Abstract:
Device for extruding hollow strands from thermoplastic material, with an extruder head, having a mandrel, and a calibrating device, for making a dimensional change while production is in progress, and with a radially adjustable inlet, at least one Ranque vortex chamber being formed in the mandrel, the cooling air outlet of which chamber leads into a cooling tube, which extends as an axial extension of the mandrel through the inlet of the calibrating device and has a cooling air outlet opening out into the calibrating device. This device achieves the object of providing a device with which effective interior cooling is achieved in calibrating devices designed for making a dimensional change while operation is in progress.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the priority of German application no. 10 2006 051 104.2, filed Oct. 25, 2006, and which is incorporated herein by reference. 
   FIELD OF THE INVENTION 
   The present invention relates to a device for extruding hollow strands from thermoplastic material. 
   BACKGROUND OF THE INVENTION 
   In DE 697 13 645 T2, a device for cooling the interior of a hollow profile, in this case a plastic pipe, by means of cooling air is described. For this purpose, a hollow cylinder that is closed at the front is pushed into the hollow mandrel of an extruder head and protrudes into the following calibrating unit, an annular gap remaining between the outer wall of the hollow cylinder and the inner wall of the extruded pipe. The hollow cylinder is double-walled and is supplied with cooling water through a central feed line, which opens out into the front end wall of the hollow cylinder. This cooling water flows from its inlet point in the front end wall of the hollow cylinder radially outwards and then back through the cylindrical double casing to the extruder head. In the region of the mandrel, the double casing is bent conically inwards and comes to lie against the circumference of the central cooling water feed line. The cooling air is blown into the hollow cylinder in the direction of extrusion and deflected outwards at the cone formed by the double casing onto the wall of the hollow cylinder. Provided there are through-openings, through which the cooling air flows into the annular gap. There it passes over the inner wall of the extruded pipe and cools it down. The heat taken up by the cooling air is removed again, at least partially, by the cooling water flowing in counterflow in the double casing, so that the cooling air can remove heat from the extruded pipe over the entire length of the annular gap. 
   In U.S. Pat. No. 4,545,751, a device for cooling the interior of a corrugated tubing produced on an extrusion line is described. Screwed as an extension onto the mandrel of the extruder head of this device is a housing, which reaches into a peripheral mould for creating the corrugation of the tubing to be produced. Arranged in the housing is a Ranque vortex chamber, the cooling air outlet of which opens out into the housing. The latter has in turn radial outlet openings, through which the cooling air flows into the extruded hollow strand lying against the mould and cools it from the inside. 
   For some years, equipment that makes it possible to change the dimensions of an extruded plastic profile while the production process is in progress has been available. This includes calibrating sleeves, the cross section of which can be changed within relatively wide limits and which have an inlet that can is radially adjustable to match the changing cross section. Such a calibrating sleeve is described in DE 10 2005 002 820 B3. 
   In particular on account of their radial dimensions, the prior-art devices for cooling the interior of extruded hollow strands in a calibrating device described at the beginning cannot be used in calibrating devices designed for making a dimensional change while operation is in progress, in particular in the case of small cross sections of the hollow strands. 
   This also applies to a device for extruding hollow strands from thermoplastic material that is disclosed in the subsequently published DE 10 2005 031 747 A1. This has, inter alia, an extruder head with a mandrel and also a calibrating device. Formed in the mandrel is a least one Ranque vortex chamber, the cooling air outlet of which leads into the interior space of the extruded hollow profile. 
   OBJECTS AND SUMMARY OF THE INVENTION 
   The object of the present invention is to remedy this situation and provide a device with which effective interior cooling is achieved in calibrating devices designed for making a dimensional change while operation is in progress. 
   This object is achieved according to the invention by a device for extruding hollow strands from thermoplastic material, including an extruder head having a mandrel, and a calibrating device, for making a dimensional change while production is in progress, and with a radially adjustable inlet. At least one Ranque vortex chamber being formed in the mandrel, a cooling air outlet of which chamber leads into a cooling tube, which extends as an axial extension of the mandrel through the inlet of the calibrating device and has a cooling air outlet opening out into the calibrating device. 
   The present invention uses the known phenomenon of the Ranque vortex tube to provide a simple way of producing a cooling gas which is used for cooling the interior of an extruded hollow strand. In this case, the vortex tube does not require any additional space ahead of the extrusion die, since it is situated in its mandrel. There, the hot air generated in the vortex tube can also be meaningfully used, for example by the mandrel being additionally heated. Cooling gas produced in the vortex tube is transferred via the cooling tube, to a certain extent as with an injection needle, into the calibrating device and is available there for effective interior cooling. Since the cooling tube only has to be designed in its cross section for the amount of cooling gas to be transported, its radial dimensions can be kept small, so that it does not hinder the radial adjustment displacements of the calibrating device that are required in the case of a dimensional change, or make them impossible. 
