Abstract:
A needle valve nozzle for an injection mould has a nozzle body with at least one melt channel that terminates in a nozzle mouthpiece and a fluidic connection to a mould cavity formed by a mould insert. A shut-off needle penetrates the melt channel and nozzle mouthpiece and can be displaced between open and closed positions. At least one infeed cone is provided upstream of a seal seat to centre the shut-off needle, the lower end of which forms a shut-off part. The nozzle mouthpiece consists of a material with high thermal conductivity, with the infeed cone configured in a centering body consisting of wear-resistant material and running concentrically with the longitudinal axis of the needle valve nozzle. The entering body is held against and/or in the nozzle mouthpiece in a positive-fit and its end section forming an outlet for the melt can be engaged with the mould insert.

Description:
FIELD OF THE INVENTION 
   The invention relates to a needle valve nozzle for an injection mould. 
   BACKGROUND OF THE INVENTION 
   Needle valve nozzles are used in injection moulds for supplying a flowable melt at a pre-determinable temperature at high pressure to a separable mould block (mould insert). The shut-off needles, in general pneumatically or hydraulically actuated, are designed to periodically open and close outlet openings for the melt, for example when a plastic material is to be injected in a segmental manner (cascade moulding). 
   Each shut-off needle is arranged in the mould-side area of the needle valve nozzle in an axially displaceable manner, and in the nozzle-side area centrically running through a melt channel (see DE 32 49 486 C3 or DE 34 03 603 A1, for example). Typically, the melt channel terminates in a nozzle mouth piece which forms on its end the outlet opening for the melt. In closed position, the lower end of the shut-off needle which has a generally cylindrical section engages into a likewise cylindrical sealing seat which is formed in the nozzle mouthpiece or in the mould insert. 
   In order to be able to maintain the melt at a constant temperature on its way through the tempered melt channel to the mould insert, the nozzle mouthpiece typically consists of a highly thermally conductive material. It is directly screwed into the nozzle body of the needle valve nozzle from the bottom side or, as provided in DE 197 17 381 A1, firmly secured by means of an outer screw socket made from a material of low thermal conductivity. The outer end of the screw socket engages into a corresponding cylindrical seat in the mould insert, thus centring the outlet opening with respect to the sealing seat. A narrow air gap formed between the highly thermally conductive nozzle mouthpiece and the mould insert provides the required thermal separation between the needle valve nozzle and the mould. 
   Due to the effects of extremely high pressures, for example far greater than 1,000 bar, a precise sealing in the closed position as well as an accurate needle guidance are essential. To this end, the nozzle mouthpiece is provided, above the outlet opening, with an infeed cone for the shut-off needle to allow the latter to centrically enter the sealing seat during closing. However, this presents the problem that the shut-off needles use to strike with their sealing edges the infeed cone in the nozzle mouthpiece. As a result, damages and in the long run leakages may occur. Precise sealing requires compliance with strict tolerance limits, in particular in the case of long nozzles and deep bores, and thus entails increased costs for manufacture and repair. 
   To avoid these drawbacks, the document DE 32 45 571 C2 provides a shut-off needle with a stepped lower end, such that a striking edge of increased diameter is formed ahead of the actual shut-off part of the needle. The axial length of the preferably cylindrical shut-off part, i.e. the axial distance between the front sealing edge of the shut-off part and the striking edge, is selected so as to provide an angle formed by a connection line from the sealing edge to the striking edge extending through connection points distributed on parallel diameters, which is greater than the cone angle of the feed hopper in the nozzle mouthpiece, in each case relative to the longitudinal axis of the melt channel. Thus the nozzle mouthpiece serves as a pre-centring body for the shut-off needle. In fact, whenever the shut-off needle is deflected from its central position during the closing procedure, only the striking edge will contact the infeed cone, whereas the sensitive sealing edge of the shut-off needle is introduced in a contactless manner into the nozzle mouthpiece. 
