DIELECTRIC HEATING

Another form of high frequency heating is that known as dielectric heating, which is applied to materials that normally are nonconductors of electricity, and usually nonconductors of heat.  The process of dielectric heating differs from induction heating in the a much higher frequency is required.  The frequencies usually applied for the heating of metals have little or no effect on dielectric materials, and vice versa.

Induction heating provides to for the transfer of eddy currents into a material which must be electrically conductive.  The surrounding  coil, which carries the high frequencies current, forms the primary of what may be termed a transformer, while the metal part to be heated forms the secondary.  This type of heating is used most to heat localized surface of a metal part.  The depth of heat penetration may vary from a rather thin skin to a relatively deep area, as may be determined by the frequency used and the time cycle employed.

With dielectric electrostatic heating, as it often is called, the material to be heated is placed between two electrodes, usually spaced the same distance apart at all points.  Energy is applied to the plates and heat is produced simultaneously throughout the area of the charge, usually at a rather rapid rate.

Dielectric heating is used in the pre-curing of practically all types of plastics, cellulose fibers, and celluloid.  Its field also covers the heating and treatment of rubber, wood paper, ceramics, cork, textiles, and many other dielectric materials.

Cause of Heating.  All materials are made of molecules which, when rubbed together, create a certain amount  of frication that produces heat.  The more intense the friction, the more pronounced the heat.   A small strand of wire, for example, twisted rapidly back and forth will create heat where it is twisted, because of the molecular disturbance at that point.

When an alternating current is subjected to a dielectric material, the molecules try to align themselves to the alternating field, rubbing themselves together with each cycle, or reversal of polarity, an a pronounced molecular friction is set up.  When the alternating field is reversed several million times a considerable amount of heat is naturally created because of the molecular friction that takes place.  This, briefly, is the principle involved in dielectric heating.

Materials normally considered insulators, often referred to as dielectrics, will absorb some electric energy when placed between electrodes of a high frequency field.  If a material were an absolute insulator, heating would be difficult, but most materials have a sufficiently high loss factor to be heated by high frequency currents.  From this it will be seen that the loss factor bears a direct relation to heating results; the lower the loss factor the slower the generation of heat, and the higher this factor the more pronounced the heating effect.

Nonmetallic materials are heated by placing them between two electrodes.  The material forms the dielectric of a condenser and therefore the dielectric losses cause the material to heat.  The power factor is extremely important, because on it will depend the possibility of heating a material.

To induce ordinary current to flow through non-conducting materials would be impossible unless the resistance to the flow of current could be reduced.  However, by increasing the frequency of the current it is possible to cause the passage of current through non-conducting materials.  The amount of heat produced, then, will depend on the physical characteristics of the material rather than its electrical properties.

A typical high frequency generator for dielectric heating has an output rating of 3 1/2 kW and can be operated at various frequency of 10 to 50 megacycles.  For most woods and plastic materials, heat can be produced satisfactorily at frequencies of 5 to 20 megacycles.  With other materials having a lower power factor, it is necessary to apply higher frequencies ranging from 20 to 50 megacycles, or even 100 megacycles or more.

The internal construction of the generator is in the lower section of the unit houses the power supply, whereas in the upper section is located the oscillatory portion of the circuit.

High frequency generators for dielectric heating are available in various sizes from as low as 1/2 kW. to 200 kW and for special applications may even exceed this maximum power output.

Vacuum tube oscillators for dielectric heating utilize three element radio tubes.  Two current sources are used; one is for the filament of the tubes, usually ranging from 5 to 22.5 volts, and the other is a high voltage direct current, which may vary from 2,000 to 20,000 volts, depending on the size of the generator. 

The alternating current input power is brought in to a center tapped transformer, where the voltage is stepped up as required, usually to about 10,000 volts.  The output current of the transformer is rectified in order to supply direct current to the two tube push pull oscillator.  The oscillator then generates current at a frequency of 5 to 50 megacycles,  or higher as may be required.  Power is taken from the circuit to the electrodes.  The material to be heated is placed between these electrodes, which in turn are connected into the tank circuit by taps on the tank circuit coil.

