Abstract:
The present invention concerns a transfer element ( 1, 9 ) for an injection molding system comprising a flow lumen for a plasticised plastic material. To provide a transfer element for an injection molding system, which makes it possible to further increase the material throughput of the injection molding system without having to tolerate increased wear of seals and other system components, it is proposed in accordance with the invention that it has a device for degassing of the flow lumen.

Description:
BACKGROUND OF THE INVENTION 
     The present invention concerns a transfer element, in particular a sprue bush or a machine nozzle, for an injection molding system, having a flow lumen for a plasticised plastic material. 
     The invention further concerns a hot runner system having a sprue bush and an injection molding machine having a machine nozzle. 
     A sprue system or runner is the connection from the machine nozzle of an injection molding machine to what is referred to as the ingate. The ingate forms the transition from the sprue runner into the mold portion cavity configuration, referred to as the mold cavity. The state of the art discloses hot runner systems for injection molding machines. Such hot runner systems represent the technologically most developed sprue systems or runners for injection molding molds. Hot runner systems serve to distribute the plastic material which is prepared in the injection molding machine in a molten state from the machine nozzle of the injection molding machine to the individual cavities in the injection molding tool. In that respect the hot runner system is temperature-controlled in such a way that the plastic material is kept in the molten state throughout the entire injection molding cycle in the hot runner system. Therefore hot runner systems are frequently also considered as a prolongation of the machine nozzle. 
     A certain amount of gas is produced during the processing process, during melting and plasticization of plastic materials, in particular PET materials. Presumably the gas contains predominantly substances which are due to additives added to the initial PET material to specifically alter the properties thereof. In that case the gas is produced in particular upon aggregate conversion of the molten material during plasticization by a machine screw by virtue of shearing, heat and overheating, the gas condensing at cooler locations in the injection molding system in the form of dust. That dust is aggressive to a high degree and is deposited in particular at colder parts of the hot runner system. In that case deposit of the dust, in particular in the plunger housings of the needle valves which seal off the hot runner system in relation to the injection molding tool, leads to severe wear of the plunger seals. Therefore many needle valve systems for hot runner systems have vent openings by way of which the dust can escape from the hot runner system before it is deposited in the region of moving parts, for example at the plunger housings of the needle valves, and there gives rise to increased wear. 
     The amount of dust in the injection molding system increases with the material throughput through the system, that is to say the plasticised amount of plastic material per unit of time. Modern injection molding systems are designed for an increasingly higher level of material throughput which is required in particular by virtue of the increase in the number of mold cavities or molding spaces in the injection molding tools and due to a reduction in the cycle times. Wear in particular in the plunger housings of the valves of the hot runner plate also increases therewith as the increased amount of dust can no longer adequately escape from the system through the vent bores in the plunger housings of the valves. 
     BRIEF SUMMARY OF THE INVENTION 
     Therefore the object of the present invention, in relation to that state of the art, is to provide a transfer element for an injection molding system which makes it possible to further increase the material throughput of the injection molding system without having to tolerate increased wear of seals and other system components. 
     According to the invention that object is attained in that there is provided a transfer element for an injection molding system comprising a flow lumen for a plasticised plastic material, wherein it has a device for degassing of the flow lumen. 
     In that case the transfer element can be the sprue bush of a hot runner system or the machine nozzle of an injection molding machine. In that respect alternatively the sprue bush and the machine nozzle of the system can also be equipped in accordance with the invention. 
     The sprue bush forms the transfer element at the hot runner side for introducing the hot plasticised plastic material from the injection molding machine into the hot runner system. In that case the sprue bush is provided for it to cooperate with the machine nozzle of the injection molding machine, in which case a sealing surface of the sprue bush generally comes into engagement with a sealing surface of the machine nozzle in such a way that no plastic material escapes when the plasticised plastic material is transferred from the machine nozzle into the sprue bush of the hot runner system. 
     In particular, the invention preferably is a transfer element for an injection molding system having a flow lumen for a plasticised plastic material, wherein it has a device for degassing of the flow lumen. The transfer element preferably has a substantially hollow-cylindrical main body having an inner peripheral surface and an outer peripheral surface, wherein the degassing device has at least one gas-permeable passage connecting the inner peripheral surface and the outer peripheral surface. 
