- 

NASA CASE NO. LAR 179-2 

PRINT FIG. 1 

The invention disclosed in this document resuited from research in aeronautical and 

space activities performed under programs of the National Aeronautics and Space 

Administration. The invention is owned by NASA and is, therefore, available for licensing 

in accordance with the NASA Patent Licensing Regulation (14 Code of Federal 

Regulations 1245.2). 

To encourage commercial utilization of NASA-owned inventions, it is NASA policy to grant 

licenses to commercial concerns. Although NASA encourages nonexclusive licensing to 

promote competition and achieve the widest possible utilization, NASA will provide the 

necessary incentive to the licensee to achieve early pradical application of the invention. 

Address inquiries and all applications for license for this invention to NASA Patent 

Counsel, Langley Research Center, Code 143, Hampton, Virginia 236654225. 

Serial No.: 07/790,731 

10-31 -91 LaRC 

Unc 1 as 
NASA Case No. LAR 14679-1 

ING FOR INDUCTION 

BEATING APPARATUS AND METHOD 

The present invention relates generally to induction heating, and 

more specifically, to a flexible heating head and various 

manufacturing processes using the flexible heating head. 

Referring to FIG. 1, a length of wire having two opposite ends 12c 

and 12d are wound into a flat spiral coil 12 which is embedded in 

a flexible, elastomeric material. The two ends 12c and 12d are 

coupled to a capacitor 16 to form a tank circuit. The tank circuit 

is powered through a power oscillator so that, when a susceptor is 

juxtaposed to an outer circuit of the head, the susceptor is heated 

by induct ion heating . 

The present invention has utility for various manufacturing 

processes in which it is necessary to place the heating head on a 

curved surface. Due to the flexibility of the heating head, the 

head can conform to the curved shape of the surface. 

Robert H. Coultrip 

1016 Dare Road - Yorktown, VA 23692 

Inductron Corp 

Carl E. Copeland 

207 Hodges Cove Road 

I 

Yorktown, VA 23692 

- 

Inductron Corp 

Samuel D. Johnson 

103 Old Dare Road 

rnductron Corp 

W. Morris Phillips 

12 Langhorn Road 

, VA 23606 

Inductron Corp 

Robert L. Fox John D. Buckley 

Route 3, Box 1168 23 East Governor Drive 

s, VA 23602 

NASA LaRC NASA LaRC 

Evaluator : Samuel S. Harris, Jr. 

Serial No.: 07/790,731 

Filed: October 31, 1991 
LAR 1 4679- 1 -1- PATENT APPLICATION 

FLEXIBLE HEATING HEAD FOR INDUCTION HEATING 

APPARATUS AND METHOD 

Oriain of the Invention 

5 

The invention described herein was jointly made by employees of the 

U.S. Government and employees of the lnductron Corporation and may be 

manufactured and used by or for the Government for governmental 

purposes without the payment of any royalties thereon or therefor. 

10 

Backaround of the Invention 

Field of the Invention 

15 The present invention relates generally to electromagnetic heating 

devices and, more specifically, to a heating head capable c; heating a 

relatively large zone, particularly on curved surfaces. 

Descri~tion of the Related Art 

20 

There currently exists a wide number and variety of induction heating 

devices. One such induction heating device is described in U.S. Patent No. 

4,521,659, issued to Buckley ef at. on June 4, 1985. The heating gun 

described therein uses a tank circuit which includes a capacitor and an 

25 inductor coil. The inductor coil is wrapped around a U-shaped pole piece 

which has a gap formed between the ends thereof. The capacitor of the 

tank circuit is connected to a power supply which provides an alternating 

current of predetermined frequency. 

The aforementioned patent deszribes a technique of joining two 

30 sheets of material by placing a susceptor, such as a wire screen, between 
LAR 14679-1 -2- PATENT APPLICATION 

!he sheets with adhesive therebetween. The heating gun is positioned 

above the two sheets and the screen with the ends of the pole piece 

touching one of the sheets and with a gap of the pole piece located above 

the area where the sheets are to be joined. An alternating current from the 

5 power source enters the tank circuit and the capacitor increases the Q of the 

tank circuit, in turn increasing the current flow through the inductor coil. 

