US2526697A - Dielectric heating method and apparatus - Google Patents

Description

Out.4 24, 1950 G. w. scoTr, JR 2,526,697 DIELECTRTC HEATING METHOD AND APPARATUS Filed June 2l, 1946 y 3 Sheets-Shoot 1 /42 INVENTOR Oct. 24, 1950 G. w. sco-r1', JR I 2,525,597

DIELECTRIC HEATING METHOD AND APPARATUS Filed June 2l, 1946 3 Sheets-Sheet 2 Oct. ,24, 1950 G. W. SCOTT, J R DIELECTRIC HEATING METHQD AND APPARATUS 5 Shoots-Shut 5 Filed June 2l, 1946 www@ FIG. 4

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Patented ct. 24, 1950 DIELECTRCr HEATING METHOD AND APPARATUS GeorgeW. Scott, Jr., Lancaster, Pa., assignor to Armstrong Cork Company, Lancaster, Pa., corporation of Pennsylvania v Application June 21, 1946, Serial No. 678,214

` 21 Claims. l f

.This invention relates to a ymethod and apparatus for dielectricy heating. In particular, it concerns the compensation of heat losses resulting from convection, conduction or radiation of heat, or a combination thereof, yfrom one or more surfaces of the mass being heated, so as to obtain a substantially uniform curing or heat activation throughout the mass, without overcuring or `otherwise deleteriously affecting the interior thereof.

In heating dielectric materials by a high-fre-r quency electric field, there is necessarily an ascending temperature gradient from the outside toward .the inside ofthe mass ofmaterial being heated-resulting from surface heat losses by conductionor radiation or both. Where the exposed surface per unit` volume of the material under treatment is large, the temperature existing adjacent the electrodesand the exterior surfaces of the mass may `differ widely from that within the interior. The heat losses in some` instances may be so great as to prevent the proper activa-v, tion of the material adjacent the exterior without over-activation of yinteriorportions of the mass. c

In curing a block of cork composition made up of cork granules coated with a heat-activatable binder by dielectric heating, for example, experience has shown that the outer portions of the block are aptto be left undercured as evidenced by a poor bond andcrumbling away of the surface portions. This necessitates trimming toremove such. undercured portions lprior to cutting the blocks into sheets orother articles. In addition, the density of the cured product is not uniform throughout the mass but is greater in theV portion where the temperature is a minimum. In someportions, an absence of Lbond may exist, making it impossible to determine the density. This is highly undesirable. The condition der scribed cannot be remedied by over-heating the interior of the mass. It is known that, witha constant `applied voltage, the rate of dielectric heating in general increases as the temperature of the body being heatedincreases. Thus, when heat losses occur at the surfaces of a body under treatment in excess of or unaccompanied by losses from the interior of the mass, the interiory rises to a higher temperature and will generate added heat4 at a higher rate. As heat losses' from the surface ycontinue while the temperature of the interior of the mass rises, the heating effect will avalanche, so to speak, in the interior andy that portion may rapidly heat up to a temperature exceeding the desired value causing overactivation or even charring of the interior, while the surface portions may not even attain activation temperature Compensation for heat loss, therefore, is a very important factor in the manufacture of manyr products by dielectric heating.

Another example of the objectionable results of excessive heat losses at the surface is observed in the 'case of dielectrically heating preforms of molding material. When a preform has been thus heated, the outer surface thereof is at a temperature substantially lower than that of the interior of the mass, When subjecting the preheated preform to molding pressure an unsatisfactory product results, particularly when a short curing time is employed, because of the lack of a uniform temperaturethroughout the preform. This is evidenced by a flaky exterior surface on the finished article or a preset interior which interferes with closing of the mold within the desired time interval. l i

, In some products, it is essential that the heating be substantially uniform throughout the mass in order to obtain a final product of uniform character, while in others it may be desirable to havek the outer surface `areas heated to a higher temperature than the interior of the mass `in order to maintain the interior at a desired minimum temperature for a protracted period of time after discontinuing application of the highfrequency field.

I have inventeda novel method of dielectric heating whereby the aforementioned difficulties are overcome `and a uniform temperature obtained throughout the mass or, at least, the heating of' all portions to the minimum activation temperature necessary for proper processing. kIn apreferred practice, I subject the mass of material to selective, differential or controlled dielectric heating, i. e., I effect dielectric heating in such a mannerA that a greater amount of heat is generated in the exterior portions of the mass than on the interior thereof. Specifically,

I subject the lateral peripheral surface portions4 which are subject to surf-ace cooling to greater electric stress (kilovolts per inch) than the interior. Thiscauses such portions to be more rapidly and intensively heated than the interior of the mass. A relatively small increase in the electric stress sufces to accomplish the desired result, because of the avalanche effect mentioned above.

The losses'from the exterior of a mass undergoing heating will vary depending upon the type of material under treatment, its geometry, the ambient temperature, and other variable factors.

