Abstract:
The present invention relates to an external cooling system for a molten film tube produced by a blown film tubular extrusion process, comprised of a divergent cooling element with a divergent cooling interface containing a cooling gas deflector spaced adjacent to the molten film tube and providing an expelled cooling gas (i) in a path opposing the flow of the molten film tube toward a first exit gap and (ii) in a path with the flow of the molten film tube toward a second exit gap. A minimum gap between the divergent cooling interface and the molten film tube occurs at the first exit gap and/or the second exit gap. Advantageously, the divergent cooling interface is provided with one or more compound angles to maximize stability and cooling efficiency. Additionally, multiple cooling elements can preferably be arranged in a stackable configuration to achieve higher throughput rates. Operation is characterized by improved film holding forces without the presence of high noise levels or detrimental vibration, flutter, and drag. Additionally, employing simplified single air delivery channels, and a stackable design, significantly reduces complexity and manufacturing costs.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    Field of the Invention 
         [0002]    The present disclosure relates to a method and apparatus for cooling. The present disclosure relates more particularly to a method and apparatus for high performance cooling. 
         [0003]    Description of Related Art 
         [0004]    Various methods to manufacture thermoplastic blown films are well known in the plastics art, and typically involve forming a continuous, vertically oriented, seamless, annular plastic film commonly referred to as the “tube” or “bubble”. Thermoplastic material is melted and pumped by an extruder through a blown film die (die), exiting as an annular flow of a molten film, continuously drawn upward by a pair of driven squeeze rollers. Gas is typically manually injected through the die to the interior of the exiting annular flow of molten film. The driven squeeze rollers act to prevent gas from escaping, trapping the injected gas inside, forming a molten film tube which is inflated by the injected gas until at the desired size and the die is sealed off to prevent inflation gas from escaping. The molten film tube is pulled upward by the driven squeeze rollers, flowing generally upward from the die through a cooling system, where it stretches, expands, and cools around the now trapped column of injected gas until it solidifies at a frost line into a solidified film tube. The solidified film tube passes through various stabilizers and enters a flattening device, which converts the tube into a flattened double thickness thermoplastic sheet of film known as “lay-flat”. The lay-flat passes through the driven squeeze rollers, and is conveyed to downstream converting equipment such as winders and bag making machines for further processing. 
         [0005]    To remain competitive, manufacturers of blown film must maximize throughput rate and quality, however cooling system performance is a significant limiting factor. The weight of thermoplastic that is extruded per unit time divided by the circumference of the die exit, provides a commonly used measure of throughput performance, and is expressed in units of PPH/Inch, Pounds Per Hour per Inch of die exit circumference. Many different cooling systems have been developed and employed, both external and internal to the tube, and to varying degrees these systems have achieved commercial success. 
         [0006]    Blown film cooling systems provide a flow of cooling gas typically external, but in many cases also internal to the molten film tube. Cooling systems are designed using well known Bernoulli and Coand{hacek over (a)} principles, and in many cases, apply the cooling gas to flow generally along the surface of the molten film tube to create holding forces on the molten film tube, providing for both stability and cooling of the molten film tube. Excessive holding forces, if present, can cause vibration, flutter, and high noise levels in the process, and can pull the molten film tube into undesirable contact with the cooling element, creating drag and causing marks and instability in the molten film tube. In other cases, cooling gas is instead applied generally against the surface of the molten film tube, typically creating turbulent cooling with repelling forces, requiring a separate means to stabilize the molten film tube. 
         [0007]    External cooling systems, generally provide the primary means for stabilization and cooling of the molten film tube, are generally easy to operate and used on most blown film extrusion processes. External cooling systems provide a flow of cooling gas along the outside surface of the molten film tube that typically generates holding forces while cooling the molten film tube, until the cooling gas dissipates into the surrounding atmosphere. Less typically, cooling gas is aimed generally inward generating repelling forces while cooling the molten film tube, undesirably requiring a separate means to hold and stabilize the molten film tube. 
