Abstract:
The manufacture of a plastic material, particularly provided with fibers, is frequently carried out in an extruder ( 10 ). Various problems have arisen in this connection. For example, problems arise in working long or endless reinforcement fibers, for example rovings ( 27 ), into the plastic. On the one hand, the fibers are so badly broken up that they have only very small lengths. On the other hand, it has proved difficult to impregnate the fibers sufficiently. According to the invention, in order to feed in fibers, the plastic is moved in batches past a pre-plastifying worm ( 11 ). This is achieved by a secondary worm ( 18 ) located next to the pre-plastifying worm ( 11 ). The secondary worm ( 18 ) enables the plastic to be provided in a controlled manner with fibers outside the pre-plastifying worm ( 11 ), and enables the secondary worm ( 18 ) to be designed in such a way that it satisfies the conditions for a good combination and mixture of the plastic with the additives, and particularly a good impregnation of the fibers.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a method of manufacturing a plastics material, particularly provided with additives, and to an extruder for carrying out the method. 
     Plastics, such as for example thermoplastics, duroplastics and elastomers, are frequently provided with additives in order to improve their properties. In particular, plastics are reinforced by fibres such as glass fibres, carbon fibres, aramide fibres, natural fibres and plastic fibres. Such fibres are added either as endless fibres, chopped fibres or matting portions to the plastics. 
     Various methods are known for manufacturing plastics with additives. The invention relates to one of these known possibilities, by means of which the plastic is provided with additives during melting in the extruders. The extruders serving to carry out known methods of this type have various drawbacks. On the one hand there is a lack of homogeneity and impregnation of the plastics or additives. On the other hand added fibres are broken, so that insufficient reinforcement of the plastic is achieved. 
     SUMMARY OF THE INVENTION 
     The object underlying the invention is to provide a method of manufacturing a reinforced plastic and a corresponding extruder, so that on the one hand homogeneous plastics with additives may be manufactured and on the other hand no appreciable breakage occurs of the fibres serving for reinforcement. 
     According to this method, the plastic is moved in batches past the pre-plastifying worm in order to disperse and/or feed in additives. This provides the opportunity of treating the plastic and/or the additives in a controlled and intensive manner outside or next to the pre-plastifying worm and to bring them into combination with one another (to impregnate them). According to a further development of the method according to the invention, in order to move the plastic past the pre-plastifying worm, a secondary worm is used. The preferably separate secondary worm has the advantage that it can be designed differently from the pre-plastifying worm and thus can be better adapted to requirements than said pre-plastifying worm. 
     In a preferred method, fibres serving for reinforcement, in particular endless fibre skeins, are fed to the plastic in the area of the secondary worm. Here the secondary worm affords the possibility of introducing endless fibres into the molten plastic mass and to work them into the mass in such a way that on the one hand they have a maximum possible length and on the other hand are intensively impregnated. 
     It has proved particularly advantageous to associate with the secondary worm a pressure-free zone, in which fibres, particularly endless fibres, are introduced into the molten plastic mass. Above all, in this way endless fibres can pass around the secondary worm and thus be introduced into the molten plastic mass. The pressure-free zone preferably extends as far as the pre-plastifying worm, ensuring gentle transfer of the endless skeins from the secondary worm to the pre-plastifying worm. In this connection, and also in the following, the term “pressure-free zone” is taken to mean a zone in which there exists only sufficient pressure to transport the plastic onwards. In this zone there is however no negative pressure, permanent pressure and/or back pressure. In another method, by virtue of the fact that, in the area of the pre-plastifying worm, additives such as for example, fibres or rovings are fed to the plastic continuously, and in particular in endless form, there results an effective combination of the reinforcements with the plastic. In particular however the additives are prevented from being broken up to the extent that only small particles remain in the plastic. 
     According to a further development of the method, the additives are fed to the at least partly melted plastic in the area of a pressure-free zone of the pre-plastifying worm, In this way it is on the one hand possible to provide the pre-plastifying worm with an opening, open to the exterior, for endless feed of the fibres or rovings. On the other hand, particularly good and effective impregnation of the fibres or rovings is achieved over their entire length. 