3 i determine the heat losses which occur by measuring the temperature attained at the surface and also at spaced points within the interior. By such method, the heat losses may be Lietomined and the temperature distribution plotted. From these data I determine in which areas of the mass to be heated the generation of heat should be in creased in order to compensate for the heat losses. In some materials, particularly cori; composition, in which the cork granules are bonded with a heat-activatable binder such as a plasticized phenol-formaldehyde resin which is thermalliI reactive, it is desirablel to generate more heat in the surface areas where losses occur, than in the interior, in order to'raise the temperature in the surface areas somewhat above r the temperature of the interior of the mass. When this practice is followed, on discontinuing the heating, the high temperatures attained by the material in the surface portions where heat los-ses are greatest and where cooling is most rapidly effected, will serve to maintain the interior of the mass at the activation or curing tempera.-` ture for a substantially longer period of time thairwould be possible if the whole mass were at a uniform temperature at the end of the heatstage. the portions of the mass adjacent the surfaces prevents for a time the conduction ci heat from theirterior of the mass.' Since the surface portions are at higher temperatures than the interior of the mass, heat dissipation from the surrace may occur for a substantial period of time before the temperature thereof falls below that atthe interior.

A complete understanding of my invention may b had by referring to the accompanying drawings illustrating a preferred practice and embodiment of my invention. In'the drawings,

Figure l is a vertical section through a typical apparatus for the dielectric heating of materialsgutilizing flat, parallel electrodes disposed between the platens of a press;

l Figure 2 is a view similar to Figure l showing shaped electrodes effective for increasing heatinG effect in certain areas;

Figure 3 is a perspective view of one form of a mold suitable for making cork composition clocks or other molded products, illustrating one practical application of the present invention;

Figure i is a vertical sectional Viewta-ken on theplane of line IV-IV of Figure 3;

Figure 5 is a view similar to Figure l illustrating'a further modication of the invention; and,

`Figure 6 is a set of curves including the voltage giuient and temperature distribution plotted along one dimension of a specimen of dielectric material.

vReferring to Figure l, the upper and lower platens of apress of any suitable type are indicated at 2 and 3. A mass of dielectric material is disposed between the platens 2 and 3 for high-frequency dielectric heating. Electrodes 5 and 5 of copper sheet or other metal are disposed above and below the mass and are insulated from the press platens 2 and 3 by layers of insulation i and 8 which prevent grounding of the electrodes. The electrodes are connected as the plates of a condenser, tuned with an inductance, and coupled to an oscillator circuit in the known manner.

In such an arrangement for dielectric heating it is obvious that considerable loss of heat will In other words, the heat stored in.

4 occur by atmospheric cooling, i. e., radiation and convection of heat from the mass to the air, at the peripheral surface 9 of the mass 4. The surfaces in contact with the electrodes 5 and 6 will also lose heat by conduction. Temperature measurements taken at two points at the surface of the mass 4' and within the interior thereof show that the distribution from edge to edge of the mass 4 is about as indicated by the dotted line T of Figure 1. It will be noted that the temperature is uniform across the greater portion of the width of the mass but falls off rapidly adjacent the lateral surfaces.

In a preferred practice of my invention, I compensate for heat losses adjacent the periphery 0f the mass by reducing the spacing between the electrodes in the region where the heat losses heating eifect in the peripheral region greater than that produced between' the central portionsv of the electrodes where the spacing therebetween is greater'as indicated at d'.1 Thus selectiveor differential heating results from the greater elec-Y tric stress applied to the lateral or lperipheral surface portions ofthe mass and jacent thereto. l Y A v In orderto provide a plane-surface to engage the mass 9, the central portions of the electrodesv llland'll may be filled with electrically and thermally insulating material asv indicated at I2 and i3.

aord solidffbacking throughout the area of the electrodes.

In the embodiment illustrated in Figure 2, the temperature distribution T plotted from data obtained as above indicated, constitutes a sub'- stantially straight line showing that, by shaping the electrodes, heat losses at'the peripheral surfaces have been completely compensated for.

The diagrammatic showings of Figures l and 2 sufce to illustrate the theory of operation underlying the present invention. Figures 3 and 4 show the details of an actual embodiment of the invention which has been employed as a mold for the-'manufacture ofvblocks of cork compositions. -The mold is in the form of a Wooden box open at the topand bottom, having four sides I6, Il', I8, and I9 which are held in assembled relation by binding frames 2D and 2| composed of lengths of angle iron welded with mitered corners into a rigid rectangle. The wooden sides ofthe mold are secured to the binding frames by fasteners 22.

As shown-in Figure 4, the mold is adapted toy regions ad-v Similar material may be provided be-i tween the edges of the electrodes and the insula-r tion layers l and, as shown ati-I4 and I5 to similar tothe electrode L24, forms the top of the m01d, 'i w r/In thfoperation of molding a cork composition blockin thefmoldshown in Figures 3 'and 4, a filling" boot (not shown) is mountedontop of th'emold, proper registry being obtained by pins on `the 'bottom of the boot which enter holes 28 in the upper angle frame 2l. After the corkcomposition which maybe composed of cork granules and a plasticized phenol-formaldehyde resin binder, as previously mentioned, has been charged into the boot, the electrode 21 is placed on top of the mass and pressure is applied to compress thecharge between the electrodes 24 and `21 and reduce the volumev until the cork composition has the desired density.r Thereupon, upper locking pins 29 and 30 similar to pins-25 and 26, are

passed through holes in the sides of the mold near the top. The externally applied pressure is thenreleased leaving the mass held under pressure between the electrodes by the upper and lower through pins, with the lateral peripheral surfaces of the mass in engagement with the coniiningwalls I6, I1, I8, and I9 of the mold. A massof cork composition has substantially uniform dielectricproperties and will normally heat at a uniform average rate so long as theltemperature of the mass increases at a uniform rate and the avalanche effect referred t0 is not a factor.

