Abstract:
An improved air cooling method for the production of cast film is shown. The device is intended to increase production rates for film without reducing the level of film gauge control. The devices utilizes a soft box for the bulk of the cooling and a directional source of pressurized air to deflect exhaust air from the soft box from striking the web curtain between the die and chill roll.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    The present application is a division of U.S. Ser. No. 09/560,491, filed Apr. 28, 2000, which application is incorporated herein by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The current invention concerns an enhanced fluid operated pinning and cooling assist device and method used for the production of thermoplastic film by the cast film process.  
           [0004]    2. Prior Art  
           [0005]    It has long been recognized that better contact between the molten web and the chilled casting roll used to produce thermoplastic film by the cast film process results in better web cooling rates, improved production rates, improved properties dependant upon a fast cooling rate, and improved uniformity of gauge of the resulting film. To promote better contact between the molten web and chilled cooling roll numerous devices have been tried, all successful to a greater or lesser degree, to pin the melt curtain to the casting roll and/or eliminate or minimize the layer of air trapped between the melt curtain and the casting roll.  
           [0006]    Initially, some forms of electrostatic pinning devices were used to secure the web to the chill roll. In U.S. Pat. No. 3,102,302 a full width electrode for pinning the web to the chill roll is disclosed and claimed, while on the other hand U.S. Pat. No. 3,223,757 discloses and claims an electrostatic devices for pinning the edges of the web to the chill roll. Eventually, however, increasing web speeds revealed shortcomings to the electrostatic pinning devices; as web speeds increased the tendency to entrain air between the web and the chill roll also increased.  
           [0007]    To overcome this high-speed air entrainment problem other types of devices were introduced. Typically, these devices move air either at or away from the roll and melt curtain. One of the first such devices was an air knife as shown in U.S. Pat. No. 4,874,571. This device delivers a linear stream of air against the melt curtain at or near the location at which the melt curtain strikes the chilled casting roll, thereby forcing entrapped air from between the melt curtain and the casting roll surface. Additionally, vacuum boxes designed to pull air away from the melt curtain/roll surface interface, i.e. from under the web, as shown in U.S. Pat. No. 4,310,295 were also used. To further improve contact between the web curtain and the chill roll, another device, hereinafter referred to as a soft box, was eventually introduced. The soft box approached the problem of improving contact by directing a diffuse stream of pressurized air over the web curtain. Unlike a conventional air knife, however, which delivers the air stream essentially in a linear pattern, the stream from the soft box is more diffuse and is applied over a significant portion of the roll circumference. See U.S. Pat. No. 5,091,134 for one configuration of what is herein referred to as a soft box.  
           [0008]    While the soft box has resulted in improvements in rate of film production and heat transfer related properties of the resulting film such as clarity, it has also inadvertently resulted in a reduction in film gauge uniformity. Specifically, the problem with reduced gauge uniformity appears to be related to flutter introduced into the web in the unsupported span between the die and the chill roll by air escaping from the soft box. It is therefore an object of the present invention to provide an enhanced cooling device for cast film production which will eliminate the problems associated with air discharge from conventional soft boxes. It is still another object of the present invention to provided an improved cooling device for cast film production which will result in higher production rates for cast film than heretofore seen while improving or at least maintaining acceptable gauge performance.  
         SUMMARY OF THE INVENTION  
         [0009]    In accordance with the present invention, there is a new cooling apparatus for cast film comprising a first plenum for supplying a diffuse stream of air to the surface of a melt curtain on a casting roll and a second plenum for producing a directional source of pressurized air so located at to discharge pressurized air into the air discharge stream from the upper portion of the first plenum whereby the air discharge from the first plenum does not strike and induce flutter in the melt curtain between the die and the chill roll. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0010]    [0010]FIG. 1 shows a cast film die and stand incorporating the cooling device of the present invention.  
         [0011]    [0011]FIG. 2 shows a cutaway view of the cooling device according to the present invention.  
         [0012]    [0012]FIG. 3 shows a detailed view of the linear air discharge nozzle used in the present invention.  
         [0013]    [0013]FIG. 4 shows a schematic view of the airflow from the cooling device over the casting roll and web. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0014]    As shown in FIG. 1, melt curtain  1  is extruded from slot die  2  onto casting roll  3 . A vacuum box  4  of typical construction is located adjacent to but against the direction of rotation from the point at which melt curtain  1  strikes the surface of casting roll  3 . Vacuum box  4  is used to evacuate air from between melt curtain  1  and surface of casting roll  2  to provide better heat transfer from melt curtain  1  to casting roll  3 . Following melt curtain  1  in the direction of rotation of casting roll  3  from the location at which it strikes casting roll  3  is the cooling device  10  which is the subject of the present invention. As shown in this FIG. 1, there are two major assemblies, a first plenum assembly  20  and a second plenum assembly  30 . Each plenum assembly  20  and  30  is provided with its own source of pressurized air,  7  and  8 , which typically will be a centrifugal blower. Additionally, as shown in FIG. 1, plenum  20  partially overlies casting roll  3  and melt curtain  1 , as side pieces  25  and  25 ′ of first plenum assembly  20  project inwardly and parallel to the ends of casting roll  3 . For purposes of future reference, a tangent to the surface of casting roll  3  at the point at which air discharged from second plenum assembly  30  strikes roll  3  is identified as  50 .  