   To improve service life, DE 32 45 571 C2 further proposes a nozzle mouthpiece made from a wear-resistant material. Such materials, however, are poor thermal conductors and therefore impair the temperature distribution in the unheatable nozzle mouthpiece. 
   The same proposal is made in the document DE 41 09 122 C1, except that the wear-resistant nozzle mouthpiece is not screwed into the nozzle body but axially pressed against the latter by means of a tension ring which encloses the mouthpiece and is screwed onto the nozzle body. A further drawback is the lateral introduction of the shut-off needle into the melt channel which entails great constructional expense. Furthermore, the shut-off part of the needle is not flush with the nozzle mouthpiece and therefore produces objectionable gate marks. 
   BRIEF SUMMARY OF THE INVENTION 
   It is an object of the invention to overcome these and further drawbacks found in the prior art and to provide an improved needle valve nozzle realized with simple means in a cost-effective manner and ensuring a constantly accurate needle guidance and sealing. Also, impairments of the temperature distribution as well as damages on the shut-off part of the shut-off needles and/or the nozzle mouthpiece are to be avoided. 
   In a needle valve nozzle for an injection mould for producing injection moulded parts, comprising a nozzle body that contains at least one melt channel for a melt, said melt channel terminating at or in a nozzle mouthpiece and having a fluidic connection to a mould cavity of the injection mould that is composed of at least one mould insert, and comprising a shut-off needle which penetrates the melt channel and the nozzle mouthpiece in such a manner that it is longitudinally displaceable and suitable to be moved from an open position into a closed position, with said shut-off needle comprising or forming on its lower end a shut-off part which engages in closed position into a sealing seat, and with at least one infeed cone provided ahead of the sealing seat for centring the shut-off needle, the invention provides that the nozzle mouthpiece is made from a highly heat-conductive material and substantially continues the melt channel cylindrically, that the or each infeed cone for the shut-off needle is formed in a centring body made from a wear-resistant material which is mounted on and/or in the nozzle mouthpiece by positive fit and that an end section which forms an outlet opening for the melt is engageable with the mould insert. 
   The nozzle mouthpiece made from a highly heat-conductive material provides a good temperature distribution well into the gate area, and its simple geometry permits economical manufacture. It allows for an accurate processing, in particular from the gate side, and therefore contributes to reducing the production costs. The centring body is manufactured with high precision as a separate part. It causes the shut-off needle to enter the sealing seat in a constantly centrical manner, with the wear-resistant material ensuring a reliable needle guidance and sealing for a long time. If leakages should nevertheless occur or the centring body become worn, the latter can be quickly and easily replaced without the need to disassemble the needle valve nozzle or the nozzle mouthpiece. Since it is possible at all times to insert another centring body with a corresponding outlet opening into the nozzle mouthpiece, bigger or smaller shut-off needles may also be used without any problem. Another advantage of the solution according to the invention resides in the fact that the centring body provides the required thermal separation between the needle valve nozzle and the mould insert. This obviates any need for additional isolation or separation elements. 
   The centring body is formed concentrically with respect to the longitudinal axis of the needle valve nozzle and comes to rest against the nozzle mouthpiece with its front. So it immediately abuts the nozzle mouthpiece, extending the nozzle towards the mould cavity, with the shut-off needle being inserted into the sealing seat in a constantly concentrical manner relative to the longitudinal axis. 
   The centring body comprises a cylindrical collar portion in the area of which the infeed cone for the shut-off needle is formed. Introducing the centring body with this collar portion into the nozzle mouthpiece ensures a reliable guidance. This is particularly advantageous if the centring body is positioned in the nozzle mouthpiece in a longitudinally displaceable manner. The shut-off needle cannot get in contact with the highly thermally conductive material of the nozzle any longer. Any damages on the nozzle mouthpiece are efficiently avoided. The needle valve nozzle has an all-in-all long service life. 