Direction of the current along the inductor coil reverses at each cycle of 

alternating current. The current flow in the inductor coil creates a magnetic 

flux within the turns of the coil. The flux is picked up by the pole piece and 

10 carried to either of the two ends. The flux then jumps to the susceptor 

rather than across the gap to the other of the two ends. Since the sheets of 

material are transparent to magnetic flux, the flux is easily transferred to the 

screen through the sheets. The rapidly changing direction of current in the 

coil causes the flux to change constantly in magnitude and direction. This is 

15 also true in the metal screen as well as in the pole piece. Hysteresis 

creates eddy currents in the screen which result in heat being generated in 

the screen. 

While the aforementioned heating gun is suitable for laying down a 

single heat zone, whereby a seam can be created between two pieces of 

20 layered material, particularly for flat surfaces, it is sometimes desirable to 

heat curved surfaces to form multi-ply composites. 

Prior methods have employed inductior~ heating of high reluctance 

metal screens or perforated metal sheets sandwiched between two pieces of 

thermoplastic or screen susceptors or metal sheet susceptors encapsulated 

25 with adhesives sandwiched between non-thermoplastic adherences. These 

methods generally require the use of a susceptor in the bondline which, in 

some bonding processes, is considered unacceptable because of possible 

corrosion or galvanic action. With conductive susceptors, it is difficult to use 

conventional non-destructive evaluation of joints and seam welds or bonds30 In the past, electric heat blankets have been used to process adhesives in 
LAR 14679-1 -3- PATENT APPLICATION 

bondlines. Heat blankets are limited to process temperatures of about 

450°F, however. 

Additionally, the use of induction heating devices for patch repair of 

damaged aircraft structures, such as wind screens, wing surfaces, etc., 

5 normally require vacuum bagging. Vacuum bagging assures proper 

pressure between patches and the damaged surface area while maintaining 

patch stability dur~ng the bonding process. Typically, vacuum bagging 

removes excessive air from the bondlineladhesive resulting in a higher 

strength repair. 

10 Vacuum bagging typically includes applying a ring or layer of sealant, 

vacuum tape, to surfaces being repaired. A suction end of a vacuum hose 

is positioned inside the sealant ring and held in place by additional sealant at 

the point where the hose crosses the sealant ring. A layer of material such 

as Kypton film is placed over and in contac! with the sealant ring. A vacuum 

15 is created in the patch area when sudon is applied to the vacuum hose. In 

conventional vacuum bagging, as described above, the process is time 

consuming and requires a variety of materials. Moreover, it is extremely 

difficult where overhead patches are required or when working in harsh 

weather conditions. 

20 The heating heed itself is an important consideration. Previous 

induction heating devices utilize heating heads with rigid coil core material 

for focusing the heat-generating flux to a specific area. This is unacceptable 

when trying to apply the heat to a curved surface since the core material 

would prevent the coil from conforming to the shape of the surface. Since 

25 the flux transferred from the coil to any point on the susceptor is a function 

of the distance between that point and the coil, uneven heating of the 

surface will result. 
LAR 14679-1 PATENT APPLICATION 

Summarv of the Invention 

An object of the present invention is therefore to provide a flexible 

heating head for use in induction heating devices and methods, which is 

5 lightweight, portable, and simple to use. 

Another object of the present invention is to provide a flexible heating 

head for use in an induction heating device in which the heat generated by 

the head can be reliably and effectively distributed under all weather 

conditions, on curved surfaces as well as flat surfaces. 

10 These and other objects of the invention are met by providing an 

induction heating head which includes a length of wire having first and 

second opposite ends and being wound in a flat spiral shape to form an 

induction coil, a capacitor connected to the first and second ends of the wire, 

the induction coil and capacitor defining a tank circuit, and a flexible, 

15 elastomeric body molded to encase the induction coil. When a susceptor is 

placed in juxtaposition to the body, and the tank circuit is powered, the 

susceptor is inductively heated. 

