Abstract:
A dispenser heats and dispenses a melted material through an orifice. A pusher is slidably received within an interior of a cylindrical body of the dispenser and is movable relative to the forward end of the cylindrical body. The pusher is made up of an internally threaded cylinder that receives a driver screw and engages a stick of meltable material. The pusher is used to advance the meltable material toward the forward end of the body. The pusher retracts when the meltable material is exhausted. An induction heated system has a susceptor plate with apertures that lead to the nozzle. In use, the motor automatically retracts the pusher a slight increment when the motor is turned off, which relieves static pressure on the elastic zone of the stick. A PC board has a control circuit to control the advancing and retracting of the pusher and for controlling the temperature of the inductor by regulating the flow of power to the inductor.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefits of provisional application serial No. 60/104,365, filed Oct. 15, 1998, in the United States Patent &amp; Trademark Office. 
    
    
     TECHNICAL FIELD 
     A method and apparatus for delivering melted material. More particularly, the apparatus is a glue gun utilizing a method of delivery of molten glue without unwanted drips and at a controlled temperature. 
     BACKGROUND OF THE INVENTION 
     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 liquified 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 housing 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. 
     A difficulty with prior devices is that once the meltable materials have been melted and dispensed, it is difficult to cease flow of the meltable material without additional and unwanted drips emerging from the nozzle. The additional flow is partially due to a large orifice in the nozzle and to an area of high pressure resulting from compression of the meltable material from the pusher used to force a material towards the heating elements. 
     SUMMARY OF THE INVENTION 
     A method and apparatus are provided for heating and dispensing a melted material. A glue gun has a cylindrical body with a trigger mechanism provided on an under side of the body. A motor is located in the interior of the body at a rearward end of the cylindrical body. A gear head is operatively connected to a forward end of the motor. An externally threaded driver screw is rotationally connected to the gear head. A pusher is slidably received within the interior of the cylindrical body and is movable relative to the forward end of the cylindrical body. The pusher is made up of an internally threaded cylinder that receives the driver screw and has an end surface on its forward end that engages a meltable material such as glue, preferably in stick form. The pusher is used to advance the meltable material toward the forward end of the body. The pusher retracts when the a stroke limit of the pusher is reached and the meltable material is substantially exhausted. 
     A nose assembly is positioned on the forward end of the cylindrical body and is made up of a conical housing, a conical inductor, a conical susceptor, and a nozzle positioned within a central orifice of each of the conical members of the nose assembly. The inductor is preferably a coil that surrounds the susceptor for heating the susceptor. The nozzle permits a flow of a meltable material through a plurality of peripheral passages. The peripheral passages are sized to permit a flow of meltable material under pressure but not to permit a flow of material when not under pressure. 
     The induction heated system of the invention is dripless and operates without a valve for several reasons. A main reason is that the motor automatically retracts the pusher a slight increment when the motor is turned off, which relieves static pressure on the elastic zone. The expansion of the compressed zone moves the stick back instead of pushing liquid material downstream. Second, the initial start up heat mass is maintained to be as low as possible to shorten the time from trigger pull to material delivery, preferably less than two seconds. Third, the latent heat mass is minimized to diminish “off” cycle melting at the stick melt phase. 
     An electrical cable connects the inductor with a power source. A PC board has a control circuit to control the advancing and retracting of the pusher and for controlling the temperature of the inductor by regulating the flow of power to the inductor. 
     The control circuit also automatically detects whether a newly loaded stick of meltable material, such as glue, is positioned within the cavity of the cylindrical body. If so, the pusher is advanced to lock the stick in the cavity. The control circuit will allow full retraction of the pusher only after the chamber is empty or when the pusher is at full stroke. The control circuit automatically detects when the pusher is at full stroke and ceases delivery of power to the inductor and returns the pusher to the reload position. Additionally, the control circuit is designed to determine whether an amount of current is being used that indicates a resistance to pusher movement provided by a loaded stick and whether the thermocouple is in operation. If either of these conditions is not satisfied, then power is automatically shut off to the inductor. Otherwise, the inductor is heated and the pusher is advanced to force liquid meltable material out of the nozzle. If the temperature of the inductor is above a target temperature, then power will be shut off to the inductor and cycled on and off to maintain the approximate target temperature. When the trigger is released, power is shut off to the inductor and the pusher is retracted a slight increment to relieve static pressure on the elastic zone. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side cross-sectional view of a glue gun of the invention, wherein the pusher is partially advanced. 
     FIG. 2 is an exploded cross-sectional view of the glue gun of FIG.  1 . 
     FIG. 3 is an enlarged cross-sectional view of the nose assembly of the glue gun of FIGS. 1 and 2. 
     FIG. 4 is a flow diagram of the logic associated with the method of operation of the glue gun of the invention. 
