Abstract:
An apparatus for continuously forming thermoplastic adhesive cartridges. A molten thermoplastic adhesive supply provides a supply of molten adhesive to a pump downstream from the molten thermoplastic adhesive supply. The pump then provides a molten thermoplastic adhesive melt stream which passes through a screen filter downstream from the pump to prevent the discharge of impurities in the molten thermoplastic adhesive melt stream from the pump. A temperature-controlled conduit downstream from the pump then lowers the temperature of the molten plastic melt stream to closely above its softening point. A continuous molder located downstream from the melt pump for receives the molten plastic melt stream from the melt pump to form the thermoplastic adhesive cartridge continuously.

Description:
BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention relates generally to extrusion of plastic materials and, more particularly, to an apparatus for continuous forming hot melt adhesive cartridges. 
     (2) Description of the Prior Art 
     Commercial hot melt adhesives are used extensively in a variety of manufacturing applications. These applications include the attachment of furniture trim, packaging labels and profile caps. Typically, these adhesives are provided in cartridge form comprising cylinders of hardened adhesive having a diameter of between about 10 and 20 mm. Depending on the chemical makeup of the adhesive, the cylinders are extruded in continuous fashion through a conventional extruder using a similar process to that of plastic extrusion. However, this has only been true for the smaller sizes. The smaller sizes do not usually require external support since most of the compositions cool and harden quickly. Larger diameter cartridges, greater than about 43 mm, have required the use of single cavity mold to form the desired shape and allow sufficient time for cooling. For example, adhesives such as low tack ethylene vinyl acetate (EVA) are first pelletized and then fed into an extruder. 
     However, some commercial hot melt thermoplastic adhesives, such as high tack ethylene vinyl acetate (EVA) or amorphous polyalphaolefins (APAO), can not be extruded conventionally. This is because these compositions are so “tacky” that the pellets lump together and the plastic can not be extruded. That is, pellets formed from these adhesives do not flow readily but rather form a conglomerate mass that plugs the extruder feed chute. These types of adhesives must be formed using a batch process that involves filling a series of cylindrical molds with molten adhesive. The molds are cooled and the adhesive is trimmed and removed from the mold. 
     Even with best automation equipment available, the maximum through put using this batch process is between about 70 and 100 pounds per hour. In addition, the batch process is labor intensive and generates a large amount of scrap adhesive that must be recycled. In addition, it would be desirable if the palletizing step could be eliminated. 
     Thus, there is a need for an apparatus for forming thermoplastic adhesive cartridges which operates continuously while, at the same time, does not require that the thermoplastic adhesive be pelletized before use. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an apparatus for continuously forming thermoplastic adhesive cartridges. A molten thermoplastic adhesive supply provides a supply of molten adhesive to a pump downstream from the molten thermoplastic adhesive supply. The pump then provides a molten thermoplastic adhesive melt stream which passes through a screen filter downstream from the pump to prevent the discharge of impurities in the molten thermoplastic adhesive melt stream from the pump. A temperature-controlled conduit downstream from the pump then lowers the temperature of the molten plastic melt stream to closely above its softening point. 
     A continuous molder is located downstream from the melt pump for receiving the molten plastic melt stream from the melt pump to form the thermoplastic adhesive cartridge continuously. The continuous molder includes a molten mold material supply and a die head for receiving the molten mold material and forming a continuous mold. A cooling zone upstream from the discharge of the melt pump and downstream from the die head cools the molten mold material below its softening point temperature to form the continuous mold prior to receiving the molten thermoplastic adhesive. 
     The cooling zone includes a vacuum chamber for receiving the molten mold material from the die head and expanding the outer wall of the continuous mold outwardly. A calibrator assembly surrounds the continuous mold for controlling the outward expansion of the outer wall of the continuous mold. Finally, a cooling fluid surrounds the molten mold material to cool and solidify the mold material. A second cooling zone downstream from the discharge of the melt pump then cools the molten plastic melt stream discharged into the continuous mold below its softening point temperature to form the thermoplastic adhesive inside the continuous mold. 
     In addition, in the preferred embodiment, a conveyor downstream from the cooling zones moves the continuous mold and adhesive downstream at a speed proportion to the rate of discharge of the molten plastic material into the continuous mold. 
     Finally, a cutter downstream from the conveyor removes the continuous mold from the thermoplastic adhesive and cuts the adhesive into predetermined lengths for subsequent use. 