   In order to align the needle valve nozzle with respect to the mould cavity, the centring body engages with its end section into a cylindrical or conical seat in the mould insert. The end section engages into the seat in a longitudinally displaceable manner. The centring seat is preferably concentrical with respect to the gate, i. e. the melt will pass into the mould cavity by easy flow without hinderance. Simultaneously, a constantly reliable sealing of the centring body in the mould insert is ensured. 
   The sealing seat for the shut-off part of the shut-off needle can be formed in the mould insert or in the end section of the centring body. Moreover, the end section of the centring body can confine a part of the mould cavity. 
   The shut-off needle can taper towards the shut-off part. On the one hand, this has positive effects on the flow conditions in the melt channel. On the other hand, the needle guidance is improved. Also contributing to these benefits is a conical and/or rounded transition from the needle portion of greater diameter to the shut-off part of smaller diameter. 
   In a further important embodiment of the invention, a striking edge is formed between the shut-off part and the needle portion of greater diameter whose diameter exceeds that of the needle shut-off part at its sealing edge. The axial distance between the sealing edge of the shut-off part and the striking edge is selected so as to provide an angle formed by a connection line from the sealing edge to the striking edge, which connection line extends through connection points distributed on parallel diameters, said angle being greater than the angle of the infeed cone in the centring body, each angle extending relative to the longitudinal axis of the needle valve nozzle. This ensures that the rather sensitive sealing edge of the shut-off needle is prevented from striking the centring body as it enters the latter. In fact, the shut-off part of the needle will remain contactless. The service life of the needle valve nozzle is further increased. 
   In another aspect of the invention, the difference between the angle of the connection line from the sealing edge to the striking edge and the angle of the infeed cone is greater than the greatest possible achievable deflection angle of the shut-off needle relative to the longitudinal axis of the needle valve nozzle before the striking edge of the shut-off needle gets in contact with the inner wall of the centring body. Thus the sealing edge of the shut-off needle is prevented from striking the centring body. This is efficiently prevented by the striking edge which may be rounded. In this way, the shut-off needle is allowed to glide with low friction into the centring body. 
   In order to prevent the build-up of undesired backpressure in the melt during the closing process, the infeed cone is formed by ribs, axial fillets or the like, which enclose the shut-off needle in a concentrical slideable manner. The melt which is necessarily displaced as the shut-off needle enters the outlet opening and the sealing seat, respectively, can easily escape into the melt channel, i. e. the material compression or the pressure increase generated during the closing process will automatically be compensated. Alternatively, the shut-off needle may comprise lateral bulges, flattenings, recesses, or similar, which likewise reduce the pressure. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further features, details and advantages will become apparent from the reading of the claims and from the following description of embodiments in connection with the drawings, wherein: 
       FIG. 1  is an enlarged axial sectional view of the lower end of a needle valve nozzle, and 
       FIG. 2  is an enlarged axial sectional view of the lower end of another embodiment of a needle valve nozzle. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The needle valve nozzle, in  FIG. 1  designated generally by the reference numeral  10 , forms part of an injection mould (not shown in detail). It has a preferably externally heated nozzle body  20  in which a material duct (not shown) is formed which delimits a melt channel  30  concentrically with respect to the longitudinal axis L of the needle valve nozzle  10 . A nozzle mouthpiece  40  made from a highly thermally conductive material is inserted, preferably screwed, from the bottom side into the nozzle body  20 , with said nozzle mouthpiece  40  cylindrically continuing the melt channel  30  downwards without modification of the cross-section. 
   A melt, for example a metal, silicone or plastic melt, is supplied via the melt channel  30  into a mould cavity (not shown in detail) for processing. Said mould cavity is formed between at least two mould inserts  50  which are mounted on a moulding plate (not shown) and confine a gate  51  concentrically with respect to the longitudinal axis L of the hot runner nozzle  10 . 
