Abstract:
A flow mixer apparatus and method in an injection molding system which transitions a flowing medium around an obstruction and/or a degree change in direction, said flowing medium exhibiting reduced stagnation points and substantially uniform flow characteristics downstream of the obstruction and/or change in direction.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This is a Continuation-in-Part of co-pending application entitled “Mixer to Improve Homogeneity in Injection Molding Machines and Hot Runners”, filed Jun. 28, 2000 Ser. No. 09/605,763 which is a Continuation-in-Part of application entitled “Nozzle with Weld Line Eliminator”, now issued as U.S. Pat. No. 6,089,468, both incorporated herein by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    This invention relates to an apparatus and method for converting the circular flow inside a melt channel to a uniform annular flow. More specifically, this invention relates to an apparatus and method for improving uniform melt flow and elimination of stagnation points as it passes through an injection molding system and/or hot runner system.  
           [0004]    2. Summary of the Prior Art  
           [0005]    The large number of variables in the injection molding process creates serious challenges to creating a uniform and high quality part. These variables are significantly compounded within multi-cavity molds. Here we have the problem of not only shot to shot variations but also variations existing between individual cavities within a given shot. Shear induced flow imbalances occur in all multi-cavity molds that use the industry standard multiple cavity “naturally balanced” runner system whereby the shear and thermal history within each mold is thought to be kept equal regardless of which hot-runner path is taken by the molten material as it flows to the mold cavities. These flow imbalances have been found to be significant and may be the largest contributor to product variation in multi-cavity molds.  
           [0006]    Despite the geometrical balance, in what has traditionally been referred to as “naturally balanced” runner systems, it has been found that these runner systems can induce a significant variation in the melt conditions delivered to the various cavities within a multi-cavity mold. These variations can include melt temperature, pressure, and material properties. Within a multi-cavity mold, this will result in variations in the size, shape and mechanical properties of the product.  
           [0007]    It is well known that providing for smooth flow of pressurized melt is critical to successful molding of certain materials. Sharp bends, corners or dead spots in the melt passage results in unacceptable residence time for some portion of the melt being processed which can cause too much delay on color changes and/or result in decomposition of some materials or pigments of some materials such as polyvinyl chloride and some polyesters or other high temperature crystalline materials. In most multi-cavity valve gated injection molding systems it is necessary for the melt flow passage to change direction by 90° and to join the bore around the reciprocating valve stem as it extends from the manifold to each nozzle.  
           [0008]    These problems necessarily require fine tolerance machining to overcome and it is well known to facilitate this by providing a separate bushing seated in the nozzle as disclosed in U.S. Pat. No. 4,026,518 to Gellert. A similar arrangement for multi-cavity molding is shown in U.S. Pat. No. 4,521,179 to Gellert. U.S. Pat. No. 4,433,969 to Gellert also shows a multi-cavity arrangement in which the bushing is located between the manifold and the nozzle. Also shown in U.S. Pat. No. 4,705,473 to Schmidt, provides a bushing in which the melt duct in the bushing splits into two smoothly curved arms which connect to opposite sides of the valve member bore. U.S. Pat. No. 4,740,151 to Schmidt, et al. shows a multi-cavity system with a different sealing and retaining bushing having a flanged portion mounted between the manifold and the back plate.  
           [0009]    U.S. Pat. No. 4,443,178 to Fujita discloses a simple chamfered surface located behind the valve stem for promoting the elimination of the stagnation point which would otherwise form.  
           [0010]    U.S. Pat. No. 4,932,858 to Gellert shows a separate bushing seated between the manifold and the injection nozzle in the melt stream which comprises a melt duct with two smoothly curved arms which connect between the melt passage in the manifold and the melt passage around the valve stem in an effort to eliminate the stagnation points.  
           [0011]    U.S. Pat. No. 5,916,605 to Swenson et al. shows an injection nozzle insert with a spiral channel formed therein. This insert is positioned adjacent the nozzle tip and requires the melt to enter through a hole at the very top of the insert. This invention therefore does not disclose a means for conveying the melt through a 90° turn as the melt transitions from the hot runner subsystem to the injection nozzle.  
           [0012]    Referring to FIGS. 5 and 6, hot runner subsystems according to the prior art are generally shown. FIG. 5 depicts a hot runner subsystem configuration  110 ′ with what is commonly referred to as a valve gated nozzle. In this configuration, a heated nozzle assembly  108 ′ comprises a nozzle bushing  112 ′ sealingly abutting against a lower flange of a bushing  148 ′. The bushing  148 ′ is inserted in a bore of a hot runner manifold  138 ′ and directs the melt flow from melt channel  142 ′ through a bushing channel  144 ′ to a nozzle tip  128 ′. The bushing channel  144 ′ is formed internal to the bushing  148 ′ and directs the melt flow through a 90 degree change in flow direction. A valve stem  126 ′ is inserted co-axially in bushing  148 ′ and extends through bushing channel  144 ′.  
