US 3742180 A
An apparatus for applying pressure and a high frequency electric field to material comprising a cavity for containing the electric field which encloses platens relatively movable to apply pressure to material placed between the platens. The cavity electrically communicates with the platens for supplying the contained electric field to the platens which, in turn, cooperate with each other to apply the field to the material between the platens. The cavity and platens are also directly coupled into the means for generating the electric field as a resonant circuit.
Description (OCR text may contain errors)
United States Patent 1191 Bradley June 26, 1973 HIGH FREQUENCY CAVITY PRESS [75 Inventor: Robert W. Bradley, Marblehead, m Goldberg Mass Assistant Examiner-Hugh D. Jaeger Att0rney-Richard A. Wise, Richard B. Megley and  Assignee: USM Corporation, Boston, Mass. Willi m R, E n
 Filed: May 26, 1971 21 Appl. No.: 147,083  ABSTRACT An apparatus for applying pressure and a high frequency electric field to material comprising a cavity for  US. Cl. 219/10.81, 156/273 containing the electric field which encloses platens rah hat. Cl. atively movable to pressure to material placed  Field of Search 219/10.5 5, 10.61, between the platens The cavity electrically communi 219/1063 1 156/273 cates with the platens for supplying the contained electric field to the platens which, in turn, cooperate with  References C'ted each other to apply the field to the material between UN TED ST S PATENTS the platens. The cavity and platens are also directly 3,205,114 9/1965 Gross 156/273 X coupled into the means for generating the electric field 3,469,054 9/1969 Serota 2l9/l0.6l as a resonant circuit. 3,401,248 9/1968 Kim 156/273 x 3,357,108 12/1967 Bennett 219 1061 x 14 Clams, 3 Drawing Flgures ll 1 7 o [llllll llllllillllll HIGH FREQUENCY CAVITY PRESS BACKGROUND OF THE INVENTION This invention relates to an apparatus for applying pressure and a high frequency electric field to material.
Machines for applying a high frequency electric field to material are well known for their utility in heating dielectric material. Frequently, such high frequency heating machines are used on dielectric material which is heat softenable. Usually, a die is applied to the softened material to emboss the surface of the material.
The high frequencies which have been found useful for heating material are in the mega-Hertz range. These frequencies have corresponding wavelengths of a few meters. Such wavelengths are in the same order of magnitude as the physical dimensions of conveniently sized machines for working on the material. It is known that electromagnetic waves which have wavelengths similar in size to the physical dimensions of conductive parts of a machine on which they are impressed tend to radiate from the conductive parts.
Only the conductive parts of high frequency heating machines adjacent the material are designed to pass electric currents through the material to heat the material. However, some electric voltages impressed on the work do not pass through the material but instead radiate as waves into space and perform no useful function. The radiated waves thus reduce the efficiency of the machine and often interfere with radiation frequencies reserved for communications and other purposes requiring such radiation. Since radiation from a high frequency heating machine is an unwanted by-product, the frequencies and signal strengths at which such radiation is permitted are strictly limited by government regulations.
In many instances it has been found impossible to prevent inefficient and illegal radiation from conventional high frequency machines. One solution to this problem has required placing the machine in a special electrically shielded room. Not only are such rooms expensive to construct, but the location of the machine in the room may interfere with the logistics of production line operation. These problems have severely hampered the use of high frequency heating machines. Another solution involves operating high frequency machines at fixed, governmentally approved frequencies and signal strengths; this may be inconveniently limiting.
Solving still another problem requires impedance matching or tuning the electric field generating and field applying components of a machine to each other. It has long been known that such tuning reduces the reflection of electric energy from the interface of electrical components toward the high frequency source. Here, reflected energy does not reach the'field applying means and thus reduces the efficiency of the machine. The size and the type of material between the platens effects the impedance and thus the reflection of the machine. Each change of material thus requires tuning of the machine for efficient operation of conventional machines.
SUMMARY OF THE INVENTION Accordingly, it is an object of the invention to provide a high frequency machine which does not radiate electromagnetic waves into the surrounding space. It is a further object of the invention to eliminate the need to tune the machine components to each other for varying conditions of machine use. Still a further object of the invention is to provide a machine in which the elapsed time to heat material from one temperature to another is independent of the surface area of the material.