' Electrodes 24fand 21 are at and parallel and are connected toa source of high-frequency alternating electric `voltage by leads 3I and 32. As seen in Figures 3 and 4, the electrodes are substantially coextensive/with the area of the mold opening. Upper and lower peripheral or ring conductors or electrodes 33 and 34 extend around the exterior of the mold, being mounted thereon by clips 35 and 31, respectively, in positions below the upper electrode and above the lower electrode. These conductors are connected to their respective electrodes and constitute extensions thereof. The connections include straps 36 and 38 secured to the electrodes and the binding frames, and theclips 35 and 31 which are also secured to the frames. k'The application of a highfrequency alternating electric voltage to the electrodes and conductors establishes a high-frequency alternating electric eld which embraces both themass and the mold walls in contact with the mass. f Y

The positions of the conductors 33 and 34 with respect tothe electrodes 21 and 24 are determined yby-temperature measurements -made in the median plane through the block 23, between and parallel to the electrodes as shown. The conductors 33 and 34 are so positioned-that the distance d between them is less than the distance d between the electrodes 24 and 21. A"The reduced-spacing between the conductors 33 and 34 as compared with that between electrodes 24 and 21 serves to cause dielectric heating of the mold walls I6, I1, I8 and I9 themselves and the portions of the shaped mass adjacent thereto and in contact therewith at a rate greater than would be obtained without the conductors.y The increased heating of the mold walls establishes what might be considered a heat barrier or dam preventing loss of heatfrom the lateral surface portions of the mass. The confining mold walls which contact the mass are thus heatedfto a temperature which is at leastashigh as that of the mass in contact therewith. In fact, there may even ybe a flow of heat from the mold walls to the lateral surface portions of the mass, counteracting any tendency of the mass to lose heat to the mold walls. The conductors 33 and 34may, therefore,

be said to make theeffective shape of the electrodes 24 and 21 similar to that of those of Figure l2. The effective shape is indicated by the chain lines X in Figure 4. From this it will be clear that the mold body will be selectively subjected to a high-frequency alternating electric stress greater than that applied to the mass withiny the mold, for the electrodes 24 and 21 and the conductors 33 and 34 will create a nonuniform high-frequency alternating electric eld. The temperature of the mold walls in engagement with the mass, rising at a rate greater than the rate of rise of the mass, will avoid substantial loss of heat from the mass to the mold walls and will so maintain a substantially uniformly rising temperature throughout the Whole of the mass which will be continued until the mass has attained a desired temperature for activating the binder.

The location of the conductors 33 and 34 may be varied to suit the particular heating problems involved. If compensation for heat losses only is desired, the conductors may be located accordingly. If it is desired to effect greater heating in the surface areas than within the interior of the block, the distance d" should be reduced. The exact spacing of the conductorsmay be easily determined by experiment.

The ' conductors 33 and 34 have anotherimportant'advantage in that they prevent localized overheating at the corners of the block 23. It is well known that, when a block of material is heated dielectrically between at electrodes vwith sharp corners, there is a concentration of the eld immediately adjacent the corners which results in localized overheating and even charring of the material under treatment. The conductors 33 and 34, as shown, have curves of appreciable radius at their corners which prevent the objectionable concentration of the eld. This is particularly important in the treatment of cork compositionblocks as described above, where the material is charged into a mold and held under pressure b5 iiat plates which serve as electrodes. It is not desirable, in such a structure, to round the corners of the electrode plates to minimize or eliminate this concentration of eld, but this effect is obtained byshaping the conductors 33 and 34 to a curve of suitable radius at their corners.

Although the mold of Figures 3 and 4 may be made of various dielectric materials, its Walls are preferably composed of maple or other hard wood completely free from moisture and impregnated with a Waterproof material such as ceresin wax or other wax having similar properties. Such a material is not subject to arcing, and has a long useful life. The mold is more fully disclosed and claimed in my copending application, Serial No. 678,215, filed June 21, 1946. It is t0 be understood, however, that the invention disclosed herein is not limited to any particular mold structure but may be carried out with widely varying types of apparatus depending upon the type of product to be made.

A method cf offsetting or preventing heat losses due to thermal conduction, convection, or radiation from the surface of a body of dielectric material to a mold wall, or the like, in which the mold wall has a higher loss factor than the body of dielectric material under treatment, is disclosed and claimed in the copending application of George E. Gard, Serial No. 678,217, led June 2l, 1946, and entitled Method of Compensating for and Preventing Heat Losses from Material During Dielectric Heating Thereof.