           [0013]    As the figure shows, the melt flow goes through both a 90 degree change in direction while it also flows around the valve stem  126 ′. These flow disturbances result in flow imbalances and stagnation points which acts to degrade the melt. In addition, the valve stem  126 ′ is not adequately supported and will be subjected to wear by the often times abrasive flowing melt. It should also be noted that bushing channel  144 ′ is expensive and time consuming to produce.  
           [0014]    Referring now to FIG. 6, which depicts a prior art hot runner subsystem  208 ′ incorporating what is commonly referred to as a “hot tip” nozzle assembly  208 ′. In this configuration, the valve stem and bushing have been removed. A nozzle bushing  212 ′ sealingly abuts directly against a lower surface of the hot runner manifold  238 ′. Melt channel  242 ′ undergoes a 90 degree change in direction to line up with the nozzle channel  220 ′. As the melt flows around the corner as shown by arrow A, stagnation points form at areas denoted  250 ′. These stagnation points impede color changing as well as degrade the melt and cause flow imbalances.  
           [0015]    There exists a need for a method and apparatus that substantially reduces the flow imbalances and stagnation points in an injection molding system and/or hot runner system that occurs as a result of the flow being diverted through a change in direction and/or around a melt flow obstruction such as a valve stem, a nozzle, a nozzle tip, a valve stem guide, a torpedo, etc.  
         SUMMARY OF THE INVENTION  
         [0016]    The primary objective of the present invention is to provide a mixer in a melt channel that creates a substantially uniform annular flow velocity profile.  
           [0017]    Another object of the present invention is to provide a mixer in a melt channel that eliminates stagnation points in the channel that occurs when the melt flows around an obstruction and/or a change in direction in the channel.  
           [0018]    A further object of the present invention is to provide a means for fast color change-over in an injection molding system, thereby reducing machine downtime between color changes.  
           [0019]    Still another object of the present invention is to provide a means for conveying heat sensitive materials through an injection molding system with reduced degradation caused by stagnation points in the melt stream.  
           [0020]    Yet another object of the present invention is to provide substantially uniform annular flow to the mold cavity which leads to improved part quality.  
           [0021]    Still yet another object of the present invention is to provide improved valve stem guidance and support in an injection molding machine/hot runner system, thereby resulting in a higher quality molded part and a valve stem with a longer usable life.  
           [0022]    Yet another object of the present invention is to provide an improved, cost effective means for turning the melt flow through various angles as it flows from the machine to a mold cavity.  
           [0023]    Still another object of the present invention is to provide a means for improving melt homogeneity as it flows through an injection molding system.  
           [0024]    The foregoing objects are achieved by providing a mixer located in a melt channel of an injection molding system, preferably around a valve stem or other flow obstruction, where the melt navigates a change in flow direction and flows around the obstruction. One preferred embodiment comprises a cylindrically tapered insert with a helical or spiral groove disposed on its outer surface. The groove is formed to be decreasing in depth and width, so as the melt flows into the groove, it gradually spills out of the groove. As the melt travels through the helical groove, it is mixed and changes direction in the hot runner manifold. The helical groove helps direct the melt around the back of the mixer which helps to eliminate stagnation points behind the flow obstruction while also providing uniform annular flow of the melt.  
           [0025]    Further objections and advantages of the present invention will appear hereinbelow.  
       
    
    
     BREIF DESCRIPTION OF THE DRAWINGS  
       [0026]    [0026]FIG. 1 is a simplified cross-sectional view of a preferred embodiment of the present invention;  
         [0027]    [0027]FIG. 2 is a simplified cross-sectional view of a preferred embodiment of the present invention showing a mixer housing installed in a hot runner manifold;  
         [0028]    [0028]FIG. 3 is a simplified cross-sectional view of another preferred embodiment of the present invention;  
         [0029]    [0029]FIG. 4 is a simplified cross-sectional views of another preferred embodiment of the present invention;  
         [0030]    [0030]FIG. 4 a  is an enlarged cross-sectional view of a preferred embodiment of a mixer bushing in accordance with the present invention;  
         [0031]    [0031]FIG. 4 b  is a simplified cross-sectional view of a hot tip injection nozzle in accordance with a preferred embodiment of the present invention;  
         [0032]    [0032]FIG. 5 is a cross-sectional view of a hot runner subsystem with a valve gated nozzle in accordance with the prior art;  
         [0033]    [0033]FIG. 6 is a cross-sectional view of a hot runner subsystem with a hot-tip nozzle in accordance with the prior art;  
         [0034]    [0034]FIGS. 7 and 8 are cross-sectional views of a hot runner subsystem of another preferred embodiment of the present invention installed in a hot runner manifold;  
         [0035]    [0035]FIG. 9 is a simplified cross-sectional view of a preferred embodiment of a mixer in accordance with the present invention installed in a hot runner manifold;  
         [0036]    [0036]FIG. 10 is a cross-sectional view of an injection nozzle with a mixer bushing in accordance with a preferred embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0037]    Referring first to FIG. 1, a preferred embodiment  10  in accordance with the present invention is generally shown. A hot runner valve gate system for injecting plastic material into a mold or the like is illustrated. The system includes a backing plate  102  and a manifold plate  104 . A mold base  106  is further attached to the manifold plate  104 .  
