable flux losses and to also use the core as a -- ~ ' :
, ', ", prcss~lIe applying device wl~cn the article to be heated is an adhesive backed element to be secured to a prim~ry work surface.

While such a technique is reasonably successful for many applications, it does inherently permit a certain amount of heat generated in the magnetic article to be conducted back into the core.
This back conduction phenomenon increases the total heating time required to activate the adhesive as well as creates undesirable heat level in the core.

Fasteners secured to a support structure with an adhesive layer such as that described above should be heated uniformly or the strength of the bond will differ substantially from one extremity of the fastener to the other. The adhesive coated fasteners may be of a variety of sizes, including those where a nonadhesive carrying feature i s adjacent one extremity. Such an irregularly configured fastener creates an undesirable problem because the nonadhesive carrying feature acts as a heat sink and thus draws the heat from the adhesive carrying structure into the nonadhesive carrying structure. This results in a nonuniform heating pattern for the adhesive layer.
, Accordingly, the inYentic~r~ seeks to provide a U-shaped core structure which prevents conduction of heat Irom the article being heated back into the core, , Another aspect of the invention seeks to provide an adhesive fastening system which minimlzes heat losses from the adhesive carrying article being heated, thus reducing the time cycle to secure an article.
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A further aspect of the invention seeks to provide a particularly configured U-shaped induction heating core which will uniformly heat an adhesive layer on an irregularly shaped article.
The invention further seeks to provide a core structure and heating system which reduces the wear on the core when it is used as a pressure applying device.
The invention broadly comprehends a core structure for use in efficiently heating by induction a metal article. The core structure comprises a U-shaped lamina-ted core with flat end surfaces at the extremities of each leg adapted to be associated with a surface of an article to be heated. Discrete non-conductive means are carried by the flat end surfaces protruding slightly beyond the associated end surfaces, and adapted to create an accurately defined, predetermined slight gap between each end surface and an associated metal article when the means firmly abut a surface of the metal article.
The invention further teaches an adhesive fastening system. The system comprises a U-shaped core having inductor windings wrapped around a region thereof and a discrete, ferromagnetic article completing a magnetic circuit between the legs of the ~-shaped core. The end faces of the legs of the core and laterally spaced predetermined upper surface regions of the article are configured to form pairs of opposing substantially flat surfaces creating the juncture regions between the core and article, thereby facilitating the formation of a closed magnetic flux path. The system further comprises discrete non-conductive spacer means of limited height formed on one of the opposing flat surfaces of each pair to maintain the opposing surfaces in carefully controlled, slightly spaced relationship to each other. The lower surface of the article is coated with a heat actlvable adhesive wherein the magnetic flux generated in the article generates heat sufficient to activate the adhesive for association with a support surface.
An induction heating system is aIso taught by the present bi' i ~ _3_ .

invention. The system comprises an irregular shaped discrete metal article and a core structure for use in induction heating.
A support surface upon which the article is to be accurately secured is provided and the article includes a primary attachment section carrying a layer of ac-tivable adhesive therethrough and a second non-adhesive carrying secondary attachment section extending generally upwardly from adjacent a perimeter region of the first section. The core struc-ture comprises a U-shaped core with end surfaces at the extremities of each leg adapted to be associated with laterally spaced side marginal upper surface regions of the primary adhesive carrying section, so that the adhesive carrying section completes a closed magne-tic flux circuit beneath the core. The legs define front and rear surfaces in planes generally parallel to the plane of the bight portion of the U-shaped core. The flat end surfaces of each leg are coplanar and intersect the planes of the front and rear leg surfaces. The end surfaces of the core carry non-conductive spacer means to abut the spaced surface regions of the metal article and non-conductively position the end surfaces in spaced juxtaposed relationship to the associated spaced surface regions. The extremity of the end surfaces located adjacent the non-adhesive carrying section is spaced from the metal article a distance less than the distance of spacing between the other extremity of the end surfaces from its associated upper surface regions on the adhesive carrying section. The flat end surfaces of the legs tilt out of coplanar relationship with the upper surface regions of the adhesive carrying section of the metal article. The spacer means thereby compensates for the heat dlssipated into the non-adhesive carrying section so that the adhesive layer will be uniformly heated between the discrete metal article and support surface.