     In further development of all methods, the fibres or rovings fed in in endless form are chopped, during feeding to the pre-plastifying worm, into rovings or fibres of finite length. In the simplest case, this may be effected at the inner edge of the feed located in the housing for the endless fibres or rovings to the worm. For this purpose the rovings are always chopped at this inner edge of the feed opening in the housing when a worm spiral of the worm passes along under the feed opening. The endless fibres or rovings are in this way chopped in a controlled manner. The length of the fibres or rovings results from the worm geometry, in particular the pitch of the worm spirals of the pre-plastifying worm or secondary worm. 
     By means of the secondary worm, in conjunction with means for transferring the plastic from the pre-plastifying worm to the secondary worm and back (with additives) a type of secondary area is provided for the plastics which enables the plastic to be treated in a controlled manner, to provide it with additives under the conditions required, and to work the additives into the plastic in the necessary way, particularly impregnating, dispersing and/or mixing. 
     The secondary worm, extending preferably parallel to the pre-plastifying worm, has its own secondary worm housing, which is connected at points with the plasticising housing. Thus each worm has its own housing, which separates the secondary worm from the pre-plastifying worn. Both worms in this way represent independent constructive units, which may be separately repaired as required. 
     The means for transferring the plastic, or plastic provided with additives, from the pre-plastifying worm to the secondary worm or vice versa, in a preferred further development of the device, are in the form of transitional openings. The transitional openings are more appropriately disposed in the housing wall of the plasticising housing and of the secondary worm housing, in such a way that, by means of apertures aligned towards one another in the walls of both housings, the plastic (if necessary with the additive) can pass to the secondary worm and then from this point back to the pre-plastifying worm. 
     In one design of the extruder, the secondary worm housing can have an opening for supply of fibres, particularly endless fibres, to the secondary worm. This ensures controlled feed of the fibres, particularly endless fibres, to the extruder and, at a point where a corresponding design of the secondary worm, fulfils the conditions for non-destructive feed of the fibres to the plastic and intensive impregnation. 
     Alternatively, it may be envisaged that the secondary worm may be provided with a dispersing worm. This serves to disperse plastic mixtures; the secondary worm reinforcing this procedure as dispersal is carried out more effectively in the area of the secondary worm than in the area of the pre-plastifying worm. 
     Preferred embodiments given by way of example of extruders and methods for manufacturing plastics according to the invention, will be explained in more detail with reference to the drawing, which shows: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG.  1 : a schematic longitudinal section of a first embodiment of an extruder, 
     FIG.  2 : a second embodiment of an extruder in a longitudinal section similar to FIG. 1, and 
     FIG.  3 : a third embodiment of an extruder in a longitudinal section similar to FIGS.  1  and  2 . 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIGS. 1 to  3  show schematically a portion of an extruder. The extruders shown serve for melting plastics of various types, particularly thermoplastics, duroplastics and elastomers. The extruder simultaneously serves to provide plastics with one or more additives or to treat plastics in any manner during melting, particularly to disperse them. The plastics, which are molten and provided or treated with additives, are expelled under pressure from the extruder from a closable mouthpiece. 
     FIG. 1 shows schematically a portion of an extruder which serves to melt plastic and to provide it with additives in the form of fibres serving to reinforce the plastic. Involved here are both endless fibres and also chopped fibres or also portions of matting or textile and wood flour or the like. 
     The extruder  10  has a pre-plastifying worm  11 , which is mounted to be driven by a motor  12  shown schematically in a plasticising housing  13 . The pre-plastifying worm  11  is provided on its external circumference with a worm spiral  14 , the pitch of which is so designed that, in conjunction with a corresponding rotary driving direction of the motor  12 , the plastic, including fibres added later, can be transported in the extrusion direction  15  (i.e. in FIG. 1 from right to left) to the closable mouthpiece  16 . The still un-melted plastic is fed for example in the form of granulate to the input of the pre-plastifying worm  11 , i.e. the end of the pre-plastifying worm  11  opposite the closable mouthpiece  16 , and in the embodiment shown through a hopper  17 . 
     According to the invention, a secondary worm  18  is associated with the pre-plastifying worm  11 . The secondary worm  18  is disposed in a separate secondary worm housing  19 . The secondary worm housing  19  and the plasticising housing  13  are connected together in such a way that they lie against one another with their walls oriented towards one another. In this way the longitudinal medial axis of the pre-plastifying worm  11  and of the secondary worm  18  extend parallel to one another at a spacing apart. The secondary worm  18  is shorter in length than the pre-plastifying worm  11 , i.e. is about half its length. The shorter secondary worm  18  is disposed in the central area of the longer pre-plastifying worm  11 . The secondary worm in the embodiment shown has its own separate drive, i.e. a motor  20  likewise shown only schematically. 