         [0008]    Present art external cooling systems are made up of various types of cooling elements. The earliest cooling element, known as a “Single Flow air ring”, still in common use today, applies a single flow of cooling gas around the molten film tube. Single Flow cooling elements typically produce good film quality, but at lower throughput rates. Additional flows of cooling gas have been added to cooling elements over time to create various multiple flow designs, such as “Dual Flow”, “Triple Flow” or “Quad Flow” designs, and some external cooling systems pair cooling elements into various configurations, depending on the application, to form what is generically known as a “Tandem” air ring. External cooling systems are typically fixed in place, but can be made adjustable in height above the die to allow extending the cooled surface area along the molten film tube, producing higher throughput, but also resulting in greater unsupported surface area between the cooling element and die, which is the hottest and weakest portion of the molten film tube, which can lead to degraded stability, making it more difficult to operate and potentially leading to a narrower range of film sizes. 
         [0009]    In contrast, internal cooling systems typically do not provide primary stabilization, and are selectively used typically to generate additional throughput beyond the capability of an external cooling system. Internal cooling systems replace manual gas injection and inflate the molten film tube with a flow of an internal supply gas that enters through the die. Although some recent high throughput internal cooling systems apply cooling gas to create holding forces, more typically cooling gas is directed against the inside surface of the molten film tube, acting to generally repel and cool the inside surface of the molten film tube. The flow of internal supply gas is trapped inside the bubble and cannot dissipate into the atmosphere, therefore complex control systems are used to balance a flow of internal exhaust gas that exits through the die to maintain a constant bubble size as is well known and understood by those skilled in the art. Internal cooling systems can be difficult or even impossible to use depending on such factors as operator skill, thermoplastic material properties, and the physical size and design of the associated die. 
         [0010]    It is highly desired to overcome the drawbacks of prior art thermoplastic cooling systems and provide a cooling system that significantly increases throughput rate, maximizes aerodynamic holding forces, allows relatively large unsupported regions of the molten tube with good stability, produces a wide range of film sizes, prevents drag on the molten film surface, minimizes turbulence, vibration and flutter, does not produce high sound power levels, and is simple and easy to control. 
       SUMMARY OF THE INVENTION 
       [0011]    In view of the foregoing, it is an object of the present disclosure to provide a method and apparatus for cooling. 
         [0012]    A first exemplary embodiment of the present disclosure presents an apparatus for cooling. The apparatus includes at least one divergent cooling element for receiving a flow of a molten film tube, the at least one divergent cooling element including a divergent cooling interface operable for expelling a cooling gas (i) in a path opposing the flow of the molten film tube toward a first exit gap and (ii) in a path with the flow of the molten film tube toward a second exit gap, wherein at least one of the first exit gap and the second exit gap define a minimum gap between the divergent cooling interface and the flow of the molten film tube. 
         [0013]    A second exemplary embodiment includes wherein the divergent cooling interface includes a cooling gas deflector for directing expelled cooling gas along the path opposing the flow of the molten film tube and along the path with the flow of the molten film tube. 
         [0014]    A third exemplary embodiment includes wherein a portion of the divergent cooling interface in the path opposing the flow of the molten film tube forms one or more compound angles, and wherein a portion of the divergent cooling interface in the path with the flow of the molten film forms one or more compound angles. 
         [0015]    A fourth exemplary embodiment of the present disclosure presents a method for cooling. The method includes receiving, by at least one divergent cooling element, a flow of a molten film tube. The method further includes cooling, by the at least one divergent cooling element, the flow of the molten film tube, wherein the at least one divergent cooling element comprises a divergent cooling interface operable for expelling a cooling gas (i) in a path opposing the flow of the molten film tube toward a first exit gap and (ii) in a path with the flow of the molten film tube toward a second exit gap, wherein at least one of the first exit gap and the second exit gap define a minimum gap between the divergent cooling interface and the flow of the molten film tube. 