     Further, the main body is preferably of a two-part structure having an upper part and a lower part which are substantially hollow-cylindrical and each have at least one main face, wherein the upper part and the lower part are so arranged that the main faces at least partially butt against each other. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  shows a sprue nozzle of a hot runner system from the state of the art, 
         FIG. 2  shows a sectional view of an embodiment according to the invention of a sprue nozzle, 
         FIG. 3  shows a plan view of the main face or separation face in relation to the lower part of the sprue bush upper part of  FIG. 2 , 
         FIG. 4  shows an isometric exploded view of the main body of the sprue bush of  FIGS. 2 and 3 , 
         FIG. 5  shows a plan view of the main face or separation face in relation to the lower part of the sprue bush upper part of an alternative embodiment, 
         FIG. 6  shows a further embodiment of the sprue bush according to the invention, 
         FIG. 7  shows an enlarged view of the sprue bush of  FIG. 6 , 
         FIG. 8  shows a plan view of the main face or separation face in relation to the lower part of the sprue bush upper part of  FIGS. 6 and 7 , and 
         FIG. 9  diagrammatically shows the structure of a machine nozzle according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The largest proportion by volume of dust-bearing gas is produced in the plasticization operation in the injection molding machine itself so that it is advantageous if that dust-bearing gas from the injection molding machine can already escape from the flow lumen before the plastic material enters the hot runner system. In that way the gas pressure in the flow lumen of the hot runner system is reduced, that is to say decompression takes place. 
     In a preferred embodiment of the invention the transfer element has a substantially hollow-cylindrical main body having an inner peripheral surface and an outer peripheral surface, wherein the degassing device has at least one gas-permeable passage connecting the inner peripheral surface and the outer peripheral surface. The gas can escape from the transfer element by way of that passage before the gas passes into the hot runner system so that substantially no dust can be deposited in the hot runner system. 
     Preferably the sprue bush is screwed at its hot runner end to the hot runner system while the machine end of the bush is held in centered relationship by a centering ring in an opening in the plate. In that respect it is particularly advantageous if the gas-permeable passage opens in a region of the outer peripheral surface which is arranged displaced towards the injection molding machine outside the centering ring of the sprue bush. In that way the dust can escape without being deposited at functional elements of the hot runner system. 
     In a particularly preferred embodiment the main body of the transfer element is at least of a two-part structure comprising an upper part and a lower part which are substantially hollow-cylindrical and each have at least one main face, wherein the upper part and the lower part are so arranged that their main faces preferably concentrically butt against each other. Such a configuration of the main body of the sprue bush is advantageous as it makes it possible for the passage to be arranged in the separation plane between the upper and lower parts of the main body so that the passage or parts thereof can be produced by grinding it free in the main face of the upper and/or lower part of the main body. 
     In practice the diameters of the sprue bushes of the hot runners are typically so selected that they match the diameter of the machine nozzle of the injection molding machine for which the hot runner system is provided. If a different hot runner system is to be used on the same machine at a later time, then in the systems known from the state of the art the machine nozzle or the entire sprue bush of the hot runner system which is to be newly employed has to be replaced, which is complicated and expensive, in order to ensure functioning of the combination of the injection molding machine and the hot runner system. In comparison the two-part structure of the main body according to the invention has the advantage that the upper part of the sprue bush can be easily exchanged and can serve as an adaptor to permit connection of the sprue bush of a hot runner of any dimension to a predetermined machine nozzle. 
     In that case it is desirable if the main face of the upper part and/or the lower part has a free-ground clearance so that at least one gap is formed between the main faces, which at least as a portion of a gas-permeable passage connects the inner peripheral surface and the outer peripheral surface of the sprue bush. 
     In that respect the term free-ground clearance is used to denote a region of small depth, which is recessed out of the main face of the lower or upper part, being produced for example by grinding or milling. 
     It is crucial in terms of the mode of operation of the degassing device that in the radial direction it has at least one portion which is permeable for the gas while the plasticised plastic material cannot issue through that portion. For that purpose the lumen of that portion which in the present application is referred to as the free-ground clearance must be of a very small gap width at least in one direction. Such a gap is of a small dimension at least in a direction parallel to the axis of the transfer element, that is to say parallel to the through-flow direction, or it is of a small dimension in the peripheral direction. In that respect the reference to a small dimension is used to mean preferably a gap width of less than 0.04 mm, particularly preferably 0.03 mm. 
