Abstract:
A method for dispensing or depositing particulate material includes accumulating particulate material in a nozzle having an opening at the bottom for discharging the particulate material, and applying positive air pressure to a buffer chamber positioned in communication with the accumulation of particulate material to start and control the flow of particulate material through the opening and applying negative pressure to the buffer chamber to stop the flow of particulate material. An unobstructed interface can exist between a surface of the particulate material and air in the buffer chamber. In one embodiment, roofing shingle granules are applied intermittently to a moving coated asphaltic sheet through a slot to form strip roofing shingles.

Description:
This is a continuation, of application Ser. No. 08/144,371, filed on Nov. 2, 1993, now abandoned. 
    
    
     TECHNICAL FIELD 
     This invention pertains to dispensing particulate material, such as granular material. In one of its more specific aspects, this invention relates to controlling the application of particulate material to substrates. 
     BACKGROUND OF THE INVENTION 
     Various means have been employed in the past to dispense particulate materials. These materials can be rather coarse, as in granular material, such as shingle granules. There is also a need to dispense finely divided particulate material, such as powders. Typical means for dispensing particulate materials include direct feed nozzles and open hoppers. 
     Some processes require a periodic starting and stopping of the dispensing process. For example, dispensing particulate material into continuously indexing containers, and depositing particulate material onto substrates in patterns. 
     Where there is a need for starting and stopping the dispensing of the particulate materials, a mechanical means, such as a fluted roll, is usually inserted at the bottom of the nozzle. Rotation of the fluted roll causes a predetermined amount of the particulate material to be dropped through the nozzle opening. 
     There is a need for a more accurate way to control the dispensing of particulate material, particularly where the dispensing is to be intermittent. As process speeds increase it becomes more and more difficult to precisely control the starting and stopping of the discharge of particulate material. Mechanical devices have been found to be deficient in accurately starting and stopping the process. 
     Lack of precise control in a process for depositing particulate material onto a substrate results in an imprecise or fuzzy boundary between the applied portion of the substrate and the non-applied portion of the substrate. Lack of precise control in a process for depositing particulate material into containers, compartmented receptacles or packages passing beneath the nozzle manifests itself in spillage of the particulate material outside the containers. 
     One cause of the impreciseness of typical particulate material depositing techniques is that the nozzles depend on gravity exclusively, not only for directing the particulate material from the nozzle, but also for movement of the particulate material within the nozzle itself. The use of gravity to move the particulate material within the nozzle, or discharge apparatus itself, has feed rate limitations, and there is no easy way to control the rate of flow of the particulate material. 
     An improved means and method for dispensing particulate material from a nozzle would eliminate the lack of preciseness inherent in the mechanical action of mechanically operated apparatus, such as a fluted roll. Also, the ideal system would provide a means for enhancing gravitational forces in starting and stopping flow and would enable some means for controlling the flow rate of particulate material during discharge. 
     SUMMARY OF THE INVENTION 
     There is now been developed a particulate material discharge device which solves the problems of accurate, relatively instantaneous control of the flow of the particulate material. The method and apparatus of this invention starts, stops and controls the flow rate of particulate material by providing pneumatic pressure changes in a buffer chamber positioned adjacent a pile or an accumulation of the particulate material in a nozzle. The opening in the nozzle through which the material flows is sized with respect to the size of the particles so that slight pressure variations in the buffer chamber will start, accelerate or stop the flow of material through the nozzle opening. 
     According to this invention, there is provided apparatus for dispensing particulate material comprising a nozzle for holding an accumulation of particulate material, an opening at the bottom of the nozzle for discharging the particulate material onto the coated asphalt sheet, a buffer chamber positioned in communication with the accumulation of particulate material, and vacuum means for reducing the pressure in the buffer chamber to stop the flow of particulate material through the opening. 
     In a specific embodiment of the invention, pressure means, such as a fan, is also supplied to increase the air pressure in the buffer chamber to initiate a flow of particulate material through the opening. In a particular embodiment of the invention the pressure means comprises a pressure fan and a valve positioned between the pressure fan and the buffer chamber. 
     In yet another embodiment of the invention the accumulation of particulate material in the nozzle is supplied by a hopper, and the ratio of the height of the particulate material in the hopper to the height of the particulate material in the nozzle is greater than 1:1. In a particular embodiment of the invention the ratio is greater than or equal to about 3:1. 