     FIG. 5 is a schematic diagram of circuitry used to control the glue gun of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIGS. 1-3, a glue gun designated generally  10 , is shown. Glue gun  10  is used for heating, liquefying and dispensing meltable material, preferably solid sticks of glue that typically measure one inch in diameter and three inches in length. Glue gun  10  has a body  12 , which is preferably approximately cylindrical in shape and is made up of a top half  14  and a bottom half  16 . Body  12  has a forward end  18  and a nose assembly  20 . A trigger mechanism  22  controls heating and dispensing of the hot glue. A power cord extends from body  12  and connects to a power supply (not shown), which is preferably a 110 volt AC power source. Power is preferably controlled by a power supply PC board (FIG.  5 ). 
     Pusher  24  provides a means for pushing a glue stick towards nose assembly  20 . Pusher  24  is slidably received within an interior cavity  26  of body  12  and has a forward end  28  and a rearward end  30 . When the pusher  24  is fully retracted, cavity  26  is accessible for loading a glue stick or other meltable material (not shown). The pusher  24  is made up of an internally threaded cylinder  32  having internal threads  34  and an end surface  36  for engaging a meltable material and advancing the meltable material toward the nose assembly  20 . The pusher  24  is advanced and retracted by an externally threaded driver screw  38 , which engages internal threads  34  of internally threaded cylinder  32 . Externally threaded driver screw  38  is provided with external threads  40 . The externally threaded driver screw  38  is rotated by motor  42 , which is preferably a 24 volt electric motor. Motor  42  receives power by a power cord (not shown). Motor  42  is operatively connected to gear head  46 , which is affixed to externally threaded driver screw  38 . 
     Nose assembly  20  is affixed to a forward end  18  of body  12  and may be seen in greater detail in FIG.  3 . Nose assembly  20  is made up of a conical housing cone  48  having a central orifice  50  formed therein. A conical inductor  52  is received within the conical housing  48 , which defines a central orifice  54 . Preferably, a low resistance coiled inductor is used for efficiency. A conical susceptor  56  is received within the conical inductor  52  and has a plurality of holes  58  formed therein and defines a central orifice  60 . Preferably, susceptor  56  is fabricated from a 22 gage low carbon steel perforated sheet that has a surface area of 3.2 square inches and a weight of 0.130 oz. The high ratio of surface area to weight provides a rapid transfer of energy from the susceptor  56  to the meltable material while minimizing latent heat when energy transfer is stopped. Additionally, the susceptor design speeds the initial flow and successive flow recoveries. In this embodiment, the susceptor  56  is constructed with a secondary element, a steel conical housing  48 , designed specifically to contain radio frequency emissions. 
     A nozzle  62  is positioned within central orifices  50 ,  54  and  60  to deliver melted material for a user&#39;s application. The nozzle  62  is provided with a plurality of peripheral passages  64  that are sized to permit flow of meltable material under pressure, but prevent flow of melted material that is not under pressure. Most flow through the nozzle enters through the peripheral passages  64 , since peripheral passages  64  communicate with an area that defines a gap between the susceptor  56  and conical housing cone  48 , which contains most of the melted material. Although a small amount of material enters through passage  60 , most of the material in this area is not melted enough to reduce the viscosity of the material sufficiently to enable flow into passage  60 . The dripless “off” cycle is achieved by first relieving elastic pressure at the melt phase  63  in the upstream or rearward direction, and second by minimizing a volume above the orifice in any gun position. Preferably, the gap between the susceptor  56  and conical housing cone  48  at the apex is approximately 0.060″. Thirdly, the dripless “off” cycle is achieved by passing the liquid material through a plurality of small peripheral passages  64  at the entry of the delivery passage in nozzle  62 . The aggregate area of peripheral passages  64  needs to exceed the delivery orifice area so that the peripheral passages  64  do not impede the volume delivery at the design pressure resulting from force applied by the pusher  24 . The combination of the motor  42  and gear head  46  results in a motor gear head speed/torque combination that provides an adequate force to a 1″ diameter stick face to deliver 8#/hr of a specified viscosity material through a perforated susceptor and a variable diameter delivery nozzle. The force on the pusher  24  is not to exceed the ability of the continuous high frequency power available at the melt phase to raise the temperature of the stick to a design point (preferably 400° F.). The force on pusher  24  should also not exceed a level of safety with respect to a possible finger pinch point in the open cavity  26  of the body  12 . The peripheral passages  64  need to be small enough in individual size to provide a capillary action for the static liquid hot melt, which typically has a 2,000-6,000 CPS viscosity at the delivery temperature. Preferably, peripheral passages  64  are small holes drilled perpendicular to the nozzle axis. 
