WO1999018027A1

WO1999018027A1 - Bulk system for hot melt adhesive application

Info

Publication number: WO1999018027A1
Application number: PCT/US1998/020925
Authority: WO
Inventors: Peter S. Melendy; Richard A. Belanger; Charles R. Sperry
Original assignee: Adhesive Technologies, Inc.
Priority date: 1997-10-08
Filing date: 1998-10-08
Publication date: 1999-04-15

Abstract

A system for the supply of hot melt adhesive includes a heating tank (2) for holding, and heating a large amount of glue (3). The tank (2) is maintained at a temperature whereby the glue (3) is liquid but does not char. A heat grid (36) is located in the bottom of the tank (2) to heat the glue (3) to the application temperature only when the glue is being applied. The liquid glue is supplied to a double acting pump (18) that is heated and then to a heated hose (60). A heated nozzle at the end of the hose (60) is operated by the user to dispense the glue. A control circuit (11) controls the operation of various heating elements to maintain the set temperatures.

Description

BULK SYSTEM FOR HOT MELT ADHESIVE APPLICATION

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of United States Provisional Application serial number 60/061 ,353, which was filed on October 8, 1997.

TECHNICAL FIELD

This invention relates to the art of hot melt adhesive systems. In particular, this invention relates to a bulk-type hot melt adhesive system.

BACKGROUND ART

Known hot melt glue systems are provided with two general types of glue supply. The first type of system is designed to be hand-held and has a heating element for melting the glue provided to the gun in discrete sticks or slugs, depending on the particular design. This type of gun is usually used for smaller tasks, and the amount of giue held in the liquid state is small. Moreover, the glue sticks or slugs are not large and must be regularly replaced. Another type of glue system is known as a bulk system. This type of system is directed toward those uses that require a larger output of glue as a function of time, e.g. five or more pounds per hour, and the amount of glue held in the liquid state is much larger. Glue in the bulk systems is supplied from a chamber that is capable of melting the larger amount of glue at one time and the melted glue is then pumped to a dispensing nozzle.

Known bulk systems use heated glue pots that maintain the glue in a liquid state. These systems generally suffer from the disadvantage that the glue in the pot chars over time, and the char becomes entrained in and contaminates the liquid glue upon application. Thus, there is a need for bulk systems that avoid charring of the melted glue.

The hand held glue systems are generally of lower cost, while the bulk systems are more complicated and expensive. Applicant's have determined, however that there is a need also for a hot melt glue system that provides the advantages of bulk systems, but at a lower cost. SUMMARY OF THE INVENTION

In accordance with a preferred embodiment of the invention, a bulk melting and delivery system is capable of delivering a relatively large amount of melted glue (e.g. eight to ten pounds per hour). The system comprises a melting reservoir, a unique perforated, high-wattage melting grid, and a precision temperature control system for each that provides efficient melting in a manner that prevents charring of the glue. The reservoir may be of a variety of materials, but the preferred is cast aluminum.

Charring occurs through burning or oxidation of the glue and is a problem because semi-solid particles of charred adhesive contaminate the glue and cause problems, for example, by clogging the nozzle, check valves and the like. And, with time, this built up layer of oxidized adhesive affects the efficiency of the melting because it becomes a thermal insulator between the heated surfaces of the tank and the solid adhesive. Many conventional adhesive systems provide for excellent melt capacities and excellent delivery methods but do not adequately protect against charring.

The prevention of charring while maintaining a ready reservoir of melted glue is a primary objective of this invention and is accomplished though a combination of features that allow glue to be maintained in a reservoir tank such that it is essentially solid in the upper part of the tank, where new glue "slabs" are supplied to the tank, and melted in the lower part of the tank, where the liquid glue is drawn off. By this arrangement, the upper surface of the melted glue is mostly in contact only with the solid glue and is, thus, protected from contact with air. As well, the temperature control system maintains the temperature of the glue below that at which char occurs. This temperature profile is possible by providing a heated tank and a further heating element in the lower part of the tank whereby the temperature of the glue is higher at the bottom and decreases to be that of the solid glue at the top. Thus, relatively high temperatures may be obtained in the lower part of the tank, adjacent the outlet, and cooler temperatures that preclude charring are obtained in the remainder of the tank.