         [0016]    The following will describe embodiments of the present invention, but it should be appreciated that the present invention is not limited to the described embodiments and various modifications of the invention are possible without departing from the basic principles. The scope of the present disclosure is therefore to be determined solely by the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is a cross sectional view of a device suitable for use in practicing exemplary embodiments of this disclosure. 
           [0018]      FIG. 2  is a close-up cross sectional view of an exemplary cooling element suitable for use in practicing exemplary embodiments of this disclosure. 
           [0019]      FIG. 3  is a close-up cross sectional view of an alternative exemplary cooling element suitable for use in practicing exemplary embodiments of this disclosure. 
           [0020]      FIG. 4  is a cross sectional view of an alternative device suitable for use in practicing exemplary embodiments of this disclosure. 
           [0021]      FIG. 5  is a logic flow diagram in accordance with a method and apparatus for performing exemplary embodiments of this disclosure. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0022]    Exemplary embodiments of the present disclosure relate to a high performance cooling system for the blown film tubular extrusion process providing increased throughput rate at high quality. Embodiments of the high performance cooling system include one or more cooling elements, capable of being stacked to achieve higher throughput, wherein at least one of the one or more cooling elements is a divergent cooling element incorporating a cooling gas deflector and a pair of opposed cooling members having respective opposed air foil surfaces (surfaces). The surfaces and cooling gas deflector form a divergent cooling interface that directs cooling gas to flow in opposite directions, creating suction forces. These suction forces stabilize and hold the molten film tube (melt) in cooling proximity with the divergent cooling element. Oppositely directed cooling gas flows are expelled between the divergent cooling interface and the melt (i) in a path opposing the flow of the molten film tube along a first surface toward a first air foil exit tip to form a first exit gap with the melt and (ii) in a path with the flow of the molten film tube along a second surface toward a second air foil exit tip to form a second exit gap with the melt. 
         [0023]    The divergent cooling interface includes air foil exit tips that are angled inward toward the surface of the molten film tube, and extend closer to the molten film tube than any other portion of the divergent cooling element. All portions of the divergent cooling interface are recessed within the air foil exit tips away from the molten film tube, preventing hang-up inducing drag. The angled in air foil exit tips act to compress the cooling gas stream, providing a cushioning effect that dampen vibrations and flutter in the molten film tube, and eliminate excessive sound power levels. Additionally, the angled in air foil exit tips act similar to a venturi, to accelerate the cooling gas streams flowing along the molten film tube to a higher velocity where the cooling gas streams emerge from the influence of associated cooling members at the air foil exit tips and flow along the molten film tube. This higher velocity cooling gas flow translates into higher cooling efficiency and throughput. 
         [0024]    Further, one or more compound angles are preferentially employed along the air foil surfaces. Larger surface angles (up to about 45 degrees from parallel to the molten film surface) provide greater compression of the cooling gas, which desirably improves cooling efficiency, but unfortunately, also can cause an undesirable reduction in holding force. Use of compound angles wraps the cooling gas flow smoothly around cooling element air foil surfaces, which are advantageously arranged to change from a larger to a smaller angle relative to the molten film tube, in the direction of cooling gas flow, prior reaching the air foil exit tips. This approach allows larger initial cooling member air foil surface angles which act to aggressively compress the cooling gas streams for maximum cooling efficiency, followed by smaller surface angles, just prior and up to the air foil exit tips, which act to accelerate the cooling gas streams, restoring holding force to a maximum. Divergent cooling elements with compound angles in the cooling member air flow surfaces, exhibit very high cooling efficiency, maximum holding force, and excellent stability, without vibration, flutter or high sound power levels. 
         [0025]    Advantageously, a simplified single air delivery channel is provided between first and second cooling members, feeding an inward radial flow of cooling gas arranged to impinge on an outer wall of a cooling gas deflector, preventing direct cooling gas flow against the blown film tube and separating the flow into oppositely directed first and second annular cooling gas streams. The cooling gas deflector generally arranged intermediate between the cooling members and the molten tube such that the inner wall of the cooling gas deflector is spaced further from the molten tube than the associated cooling member tips to prevent drag. Additional flows of cooling gas can be advantageously added, but are not required. 
