Abstract:
A longitudinal stream of cooling air is caused to flow around the cylinder (10) and over the length of each of one or more heating-and-cooling zones. This longitudinal flow is facilitated by specially designed cooling fins (22; 25). The performance of the extruder is improved in that the temperature over the cross-section of the cylinder is more uniform. Moreover, wear and tear on the extrusion screw and on the cylinder are greatly reduced.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to extrusion equipment, and more particularly to a method of air-cooling an extruder cylinder, the length of which includes at least one heating-and-cooling zone, as well as to a device for implementing this method. 
     2. Description of Related Art 
     In order for plastics, usually in the form of pellets or granules and coming from a hopper, to be worked in an extruder, they are fed in at one end of an extrusion cylinder and are then led all along the cylinder by means of an extrusion screw to emerge in a pasty form through a die mounted at the other end of the cylinder. The plastic material therefore passes from a solid state to a pasty state over a given distance, this transformation of material involving substantial exchanges of heat. In order for transformation of the material to be complete, it must attain a given temperature before passing through the die; for that purpose, the temperature of the cylinder surrounding the screw must be precisely controlled, this temperature depending essentially on the type of plastic being extruded and the type of screw being used. In order to maintain the cylinder at this given temperature, heating elements and cooling devices are disposed along the cylinder. In general, the latter is divided into several successive independent heating-and-cooling zones, whereby a temperature profile along the cylinder is obtained. Before the machine is started up, the cylinder must be heated by means of the heating elements in order to bring it up to its set temperature, whereas thereafter, the kneading and mechanical constriction imposed on the plastic by the screw supply the system with an amount of heat which may be sufficient or even in excess of what is needed to keep the system at the set temperature. It may therefore be necessary to add a cooling system to the cylinder so that, by combining the heating and cooling cycles, the cylinder, and hence the plastic, remains at an approximately even temperature over a given zone of the cylinder. 
     Various cooling methods and systems have already been proposed, e.g., those using a cooling liquid such as water or oil. The main drawback of these systems resides in the complexity of the cooling circuit, as well as the particular machining of the extrusion cylinder in order to have the cooling ducts pass through it. 
     Other systems proposed use a stream of air as the coolant. The advantage as compared with the liquid cooling systems is the simplicity of the circuit and of the cylinder. Such a prior art system for cooling by air circulation is illustrated in FIG. 1, which is a cross-section through the cylinder 10 of an extruder 1. Cylinder 10 comprises a cylindrical interior space in which the screw (not shown) rotates. The cooling circuit 2 is composed, firstly, of a cool-air supply, here represented by a blower 20 causing air, generally drawn from the ambient atmosphere or from a cool-air source, to circulate within an enclosure 21 which completely surrounds the relevant heating-and-cooling zone of cylinder 10 over its entire length. Several systems such as the one shown here are usually installed, one after another all along cylinder 10, each of them constituting a heating-and-cooling zone. Heat-exchange means 22 surround cylinder 10, with various designs such as copper tongues 22A, disks 22B, or wedges 22C being shown in the drawing. Heat-exchange means 22 are associated with the heating circuit 3, heating sleeves 30 of which are merely indicated diagrammatically. As may be seen in this drawing, the circulation of cooling air takes place with a rising movement, coming from blower 20, circulating on both sides of cylinder 10 while passing through heat-exchange means 22, and escaping through a grid 23 or openings made at the top of enclosure 21, as indicated by arrows. 
     Owing to the rising movement of the cooling air, due particularly to the chimney effect, cylinder 10 is not cooled homogeneously; for the air in contact with the lower part of cylinder 10, designated as region 11, is still cool, whereas it warms up while passing over the sides of cylinder 10 to attain its maximum temperature toward the top of cylinder 10, i.e., the region designated 12. Inasmuch as the air is warmest at the top of cylinder 10, region 12 is less efficiently cooled than region 11; this leads to deformation of cylinder 10 since the elongation of the top part 12 of cylinder 10 is greater than that of the bottom part 11. In view of the longitudinal curvature taken on by cylinder 10, it is no longer exactly coaxial with the screw; the slight clearance existing between the screw and the inside surface of the cylinder is undesirably modified, which leads to premature wear and tear on the screw and on the cylinder, as well as to irregular working of the plastic being kneaded. 
     Another cooling device is disclosed in U.S. Pat. No. 4,763,722. As in the device described above, the air circulates in a direction perpendicular to the axis of the extrusion cylinder; and although in this case the air is returned for a second passage in the opposite direction, the movement of air always remains perpendicular to the axis of the cylinder, bringing about unequal cooling of the top and bottom parts of the cylinder. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of this invention to provide an improved method of cooling the cylinder of an extruder which avoids the drawbacks mentioned above, i.e., which allows uniform cooling of the cylinder over its entire cross-section. 
     Another object of this invention is to provide an improved device for cooling the cylinder of an extruder which is suitable for carrying out the aforementioned method. 
     A further object is to provide such a device which can easily be installed on an existing extruder or can be mounted on a new machine from the outset. 
     To this end, in the cooling method according to the present invention, for each heating-and-cooling zone, air-delivery means inject cool air at at least one point of entry within an enclosure surrounding the heating-and-cooling zone, the air then being directed in order to be distributed over the periphery of the cylinder and to flow substantially longitudinally along the zone, then to be exhausted at at least one point of exit from the enclosure. 