   Further advantageous refinements of the invention are provided as set forth in detail herein, such as below, and in the claims and abstract. 
   The invention is explained in more detail below on the basis of exemplary embodiments of a pipe extrusion line. 
   Relative terms, such as left, right, up, and down, are for convenience only, and are not intended to be limiting. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a schematic side view of an extrusion line, 
       FIG. 2  shows an enlarged schematic detail A according to  FIG. 1  in a sectional representation, in the case of a device in a first operating state, 
       FIG. 3  shows an enlarged detail from  FIG. 2 , 
       FIG. 4  shows a representation according to  FIG. 3  in the case of a device in another operating state, 
       FIG. 5  shows a schematic cross section through the mandrel of an extruder head in a first embodiment of a Ranque vortex chamber, and 
       FIG. 6  shows a representation according to  FIG. 5  in a second embodiment of the Ranque vortex chamber. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The extrusion line for producing pipes that is represented in  FIG. 1  comprises an extruder unit  1  with a feed hopper  2 , an extruder screw, which cannot be seen in the drawing, and a pipe extrusion head  3 . A thermoplastic material  4  in the form of granules or powder is fed to the extruder unit  1  via the feed hopper  2 . In this extruder unit, the granules or powder is/are heated, kneaded and plasticated. Subsequently, the plastic  4  is conveyed as a mouldable compound by the extruder screw into the pipe extrusion head  3  and forced there through an annular gap  15  (see  FIGS. 2 to 4 ). 
   After emerging from the annular gap  15 , the hot, still deformable pipe  5  is drawn by means of a tracked take-off unit  6 , arranged at the end of the extrusion line, through a calibrating and cooling unit  7 , which has a vacuum tank  8  with a calibrating sleeve  9  arranged at its inlet. The calibrating sleeve  9  is infinitely variable in diameter, so that the extruded, still mouldable pipe  5  can be fixed to the desired outer diameter. After leaving the calibrating and cooling unit  7 , the pipe  5  enters a cooling zone  10 , in which it is cooled down to room temperature. Arranged between the cooling zone  10  and the tracked take-off unit  6  is an ultrasonic scanner  11 , with which the diameter and the wall thickness of the extruded pipe  5  are recorded. The tracked take-off unit  6  is adjoined by a separating saw  12 , in which the pipe  5  is cut to length. To maintain a negative pressure in the calibrating and cooling unit  7 , the cooling zone  10  and the ultrasonic scanner  11 , seals  13  are provided, enclosing the pipe  5  running through with a sealing effect. 
   Since the extruded pipe  5  is only cured, i.e. becomes dimensionally stable, after it leaves the cooling zone  10 , before that it must be supported to avoid it sagging and thereby deforming. For this purpose, two pipe supports  14  are provided in the cooling zone  10  and one is provided in the calibrating and cooling unit  7 . 
   The calibrating sleeve  9  has an annular inlet head  16  and an annular outlet head  17 . While the inlet head  16  is arranged outside the vacuum tank  8 , the outlet head  17  is in the vacuum tank  8  ( FIG. 1 ). The outlet head  17  has a fixed inner diameter, which corresponds at least to the greatest pipe diameter to be handled in the extrusion installation. It can be displaced with respect to the fixed inlet head  16  in the axial direction of the calibrating sleeve  9 , in order to change its diameter. For this purpose, at least two spindle units  18  are provided, the threaded spindles of which are motor-driven. 
   The inlet head  16  has radially adjustable segments  19  ( FIGS. 2 to 9 ), which are arranged uniformly over the circumference of the pipe  5  to be calibrated and form a conical inlet of the calibrating sleeve  9 . For the further construction of the calibrating sleeve  9 , reference is made to DE 2005 002 820 B3, the relevant disclosure of which is hereby made the subject matter of these exemplary embodiments. This calibrating sleeve  9 , in the same way as the other equipment of the extrusion line too, is suitable for making a dimensional change while production is in progress. 
   In the front end of the pipe extrusion head  3  that is shown in  FIGS. 2 to 4 , the polymer melt  41  conveyed by the extruder screw is divided in an annular manner. Provided for this purpose is a mandrel support tip  20 , which protrudes conically into the stream of polymer melt  41 . The mandrel support tip  20  is adjoined by a mandrel support spider plate  21 , by means of which a mandrel  22  of the pipe extrusion head  3  is connected to the mandrel support tip  20  by screwing. The mandrel  22  goes over at its front end into a hollow cylinder  23 , in the region of which the mandrel  22  is surrounded by a hollow-cylindrical die ring  24  while leaving the annular gap  15 , said die ring being connected to the mandrel  22  by screwing. The annular gap  15  continues through the mandrel  22  to the mandrel support tip  20 . In the mandrel support spider plate  21 , the annular gap  15  is interrupted every 90 degrees by webs of material (not represented), which however do not disturb the flow of the polymer melt  41 . 
