Search Images Maps Play YouTube News Gmail Drive More »
Sign in
Screen reader users: click this link for accessible mode. Accessible mode has the same essential features but works better with your reader.


  1. Advanced Patent Search
Publication numberUS2179261 A
Publication typeGrant
Publication dateNov 7, 1939
Filing dateAug 11, 1937
Priority dateAug 11, 1937
Publication numberUS 2179261 A, US 2179261A, US-A-2179261, US2179261 A, US2179261A
InventorsArthur C Keller
Original AssigneeBell Telephone Labor Inc
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Method and apparatus for heating dielectric materials
US 2179261 A
Abstract  available in
Previous page
Next page
Claims  available in
Description  (OCR text may contain errors)

Nov. 7, 1939. A. c. KELLER 2.179.261

METHOD AND APPARATUS FOR HEATING DIELECTRIC MATERiALS Filed Aug. 11, 1957 INVENTOR A. C. KELLER A TTORNE Y Patented Nov. 7, 1939 UNITED STATES PATENT OFFICE METHOD AND APPARATUS FOR HEATING DIELECTRIC MATERIALS Arthur 0. Keller, Mount Vernon, N. Y., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application August 11, 1937, Serial No. 158,551

2 Claims.

heat such materials quickly and uniformly throughout their mass.

In general, non-conducting molding materials are poor conductors of heat and it is, therefore, difficult to raise the inner portions of such materials quickly to molding temperature with ordinary heating methods without subjecting the outer layers of the material to excessive temperatures. With molding materials which are critical as to thermal history it is important not only to avoid excessive temperatures in any part of the material but also to reduce to a minimum the time interval during which the material is in a plastic condition.

According to this invention, such materials are heated uniformly and simultaneously throughout their mass by means of the dielectric loss produced in the material itself when it is subjected to a relatively high voltage, high frequency field, the heating circuit preferably being tuned to the frequency of the high frequency field. In one embodiment of the invention the material and the electrodes constitute a tuning condenser for a power oscillator supplying the heating current. In such a system any changes in the capacity of the heating circuit due to variations of the dielectric constant of the molding material as the heating progresses, produce corresponding changes in the frequency of the oscillator, thereby keeping the dielectric loss in the material at a maximum throughout the heating operation. The frequency of the oscillator at any instant is, therefore, a measure of the temperature of the material at that time and when the pieces of material to be heated are preformed to standard dimensions, the operator can determine when the molding temperature has been reached by observing a wavemeter which is loosely coupled to the oscillating circuit in the usual manner.

Alternatively, the heating current may be derived from a fixed frequency source and the frequency used may be so chosen with respect to the initial capacity of the heating circuit that the capacity changes effect the required degree of automatic regulation in the time rate of heat generation in the material. I

These and other features of the invention will be more clearly understood from the following detail description and the accompanying drawing in which:

Fig. 1 shows a system in which the frequency of the field is controlled by the capacity of the heating circuit;

Fig. 2 shows a system in which the frequency of the field is independent of the capacity of the load circuit; and

Figs. 3 to 5 show various electrode arrangements.

It can be shown that the power loss in molding materials which are relatively good dielectrics and in which the direct current conductance is negligible is approximately P=21rfE C where P is the power loss in watts,

f is the frequency of the power source in cycles per second,

E is voltage applied to the electrodes between which the material is placed,

C is the capacity in farads of the condenser. 0 formed by the material and the electrodes, and

is the power factor of the material being used.

Since the dielectric loss is directly proportional to the frequency of the supply, this frequency 25 should be very high (of the order of 1 to 50 megacycles) to avoid the necessity of using an unduly high voltage to reduce the heating time tothe value required.

In molding phonograph record pressings, for examples, the powdered molding material is sometimes first made into a preform or disc about 8 inches in diameter and about inch thick. Such a disc of one suitable material requires about 300 watthours to bring it to molding temperature but due to the poor heat conductivity of the material and the necessity of avoiding excessive temperatures in the outer portions of the disc, the heating cycle must be of the order of 20 to 30. minutes when ordinary oven methods are used. With the electrostatic method of this invention the heating cycle may be readily reduced to 2 or 3 minutes or less to keep step with the molding cycle so that the material is in the heated condition for only a very short time and the deterioration of themeterial is, therefore, a minimum.

In the system of Fig. 1 the tube I0 is a power oscillator of sufiicient capacity for the particular application and operates directly from the alternating current supply ll, through a filament transformer l2 and a plate transformer I3. The tuning condenser l4 shunting the grid and plate circuit inductances l5 and It comprises electrodes I1 and IS with the material l9 to be heated disposed between them. The grid circuit is provided with a grid leak 20 and condenser 2| and the plate circuit with a radio frequency choke 22 and a condenser 23 shunting the sec- 5 ondary of the transformer l3 to keep the radio frequency energy out of the transformer. While the circuit shown is very simple, eflioient and convenient to use it will be understood that any other well-known type of oscillating circuit may be employed though obviously from an emciency standpoint the tuning condenser should be in the plate circuit.