         [0038]    The system further includes a nozzle assembly  108  for introducing molten plastic material into a mold (not shown) and a manifold/mixer housing arrangement  110  for communication of plastic material from a source (not shown) to the nozzle assembly  108 . A manifold heater  139  is shown inserted in a manifold  138 , thereby heating the manifold  138  which in turn heats the flowing plastic within a melt channel  142 . The mixer housing  130  is inserted in a bore  143  of the manifold  138 .  
         [0039]    As shown in FIG. 1, the nozzle assembly  108  consists of a nozzle body  112 , a tip  114 , a nozzle heater  116 , a spring means  118 , and a nozzle insulator  113 . The nozzle body  112  is typically made of steel, while the tip  114  may be formed from any suitable highly heat-conductive material known in the art such as beryllium/copper or tungsten carbide. The nozzle body  112  has an axial channel  120  through which molten plastic material flows. The tip  114  surrounds a terminal part of the axial channel  120 .  
         [0040]    If desired, the nozzle tip  114  may include a sheath  122  for thermally insulating the downstream end of the nozzle tip  114 . The sheath  122  may be formed from a resinous material which may be prefabricated. Alternatively, the sheath  122  may be formed from an overflow of injected resin in the first operating cycle or cycles. The nozzle insulator  113  is installed within a cavity of the manifold plate  104  and acts to reduce the thermal communication between the nozzle body  112  and the manifold plate  104 , thereby maintaining the high temperature of the molten plastic material as it flows through the axial channel  120 . The nozzle insulator  113  may be formed from any suitable insulating material, typically known in the art such as titanium.  
         [0041]    The nozzle heater  116  may be any suitable electric heater known in the art to which current is admitted by way of a cable  124 . As shown in FIG. 1, the nozzle heater  116  surrounds a portion of the nozzle body  112 .  
         [0042]    A valve stem  126  is provided to permit opening and closing of the gate  128  in the nozzle body  112 . The valve stem  126  may be formed by a steel rod that extends through a passageway  20  in the mixer housing  130  and into the nozzle body  112 . The end of the valve stem  126  opposite to the gate  128  is connected to a piston head  131  by a set-screw  154 .  
         [0043]    The piston head  131  is housed within a cylinder housing which comprises the upper distal end of mixer housing  130  and formed by cylindrical wall  134 . Downstroke of the piston head  131  causes the valve stem  126  to move into a position where it closes or reduces the cross sectional area of the gate  128  so as to restrict flow of the molten plastic material. Upstroke of the piston head  131  causes the valve stem  126  to move so as to increase flow of the molten plastic material through the gate  128 .  
         [0044]    The hot runner system of this preferred embodiment also includes the manifold/mixer arrangement  110  consisting of the manifold  138  and the mixer housing  130  inserted into bore  143  therein. A locating pin  129  fixes the alignment of the mixer housing  130  to the melt channel  142 . The manifold  138  is formed by a distribution plate housed between the plates  102  and  104  but separated therefrom by an air gap  140 . The backing plate  102  is rigidly affixed to the manifold plate  104  by a plurality of high strength bolts (not shown) which must withstand the large forces generated during the cyclic molding process.  
         [0045]    The manifold includes the melt channel  142  forming part of the hot runner system for transporting molten plastic material from a source (not shown) to the gate  128  associated with a respective mold or molds. The manifold further includes the bore  143  into which mixer housing  130  is inserted. The manifold  138  may be formed from any suitable metal or heat conducting material known in the art. The manifold heater  139  is well known in the art and typically comprises a wire/ceramic resistive type heater with a cylindrical cross section that is seated into a groove of the manifold  138 .  