     Such a free-ground clearance can be produced in particular of very small depths in the axial direction. The gap formed by a clearance of small depth permits gas to escape without the plastic material being able to escape through such a gap of the sprue bush or the machine nozzle. In that respect a depth for the clearance of less than 0.04 mm and particularly preferably a depth of about 0.03 mm has proven to be advantageous. Such a design configuration permits the escape of a sufficiently large amount of gas while nonetheless no plastic material can issue from the transfer element. 
     In that respect it is sufficient if the free-ground clearance of the preferred small depth forms the gas-permeable passage only in the region directly adjoining the inner peripheral surface of the main body while the gas passing through the gap is guided further radially outwardly by a passage of larger cross-section or a groove as the small dimension of the degassing passage is required only in the interior of the main body to prevent the plasticised plastic material from also issuing from the transfer element. 
     A preferred length for the free-ground clearance in the radial direction is between 2 and 3 mm, particularly preferably being 1.5 mm. 
     In a particularly preferred embodiment the free-ground clearance in the main face of the upper and/or lower part is so arranged that a plurality of gas-permeable passages are formed distributed over the periphery, the passages connecting the inner and outer peripheral surfaces of the main body. In that respect it is particularly advantageous if the individual passages which are at least partially formed by the free-ground clearance are distributed in a star form in the peripheral direction of the transfer element so that gas discharge is possible in all directions. 
     In a particularly preferred embodiment the free-ground clearance has a substantially circular region which adjoins the inner peripheral surface and which is continuous in the peripheral direction. In such a circular gap gas can issue from the plastic material in all radial directions and can be discharged from the transfer element. 
     In that respect in an embodiment the connection between the gap formed by the free-ground clearance and the outer peripheral surface of the main body can be made by one or more regions with a free-ground clearance. 
     Alternatively or additionally there can be provided bores or grooves which extend in the radial direction and which outwardly vent the circular free-ground clearance. The grooves or bores are of a diameter which is larger than the depth of the free-ground clearance. In that way, through bores or grooves involving a lesser consumption of surface area in the peripheral direction, it is possible to discharge the same amount of gas or a larger amount of gas, than through comparable free-ground clearances, in a radial direction. In that way therefore a larger contact surface is afforded between the upper part and the lower part of the sprue bush. 
     In a further embodiment the peripherally extending free-ground clearance is concentrically surrounded by a degassing groove which is of a larger flow cross-section than the free-ground clearance and which serves to collect the outflowing gas. The gas which issues from the clearance into the peripherally extending degassing groove is guided in the groove to radially extending passages connecting the groove to the outer peripheral surface of the bush. 
     In a particularly preferred embodiment the upper part and/or the lower part has dowel pins which project from the main face and which engage into bores provided for same in the main face of the respective other part. It is possible in that way to provide for exact assembly of the upper and lower parts of the main body. 
     Alternatively precise assembly of the upper and lower parts in that way can be achieved by the provision of a mating diameter on the upper and lower parts. With such a mating diameter a portion of the one part which is of an inside diameter that is equal to the mating diameter and a portion of the other part having an outside diameter which is equal to the mating diameter engage into each other and center the parts which are fitted together. 
     In a further embodiment the main body of the transfer element has more than two parts which are substantially hollow-cylindrical and each have at least one main face, wherein the main faces of two parts at least portion-wise butt against each other, wherein a respective one of the mutually butting parts forms an upper part and the other forms a lower part in accordance with this invention. In that way as described hereinbefore degassing passages for the transfer element can be provided in the butting or separation regions between the parts in more than one plane. 
     The present object is also attained by a hot runner system having a sprue bush having the above-described features and by an injection molding machine having a machine nozzle having the above-described features. 
     Further features, advantages and possible uses of the present invention will be apparent from the accompanying Figures and the description relating thereto. 
       FIG. 1  shows a sprue bush  1  for a hot runner system from the state of the art. In that case the sprue bush  1  is let into the cover plate  2  of the hot runner system and opens into the hot runner block  3 . The sprue bush  1  has a main body  4  which is of a substantially hollow-cylindrical shape, with an outer peripheral surface  5  and an inner peripheral surface  6 . The hollow-cylindrical main body  4  of the sprue bush  1  is annularly surrounded by heating elements  7  so that the fluid plasticised plastic material does not cool down in the main body  4  of the sprue bush  1 . 
     The main body  4  of the sprue bush  1  is screwed at its runner end to the hot runner distributor by means of a flange  27  while the machine end of the main body  4  is held in centered relationship in a suitable opening in the hot runner cover plate by means of a centering ring  8  screwed to the hot runner cover plate  2 . 
