A method and apparatus are provided for heating a solid material and dispensing the material as a liquid. A central housing has an inlet, a dispensing orifice and a flow passage extending through the central housing for passing the material from the inlet to the dispensing orifice. A susceptor and induction coil are disposed within the flow passage for immersion within the material after it is liquified. The susceptor includes a conically shaped flow section which extends across the flow passage, and a plurality of flow ports for passing the material. The susceptor further includes a cylindrical section which extends downstream from the flow section for receiving the material from the flow section and passing material to the dispensing orifice. The induction coil is aligned with and spaced downstream from the flow section of the susceptor, surrounding part of the susceptor for electromagnetically inducing electric currents to flow within the flow section.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates in general to heating and dispensing materials, and 
in particular to devices for electromagnetically heating and dispensing 
materials. 
2. Description of the Prior Art 
Prior art devices have been utilized for heating and dispensing materials, 
such as for heating a solid material until it melts and then dispensing 
the material as a liquid. For example, hot glue guns are used for heating 
an end of a solid glue stick to a transition temperature at which the glue 
is liquefied and then dispensing the melted glue through a dispensing 
orifice. Typically, a housing is provided having an interior flow path 
through which the material is pushed as it is heated. Resistance heating 
elements are commonly used. The resistance heating elements have been 
mounted to the housing outside of the flow path, and often outside of the 
housing. 
Other devices have utilized induction heating to heat materials for 
dispensing. A housing is usually provided having an interior flow path 
through which the material is pushed as it is heated. An 
electromagnetically heated susceptor is located either directly in or 
immediately adjacent to the material flow path. Induction coils have been 
mounted outside of the housings for inducing eddy currents to flow within 
the susceptors to generate heat for transferring to the materials. Often 
an external shroud is provided around the induction coil to protect an 
operator. Heat from passing current through the induction coil usually has 
to be removed to prevent overheating of the coil. Forced cooling is often 
used, resulting in wasted energy. External shrouds and cooling devices for 
induction coils also add additional weight and size to such prior art 
devices. 
Inductive heating devices having large material flow capacities require 
that a large surface of the material be heated at one time. For melting 
materials, this results in susceptors having large heat transfer surface 
areas for contacting materials at melt faces for the materials. In order 
to prevent cold spots over the large heat transfer surface areas of such 
susceptors, the susceptors are made to have high heat capacities and high 
thermal conductivities. Although susceptors having high heat capacities in 
combination with high thermal conductivities add additional weight to 
prior art devices, they provide substantially uniform temperatures across 
the heat transfer surface areas, even those portions of the surface areas 
which are more remote from induction coils than others. However, when 
inductive heating of the susceptor is stopped, the large heat capacity of 
such susceptors will result in continued heat transfer to the material, 
often to a significant depth within the material beyond the melt face. 
This not only wastes energy, but may also result in waste of the material 
being heated. 
SUMMARY OF THE INVENTION 
A method and apparatus are provided for heating and dispensing a material. 
A central housing has an inlet, a dispensing orifice and a flow passage 
extending through the central housing for passing the material from the 
inlet to the dispensing orifice. A susceptor and induction coil are 
disposed within the flow passage for immersing within the material. The 
susceptor includes a conically shaped flow section which extends across 
the flow passage, and a plurality of flow ports for passing the material. 
The susceptor further includes a cylindrical section which extends 
downstream from the flow section for receiving the material from the flow 
section and passing material to the dispensing orifice. The induction coil 
is aligned with and spaced downstream from the flow section of the 
susceptor, surrounding part of the susceptor for electromagnetically 
inducing electric currents to flow within the flow section. The induced 
electric currents are substantially uniform across the flow section to 
provide a substantially uniform thermal transfer from the flow section to 
a melt face for the material. The flow section has a limited heat capacity 
such that the flow section will not contain an amount of heat sufficient 
to significantly raise the temperature of the material adjacent to the 
flow section when the electric currents are stopped, preventing thermal 
transfer from the susceptor to a significant portion of the material 
beyond the melt face.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 is a side elevational view of hot glue gun 11 of the present 
invention. Gun 11 is used for heating, liquefying and dispensing solid 
sticks of glue which nominally measure two (2) inches in diameter and 
eight (8) inches in length. Gun 11 has a body 13 and a nozzle tip 15. Grip 
handle 17 is provided for holding gun 11, and includes a trigger type of 
button 19 for controlling heating and dispensing of the hot glue. Power 
cord 21 extends from handle 17 and connects to power supply 23, which 
preferably is a 110 volt AC source. 