         [0046]    The mixer housing  130  surrounds and guides a portion of the valve stem  126 . This is an important advantage of the present invention because this increased valve stem support reduces valve stem wear and will significantly increase the life of the valve stem. Increased valve stem life will result in reduced maintenance costs and machine downtime.  
         [0047]    The mixer housing  130  is formed from any suitable material known in the art (usually steel) and is designed to be inserted into the manifold  138  from the top. As shown in FIG. 1, a helical channel  19  mates with the melt channel  142  in the manifold  138  and the axial channel  120  in the nozzle assembly  108 .  
         [0048]    As the melt flows from melt channel  142  to a flow inlet  18 , it strikes the helical channel  19  substantially perpendicular to valve stem  126  longitudinal axis. If helical channel  19  were not present, the melt would tend to flow mainly down along the face of the valve stem  126 , thereby causing stagnation points behind the valve stem  126 . As a result of stagnation points, parts would not fill uniformly and the melt would degrade due to prolonged exposure to elevated temperatures. However, in this preferred embodiment, the melt flows into helical channel  19  and is directed to flow around the mixer housing  130 , thereby eliminating the formation of stagnation points behind the valve stem  126 . As the melt flows through helical channel  19 , the cross-sectional area of the groove decreases so as to force more and more of the melt out of the helical channel  19 . This gradually transitions the flow to annular flow so that by the time the melt reaches an exit  17 , stagnation points have been eliminated and a substantially uniform velocity profile has been established which results in the formation of high quality molded parts. In addition, the helical channel  19  has transitioned the flow through a 90° turn without the need for expensive bushings that are currently used in the art (FIG. 5).  
         [0049]    Referring now to FIG. 2 (where like features have like numerals), another preferred embodiment in accordance with the present invention is generally shown installed in a hot runner manifold  138 . In this embodiment, the mixer housing  130  is a singular bushing that is inserted in the bore  143  of the manifold  138  from the top. In this embodiment, the bore  143  is tapered at its lower end. The angle of the taper of the bore  143  is such that the gap between the bore surface and the helical channel  19  increases as the melt flows toward the exit  17 . The mixer housing  130  further comprises the passageway  20  for insertion of the valve stem (not shown). The flow inlet  18  is aligned with the melt channel  142  by locating pin  129 .  
         [0050]    Referring now to FIG. 3, an alternate preferred embodiment of the present invention is shown where the mixer housing  130  is divided into two distinct pieces, a piston housing  130   a  and a mixer insert  130   b.  In this embodiment, the mixer insert  130   b  is installed in the hot runner manifold  138  from the bottom and bore  143  has a shoulder  150  where the insert  130   b  will seat. The piston housing  130   a  is installed over the top distal end of the insert  130   b  and a fastener  149  securely fastens the assembly as shown. A slot  152  in the insert  130   b  interfaces with an alignment device  147  that is installed in a hole  145  located in manifold  138 . This feature maintains alignment of flow inlet  18  to the melt channel  142 . This alignment feature is subject to many modifications that become apparent to one familiar with this art. For example without limitation, any type of alignment feature such as a key and keyway, or a D-shaped hole on either the mixer insert  130   a  or the manifold  138  could be employed.  
         [0051]    In this embodiment, the valve stem  126  is inserted through the mixer insert  130   a,  thereby supporting and guiding the valve stem  126  while also directing the melt through a 90 degree turn and around the back of the valve stem  126 . The helical channel  19  mixes the melt and converts the melt from circular to annular flow, thereby creating a substantially homogeneous melt exhibiting a uniform velocity profile at the exit  17 .  
         [0052]    Referring now to FIGS. 4 and 4 a  (where like features have like numerals), another preferred embodiment in accordance with the present invention is generally shown installed in a hot runner manifold  138 . A mixer bushing  152  is inserted in the bore  143  from the bottom of the manifold  138  and securely affixed therein by fastener  149 . The mixer bushing  152  further comprises a flow inlet  18  and a flow exit  17  and an internal helical channel  64  communicating the melt flow therethrough. The valve stem  126  is inserted co-axially with the helical channel  64  such that the melt flow is directed around the valve stem. Mixer bushing  152  acts as a guide for valve stem  126  where the valve stem is contacted by lands  70  at contact area  72 . Downstream of the contact area  72 , the contact ceases as the helical channel  64  depth decreases and land clearance  74  from the valve stem steadily increases in the direction of the melt flow.  