   The hollow cylinder  23  is closed at its front end by a diaphragm  39 , which has a central outlet opening  25 , which opens out into a collecting chamber  26 . At the opposite extreme end of the hollow cylinder  23 , a diaphragm  27  is likewise provided, leaving an annular outlet opening  28  at its circumference. Arranged in the mandrel support spider plate  21  is an air supply bore  29 , which is angled away at right angles in relation to the mandrel  22  in the vicinity of the centre axis of the mandrel  22 , and is continued in the latter to the front end of the hollow cylinder  23 . There, the air supply bore  29  opens out tangentially into the hollow cylinder  23 . On account of this tangential introduction of air and the outlets  25  and  28 , the hollow cylinder  23  acts as a Ranque vortex tube. This is supplied with compressed air at a pressure of approximately 7 bar and a temperature of about 20° C. by means of the air supply bore  29 . 
   On account of this air supply into the hollow cylinder  23 , two air flows form in the latter: a hot air flow  30  at the wall of the hollow cylinder  23  and a cold air flow  31  in the vicinity of the centre axis of the mandrel  22 . The hot air flow  30  leaves the hollow cylinder  23  via the outlet opening  28  and flows from there via an air discharge bore  32 , which continues in the mandrel support spider plate  21 . The hot air flow  30  has a temperature of up to 110° C. The temperature of the cold air flow  31  is approximately 0° C. to 5° C. and flows via the outlet opening  25  into the collecting chamber  26 . From the collecting chamber  26 , the cooling air flows into a cooling tube  33 , which extends as an axial extension of the mandrel  22  through the segments  19 , i.e. through the inlet of the calibrating sleeve  9 , and has a cooling air outlet  34  opening out into the calibrating sleeve  9 . The cooling air  31  flowing out from the cooling tube  33  cools the extruded pipe  5  on its inner side in a very effective way in addition to the exterior cooling taking place in the vacuum tank  8 . In order to prevent heating of the cooling air  31  on its way into the calibrating sleeve  9 , the collecting chamber  26  and the cooling tube  33  are insulated. 
   To make the cooling more intensive, water is mixed with the cooling air  31  flowing out from the cooling tube  33 . For this purpose, a water supply bore  35  is provided, extending through the mandrel support spider plate  21  and the mandrel  22  into the front diaphragm  39  of the hollow cylinder  23  and going over there into a thin pipeline  36 , which runs centrally through the collecting chamber  26  and the cooling tube  33  and ends at the cooling air outlet  34 . 
   In order to bring the moist cooling air flow effectively into the region of the inner wall of the extruded pipe  5 , a corresponding air directing device  37  is provided ahead of the cooling air outlet  34 , and in this exemplary embodiment is configured as a cone. 
     FIG. 3  shows the production of a pipe  5  with a large diameter,  FIG. 4  shows the production of a pipe  5  with a small diameter. A comparison of the two representations shows that the cooling tube  33  and the collecting chamber  26  neither hinder the segments  19  in their radial adjustability nor adversely affect the melt cone  40  formed between the pipe extrusion head  3  and the calibrating sleeve  19 . 
   In  FIGS. 5 and 6 , two exemplary embodiments of how Ranque vortex tubes are formed in the mandrel  22  are shown. The example shown in  FIG. 5  corresponds to the exemplary embodiment explained above according to  FIGS. 2 to 4 . Here the mandrel  22  has been drilled with a bore of large diameter, so that the hollow cylinder  23  formed as a result acts as the one and only Ranque vortex tube. In this case, the collecting chamber  26  represented in  FIGS. 2 to 4  is not absolutely necessary, i.e. the cooling tube  33  may directly adjoin the outlet opening  25 . 
   In the exemplary embodiment according to  FIG. 6 , seven bores  38  of smaller diameter have been made in the front region of the mandrel  22  and each of the seven bores  38  acts independently as a Ranque vortex tube. In other words, there is a plurality of vortex tubes. Each vortex tube then has of course its own tangential air supply and its own outlet openings for the cold and warm air flows, the outlets for the cold air opening out into the collecting chamber  26 . 
   While this invention has been described as having a preferred design, it is understood that it is capable of further modifications, and uses and/or adaptations of the invention and following in general the principle of the invention and including such departures from the present disclosure as come within the known or customary practice in the art to which the invention pertains, and as may be applied to the central features hereinbefore set forth, and fall within the scope of the invention or limits of the claims appended hereto.