Brief Descrlptlon of the Drawinas 

20 

Fig. 1 is a top view of an induction coil and capacitor, which 

comprises a tank circuit, according to the present invention; 

Fig. 2 is a section view taken along line 11-11 of Fig. 1 ; 

Fig. 3 is perspective view of an induction heating head according to 

25 the present invention; 

Fig. 4 is a perspective view of a power supply unit of the present 

invention, used in conjunction with the heating head of Fig. 3; 

Fig. 5 is a side elevational view of a second embodiment of a heating 

head (without illustrating the capacitor) according to a second embodiment 

30 of the present invention; 
LAR 1 4679-1 -5- PATENT APPLICATION 

Fig, 6 is a bottom view of the induction heating head of Fig. 5; 

Fig. 7 is an enlarged, sectional view taken along line VII-VII of Fig. 6; 

and 

Fig. 8 is a schematic, exploded view showing a method of induction 

5 heating according to the present invention. 

Detailed Description of the Preferred Embodiments 

Referring to Figs. 1-3, a flexible heating head according to the present 

10 invention is generally referred to by the numeral 10 and includes an 

inductive coil 12 embedded i? a high temperature resistance elastomeric, 

flexible body 14 which is molded into the shape of a disk. The coil 12 

illustrated in Fig. 1 is shown prior to molding and may consist of, for 

example, a length of insulated wire wound in an involute spiral to form a 

15 plurality of equidistantly spaced, concentric windings which are held by a 

suitable jig (not shown), or other means during molding so that center axes 

of the winding are mutually coplanar. The wire, as shown in Fig. 1 (a), has 

preferably a multi-stranded copper core 12a in an insulative sheath 12b. 

One example of a commercially available wire is CAROL SUPER VU-TRON 

20 ll 600V electric cable, which is particularly suitable for the purposes of the 

present invent~on because of the ease with which a piece of this wire bends 

and flexes, without retaining the bent shape. As a further example, twelve 

feet of such wire when coiled will form a circle of about a nine inch diameter. 

The coil 12 is completely encapsulated in the body 14 by any 

25 conventional means of molding. The elastomeric rrsterial which forms the 

body 14 can be any of a variety of known materials which can be applied to 

a mold in liquid form and then after setting forms a solid material which is 

flexible and resilient. The material thus provides a high temperature coating 

which protects the windings from heat generated during induction heating 

30 and which holds the windings in their precasting positions illustrated in Fig. 
LAR 1 4679- 1 -6- PATENT APPLICATION 

1. An example of a commercially available plastic material suitable for use 

in forming the body 14 is RTV 60. 

When the body 1 4 is placed on an uneven surface, the body 14 can 

be pressed by hand to conform to the shape of the uneven surface. This 

5 ailows the windings within the body 14 to remain at an equal distance from 

the surface. 

The two opposite ends 12c and 12d of the length of wire which forms 

the coil 12 extend out of the side of the body 14 and are connected to a 

capacitor 16 (or plural capacitors connected in parallel). The capacitor 16 

10 and the induction coil 12 thus form a tank circuit. Preferably, the capacitor is 

mounted in a housing 18 and the tank circuit is energized via a power 

source which is incorporated into a portable control unit 20. The control unit 

20 is connected to the tank circuit through a power cord 22 having a coupler 

24 that plugs into a socket 26 of the control unit 20. The control unit may be 

15 provided with AC or DC current derived from any convenient source, such 

as household current or a D.C. battery connected at input jacks 28. 

The control unit 20 includes a self-timing solid state power oscillator 

which produces a KHz output which is delivered to the capacitor via power 

lines 30 and 32 which run through the power cord 22. The frequency of the 

20 power output can be varied by turning a power output control knob 34 of the 

control unit 20 to achieve a desired power with a .47 M, 800 VDC 

(SPRAGUE Model 710P) capacitor. The power output of the oscillator at 

2000 watts can reach about 25 KHz. 