         [0053]    In operation, when the valve stem  126  is retracted by piston  131 , the melt flows from melt channel  142  onto one or more of the helical channels  64  which induces a helical flow pattern. As the melt flow progresses toward exit  18  more and more of the melt spills over the lands  70  as the land clearance  72  gradually increases. In this manner, the helical flow is gradually transitioned to substantially uniform annular flow around the valve stem  126 . Additionally, the melt flow has also undergone a change of direction of at least 90 degrees without the creation of preferential flow which has also been known to degrade molded part quality.  
         [0054]    Referring to FIG. 4 b,  where like features have like numerals, an injection nozzle provided with a hot tip  128 ′ configuration is shown. In this embodiment, the elongated valve stem has been removed and replaced by a shortened pin  126 ′ which extends from the top of fastener  149  to adjacent exit  17 . The mixer bushing  152  is identical to the one described in FIGS. 4 and 4 a.  The pin  126 ′ is secured by either a press fit or other suitable means, and is fixed inside the mixer bushing  152 . In this configuration, an improved hot tip injection nozzle is provided wherein flow imbalances and stagnation points in the melt stream have been substantially removed.  
         [0055]    Referring now to FIG. 7, where like features have like numerals, another preferred embodiment of the present invention is shown. A mixer housing  130  is inserted in a hot runner manifold  238 ′ that directs the melt flow to a “hot tip” injection nozzle assembly  208 ′. The mixer housing  130  has a tapering helical channel  64  formed thereon, with a flow inlet  18  aligned with melt channel  242 ′ by locating pin  129 . A cover  154  is fastened to the manifold  238 ′ using a plurality of fasteners  156  to affix the mixer housing  130  in the manifold. While the figure shows the mixer housing  130  and the cover  154  as separate pieces, combining these pieces is also contemplated. Similar to previously discussed embodiments, lands  70  are formed in a tapered fashion so that the gap  74  between the mixer bushing  130  and the manifold  238 ′ gradually increases. In operation, the melt flows from melt channel  242 ′ to flow inlet  18  where it enters the helical channel  64 . As the melt flows through the helical channel, more and more melt spills into gap  74  such that the melt flow has undergone significant mixing and a change in direction without the creation of stagnation points.  
         [0056]    Referring to FIG. 8, where like features have like numerals, the mixer housing  130  is shown installed upstream in a hot runner manifold  238 ′. In this embodiment, mixer housing  130  prevents the creation of stagnation points that commonly occur inside the melt channels of a hot runner as the melt undergoes a change in direction.  
         [0057]    Referring now to FIG. 9, where like features have like numerals, a mixer bushing  130  is installed in a manifold  238 ′ in accordance with an alternative embodiment of the present invention. In this embodiment, the mixer bushing  130  has an internally formed helical channel  64  with a coaxially extending elongated headed pin  158  inserted therein. The pin and the bushing are seated in a counter bore in the manifold  238 ′. The mixer and pin are trapped in the manifold by a cover  154  and at least one fastener  149 . A locating pin  129  is inserted in the manifold  238 ′ and is received by the mixer bushing  130  for maintaining alignment of the flow inlet  18  with the melt channel  242 ′. In this configuration, flowing melt enters flow inlet  18  from melt channel  242 ′ and travels down the mixer bushing  130  through helical channel  64 . A gradually expanding gap  74  is created between a series of lands  70  and the pin  158 . As the melt flows through the helical channel, more and more of the melt is allowed to spill over the lands  70  and into the gap  74  such that the melt flow is converted from helical to annular flow. This gradual transition substantially reduces stagnation points and increases melt homogeneity.  
         [0058]    Referring now to FIG. 10, where like features have like numerals, an injection nozzle assembly in accordance with a preferred embodiment of the present invention is generally shown. A mixer bushing  130  is inserted co-axially into a nozzle housing  24  with a locator pin  34  maintaining alignment between the parts. As in previous embodiments, an internal helical channel  64  having an inlet  18  and an exit  17  is formed in the mixer bushing  130  and a movable elongated valve stem  126  extends through the helical channel to a nozzle outlet  128 . A melt channel  142  in the manifold  104  is in fluid communication with a passageway  28  in the mixer bushing  130  and then a second passageway  30  formed in nozzle housing  24 . The flowing melt enters the inlet  18  substantially perpendicular to the longitudinal axis of valve stem  126  and is directed into the helical channel  64 . As the melt flows through the helical channel  64 , more and more of it will spill over the lands  70  into gap  74  thereby gradually transitioning the melt from helical to annular flow and improving melt homogeneity.  
         [0059]    It is to be understood that the invention is not limited to the illustrations described herein, which are deemed to illustrate the best modes of carrying out the invention, and which are susceptible to modification of form, size, arrangement of parts and details of operation. The invention is intended to encompass all such modifications, which are within its spirit and scope as defined by the claims.