       FIG. 1 , besides the sprue bush  1 , shows a further transfer element, namely the tip of a machine nozzle  9 . The machine nozzle  9  and the main body  4  of the sprue bush  1  of the hot runner system butt against each other in such a way that a flow of plasticised plastic material  10  can pass without loss from the machine nozzle  9  into the main body  4  of the sprue bush  1 . For that purpose at its upper machine end the main body  4  of the sprue bush  1  has a concentric sealing surface  11 . In this case the sealing surface  11  is curved. The machine nozzle  9  also has at its front tip a concentric sealing surface with a curvature. In this case the curvature of the sealing surface  12  of the machine nozzle  9  is greater than the curvature of the sealing surface  11  of the main body  4  of the sprue bush  1 . Therefore, when the two transfer elements  4 ,  9  come into engagement with each other, a line-shaped seal is afforded with a high level of sealing integrity between portions of the sealing surfaces  11 ,  12 . 
     In following  FIGS. 2 through 4  showing a preferred embodiment of the present invention the components identical to the sprue bush  1  and the machine nozzle  9  in  FIG. 1  are respectively indicated by the same references. 
     In the sprue bush  1  according to the invention as shown in  FIG. 2  the main body  4  is made up from an upper part  4   a  and a lower part  4   b . In this case provided in the butting junction region  15  between the upper part  4   a  and the lower part  4   b  of the main body  4  is a passage for degassing of the flow lumen formed by the inner peripheral surface  6  of the main body  4 . That passage in the butting region  15  is of such a configuration that its gap width allows gas or dust to pass therethrough while the plasticised plastic material  10  cannot leave the main body  4  of the sprue bush  1 . 
     It can be clearly seen from  FIG. 2  that the butting region  15  between the upper part  4   a  and the lower part  4   b  of the main body  4  is arranged above the centering ring  8  in the main body  4  of the sprue bush  1 . In this arrangement the butting region  15  is displaced with respect to the centering ring  8  in the direction of the machine nozzle  9 . That avoids the dust issuing through the through passage being deposited in outside regions of the sprue bush in which it adversely affects operation of the sprue bush. 
       FIG. 3  shows a view from below onto the main face  16  of the upper part  4   a  of the main body  4  of the sprue bush  1 . The main face  16  of the upper part  4   a  has a free-ground clearance  17  which, in relation to the faces  16  which come into engagement with the main face  18  of the lower part  4   b , involves a difference in height of 0.03 mm. When the upper part  4   a  and the lower part  4   b  of the main body  4  are screwed together, then the free-ground clearance  17  results in the provision of a system of passages, which permits degassing of dust from the flow lumen  18  of the main body  4 . 
     In the illustrated embodiment the clearance  17  has an annular region  19  which directly adjoins the inner peripheral surface  6  of the main body so that the gas can escape from the flow lumen  20  in all directions. 
     In the view onto the main face of the upper part  4   a  in  FIG. 3  it is also possible to see end bores  20 ,  21 . In that arrangement the bores  20  of larger diameter serve to receive centering pins  22  while the bores  21  of smaller diameter are provided for passing fixing screws  23  therethrough. 
     Screwing and centering of the upper part  4   a  and the lower part  4   b  of the main body  4  can be clearly seen in the isometric view in  FIG. 4 . While centering pins  22  are received in centering bores  24  in the lower part  4   b  and centering bores  20  in the upper part  4   a  the fixing screws  23  engage through the bores  21  in the upper part  4   a  and are screwed into the screwthreaded bores  25  in the lower part  4   b . The screw means and the high pressing force  28  with which the machine nozzle  9  is pressed against the sprue bush  1  provide that the parts  16  of the main face of the upper part  4   a , that project with respect to the clearance  17 , are pressed in force-locking relationship against the main face  18  of the lower part  4   b  so that only the passages afforded by the clearance  17  permit degassing of the flow lumen  18 . 
       FIG. 5  shows a plan view of the main face of an upper part  4   a ′ of an alternative embodiment of the sprue bush  1  according to the invention. Like the above-described embodiment the main face  16 ′ of the upper part  4   a ′ has a circular free-ground clearance  19 ′ directly adjoining the flow lumen  26  of the upper part  4   a ′. Degassing of the clearance is effected by way of four degassing grooves  29  arranged in a star form around the flow lumen  26 . In this arrangement the degassing grooves  29  are of a diameter greater than the depth of the clearance  19 ′. It is found to be advantageous in this variant that it provides a larger contact face between the upper part ( 4   a ′) and the lower part of the sprue bush. 