     In yet another embodiment of the invention the vacuum means comprises a vacuum fan and a valve connecting negative gauge pressure air from the vacuum fan to the buffer chamber. 
     In another specific embodiment of the invention the opening is a slot. Most preferably, the slot, nozzle and buffer chamber are arranged transverse to the machine direction of the moving substrate, and a source of both pressurized air and negative gauge pressure air is connected to each end of the buffer chamber. 
     In a particular embodiment of the invention the opening is a slot having a width within the range of from about 0.06 to about 1.25 inches (about 0.15 to about 3.2 cm). Preferably, the width of the slot is within the range of from about 0.25 to about 0.75 inches (about 0.64 to about 1.9 cm). 
     In yet another embodiment of the invention flexible members are connected to the opening to help stop the flow of particulate material through the opening. 
     In yet another embodiment of the invention the size of the opening is within the range of from about 1.5 to about 20 times the average size of the particulate material. Preferably, the size of the opening is within the range of from about 3.0 to about 12 times the average size of the particulate material. 
     In a preferred embodiment of the invention the ratio of the area of the opening to the area of the surface of the accumulation of particulate material in the nozzle is greater than about 1:4. 
     According to this invention, there is also provided a method of dispensing particulate material comprising accumulating particulate material in a nozzle having an opening at the bottom for discharging the particulate material, and changing the air pressure in a buffer chamber positioned in communication with the accumulation of particulate material to control the flow of particulate material through the opening. 
     In a particular embodiment of the invention the step of changing the air pressure comprises reducing the pressure in the buffer chamber to stop the flow of particulate material through the opening. The air pressure in the buffer chamber is preferably decreased to a pressure within the range of about -5 to about -10 inches of water gauge pressure (about -9.3 to about -37.3 mm Hg) to stop the flow of particulate material through the opening. 
     In yet another embodiment of the invention the step of changing the air pressure comprises increasing the air pressure in the buffer chamber to initiate a flow of particulate material through the opening, and reducing the pressure in the buffer chamber to stop the flow of particulate material through the opening. 
     In a specific embodiment of the invention, the flow rate of particulate material through the opening is changed to accommodate changes in the speed of a substrate. 
     In yet another embodiment of the invention, a control means, operatively connected to the supply of pressurized air to the buffer chamber, is operated to vary the flow rate of particulate material through the opening to accommodate changes in the speed of moving containers or a moving substrate. 
     In an additional embodiment of the invention, the size of the opening is changed to vary the flow rate of particulate material through the opening to accommodate changes in the speed of moving containers or a moving substrate. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a schematic view in perspective of apparatus for dispensing granules according to the principles of the invention. 
     FIG. 2 is a schematic view in elevation of a cross section of the granule dispensing apparatus of FIG. 1. 
     FIG. 3 is a schematic view in elevation of the granule dispensing apparatus of claim 2 taken along lines 3--3. 
     FIG. 4 is a schematic cross-sectional view in elevation illustrating the use of flexible flaps on the nozzle of the invention. 
     FIG. 5 is a schematic view in perspective illustrating an embodiment of the invention using a series of orifices rather than a slot in the dispensing nozzle. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     For purposes of illustration this invention will be described in terms of a process for dispensing shingle granules onto a coated asphalt sheet to form strip roofing shingles. It is to be understood that the invention encompasses dispensing many other kinds of particulate material, such as inorganic particles like powders, frits, spheres, pellets, and microspheres. Also encompassed are organic particles, such as powders, spheres, microspheres, grains, seeds, etc. 
     As shown in FIGS. 1 and 2, the apparatus for dispensing particulate material is generally comprised of hopper 10 and nozzle 12. The hopper can be any suitable means for supplying granules to the nozzle to form a pile or accumulation 14 of granules 16. The exit or throat 18 of the hopper narrows down to be considerably smaller in cross-sectional area than surface area 20 of the accumulation of granules. The surface of the accumulation of granules is the interface between the granules and the air in the buffer chamber. 