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A preferred embodiment of the invention provides a U-shaped core including an exciter coil wound about the bridge section of the core and with protuberance means formed between the article to be heated and the end face of the core. The -3a-:
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L~ ~3 protuberance means are of a limited height and may be described as being rod-like, non-conductive elements embedded in the end faces of the core. For heating irregular shaped articles, the end surfaces of the core are provided with non-conductive means which extend a greater distance from the end surface at one extremity than at the opposite extremity.
An alternate embodiment of the inven-tion considers the formation of a wedge-shaped insulative coating on the end faces so that the end faces are spaced a greater distance from the article in a region xemote from the non-adhesive carrying tab than the region closely adjacent the adhesive carrying tab.
Many other objects, features and advantages of the invention will be more fully realized and understood from the following detailed description when taken in conjunction with the accompanying drawings wherein like reference numerals throughout the various views of the drawings are intended to designate similar elements or components.

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Brief Description OI the Drawings ~
. . . _ - 1 Fig. 1 is a perspective view of the core structure of the present invention. ~-Fig. 2 is a bo-ttom plan view of the core structure of the invention.

Fig. 3 is a side elevational view of the core structure of the invention shown in association with a workpiece.

Fig. 4 is a front end view of the fastening system shown in Fig. 3.

Fig. 5 is an enlarged partial side elevational view showing the relationship of the end faces of the core with the workpiece being heated .

Fig. 6 is a perspective view of an irregularly shaped adhesive fastener typical of the type which may be secured using this invention.
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Fig, 7 is a perspective view of another U-shaped induction core in accordance with the invention.
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Fig. 8 is a rear erid elevational view of the core used in association with the metal article of Fig. 6, Fig. 9 is a side elevational view of the invention shown in Fig. 8.

~ig. 10 is an enlarged partial side elevational view of the invention of Fig. 9 showing the relationship of the end face of the core to the metal article, .
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Fig. 11 is a side elevational view of an alternate embodiment of the invention.

Detailed Description of the Pr_ferred EmbodIment Referring now to the drawings, reference numeral 10 denotes the U-shaped core struc-ture which is the subject matter of the invention.
The core 10 basically is configured to include a bridge section lZ
interconnecting leg regions 14 with an exciter coil 16 wound about the bridge section. As shown clearly in Figs. 1 and 2, the core structure 10 will also be defined as including substantially flat end faces 18 at the extremities of the legs as well as front and rear surfaces 24 and 26, respectively, and inner leg surfaces and outer leg surfaces 20 and 22, respectively.

Figs. 3-5 describe the core 10 in assoclation with an article to be heated. It will be shown that the invention is particularly effective in heating a discrete, ferromagnetic article 34 which will include a flat base 36 carrying a layer of heat activable adhesive 40 therebeneath. The article may typically include a secondary fasten-ing region, such as tab-like extension 42.
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Heating systems of this general type rely upon a flux density created in a metal article to produce heat therein by eddy currents and hysteresis when the metal article completes a magnetic flux circuit between the legs of the core. The core IS preferably configured to have a relativeLy large cross-sectional area compared to the article to be ùeated and the metal article will be of a high loss material with :

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the core itself being of a relatively low loss material. These situations maximize the heat created in the article by the hysteresis and eddy current effect, thus producing a heat level in a metal article high enough to activate the adhesive. In many industrial applications, it is very important that the time required to effect such a fastening be minimized so the heating process should be as efficient as possible.

As noted above, prior art heating systems tend to permit the heat generated in the ferromagnetic article to be conducted back into the core. It is with this condition in mind that reference is now made to spacer elements 28 and 29 formed in the end surfaces of the core.
In the preferred embodiment, these spacer elements are rod-like, nonconductive elements extending in a airection from the inner surfaces 20 of the leg toward the associated outer sur~aces 22. These spacer elements are embedded in the end surfaces of the laminated core and are suitably fixed therein, for example by adhesive. Elements 28 and 29 will protrude from the end surfaces 18 a carefully and accurately controlled short distance. The distance of protrusion should be uniform throughout the length of each element. The distance that end surfaces ' 18 are spaced from the metal article should not be so great as to permit a significant flux loss to occur. Preferably this distance will be in the range of . 002 in. -. 005 in. but the exact dimensioning will, OI course, vary with the application and article being heated.
' Since the core lO will typically be used as a pressure applying element in the adhesive fastenlng system, the nonconductive spacer elements 28 and 29 should also be of a hard, wear-resistant material~

It has been found tllat a ceramic material or aluminum oxide material satisfies the insulating as well as wear-resistant requirements of this invention.