   A centring body  80  made from a wear-resistant material is arranged between the nozzle mouthpiece  40  and the mould inserts  50 , said centring body  80  centring the needle valve nozzle  10  with respect to the gate  51  and forming together with the air chamber  22  which encloses the nozzle mouthpiece  40  and the nozzle body  20  a thermal separation between the needle valve nozzle  10  and the mould inserts  50 , i. e. the highly heat-conductive nozzle mouthpiece  40  allows the flowable melt to be kept at a constantly high temperature on its way to the mould cavity. In addition, the centring body  80  prevents heating of the cool mould inserts  50 . 
   As illustrated in  FIG. 1 , the centring body  80 , which is all-in-all rotationally symmetrical with respect to the longitudinal axis L, comprises a broad flanged edge  81  and a narrow cylindrical collar portion  82 . The latter is inserted into the melt channel  30  and in the nozzle mouthpiece  40 , respectively, from the bottom side and reciprocates in them in longitudinal direction. 
   At its end facing the mould cavity, the centring body  80  forms a cylindrical end section  84  which engages in a longitudinally displaceable manner into a cylindrical seat  54 . Said centring seat  54  formed by the mould inserts  50  is concentrical with respect to the gate  51  and may have a slightly conical form in an upper section  55  in order to facilitate the introduction of the centring body  80  with its end section  84  during assembly. Its outer diameter is selected so as to provide a pressure-tight connection and to prevent the melt from entering the air chamber  22 . Simultaneously, longitudinal movements of the centring body  80  in the seat  54  are possible so that thermally induced changes in length of the needle valve nozzle  10  can be compensated, if necessary. The end face  85  of the end section  84  is preferably flat, just as the bottom surface (not specified) of the centring seat  54 , so as to provide an accurate fit. 
   It will be noted that the melt channel  30  is continued through the centring body  80  whose end section  84  forms an outlet opening  44  for the melt. The centrical alignment of the hot runner nozzle  10  permits unhindered flow of the melt through the gate  51  into the mould cavity. 
   An axially displaceable shut-off valve  60  is provided for opening and closing the outlet opening  44 . The shut-off valve  60  reciprocates in the melt channel  30  in longitudinal direction and can be moved by a pneumatic drive (not shown) from an open position into a closed position. The shut-off valve  60 , which is at least sectionally cylindrical and comprises several stepped diameters along the longitudinal axis L, has at its end a shut-off part  70  designed to engage in closed position through the end section  84  of the centring body  80  into a sealing seat D. In the embodiment shown in  FIG. 1 , said sealing seat D is provided ahead of the gate  51  in the mould inserts  50  and is of tapered form in its upper portion  57  so as to allow the shut-off part  70 , supported by the accumulated melt, to enter the sealing seat D in a low-friction manner. 
   The transitions from the needle portions of greater diameter to the sections having a smaller diameter may be conical or rounded, with a striking edge  74  being formed between a needle portion  64  and the abutting shut-off part  70  whose diameter exceeds that of the sealing edge  72  at the end of the shut-off part  70  of the shut-off needle  60 . 
   In order to prevent damages on the sensitive shut-off part  70  and to keep the outlet opening  44  permanently pressure-tightly sealed, the striking edge  74  is designed to align the shut-off needle  60  by means of the centring body  80  in case that said shut-off needle  60  were deflected from its concentric position in the melt channel  30 . To this end, an infeed cone  46  is provided in the collar portion  82  of the centring body  80 , said infeed cone being concentrical relative to the longitudinal axis L and continuing by a cylindrical guiding portion  83  whose inner diameter is insignificantly greater than the outer diameter of the needle portion  64  of the shut-off needle  60 . A cone  86  which is formed ahead of the outlet opening  44  in the centring body  80  reduces the inner diameter of the guiding portion  83  to the diameter of the shut-off part  70  of the shut-off needle  60 . 