     Granules can be fed to the hopper by any suitable means, such as granule feeder 22, many designs for which are well known in the art. When the granules exit the nozzle they exit through an opening, such as slot 24 and are deposited on moving coated asphalt sheet 26. The granules are deposited onto the sheet in an intermittent manner to form a series of prime granule application areas or blend drops 28 which are separated by a series of background color areas, such as background color areas 30. Usually the background color granules are dropped onto the coated asphalt sheet after the blend drops are deposited, as is well known in the art. 
     As shown in FIGS. 2 and 4 in particular, granules 16 fed from feeder 22 form a head within hopper 10. That head provides a relatively constant feed via gravity discharge to the accumulation 14 of granules formed within the nozzle. That is, as a flow of granules 16 is discharged from the accumulation 14 through slot 24, a like volume of granules automatically is supplied to the accumulation 14 by gravity flow from the hopper 10. As shown in FIGS. 2 and 4, the outlet of the hopper 10 contacts the accumulation surface 20. Granules will initially flow into the accumulation 14 until the surface 20 reaches the level of the hopper outlet, plugging the same against further flow. This is a well known plug-feed type of supply, which is used herein to maintain the volume of the accumulation 14 relatively constant. The preferred ratio of the head to the accumulation of granules is discussed in more detail hereafter. 
     It should be understood that the invention encompasses apparatus for dispensing particulate material for any purpose, including filling containers. It is not necessary that the particulate material be deposited onto a moving substrate. 
     As shown more clearly in FIG. 2, there is an open area, buffer chamber 32, positioned above the surface of the accumulation of granules in the nozzle. It is changes in the pressure of the buffer chamber which affect the flow of granules through the slot. It is to be understood that the buffer chamber is positioned adjacent the accumulation of granules in the nozzle. It need not necessarily be positioned above the granules. Also, a screen or perforated plate can be positioned at the surface of the accumulation of granules to separate the buffer chamber from the accumulation of granules. In FIG. 2, and unobstructed interface free of a screen or perforated plate exists between a surface of the particulate material and air in the buffer chamber. 
     During the start up of the granule application process, it may be necessary to close off the slot in the nozzle to provide sufficient back pressure to enable the granules to be stopped from flowing through the nozzle. Accordingly, a means, such as start-up plug 34, is provided to temporarily plug the slot during initiation of the process. 
     As shown in FIG. 3, the buffer chamber can be adapted to extend beyond either end of the nozzle, so that the buffer chamber is in communication with the top surface of the accumulation of granules in the nozzle. Positioned in communication with the buffer chamber are two other chambers which affect the pressure within the buffer chamber. These are pressure chamber 36 and vacuum chamber 38. The vacuum chamber is in communication with the buffer chamber through any suitable means, such as vacuum opening 40. 
     The flow of air from the buffer chamber to the vacuum chamber can be controlled by any suitable device, such as by vacuum plate 42 operated by vacuum solenoid 44. Any means, such as vacuum fan 46, can be put in communication with the vacuum chamber in order to produce a negative gauge pressure in the vacuum chamber. A vacuum fan is not the only possibility for creating the negative pressure within the vacuum chamber. Other devices include the use of a venturi or a pump. 
     The vacuum fan is operatively connected to the vacuum chamber by any suitable conduit, such as vacuum piping 48. Further, an accumulator, such as vacuum accumulator 50, can be used to dampen surges in demand and supply of the negative gauge pressure air. It can be seen that the opening and closing of the vacuum plate against the vacuum opening by action of the vacuum solenoid will affect the communication between the negative gauge pressure vacuum chamber and the buffer chamber. The application of negative gauge pressure to the buffer chamber will create a sufficient pressure drop over the accumulation of granules to stop the flow of granules through the slot. 
     When a negative pressure is applied to the vacuum chamber and through the vacuum opening to the buffer chamber, there is produced an upward flow of air through the slot and through the granules that have accumulated in the nozzle. The upward flow of air provides an upwardly oriented drag force on the granules in contrast to the downward pull of gravity on the granules. If the proper amount of negative pressure is applied to the buffer chamber, the drag force from the upward flow of air through the slot will balance the pull of gravity on the granules, and the granules will be held in place rather than continue falling down through the slot. The granules are held in place by the upward flow of air. 