     The secondary worm  18  is connected to the pre-plastifying worm  11  by means for introducing the plastic from the pre-plastifying worm  11  to the secondary worm  18  and for removing the plastic reinforcement fibres from the secondary worm  18  to the pre-plastifying worm  11 . In the embodiment shown, these means are two transitional openings  21  and  22  following one another at a spacing apart in the extruding direction  15 . The transitional openings  21  and  22  are formed by corresponding apertures in the adjacent walls of the plasticising housing  13  on the one hand and of the secondary worm housing  19  on the other hand. 
     According to the invention, the pre-plastifying worm  11  is provided with a blocking zone  23  located between the two transitional openings  21  and  22 . The blocking zone  23  is formed by a particular geometry of the worm spiral  14  of the pre-plastifying worm. In the embodiment shown the blocking zone  23  is formed by a contrary-running pitch of the worm spiral  14 . The blocking zone  23  achieves a situation in which the plasticised plastic is passed from the pre-plastifying worm  11  via the first transitional opening  21  in the extrusion device  15  to the secondary worm  18 , because in the area of the blocking zone  23 , the pre-plastifying worm  11  permits no passage of the molten plastic along the pre-plastifying  11  in the extrusion direction  15 . 
     In the pre-plastifying worm  11 , the worm geometry in front of the first transitional opening  21  is preferably identical with the worm geometry behind the second transitional opening  22 . Accordingly the worm spirals  14  of the pre-plastifying worm  11  before and after the transitional opening  21  have roughly the same pitches. It may also be envisaged that the pre-plastifying worm  11  may be provided with a worm geometry behind the transitional opening  22  different from the worm geometry in front of the transitional opening  21 . 
     The secondary worm  18  has counter-running worm spirals. A first area of the secondary worm  18 , in the extrusion direction  15 , has a worm spiral  24  which extends in the extrusion direction  15 . A subsequent second worm spiral  25 , aligned towards the closable mouthpiece  16 , has a contrary configuration, i.e. is designed to convey contrary to the extrusion direction  15 . The worm spirals  24  and  25  with contrary pitch meet one another roughly in the area of the transitional opening  22  lying foremost in the extrusion direction  15 , i.e. facing the closable mouthpiece  16 . In the embodiment shown, the worm spirals  24  and  25  of the secondary worm  18  meet one another at the rear end of the transitional opening  22 . The worm geometry of the first worm spiral  24  in the extrusion direction  15  is so designed that, before the meeting of the worm spirals  24  with the oppositely-directed worm spirals  25 , a pressure-free zone  26  results. This pressure-free zone  26  begins behind the first transitional opening  21  in the extrusion direction  15  and terminates before the point of meeting of the differently-aligned worm spirals  24  and  25 . In addition, the pressure-free zone  26  may also extend over the area of the second transitional opening  22  in the extrusion direction  15 . If necessary, the pressure-free zone  26  may even extend over a short section of the following section of the pre-plastifying worm  11  following in the extrusion direction after the blocking zone  23 . 
     According to the invention, the secondary worm housing  19  has an outwardly-facing feed opening  29 , which opens into the pressure-free area  26  and serves to feed endless fibres, for example a roving  27 , in an uninterrupted manner to the melted plastic. The roving  27  is continuously withdrawn from a roving roll  28  and passed through the feed opening  29  in the secondary worm housing  19  of the secondary worm  18  between two successive spirals. Thus the endless roving  27  is worked into the molten plastic and impregnated. 
     In addition, the extruder in FIG. 1 has a storage container  30  for supplying chopped fibres or other additives such for example chopped matting or textile portions or wood flour. This storage container  30  is in turn associated with the secondary worm  18 , in such a way that the material passes from the container  30  through the secondary worm housing  19  to the worm spiral  25  conveying contrary to the extrusion direction  15 . In this way the chopped fibres or other additives are transported from the container  30  to the material in the worm spiral  24  of the secondary worm  18  conveying in the extrusion direction  15  and at this point unified and mixed with the plasticised plastic provided with endless fibres. This mixture of materials passes through the second transitional opening  22  to the section of the pre-plastifying worm  11 , lying behind the blocking zone  23 , in which the material is transported onward to the closable mouthpiece  16 . 