   When moving from its open position into its closed position, the shut-off needle  60  is automatically aligned by the striking edge  74  and the infeed cone  46  so as to be concentric with respect to the longitudinal axis L, with the needle portion  64  being further advanced within the guidance section  83  of the centring body  80  without changing its alignment until the shut-off part  70  reaches the sealing seat D. The sensitive sealing edge  72  of the shut-off needle  60  is prevented from striking the nozzle mouthpiece  40  or the centring body  80 . Damages to the shut-off needle  60  are thus avoided. Since the preferably slightly rounded striking edge  74  of the shut-off needle  60  is relatively insensitive, and the centring body  80  is made from a wear-resistant material, there is no problem in the case that the striking edge  74  gets in contact with the infeed cone  46 . 
   In order to prevent the build-up of undesired backpressure in the melt during the closing process of the shut-off valve  60 , the infeed cone  46  in the centring body  80  is formed by ribs, axial fillets (not specified) or the like, which enclose the shut-off needle  60  in a concentrically slideable manner. Additionally or alternatively, the shut-off needle  60  may be provided in the area of the needle portion  64  with lateral bulges or recesses  66  so as to allow the melt displaced by the shut-off needle  60  to freely flow back into the melt channel  30 . 
   In the embodiment shown in  FIG. 2 , the end section  84  of the centring body  80 , which is provided with a convex end face, confines a portion of the mould cavity. The centring seat  54  for the centring body  80  is still provided by the mould inserts  50 , whereas the sealing seat D for the shut-off part  70  of the shut-off needle  60  is arranged in the end section  84  of the centring body  80 . The outlet opening  44  and the gate  51  coincide with one another, i.e. the melt passes directly through the centring body  80  into the mould cavity. 
   It will be noted in  FIG. 2  that the centring body  80  reciprocates in the nozzle mouthpiece  40  just as in the seat  54  of the mould inserts  50  in a longitudinal direction, so that a narrow motion-induced gap may form between the flanged edge  81  and the end face  41  of the nozzle mouthpiece  40 . This ensures that the needle valve nozzle  10  can elongate when being heated without modifying the cavity boundary. 
   It is important to select the axial distance A between the sealing edge  72  of the shut-off part  70  and the striking edge  74  in such a manner that the angle α of a connection line V from the sealing edge  72  to the striking edge  74  which extends through connection points distributed on parallel diameters is greater than the cone angle β of the infeed cone  46  in the collar portion  82  of the centring body  80 , each angle being relative to the longitudinal axis L of the hot runner nozzle  10 . Thus the shut-off needle  60  is in any case stopped with its striking edge  74  on the infeed come  46  and guided through the guiding portion  83  so as to allow the shut-off part  70  to enter the sealing seat D in a contactless manner. Damages on the sealing edge  72  are almost completely precluded. 
   The invention is not limited to any of the embodiments described above, but includes many variations and modifications. For example, the centring body  80  is preferably made from a powder metallurgical material. It is conceivable, however, to use other materials having a high wear resistance. Depending on the particular embodiment, the flanged edge  81  of the centring body  80  may encompass or enclose the lower end of the nozzle mouthpiece  40 . Furthermore, the shut-off needle  60  may be flush with the injection moulded part (not shown) or leave a mark on it. The embodiment shown in  FIG. 2  allows in any case and at any time for easy replacement of the gate point if the centring body  80  is worn. 
   All features and advantages which become apparent from the claims, the description and the drawings, including design details, spatial arrangements and process steps, may be essential to the invention, both individually and in a great variety of combinations. 
   REFERENCES 
   
       
       α,β angles 
       A distance 
       D sealing seat 
       L longitudinal axis 
       V connection line 
         10  needle valve nozzle 
         20  nozzle body 
         22  air chamber 
         30  melt channel 
         40  nozzle mouthpiece 
         41  end face 
         44  outlet opening 
         46  infeed cone 
         50  mould insert 
         51  gate 
         54  centring seat 
         55  upper portion (centring seat) 
         57  upper area (sealing seat) 
         60  shut-off needle 
         64  needle portion 
         66  recess 
         70  shut-off part 
         72  sealing edge 
         74  striking edge 
         80  centring body 
         81  flanged edge 
         82  collar portion 
         83  guiding portion 
         84  end section 
         85  end face 
         86  cone