. Figure illustrates a block 40 being subjected to dielectric heating while conned between press platens 4I and i2, and shows how the effective distance between the electrodes in such case may be reduced adjacent the periphery by the use of a conductor 39. This conductor, like the conductors 33 and 3L! is located adjacent the periphery of the block 40. The conductor 39 is connected to the upper electrode 43 by connectors 44. The distance between the conductor 39 and the lower electrode 45 is less than the distance between the electrodes i3 and t5. The spacing of the conductor 39 with respect to the lower electrode 45 may be determined by the amount of heat desired at the peripheral surface of the block` The conductor may be embedded in a layer of insulating material 46 which provides a flat surface for Contact with the block.

Referring now to Figure 6, the solid curve V represents the voltage gradient from edge to edge of a '-inch specimen of dielectric material, heated in accordance with the present invention. It will be noted that the voltage gradient from the outer edges falls sharply from a maximum point on each surface to a low value which is substantially constant throughout the major portion of the width of the specimen. The maximum values, of course, correspond to the minimum spacings between electrodes. In the specimen under consideration, the curve falls from approximately 5.6 kilovolts per inch at the edges to about 2.5 kilovolts per inch in the interior and remains substantially constant at the latter value throughout the greater portion of the width of the specimen.

Curve T1 in Figure 6 shows the variation of temperature across the width of the specimen and indicates that the temperature follows the voltage gradient very closely. The curve clearly shows the effect of peripheral conductors in effecting greater heating of the block adjacent the exterior than in the interior, in order t0 compensate for losses. shows temperatures adjacent the edges of the block substantially higher than that existing in the interior of the block. As pointed out above, this may be desirable in certain instances. In

other cases, it may sufce to heat the exterior to a temperature only slightly above that of the interior. The spacing of the peripheral conductors determines the extent to which the final temperature of the exterior exceeds that of the interior. By properly determining the spacing of the conductors, it is possible to maintain the body throughout its entire volume at a minimum temperature, at least, despite heat losses which occur at the surfaces.

For the purpose of comparison, curve T2 of Figure 6 shows temperature conditions in a body of the same material dielectrically heated between plane electrodes without the use of my invention. It will be noted that the temperature drops sharply adjacent the edges. This is the result of surface cooling in the absence of compensation for heat looses. If the minimum temperature to be attained in the block under treatment is in the neighborhood of 220 F., curve T1 shows that, by the aid of my invention, all portions of the mass attain such temperature. Curve T2 shows that, with conventional eletrodes, the edge portions fail by a substantial margin to reach such minimum temperature. It is for this reason that, in curing a cork corn-r It will be noted that Figure 6 position containing a synthetic resin binder, as explained above, the edge portions of the block for about one inch from the outside towardthe center areA undercured in conventional processing by dielectric heating. As stated, this is evi'- denced by improper bonding and consequent crumbling of the mass upon removal from the mold.

In actual practice, I have used as a source of high-frequency power, an oscillator operating at a frequency of approximately 10 megacycles, with approximately 15 kilowatts output. The appropriate frequency and power required with vary, however, depending on the particular material being processed and the size and shape of the mass. In the claims, the term high-frequency is intended to mean a frequency of 1 megacycle or above and preferably above 5 megacycles, 13.66 megacyoles being a suitable commercial frequency for dielectrically heating objects of a size of the order of 26" x 50" x 6".

While I have illustrated and described a preferred embodiment and practice of my invention it will be understood that the same it not limited thereto but may be otherwise embodied and practiced within the scope of the following claims.

I claim:

l. In a method of dielectrically heating material having lateral surfaces subject to surface cooling by conduction of heat from the material to a mold body, by radiation and convection of heat from the material to the air, or the like, the steps including: subjecting the material to high-frequency alternating electric stress throughout the whole of the volume of the material to elevate the temperature'of thermaterial and simultaneously selectively subjecting all of the lateral surface portions of the material subject to surface cooling and regions adjacent thereto to a high-frequency electric stress greater than that applied to the remainder of the material to heat all of the lateral surfaces of the material subject to surface cooling to a temperature not substantially lower than the temperature of the remainder of the material and compensate for surface cooling thereof.

2. In a method of dielectrically heating and curing a formed mass of dielectric material intermixed with a thermally reactive binder and having lateral surfaces subject to surface cooling by conduction of heat from the material to a mold body, by radiation and convection of heat from the material to the air, or the like, the steps including: placing the mass between substantially plate-type electrodes in substantially parallel lspaced relationship, impressing a high-frequency alternating electric Voltage across the electrodes to subject the material to high-frequency alternating electric stress throughout the whole of the volume of the material to elevate the temperautre of the thermally reactive binder to activation temperature, and simultaneously selectively subjecting the lateral surface portions of the material normally subject to surface cooling to a high-frequency electric stress greater than that applied to the remainder'of the material.