In the preferred embodiment, the invention includes a holding tank that is approximately 10 ^Λ inches long by 10 ^ΛA inches wide and 4 inches deep. Other shapes and sizes will be useful, but this shape and size have been found to be useful. The tank is designed to receive solid, formulated glue "slabs" that are approximately 10 inches square and about 5/8 inch thick, thus providing about one kilogram (2.2 lb.) of glue in each slab.

The tank is accurately heated to a temperature that is just greater than the melting point of the adhesive. In the case of a well known packaging hot melt formulation which is best extruded at 300° F, the tank temperature is maintained at about 220° F. A perforated melting grid is located within the tank and designed to minimize heat exchange with the tank. This grid is approximately 9 /₂ inches square in plan and tapers downward toward a center cylindrical section at an angle of about 10°. The grid includes a heater element for heating the grid when liquid glue is being pumped from the tank and "cold" glue contacts it and cools it below its set point. Typically, the temperature of the melt grid is controlled to be less than about 300° F.

A ball check assembly is attached to the outlet of the tank between the tank and a pump assembly that will be described further below. In the preferred embodiment, the ball check assembly is not a true ball check because a gap (about 1/8") is provided intentionally between the top of the ball and its seat. This gap facilitates the filling of an empty, or cavitated, machine by allowing liquid glue to flow into the pump even in the absence of pumping activity. When the pump draws liquefied glue from the tank, however, this ball check valve, which is already partly open, opens yet further to allow a free flow of glue from the tank in the required volume. The pump is preferably a double acting pump, and when the pump stroke applies pressure to the valve, the pressure causes the ball to engage the seat, thus sealing it and forcing glue to flow from the pump. An alternative to providing the gap is to provide a very weak spring for the ball, but the gap is preferred because it allows the glue to flow into the pump by gravity which assists initial filling of the machine.

The timing and control of the heating of the tank, grid and other parts of this invention are important to its proper functioning. This "logic" will be described in detail below.

A double-acting, air-driven pump is fitted to the bottom of this tank assembly to pump the melted glue through a hose and to a nozzle. The glue side of this pump uses a piston of relatively small diameter (e.g., 7/8") that is driven by a commercially-available air cylinder having, for example, 2" bore and a 3" stroke. The general structure of this pump is known in the art, and double-acting feature on the glue side provides, essentially, an uninterrupted flow of adhesive during pumping. The piston itself includes an integral ball check in its center so that when the pump reverses direction, the glue is pumped through the piston rather than ahead of it. This air/glue pump combination is designed to have a considerable pressure multiplier, in the order of about 8:1 based on the respective cross sectional areas of the pistons. This high ratio is preferred to reduce the air pressure required to drive the piston to a reasonable level, e.g., about 50 psi., while maintaining an adequate glue pressure (about 400 psi) for extruding the liquid glue with the desired force.

While logic designed into the machine must be satisfied before pumping can be initiated, once the machine is pumping, the air pump is automatically and continuously cycled by use of an outrigger magnetic piston, which actuates two reed-type limit switches. These switches provide electric signals that are supplied to a 4-way air solenoid valve, which controls the direction of piston travel. This cycling function could be mechanical as well, but electrical signaling is best because of the simplicity with interfacing with the electrical "logic control board".

As the glue exits the double acting pump, it is directed to a heated hose that is preferably about 8 feet in length. This hose is a commercially available 5/16" I.D. hydraulic hose with stainless steel braid over a Teflon tube core. Over this heated core is a 3/8" inch thick tubular, silicone rubber foam insulator and a braided polyester cover. The hose is heated with a length of high-temperature, coated resistance wire formed in a single piece, but doubled back in a loop. A length of 16 feet is used to provide an 8-foot length when doubled back. This wire is snaked through the empty hose and connected electrically to a voltage limiting circuit very similar to a household dimmer switch. The limiting switch does not "clip" voltage, per se, but varies the cycling rate. This voltage allows use of wire of reasonable size, i.e., one that provides mechanical strength adequate to endure the stress and strain of being in direct contact with high-pressure, fast- flowing melted adhesive. Most heated hoses commercially available use external wraps of resistance wire rather than heated wire within the hose. This temperature of the heated hose is ultimately controlled by the control circuit board.