The heating head of the present invention can also use the power 

25 source described in U.S. Patent No. 4,521,659, which is incorporated herein 

by reference. 

The alternating current supplied to the tank ciicuit and passing 

through the coil produces a magnetic flux inside and around the induction 

coil 12. The flux jcmps to a susceptor if placed in juxtaposition to the 

30 induction coil 12. Hysteresis creates eddy currents in the susceptcr which 
LAR 14679-1 -7- PATENT APPLICATION 

result in heat generated in the susceptor. 

The heat generated by the heating head facilitates the curing of large 

adhesive bond areas in a single operation. Moreover, since the heating 

head is flexible, it can conform to complex, curved surfaces common to 

5 aircraft structures, [hereby assuring even curing of adhesive bonding repairs 

of windscraens, wing surfaces, etc. 

The control unit 20 may include a timer 36 which can be set so that 

induction heating can occur for a predetermined length of time. Also, the 

control unit 20 may include a temperature controller 38 which has a digital 

10 read out 40 consisting of a four digit number read out. A push button 42 is 

provided for each digit so that a predetermined temperature can be selected 

and input into the control unit 20. The temperature controller 38 is used in 

conjunction with a thermocouple provided at the area to be heated so that 

the thermocouple leads are connected to the control unit at the 

15 thermocoupla jack 44. The control unit 20 includes internal circuitry which 

includes a microprocessor so that the sensed temperature value can be 

compared to the input temperature provided at the temperature controller 38. 

For example, the controller can be programmed so that once the 

predetermined temperature is achieved, the controller will maintain the 

20 predetermined temperature for a predetermined period of time. Thus, the 

maximum temperature can be preset by means of the temperature controller 

38. This is particularly advantageous for curing adhesives which specify a 

predetermined temperature level for a predetermined period of time in order 

to effect a cure of the adhesive. 

25 Other aspects of the control unit include a power on light 46, a fuse 

48 and a power onloff toggle switch 50. The temperature controller is 

commercialiy available from OMEGA and can be suitably wired into the 

power circuit so that the power output is controlled to maintain the 

predetermined temperature. Similarly, timers are commercially available and 

30 can be suitably wired to the power circuit so that the power is autcmatically 
LAR 14679-1 -8- PATENT APPLICATION 

cut-off to the tank circuit after expiraticn of the predetermined period of time. 

Another embodimen! of the present invention is illustrated in Fig. 5, in 

which the disk-shaped body 52 is similarly molded to envelope a cod 54 

having opposite ends 54c and 54d which exter;d out a zide thereof for 

5 connection to a capacitor (or capacitors). In the embodiment of Fig. 5, 

the overall aiameter of the disk-shaped body 52 is made slightly larger such 

that, for example, if tne coil 54 has a disrneter of about 9 iiches, :he overall 

diameter of the body 52 will be about 11 inches, to provide an additional 

inch around the periphery of the disk. A circular recess 56 is then formed ir! 

10 the surface of the body 52 which will overlay objects to be heated. The 

circular recess 56 defines an annular flange 58 extending circumferentially 

around the body 52. When the body is placed on a surface to be worked, a 

suction chamber 60 is defined betweer; ihe surface 62 to be worked and the 

surfaces of the recess 56. These surfaces include a flat, circular surface 

15 56a and a cylindrical surface 56b. 

Once the body 52 is placed on the surface !o be worked, a vacuum 

pump 64 is activated to renlove air from the chamber 60 thus creating a 

vacuum which draws the flexible body 52 downwardly onto whatever is 

placed in the chamber that requires heating, such as an adhesive material 

20 which is used to attach a patch or other adherend to the underlying surface 

62. The vacuum is introduced to the chamber 60 through a fitting 66 which 

is mounted in a hole formed through the body 52. The fitting can be of any 

suitable type to which a vac~um~hose 68of the vacuum pump 64 can be 

attached. In the embodiment illustrated, the fitting 66 includes a threaded 

25 stem 68, a nut 70 and washers 72 and 74. Air is removed from the 

chamber 60 through a central bore 76 of the stem 68. 