       FIGS. 6 through 8  show a particularly preferred embodiment of the invention in which the upper part  4   a ″ and lower part  4   b ″ of the main body of the sprue bush have a mating diameter for centering of the upper part  4   a ″ and the lower part  4   b ″. By means of such a mating diameter it is possible to absorb high lateral forces which can occur for example due to lateral displacement between the sprue bush and the machine nozzle. For that purpose the lower part  4   b ″ has an axially projecting portion  34  whose outside surface  30  is of the same or a slightly smaller diameter than the inside surface  31  of the end  35  of the upper part  4   a ″, that is towards the hot runner. After assembly of the upper part  4   a ″ and the lower part  4   b ″ the axially projecting portion  34  of the lower part  4   b ″ engages into the hot runner end of the upper part  4   a″.    
     The configuration of the degassing device is shown in detail in  FIG. 7  illustrating an enlarged view of part of the assembled elements of  FIG. 6 . It is possible to clearly see a circular free-ground clearance  19 ″ in the main face of the upper part  4   a ″, by which a degassing gap is provided in the butting or separation plane  15 ″ between the upper and lower parts after assembly of the upper part  4   a ″ and the lower part  4   b ″. The clearance  19 ″ opens in the radial direction into a peripherally extending annular degassing groove  32  concentrically surrounding the clearance  19 ″. There the gas escaping from the flow lumen of the sprue bush is collected and is discharged from there outwardly by way of degassing bores  33  which extend in the radial direction and which are distributed in a star form around the periphery of the bush. 
       FIG. 8  shows by way of explanation a view from below onto the main face of the upper part  4   a ″. It is possible to clearly see the concentric arrangement of the clearance  19 ″ and the peripherally extending degassing groove  32 . This Figure also clearly shows the arrangement of the four radial degassing bores which are arranged at 90° relative to each other and which open into the peripherally extending degassing groove. 
       FIG. 9  show a diagrammatic view corresponding to  FIG. 2  to illustrate the arrangement and cooperation of a machine nozzle and a sprue bush. In this case, contrary to  FIG. 2 , it is not the sprue bush but the machine nozzle that is of a two-part configuration so that the degassing device is provided in the machine nozzle. The main body  9  of the machine nozzle comprises an upper part  9   a  and a lower part  9   b , wherein the nozzle upper part in the illustrated embodiment has a free-ground clearance in its main face  36 . A plan view onto the main face of the machine nozzle upper part  9   a  shows, apart from suitably adapted radii, the same structure as the main face of the sprue bush upper part in  FIG. 3 , with the same functionality as described hereinbefore for the sprue bush. All other configurations of the degassing device for the sprue bush can also be transferred to the machine nozzle. 
     LIST OF REFERENCES 
     
         
           1  sprue bush 
           2  cover plate 
           3  hot runner block 
           4  hollow-cylindrical main body 
           4   a ,  4   a ′,  4   a ″ upper part of the main body of the sprue bush 
           4   b ,  4   b ′,  4   b ″ lower part of the main body of the sprue bush 
           5  outer peripheral surface 
           6  inner peripheral surface 
           7  heating element 
           8  centering ring 
           9  machine nozzle 
           9   a  upper part of the machine nozzle 
           9   b  lower part of the machine nozzle 
           10  plastic material 
           11  sealing surface of the sprue bush 
           12  sealing surface of the machine nozzle 
           15 ,  15 ″ butting plane 
           16 ,  16 ″ main face of the upper part 
           17  free-ground clearance 
           18  main face of the lower part 
           19 ,  19 ′,  19 ″ circular region of the free-ground clearance 
           20  bores in the upper part 
           21  bores in the upper part 
           22  dowel pins 
           23  fixing screws 
           24  bores in the lower part 
           25  screwthreaded bores 
           26  flow lumen 
           27  flange 
           28  machine nozzle pressing force 
           29  degassing groove 
           30  outside surface with mating diameter 
           31  inside surface with mating diameter 
           32  peripherally extending degassing groove 
           33  radial degassing bore 
           34  projecting portion 
           35  hot runner end of the upper part  4   a″   
           36  main face of the upper part  9   a  of the machine nozzle