Abstract:
The invention relates to a device for the production of multi-layer, co-extruded, tubular pre-forms made from thermoplastic plastic. A co-extrusion head ( 10 ) comprises co-axially arranged flow channels (FK 1 , FK 2 ), each supplied by a single inlet opening (ZF 1 , ZF 2 ) with a material melt, distributed annularly in a distribution ring ( 26, 28 ). The gap width in each distribution ring ( 26, 28 ) is greater in the vicinity of the inlet opening (ZF 1 , ZF 2 ) than the gap width (s 2 , s 6 ) in the region of the side opposing the inlet opening (ZF 1 , ZF 2 ). The flow channels (FK 1 , FK 2 ) are also asymmetric with regard to the gap widths.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention concerns devices for producing multilayer, coextruded, tubular preforms made of thermoplastic material. A device of this type has a coextrusion head with several, essentially coaxially arranged, flow channels, each of which is fed from an individual inlet opening with a material melt, which is annularly distributed and flows along an annular conical frustum, wherein the flow channels become a common annular flow channel that widens like a funnel. A displaceable annular piston can reciprocate in an annular accumulation chamber, which is followed by an annular discharge channel with an annular extrusion orifice that can be closed. 
     2. Description of the Related Art 
     EP 0 326 584 B1 of the same applicant discloses a method and a device for producing large-volume hollow plastic bodies with multilayer walls. The gap widths in each flow channel are constant along the circumference of the ring. Due to the one-sided supply of the material melt, different pressure conditions thus develop along the ring and along the annular conical frustum, which impair uniform flow of the material melt, so that material mixing occurs, which leads to loss of quality of the preform. 
     In addition, DE 195 45 441 A1 of the same applicant discloses a device for producing dish-shaped molded parts made of a thermoplastic material. This device also uses a coextrusion head, in which the material melt flows in several layers, from which a multilayer tubular preform is produced. 
     SUMMARY OF THE INVENTION 
     The present invention builds on the prior art disclosed in the two documents cited above. The content of these documents is herewith incorporated in the disclosed content of the present application. 
     The objective of the invention is to specify a device of the aforementioned type, in which uniform flow of the multilayer material melt in the coextrusion head is realized. 
     A further objective of the invention is to specify a device with a simple feed device for supplying the material melt. 
     Yet another objective of the invention is to specify a device, in which the annular accumulation chamber is filled with material melt gently and with a high degree of uniformity. 
     These objectives are achieved by the objects specified in the claims. Advantageous refinements are specified in the dependent claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       The invention is explained below with reference to specific embodiments. 
         FIG. 1  shows a longitudinal section through a coextrusion head. 
         FIG. 2  shows a cross section through the first distributor ring. 
         FIG. 3  shows a cross section through the first flow channel. 
         FIG. 4  shows a cross section through the second distributor ring. 
         FIG. 5  shows a cross section through the second flow channel. 
         FIG. 6  shows a cutaway section to show the gap widths. 
         FIG. 7  shows a longitudinal section through the coextrusion head with baffle and load-relieving cylinder. 
         FIG. 8  shows a longitudinal section through the coextrusion head with a first feeding device. 
         FIG. 9  shows a longitudinal section through a second feeding device. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a partial longitudinal section of a coextrusion head  10 , which comprises an accumulator jacket  8 , which surrounds an annular accumulation chamber  14 , which holds the material melt to be discharged. The accumulator jacket  8  is connected with a housing (not shown). A first material melt is fed through an inlet opening ZF 1  to a first distributor ring  26 , which is part of an annular piston  16 . The annular piston  16  can be moved along its longitudinal axis and slides along a torpedo  6  and the accumulator jacket  8 . The annular piston  16  is connected by piston rods K 1 , K 2  with a hydraulic system (not shown). The torpedo  6  is likewise connected at its upper end with a hydraulic system (not shown). 
     The first material melt is conveyed horizontally along the first distributor ring  26  and at the same time flows downward along an annular flow channel FK 1 , which is designed as an annular conical frustum. The downward flowing first material melt then enters a first cylindrical ring  22  and from there reaches a orifice_ 30 , into which a second material melt opens. This second material melt is supplied through a second inlet opening ZF 2 , which is arranged diametrically to inlet opening ZF 1 . The second material melt is likewise distributed from the second inlet opening ZF 2  by an associated peripheral second distributor ring  28  and enters the second flow channel FK 2 , which is designed as an annular conical frustum. The second material melt flows from there into a second cylindrical ring  24 , from which it reaches the orifice  30 . 
