Abstract:
In the extrusion of plastic pipe wherein the pipe is in a self supporting but semi-molten state on exiting a die orifice, the inner surface of the extrudate is cooled by a device which applies a suction for establishing flow of a cooling fluid which contacts the inner surface of the extrudate. In a further embodiment the flow of cooling fluid established for cooling the extrudate additionally cools a cavity within a mandrel of the die orifice.

Description:
This invention relates to the extrusion of plastic pipe. In one aspect this invention relates to a method of and an apparatus for extruding thermoplastic materials to form pipes or other hollow elongated articles having preselected cross sectional dimensions. In another aspect the present invention relates to improved apparatus and method for cooling the plastic material as it moves through and out of the die. 
     BACKGROUND OF THE INVENTION 
     Extruders commonly convert granular resin feeds into sheets, films and shapes such as pipes. Normally plastic pipe is produced by extruding molten plastic through a pipe shaping die orifice of an extruder. As used herein a pipe shaping die orifice is an elongated annular opening formed between an internal mandrel and a surrounding bushing which is spaced apart from the internal mandrel. 
     Since the viscosity of the molten plastic is temperature dependent, temperature measurement is an important operational guide to extruder operation. Temperature control schemes generally divide the extruder barrel into temperature control zones, with a temperature control loop including a controller, temperature sensor, heater and an over temperature alarm for each zone. Band-type electrical resistance heaters of two piece construction to facilitate removal are generally employed for directly heating the barrel. 
     Extruders require large heater ratings to decrease heating time. After extrusion has begun, however, heat is internally generated from friction, and at high production speeds the internal temperature, if uncontrolled, rises above the accepted maximum. 
     One of the factors which limits the production rate of an article formed by extruding a plastic material through an elongated die orifice is the rate at which heat can be removed from the article as it moves through the die. For an extrudate to be formed so that it is self-supporting when it emerges from the die, the plastic must be cooled below its solidification temperature before exiting the die. In the past this cooling has been accomplished by a variety of ways including, for example, circulating a cooling fluid through the outer portion or bushing of the die as well as through the mandrel. While cooling fluid circulation through the die, and other prior methods have proved effective for extruding relatively small diameter thin wall pipes, the prior art systems lack effectiveness in cooling larger diameter thicker wall pipes. This ineffectiveness in heat transfer from the plastic is due to the increased wall thickness of the large diameter pipe and thus lengthened heat conduction path for heat to escape from the extrudate so that heat is trapped around the mandrel of the die. The excessive temperature results in a condition known as control system temperature override which at least requires a reduction in extrusion speed and hence production rate to satisfy the temperature requirements. 
     Accordingly, it is an object of this invention to provide an improved method and apparatus for extruding plastic material. 
     Another object of this invention is to improve the rate of cooling the extrudate as it passes through the die of an extruder. 
     Still another object of this invention is to improve the production rate for extruding relatively large diameter pipes. 
     A yet more specific object of this invention is to provide a device for removing heat from the mandrel and from the inner surface of the extrudate passing from the die of an extruder. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with this invention, it has been found that it is possible to cool the extrudate advancing through the die of an extruder by applying a suction for establishing flow of cooling fluid which contacts the inner surface of the plastic pipe. 
     In a preferred embodiment of this invention a semi-molten but self-supporting plastic pipe emerging from the pipe shaping orifice of an extruder is guided, while it is still hot and in a pliable state, to a sizing sleeve which can have a range of diameters which can be a smaller diameter or a larger diameter than the die orifice of the extruder. After being sized the extruded pipe is cooled by spraying a cooling fluid on the outer surface of a section of the pipe and further cooled along the inner surface of the entire length of the pipe by cool ambient air induced to flow toward the mandrel along the inner surface of the plastic pipe by the suction action of a venturi blower/exhauster. The venturi blower/exhauster is positioned along the longitudinal axis of the mandrel with the venturi blower outlet aligned so as to discharge in the axial direction with advancement of the plastic pipe. As used herein a venturi blower/exhauster is a venturi device in which a motive gas, such as compressed air, entering a motive gas inlet creates a vacuum at the suction inlet of the blower/exhauster which induces large volumes of surrounding air to enter the venturi blower/exhauster through the suction inlet and discharge rapidly through the venturi discharge outlet. 
     Additional objects, and advantages of the invention will be apparent from the following detailed description of the preferred embodiment of the invention as illustrated by the drawings in which. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic illustration of an extruder apparatus which can be used with the improved die of the present invention. 
     FIG. 2 is a cross sectional elevation view of an extrusion die according to the present invention. 
     FIG. 3 is a cross sectional view taken along line 3--3 of FIG. 2. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     It will be appreciated by those skilled in the art that, since FIG. 1 is schematic only, many items of equipment which would be needed in a commercial plant for successful operation have been omitted for the sake of clarity. Such items of equipment would include, for example, pressure measurement instruments and corresponding controllers, additional temperature measurement instruments and temperature controllers, additional heat exchangers and valves, switches, etc., all these items would be provided in accordance with standard engineering practice and form no part in the explanation of the present invention. 
     Although the present invention is illustrated and described in terms of plastic pipe extrusion the present invention is applicable to the extrusion of any hollow elongated article of preselected cross sectional configuration which can be produced from a thermoplastic material. The preferred thermoplastic materials include but are not limited to polyethylene, polypropylene, polymethylpentene, polyhexene, and mixture of any two or more thereof. 
     Referring now to the drawings, and particularly to FIG. 1, the extrusion die of this invention, shown generally by reference numeral 10, is illustrated in conjunction with a single screw type extrusion apparatus, including gear box 12, which supports an extrusion barrel 14. A motor 18 and a coupler 20 which would drive the gear box 12 are also illustrated in FIG. 1. 
