Abstract:
Multi-cavity injection molding apparatus includes a plurality of injection nozzles, each of which has a two-piece tip assembly in the nature of a replaceable tip and a retainer that detachably secures the tip to the nozzle body. The retainers are constructed from material having a lower coefficient of thermal conductivity than the tips so that the retainers also serve to insulate the tips from the relatively cold, surrounding heat sink presented by the mold. Although each retainer prevents its tip from being unintentionally axially released from the nozzle body, the retainer engages the tip at only two axially spaced locations along its length so as to present an insulating air gap in surrounding relationship to much of the tip. The otherwise exposed end of the air gap is sealed off when the nozzle is hot by virtue of a sealing collar on the retainer that progressively tightens against the cooperating, beveled surface of the tip as the tip heats up. Each nozzle is mounted on the manifold block in a ball and socket relationship so that the nozzles can swivel and self-align as the manifold block and nozzles heat up from room temperature to operating temperature at the commencement of the injection process.

Description:
TECHNICAL FIELD 
     The present invention relates to injection molding apparatus and, more particularly, to improvements in the construction of nozzles that deliver hot melt into the mold cavities of such equipment. 
     BACKGROUND 
     It is known in the art to provide injection nozzles with two-piece tip assemblies comprising a replaceable inner tip or insert and a collar-like retainer that detachably secures the tip to the main body of the nozzle. See, for example, Gellert U.S. Pat. No. 5,299,928. 
     It is also known to make the retainer from a lower thermally conductive material than the tip itself so that the tip, through which the hot melt travels on its way to the mold cavity, is thermally insulated by the retainer from adjacent portions of the relatively cold mold. The &#39;928 patent, for example, describes constructing the tip from a highly thermally conductive material such as a beryllium copper alloy while forming the outer retainer from a much less thermally conductive material such as a titanium alloy. 
     While using the retainer to insulate the hot nozzle tip from proximal portions of the cold mold is helpful in increasing the thermodynamic efficiency of the apparatus, the extent of direct physical contact between the insulating retainer and the hot tip also has a direct bearing on heat loss. Because the retainer is not a perfect insulator, there is still a significant amount of heat loss from the tip to the cold mold via the retainer, particularly across regions where the tip and the retainer are in intimate physical contact with one another. 
     Furthermore, as the manifold block and the nozzles attached to the block heat up as they are prepared for dispensing the hot melt, and during the injection process itself, dimensional changes take place involving the nozzles. Generally speaking, while the nozzles and manifold block tend to grow or expand as they become hot, the mold remains much cooler and dimensionally stable such that the nozzles can become misaligned with the mold cavities. For example, while the center-to-center distances between gates in a multi-cavity machine remains essentially constant at all times, the center-to-center distance between the base ends of the nozzles can increase significantly as the metal manifold block expands under high heat conditions. Consequently, while the nozzles may be in perfect registration with the gates when the apparatus is cold, the base ends of the nozzles may move out of axial registration with the mold cavity as the manifold block and nozzles heat up, placing bending loads on the nozzles as their discharge ends are retained in place by surrounding portions of the mold. This obviously places undue stress on the nozzles and can lead to premature wear and fatigue, as well as having adverse effects on the proper injection of hot melt through the gate and the ability to produce a preform product having only a minimal gate vestige at the completion of the forming cycle. Furthermore, if the manifold and the mold are pulled apart for maintenance purposes or adjustment, once the discharge ends of the nozzles are released by the mold they tend to spring back into alignment with their bases, which means that the discharge ends are now out of registration with the receiving wells in the mold and cannot be reinserted into the mold until after they have been cooled down. This can result in a significant amount of downtime in an industry where it is crucially important to keep the molding apparatus in continuous productive operation as much as possible. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a two-piece tip assembly on an injection molding nozzle has the insulating sleeve of the retainer surrounding the nozzle tip in radially spaced relation thereto so as to form an insulating air gap between the retainer and the tip along a significant portion of the length of the tip so as to reduce heat loss from the tip to the cold mold. The bore through the insulating retainer is constricted at its outer end so as to form a collar on the retainer that is very slightly spaced from the adjacent surface of the tip when the tip is cold. However, when the tip is hot such as during injection operations, expansion and growth of the tip relative to the retainer causes the collar to tighten around the tip so as to effectively seal off the insulating air gap from hot melt that might attempt to back fill into the air gap from beyond the nozzle. Direct physical contact between the retainer and the tip is limited to only two points, i.e., the seal at the sealing collar, and the abutment at the inner end of the retainer where it overlies and engages an outwardly facing shoulder on the tip. 