' For most molding materials, the dielectric constant will increase in magnitude as the temperature of the material rises but since the capacity of the tuning condenser l4 determines the oscillator frequency, the load circuit is always resonant to the oscillator frequency. The reduction of the oscillator frequency during the heating operation will, of course, vary with the nature of the material used and the temperature range. Since, as pointed out above, the power loss is a function of the frequency, this reduction in frequency gradually reduces the power dissipated as the material heats and permits a somewhat higher initial rate of dissipation than would otherwise be practical.

When, as in the case of molding records, the heating operation is carried out repeatedly with 0 identical preforms, a conventional wavemeter 24 may be disposed with its pick-up circuit in the field of the oscillator and the reading of the meter when the molding temperature is reached determined empirically. When this has been done, the operator may then use the meter as a thermometer for determining when molding temperature 'has been reached in all subsequent operations.

In the system of Fig. 2, the oscillating tube 3| i may be of the low voltage, low power capacity type and is tuned to the desired frequency by the variable condenser 32. By means of the winding 33 a potential of the oscillator frequency is applied to a conventional amplifier 34 which may have as many stages as necessary to provide the power required. The amplifier output is supplied through a transformer 35 to the electrodes 36, 31 between which the molding material 38 is located to form a condenser which tunes the secondary circuit of the transformer to the Osoillator frequency. With this circuit the oscillator frequency will be essentially fixed by the condenser 32 and it is, therefore, independent of the capacity variations incident to temperature changes in the molding material. In most cases it will be desirable to adjust the inductance of the heating circuit to a resonant frequency somewhat below the oscillator frequency so that as the material 38 heats up, the circuit passes through resonance. This gives an increasing dissipation up to resonance and then a decreasing dissipation thereby] affording a measure of automatic heat regulation which reduces the danger of impairing the material by overheating.

In some cases, particularly for materials having a wide variation of dielectric constant with temperature, it may be desirable to space one or both of the electrodes a short distance from the material as shown in Figs. 3 and 5, respectively. While the use of an air-gap makes a higher voltage necessary to obtain the same heating effect, the power loss in the gap is very small and the change in the capacity of the condenser as the 'fmaterial heats may be readily reduced to any de- .SiiBd value. For example, with one well-known 7 material, if the preform 39 of Fig. 3 is inch thick and has a 100 per cent variation of its dielectric constant when heated to molding temperature, a inch gap 40 reduces this variation to about 10 per cent.

If the gap is evenly divided into two gaps 53 and 54 on opposite sides of the preform 51 as shown in Fig. 5, the same result is obtained with the further advantage of areduction or elimination of any tendency toward the spotty heating which sometimes occurs due to poor contact between the electrodes and the material. When the material 51 has non-planar surfaces more uniform heating will be obtained by contouring the opposed surfaces of the electrodes 55, 56 to give constant impedance for all paths between them. While in general this requires that the electrode surface follow the contours of the surface of the material, bestresults will be obtained by determining the final shape by experiment.

When the surface of the material to be heated is somewhat irregular and the use of air-gaps isundesirable, improved contact arid more uniform heating can be obtained, as shown in Fig. 4, by using thin metal electrodes 45, 46 which are deformed and pressed into firm contact with the material 41 by resilient material 48 such as sponge rubber held in place by the backing plates 49, 50.

, In general, however, it will be found preferable to use an air-gap to avoid excessive heating in certain parts of the material but when very rapid heating of irregular shapes is required, the twogap system with empirically contoured electrodes,

as shown in Fig. 5, gives best results.

While the invention has been described with reference to particular embodiments for the purpose of illustration, it will be understood that the invention is to be limited only by the scope of the following claims.

What is claimed is:

1. In an electrostatic heating system, heating electrodes, material to be heated between the electrodes and an oscillation generator having a frequency determining circuit comprising inductance and capacity, the capacity consisting substantially entirely of that between said electrodes.

plate electrodes, a source of high frequency energy connected to the electrodes, and means for deforming and pressing the electrodes into firm contact with the opposite sides of the material to be heated.

4. The step in the method of heating materials by means of the dielectric loss produced in the material when it is subjected to a high frequency field which comprises tuning the heating condenser to an initial resonant frequency such that the variation in the dielectric constant of the material with temperature causes the circuit to resonate at the frequency of the field at an intermediate temperature in the heating range.

5. The method of heating materials having low conductivity and a dielectric constant which varies with temperature, which comprises subjecting the material to a high frequency field, varying the frequency of the field in accordance with the variation in the dielectric constant of the material, determining the relationship between the controlled frequency and the tempera- 10 the frequency of the generator oscillations.

7. The method of heating materials which comprises premrming the materials to standard dimensions, subjecting the preformed material to a high frequency field to produce a dielectric loss in the material and utilizing the change in the dielectric constant of the material with temperature to change the frequency 01' the held so as to maintain the loss in the material substantially constant as the heating progresses.