In the illustrated embodiment, the heating head weights approximately 

two pounds, and is thus advantageous as a light weight, easy t~ use device. 

This is based on the nine inch diameter induction coil which is encapsulated 

30 in a body having a diameter of about eleven inches and a thickness of about 
LAR 14679-1 -9- PATENT APPLICATION 

one half inch. The body is molded to encapsulate the coil and is made of 

!he aforementioned high temperature, highly flexible plastic material, such as 

RTV 60. The flange of the annular step is about one quarter of an inch 

around the outer perimeter and thus provides the peripheral seal for the 

5 suction chamber. With suction applied to the vacuum fitting, and the head 

positioned over a patch on a surface to be repaired, the quarter inch 

shoulder or step around the heating head contacts the surface being 

repaired and a vacuum seal results. The vacuum pulls the nine inch heating 

area of !he head zgainst the patch to assure good patcwadhesive to surface 

10 pressure and removes excessive air from the adhesive. This also permits 

holding of the heating head in proper position for the duration of the repair 

process. 

In most cases surfaces requiring repairs or patches are smooth 

enough so that with suction applied to the heating head and with the quarter 

15 inch shoulder of the heating head in contact with the surface being repaired, 

a vacuum seal will result and be maintained. A thin coating of commercial 

vacugm grease applied to the face of the heating head shoulder may be 

used to supplement the vacuum seal. In cases where surfaces to be 

repaired are unusually roug5, or where riveffscrewheads obstruct good 

20 heating head to surface contact, a layer of sealant tape can be applied to 

the heating head shoulder to assure proper seal. 

Referring to Fig. 8, a patching technique using the heating head of 

the present invention is illustrated aschematically. A first adherend 78 has 

some need of repair, which is illustrated as a hole 78a which requires 

25 patching. First, an adhesive strip 80 is placed on the surface of the 

adherend 78 over the hole 78a. The adhesive may be of the type which is 

thermoplastic and/or thermosetting and is cut to fit from a sheet. A suitable 

type of adhesive which is commercially available is known as PEEK, which 

is a trademark for the ICI Corporation and is essentially a poly(pheny1ene 

30 ether ether) ketone. Prior to heating, the PEEK sheet is a flexible, solid 
LAR 14679-1 -10- PP.TENT APPLICATION 

sheet which is essentially transparent. At about 720°F, the sheet melts and 

is thus thermoplastic. It eventually cures to form a strong bond between the 

adherend 78 and a superimposed adherend 82. 

As mentioned above, Fis. 8 is schematic, and the dimensions of the 

5 various materials are exaggerated for the purposes of illustration. Typically, 

a sheet of PEEK has a thickness of less than a millimeter and thus 

exaggerated thicknesses are required for a better understanding of the 

laminated structures created by induction heating. 

A second adherend 82 is, generally speaking, the patch which covers 

10 the hole 78. The adhesive 80 is thus required to bond the adherend 82 to 

the adherend 78. 

Prior methods of induction heating have used metal screens or 

perforated metal sheets sandwiched between two pieces of thermoplastic in 

order to generate the required induction heating. Also, thermoplastics have 

15 been used in the past that have conductive particles formed therein to 

provide particle-susceptors for joining plastics, ceramics, and composites. 

These techniques all require a susceptor in the bondline which in some 

bonding processes is considered unacceptable because of possible 

corrosion or galvanic action. 

20 According to the present invention, a flexible ceramic susceptor 84 is 

used outside the bondline in order to join metals, plastics and ceramics 

using thermoset/thermoplastic adhesives. Prior uses of metallic susceptors 

in the bondline region creates a problem of unacceptable bond line thickness 

or other problems, such as a radar signature which is generated by virtue of 

25 havin~ the metallic susceptor becoming part of the composite structure. 