3. In a method of dielectrically heating material having lateral surfaces subject to vsurface cooling by conduction of heat from the material to a mold body, by radiation and convection of heat from the material to the air, or the like, wherein the mass to be treated is positioned between spaced electrodes with portions of the electrodes adjacent the lateral surfaces and conmaterial' to high-frequency alternating electric stress throughout the` whole of the volume of the mass 4to elevate the temperature of the material, and (b) simultaneously selectivelysubjecting all of the lateral surface portions of the material subject to surface cooling and regions adjacent thereto to a high-frequency electric stress greater than that applied to the remainder of the material, said steps (a) and (b) yboth being performed by impressing a high-frequency alterknating electric voltage upon said spaced electrodes between which the material is disposed, whereby all of the lateral surfaces of the material subject to surface cooling are heated to a temperature not substantially lower than the temperature of the remainder of the material.

y 4. In a method of dielectricallyheating and curing a mass `of dielectric material which has substantially uniform average dielectric properties, said mass having lateral peripheral surfaces subject to surface cooling by conduction ofheat Vfrom the mass toa moldbody, by radiation and convectionpfl gheat from the material .to1the iair, or thea-like; the steps comprising: positioning a mass :of dielectric material which `has substantially uniform average dielectric `properties between 4spaced electrodes having closer spaced portions adjacent the periphery of the mass, impressing a uniform high-frequency alternating electric voltage on the electrodes to create an electric stress in the whole of said mass, witha lesser electric stress in the inner portions ingof said mass than' the electric stress created in the periphery and adjacent parts of said mass,

vheating the innergportions of the mass to curing .temperature by dielectric loss resulting from such lesser electric stress, and more highly heating the periphery and adjacent parts of the` `mass by dielectric loss resulting from the greater electric rstress created in the periphery and adjacent parts of the mass, whereby the periphery rand adjacent partsv of the mass are more highly heated notwithstanding that the mass has substantially uniform average dielectricproperties. 5. In a method of dielectrically vheating and curing a formed mass of dielectric material intermixed with a thermally reactive binder and having lateral surfaces subject to surface cooling by conduction of heat from the material toa mold body, by-radiation and convection of heat from the material to the air, or the like, wherein the massgtov be treated is disposed between spaced electrodes disposed in substantially parallel relationship and wherein conductors are positioned adjacent lateral surfaces of the mass normally subject to heat loss during dielectric heating, the conductors being disposed in substantially parallel spaced relationship and closer to each other than the electrodes and being connected electrically to the electrodes, the steps including: (a) subjecting the material to high-frequency alternating electric stressthroughout the whole of the lvolume of the mass to elevate the temperature of the mass, and (b) simultaneously selectively subjecting all of the lateral surface portions of the material subject to surface cooling and regions adjacent thereto to a high-frequency electric stress greater than that applied to the remainder of the material, said steps (a) and (b) both being performed by impressing the same high-frequency alternating electric voltage upon said spaced electrodes and said spaced conductors, whereby all of the lateral surfaces of the material subject to surface cooling are heated to a temperature not substantially lower than the temperature of the remainder of the material.

6. In a method of dielectrically heating and curing a shaped mass of dielectric material within a confining wall in which the lateral surfaces of the mass are in engagement with the confining wall and are normally subject to surface cooling by conduction of heat from the mass to the coniining wall, the steps comprising: subjecting the shaped mass to high-frequency alternating electric stress throughout the whole of the volume of the mass to heat it bydielectric loss to an activating temperature and simultaneously therewith selectively subjecting the confining wall and portions of the shaped mass adjacent thereto and in contact therewith to a greater high frequency alternating electric stress than applied to the remalnder of the mass within the confining wall to heat such conning wall in contact with the mass to a temperature at least as high as that of the mass in contact therewith to maintain the portions of the mass adjacent to the confining wall at substantially the same activating temperature asv the remainder of the mass.

'7.- In a method of dielectrically heating a mass of cork granules coated with a thermally activatable binder in a mold of dielectric material in which the lateral surfaces of the mass are in engagement with the mold walls and are normally subject to surface cooling by conduction cf heat from the mass to the mold walls and in which the mass has substantially uniform average dielectric properties and will normally heat at a uniform average rate upon the application of a uniform high-frequency alternating stress thereto only so long as the temperature of the mass increases at a subsstantially uniform rate throughout, the steps comprising: subjecting said mass of binder-coated cork granules to high-frequency alternating electric stress throughout the whole of the volume of the mass to cause the temperature of the mass to rise, simultaneously subjecting said mold to a greater high-frequency alternating electric stress to cause the temperature of the mold walls in engagement with the mass to rise at a rate not substantially less than the rate of rise of the mass to avoid substantial loss of heat from the mass to the mold walls and thus maintain a substantially uniformly rising temperature throughout the whole of the mass, and continuing the application of such high-frequency. alternating electric stress to the mass and to the mold to raise the temperature of the mass substantially uniformly throughout until .the same has attained a desired temperature for activating said binder. y

8. In a method of dielectrically heating a mass #of dielectric material in a mold in which the lateral surfaces of the mass are in engagement with the mold and are normally subject to surface cooling by conduction of heat from the mass to the mold body, the step comprising: positioning the mass within a mold formed of dielectric material, subjecting the mass to high-frequency alternating electric stress throughout the whole of the volume of the mass to elevate the temperature of the mass, and simultaneously therewith selectively subjecting the mold body in contact with the mass to a high-frequency alternating electric stress greater than that applied to the mass within the mold to heat such mold body in contact with the mass to a temperature at least as high as that of the mass within the mold and in contact therewith, whereby the lateral surfaces of the mass normally subject to surface cooling are in contact during heating of the mass with mold surfaces which are at a temperature at least as high as the temperature of the mass and loss of from the mass is avoided.