To this end a high frequency machine is provided with a cavity for containing an electric field and enclosing the parts of the machine which have high frequency energy on them. The cavity is electrically conducting on its interior surface and designed so that substantially no high frequency energy appears on its exterior surface. The cavity has an opening through which pieces of material to be operated upon may be placed between platens in the cavity. A cover is positioned relative to the opening to contain the electric field at the opening. Platens are movably mounted within the cavity and cooperate to press the material placed between them. The platens are also electrically connected to the cavity to apply the high frequency field contained in the cavity to the material between the platens. Means for supplying a high frequency field to the cavity are provided.
The means for supplying the electric field to the cavity directly couple a high frequency power supply to the machine which itself serves as a resonant tank circuit to form the high frequency generator. The material in the machine forms an integral part of the resonant circuit directly coupled into the high frequency generator. Thus changes in the material merely change the frequency of the machine. A range of frequencies is acceptable for treating the material and, since the cavity prevents radiation,' governmental frequency regulations are not a problem. The direct coupling of the machine or field applying component as part of the high frequency generator or field generating component eliminates energy reflection problems between field applying and generating components by combining them as one integral component. The cavity and direct coupling thus eliminate the need to tune the machine.
It is additionally possible and preferable to select impedence characteristics for the machine resonant circuit relative to the internal impedance characteristics of the high frequency power supply such that the electric power of the high frequency field applied to the material is directly proportional to the effective area of the platens for a wide range of effective areas, while the potential difference across the platens or from one platen to the other is held constant. The effective area of each platen is that surfacearea of each platen which affects the material between the platens. The relative values of the characteristics which produce the direct proportionality are empirically determined. The direct proportionality results in the application to pieces of the material uniform except in area of constant electric field power per unit area regardless of the area of mate'- rial between the platens. In a high frequency heating machine the power per unit area is the calories per second per unit area applied to the material; thus the elapsed time to heat the material from one temperature to any other temperature is independent of the area of the material. The constant potential difference across the platens may conveniently be set below that potential which will permit arcing across the platens, a value dependent on ambient conditions at the press. In a high frequency heating machine such arcing may damage the material being heated.
BRIEF DESCRIPTION OF THE DRAWINGS The above and other features of the invention will now be more particularly described with reference to the drawings of a preferred embodiment of the invention in which:
FIG. 1 is a perspective view, partly in section, of an apparatus embodying the invention;
FIG. 2 is a side view, partly in section, of the'apparatus shown in FIG. 1; and
FIG. 3 is a side view, partly in section, of a support portion of the apparatus shown in FIG. 2.
DESCRIPTION OF THE PREFERRED EMBODIMENT As seen in FIG. 1, a high frequency heating machine embodying the instant invention has a cavity generally indicated at enclosing an operating portion of the machine and electrically conducting on its interior surfaces. The cavity has an opening indicated generally at 12 to provide access into the cavity for pieces of material 14 to be heated in the machine. In FIG. 2 a platen 16 is seen to be mounted within the cavity on a support structure indicated generally at 18. Another platen is mounted for movement relative to the first platen to press material 14 between the platens in the cavity. A composite cover 22 for the cavity opening 12 is secured to the movable platen. When the platens engage and press the material, the cover closes the opening 12 to contain an electric field in the cavity. The electric field is applied to the cavity through input link 23 from a high frequency power supply (not shown) directly coupled to the cavity. The platen l6 electrically communicates with the cavity and the second platen also electrically communicates with the cavity through the composite cover to apply the electric field in the cavity to the material 14 between the platens. The electric field applied to the material by the platens dielectrically heats lossy material. Preferably, a die 25 is placed between the platens to emboss the heated material when it is pressed by the platens.
The cavity 10 is a box-like structure having interior walls 40 and a bottom 42 made from an electrically conducting material such as copper. At the top of the box is the opening 12 through which material to be heated is inserted into the cavity. Additional walls 44 project from a central portion of the cavity bottom 42 to enclose a central support shaft 46 of the platen support structure 18. The walls 44 shield the shaft 46 from the electric field in the cavity. Supporting walls 48 physically and electrically connect the upper end of walls 44 to the periphery of the first platen 16. The physical connection of the walls 48 to the platen 16 supports the entire cavity 10 which is not supportingly connected to any other part of the machine to isolate the cavity and contain the electric field in the cavity. The contained electric field does not substantially radiate into the space adjacent the machine.