Comparative Heating Methods.  In using hot plates heat must be conducted from the exterior to the interior, and the time required may run into several minutes.  Heating depends entirely on conduction since heating begins on the outside, nearest to the source of heat, and moves inward by thermal conduction.  With this method the material does not become heated uniformly.  Assuming a temperature of 225˚F were wanted, the outside may attain this heat but the inside may reach only 150 to 220˚F, depending on the nature of the material and the duration of heating time.

Dielectric heat, obtained is produced in the material itself and is uniform from the inside to the outside of the material.  Whatever variation may take place is due to radiation and conduction losses at the surface which may possibly cause slight outward flow of heat.

contrasted with hot plate heating, in the dielectric method a temperature of 225˚F can be obtained throughout the entire area of the material.  Heating will be uniform and the temperature rise will be the same throughout the thickness.

Heating Plastic Preforms.  One of the outstanding applications for dielectric heating is in the precuring of plastics, as required for transfer molding.  This is carried out by first preforming the plastic material to a flat shape, then placing it between the electrostatic electrodes for the precuring or preheating cycle, after which the preform is placed in the mold and cured.  From this it will be seen that dielectric heating is primarily for the preheating of the preform prior to molding.  since a uniform heat is produced throughout the preform, marked improvements in the molding  properties are obtained.

As a comparison with former methods, let us assume that a preform was heated by conduction and that 6 min. were required to preheat it to 250˚F.  The molding time may then require 6 min. to cure the form completely.  With dielectric heating the preform may require only from 40 sec. to 1 min. for the preheating cycle, and since a uniform temperature has been produced throughout it area the molding time may be only 3 min.  This reduction in molding time is the result of improved flow properties in dielectric heating.

Due to the high voltage across the electrodes of a dielectric heating unit, it is necessary to provide a screen or shielding around the plates as a precaution against injury to the operator.  Such a unit may be at the right hand side of a high frequency generator.  The lower electrode is fixed and the upper one is adjustable to the thickness of the plastic preform.  When the front door of the heating unit is elevated the upper electrode is automatically raised slightly to facilitate the removal and the insertion of the preforms.  When the front door of the heating unit is elevated the upper electrode is automatically raised slightly to  facilitate the removal and the insertion of the preforms.  Also connected with the movement of the door is a limit switch which breaks the circuit when the door is opened and automatically closes the circuit when the door is lowered or closed.  After the proper curing time has been established for a given preform the cycle is controlled by a timer, so that repeated heats may be uniform.  Starting of the cycle is by means of the push button.

The preheating and molding times will vary, depending upon the type of plastic and the source of high frequency power,  but substantial time reductions are made possible, sometimes exceeding 50 per cent of former curing requirements.

Dielectric heating for plastics offers other indirect savings.  One is the fact that thicker sections can be molded, with less change of uncured sections or the formation of open seams.  There also is the possibility of reducing the molding pressure as much as one-third, so that press costs and maintenance requirements are lessened.  Likewise, available presses can handle larger molds, or a press of a given size will accommodate a wider range of molds than was possible before.

Power Requirements.  In heating plastics having an average specific heat of 0.4 to 0.5, it usually requires about 2 kW. to heat 1 lb. in 1 min. to 300˚F.  The formula used for dielectric heating is similar to that applied to induction heating in that a certain mass of material having a given specific heat must be heated to a given temperature, and that so many B.t.u. will be required.  The formula would be:

W X S X T = B.t.u.

Where W = the weight of material.

             S = the specific heat.

             T = the temperature rise desired.

This formula, of course, does not take into consideration the voltage and frequency, but as a rule will serve as a quick means of determining the approximate power required to heat dielectrics.  With this can be included:

3413 B.t.u.

1 kW. output = 60  = 56.71 B.t.u. per min.

Assuming that apiece of preformed plastic weighting 2 1/2 lb. requires heating to 250˚F and that the material has a coefficient of specific heat of 0.4, we would have:

2.5 X 0.4 X 250 - 250 B.t.u.

If a generator with an output capacity of 5 kW were used it would produce:

56.7 X 5 = 283.5 B.t.u. per min.

as its available power, from which the heating time for the 2 1/2 lb. mass would be:

                                                                            283.5

250 = 0.88 min.