Referenced by
Citing PatentFiling datePublication dateApplicantTitle
US2421098 *Dec 16, 1943May 27, 1947VogtRepair of tires
US2423915 *May 31, 1943Jul 15, 1947Clearing Machine CorpMethod for heating and packaging thermoplastic molding material
US2429819 *Mar 28, 1944Oct 28, 1947Gen ElectricHigh-frequency heating apparatus
US2433067 *Jun 26, 1942Dec 23, 1947George F RussellMethod of and apparatus for highfrequency dielectric heating
US2436999 *Apr 23, 1943Mar 2, 1948Hpm Dev CorpMethod and apparatus for plastic injection
US2439286 *Feb 16, 1944Apr 6, 1948Rca CorpOscillation generator
US2449451 *Sep 28, 1944Sep 14, 1948Westinghouse Electric CorpHigh-frequency dielectric heating apparatus
US2457498 *Dec 11, 1945Dec 28, 1948Mann Julius WRadio-frequency parallel bonding
US2458012 *Apr 3, 1946Jan 4, 1949Westinghouse Electric CorpApparatus for high frequency dielectric heating of condenser bushings
US2465102 *Oct 4, 1943Mar 22, 1949Rca CorpRadio-frequency heating apparatus
US2471744 *May 29, 1944May 31, 1949Rca CorpMethod of and means for measuring microwave power
US2472370 *Jan 8, 1945Jun 7, 1949Cutler Hammer IncElectrode for high-frequency heating of insulation preforms
US2473041 *Aug 9, 1945Jun 14, 1949Swift & CoHigh-frequency electrostatic field apparatus for egg pasteurization
US2473188 *Jun 17, 1944Jun 14, 1949Rca CorpRadio-frequency dielectric heater with constant heating rate control
US2474420 *Jul 16, 1945Jun 28, 1949Ross M CarrellHigh-frequency dielectric heating apparatus
US2477258 *Jul 1, 1944Jul 26, 1949Hpm Dev CorpTurret injection machine
US2478005 *Jul 2, 1943Aug 2, 1949Borden CoInjection molding apparatus
US2504109 *Oct 4, 1946Apr 18, 1950Westinghouse Electric CorpDielectric heating with cavity resonator
US2505025 *Jun 1, 1945Apr 25, 1950Girdler CorpHigh-frequency treating system
US2508751 *Nov 10, 1945May 23, 1950Cutler Hammer IncOscillator circuit for high-frequency dielectric heating
US2516324 *Feb 15, 1946Jul 25, 1950Rca CorpConstant potential gradient dielectric heating device
US2526283 *Apr 2, 1947Oct 17, 1950Henning Windfeld MadsenDevice for permanent waving of hair
US2526697 *Jun 21, 1946Oct 24, 1950Armstrong Cork CoDielectric heating method and apparatus
US2575251 *Sep 9, 1943Nov 13, 1951Orlan M ArnoldMethod of welding bodies
US2590580 *Jul 26, 1946Mar 25, 1952Ben J ChromyHigh-frequency corn popping apparatus
US2592691 *Aug 31, 1946Apr 15, 1952United Shoe Machinery CorpAvoiding effect of moisture during high-frequency dielectric heating
US2594420 *Jun 29, 1948Apr 29, 1952Rca CorpHigh-frequency dielectric heating system
US2595501 *Jul 27, 1946May 6, 1952Allis Chalmers Mfg CoMethod of molding insulating material
US2607880 *Sep 21, 1945Aug 19, 1952Lord Mfg CoElectrostatic heating
US2626344 *Feb 14, 1950Jan 20, 1953Westinghouse Electric CorpApparatus for dielectrically heating irregularly shaped objects under pressure
US2678138 *Aug 8, 1950May 11, 1954Gen Motors CorpFeeding apparatus
US2695475 *Oct 21, 1949Nov 30, 1954American Optical CorpMeans and method of hardening glass articles
US3021270 *Oct 6, 1958Feb 13, 1962Union Carbide CorpTreating plastic articles
US3147325 *Nov 28, 1960Sep 1, 1964Gen Motors CorpProcess for making bonding pulleys
US4016886 *Nov 26, 1974Apr 12, 1977The United States Of America As Represented By The United States Energy Research And Development AdministrationMethod for localizing heating in tumor tissue
US4420670 *Mar 5, 1982Dec 13, 1983Cincinnati Milacron Industries, Inc.Control for dielectric heating in blow molding machine
US4441876 *Sep 11, 1981Apr 10, 1984Michel MarcFlow molding
US8099982 *Mar 26, 2008Jan 24, 2012National Institute Of Advanced Industrial Science And TechnologyMethod of molding glass parts and molding apparatus
US20100112341 *Mar 26, 2008May 6, 2010Hideki TakagiMethod of molding glass parts, molding apparatus, and molded product of glass material
DE912988C *Oct 2, 1948Jun 8, 1954Telefunken GmbhDielektrische Hochfrequenzerwaermungseinrichtung
DE2156084A1 *Nov 11, 1971May 18, 1972 Title not available
U.S. Classification219/777, 219/779, 65/DIG.400, 264/DIG.460, 219/780
International ClassificationH05B6/50
Cooperative ClassificationY10S264/46, Y10S65/04, H05B6/50
European ClassificationH05B6/50