     Accordingly, one aspect of the present invention is to provide an apparatus for forming a plastic body. The apparatus includes: a molten plastic supply; a pump downstream from the molten plastic supply for providing a molten plastic melt stream; and a temperature-controlled conduit downstream from the pump for lowering the temperature of the molten plastic melt stream to closely above its softening point. 
     Another aspect of the present invention is to provide a melt pump for an apparatus for forming a thermoplastic adhesive cartridge. The apparatus includes: a molten thermoplastic adhesive supply; a pump downstream from the molten thermoplastic adhesive supply for providing a molten thermoplastic adhesive melt stream; a screen filter downstream from the pump for preventing the discharge of impurities in the molten thermoplastic adhesive melt stream from the pump; and a temperature-controlled conduit downstream from the pump for lowering the temperature of the molten plastic melt stream to closely above its softening point. 
     Still another aspect of the present invention is to provide an apparatus for continuously forming thermoplastic adhesive cartridges. The apparatus includes: a molten thermoplastic adhesive supply; a pump downstream from the molten thermoplastic adhesive supply for providing a molten thermoplastic adhesive melt stream; a screen filter downstream from the pump for preventing the discharge of impurities in the molten thermoplastic adhesive melt stream from the pump; a temperature-controlled conduit downstream from the pump for lowering the temperature of the molten plastic melt stream to closely above its softening point; and a continuous molder downstream from the melt pump for receiving the molten plastic melt stream from the melt pump. 
     These and other aspects of the present invention will become apparent to those skilled in the art after a reading of the following description of the preferred embodiment when considered with the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of an apparatus for continuously forming thermoplastic adhesive cartridges constructed according to the present invention; and 
     FIG. 2 is an enlarged, partial sectional view of the cooling chamber of the present invention shown in FIG.  1 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following description, like reference characters designate like or corresponding parts throughout the several views. Also in the following description, it is to be understood that such terms as “forward,” “rearward,” “left,” “right,” “upwardly,” “downwardly,” and the like are words of convenience and are not to be construed as limiting terms. 
     Referring now to the drawings in general and FIG. 1 in particular, it will be understood that the illustrations are for the purpose of describing a preferred embodiment of the invention and are not intended to limit the invention thereto. As best seen in FIG. 1, an apparatus for continuous forming thermoplastic adhesive cartridges, generally designated  10 , is shown constructed according to the present invention. The system  10  includes two major sub-assemblies: a melt pump  12  and a continuous molder  14 . 
     The melt pump  12  includes a molten thermoplastic adhesive supply comprising an adhesive mixer  22  in which the adhesive components are initially melted to form an adhesive melt. Depending on the particular adhesive being prepared, the temperature in the mixer may vary between about 200° F. and 450° F. The melt is transferred via pump  30  to a melt holding tank  28 . Both the mixer  22  and the holding tank  28  are insulated and may be provided with hot oil jacket heaters to maintain the adhesive in a molten state. A second pump  18  is connected to a holding tank  28 . The holding tank  28  desirably is installed at a higher elevation than the pump  18  to create a gravity feed to the pump and to ensure that the pump  18  is provided with a sufficient net positive suction head. A pre-filter  29  may be provided on the suction side of pump  18 . Preferably, the pump  18  is a positive displacement pump, and more preferably, a gear pump. Gear type pumps offer the advantage of precise volume control, which is important to the practice of the present invention. 
     The pump  18  discharges to downstream filter  20 , which desirably is a changeable dual screen, pack-type filter well known in the plastic extrusion field. The filter serves to remove any unmelted adhesive components or particulate matter from the melt stream. Alternatively, a continuous filter, such as that available from Gneuss of Bad Oeynhausen, Germany and Matthews, N.C. may be used. The advantage of a continuous filter is that its use avoids momentary flow interruptions that may cause problems downstream of the filter. 
     The still molten adhesive then travels to a temperature control system which includes a first heat zone located at the inlet to heat exchanger  34 . Although it is expected that any suitable heat exchanger may perform adequately, a particularly desirable construction is a tube and shell heat exchanger that uses a water cooling medium to cool the melt stream to just above the softening point of the molten adhesive. In a preferred embodiment, thermal oil may be used a heating medium and water as a cooling medium. Thus, the heat exchanger creates a second, lower temperature zone at its exit. 
     The term “softening point” refers to the “ring and ball” test as determined by ASTM test procedure E28 (06.03). Typical softening points for various hot melt adhesives that may be prepared according to the present invention are shown in Table 1 below. These compositions are available from Jowat Corporation of High Point, N.C. 
     
