Abstract:
Thermoplastic pellitized materials are melted in gravity flow through coaxially oriented perforated cylindrical metal susceptors. The susceptors are equally energized by the interception of a common magnetic field formed by a high frequency powered inductor coil.

Description:
FIELD OF THE INVENTION 
     Cylindrical susceptors intercept a high frequency magnetic field to melt pellet form thermoplastic materials. A multi-turn magnetic induction coil and two perforated metal susceptors are vertically oriented on the same axis. A smaller diameter susceptor is placed in the coil interior and a larger diameter susceptor is placed on the coil exterior in coaxial location. When a current flows in the inductor coil, a toroid shaped magnetic field is formed. A current is induced in the field susceptors that generates controlled heat. Pelletized thermoplastic material is continuously gravity fed to fill the interior susceptor. Material is similarly fed to cover the exterior surface of the outer susceptor. Heat induced in the susceptors melts the material in contact with both surfaces. Melted material flows in the annulus between the susceptors to exit at the bottom end with minor thermal exposure time. 
     BACKGROUND OF THE INVENTION 
     Current methods of melting pelletized thermoplastic adhesive materials utilize a tank that is resistance heated to melt by heat conduction from the walls of the tank. Thermoplastic materials are poor thermal conductors. Extensive time is required to melt the entire body of material and additional electrical power is required to maintain the material in a liquid state. If tank wall surface temperatures are allowed to exceed the material application temperature to expedite melting, material degradation will occur. Many materials held at application temperature for an extended period will degrade in performance and foul the application apparatus. 
     Large tanks of colored polymer are propane fired or melted by heat exchange from heated oil and stirred to maintain a large batch of road striping material for intermittent application. Large tanks of asphalt are fired by propane, or resistance element heated to melt for roofing operations. Both of these applications experience overheating and start up delay, and are energy inefficient. 
     SUMMARY OF THE INVENTION 
     Magnetic induction heating of an intermediary susceptor is a method of heat transfer employed to impart heat by conduction or radiation to electrically non-conductive materials. When a susceptor having a properly arranged plurality of holes is presented to a high frequency magnetic field an electrical current will flow with even distribution around the holes and result in an evenly distributed heat. The system requirements of inductor coil form and placement, choice of electrical frequency applied, susceptor material choice and thickness, and power control are all subjects well known to those skilled in the art of induction heating process. Materials such as hot melt adhesives, asphalt, and plastisols in the form of pellets, prills, tack blocked particulate, and small chiclets are melted efficiently and on demand in the apparatus of this invention. 
     The apparatus of this invention presents a continuous melting method for electrically non-conductive particulate materials that can be started and stopped, as melted material demand is required. The process requires less power and does not degrade the material in the melting apparatus. When the heat of the susceptor is maintained at the target melt temperature of the material, flow volume is dependent on the viscosity of the melted material. Material presented to a surface of the perforated susceptor will flow through this interface only as fast as the material thermal conductivity will allow. Applying pressure to the material at this interface is of minor consequence to aid the speed of the process. Therefore, the process maximum volume is directly related to the surface area of the susceptor in contact with the material. The invention maximizes the melt surface area within a small envelope. 
     The use of melting susceptors intercepting substantially all of the empowering magnetic field is taught in Lasko U.S. Pat. No. 7,755,009. It utilizes the second susceptor to mix and add heat to the gravity flowing liquid of the melt susceptor. The multiple susceptor form of the present invention presents a second primary melt face that increases the melt surface in the same space. The use of folded susceptors is taught in Lasko U.S. Pat. No. 6,230,936. These susceptor forms are uniquely joined in this invention to provide a method of utilizing the advantages of both. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a vertical section of the melting system having cylindrical susceptors. 
         FIG. 2  is a top view of the melting system having cylindrical susceptors. 
         FIG. 3  is a top view of the melting system having folded cylindrical susceptors. 
         FIG. 4  is a vertical section of a melting system for combining materials. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The major elements of this invention are illustrated in proportion and position in cross sectional view  FIG. 1  and top view  FIG. 2 . Thermoplastic pellets  1  are continuously fed to a cylindrical containment vessel  2   b  with extension  2   a  acting as a removable reservoir. An inner susceptor  3 , constructed of 20 ga. perforated steel, shaped as a cylinder, is suspended by three steel rods  4  that nest in locating slot  5  on support platform  6 . An outer susceptor  7  of similar construction is coaxially positioned by support platform  6 . A magnetic field inductor coil  8  is suspended in the annulus between susceptors  3  and  7  by three spacers  9  that rest on the upper edge of the outer susceptor  7 . The thickness of the susceptor material is chosen to minimize the latent heat on power off. It dissipates into only those pellets contacting the susceptors. This allows an initial and subsequent restarts of melt flow within a few seconds. 
     Inductor coil  8  is constructed of solid  14  ga. bare copper wire with spaces between the turns adjusted to present a magnetic field to the susceptors that will result in an evenly induced current flow. The diameter of inductor coil  8  is chosen to be in close proximity to the inner surface of outer susceptor  7  to impart energy in proportion to its greater mass. These are coil design methods that are well known to the practice of induction heating. 
     High frequency power is applied to the coil by flexible cable at connector  10 . The power level is controlled by thermocouple  11  to hold the susceptors at the melt target temperature as melting material passes from the pellet exposed surfaces of susceptors  3  and  7  through their perforations. The melted material flows through annulus  12  to exit at the bottom. A wireless transmitter  13  reports the thermocouple signal to the system controller to avoid RF interference and eliminate wiring for a single control signal. 
     End cap  14  directs receding pellet material to the susceptor melting surfaces. Interior flow baffle  15  and exterior flow baffle  16  are 45° Teflon cones that direct material at the column bottom to prevent the slowing of material flow at this point that would cause localized over heating of an equally energized the susceptor. 
     Liquid material  17  gravity flows from annulus  12  to gather as a single stream of material  18 . Exterior flow baffle  16  is extended to provide the gathering cone for material stream  18 . 
     Another embodiment of this same melting process doubles the flow capacity by folding the susceptors as shown in top view  FIG. 3 . The numbers of folds, of the inner susceptor  19 , are calculated to provide a total peripheral length equal to two times the diameter at the tips of the folds, thereby doubling its surface area. The surface area of the outer susceptor  20  is forced to equal the surface area of the inner susceptor by calculating the greater included angle of the fold  21  that will yield the same peripheral distance, thereby yielding a susceptor of equal mass. In this example a further refinement yields opposing 90° angles that form a chain of squares that are end caped with pyramid shapes of Teflon  22  to deflect the pellet flow. The containment vessel is the same as used in the previous example. The power applied is increased to yield two times the melt rate in the same space. 
     A major advantage of this folded form allows the inductor coil  8  to be positioned without concern for the greater mass normally presented by the greater diameter outer susceptor to the same magnetic field. The induced current flow in the folded susceptor follows the shape of the periphery with the same current intensity at the valleys and the tips of the folds. Therefore, the inductor coil  8  turns need be spaced in only one dimension to yield an energy distribution consistent with the materials flow characteristics. 
     Sectional drawing  FIG. 4  is another embodiment of the invention that adds a containment cylinder  23  that provides an isolation of a different material  24  introduced to interior susceptor  3 . The perforation size and thickness of susceptor  3  are chosen to accommodate the different viscosity and melt temperature of material  24  in desired proportion to material  1 , while maintaining an equivalent susceptor mass. 
     End cap  14  is removed and cylinder  25  is added to the upper end of susceptor  7  to extend annulus  12 , so that a sold particulate material can be added to the mix at entrance  26 .