9. In a method of processing and curing a sube stantially uniform mixture of a dielectric material and a thermally reactive binder, said mixture having substantially uniform average dielectric properties, the steps comprising: charging said mixture into a mold of dielectric material, compressing the mixture to a mass of the desired configuration, subjecting the compressed mass to a high-frequency alternating electric stress to elevate the temperature of the thermally reactive binder to curing temperature, applying a greater high-frequency alternating electric stress to the lateral surfaces of the mixture, and maintaining said differential high-frequency al ternating electric stresses in the mass until curing of said binder has been effected.

l0. In a method of dielectrically heating a mass of material having lateral surfaces subject to surface cooling by conduction of heat from the mass to a mold body, by radiation and convection of heat from the mass to the air, or the like, the steps including: subjecting the material to high-frequency alternating electric stress throughout the whole of the volume of the material and simultaneously selectively subjecting all of the lateral surface portions of the material subject to surface cooling and regions adjacent thereto to a high-frequency electric stress greater than that applied to the remainder of the material by establishing a voltage gradient within the material which increases from the interior of the mass toward all of the lateral surface portions of the mass subject to surface cooling and regions f adjacent thereto to heat all of the lateral surfaces of the material subject to surface cooling to a temperature not substantially lower than the temperature of the remainder of the material and to compensate for surface cooling thereof.

1l. In a method of dielectrically curing a mass of cork granules and a binder which mass is subject to crumbling if the binder is not elevated to a minimum activation temperature and subject to overcuring and charring if the binder is elevated above a maximum temperature, the steps comprising: depositing a loose mass of cork granules and binder in a mold formed of dielectric material with electrodes disposed on opposite sides of the mass within the mold, the effective field of said electrodes embracing both the mass and the mold walls in contact with the mass with a more intensive effective field within said mold walls in contact with the mass than in the mass, establishing a voltage gradient within the mass and the mold walls which increases toward the outer surfaces of the mold walls by applying a high-frequency alternating electric voltage across said spaced electrodes, and maintaining said nonuniform voltage gradient within the mass and mold walls until the mass has been heated to a desired temperature above said minimum and below said maximum temperatures to cure the mass substantially uniformly throughout its extent.

12. In a method of dielectrically heating a mass of material in a mold in which the mass is in Contact with mold surfaces at the lateral surfaces of the mass and in contact with plate electrodes at the other surfaces of the mass to heat the whole of the mass to a desired temperature without overheating or underheating at the lateral surfaces in contact with the mold Walls, the stepsV comprising: positioning the mass to be heated in a mold of dielectric material and between a pair of spaced electrodes which extend beyond the walls ofthe mold, said electrodes being spaced more closely together in the portions beyond the mold than in the other portions, and applying a high-frequency alternating electric voltage across said spaced electrodes, thereby establishing a voltage gradient within the mass and the mold walls which increases' toward the outer surfaces ofthe mold walls, whereby the mass is heated uniformly throughout without substantial overheating or underheating at the lateral surfaces in contact with the mold.

13. In a method of dielectrically heating a mass of dielectric material in a mold between a pair of. spacedparallel plate electrodes which are substantially coextensive with the area of the mold opening, the steps comprising: positioning a mass of dielectricmaterial to be heated within a mold formed of dielectric material with the periphery of the mass in engagement with the mold walls, confining said mass between a pair of plate electrodes which` are disposed substantially paral- .lelly to one another and are substantially coextensive in area with the area of the surfaces of the mass which are not in engagement with the mold walls thereby surrounding substantially the entire mass to be heated, subjecting the mass to high-frequency alternating electric stress throughout the Whole of the volume of the mass by impressing a high-frequency alternating electric voltage upon said plate electrodes to thereby elevate the temperature of the mass, and simultaneously subjecting the mold to high-frequency alternating electric stress greater than that applied to the mass within the mold by impressing the same high-frequency alternating electric'voltage upon a pair of parallel ring electrodes electrically connected one with each of the plate electrodes and spaced a distance from each other less than the distance from one plate electrode to the other to thereby elevate the temperaure of the mold body adjacent the mass to a temperature at least as highY as that of the mass within the mold and in contact therewith, whereby the lateral surfaces of the mass normally subjected to surface cooling are in contact during heating of the mass with mold surfaces which are at a temperature at least as high as the temperature of the mass and loss of heat from the mass is avoided.

14. In a method of dielectrically heating a mass of material of substantially uniform cross-sectional shape, such as a rectangular parallelepiped, said mass including a heat-activatable binder which must attain a minimum temperature for proper activation and said mass having peripheral surface portions, such as the lateral surfaces of said rectangular parallelepiped, subject to surface cooling by conduction of heat from the mass to a mold body, by radiation and convection of heat from the mass to the air, or the like, the steps including: subjecting the whole of said mass to high-frequency alternating electric stress with a greater high-frequency stress simultaneously and selectively applied in regions adjacent to all of said peripheral surface portions subject to surface cooling to cause said mass to heat at said peripheral surface portions at a rate not substantially less than the rate of heating of the interior y0f the mass, and continuing the application of 13 said high-frequencyalternating electric stress until said minimum activation temperature has been reached throughout 4the whole of said mass including said peripheral surface portions.