     Band type electrical resistance heaters 16, which are spaced along the length of the barrel 14 to insure that the plastic material is molten as it enters the extrusion die 10, are illustrated. Other types of heating can also be employed, for example, induction heating wherein the extruder barrel steel is heated by internal circulating electric currents may be employed, if desired. Temperature sensors 25 which are spaced apart along the length of the barrel are illustrated in FIG. 1. These temperature sensors 25 would be utilized in conjunction with temperature controllers, not illustrated in FIG. 1. 
     A thermoplastic material in particle form can be charged to a feed hopper 23 by any suitable means. It should be evident that the extrusion apparatus illustrated in FIG. 1 is for illustration only and that the extrusion die of the present invention can be used with any extruder including dynamic extruders, single or multiple screw extruders having multiple dies, and the like, provided they have the pumping capacity necessary. 
     Referring now to FIG. 2 of the drawings, the novel extrusion die of the present invention will be described in more detail. It should be obvious that the invention is not bound by the exact configuration illustrated in FIG. 2 since the novel concept of providing a venturi blower/exhauster for cooling the inner surface of the extrudate can be applied to various extrusion dies having various modifications. 
     The die 10 includes a mandrel 22 and a bushing 24 positioned around the mandrel and spaced apart therefrom. The annular opening between the bushing 24 and the mandrel 22 defines the pipe shaping orifice of the die 10. The relative size of the mandrel 22 and the bushing 24 which define the pipe shaping orifice of the die 10 may vary over wide limits. 
     In operation essentially liquid plastic material such as high density polyethylene 32, enters the die 10. Plastic pipe 34 which is in a semi-molten or expandable state with a plastic memory exits the end of the orifice in the die 10. This state of the plastic pipe is characterized by a temperature below the melting point in the case of crystalline plastic materials but not so far below the melting point that the material cannot be expanded and shrunk. The pipe when expanded and allowed to shrink returns to its original diameter. 
     On exiting the end of the orifice in the die 10 the semi-molten pipe 34 is guided through a sizing sleeve 36 which as previously stated may be a larger diameter or a smaller diameter than the pipe shaping orifice. The sleeve 36 illustrated in FIG. 2 is a smaller diameter than the pipe shaping orifice of die 10. U.S. Pat. No. 4,314,958, the disclosure of which is incorporated herein by reference, discloses an external expanding mandrel for increasing the diameter of the pipe to a maximum value significantly above the inner diameter of the die bushing, so that a preselected pipe diameter which is greater than the diameter of the bushing can be produced without changing the size of the pipe shaping orifice. 
     After passing through the sizing sleeve 36 the pipe 34 is rapidly cooled by means of a quench fluid sprayed onto the outer surface of the hot pipe 34 from quench nozzles 38. Preferably the quench fluid is sprayed onto the pipe within a vacuum tank 40. The cooled pipe 35 exiting the vacuum tank 40 is in a rigid state. 
     As illustrated in FIG. 2 and FIG. 3, a venturi blower/exhauster 26, generally aligned with the longitudinal axis of the mandrel 22, is attached to the exit end 39 of the mandrel 22 by any suitable means. In a preferred embodiment an annular mounting flange 33 of the blower/exhauster 26 is supported by thin rigid members 31 bolted to the exit end of the mandrel 22 thus providing an opening 29 between the venturi blower/exhauster 26 and the cavity 44 of the mandrel 22. The size of the opening 29 may vary over wide limits. 
     The venturi blower/exhauster 26 is a well known component such as is described in Perry&#39;s Chemical Engineering Handbook, 5th Edition, Chapter 6, and is commercially available, for example, under the name Jet-air Venturi Blower/Exhauster from McMaster-Carr, Chicago, Ill. 
     The following specific example is intended to further illustrate this invention but not to limit the scope thereof unduly. 
     EXAMPLE 
     In an apparatus as shown in the drawings, plastic pipe was produced from a high density ethylene-hexene-copolymer known as Marlex® TR480-01 manufactured by Phillips Petroleum Company, Bartlesville, Okla., which is a high density polyethylene having a density in the range of 0.945-0.952 g/cc determined in accordance with ASTM D-1505 and having a melt index of 13 g/min. The outer diameter of the pipe shaping orifice was 42 inches. The semi-molten pipe with a wall thickness of 3.5 inches was reduced in size and cooled according to this invention. The outer diameter of the resulting pipe was 32 inches with a nominal wall thickness of 2.91 inches. 
     For operation of the blower/exhauster 26, compressed air in a range from about 46 SCFM to about 315 SCFM and at a pressure in a range from about 20 psi to about 150 psi can be supplied to the motive gas inlet of blower/exhauster 26 with a resulting discharge flow in a range from about 1300 SCFM to about 8000 SCFM. In a presently preferred embodiment about 63 SCFM of compressed air at a pressure of about 80 psi was provided via conduit 28, and an exit volume of about 1400 cfm was directed along the center of the extruded pipe. Further in operation of the blower/exhauster 26 the volume of air entering the inlet 39 of blower/exhauster 26 was drawn from relatively cool ambient air surrounding the pipe 35 and drawn into the end of the pipe 42. The cooling fluid thus flowing along the inner surface of the pipe 34 countercurrent to the air flow indicated by the arrow 27 which is discharged from blower/exhauster 26. As illustrated in FIG. 2 the cooling air flows into the mandrel cavity 44. This cooling air flow is illustrated by the arrows 46 in FIG. 2. 
     Reasonable variations and modifications such as repositioning the blower/exhauster 26 within the cavity 44 of the mandrel 22 or within the pipe 34, which will become apparent to those skilled in the art, can be made from this invention without departing from the spirit and scope thereof.