Feed assembly 25 provides a means for pushing a glue stick into nozzle tip 
15. Feed assembly 25 includes a stepper motor 27 which is connected by 
means of gear 29 to rack 31. Stepper motor 27 and gear 29 are mounted to 
driven member 33, which is moved in direction 34 within cavity 35. An 
intermediate position for driven member 33, stepper motor 27 and gear 29 
is depicted in FIG. 1. A rearward position 36 is depicted in phantom for 
driven member 33, stepper motor 27 and gear 29. Glue stick 37 is placed in 
cavity 35, forward of driven member 33. Glue stick 37 has a forward end 39 
for pressing into nozzle tip 15. Stepper motor 27 is actuated to move 
driven member 33 forward in direction 34, from position 36 to the 
intermediate position depicted in FIG. 1. This presses the forward face 39 
of glue stick 37 into the rearward end of susceptor 53. 
FIG. 2 is a sectional view depicting nozzle tip 15 in more detail. Nozzle 
41 is formed from aluminum and has a dispensing orifice 43. A housing 45 
of a plastic material, such as teflon, extends rearward of nozzle 41, and 
has a conical shape. A cylindrical member 47 extends rearward of housing 
45. Nozzle 41, housing 45, and cylindrical member 47 together define a 
central housing 49 having interior bore 51. Bore 51 provides a flow 
passage for passing glue through housing 49. 
Susceptor 53 extends within housing 49, across a rearward section of bore 
51. Susceptor 53 includes a conical flow section 55, having a thin cross 
section with a heat capacity which is not substantially greater than a 
thin section of the material extending across the melt face at forward end 
39 of glue stick 37. Conical flow section 55 has an outer diameter of two 
(2) inches. Holes 57 extend through the rearward portion of susceptor 53 
to provide flow ports through flow section 55. Holes 57 are parallel to 
central longitudinal axis 58. 
FIG. 3 is a sectional view taken along section line 3--3 of FIG. 2, and 
depicts holes 57 extending through the conically shaped, rearward facing 
end of susceptor 53. In this embodiment of the present invention, 
approximately 51% of the rearward facing surface end of susceptor 53 is 
holes, providing a reduced heat capacity for susceptor 53. The solid 
portion 60 of the conically shaped, rearward facing end of susceptor 53 
contacts forward face 39 of material 37 to define a melt face. The melt 
face also extends within holes 57 when solid material is pushed into holes 
57. Thus the effective heat transfer surface area for susceptor 53 at the 
melt face includes both solid portion 60 of the rearward facing end of 
susceptor 53 and at least a portion of the periphery of holes 57. 
Referring to FIG. 2, susceptor 53 further includes cylindrical section 59 
and thermal transfer member 61. In the preferred embodiment, flow section 
55 and cylindrical section 59 are formed from various materials within 
which an electric current can be electromagnetically induced to flow. 
Thermal transfer member 61 is formed from a non-ferrous material, and 
provides a means for transmitting electromagnetically induced heat forward 
from the rearward portion of flow section 55 so that restarting of glue 
flow from gun 11 can be more quickly accomplished than if member 61 were 
not included. The components of susceptor 53 may be formed of other 
materials, so long as flow section 55 is formed from materials within 
which may be electromagnetically heated by inducing eddy currents to flow 
therein. 
The exterior of cylindrical section 59 is threaded. The rearward end of 
nozzle 41 is threaded and secures to cylindrical section 59, and the 
forward end of housing 45 is also threaded for coupling to cylindrical 
section 59. Cylindrical section 59 will conduct high frequency electric 
current from flow section 55 to nozzle 41, which is also conductive. 
Annular space 63 extends between cylindrical section 59 and thermal 
transfer member 61 of susceptor 53. Four flow ports 65 and four flow ports 
67 extend through cylindrical section 59 to connect annular space 63 to 
annular space 69, which extends between housing 45 and flow section 55. 
Flow ports 65, 67 are offset both angularly and longitudinally along a 
central axis for central housing 49. Annular space 69 has a conical shape, 
which extends with a narrower width at outermost portion 71 than at inner 
portion 73. Inner portion 73 is wider to provide a constant cross 
sectional flow area per unit amount of glue flowing through annular space 
69. Annular space 69 is formed between housing 45 and flow section 55 of 
susceptor 53. The forward face of flow section 55 is at a 45 degree angle 
to central longitudinal axis 58 for flow passage 51 in central housing 49. 
The interior, conically shaped surface of housing 45 is at a 30 degree 
angle to longitudinal axis 58 for flow passage 51 and central housing 49. 
Induction coil 75 is conically shaped and located within conically shaped 
annular space 69. Forward end 77 of coil 75 is welded to the forward end 
for flow section 55 of susceptor 53. Wire 79 extends from the rearward end 
of coil 75 to electrically connect coil 75 to power supply 23 (shown in 
FIG. 1). Wire 81 extends through housing 45 to ground screw 83 and nozzle 
41. This provides an electrical connection for connecting power supply 23 
to the forward end 77 of coil 75, which is welded to susceptor 53. 