A pressure-relief valve is provided to protect against excessive pressure buildup in the pump, as by thermal expansion when heating. This valve is fitted to the bottom of the holding tank and is calibrated for about 1000 psi when the operating pressure of the pump is 400 to 500 psi. This valve is, seldom, if ever, actuated, as when the adhesive cools it shrinks, thus providing a place to expand upon subsequent expansion during heat up. But, in the event that high air pressure is inadvertently fed to the pump, with its multiplier effect, this pressure relief valve will prevent destruction of the hose, for example, or the possibility of a dangerous "explosion" of hot adhesive.

An independently-heated and controlled spring-biased, mechanically actuated nozzle assembly is connected to the other end of the hose. Flow through the nozzle is controlled by engagement between a conical tip of an inlet tube and a conical bore of a nozzle tip that is movable with respect to the inlet tube. This basic construction is generally conventional in design, and the actuation of the nozzle is accomplished by a lever which fits between a stop on the inlet tube and the movable nozzle tip to provide a mechanical advantage that renders the nozzle relatively easy to open for the user. This lever is covered with an insulated arm to protect the user's fingers from the temperatures of the nozzle. The temperature of the nozzle is controlled by the control circuit board.

The tank provides a letter-slot like opening to allow one-by-one insertion of the "slabs" of adhesive, when necessary. A switch senses the glue level and provides a signal when additional slabs are needed.

The unit preferably operates on ordinary household voltage and current (120 vac, 15 amp), and uses about I cfm at 50 psi, which is relatively low air consumption.

The electronic control board is important to the efficient functioning of this bulk glue system. In accordance with one aspect of the invention seven temperature control zones are identified in the machine, and they are controlled by a single printed circuit board. The controls utilize a commercially available Zero Voltage Switch IC with NTC thermistor inputs specifically designed for AC temperature control. The temperature for each zone can be maintained within a few degrees F, and the design is inherently adjustable The seven zones are: the melt grid, the tank, the pump, two hoses (one or two could be provided), and two nozzles (one for each hose). In other embodiments any number of hoses can be used.

When the operator first activates the unit, all zones immediately begin heating for an initial period of 15 to 20 minutes. During this initial period, which is adjustable from zero to about 20 minutes, full power is delivered to every zone, including the melt grid. During this time, referred to as the "Power-Up Delay Phase," the machine is heating each zone to normal operating temperature. The circuit is switched to allow power to be delivered to the electrical 4-way solenoid valve only after the tank reaches the predetermined temperature, Thus, the machine cannot pump adhesive until the tank has reached the required temperature.

An important aspect to the proper functioning of the machine without overheating and charring is the controlled application of electrical power to the heating elements of the melt grid. Application of this power is initiated by movement of the adhesive pump, which movement is detected in the preferred embodiment by a reed sensor attached to the air cylinder. This sensor delivers a timed voltage output to the grid typically for 15 to 20 seconds. If the pump continues operating, then the grid will be powered continuously until the pump stops, plus a period of 20 seconds, for example. During this "on time" the grid is still controlled so that it does not exceed its set temperature, which is typically 300° F. In the event of a standby condition, i.e., the pump is not operating, the grid is not supplied with power, and it cools. This allows the machine to standby indefinitely without heating to excessive temperatures, which results in charring. But, the grid is sufficiently powered, in the order of 900 watts, so that it can recover almost immediately upon subsequent actuation of the pump.

All aspects of the control board are adjustable: temperatures, timing, power up delay, voltage to the hoses, etc. to allow adjustment for a wide range of adhesives and to compensate for changing ambient conditions. A unique feature of this control board is that a separate, smaller circuit board card has been designed to be detachable from the mother board. This card contains precision, set point resistors for each zone, thus setting a machine's particular overall calibration. An operator may adjust for different adhesives by simply unplugging this card and replacing it with a card set up with the resistors designed for the new adhesive. This makes changes in the field very simple.