     In the device according to the invention, the improvement comprises at least one air-delivery means connected to at least one point of entry of an enclosure surrounding the cylinder, means for guiding the stream of air within the enclosure, disposed so as to distribute the stream of air around the cylinder, then to cause it to flow substantially longitudinally along the heating-and-cooling zone, heat-exchange means disposed on the periphery of the cylinder, and at least one point of exit of air. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the invention will now be described in detail with reference to the accompanying drawings, in which: 
     FIG. 1 is a cross-section through an extrusion cylinder equipped with a prior art cooling device, 
     FIG. 2 is a longitudinal section through part of an extrusion cylinder having a heating-and-cooling zone, equipped with a cooling device in a first embodiment of the invention, 
     FIG. 3 is a section similar to that of FIG. 2, with a cooling device in another embodiment of the invention, 
     FIG. 4 is a perspective view of part of a heating-and-cooling zone provided with cooling fins according to the present invention, 
     FIG. 5A is a perspective view of a cooling fin mounted on a heating collar, and 
     FIG. 5B is an end-on view of the aforementioned fin. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 2, a longitudinal section of part of a cylinder 10 of an extruder, shows one heating-and-cooling zone and portions of two adjacent ones. The extrusion screw disposed within cylinder 10 is omitted in order not to clutter the drawing. A cooling circuit 2 of this heating-and-cooling zone also comprises a blower 20 which feeds air to the outside surface of cylinder 10, jacketed in an enclosure 21. According to the present invention, and contrary to the prior art system illustrated in FIG. 1, the air circuit here is so arranged that after the air has been distributed over the periphery of cylinder 10, it flows substantially longitudinally along the cylinder portion. For this purpose, baffles 21A guide the stream of air coming from blower 20 in order to distribute it and to cause it to flow longitudinally into the heat-exchange or cooling means 22, shown diagrammatically in FIG. 2. These means 22 may be of the same type as previously described with reference to FIG. 1, or they may be specially designed for longitudinal air circulation as will be explained below. 
     As in the prior art design, the device includes a heating circuit 3, which may be composed of sleeves 30. In order to avoid the chimney effect, the hot air is exhausted from enclosure 21 through one or more openings 24 preferably disposed toward the bottom of the enclosure. Thus, the air flowing longitudinally along the heating-and-cooling zone causes uniform cooling of the cross-section of cylinder 10, the top part of this cylinder being cooled in the same way as the bottom part. In the embodiment illustrated in FIG. 2, the cool air coming from blower 20 is supplied at an approximately central location of the heating-and-cooling zone and is exhausted at the ends, whereas in the embodiment of FIG. 3, the cool air is supplied at one end of the zone and exhausted at the other. 
     The means for supplying the cool air are shown here in the form of a blower 20 associated solely with the particular heating-and-cooling zone. It will be understood, however, that one blower may feed several heating-and-cooling zones, just as the air-supply means may take the form of a cool-air line feeding a number of machines, a whole factory, or part of a factory. 
     As stated above, the cooling means 22 shown in FIG. 1, viz., tongues 22A, wedges 22C, or others not yet shown--except for disks 22B which are too specifically provided for a prior art cooling method--may be used with this novel longitudinal cooling method according to the present invention, even though their operation may not be optimal. 
     In order to increase the cooling efficiency of the system, cooling fins 25 may be disposed around cylinder 10 as depicted in FIG. 4. This drawing figure shows fins 25 on only part of the periphery of cylinder 10, but it will be understood that, in practice, fins are disposed over the entire periphery of the cylinder. Cooling fins 25 are preferably made up as indicated in FIGS. 5A and 5B. Each fin 25 has the general shape of a relatively thin rectangular parallelepiped, on the order of a few millimeters thick, some 40-50 mm high, and about twice as long, these dimensions and proportions being of a purely indicative nature. Fins 25 are made of a material which is a good heat conductor, such as copper or aluminum. 
     As may be seen in FIG. 5A, fin 25 includes an aperture 25A and a wide center slot 25B, both intended to receive and fix a conventional heating sleeve or collar. The underside 25C of fin 25, which is to be in contact with the outside surface of cylinder 10 and hence to provide the heat exchange with the latter, may be either plane or cut in a concave manner, as shown in FIG. 5B, so as to improve both the heat exchange and the alignment of fin 25 along a generatrix of cylinder 10. 
     As is apparent from FIG. 4, the side-by-side arrangement of fins 25, the underside 25C of each being in contact with cylinder 10, creates a sort of fan, leaving an air guide in the form of longitudinal channels free through which the cooling air can circulate longitudinally. The heating sleeves have been omitted from FIG. 4 in order not to clutter the drawing. FIG. 4 shows a single fan of cooling fins 25; there may be several such fans disposed one after another in a heating-and-cooling zone. 
     Cooling fins 25 are very simple to manufacture because they may be stamped out of a strip of material in order to cut them to the required dimensions, to make aperture 25A and slot 25B, and possibly to machine contact surface 25C. Fins 25 are preferably all of the same size, with the number of fins disposed around a given cylinder depending on the outside diameter thereof. The heating sleeves disposed in apertures 25A serve to hold fins 25 in place about cylinder 10; however, other means may equally well be provided for positioning and fixing fins 25 about the cylinder. 
     Owing to the simplicity of their concept, the cooling method and the device represented by cooling fins 25 may easily be adapted to all extrusion cylinders, so that either existing extruders may be modified in order to improve their performance, or the inventive device may be installed on new machines from the outset.