         [0026]    Embodiments of a divergent flow, high performance cooling element of the present invention include a simplified air delivery channel feeding one or more compound angle cooling member air foil surface with an initial 22.5 degree angle followed by a 7.5 degree air foil exit tip angle, provide an increased holding force, reduced vibration and flutter resulting in measured sound power levers 18 db lower (64 times less) than divergent cooling elements with a 0 degree air foil exit tip angle. Embodiments of the present disclosure include one or more compound angle cooling member air foil surface with an initial angle between 15 and 25 degrees, followed by an exit tip angle between 5 and 15 degrees with excellent stability, efficiency gains, increased holding force, reduced vibration and flutter. However, it should be appreciated that embodiments include a cooling gas foil surface and an exit tip angle with any combination of compound angles that aid in increasing throughput, stability, and in reducing vibration and flutter. 
         [0027]      FIG. 1  shows a cross sectional view of a typical blown film extrusion process employing a short stack cooling system with divergent cooling elements of the present invention. In  FIG. 1 - FIG. 4 , all thin arrows indicating a direction are for illustrative purposes only, labeled for example as AF, and indicate a direction flow of a fluid (e.g. cooling gas). Further, Thick arrows indicating a direction are for illustrative purposes only, labeled for example as MF, and indicate a direction flow of a plastic film material (e.g. molten film tube). Thermoplastic resin is introduced through feed hopper  2  into extruder  4  where the resin is melted, mixed and pressurized. Molten resin is conveyed through melt pipe  6  into a die means  8  that forms it into an annular molten flow that exits generally from the top surface of die means  8  as a molten film tube  12 . 
         [0028]    Internal gas supply conduit  10  operably provides an internal cooling/inflating gas through die means  8  to the interior of molten film tube  12  and solidified film tube  16 . Internal gas exhaust conduit  9  operably removes internal cooling/inflating gas through die means  8  as required to maintain a desired trapped tube volume of gas inside molten film tube  12  and solidified film tube  16 , further contained by nip rollers  20 . Gas flow through Internal gas supply conduit  10  and Internal gas exhaust conduit  9  are controlled by methods commonly understood by those skilled in the art. Molten film tube  12  expands outwardly around the trapped tube volume of gas and is drawn upwardly by nip rollers  20  while being cooled to solidify at freeze line  14  forming solidified film tube  16 . Solidified film tube  16  is collapsed by flattening guides  18  before passing through nip rollers  20  forming flattened film  22 . Flattened film  22  is then conveyed to downstream equipment for conversion into usable products as desired. 
         [0029]    Annular cooling elements  23 ,  24   a - d , and  26  are arranged coaxial with and in the direction of flow of molten film tube  12 . Cooling elements  23 ,  24   a - d , and  26 , each supplied with cooling gas from a suitable external source, direct associated cooling gas alongside molten film tube  12 , generally in the same and/or opposite direction to the flow of molten film tube  12 , acting to stabilize and cool molten film tube  12 . 
         [0030]    Upward cooling gas traveling generally in the direction of flow of molten film tube  12  from cooling elements  23  and  24   a - c , and downward cooling gas traveling generally opposite the direction of flow of molten film tube  12  from cooling elements  24   a - d  flows directly into a cavity C around molten film tube  12 . Cavity C is contained and isolated from the surrounding atmosphere by enclosure  28  with additional extents formed by the portion of the cooling elements  23  and  24   a - d  in contact with cavity C (cooling element cavity portion), and the portion of the molten film tube  12  in contact with cavity C (molten film cavity portion). Cooling gas entering cavity C flows alongside and cools molten film tube  12 , and exhausts between cooling elements  23  and  24   a - d  to the surrounding atmosphere. Generally upwardly directed cooling gas from cooling element  26  flows unrestricted, along molten film tube  12 , directly influenced by the surrounding atmosphere, while cooling and allowing for free expansion of molten film tube  12 . 