     The two material melts should flow in such a way that the interface between the two material melts runs as smoothly as possible and is not disturbed by turbulence. The orifice  30  is followed by a region of quieted flow, which is formed as a common cylindrical ring  34 . This region of quieted flow ensures that after the material melts come together at the orifice  30 , uniform flow of the material melts is obtained, so that smooth running of the interface between the two material melts is produced. The two material melts then flow to a point of widening  32 , where they enter a widening, common flow channel  12 . This flow channel  12  has a funnel-shaped cross section and is annularly formed in the annular piston  16 . 
     At the beginning of the filling operation, the annular piston  16  is in its lower position, as is schematically indicated by broken lines in connection with reference number  15 . The funnel-shaped common channel  12  is still filled with the two material melts from the previous production operation for producing a tubular preform. The resumption of flow of the two material melts causes the annular piston  16  to move upward. Due to the funnel shape of the common flow channel  12  and the other geometry of the material melt supply system, the interface that forms between the two material melts remains largely smooth and is not swirled into a turbulent state. 
     When the annular piston  16  has reached its upper operating point, the torpedo  6  is moved downward, and its nozzle pintle  6   a  opens an annular extrusion orifice  20 , so that during a downward movement of the annular piston  16 , the tubular preform with the interface between the two material melts is discharged. To maintain smooth running of the interface, an annular discharge channel  18  is provided with a special geometry. When the discharge operation has been completed, the torpedo  6  is moved back up and closes the extrusion orifice  20 , whereupon a new filling operation for the annular accumulation chamber  14  begins. 
     The geometric features of the design of the annular piston  16  that are responsible for the uniform flow of the material melt that is supplied are explained below. As  FIG. 1  shows, the first distributor ring  26  is not formed symmetrically with respect to the center axis m, but rather it has a greater cross-sectional area in the vicinity of the first inlet opening ZF 1  than it does on the opposite side from the inlet opening ZF 1 . This is illustrated in  FIG. 2  by a cross section along line A-A. The gap width s 1  of the first distributor ring  26  is greater in the vicinity of the first inlet opening ZF 1  than the gap width s 2  on the opposite side. The material melt flows from this distributor ring  26  into the annular flow channel FK 1 .  FIG. 3  shows a cross section through this flow channel along line B-B. The effective gap width s 3 , i.e., the gap width in the direction of the normal, approximately in the center of the annular conical frustum, is smaller on the side of the first inlet opening ZF 1  than the gap width s 4  on the opposite side. The annular cross-sectional area F 26  in the center of the first distributor ring  26  along the sectional line AA is greater than the mean effective cross-sectional area FFK 1  of the first flow channel FK 1  along the sectional line BB. Accordingly, the material melt flow is throttled, and there is an increase in pressure. 
       FIGS. 4 and 5  show the conditions with respect to the second material melt. The gap width s 5  of the second distributor ring  28  is greater on the side of the second inlet opening ZF 2  than the gap width s 6  on the opposite side. In the second flow channel FK 2 , the gap width s 7  on the side of the second inlet opening ZF 2  is smaller than the gap width s 8  on the opposite side. Here again, there is an increase in pressure when the material melt flows from the inlet opening ZF 2  into the second flow channel FK 2 , since the annular cross-sectional area F 28  is greater than the cross-sectional area FFK 2  of the second flow channel FK 2 . 
     After passing through the first flow channel FK 1 , the first material melt enters the first cylindrical ring  22  (see  FIG. 1 ). This cylindrical ring  22  has a constant gap width s 9  along its length and circumference. This gap width s 9  is configured in such a way that the cross-sectional area F 22  of the cylindrical ring  22  is greater than the cross-sectional area FFK 1  at the end of the first flow channel FK 1 , which makes a transition into the cylindrical ring  22 . F 22  is preferably twice as large as FFK 1 . Similar conditions apply to the second material melt, which flows from the second flow channel FK 2  into the cylindrical ring  24 . This cylindrical ring  24  has a constant gap width s 10 , which is configured in such a way that the cross-sectional area F 24  is greater than the cross-sectional area FFK 2  at the end of the second flow channel FK 2 . The specified geometric configuration and the resulting asymmetrical guidance and throttling of the flow of the material melt provide uniform flow from the distributor rings  26  and  28  via the flow channels FK 1  and FK 2 , which are shaped like conical frustums, to the cylindrical rings  22  and  24 . 