Susceptor 53 will conduct the high frequency current to nozzle 41 and 
ground screw 83. 
FIG. 4 is a schematic diagram depicting an electromagnetic circuit which 
includes power supply 23, susceptor 53 and induction coil 75. Power supply 
23 includes high frequency power supply 85 which is connected by means of 
power cord 21 to an external power source. Power supply 23 nominally 
operates at frequencies of 50 kHz, with the frequency typically being 
lowered for susceptors of larger dimension, and can be powered from a 20 
amp 110 volt a.c. outlet. Transformer 87 is electrically connected between 
high frequency power supply 85 and induction coil 75 by means of wires 79, 
81. Thermocouple 89 is provided for controlling the temperature of 
susceptor 53. Power supply 23 has a variable temperature set point for 
accommodating glues of different melting temperatures. 
Referring to FIG. 2, in operation, high frequency electrical current 
flowing through induction coil 75 causes an electromagnetic field, 
depicted as the lines of electromagnetic flux 91 passing through susceptor 
53. Electromagnetic flux 91 causes eddy currents to flow within susceptor 
53, which generate heat. The forward end 39 of glue stick 37 is pressed 
inward to susceptor 53 by feed assembly 25 (shown in FIG. 1). This causes 
the end face 39 of glue stick 37 to melt and flow through ports 57 into 
conically shaped annular space 69. The melted glue then flows from annular 
space 69 through flow ports 65, 67, into cylindrically shaped annular 
space 63, and through dispensing orifice 43 of nozzle 41. Melted glue 
flowing past induction coil 75 removes heat from coil 75, cooling coil 75. 
It should be noted that the cross-sectional flow area for the total 
combined flow ports 57 in susceptor 53 is equal to the effective 
cross-sectional flow area of annular space 69, flow port 65, 67, and 
annular space 63 after coil 75 and susceptor 53 are installed within 
central housing 49. This prevents flow restrictions from occurring as the 
melted glue passes through flow passage 51. 
It should be noted that after holes 57 are formed into flow section 55, the 
heat capacity for flow section 55 is limited such that it is capable of 
only containing enough heat for melting only a very fine, thin layer of 
the face 39 of glue stick 37. The low heat capacity for flow section 55 
will not contain an amount of heat sufficient to raise the temperature of 
a significant portion of the glue material adjacent to the flow section 
beyond the melt-phase transition temperature, that is beyond the 
temperature at which the glue melts. This provides for a very finely 
controlled, thin melt face for glue stick 37. Thus, once the high 
frequency electric current is turned off from flowing within induction 
coil 75, the glue at melt face 39 almost immediately stops melting. 
Cylindrical section 59 is formed from a ferrous material and receives some 
of the electromagnetic field flux 91 from induction coil 75. This causes 
eddy currents to flow in cylindrical section 59, generating heat for 
transferring to the material adjacent to section 59 in annular space 63. 
Additionally, thermal transfer member 61 transfers heat to the glue within 
annular space 63 to help liquefy the material to initiate flow as glue gun 
11 is cycled back on to dispense more glue through orifice 43. Heat from 
coil 75 and heat induced within flow section 55 will quickly liquefy any 
glue that solidifies within annular space 69 when gun 11 is cycled off. 
Other embodiments of the present invention may be made for heating and 
dispensing materials. It should be noted that in other embodiments of the 
present invention, susceptors may be made from materials other than 
ferrous materials, such as ceramic and carbon materials capable of having 
electric currents induced to flow therein. One such example is a susceptor 
having a carbon core which is coated with silicon carbide. Such materials 
will allow use of the present invention at temperatures which are much 
higher than those for melting glue. 
The present invention provides several advantages over prior art devices 
for heating and dispensing materials, such as glue. The present invention 
provides a very finely controlled, thin melt face transition by providing 
a susceptor having a low heat capacity so that any thermal transfer from 
the susceptor to the melt face will be quickly absorbed by the adjacent 
material at the melt face. Also, the induction coil according to the 
present invention surrounds and extends along a portion of the susceptor 
so that uniform currents can be generated across different sections of the 
susceptor. The induction coil is within a flow passage and immersed within 
the material to both cool the induction coil and use heat which is 
normally lost by exteriorly mounted induction coils. Additionally, a 
thermal transfer member extends forward of the flow section of the 
susceptor for transferring induced heat forward to improve recovery times 
when material flow is cycled back on. 
Although the invention has been described with reference to a specific 
embodiment, this description is not meant to be construed in a limiting 
sense. Various modifications of the disclosed embodiment as well as 
alternative embodiments of the invention will become apparent to persons 
skilled in the art upon reference to the description of the invention. It 
is therefore contemplated that the appended claims will cover any such 
modifications or embodiments that fall within the true scope of the 
invention.