The support structure 18 supports the first platen l6 and thus also supports the cavity 10 connected to the platen. The support structure 18 comprises the central support shaft 46 and additional support members 50 which project through close fitting holes 51 (FIG. 3) in the cavity base 42 and supporting walls 48 to support the first platen 16 near its periphery. Although it is intended, in this preferred embodiment of the invention, that the central support 46 provide the main physical load bearing support while the members prevent deflection of the platen 16 near its periphery, supports of either the types illustrated by 46 and 50 may be used separately or in any other desired combination.
The support shaft 46 may be made from any material selected for its supporting strength because the enclosing walls 44 and 48 of the cavity prevent any of the electric field in the cavity from reaching the support. Thus, the central support will not conduct any part of the electric field from the cavity to the space surrounding the machine. The support members 50 must be made from an electrically non-conducting material such as ceramic to prevent them from conducting the electric field from within the cavity to the space outside the cavity. The holes 51 in the cavity walls 42 and 48 through which the support members 50 project should be sufficiently close fitting to cooperate with the members 50 to contain the electric field in the cavity.
The platen 116 is mounted on the support structure 18 to provide a rigid platform against which the movable platen 20 can press the material 14. The platen 16 may be made from a strong, rigid material such as steel. Preferably, the platen 16 supports on its upper surface a panel 60, of silicone glass or the like, which extends beyond the edges of the platen to the walls 40 of the cavity. The panel thus serves as an insulator plate and as a cover for the opening in the cavity to exclude dust and foreign matter. The platen i6 and panel 60 preferably support a second panel 62 which is substantially coextensive with the area of the platen. The second panel 62 may be made of silicone rubber or the like and provides a resilient surface on which the material 14 may be placed. The first and second panels also affect the electrical capacitance between the platens 16 and 20.
The platen 20 is mounted for vertical movement relative to the platen 16 and is moved by a fluid operated cylinder 63 to press the work piece 14 between the platens. Electrically operated thermal-heating elements 64 are embedded interior the platen 20 and are directly connected to a source of electric power (not shown) to thermally heat the second platen. The elements 64 are embedded in the platen 20 sufficiently far from the inner surface 65 of the platen 20 so that no high frequency energy reaches the elements due to the known skin effect phenomena in which high frequency energy appears only on the surface of conductors. Thus no filters or other devices are needed to protect electrically connected external components, here the electric power source for the heating elements 64, from the high frequency energy in the cavity.
A die 25 is secured to the movable platen 20 to engage the material 14 when the platen 20 is moved toward the fixed platen 16. The material engaging surface 66 of the die may be embossed with any characteristics which it is desired to impress upon the material 14. Because the die surface 66 more closely approaches the fixed platen 16 than does the movable platen 20, the area of the die surface 66 alters the capacitance between the platens by altering the capacitively effective area of the parallel plate platen structure creating the capacitance. The die area thus forms the effective platen area. A flange 69 electrically connects the die to the cover 22.
The composite cover 22 is secured at an inner edge 70 to the platen 20 and flange 69 and is secured at its outer edge by a bracket 68 to an outer cover 71. The
cover 22 is made from electrically conducting material and has a jog 72 intermediate its edges to permit the cover to bend to accommodate thermal expansion of the platen 16 when it is thermally heated by the heating elements 64. The support bracket 68 also electrically connects the cover 22 to contact members 74 which extend around the entire periphery of the cover. When the cover is closed and the die 25 engages the material 14 on the panel 62, the contact members electrically connect the cover with cooperative contact members 76 electrically connected to the walls 40 of the cavity. Through cooperative contacts 74 and 76 the cover electrically connects the walls 40 of the cavity to the movable platen 20 and die 25 to close the cavity opening 12 and contain the electric field within the cavity.