     In addition, the present invention contemplates having the base ends of the injection nozzles swivel-mounted in the manifold block so that the nozzles can self-adjust or self-compensate as the manifold block grows and expands when heated. To this end, the manifold block is provided with a number of concave seats that matingly receive the lower halves of spherical base portions of the nozzles. Clamp-down structure attaching the nozzles to the manifold block is provided with internal concavities that matingly receive the upper halves of the spherical bases. The retainer at the discharge end of each nozzle is configured to present a laterally outermost edge that resides in close proximity to a surrounding wall portion of the nozzle-receiving well in the mold so that the discharge end of the nozzle stays properly located and registered with the gate while permitting swiveling action at the lower end. Opposing surfaces of the mold and the nozzle tip assembly are configured in such a manner as to maximize delivery of hot melt into and through the gate while minimizing the amount of excessive back fill of melt into the void area between the mold surface and the nozzle tip assembly surface. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a front elevational view of a manifold utilizing injection molding nozzles constructed in accordance with the principles of the present invention; 
     FIG. 2 is a top plan view of the manifold; 
     FIG. 3 is an enlarged, fragmentary, cross-sectional, schematic illustration of the relationship between the injection nozzles and the mold while the nozzles and distribution manifold are relatively cold before commencement of the molding process; 
     FIG. 4 is an illustration of the nozzles and mold similar to FIG. 3 but illustrating the way in which the nozzles have swivelled into axially aligned relationship with the mold cavities after the manifold and nozzles have been heated up; 
     FIG. 5 is an enlarged, fragmentary cross-sectional view of the discharge end of one nozzle and associated mold structure illustrating the relationship of parts; and 
     FIG. 6 is an exploded, isometric view of the nozzle tip and retainer therefor in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION 
     The present invention is susceptible of embodiment in many different forms. While the drawings illustrate and the specification describes certain preferred embodiments of the invention, it is to be understood that such disclosure is by way of example only. There is no intent to limit the principles of the present invention to the particular disclosed embodiments. 
     The manifold  10  in FIGS. 1 and 2 includes a base plate  12  that supports an upright manifold block  14  which receives hot melt through an inlet sprue  16 . Internal passages within block  14  in turn distribute the hot melt to a number of injection nozzles  18 , here shown as being six in number corresponding to a six cavity mold. 
     As illustrated in FIGS. 3,  4  and  5 , during use of the manifold  10  nozzles  18  project into corresponding receiving wells  20  within a mold  22  for the purpose of delivering hot melt into corresponding cavity spaces  24  within cavities  25  of mold  22 . Each space  24  receives a core  26  that cooperates with a cavity  25  to define the appropriate configuration of space  24  prior to and during the reception of the hot melt. Cores  26  are withdrawn from the spaces  24  at the completion of each injection cycle. Cooling channels  28  surrounding each space  24  are supplied with suitable coolant for the purpose of cooling cavity  25  and the product therein. Nozzles  18  and manifold block  14  are heated by suitable heating apparatus not shown, but well understood by those skilled in the art. 
     As illustrated particularly in FIG. 5, each nozzle  18  includes an elongated, tubular nozzle body  30  having a central passage  32 . A coaxial socket  34  is recessed into the discharge end of body  30  in communication with passage  32 . Socket  34  includes a cylindrical smooth-walled inboard portion  36  that is somewhat larger in diameter than passage  32  so as to present an annular shelf  38  that circumscribes the outlet of passage  32 . Socket  34  also includes an outboard portion  40  that is coaxial with but larger in diameter than inboard portion  36  so as to present a second shelf  42  at the intersection of inboard and outboard portions  36  and  40 . An inner stretch of the wall surface defining outboard socket portion  40  is internally threaded, while an outer stretch thereof is smooth-walled. 
     Socket  34  removably receives a replaceable nozzle tip  44 , shown in an isolated condition in FIG.  6 . Tip  44  is tubular, having a central passage  46  therethrough for receiving melt from passage  32  when tip  44  is received within socket  34 . Tip  44  is of generally cylindrical overall configuration and has a bottom end  48  that abuts and rests upon shelf  38  when tip  44  is in place within socket  34 . Tip  44  includes a cylindrical base portion  50  of one diameter and a neck portion  52  of a lesser diameter so as to present an axially outwardly facing shoulder  54  at the intersection of base portion  50  and neck portion  52 . Most of the exterior of neck portion  52  extends parallel to the axis of passage  46 . However, the end face  58  of tip  44  surrounding outlet  56  of passage  46  presents a truncated cone, while a bevel ring  60  at a different angle than end face  58  is disposed between the straight cylindrical portion of neck portion  52  and end face  58 . Preferably, tip  44  is constructed of a highly thermally conductive material such as a suitable bronze alloy, preferably Ampco 940. 