This problem is overcome by using a flexible susceptor 84 which can 

be as thin as .0005 inches. A suitable flexible ceramic material is known as 

GRAFOIL which is a product of Union Carbide. Basically, the susceptor 84 

is heated by induction heating and the heat is transferred through the 

30 adherend 82 to the adhesive 80, thus causing the adhesive to melt 
LAR 14679-1 -1 1- PATENT APPLICATION 

thermoplastics or curs thermosetting adhesives. Temperature controllers as 

described herein can be used as a means of controlling the bondline 

temperature. 

The vacuum techniques described with respect to the embodiment of 

5 Fig. 5 can be employed to cure the adhesive 80 using a GRAFOIL susceptor 

84. Other susceptors may be employed to heat the adhesive dO through the 

adherend 82, but preferably, the susceptor is extremely thin yet of 

sufficiently high reluctance so as to be heated by induction heating of the 

heating head 88 (which may correspond to the heating heads 14 or 52 of 

10 the embodiments described herein) to facilitate a rapid heat transfer to the 

adherend 82. Vacuum bagging may also be used in conjunction with the 

flexible heating head of Eg. 3 to ensure pressure at the bondline, to 

diminate fiauring during cure and to remove volatiles or trapped air during 

adhesive curing. This will result in higher bond strengths and significant cost 

15 savings. The same advantages can be achieved by using the embodiment 

of Fig. 6 instead of using vacuum bagging. 

Since the flexible heating head 88 and the GRAFOIL susceptor 84 

are b~th flexible, the method of the present invention can be used to repair 

or manufacture complex, cutved structures. The present technique 

23 produces temperatures up to 1300°F in less than 15 seconds, using a 2,000 

wa!! power unit to power a nine inch diameter induction ail. The 

terperatures whicb are capable of being achieved by induction heating are 

generally three tines higher than that which can be achieved by using heat 

blankets. Moreover, using a susceptor outside of the bondline produces a 

25 thinner bondline which can be heated more quickly and more evenly than 

previously possible. 

Preferably, an insulative layer 86 is provided between the heating 

head 88 and the GRAFOIL susceptor 84. Also, the bondline temperature, at 

the adhesive layer 80, is less than that of the susceptor 84 because of heat 

30 transfer properties of the adherend 82. Thus, the insulation layer 86 is used 
LAR 14679-1 PATENT APPLICATION 

to prevent the induction heating head 88 from heat sinking the GRAFOIL 

susceptor 84. 

Either of the adherends 78 and 82 can be metallic, plastic, or 

ceramic. The chosen materials should permit a relatively heat transfer 

5 through the adherend 82 to the underlying adhesive layer 80. It is also 

possible to apply multiple layers of adhesive layer 80 and adherend 82 so as 

to provide a multi-ply laminated structure. This is normally the case for 

patches which require a great deal of strength. One such known technique 

is to use "pyramid" patches whereby the lowermost plies are of greater 

10 diameter than the uppermost plies in a gradually decreasing manner from 

bottom to top so as to create a "pyramid" type structure. This is particularly 

suitable for aerodynamic structures since it creates a better contour for 

aerodynamic purposes. 

Numerous modifications and adaptations of the present invention will 

15 be apparent to those so skilled in the art and thus, it is intended by the 

following claims to cover all such modifications and adaptations which fall 

within the true spirit and scope of the invention. 
LAR 14679-1 

FLEXIBLE HEATING HEAD FOR INDUCTION HEATING 

APPARATUS AND METHOD 

Abstract 

5 

An induction heating head includes a length of wire having first and 

second opposite ends and being wound in a flat spiral shape to form an 

induction coil, a capacitor connected to the first and second enas of the wire, 

10 the induction coil and capacitor defining a tank circuit, and a flexible, 

elastomeric body molded to encase the induction coil. Wben a susceptor is 

placed in juxtaposition to the body, and the tank circuit is powered, the 

susceptor is inductively heated. 

4 

3 /4 

4-5- PATENT APPLICATION 
FIG. 2 12J 

30 

FIG. 3 
68 FIG. 5 

52 ,54 54c 54d 

\ .-56 56a 
FIG. 7 

FIG. 8