Coating processes – Measuring – testing – or indicating
Reexamination Certificate
2000-10-31
2003-02-11
Bareford, Katherine A. (Department: 1762)
Coating processes
Measuring, testing, or indicating
C427S424000
Reexamination Certificate
active
06517891
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to an apparatus for dispensing viscous fluids and, more particularly, to an apparatus and method for supplying hot melt adhesives to a dispensing gun.
BACKGROUND OF THE INVENTION
The ability to precisely dispense viscous industrial materials, such as hot melt adhesives, is a necessity for manufacturers engaged in the packaging and plastics industries. Inconsistent application of adhesive onto a substrate translates into unusable and scrap product and increased costs. Therefore, the process of supplying adhesive to a fluid dispensing applicator or gun must be precisely controlled.
A typical fluid dispensing operation employs a dispensing gun to apply a fluid, for example, an adhesive, onto a substrate being moved past the dispensing gun by a conveyor. The speed of the conveyor, or line speed, is set according to such factors as the complexity of the dispensing pattern and the configuration of the gun. Fluid adhesive is normally supplied to the dispensing gun by flexible hoses. Adhesive is pumped from a reservoir by a metering pump, for example, a motor-driven positive displacement pump. A metering pump for purposes herein is a pump in which the output volume is directly proportional to the action or displacement of the pump independent of fluid viscosity, except for any fluid leakage within the pump. Therefore, with a metering pump, the flow rate of the adhesive being dispensed from the gun is a function of the speed of the motor driving the pump.
The proper application of fluid or adhesive onto a substrate requires that the flowrate of the fluid from the dispensing gun remain as constant as possible throughout the fluid dispensing process. Variations in the flowrate result in different quantities or volumes of fluid being applied at different locations across the substrate. Thus, with too little adhesive, a desired coating thickness is not achieved, and the quality of the adhesive capability is reduced. Similarly, with an excessive quantity of fluid being dispensed, the adhesive may subsequently be displaced to areas of the substrate where it is not wanted; and again, the quality of the substrate product is reduced. In either event scrap product is often the result.
In many applications, the speed of the conveyor carrying the substrate is controllable and changed in accordance with the production line's capability to produce a high quality product. For example, with a first time run of a product, a production line may be operated at a slower speed to ensure a high quality product. But over time, as the production line is tuned, it can operate at a higher conveyor speed and still produce a high quality product. Assume the fluid dispensing system is operating properly with the conveyor operating at a first constant speed. If the speed of the conveyor and the substrate is increased to a higher constant speed, the flowrate of fluid being dispensed through the gun must also be increased in order to maintain a consistent, high quality coating of fluid on the substrate. It is known to use a signal related to the conveyor speed to modify the speed of the pump motor. Hence, when the conveyor is adjusted to the higher constant speed, the speed of the pump motor increases; and the flow of fluid to the gun is increased, thereby causing the pressure within the gun to increase. The increased gun pressure causes the flowrate of fluid from the gun to increase, and thus, the flowrate of the fluid being dispensed is changed as a function of conveyor speed.
The above flow control system works relatively well while the conveyor is operating at a constant speed, however, the flow control system does not operate properly during periods when the conveyor is accelerating or decelerating. Such conveyor speed changes occur, for example, when the conveyor is initially started from rest. Known systems are unable to maintain the desired flowrate of the fluid through the dispensing gun during periods of conveyor acceleration and deceleration.
FIG. 5A
illustrates how the fluid pressure at the dispensing gun changes with respect to an acceleration and deceleration of the conveyor. When the conveyor is at a zero speed (
500
), with some systems, for example, those using a pressure relief recirculation valve, the recirculation pressure is higher (
502
) than a desired operating pressure (
504
) of the dispensing gun. Therefore, when the conveyor line is initially started (
506
) and is accelerating, the fluid dispensing occurs at an excessive pressure, thereby depositing excessive fluid and producing scrap product. The production of scrap product will continue as the pressure decreases (
508
) and the conveyor accelerates until both the conveyor speed and operating pressure reach their desired values (
509
). For purposes of illustration, the desired values of conveyor speed and operating pressure are shown as the common line (
504
). Upon being given a deceleration command (
530
), the conveyor speed decreases (
532
) to a zero velocity (
534
). However, upon the dispensing gun closing, the pressure rises (
536
) until the pressure relief valve opens and stabilizes the pressure (
538
).
In other recirculation systems, a solenoid actuated pressure relief valve is in series with a restricted orifice; and upon the recirculation valve opening, the recirculation pressure (
510
) is held at a level lower than desired operating pressure. Upon the conveyor accelerating (
506
), the gun pressure initially drops to a still lower pressure (
512
) faster than the metering pump can increase the pressure. Therefore, for a short period of time after the conveyor line starts, an excessive amount of fluid is dispensed which results in the production of scrap product. As the conveyor line accelerates, at some point (
514
), for a current conveyor speed, the correct amount of fluid is being dispensed; but continued conveyor line acceleration (
516
) with lower pressure (
518
) results in less than the desired flowrate of fluid through the dispensing gun. Thus, scrap product continues to be produced until the conveyor speed and operating pressure both reach their desired values (
504
). Upon the conveyor starting a deceleration, the recirculation valve is opened and the pressure decreases until it is stabilized at a value (
542
) determined by the restricted orifice.
As can be seen in
FIG. 5A
, with the lower recirculation pressure just described, the conveyor accelerates to its desired speed well before the dispensing gun pressure reaches its desired operating pressure. A significant contributing factor to this extended pressure recovery time is the use of flexible hoses connecting the pump with the dispensing gun. At the desired operating pressure, the hoses expand slightly; and the quantity of fluid being dispensed is small relative to the volume of the hoses. In fact, many times, the quantity of fluid dispensed is no more, and often less, than the expansion, or increased volume, of the hose at the desired operating pressure. Therefore, it takes longer for the pump to restore the desired gun pressure because the pumped fluid has to again expand the hose with fluid in order to achieve the desired operating pressure. As will be appreciated, the graphical representations of the pressure and line speed in
FIG. 5
are only exemplary. The acceleration and deceleration of the conveyor often varies nonlinearly and normally is not linear as shown. Further, the acceleration and deceleration of the conveyor may differ from day to day and may be different with different systems. Further, the the exact profile of pressure with respect to time often varies substantially on an instantaneous basis and is not in any respect related to the conveyor speed.
Therefore, there is a need for a fluid dispensing system which maintains a desired flowrate of fluid through the dispensing gun while the speed of the conveyor carrying the substrate is changing, for example, when the conveyor is accelerating from rest to its desired conveying speed.
SUMMARY OF THE INVENTION
The fluid dis
Estelle Peter W.
Saidman Laurence B.
Bareford Katherine A.
Nordson Corporation
Wood Herron & Evans L.L.P.
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