     Each nozzle  18  also includes an insulating retainer  62  that is detachably secured to nozzle body  30  and which removably holds tip  44  in place within socket  34 . Preferably, retainer  62  is constructed from a titanium alloy so as to have substantially lower thermal conductivity than tip  44 . As a primary component retainer  62  comprises a sleeve  64  having a through bore  66  that receives neck portion  52  of tip  44 . Bore  66  includes an enlarged, inboard section  66   a  that circumscribes neck portion  52  in radially spaced relation thereto so as to define an insulating air gap  68  around neck portion  52 . Bore  66  also includes a constricted outboard section  66   b  of reduced diameter relative to inboard section  66   a  so as to define a sealing collar  70  surrounding the bevel  60  on neck portion  52 . Although the diameter of outboard section  66   b  is slightly greater than that of bevel  60  when tip  44  is cold, it will be appreciated that as tip  44  becomes hot and grows in length, collar  70  comes into tight, sealing contact with neck portion  52  at bevel  60  so as to close off air gap  68  at that location. 
     A dished out recess  72  in end face  74  of retainer  62  circumscribes collar  70  so as to thin down collar  70  and provide a slight amount of flexibility thereto to facilitate sealing contact between collar  70  and bevel  60  at the appropriate time. Such recess  72  also provides less metal for retainer  62  in the immediate vicinity of its point of contact with bevel  60  so as to reduce heat loss from tip  44  in that area. End face  74  also includes an annular flat region  76  that circumscribes recess  42  and extends radially outwardly to the outermost peripheral edge  78  of an overhanging lip  80  on sleeve  64 . The exterior of sleeve  64  generally adjacent the inboard end thereof contains a set of threads  82  that mesh with the internal threads within socket  34  to detachably secure retainer  62  to the body  30 . It will be appreciated to those skilled in the art that means other than intermeshing threads may be used to effect such releaseable attachment. 
     The cavity space  24  and the nozzle-receiving well  20  are communicated with one another by a relatively short, narrow gate  82  so that, during operation, hot melt from the nozzle  18  passes through gate  82  and into space  24 . Well  20  has an innermost end surface  84  that faces nozzle  18  and cooperates with end face  74  of retainer  62  and end face  58  of tip  44  to define a relatively thin void  86 . Thus, end face  58  of tip  44  and end face  74  of retainer  62  are not in contacting engagement with end surface  84  of well  20  but are instead spaced slightly axially therefrom. Preferably, end surface  84  is configured to present a conical depression  84   a  that surrounds gate  82  and is almost complemental to the conical end face  58  of tip  44 , although it will be noted that end face  58  is slightly more sharply inclined than depression  84   a  such that void  86  becomes slightly more progressively constricted as gate  82  is approached. Depression  84   a  extends laterally outwardly to a point beyond collar  70  on retainer  62  and into general registration with recess  72 , whereupon surface  84  changes to an annular flat region  84   b  that surrounds depression  84   a  and extends generally parallel to the flat region  76  on end face  74  of retainer  62 . It will be noted that void  86  is somewhat thinner in the area between flat regions  76  and  84   b  than between depression  84   a  and end face  58  of tip  44 . 
     Well  20  is substantially larger in diameter than nozzle body  30  so as to provide a substantial amount of air space surrounding body  30  to insulate it from mold  22 . However, well  20  also tapers toward a reduced diameter dimension as gate  82  is approached, and at its inner end, well  20  is provided with a relatively short, axially extending sidewall  88  that extends parallel to the peripheral edge  78  of retainer  62  and circumscribes the same. The diameter of well  20  at sidewall  88  is only slightly larger than the outer diameter of retainer  62  at edge  78  such that sidewall  88  serves to locate and confine retainer  62  against lateral displacement, thus maintaining melt passage  46  and outlet  56  of tip  44  in axial registration with gate  82 . 