For stability in the heating system, at least two rod-like elements spaced from one another should be embedded in each end surface and as shown in the drawings.

Fig. 6 shows an adhesive carrying metal article 34 which is effectively secured to a supporting work structure through the use of the invention. It will be noted that the metal article includes a substantially flat base 36 with an undersurface coated with a layer OI
heat activable adhesive 40. One end extremity of the generally rectangular base includes a tab-like extension 42 extending upwardly and may be provided with an aperture 44 to accept a secondary attachment once the article 34 is adhesively secured to a primary surface 48~

In use, a U-shaped core of the type generally described herein is associated with the base 36 of the fastener with the end surfaces 18 of the core juxtaposed over spaced upper surface regions 38 and forming juncture regions in a completed magnetic flux circuit. As the flux flows through the base 36, heat is generated by the creation of eddy currents and through the hysteresis effect. It will be noted, however, that an upstanding tab 42 will act as a heat sink, drawing effective heat fr~m the region of the base adjacent the tab. Thus, the tab influences the development of a nonuniform heating pattern as for the adhesive layer. The present invention compensates for such an undesirable situation~ ~`

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, As shown in F'igs. 7-10, the core lO is associatecl with an irregular shaped item, such ~s article 3~, so that the protuberance 28 is adjace.nt the tab 42 while the protuberance 29 i5 positioned remote from the tab 42. Since the flux density created in an article to be heated using a heating system of this type depends greatly llpon the distance that the core is spaced above the surface to be heated, it will be apparent that the flux density m the region adjacent the tab 42 will be greater than the density in the region remote from such a tab. Thus, a greater amount of heat is provided the base 36 in the region of the tab to accommodate and compensate for the heat that is conducted into the heat sink tab 42.

A carefully controlled use of nonconductive protuberances 28 and 29 will thus create a substantially uniform heating pattern for the adhesive layer 40. It should be noted that while the rods 28 and 29 are shown to be of different diameters, the same result can be accomplished by using a rod o:E identical diameters and imbedding one rod in the end face a greater distance than the imbedment of the other.
It becomes important that the rod be of nonconducting, wear-resistant material, such as ceramic, aluminum oxide or the like, in order to ~0 prevent conduction beck into the core element itself, which would create heat loss from the side edge margins of the fastener device . .
into the core element.

. While a pre:Eerred embodiment shows the use of rods which provide a limited surface contact between the core and the article, it 2~ should be apparent that the tilting o the end faces of the core can be ': ~

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accomplished in an alternate rnanner. For example, in Fig. 11 a core 10a substantially identical to 10 includes all of the features of the primary embodiment with like reference numerals including the suffix "a" designating similar elements with the exception of the type of spacing means shown on the end surface 18a. In the alternate embodi-ment, a wedge-shaped coating of nonconductive material is formed on the end surfaces of the core. The extremity 28a of the coating is o a gLven thickness which is less than the thickness of the extremity 29a, thus providing a difference in flux density substantially identical to that of the preferred embodiment.

The invention described above thus provides a core and an adhesive fastening system of particular value in securing a small, discrete, ferromagnetic article to a work surface when a short, accurately controlled temperature level is required to activate an adhesive layer. The invention describes a preerred embodiment for spacingly ins~llating the end surfaces of a core from the surface being heated without appreciably sacrificing heat losses by flux - density loss. The invention further describes the means to reduce wear on the end ~aces when the core is to be used as a pressure applying element during the heating of the work surface.

While the invention has been described with particular reference to a pre~erred embodiment, it will be understood that it is not intended to limit the invention to that embodiment. On the contrary, it is intended to cover all alternatives, modiications and equivalents as may be included within the spirit and scope of the invention as described by the appended claims .
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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A core structure for use in efficiently heating by induction a metal article comprising a U-shaped laminated core with flat end surfaces at the ex-tremities of each leg adapted to be associated with a surface of an article to be heated, discrete nonconductive means carried by the flat end surfaces protruding slightly beyond the associated end surfaces, and adapted to create an accurately defined, predetermined slight gap between each end surface and an associated metal article when said means firmly abut a surface of said metal article.

2. The core structure of Claim 1, wherein said nonconductive means are elements of rigid material embedded in the end surfaces.

3. The core structure of Claim 2, wherein said embedded elements are rod-like members oriented to transverse the end surfaces in a direction from an inner leg surface to an outer leg surface.