     A power cable is provided in the bottom half  16  of body  12  (FIG. 1) for providing power to inductor  52 . A PC board  66  (FIGS.  2  and  5 )has electronics for controlling whether power is delivered to motor  42  for controlling the advancing and retracting of pusher  24 . PC board  66  also controls whether power is delivered over the power cable for controlling the heating of inductor  52 . A load position magnet  68  is provided on a forward end  28  of pusher  24  and a reverse position magnet  70  is provided on a rearward end  30  of pusher  24 . A Hall effect sensor  72  (FIGS. 2 and 5) is provided on PC board  66  to detect when load position magnet  68  or reverse position magnet  70  is in a position proximate Hall effect sensor  72  to determine whether pusher  24  is in a fully extended or fully retracted position. Hall effect sensor  72  is a magnetic sensing switch that may be obtained from Allegro Microsystems, Incorporated and available as part number UGN3235K Hall effect sensor  72  directs pusher  24  to advance or retract in accordance with the method of operation described below. 
     Referring now to FIG. 4, in operation, when an operator pushes trigger mechanism  22 , the electronics on PC board  66  of glue gun  10  determine that trigger mechanism  22  is being pushed as represented by step  100 . If it is determined that trigger mechanism  22  is being pushed, then a determination is made as to whether a newly loaded stick of meltable material, such as glue, is positioned in cavity  26  as represented by step  102 . If so, motor  42  is directed to advance pusher  24  to lock in the glue stick. It is preferable that a newly loaded stick is locked into cavity  26  so that glue gun  10  can be immediately operated in an inverted position without having the glue stick fall out. If it is determined in step  102  that a newly loaded stick is positioned within cavity  26 , then pusher  24  is advanced to lock in the glue stick, as represented by box  103 . If it is determined in step  102  that a newly loaded stick is not positioned within cavity  26 , then a determination is made as to whether pusher  24  is positioned at full stroke, as represented by step  104 . If pusher  24  is positioned at full stroke, then the glue stick has been extinguished. Preferably, the glue gun  10  is designed such that a partially expended stick may not be removed from cavity  26 . By removing a partially expended stick, hot material at the stick face may cause injury to an operator who is attempting to remove the stick. Additionally, by removing a stick, melted material in cavity  26  will subsequently cool, which may prevent the insertion of additional sticks. 
     If it is determined in step  104  that the pusher  24  is at full stroke, then power is no longer delivered in power cable  44  so that inductor  52  is no longer being heated. Additionally, motor  42  is directed to retract pusher  24  to the fully retracted or reload position as indicated in step  106 . If it is determined that the pusher  24  is not at full stroke in step  104 , then a determination is made whether an amount of current being used by motor  42  is indicative of the presence of resisting pressure provided by a loaded stick, as represented in step  108 . The presence of a stick in the cavity  26  needs to be sensed on each operation of gun  10  to prevent high frequency power from being delivered in the absence of a continuous load. If power were delivered to inductor  52  in the absence of a stick, then the stick “heel” remaining in the cavity  26  adjacent to susceptor  56  would melt back into cavity  26 . 
     If it is determined that the current being used indicates that a stick is not present, as represented in box  108 , then power is shut off to inductor  52  as represented by box  110 . If it is determined that a stick is present in step  108 , a determination is made as to whether the thermocouple has failed as represented by box  112 . If the thermocouple has failed, then power is shut off to inductor  52  as represented by box  110 . If the thermocouple is operational, then the glue gun  10  delivers power to heat inductor  52  as represented by box  114 . Additionally, power is provided to motor  42  to advance pusher  24  as represented by box  116 . A determination is then made whether the temperature of susceptor  56  is above a predetermined target temperature as represented by box  118 . The temperature of material at the hottest point of susceptor  56  should not exceed the melt delivery temperature as the nozzle  62  is initially cleared of the frozen plug in the fast start up of the system. If the temperature is above the target temperature, as determined in step  118 , then power is shut off to inductor  52  as represented by box  110 . The power is subsequently cycled on and off to maintain the temperature at approximately the target temperature as represented by steps  100 - 118 . 
     If the temperature is not above the target temperature, as determined in step  118 , then a determination is made whether trigger mechanism  22  has been released as represented by box  120 . If the trigger mechanism  22  is determined not to have been released, then glue gun  10  continues to operate as represented by boxes  104 - 120 . If it is determined in step  120  that the trigger mechanism  22  has been released, then power is shut off to inductor  52  as represented by box  122  and pusher  24  is reversed approximately 0.02 inches to relieve pressure on the material in the elastic zone. 