         [0031]      FIG. 2  shows a cross sectional view of the inner portion of one half of a divergent cooling element, of the present invention. Each divergent cooling element ( FIG. 1, 24   a - d ) is provided with an interior air plenum  38  of any suitable shape and size, that directs the associated supplied cooling gas to flow generally radially inward through annular channel  40  formed between cooling members  42  and  44 . Annular channel  40  feeds cooling gas toward cooling gas deflector  46 , inwardly supported from cooling members  42  and/or  44  using common, readily available fastening means such as screws and washers, not shown. Cooling gas deflector  46  prevents direct cooling gas flow against molten film tube  12  and separates cooling gas flow into oppositely directed cooling gas streams  48  and  50 . Cooling gas stream  48  flows generally opposite the direction of the flow of molten film tube  12 , between air foil surface  52  and molten film tube  12 . Cooling gas stream  50  flows generally in the same direction of flow of molten film tube  12 , between air foil surface  54  and molten film tube  12 . 
         [0032]    Air foil surfaces  52  and  54  are annularly angled inward toward molten film tube  12 , in the direction of respective air flow just prior to terminating at air foil exit tips  56  and  58 , where cooling gas streams  48  and  50  leave the influence of air foil surfaces  52  and  54  respectively and flow in cooling contact along the surface of molten film tube  12 . Compound angles are preferentially employed along air foil surfaces  52  and  54  with larger angles relative to molten film tube  12  located nearest to cooling gas deflector  46 , and smaller angles located adjacent respective air foil exit tips  56  and  58 . Importantly, no portion of the divergent cooling element having a divergent cooling interface comprised of cooling gas deflector  46 , air foil surfaces  52  and  54 , and air foil exit tips  56  and  58  is closer to the molten film tube than either one or both of the air foil exit tips  56  and  58 , to ensure that no mechanical contact with molten film tube  12  can occur. 
         [0033]    In  FIG. 3 , cooling gas deflector  46  is replaced by cooling gas deflector  46   a  located intermediate cooling members  42  and  44 , forming a pair of annular channels  40   a  and  40   b . Cooling gas from interior air plenum  38  generally flows radially inward, independently through annular channels  40   a  and  40   b,  exiting the influence of cooling gas deflector  46   a  as oppositely directed cooling gas streams  48  and  50 , respectively. Cooling gas stream  48  flows generally opposite the direction of flow of molten film tube  12 , between air foil surface  52  and molten film tube  12 . Cooling gas stream  50  flows generally in the same direction of flow of molten film tube  12 , between air foil surface  54  and molten film tube  12 . 
         [0034]    As described in  FIG. 2 , air foil surfaces  52  and  54  are annularly angled inward toward molten film tube  12 , in the direction of respective air flow just prior to terminating at air foil exit tips  56  and  58 , where cooling gas streams  48  and  50  leave the influence of air foil surfaces  52  and  54  respectively and flow in cooling contact along the surface of molten film tube  12 . Compound angles are preferentially employed along air foil surfaces  52  and  54  with larger angles relative to molten film tube  12  located nearest to cooling gas deflector  46   a,  and smaller angles located adjacent respective air foil exit tips  56  and  58 . Importantly, no portion of the divergent cooling element having a divergent cooling interface comprised of cooling gas deflector  46   a,  air foil surfaces  52  and  54 , and air foil exit tips  56  and  58  is closer to the molten film tube than either one or both of the air foil exit tips  56  and  58 , to ensure that no mechanical contact with molten film tube  12  can occur. 