       FIG. 6  shows the geometric relationships on the basis of a cutaway section with distributor ring  26 , flow channel FK 1 , and cylindrical ring  22 , and the associated size relationships for the gap widths s 1 , s 3 , and s 9  and the annular cross-sectional areas F 26 , FFK 1 , and F 22 . The first material melt flowing in at ZF 1  is uniformly distributed in the horizontal direction along the distributor ring  26  and does not flow off immediately in the vertical direction into the first flow channel FK 1 , since the latter has a reduced gap width s 3 &lt;s 4  in the vicinity of the inlet opening ZF 1 , and F 26 &gt;FFK 1 . Since s 4 &gt;s 3 , the flow rate at s 3  is greater than at s 4  at otherwise equal material volume/time. In the cylindrical ring  22  with s 9 &gt;s 3 , the material is quieted, and the material flow rate is equalized in the circumferential direction of the cylindrical ring  22 , with the result that the first material melt has the same flow rate along the circumference at the orifice  30 . 
     The dimensions of the gap widths s 1  to s 10  are to be selected as a function of the material for the first material melt and the second material melt. Typically, the cross-sectional areas F 22  and F 24  of the cylindrical rings  22  and  24  can be selected to be at most half as great as the cross-sectional areas F 26  and F 28  of the corresponding distributor rings  26  and  28 . 
     The two material melts meet at the orifice  30  and pass together through the common cylindrical ring  34 , which likewise constitutes a region of quieted flow in which turbulence of the interface between the two material melts is avoided. The cross-sectional area F 34  of this common cylindrical ring  34  is equal to the sum of the cross-sectional areas F 22  and F 24  of the cylindrical rings  22  and  24  (F 34 =F 22 +F 24 ). The length L 34  of the common cylindrical ring  34  is preferably greater than or equal to twice the sum of the gap widths of cylindrical ring  22  and cylindrical ring  24  (L 34 ≧2(s 9 +s 10 )). 
     After passing through the region of quieted flow and the point of widening  32 , the united material melts enter the common flow channel  12 , which widens like a funnel. This flow channel  12  is bounded by an inner conical frustum surface  36  and an outer conical frustum surface  38 . When viewed in longitudinal section, these conical frustum surfaces  36 ,  38  form an asymmetrical funnel, with a first angle between the vertical and the inner conical frustum surface  36  being smaller than a second angle between the vertical and the outer conical frustum surface  38 . The first angle can typically be on the order of 0°, i.e., the inner conical frustum surface  36  can be configured as a cylindrical surface. 
       FIG. 7  shows an example in which the material flow within the first flow channel FK 1  is controlled by throttling with a baffle. The flow channel FK 1  is configured as an annular conical frustum and is bounded by an outer wall  40  and an inner wall  41 . An annular groove  42  that holds the baffle  44  is set in the outer wall  40 . This baffle  44  can be moved into the corresponding annular conical frustum for throttling the flow of the material melt in the flow channel FK 1 . Preferably, an elastic baffle  44  is used, whose inside diameter can be varied by means of an adjusting device. Reduction of this inside diameter then produces throttling of the material flow.  FIG. 7  shows only a baffle  44  for the first flow channel FK 1 . It is also possible to install a throttling device of this type in only one of the two flow channels FK 1  or FK 2 , or in both of these flow channels and in other flow channels. 
       FIGS. 8 and 9  show embodiments in which a special feeding device is used to supply the material melt. In  FIG. 8 , the first inlet opening ZF 1  is connected with a feeding device  50 , which in turn is rigidly connected with the housing of the coextrusion head  10 , for example, seated in a stationary manner on the accumulator jacket  8 , supported by a column  57 . As was mentioned earlier, the annular piston  16  can move up and down. The feeding device  50  contains a recess  54 , which further conveys the material melt to the inlet opening ZF 1  during the stroke of the annular piston  16 . The recess  54  receives the material melt via an inlet channel  55 , which is rigidly connected with an extruder line (not shown). A similar feeding device  52  with a recess  56  is also provided for the second material melt, which is supplied to the second inlet opening ZF 2 . The recesses  54 ,  56  have a vertical length equal to the stroke of the annular piston  16 . The feeding device  50 ,  52  is preferably designed as an annular segment and extends along the circumference of the annular piston  16 . In this example, the extruder line does not have to perform any rotating movements during the stroke of the annular piston  16 , so that it is possible to save moving parts. The columns  57  hold the feeding devices  50 ,  52  and leave sufficient free space around the feeding device  50 ,  52 , so that adequate material, which then becomes encrusted, can be easily removed from the region of the recesses  54 ,  56 . 