     As illustrated in FIGS. 3 and 4, each nozzle  18  is provided with a generally spherical base  90  whose lower half is matingly received within a corresponding concave seat  42  in the top surface of manifold block  14 . Structure for retaining bases  90  within their seats  92  in a manner to permit swiveling of bases  90  comprises a plurality of retaining collars  94  secured to manifold block  14  by fasteners such as screws  96  (FIG.  2 ). Each retaining collar  94  has a cavity  96  on its underside that matingly receives and overlies the upper half of the corresponding spherical base  90  so as to retain the nozzle on manifold block  14  yet permit it to swivel in the manner of a ball and socket. Preferably, each retaining collar  94  is constructed from graphite impregnated tool steel that has been oil-hardened in order to provide the necessary amount of lubricity and resistance to galling. Manifold block  14  has a hot melt supply port  96  at the base of each seat  92 , and each spherical base  90  has an inlet  98  to the passage  32 . 
     Operation 
     Because the mold  22  remains relatively cool throughout the injection molding process, the center-to-center distance between gates  82  remains substantially unchanged. However, because the temperature of the manifold block  14  and nozzles  18  increases so substantially from room temperature to operating temperature, the dimensions of manifold block  14  and nozzles  18  increase correspondingly. Thus, as the manifold block heats up, the center-to-center distance between bases  90  of the nozzles  18  increases, with the smallest increase occurring between nozzles at the center of manifold block  14  and the largest increase being experienced at the outermost nozzles. 
     FIG. 3 is an exaggerated illustration of the condition that exists when manifold block  14  and nozzles  18  are at room temperature, at which time the center-to-center distance between bases  90  is slightly less than the center-to-center distance between gates  82 . Consequently, when the nozzles  18  are inserted up into wells  20 , nozzles  18  swivel slightly about their bases  90  as the retainers  62  become located within the bounds of the axial sidewall portion  88  of the well. As illustrated in FIG. 3 on an exaggerated scale, each nozzle  18  thus becomes slightly tipped, about 5°, as the outlets  56  of the nozzles come into registration with gates  82 . 
     As manifold block  14  and nozzles  18  are then heated up, as illustrated in FIG. 4, the expanding manifold block causes the axes of the nozzles to line up with the axes of the cavity spaces  24 . Due to the ball and socket relationship between the base of the nozzles and the manifold block  14 , the nozzles are free to self-adjust or self-compensate for the changing conditions, and only to the extent required by such changes. It will be noted that because the retainers  62  are captive within the sidewalls  88  of wells  20  during such change in conditions, the nozzle outlets  56  remain aligned with gates  82  throughout the process and that any misalignment occurs at the base ends of the nozzles due to rotation of bases  90  relative to seats  92 . 
     It will also be noted that each nozzle body  30  and tip  44  grows axially as the nozzle is heated up. Thus, one result is that the outer faces  58  and  74  of the nozzle tip and retainer respectively are displaced closer and closer to gate  82  and end surface  84  of well  20 . In addition, because of the different coefficients of thermoconductivity between nozzle tip  44  and retainer  62 , neck portion  52  of tip  44  projects progressively further through and out of collar  70  toward gate  82  as tip  44  gets hot. This axial growth of tip  44  causes bevel  60  to progressively present larger portions of its circumference to the constricted outboard section  66   b  of bore  66 , resulting in a progressively tighter and tighter seal between collar  70  and tip  44 . 
     Consequently, when nozzles  18  are hot, the air gap  68  surrounding each neck portion  52  is effectively sealed off against the admittance of hot melt that backfills within void  86  laterally outwardly from each gate  82 . This provides better insulation for tip  44  than would otherwise be the case and less consequent heat loss to the cold mold  22 . It will be noted also that due to the fairly constricted nature of void  86  between flat regions  76  and  84   b , backfill of the hot melt will not extend out to the outermost periphery  78  of retainer  62  but will instead terminate somewhere in the vicinity of the dished out recess  72 . This condition also aids in reducing heat loss from the nozzles  18  and facilitates cleaning out of solidified backfill material on the mold apparatus when a different melt material or color is to be injected. 
     Although preferred forms of the invention have been described above, it is to be recognized that such disclosure is by way of illustration only, and should not be utilized in a limiting sense in interpreting the scope of the present invention. Obvious modifications to the exemplary embodiments, as hereinabove set forth, could be readily made by those skilled in the art without departing from the spirit of the present invention. 
     The inventor(s) hereby state(s) his/their intent to rely on the doctrine of equivalents to determine and assess the reasonably fair scope of his/their invention as pertains to any apparatus not materially departing from but outside the literal scope of the invention as set out in the following claims.