Abstract:
An extrusion system is provided with a screen changer between in-line screw sections. An in-line seal diverts high pressure, high temperature foamable material toward the screen changer. The seal may have reverse flights to provide a visco-dynamic seal. The screw sections and the seal may be located in a common barrel housing. The screw sections and the seal may be integrally rotated as a single in-line unit. A device including an injection valve may be attached to the upstream screw section to introduce a foaming agent into the material being extruded.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to single extruder systems for processing, screening and extruding foamable plastic material. More particularly, the invention relates to a system for flowing plastic material through a screen changer at an upstream point where the material is at relatively high temperature and pressure. 
     To manufacture foam plastic products, it is known to flow molten foamable material through a screw extruder, then through a screen changer, and then through a die. The molten material is cooled as it flows through the extruder. The cooled material should have sufficient viscosity to prevent premature foaming. 
     High viscosity material, however, may require a larger or more complicated screen changer. If the screen changer is not large enough, the pressure drop across the screen changer may cause premature foaming. Also, the increased pressure demand at the end of the extruder results in more shear heat in the extrudate, lessening cooling efficiency. 
     A prior art screen changer is shown in U.S. Pat. No. 3,856,277 (Tiramani). 
     SUMMARY OF THE INVENTION 
     The problems and disadvantages of the prior art are overcome to a great extent by providing a foam extrusion system with a screen operatively located between first and second in-line screw sections. 
     In one aspect of the invention, a seal structure directs the molten material from the first screw section and toward the screen. In a preferred embodiment, the seal structure has reverse flights to provide a visco-dynamic seal. Preferably, the reverse flights cause a small amount of molten material to be recycled upstream, eliminating stagnation and eventual degradation of the extrudate. 
     In another aspect of the invention, the seal structure is rotated by the first screw section, and the rotation of the second screw section is driven by the seal structure. In a preferred embodiment, the screw sections and the seal structure are integrally connected together and located within a common barrel-shaped housing. 
     The screen may be located within a screen changer. In a preferred embodiment of the invention, a system of bypass conduits provide fluid communication between the barrel housing and the screen changer. 
     In another aspect of the invention, foamable plastic material is extruded by rotating a primary screw section, a seal section, and a secondary screw section, and by causing the material to flow through the primary screw section, then through the screen changer, and then through the secondary screw section. In a preferred method, the plastic material has a gaseous foaming agent entrained therein. As an alternative, the screen changer can be positioned at a location along the barrel before the foaming agent is injected into the process. 
     An object of the invention is to provide an economical, uncomplicated and easy to use foam extrusion system. 
     Another object of the invention is to provide an extrusion system that does not require a large screen and screen changer. 
     Another object of the invention is to locate a screen changer midway within a single extrusion line. Preferably, the plastic material is relatively cool and viscous at the downstream end of the line, near the die. The screen changer may be located at a point where the plastic material is at relatively high temperature (and hence lower viscosity) and where a pressure drop is more acceptable to the process. 
     Another object of the invention is to avoid stagnation and degradation of plastic material within the extrusion system. In a preferred embodiment of the invention, stagnation is avoided by providing the seal with reverse flights. The seal is rotated integrally with the screw sections. Consequently, the reverse flights cooperate with the interior of the barrel housing to cause a small amount of plastic material to recycle through the screen changer. 
     The present invention provides an uncomplicated drive system for an extrusion system. This object may be achieved by constructing the seal as an integral driven part of the screw sections. With this arrangement, the second screw section may be driven directly from the rotation of the first screw section. The invention avoids the need for multiple, flighted extruders. 
     The present invention may be used with a wide variety of plastic materials and foaming agents, including but not limited to, high and low density polyethylene, polystyrene, polypropylene, PET and the like. 
     These and other objects, features and advantages of the invention will become apparent from the following detailed description of preferred embodiments of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partially broken away side view of an extrusion system constructed in accordance with the present invention. 
     FIG. 2 is a cross sectional view taken along line  2 — 2  of FIG.  1 . 
     FIG. 3 is a cross-sectional view taken along line  3 — 3  of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring now to the drawings, where like reference numerals designate like elements, there is shown in FIG. 1 an extrusion system  10  for producing molten foamable plastic material (not illustrated). The extrusion system  10  has an inlet unit  12 , a primary screw section  14 , a heater  16 , a screen changer  18 , a secondary screw section  20 , and a cooler  22 . In operation, plastic material is fed into the inlet unit  12 , conveyed downstream (from right to left as viewed in FIG. 1) through the primary screw section  14 , and melted by the heater  16 . The molten material flows through a screen  24  in the screen changer  18 , conveyed through the secondary screw section  20 , and cooled by the cooler  22 . 
     A seal structure  26  is used to divert the flow of molten material through the screen changer  18 . The seal structure  26  is also used to integrally drive the secondary screw section  20  with the primary screw section  14 , as discussed in more detail below. 