Troubleshooting a machine of this type is now simplified. A troubleshooter simply has to check resistance and voltage along a parallel connector for both the thermistor (sensor) inputs and the high voltage outputs.

A simple lever-type micro-switch is located at the top of the reservoir to detect when the glue level drops and to signal to the operator to add another slab of adhesive. When the level of the solid slab goes down so as to require its replacement, the switch preferably actuates a "beeper" and/ or visual signal for the operator. In the event that the machine is not consistently replenished with glue, there is no catastrophic result; the machine will simply run out of glue. The tank can then be refilled, and in a short time the grid will melt enough to resume.

In accordance with a second embodiment, the current to be supplied to the heating elements is determined as a function of the glue consumed. Thus, in contrast to measurement of the temperature of the reservoir and increase of the current through the heating elements after sensing a temperature drop, the system of according to this embodiment senses when a predetermined amount of glue has been consumed and increases the current in the heating elements to provide the predetermined amount of energy required to melt that amount of glue. Because the temperature drop in the reservoir lags significantly behind the use of the glue, the system of the invention maintains the temperature of the glue reservoir far more efficiently.

Further, in accordance with the second embodiment, the current in the heaters is not reduced upon reaching the desired temperature during start up and when heating due to consumption of glue. Instead, the temperatures in the reservoir and pump are caused to exceed the nominal target temperatures, e.g., 180°F and 300°F respectively, by a predetermined amount, preferably about 12%, to account for time lags and other variables. The temperature is allowed to return to steady state after this "overshoot" is achieved, however.

In operation, a counter, which can be operated by sensing reed switches on the pump, determines the number of cycles made by the transfer pump. When a predetermined amount of glue has been dispensed, the temperature of the reservoir is increased to about 200°F, which allows the slab of glue floating on top of melted glue to descend. The temperature of the pump is controlled and can increase above the steady state temperature during start-up or as a part of other desired events.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a side view in partial cross section of a base unit of a bulk system for supplying hot melt adhesive in accordance with the invention.

Figure 2 is a vertical cross section of a preferred embodiment of a heating tank for use in the bulk glue system shown in figure 1.

Figure 3 is a plan view of the system shown in figure 2.

Figure 4 is a bottom view of the system of figure 2.

Figure 5 is an exploded view of the hose mounting block of the embodiment of figure 2.

Figure 6 is a cross section of the nozzle used in the embodiment shown in figure 2.

Figure 7 is an end view of the nozzle heater block.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

With reference to figures 1 and 2 of the drawings, a preferred embodiment of a bulk system for application of a hot melt adhesive in accordance with the invention includes a tank 2 having upstanding walls 4 and a bottom 6. The walls 4 and bottom 6 form an enclosure for receiving solid glue 8, which may be any of a variety of known compositions and is preferably in the physical form of a flat "slab." The size of the slab is such that it just fits within in the tank 2, leaving only a small gap between the edge of the slab and the side of the tank. For example, if the tank is square and has an inside transverse dimension of 10! inches, the slab will be square with a width of 10 inches. This construction minimizes the width of the gap between the edges of the glue and the sides of the tank whereby the solid slab covers substantially the entire pool of melted glue, preventing contact between the melted glue and air. Because charring is caused by oxidation of melted glue, this construction aids significantly in the prevention of charring.

A lid 7 may be employed to cover the tank while leaving a small slot 9 for receiving the slabs 8. The tank is heated by one or more electric heating elements 10, which are attached to the lower surface of the tank bottom. These heating elements are supplied with electric power from a control circuit to maintain the temperature of the lower part of the tank at a temperature that is above the melting point of the glue but below the preferred application temperature of the glue. For example, the preferred application temperature of a known glue composition is 300° F, and if that glue is to be used, the temperature of the lower part of the tank would be controlled to be about 225° F. This temperature will cause the glue slab 8 to begin to melt in the lower part of the tank but will also establish a vertical temperature gradient in the glue whereby the upper slab will remain solid. The temperature of the glue in the tank can be measured by any of several types of temperature sensors (not illustrated) and placed to detect the temperature in the desired locations.