A high frequency electric energy power supply (not shown) is directly coupled to the top surface of the first platen by a lug (not shown) of input terminal 23. The input terminal includes capacitor 79 which blocks direct current input to the cavity. Most of the high frequency energy from the input radiates as a rapidly changing electric field between the surface of the fixed platen and the die and movable platen. The die and movable platen electrically communicate with the cavity walls, the interior surfaces of which electrically communicate with a grounding plate 80. Only the interior surface of the electrically communicating elements need be considered since by a well known phenomena, commonly called skin effect, all the high frequency energy on the cavity remains on the surface of the cavity. Some of the high frequency energy circulates in the space enclosed by the cavity. Part of this energy is received by a pick up coil 82 to provide feedback for the high frequency power supply. Preferably the coil 82 is of an oriented type, such as the loop coil shown, so that the feedback may be varied by rotating the coil to affect generator performance in a well known manner.
The interior conducting surface of the cavity is an inductance at the high frequencies employed in the machine. The parallel plate structure of the platens, panels, material and die is capacitive and together with the associated capacitor 79 are in parallel with the cavity. The machine thus forms an inductive-capacitive parallel resonant tank circuit directly coupled into the high frequency generator. Direct coupling is not by transmission line, but by direct link to the power supply portion of the generator.
It has been found experimentally that direct proportionality between the effective area of the platens and the power in the machine for application to the material is achieved for a high frequency power supply cornprising a plate loaded vacuum tube having DC characteristics in which the plate current is 2 amps; grid current, one-half amp; plate voltage, 3,400 volts and gain, mu 30; and a cavity 10 having a bottom 42-l7 X l7 inches, walls 40-l0 inches high, walls 44-9 inches high and walls 48-4 k inches wide, panels 60 and 62% inches thick each and an associated capacitor 79 of 0.0005 microfarads.
It is understood that the preferred embodiment described above is merely illustrative of the invention and not a limitation to the invention.
Having thus described my invention, what I claim as new and desire to secure by Letters Patent of the United States is:
l. A machine for applying a high frequency electric field and pressure to material which comprises:
means defining a cavity for containing the field in the cavity having an opening for admitting the material into the cavity;
a cover positioned relative to the opening for containing the field in the cavity at the opening;
' platens within the cavity for receiving the material between them and for applying the field to the material;
means electrically connecting one of the platens and the cavity defining means for applying high frequency energy to the one platen;
means for supplying the high frequency energy to the other platen and cavity; and
means for supporting the platens for relative movement with respect to each other for applying the pressure to the material.
2. A machine as in claim 1 wherein the cover is physically connected to one of the platens for movement therewith.
3. A machine as in claim 1 wherein the platen support means comprises a shaft projecting through a hole in the cavity defining means to one of the platens and enclosed by additional walls of the cavity extending from the platen on the shaft to the hole.
4. A machine as in claim 3 wherein the additional walls are physically connected to the shaft supported platen and the cavity defining means for supporting the cavity defining means.
5. A machine as in claim 3 wherein the additional walls comprise the electrically connecting means.
6. A machine as in claim 1 wherein the platen support means comprises an electrically non-conducting member projecting through a hole in the cavity defining means to the platen on the member.
7. A machine as in claim 1 wherein the platens and cavity defining means electrically cooperate to form a resonant circuit.
8. A machine as in claim 7 wherein the means for applying the field comprises the resonant circuit.
9. A machine as in claim 7 wherein the resonant circuit is directly coupled to a high frequency power supply to form a high frequency generator.
10. A machine as in claim 1 wherein the power of the electric field in the machine is directly proportional to an effective surface area of each platen to apply a substantially constant field power'per unit area to pieces of material of various areas.
11. A machine as in claim 10 wherein a potential difference across the platens is constant.
12. A machine for applying a high frequency electric field to material, comprising:
a generator for high frequency electric energy; and
means directly coupled into the generator for applying the energy to the material as a field having a substantially constant power per unit area of the material for pieces of the material of various areas.
13. A machine as in claim 12 wherein the directly coupled means additionally comprise a resonant circuit portion of the high frequency generator.
14. A machine for applying a high frequency electric field and pressure to material, comprising:
a generator for high frequency electric energy; two platens electrically connected to the generator for applying the energy as an electric field and movable for applying the pressure to the material between the platens; and means electrically connected to the generator and platens for providing the power of the field in direct proportion to an effective area of the platens.