     The inlet unit  12  may have a hopper  28  for gravimetrically or volumetrically receiving and blending plastic pellets (not illustrated). The pellets flow by gravity from the hopper  28  into the primary screw section  14 . The illustrated system  10  may be used to process two hundred to eight hundred pounds (ninety to three hundred sixty kilograms) of plastic material or more per hour. 
     The primary screw section  14  has a primary screw  30  located within a barrel housing  32 . The screw  30  has a cylindrical core  34  and screw threads  36 . The screw threads  36  cooperate with the interior cylindrical surface of the housing  32  such that rotation of the screw  30  causes the plastic material to flow downstream. The housing  32  is shown in longitudinal cross section in FIG.  1 . 
     The screw  30  may be rotated by a suitable motor  38 . The motor  38  is operatively connected to the screw  30  by suitable gears  40  shown schematically within the inlet unit  12 . 
     As the material flows downstream through the primary screw section  14 , it is melted by the heater  16 . The heater  16  may be an electric resistance heater or any other suitable heating means. 
     A suitable foaming or blowing agent is injected into the molten stream of plastic material by a suitable injection valve apparatus  42 , which can be located anywhere in the primary section. 
     In the illustrated embodiment, the seal  26  is integrally connected to the primary screw  30 . This way, the seal  26  rotates in unison with the primary screw  30 . The seal  26  has reverse flights  54  on its periphery for cooperating with the interior surface of the barrel housing  32  to create a visco-dynamic seal. 
     The seal structure  26  may be integrally and drivingly connected to the cylindrical core  34  by a conical portion  52 . The smaller diameter of the conical portion  52  matches the cylindrical core  34 . The larger diameter of the conical portion  52  matches the cylindrical root of the flighted seal structure  26 . 
     The seal  26  rotates in the same direction as the primary screw  30 . Consequently, the reverse flights  54  cause a thin layer of molten plastic material to flow upstream between the seal  26  and the interior surface of the barrel housing  32 . An advantage of the illustrated arrangement is that molten material does not become stagnant between the exterior surface of the seal  26  and the interior surface of the barrel housing  32 . If such material were to become stagnant, it could become degraded by heat. 
     A bypass conduit  56  is connected to a hole  58  (FIG. 3) immediately upstream of the seal  26 . The seal  26  causes molten plastic material to flow through the bypass conduit  56  to the screen changer  18 . A second bypass conduit  60  is connected to the downstream end of the screen changer  18 . The second bypass conduit  60  is connected to a hole  62  (FIG. 2) in the barrel housing  32  immediately downstream of the seal  26 . In alternative embodiments, the screen changer  18  and bypass structure  56 ,  60  can be located anywhere along the primary section where the plastic would be molten. 
     Thus, high pressure molten material conveyed by the primary screw section  14  is diverted through the screen changer  18  by the combined operation of the bypass conduits  56 ,  60  and the seal  26 . A small amount of plastic material is visco-dynamically recycled upstream across the seal  26 . 
     The screen  24  removes impurities from the molten material in a manner known in the art. The screen changer  18  changes the screen  24  from time to time in a manner known in the art. An advantage of the invention is that the foamable material may be pressurized and highly fluid at the point where it flows through the screen changer  18 . With this feature, the screen changer  18  does not have to be large or complicated to avoid pressure loss which could cause pre-foaming. 
     Most of the molten material returning to the barrel housing  32  through the second bypass conduit  60  is conveyed further downstream by the secondary screw section  20 . The secondary screw section  20  has a secondary screw  64  with a cylindrical core  66  and threads  68 . The threads  68  cooperate with the interior surface of the cylindrical barrel housing  32 . The cylindrical core  66  is integrally and drivingly connected to the seal structure  26  by a conical portion  72 . The smaller diameter of the conical portion  72  matches the cylindrical core  66 . The larger diameter of the conical portion  72  matches the root of the seal structure  26 . 
     The cooler  22  reduces the viscosity of the molten material before it is extruded through an appropriate die  70 . The cooler  22  may be a heavy duty cooler, high flow cooling jackets or direct barrel cooling. The cooler  22  may be, for example, of the type employed by Sencorp Systems, Inc., Hyannis, Mass. The barrel housing  32  may be cooled by air or water or by any other suitable cooling means. 
     If desired, the operation of the heater  16 , the cooler  22 , the motor  38 , and other components of the system  10  may be monitored by suitable transducers (not illustrated) and controlled by a central processing unit (not shown). Information from the transducers may be presented at a suitable control panel (not illustrated). 
     The illustrated system  10  may be used to make a wide variety of end products, including, but not limited to, bottle wrap labels, anti-slip mats, medium density thin wall foam for hamburger packs, film laminated or extrusion coated foam for tableware, high density foam for egg boxes, low density foam for meat trays, low/medium density foam for paper laminated display board, and low density foam for insulation sheet, wall panels and insulation board. 
     The above descriptions and drawings are only illustrative of preferred embodiments which achieve the features and advantages of the present invention, and it is not intended that the present invention be limited thereto. Any modification of the present invention which comes within the spirit and scope of the following claims is considered part of the present invention.