         [0015]    The cooling device  10  of the present invention is shown in detail in FIG. 2. There is a first plenum assembly  20  having an air inlet  21 , a back wall  22 , a top piece  23 , a bottom piece  24 , two end pieces  25  and  25 ′, and a discharge side  27 . Additionally, there is a perforated diffuser  26  situated between back wall  22  and the discharge side  27 . Discharge side  27  is open, and in operation will be adjacent to cooling roll  3  and web  1  which lies on cooling roll  3 . The relationship between cooling roll  3 , web  1  and discharge side  27  can be seen most clearly in FIGS. 1 and 4. Immediately above the first plenum assembly  20  is a second plenum assembly  30 . Second plenum assembly  30  has an air inlet  31  and a body assembly  32  having a tubular back wall  33 , a top piece  34 , a bottom piece  35 , two end pieces  36  and  36 ′, and a linear discharge nozzle  37 . In the preferred embodiment as shown in FIG. 2, tubular back wall  33  will also serve to distribute air to the inside of the plenum and will comprises a heavy walled pipe to which top piece  34  and bottom piece  35  are attached. Tubular back wall  33  has been perforated between the points of attachment of top piece  34  and bottom piece  35 .  
         [0016]    [0016]FIG. 3 shows the detailed structure of discharge nozzle  37 , and its relationship to top piece  23  of first plenum assembly  20 , casting roll  3 , web  1 , and the discharge path  29  for air from first plenum assembly  20 . Second plenum assembly  30  is mounted directly onto the top piece  23  of first plenum assembly  20 . Mounted on top piece  34  of second plenum assembly  30  is an adjustable blade  38 . An air discharge nozzle  37  is defined between the inboard end  35 ′ of bottom piece  35  and the adjustable blade  38 . Adjustable blade  38  is mounted to top piece  34  by mounting block  39 . Any conventional means may be used to mount blade  38  to top piece  34  that will allow some movement of adjustable blade  38  with respect to bottom piece  35 . In practice it has been found that mounting block  39  to mounting top piece  34  with simple machine screws, wherein the screw holes in mounting block  39  are elongated is satisfactory.  
         [0017]    [0017]FIG. 3 also shows the relationship between the discharge zones for the first plenum and the second plenum. As shown, the discharge from the linear nozzle  37  of the second plenum discharges directly into the upper discharge stream of the first plenum. This upper discharge stream for first plenum assembly  20  is defined by top piece  23 , sidepieces  25  and  25 ′, and the outer surface of casting roll  3  and melt  1 . As shown in FIG. 3, the orientation of adjustable blade  38  is intended to strike tangent  50  on cooling roll  3  at an angle of approximately 30 degrees.  
         [0018]    [0018]FIG. 4 shows the directions of airflows associated with the present invention. Melt curtain  1  is initially struck by a linear discharge from nozzle  37  of second plenum assembly  30  and forced against casting roll  3 . Melt curtain  1  is then further forced onto the surface of casting roll  3  by the diffuse air discharge from first plenum assembly  20 . As shown in FIG. 4 and as previously discussed in conjunction with FIG. 3, air discharged from nozzle  37  is directed into the opening at the upper surface of first plenum assembly  20  formed by casting roll  3  and melt curtain  1 , side pieces  25  and  25 ′ and top piece  23 . This air stream discharged from nozzle  37  deflects air discharged from the first plenum assembly that would normally be expected to escape through this opening and redirects the flow along the direction of rotation of casting roll  3 . The net effect of the air discharges from nozzle  37  and first plenum assembly  20  is to press melt curtain  1  tightly to casting roll  3  while substantially limiting the air discharge from first plenum assembly  20  from striking melt curtain  1  between die  2  and the location at which it strikes casting roll  3 . As previously indicated, flutter or other motion of melt curtain  1  in the region between die  2  and casting roll  3  induced by air escape results in gauge variations in the finished film. It is customary for the air discharged from both the first and second plenums to be at ambient temperature; for certain applications, however, an improvement in rate may further be seen if the air discharged from the first plenum is cooled below ambient temperature.  
         [0019]    By way of example, the current invention was tested on the production of multi-layer polypropylene film. The polypropylene film had a finished width of 4300 mm and a nominal thickness of 20 microns. The film was extruded at a rate of 1000 Kg per hour. Air pressures in the first and second plenums exceeded 0.5 inches of water, and preferably were in the range of 2 to 3 inches of water and 5 to 10 inches of water respectively. Film thickness measurements made on line with an NDC Beta gauging system showed a maximum film thickness variation across the web of 1.5%.  
         [0020]    Although the invention has been illustrated by the production of polypropylene film, the invention is not to be construed as limited to the production of propylene film. All manner of thermoplastic film forming resins used in the cast film process can be processed with the current invention. The combined flows from both plenums not only reduces flutter between the extrusion orifice and the chill roll, thereby improving gauge control, but also improves rate by improved pinning of the web to the casting roll and reduced air entrainment.