4. A core structure as in Claim 1, for use in efficiently heating, by induction, an irregular shaped metal article including one section carrying a layer of heat act-ivable adhesive therebeneath and a nonadhesive carrying section extending from adjacent one edge of said adhesive carrying section, said nonconductive means located adjacent the nonadhesive carrying section being spaced from the metal article a distance less than the distance of spacing between the other extremity of the end surfaces from its associated upper surface regions on the adhesive carrying section so that the adhesive will be heated substantially uniformly across its faying surface.

5. The core structure of claim 2, including at least two spaced elements in each end surface, all elements protruding an equal amount from said end surfaces.

6. The core structure of claim 2, wherein the elements protrude an amount in the range of .002 in.-.005 in.

7. The core structure of claim 2, wherein the elements are of a hard, force-transmitting and wear-resistant material.

8. The core structure of claim 1, wherein the nonconductive means comprises a coating on the end surface of each leg of increas-ing thickness from a front surface of the core to a rear surface of the core with the exposed end surfaces of the thus formed composite core structure being coplanar.

9. The core structure of claim 1, wherein the nonconductive means comprises a plurality of spaced protuberances embedded in each end surface, one protuberance in one end surface extending a smaller distance from the end surface than another protuberance in said one end surface.

10. An adhesive fastening system comprising, in combination, a U-shaped core having inductor windings wrapped around a region thereof, a discrete, ferromagnetic article completing a magnetic circuit between the legs of the U-shaped core, the end faces of the legs of the core and laterally spaced predetermined upper surface regions of the article con-figured to form pairs of opposing substantially flat surfaces creating the juncture regions between the core and article facilitating the formation of a closed magnetic flux path, discrete nonconductive spacer means of limited height formed on one of said opposing flat surfaces of each pair to maintain said opposing surfaces in carefully controlled, slightly spaced relation-ship to each other, the lower surface of said article being coated with a heat activable adhesive wherein the magnetic flux generated in said article generates heat sufficient to activate the adhesive for association with a support surface.

11. The induction heating system of Claim 10, wherein the ferromagnetic article comprises a first, primary attachment section carrying said layer of heat activable adhesive therebeneath and a second nonadhesive carrying, secondary attachment section extending from ad-jacent a perimeter region of said first section, the discrete nonconductive spacer means being of variable, increasing height from an area adjacent the secondary attachment section to an area remote from the secondary attachment section to insure that a substantially uniform heating pattern exists in the primary attachment section.

12. An induction heating system comprising in combination, an irregular shaped, discrete metal article, a support surface upon which said article is to be accurately secured, a pressure applicating core structure for use in efficiently heating, by induction, said irregular shaped discrete metal article, said article including a primary attachment section carrying a layer of activable adhesive therethrough and a second nonadhesive carrying secondary attachment section extending generally upwardly from adjacent a perimeter region of said first section, the core structure comprising a U-shaped core with end surfaces at the extremities of each leg adapted to be associated with laterally spaced side marginal upper surface regions of said primary adhesive carrying section so that the adhesive carrying section completes a closed magnetic flux circuit beneath the core, the legs defining front and rear surfaces in planes generally parallel to the plane of the bight portion of the U-shaped core, the flat end surfaces of each leg being coplanar and intersecting the planes of the front and rear leg surfaces, said end surfaces of the core carry-ing nonconductive spacer means to abut said spaced surface regions of the metal article and nonconductively position the end surfaces in spaced juxtaposed relationship to the associated spaced surface regions with the extremity of said end surfaces located adjacent the nonadhesive carrying section being spaced from the metal article a distance less than the distance of spacing between the other extremity of the end surfaces from its associated upper surface regions on the adhesive carrying section, to tilt the flat end surfaces of the legs out of coplanar relationship with the upper surface regions of the adhesive carrying section of the metal article, the spacer means thereby compensating for the heat dissipated into the nonadhesive carrying section so that the adhesive layer will be uniformly heated between the discrete metal article and support surface.

13. The core structure of Claim 12, wherein the nonconductive spacer means comprises a coating on the end surface of each leg of increasing thickness from a front surface of the core to a rear surface of the core with the exposed end surfaces of the thus formed composite core structure being coplanar.

14. The core structure of Claim 12, wherein the nonconductive spacer means comprises a plurality of spaced protuberances embedded in each end surface, one protuberance in one end surface extending a smaller distance from the end surface than another protuberance in said one end surface.

15. The core structure of Claim 14, wherein the spaced protuberances are rib-like members each extending in a direction from an inner side surface to an outer side surface of a leg.