Aa. mold bodyby radiation and convection of heat from. the mass to the air, or the like, the steps including: subjecting the whole of said mass to a nonuniform high-frequency alternating electric stress with a greater high-frequency stress being selectively applied to all of said peripheral surface portions subject to surface cooling to causesaid mass to heat at said peripheral surface portions at a rate not substantially less than the rate of heating of the interior of the mass,

and continuing the application of said nonuniform high-frequency alternating electric stress until said minimum activation temperature has been reached throughout the whole of said mass including said peripheral surface portions.

16. In-a method of dielectrically heating a mass of material of substantially uniform cross-sectional shape, such as a rectangular parallelepiped, said ,mass including a heat-activatable binder .which must attain a minimum temperature for proper activation and said mass having peripheral surface portions, such as the lateral surfaces of said rectangular parallelepiped, subject to surface cooling by conduction of heat from the mass to va mold body, by radiation and convection of heat from the mass to the air, or the like, the steps including: placing said mass between substantially plate-type electrodes in substantially par allel spaced relationship, with portions of the electrodes adjacent the peripheral surface portions of the mass disposed in closer relationship to one another than the other portions of said electrodes, `subjecting the Whole of said mass to a nonuniform high-frequency alternating electric stress with a greater highfrequency stress being' selectively applied in regions adjacent to all -of said peripheral. surface portions subject to surface cooling by impressing a high-frequency alternating electric voltage upon said spaced electrodes between which the mass is disposed to cause said mass to heat at said peripheral surface portions at a rate not substantially less than the rate of heating of the interior of the mass, and continuing the application of said nonuniform high-frequency alternating electric stress until said minimum activation temperature has been reached throughout the whole of said mass including said peripheral surface portions.

17. In a method of dielectrically heating a shaped mass of dielectric material of substantially uniform cross-sectional shape, such as a rectangular parallelepiped within a confining mold of dielectric material, said mass including a heatactivatable binder which must attain a minimum temperature for proper activation and said mass having peripheral surface portions, such as the lateral surfaces of said rectangular parallelepiped, in engagement with said confining mold and normally subject to surface cooling by conduction of heat from the mass to the mold walls, the steps including: subjecting the said mass and mold to high-frequency alternating electric stress with a greater high-frequency stress appliedk to the mold walls than to the mass to cause the temperature of the mold walls in engagement with the mass to rise at a rate not'substantially less than the rate of rise of the mass to cause said mass to heat at said peripheral surface p0rtions at a rate not substantially less than the rate of heating of the interior of the mass, and continuing the application of said high-frequency alternatingelectric stress to the mass and mold walls until said minimum activation temperature has been reached throughout the whole of said mass including said peripheral surface portions. l

18. In a method of curing a mass of cork f granules and a binder, which mass is subject to crumbling if the binder is not elevated topa minimum activation temperature and subject to overcuring and charring if the binder is elevated above a maximum temperature, said mass being of substantially uniform cross-sectional shape, such as a rectangular parallelepiped, the steps comprising: depositing a loose mass of cork granules and a binder in a mold formed of dielectric material with electrodes disposed on opposite sides of the mass within the mold, said mass having peripheral surface portions, such as the lateral surfaces of said rectangular parallelepiped, in engagement with said mold and normally subject to surface cooling by conduction of heat from the mass to the mold walls, sub-- jecting the mass and the mold to high-frequency alternating electric stress with a greater highfrequency stress being applied to the mold walls than to the mass to cause the rtemperature of the mold walls in engagement with the mass to rise at a rate not substantially less than the rate of rise of the mass to cause said mass to heat at said peripheral surface portions at a rate not substantially less than the rate of heat`- ing of the interior of the mass, and continuing the application of said high-frequency alternat ing electric stress to the mass and mold walls until the mass has been heated to a desired Ytemperature above said minimum and below said maximum temperatures to cure the mass substantially uniformly throughout its extent.

19. In an apparatus for dielectrically heating a mass of material of substantially uniform crosssectional shape, such as la rectangular parallelepiped, said mass having peripheral surface portions, such as the lateral surfaces of said rectangular parallelepiped, subject to surface cooling by conduction of heat from said :mass to a mold body, the combination of a mold body of dielectric material, a pair of rigid plate-type electrodes disposed within said mold in substantially parallel spaced relationship to confine the mass of material to be heated and form a condenser and between which a high-frequency alternating electric eld may ybe established from a high-frequency source to which the electrodes are connected, and a conductor of high-frequency alternating electric current disposed outside of said mold and extending around substantially the entire outer periphery thereof and electrically connected to one of said electrodes, the minimum distance from said conductor to the other of said electrodes being less than the minimum dis'- tance between said electrodes. o

20. In an apparatus for dielectrically heating la mass of material of substantially uniform cross-sectional shape, such as a rectangular parallelepiped, said mass having peripheral surface portions, such as the lateral surfaces of said rectangular paralleleplped, subject to surface cooling by'conduction of heat from;said mass to amold body, the combination o a mold body of dielectric material, a pair of rigid plate-type electrodes disposed within said mold in substantially parallel spaced relationship to conne the material to be heated and form a condenser and between which a high-frequency alternating electric field may be established from a highfrequency source to which the electrodes are connected, a conductor of high-frequency alternating electric current disposed around substantially the entire outer periphery of said mold adjacent to one of said electrodes and electrically connected thereto, and a second conductor of high-frequency alternating electric current disposed around substantially the entire outer periphery of said mold adjacent to the other of said electrodes and electrically connected thereto, the minimum distance between said conductors being less than the minimum distance between said electrodes and the effective eld of said electrodes and conductors embracing both the mass and the mold walls in contact with the mass with a more intensive eiiective field within said mold walls in contact with said mass than in said mass.