     It should understood that if the velocity of the air flow through the slot exceeds a critical level, then the granules would become fluidized, and begin to move as if they were caught in a fluid medium. Fluidization of the granules means that the granules are not held in place, but are supported with sufficient drag force of upwardly moving air that they are free to vibrate or move laterally relative to each other. The fluidization of the granules within the nozzle would create churning, mixing and various air flow paths which would contain some entrained granules. If the air flow is of sufficient velocity to cause fluidization of the granules, some of the granules would fall through the nozzle. Therefore, the amount of upward air flow through the nozzle must be carefully balanced so that the drag force exceeds the weight of the granules to prevent the granules from falling without causing fluidization of the granules. 
     Another problem of fluidization can occur if upward air velocity at the surface of the accumulation of the granules creates drag force sufficient to cause some of the granules to become airborne. Airborne granules can foul the air handling system. 
     In a manner similar to the equipment shown on the vacuum side, the pressure chamber is in communication with the buffer chamber by means of pressure opening 52, and this can be controlled with any suitable device, such as pressure plate 54 operated by pressure solenoid 56. The pressure in the pressure chamber can be supplied by any suitable means, such as pressure fan 58 connected via pressure conduit 60, and employing pressure accumulator 62. It is to be understood that any number of mechanisms can be used to supply pressure to the pressure chamber, such as pumps, turbines, or bellows. It can be appreciated that the pressure plate acts as a valve between the pressure fan and the buffer chamber. Likewise, the vacuum plate acts as a valve to control the process of reducing the pressure in the buffer chamber used to stop the flow of granules through the slot. Another means for controlling the pressure in the pressure chamber is by using pressure relief valve 63. 
     In operation it has been found preferable to have sufficient height of the hopper relative to the height of the accumulation of granules in the nozzle so that pressure changes in the buffer chamber are communicated primarily to the granules in the nozzle, rather than to the granules in the hopper. Preferably, the ratio of the height of the granules in the hopper to the height of the granules in the nozzle is greater than 1:1. Most preferably, the ratio is greater than or equal to about 3:1. If the ratio were lower than about 1:1 negative pressure in the buffer chamber would have the effect of drawing air through the granules in the hopper rather than through the granules in the accumulation in the nozzle. This would mean that the application of negative pressure in the buffer chamber would be ineffective in stopping the flow of granules passing through the slot. 
     As shown in FIG. 3, there is a source of pressurized air at one end of the apparatus, and a source of negative gauge pressure air connected to the other end of the buffer chamber. Where shingles of sufficient width are being produced, such as on a 3-wide machine or a 4-wide machine, it is preferable to have a source of both pressurized air and negative gauge pressure air connected to each end of the buffer chamber. This would reduce the possibility of a time delay in having the effect of a change in air pressure cross the width of the shingle manufacturing machine. 
     The size of the width of the slot depends in part upon the size of the granules used. For granules sized as 3M No. 11 grade roofing granules, the preferred slot has a size within the range of from about 0.06 to about 1.25 inches (about 0.15 to about 3.2 cm). Most preferably, the width of the slot is within the range of from about 0.25 to about 0.75 inches (about 0.64 to about 1.9 cm). 
     In order to most completely close off the slot when the granules are supposed to be stopped, it is preferable to use flexible members, such as thin stainless steel flaps 64 to help stop the flow of granules through the slot, as shown in FIG. 4. The flexible members can be of any suitable type, sufficient to allow the flow of granules during the time when the granules are supposed to be flowing. 
     It should be understood that the shape of the opening for discharging the particulate material need not be a slot. As shown in FIG. 5, the openings can be of different shapes, such as round or oval openings 66. As can be appreciated, a series of such oval openings would create a series of particulate material streams, such as granule streams 68. These granule streams could be used to produce particularly desired patterns of discreet granules, such as discreet granule patterns 70. 
     It has been found that the surface area of the accumulation of particulate material has a critical relationship with the area of the opening. This is because if the area of the surface of accumulation of particulate material is too small, the negative pressure will create a fluidized bed situation in which the particulate material is actually floating on the air, and this would interrupt the smooth processing of the apparatus. Preferably the ratio of the area of the opening to the area of the surface of accumulation of particulate material in the nozzle is greater than about 1:4. 
     It will be evident from the foregoing that various modifications can be made to this invention. Such modifications, however, are considered as being within the scope of the invention. 
     INDUSTRIAL APPLICABILITY 
     This invention will be found to be useful in the production of granule coated discreet roofing shingles suitable for use in residential and commercial roofing applications.