       FIG. 9  shows an alternative solution for the feeding. The inlet opening ZF 1  is rigidly connected with a feed cylinder  60 . The feed cylinder  60  thus carries out the lifting movement of the annular piston  16 . A hollow feed piston  62  moves in the feed cylinder  60 . The hollow feed piston  62  is connected in a stationary manner, for example, rigidly, with the jacket ring  8 . The feed piston  62  is also rigidly connected with an extruder line  64 , which supplies the material melt. In this arrangement as well, the extruder line  64  does not have to carry out any tilting or rotating movement corresponding to the movement of the inlet opening ZF 1 . A similar feeding device can also be provided for the second inlet opening ZF 2 . 
       FIG. 7  shows another example, in which the annular piston  16  is rigidly connected with a hydraulic system  70 , which is acted upon with hydraulic fluid through a hydraulic line  72 . The hydraulic system  70  serves the purpose of load relief and works in such a way that it takes some of the weight of the annular piston  16 , which weighs on the melt in the annular accumulation chamber  14  as the latter is being filled. The annular piston  16  can weigh several tons, and the material properties of the material melt can change under this pressure. The hydraulic system  70  thus removes some of the weight during the filling of the annular accumulation chamber  14 . When the material melt is discharged from the annular accumulation chamber  14  through the annular extrusion orifice  20 , the hydraulic system is switched to an inoperative status, so that the weight of the annular piston  16  assists with the discharge. It is advantageous for the hydraulic system  70  to have a cylinder  74  with a piston  76 , with the cylinder  74  being rigidly connected with the annular piston  16 . The piston  76  is supported on the housing or the annular jacket  8 , which results in a compact configuration. It is advantageous for two units consisting of a cylinder and piston to be installed on diametrically opposite sides of the annular piston  16 . 
     In accordance with an additional measure, a collecting device  80  is installed above the annular piston  16  for collecting hydraulic oil dripping from the hydraulic systems for the annular piston  16  and the torpedo  6 . This hydraulic oil would drip from the top end of the annular piston  16 , which is hot during operation, and cause fouling and other problems. The hydraulic oil is carried away from the vicinity of the annular piston  16  by a drain line  82  and removed. 
     The examples shown here concern a coextrusion head for processing two material melts. If more than two material melts are to be processed, correspondingly greater numbers of inlet openings, distributor rings, flow channels, etc., must be provided in similar fashion. In practice, it is possible to process five or even six different material melts, which results in a complex design of the annular piston  16  and other associated structural parts. 
     LIST OF REFERENCE NUMBERS 
     
         
           6  torpedo 
           8  accumulator jacket 
           10  coextrusion head 
           12  common flow channel 
           14  annular accumulation chamber 
         ZF 1  first inlet opening 
         ZF 2  second inlet opening 
           16  annular piston 
         K 1 , K 2  piston rods 
           15  lower position of the annular piston 
           18  annular discharge channel 
           20  extrusion orifice 
           22  first cylindrical ring 
           24  second cylindrical ring 
           26  first distributor ring 
           28  second distributor ring 
           30  orifice 
           32  point of widening 
           34  common cylindrical ring 
         s 1  to s 10  gap widths 
         F 26  cross-sectional area of the first distributor ring 
         F 28  cross-sectional area of the second distributor ring 
         FK 1  first flow channel 
         FK 2  second flow channel 
         FFK 1  cross-sectional area of the first flow channel 
         FFK 2  cross-sectional area of the second flow channel 
         F 22  cross sectional area of the first cylindrical ring 
         F 24  cross-sectional area of the second cylindrical ring 
         F 34  cross-sectional area of the common cylindrical ring 
         L 34  length 
           36  inner conical frustum surface 
           38  outer conical frustum surface 
           40  outer wall 
           41  inner wall 
           42  groove 
           44  baffle 
           50 ,  52  feeding device 
           54 ,  56  recess 
           55  inlet channel 
           57  columns 
           60  feed cylinder 
           62  feed piston 
           64  extruder line 
           70  hydraulic system 
           72  hydraulic line 
           74  cylinder 
           76  piston 
           80  collecting device 
           82  drain line