Liquid glue is dispensed from an outlet at the bottom of the tank, as will be described below, and each glue slab descends further into the tank as the melted glue is withdrawn. Figure 2 illustrates a first slab 8 resting on a pool of melted glue 12, and a new slab 8' being inserted into the tank. A micro-switch 14 is supported above the pool such that a switch arm 16 extends into the tank to engage the uppermost slab 8. As the slab descends, the arm moves downward until the switch is closed, whereby an alarm (not shown), which may be visual or audible, or both, is activated to alert the operator to the necessity of adding more glue.

A double-acting pump 18 is mounted to the bottom of the tank, with its inlet connected to the tank outlet. The pump 18 includes a piston 20 that is reciprocally driven by a shaft 22, which is, in turn, driven by an air cylinder 24. The construction of the double acting pump is generally known in the art, and the piston includes a spring-loaded check valve 26. The piston rides in a channel 28 having a diameter such that when the piston moves to the left of figure 2, glue in channel 28 flows through the valve 26 and part of that glue is forced out through the pump outlet (see figure 4) while part remains in the channel. As the piston moves to the right, the channel 28 is again filled from the tank, and the glue remaining in the right hand part of the channel is expelled through the outlet. Thus, the flow if glue is essentially continuous as the piston 20 reciprocates. The outlet of the tank is fitted with a check valve 30, which allows flow into the pump under a small or no pressure difference but prevents reverse flow into the tank when the piston 20 of the pump 18 moves to the left of figure 2. As shown, the check valve includes a ball 32 and spring 34, and in the preferred embodiment the length of the spring in the absence of applied external forces provides a gap of about 1/8" between the ball and the valve seat. This allows the fluid glue to fill the pump channel 28 even in the absence of pumping to prevent cavitation. Alternatively, the spring could be very weak such that even the slightest movement of the pump would open the valve to allow entry of glue.

Returning now to the description of the tank construction, and with reference to figures 1 and 2, the preferred embodiment of the invention includes a grid 36 in the lower part of the tank for supplying a large amount of heat to the glue very quickly. The grid 36 generally comprises a plate 38 that is configured much like the bottom of the tank, e.g., with a slight tilt toward the center, and the grid is separately heated. The plate includes holes 40 for allowing melted glue to pass to the bottom of the tank. The grid includes upstanding fins 42 and a central cylinder 44. The central cylinder slides into an opening in the bottom of the tank, preferably engaging a flange extending from the bottom of the tank, and is sealed thereto by O-rings 46. The grid is heated in the embodiment shown by a number of cartridge heaters 48, which reside in vertically-extending channels in the central cylinder 44. Fluid glue flows into the outlet through channels 49.

The holes in the grid allow the glue to flow through the grid very easily to supply melted glue to the bottom of the tank and, therefore, to the pump. The grid is close to the bottom of the tank but is not in contact with it. Thus, the grid is generally thermally insulated from the tank and is capable of rapid temperature increase when necessary.

Power to the heaters 48 is controlled by a circuit board 11 , which can be mounted in any of a variety of locations in the device but is shown adjacent the tank in an insulated housing. The heaters are controlled such that they receive power only when (1 ) the pump is operating and (2) when the temperature of the grid is below a set temperature, e.g., 300° F. As well, the heaters receive power for a period of time after pumping is stopped, typically 20 seconds but adjustable for 15 to 30 seconds and possibly up to several minutes. The control of the pump will now be described. A magnet 50 is attached to the shaft 22 for movement with the shaft. A series of reed switches 52 is mounted adjacent the shaft so that they are switched upon passage of the magnet 50. In the embodiment shown in figure 2, three such reed switches are employed, and the central one is used to determine when the pump is operating. Thus, the control circuit will receive an input from the central reed switch and an input from a thermister measuring the temperature of the grid 36. When these inputs indicate that the grid is below the set temperature and the pump is operating, the control circuit will provide power to the heaters 48.

Further, the pump 18 is heated by heating elements 54 whereby the glue in the pump is maintained for example at about 275° to 300° F. As will be described below, the hose carrying the heated glue and the nozzle are also heated. The pump is also fitted with a safety valve 56 which opens upon excessive pressure in the channel 28 to return glue to the tank. This may occur, for example, if the glue expands too much on initial heating.