         [0035]      FIG. 4  depicts a cooling system employing high performance, divergent cooling elements of the present invention in a configuration similar to  FIG. 1 , but with the addition of a enclosure  28 , variable exhaust device  30 , variable controller means  32 , and flow buffer  34  with freely swinging flapper  36  as described in co-pending application titled Controlled Pressure Enclosure filed on Jan. 15, 2016 with first named inventor Robert E. Cree, filed under attorney docket number 100646.000004, the contents of which is hereby incorporated by reference. Cooling gas supply conduits  60  are also added, spaced generally inside and around the perimeter of cooling elements  23 ,  24   a,    24   b  and  24   c,  forming a common supply of cooling gas. Cooling gas supply conduits  60  also act to space apart and locate concentric to molten film tube  12  each of the associated cooling elements  23 ,  24   a,    24   b  and  24   c.  Cooling element  24   d  is advantageously shown supplied with cooling gas in common with cooling element  26 , forming a high performance triple flow air ring. Cooling element  26  is depicted with one single-flow of cooling gas, but can include more than one flow of cooling gas, forming further high performance multiple-flow versions in combination with cooling element  24   d  of the present invention, exiting to flow unrestricted, generally upward and along molten film tube  12 , directly influenced by the surrounding atmosphere, while cooling and allowing for free expansion of molten film tube  12 . Cooling element  26  may also be omitted, allowing cooling gas exiting from the upper most located high performance, divergent cooling element to either be located below frost line  14  and allow for free expansion or be located above frost line  14  and constrain the molten film tube  12 . 
         [0036]    The present invention is presented on an upward blown film extrusion process, but equally applies to horizontal or downward versions of the blown film extrusion process, without limit. Further, the present invention can be employed in a linear rather than annular configuration, and applies to collapsing frame stabilization as well as single sheet cast film prior art. 
         [0037]    Referring to  FIG. 5 , presented is an exemplary logic flow diagram in accordance with a method and apparatus for performing exemplary embodiments of this disclosure. Block  502  presents receiving, by at least one divergent cooling element, a flow of a molten film tube; and cooling, by the at least one divergent cooling element, the flow of the molten film tube, wherein the at least one divergent cooling element comprises a divergent cooling interface operable for expelling a cooling gas (i) in a path opposing the flow of the molten film tube toward a first exit gap and (ii) in a path with the flow of the molten film tube toward a second exit gap, wherein at least one of the first exit gap and the second exit gap define a minimum gap between the divergent cooling interface and the flow of the molten film tube. Block  504  relates to wherein the at least one divergent cooling interface comprises a cooling gas deflector for directing expelled cooling gas along the path opposing the flow of the molten film tube and along the path with the flow of the molten film tube. 
         [0038]    Then block  506  indicates further comprising cooling by a second cooling element stacked adjacent the at least one divergent cooling element. Block  508  specifies further comprising cooling the flow of the molten film tube by at least one of a triple flow air ring and a multiple flow air ring. Block  510  then indicates wherein a space is defined between the at least one divergent cooling element and the second cooling element to allow gas exchange with a surrounding atmosphere. Block  512  then states wherein a portion of the divergent cooling interface expelling the cooling gas in the path opposing the flow of the molten film tube forms compound angles, and wherein a portion of the divergent cooling interface expelling the cooling gas in the path with the flow of the molten film forms compound angles. 
         [0039]    Block  514  relates to wherein the expelled cooling gas from the at least one divergent cooling element sufficiently cools the molten film tube at a rate between 0.5 and 5 (pounds/hour)/(inch of die circumference). Finally block  516  then states wherein at least a portion of the cooling gas is received by at least one enclosure comprising a cavity for receiving the cooling gas from the at least one divergent cooling element, the at least one enclosure operable to maintain a predetermined pressure differential between an inside surface and an outside surface of the flow of the molten film tube. 
         [0040]    The logic flow diagram may be considered to illustrate the operation of a method. The logic flow diagram may also be considered a specific manner in which components of a device are configured to cause that device to operate, whether such a device is a blown film tubular extrusion device, controlled pressure enclosure, or divergent cooling element, or one or more components thereof. 
         [0041]    Embodiments of the present invention has been described in detail with particular reference to particular embodiments, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restrictive. The scope of the invention is indicated by the appended claims, and all changes that come within the meaning and range of equivalents thereof are intended to be embraced therein.