     Referring now to FIG. 5, a circuit diagram of a power supply PC board is shown. The timers of dual timer U 8  feed an electrical signal to a two input quad X-OR gate U 5 . Preferably quad X-OR gate U 5  is set up such that 1−0=1/0−1=1. The timers of dual timer U 8  communicate with quad with X-OR gate U 5  to create a positive output to motor controller U 11  to switch motor  42  (FIGS. 1 and 2) in reverse for either a predesignated period of time, preferably 0.026 seconds, to relax the stick compression zone after each release of trigger mechanism  22 , or 1.9 seconds for a full reversal of pusher  24 . Both timers of dual timer U 8  are set by changing the timers associated resistor/capacitor combinations. In the preferred embodiment, resistor R 19  is a 500 ohm/C 19  47 uF set at 0.026 seconds at line U 8 - 9  to relieve pressure. Resistor R 18  is preferably set at 17.2K/C 18  100 uF=1.9 seconds at line U 8 - 5  for full reversal. The output at line U 5 - 4  is matched with another positive signal in quad AND gate that is taken negative to stop the reverse of motor  42  (FIGS. 1 and 2) when Hall effect switch U 14  is actuated by load position magnet  68  (FIGS.  1  and  2 ). This action stops motor  42  during the timing cycle at the reload position or fully open position prior to a stall at the end of the rack or screw. 
     When the normally opened trigger mechanism  22  is closed to ground line, U 7 - 1  goes negative and with its permanently negative line U 7 - 2  makes line U 7 - 3  positive. This action does two things. It makes line U 6 - 5  positive and with the normally positive line U 6 — 6  (line U 7 - 5  is not receiving a positive reverse signal from the 1.9 second one shot) makes line U 6 - 4  positive, switching motor controller U 11  on for forward travel. 
     The positive line U 7 - 3  on trigger action also holds line U 8 — 8  positive during forward travel. When trigger mechanism  22  is released, line U 7 - 3  goes negative causing timer output line U 8 - 9  to deliver a 0.026 second pulse positive to line U 5 — 5  making line U 5 - 4  positive to switch motor controller U 11  into reverse for 0.026 seconds to relieve the pressure on the compressive solid/liquid interface in the interior cavity  26  (FIGS.  1  and  2 ). 
     The Hall effect sensor U 14  (FIG. 5) or  72  (FIGS. 1 and 2) is actuated by the reverse position magnet  70  (FIGS. 1 and 2) at the extent of the forward travel of pusher  24 . The normally open contact of Hall sensor U 14  closes to the ground line, placing a negative signal on line U 8 - 6  which sends a 1.9 second output to U 5 - 6  to initiate a motor controller U 11  reverse. It also sends a signal via line U 7 - 5  to interrupt forward travel of pusher  24  if the operator continues to hold trigger mechanism  22  in a closed position by making line U 6 - 4  negative. 
     If the operator has not released trigger mechanism  22  at the end of the reverse cycle, the unit will cycle forward again without heat and will continue to do so until trigger mechanism  22  is released. Automatic reverse at the full stroke at pusher  24  is the only reverse of glue gun  10 . This is designed to make the removal of a partially used stick difficult, since removal of a partially used stick can lead to performance difficulties. 
     A negative signal from line U 7 - 10  to shut down circuits of the bridge drivers U 2  and U 3  gates the power bridge to heat susceptor  56  (FIGS.  1  and  2 ). This action requires a negative signal on line U 7 - 12  reporting that an amount of current is being used in the motor circuit that would indicate the resistance force of a loaded stick. The lack of this signal prevents the melting or softening of the remaining stick heel if a stick is not loaded within chamber interior cavity  26  (FIGS.  1  and  2 ). It also requires a negative signal from thermoouple controller U 10  to the companion input line U 7 - 13 . The signal over line U 7 - 13  will oscillate to hold a predetermined susceptor set temperature as the glue stick is advanced by pusher  24  (FIGS.  1  and  2 ). 
     These actions are reported as a positive signal to the line U 6  AND gate via line U 6 - 1  along with a positive trigger signal over line U 6 - 2 . This signal is inverted at U 7 - 8 / 9 / 10  to allow the drivers U 2  and U 3  to gate the power circuit. The release of the trigger mechanism  22 , expiration of the stick, exceeding the target temperature of susceptor  56 , or failure of the thermocouple U 10  will stop the heat. 
     This invention has several advantages. Dripless operation without a valve is accomplished by providing specially sized peripheral passages. Melted material is only delivered when a specified amount of pressure is experienced by the melted material. Additionally, the automatic retraction of the pusher upon release of the trigger mechanism relieves the pressure and prevents unwanted melting of the glue stick. The combination of the above described mechanical features with the electronic control system of the invention provides dripless automated operation, precise product temperature and improved operator safety. 
     While the invention has been shown in only one of its forms, it should be apparent to those skilled in the art that it is not so limited, but is susceptible to various changes without departing from the scope of the invention.