With regard to the glue in the tank, this arrangement has the following effect. If the machine has been in the standby condition, the glue in the bottom of the tank will have been maintained at a temperature (e.g., 225° F) such that it is capable of flowing, but it is not at the preferred application temperature. The grid 36 is, therefore, below its set temperature. The pump, however, contains glue heated to the operational temperature and that glue can be supplied to the nozzle through the hose. When the pump is activated, the control circuit senses operation of the pump and immediately supplies power to the heaters 48. Because these heaters are of high power (e.g., 500-900W) heat is supplied almost instantaneously to the glue, and its temperature is increased very quickly to the desired operational temperature. In this connection, it is noted that the glue is fluid, having absorbed already the heat of fusion. Thus, the grid can raise the glue temperature very quickly. The heated glue therefore flows into the pump at the desired temperature.

In steady state operation, as when the pump is operating continuously, power to the grid is essentially a function of the heating requirements of the glue.

Referring now to figure 4, the pump 18 includes first and second outlets 58. A hose block 60 is shown attached to one of the outlets, and a second hose could be attached to the other. The hose block 60 is shown in more detail in figure 5 and is a block with an internal passage extending between inlet 62 and outlet 64. The block is mounted to the pump by bolts 66. The outlet includes a threaded nipple for receiving the inlet of a hose 68. The hose itself is of known construction but has been modified to have a heating wire 70 extending throughout its length. The wire is folded over so that the free ends can be connected to electric terminals 72. The terminals are supplied with current to maintain the temperature of the glue in the hose at the operating temperature. A thermister (not shown) is also provided in or on the hose to supply a signal to the control board to control power applied to the wire 70 through the terminals 72 to maintain the desired temperature.

The piston in the air cylinder 24 is controlled by a solenoid valve 74. The valve receives input air from a pump (not shown) and also receives signals from the outermost reed switches 52. The valve supplies air to whichever side of the cylinder 24 is necessary to reciprocate the piston, which drives the glue pump as is known in the art.

With reference to figure 6, the nozzle in accordance with the preferred embodiment includes a central inlet tube 76 with a nipple 78 for receiving the hose 68. A movable tip 80 which includes a conical seat 82 is mounted on the tube for relative movement. A spring is located between a stop element 84 and an end of the tip to urge the tip to the closed position shown in figure 6. Glue entering the tube 76 flows through the tube and exits through apertures 86. The glue is then allowed to flow around the conical end of the tube and out through the end of the tip 80 when the tip is moved out of contact with the end of the tube. This is accomplished by the user's applying force to the trigger 88, one end of which is wedged between a heater block 90 and the movable tip. When the trigger 88 is pulled toward the nozzle handle 92, the tip is lifted from the tube, and glue flows out through the threaded tip 94, which can be supplied with a directional element as known in the art.

An end view of the block 90 is shown in figure 7 and the block includes an aperture for receiving the tube 76, an aperture receiving a heater cartridge 96 and another aperture receiving a thermister 98. Signals from the thermister are supplied to the control board for controlling power applied to the heater cartridge and maintaining the temperature of the nozzle.

Modifications within the scope of the appended claims will be apparent to those of skill in the art.

Claims

We Claim:

1. A reservoir for a hot-melt adhesive dispensing system comprising: an adhesive-holding tank with an opening in an upper portion thereof adapted to receive said hot-melt adhesive in a bulk, solid state, an adhesive-melting grid in a lower portion of said tank and spaced from a bottom of said tank, and an outlet in said bottom positioned to receive said hot-melt adhesive in a liquid state.

2. A reservoir according to claim 1 wherein said tank is heated and further comprising a control circuit that maintains said adhesive-holding tank at a temperature below the char temperature of said hot-melt adhesive.

3. A reservoir according to claim 2 further comprising a pump for withdrawing fluid glue from said tank and wherein said control circuit provides heat to said grid only when said grid is below a set temperature and said pump is operating.

4. A reservoir according to claim 1 in further combination with pump for withdrasing fluid from said tank, a hose conveying said glue and a dispensing nozzle.

5. A combination according to claim 4 wherein said hose and nozzle are heated.