21. In an apparatus for dielectrically heating a mass of material having peripheral surface portions subject to surface cooling by conduction of heat from said mass to a mold body, the combination of a mold body of dielectric material, an upper, rigid, nat-faced, plate-type electrode disposed within said mold, a lower, rigid, atiaced, plate-type electrode disposed within said mold in substantially parallel spaced relationship with respect to said upper electrode, said electrodes being disposed to conne a mass of material to be dielectrically heated therebetween and form a condenser and between which a high-frequency alternating electric field may be established from a high-frequency source to which the electrodes are connected, a high-frequency alternating electric current conductor disposed around substantially the entire outer periphery of said mold and having a portion at least thereof disposed adjacent to and below said upper electrode close to the outer surface of said mold, and a second high-frequency alternating electric current conductor disposed around substantially the entire outer periphery of said mold and having a =portion at least thereof disposed adjacent tc and above said lower electrode close to the cuter surface of said mold, said conductors being'electrically connected to the respective adjacent electrodes.

GEORGE W. SCOTT, JR.`

REFERENCES CITED The following references are of record in the le or this patent:

UNITED STATES PATENTS Number Name Date 2,087,480 Pitman July 20, 1937 2,147,689 Chaiee Feb. 21, 1939 2,179,261 Keller Nov. 7, 1939 2,267,001 Toulmin Dec. 23, 1941 2,282,317 Bennett May 12, 1942 2,293,851 Rogers Aug. 25, 1942 2,303,983 Brown Dec. 1, 1942 2,304,958 Rouy Dec. 15, 1942 2,307,344 Zottu Jan. 5, 1943 2,308,995 Miess Jan. 19, 1943 2,341,617 Hull Feb. 15, 1944 2,354,714 Strickland Aug. 1, 1944 2,370,624 Gillespie Mar. 6, 1945 2,372,929 Blessing Apr. 3, 1945 2,388,824 Brown Nov. 13, 1945 2,404,191 Quayle et al July 16, 1946 2,415,025 Grell et al. Jan. 28, 1947 2,421,097 Lakso May 27, 1947 2,421,099 Vogt May 27, 1947 2,423,902 Peterson July 15, 1947 2,433,067 Russell Dec. 23, 1947 2,433,842 Griffin Jan. 6, 1948 2,441,548 Sperry May 11, 1948 2,456,611 Baker Dec, 21, 1948 2,459,225 Hickok Jan. 18, 1949 2,459,622 Cohoe et al Jan. 18, 1949 2,459,623 Cohoe et al Jan. 18, 1949 2,477,214 Story July 26, 1949 2,483,569 Baker Oct. 4, 1949 FOREIGN PATENTS Number Country Date 118,453 Australia May 11, 1944 OTHER REFERENCES Radio Frequency Technology in Wood Application, Russell and Mann, Dec. 1943.

Electronics, March 1944, pages 220 and 224.

Modern Plastics, June 1944, pages 110 and 111.

Dakin et al.: Industrial and Engineering Chemistry, March 1945, pages 273-275.

Certicate of Correction Patent No. 2,526,697 October 24, 1950 GEORGE W. SCOTT, JR.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows:

Column 3, line 8, for in creased read increased; column 4, line 17, for the Word region read regions; column 7, line 67, for looses read loses; columny 8, line 13, for with read will; line 38, for laterial read lateral; column 9, line 37, for ngof read of; column 10, line 40, for subsstantially read substantially; line 65, for step read steps; and that the said Letters Patent should be read as corrected above, so that the same may conform to the record of the case in the Patent Oice.

Signed and sealed this 19th day of December, A. D. 1950.

[SEAL] THOMAS F. MURPHY,

Assistant Uommz'ssz'oner of Patents.

Certificate of Correction Patent No. 2,526,697 October 24, 1950 GEORGE W. SCOTT, JR.

It is hereby certified that error appears in the printed specication of the above numbered patent requiring correction as follows:

Column 3, line 8, for in creased read lncreaseal; column 4, line 17, for the Word region read regions; column 7,1ne 67, for looses read loses; column 8, line 13, for with read will; line 38, for laterial read lateral; column 9, line 37, for ngof read of; column 10, line 40, for subsstantally read substantially; line 65, for step read steps; i Y and that the said Letters Patent should be read as corrected above, so that the same may conform to the record of the case in the Patent Oce.

Signed and sealed this 19th day of December, A. D. 1950.

[sur] THOMAS F. MURPHY,

Assistant Commissioner of Patents.

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