Where amore accurate calculation is required to allow for such factors as the frequency, thickness of the part, and its loss factor, the following basic formula can be applied:

H - CFL E²

                                                                                                T

where H = heat per unit volume.

            C = dielectric constant.

            F = frequency.

            L = loss factor.

            E = voltage.

            T = thickness of part.

The coefficient of the specific heat of a material is expressed by the number of B.t.u. required to raise 1 lb. of that material 1˚F.  Material having a specific heat of 0.5, for example, would mean that it would take 2 B.t.u. to raise 1 lb of material 1˚F.  The specific heat of the materials is based on the standard of 1.0 which is applied to water, which means that it takes 1 B.t.u. to raise 1 .b of water 1˚F.

High-Frequency Heating of Wood.  Another outstanding use of dielectric heating is in the joining of plywood, or laminated section which require an elevated temperature to dry the adhesive bond.  Representative of this application is the drying of plywood sheets.  An existing method used for preparing plywood for drying is several sheets are place in a press, squeezed together by hydraulic pressure, and then held intension by a series of clamps attached around the edges.  The charge then is transferred to a drying room in which from 10 to 12 hr, or even more, are required for drying to assure a suitable bond.  With this process a considerable storage space is required.

In application of high frequency heat to plywood, however, is such as might require only a few minutes to dry a similar quantity of plywood, such an installation.  The plywood sheets are placed within a few minutes the temperature throughout the mass is raised 200 to 250˚F, as required, after which the plywood sections are ready for finishing operations.  No storage is required and usually the charge can be cured in about the same time as was formerly needed to apply the holding clamps used in the other method.  In applying high frequency heat to plywood the ground lead is connected to the top plate and the base of the press, then a center electrode is applied midway between the plywood sheets.  sometimes a series of plates are used for heating, although the electrode connection is made only to the center plate.  These plates do not have any effect on the heating process.

Advantages of Dielectric Heating.  The application of high frequency heat to dielectrics has many advantages.  Basically, a more uniform heat is distributed through the part being treated; heat is applied at an exceptionally fast rate so that the output may be increased; and uniformity of heat can be produced without variation, thus reducing spoilage.  Since dielectric heat can be applied to such a variety of nonmetallic materials of different structures and compositions, various other advantages are also obtained.  These are:

• Often a better quality of product is the result of uniform heating, which can be obtained in no other way.
• Processes of manufacture can be modified in many cases, with less handling and fewer subsequent operations.
• Faster heating or drying usually reduces floor space and, as a rule, reduces inventory which might otherwise be tied up due to slower heating or drying processes.
• No delay in obtaining instant heat, as compared with other methods where drying ovens or furnaces may require considerable time to reach a given or desired temperature.
• Heat may be stopped instantaneously, which in some cases many prevent overheating and possible spoilage of products.
• Dielectric heat can often be used to obtain a desired chemical reaction.

Dielectric Dehydration.  Another form of high frequency generator with an output power rating of 3 kW, the electrodes which are made adjustable to various thicknesses of material.  In use, however, the electrodes require shielding as a protection to the operator.  This unit operates at a frequency of 15 megacycles.

Among the many uses for dielectric generators of this type is the dehydration of food.  Test setup for this purpose the samples, previously compressed at a 500 lb. pressure, are placed between electrodes over which a bell jar is placed to provide a vacuum.  Temperatures of 120 to 150˚F, depending on the type of food being dehydrated, are used and moisture content can be reduced to 1 per cent or less without burning or affecting the outside surface.  The process is relatively fast and the cost of operation comparatively low, or less than 100 watts per lb.

No attempt has been made to cover the use of dielectric heat in great detail.  The references given, however, are more for the purpose of showing the difference between induction heating and dielectric heating.  In many respects the equipment used for both methods is similar in construction, although the applications are entirely different and have little similarity.  The use of induction heating, is suited to the heating of metals, whereas dielectric heating primarily is intended for nonconductors.  Occasionally there might be a choice of either of these heating methods, but such cases will be rare since each method has its own distinctive field.