       
         
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Adhesive 
                 Softening Point (° C.) 
               
               
                   
                   
               
             
             
               
                   
                 JT 286 00 
                 110-120 
               
               
                   
                 (EVA) 
               
               
                   
                 JT 268 00 
                  95-100 
               
               
                   
                 (APAO) 
               
               
                   
                   
               
             
          
         
       
     
     Precise temperature control through the heat exchanger  34  is desirable as it has been observed that, for adhesives having a crystalline structure, the molten adhesive is more prone to solidify on the heat exchanger tubes thus forming an insulating blanket thereon. It will be readily understood that this insulator interferes with the proper performance of the heat exchange. The heat exchanger  34  may further include an internal static mixer to create turbulent flow of the adhesive therethrough and thus more efficient heat exchange. 
     From heat exchanger  34 , the softened adhesive  35  is fed to continuous molder  14 . The continuous molder  14  includes molten mold material supply  51  that is fed to a crosshead die  52  to form a continuous thermoplastic mold for forming the adhesive profile  35 . Referring to FIG. 2, desirably, the inside diameter of the thermoplastic mold  53  as it exits the crosshead die  52  is greater that that of the pump discharge line  19 . 
     Any suitable thermoplastic material may be used for the continuous mold  53 , as long as the softening temperature of the mold material is greater than the softening temperature of the thermoplastic adhesive, with low-density polyethylene being preferred. A suitable cross head extruder for this purpose is a Model DS 50H available from Davis-Standard of Somerville, N.J. 
     The thermoplastic material making up the mold exits the die head in a molten state. The softened adhesive and the thermoplastic material then enter a cooling chamber  54 , which is maintained under a vacuum. The cooling chamber  54  contains a cooling fluid bath  57  that completely covers and hardens the mold  53  immediately upon its entry into the chamber. Desirably, the cooling fluid is chilled water maintained at a temperature between about 32° F. and about 50° F. Other cooling fluids that are compatible with the adhesive and the mold material may be used. A vacuum of between about 6 and 25 inches of water is created in the chamber  54 . Lower vacuum ranges then about 6 have been found to sometimes produce an unstable mold. Higher vacuum ranges than about 25 do not appear to significantly increase mold stability. Because of the vacuum, the thermoplastic mold expands to contact the calibrator assembly as described below. 
     As best seen in FIG. 2, a first cooling zone  55  is defined in the chamber between the chamber entry point and a point downstream of the entry at which the mold has hardened sufficiently to contain the softened adhesive without melting the mold. Thus, the cooling zone is positioned at a point upstream of the adhesive pump discharge so that the softened adhesive discharges into a continuous hardened tube of mold material. Because the mold is comprised of a thin skin of thermoplastic material, it will be flexible even after it is hardened. 
     The adhesive completely fills the inside diameter of the mold because a slight backpressure is purposely created at the process startup to cause the pump discharge  19  to be submerged in adhesive. Desirably, the adhesive extends upstream past the end of the discharge tube for a short distance (i.e. the adhesive backfilled around the tube). It has been discovered that this arrangement improves tube filling. The flow rate of the adhesive is controlled within tight tolerances to maintain this amount of submergence. For example, if the adhesive flow rate is too high, over backfilling can occur and the adhesive can fill completely the air space between the outside of the pump discharge tube and the mold and break the continuous mold. 
     It has been found that gear type pumps provide sufficient performance to meet the needs described above. Gear pumps transport precise amounts of fluid with each rotation of the pump elements. Thus, controlling the speed of the pumps permits the flow rate of the adhesive to be controlled precisely. As discussed above, precise control is needed to maintain the pump discharge submergence, however future improvements in extruder designs may permit substitution for the gear pump if the flow rate can be sufficiently controlled. 
     The outside diameter of the mold and indirectly the outside diameter of the adhesive profile is controlled by calibrator assembly  58  which is comprised of a plurality of spaced apart, donut-shaped plastic rings. The centerline of each of the rings is aligned with the center of the adhesive profile. The vacuum in the cooling zone causes the mold  53  to swell so that it maintains contact with the rings. As can be seen, the ring spacing generally increases as the mold/adhesive travels through the vacuum chamber since less support is required as the mold solidifies. The vacuum chamber may also include a plurality of support rollers downstream from calibrator assembly  58  to prevent the adhesive profile from sagging. 
     A second cooling zone is defined starting at a point at which the mold is hardened sufficiently to carry the adhesive. It is in this cooling zone that the adhesive is cooled until it is hardened. The second cooling zone extends through the remaining length of the cooling chamber  54  and extends to second cooling chamber  56 . The second cooling chamber  56  is also provided with a chilled water bath. Additional cooling chambers may be provided downstream of the second cooling chamber  56  as desired. It is believed that increasing the number of cooling chambers permits the adhesive throughput to be increased. 
     A puller  60  is provided downstream of the last cooling chamber for pulling the mold encased adhesive from the pump. A suitable puller assembly is the Model 210 6P-48 available from Conaire of Pittsburgh, Pa. This puller is comprised of upper and lower flexible continuous belts that engage the upper and lower surfaces of the adhesive profile. 
     A cutter  62  cuts the continuous adhesive profile into cartridge sized sections for use in a variety of commercial glue applicators. A suitable cutter apparatus for the practice of the present invention is the Model SC5, also available from Conaire. 
     Certain modifications and improvements will occur to those skilled in the art upon a reading of the foregoing description. By way of example, inorganic and organic fillers could be added to the melt to modify the end characteristic of the cartridges. Also, EVA having a low melt index could be used to form the continuous molding skin which would allow the skin to be left on the extruded EVA adhesive cartridge. While the preferred cross-section of the cartridge is round, an oval shape could be formed. It should be understood that all such modifications and improvements have been deleted herein for the sake of conciseness and readability but are properly within the scope of the following claims.