     Plasticising of the plastic and heating of the plastic inclusive of the fed-in fibres or the like is effected by heating zones  31 , shown schematically, on the outside of the plasticising housing  13  and of the secondary worm housing  19 . 
     The method according to the invention with extruder  10  is carried out as follows: 
     The plastic, which at this stage can already be a mixture, is passed through the hopper  17  to the pre-plastifying worm  11 . This latter passes the plastic in extrusion direction  15  to the blocking zone  23 . Thus the plastic is moved past heating zones  31 , which means that it is plasticised as it reaches the blocking zone  23 . 
     The plastic is then diverted from the blocking zone  23 , passing through the first transitional opening  21  to the secondary worm  18 . 
     The secondary worm  18  transports the plasticised plastic conveyed thereto in the area of the worm spiral  24  in extrusion direction  15 . Thus the plasticised plastic reaches the pressure-free area  26 . At this point endless fibres, particularly the roving  27 , are endlessly unwound from the roving roll  28  and continuously fed through the feed opening  29  in the secondary worm housing  19  to the secondary worm  18 . Thus the roving  27  is worked into, and particularly impregnated by, the plasticised plastic. 
     Further additives, for example chopped fibres, are fed to the end of the secondary worm  18  facing the closable mouthpiece  16  from the supply container  30 . The chopped fibres are passed contrary to the extrusion direction  15  through the end area located in the area of the supply container  30  of the secondary worm  18  by means of a counter-running worm spiral  15 , to the plasticised plastic provided with the roving and is there mixed. The resultant mixture passes through the second transitional opening  22  out of the area of the second worm  18  back into the area of the pre-plastifying worm  11 , i.e. behind the blocking zone  23 , and thus to the second section of the pre-plastifying worm  11 , facing the closable mouthpiece  16 . This second section of the pre-plastifying worm  11  then transports the molten plastic provided with impregnated fibres and if necessary other additives to the closable mouthpiece  16  of the extruder  10 , where, if necessary under pressure, the plasticised and reinforced plastic is ejected. 
     FIG. 2 shows an extruder  32 , which is substantially identical with the extruder  10 . Therefore the same reference numbers are used for identical parts. 
     The extruder  32  differs from the extruder  10  in that, in the area of the secondary worm  38 , only ore storage container  30  serves to supply chopped fibres, wood flour, textile portions or other additives. By means of the extruder  32 , the corresponding short-fibre additive may be fed in the area of the secondary worm  18  to molten plastic of a molten plastic mixture, the molten plastic being combined with the additive by counter-conveying worm spirals  24  and  25  in the region of the transitional opening  22  to the pre-plastifying worm  12  and being mixed or impregnated. For this purpose the end of the worm spiral  24  facing the worm spiral  25  can also have a pressure-free area  26 . However, due to the lack of the feed opening  29  (FIG. 1) for an endless roving  27 , in the extruder  32  the pressure-free area  26  can be omitted. 
     FIG. 3 shows an extruder  33  which can also be suitable for plasticising un-reinforced plastics. The extruder  33  corresponds in its basic structure with the extruder  10 . For this reason here also the same reference numbers are used for identical parts. 
     Contrary to extruder  10  in FIG. 1, the extruder  33  has in the area of the secondary worm  18  a feed facility for any additives to the plastic. The secondary worm  18  rather serves to disperse the plastic or a plastic mixture. For this purpose the secondary worm  18  has a dispersing zone  34 . The dispersing zone  34  extends substantially between the transitional openings  21  and  22 . In the embodiment shown, the dispersing zone  34  projects slightly into the area of the first transitional opening  21 . The dispersing zone  34  is designed as is already known with pre-plastifying  25  worms  11 . 
     In contrast to the embodiments shown above, it is possible to design the secondary worm and also the pre-plastifying worm as a double worm, which may run in the same direction, or counter to one another. In addition it is feasible to propel the secondary worm at a rotational speed different from that of the pre-plastifying worm. Further it is feasible to design the pre-plastifying worm and/or the secondary worm, in contrast to the illustrations in FIG. 3, as axially movable. Finally, it can be advantageous to provide the secondary worm and/or the pre-plastifying worm with means for opening or tearing up matting or textile portions or fibre parts. This improves the capacity for impregnation of these reinforcing materials.