Abstract:
A mixer method and apparatus for use generally in injection molding machines is provided. The apparatus and method is generally comprised of a mixer insert that retains a mixing element that is sealingly inserted in the injection molding machine, for example a hot runner manifold. The mixing element reduces the melt imbalances in a flowing melt stream for the formation of improved molded parts.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a Continuation-In-Part of co-pending application Ser. No. 09/845,399 filed Apr. 30, 2001 which is a Continuation-In-Part of co-pending Ser. No. 09/605,763 filed Jun. 28, 2000 now U.S. Pat. No. 6.382,528 which is a Continuation-In-Part of co-pending Ser. No. 09/435,965 filed Nov. 8, 1999 now U.S. Pat. No. 6,089,468, all of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to injection molding machines for the transmission of various molten materials to a mold cavity or cavities. More specifically, this invention relates to a method and apparatus for the insertion of a mixer in the melt stream of an injection molding machine. 
     2. Summary of the Prior Art 
     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. 
     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. Though the effect is most recognized in molds with eight or more cavities, it can create cavity to cavity variations in molds with as few as two cavities. 
     The flow imbalance in a mold with a geometrically balanced runner is created as a result of shear and thermal variations developed across the melt as it flows through the runner. The melt in the outer region (perimeter) of the runner&#39;s cross-section experiences different shear and temperature conditions than the melt in the center region. As flow is laminar during injection molding, the position of these variations across the melt stream is maintained along the length of the runner branch. When the runner branch is split, the center to perimeter variation becomes a side to side variation after the split. This side to side variation will result in variations in melt conditions from one side to the other of the part molded from the runner branch. 
     If the runner branches were to split even further, as in a mold with 4 or more cavities, there will exist a different melt in each of the runner branches. This will result in variations in the product created in each mold cavity. It is important to note that as consecutive turns and/or splits of the melt channel occur, the difference in melt temperature and shear history is further amplified. This cumulative effect is clearly recognized in large multi-cavity molds where the runner branches split and turn many times. 
     In an attempt to reduce this variation, the prior art has been primarily directed at various mixing devices that are located within the runner nozzle which is typically just prior the mold cavity. Examples of this can be found in U.S. Pat. No. 4,965,028 to Manus et al. and U.S. Pat. No. 5,405,258 to Babin. 
     Mixers at various locations within the injection molding machine are also well known. Examples of mixers in the hot runner manifold include U.S. Pat. No. 5,683,731 to Deardurff et al., European Patent 0293756, U.S. Pat. No. 5,688,462 to Salamon et al. and U.S. Pat. No. 4,848,920 to Heathe et al. (all incorporated herein by reference). An example of mixers installed within the injection unit can be found in U.S. Pat. No. 3,156,013 to Elphee (incorporated herein by reference). 
     Within the prior art, at least as much as known, there is no retrofit apparatus or method for installation of a mixer in an already existing injection molding machine, specifically in the hot runner manifold. Attempts at alleviating runner imbalance has been directed at correcting the problem within the injection nozzle or further upstream in the machine nozzle or sprue bar. 
     There exists a need for a mixer apparatus and method that allows for the easy and precise placement of a mixer in the melt stream in an injection molding machine, for example in a hot runner subsystem. Preferably, the mixer should be installed just upstream of where the melt channel splits or divides. 
     SUMMARY OF THE INVENTION 
     One general objective of the present invention is to provide a mixer apparatus and method that can be easily and precisely placed in an injection molding machine to help alleviate non-homogenity in a melt stream. 
     Another general object of the present invention is to provide a replaceable mixer insert apparatus and method in an injection molding machine. 
     Yet another general object of the present invention is to provide a mixer apparatus and method that is completely contained within the hot runner manifold. 
     The foregoing objects are achieved in one exemplicative embodiment by providing a mixer insert that is sealing placed in a receiving bore, for example, in a hot runner manifold. The mixer insert contains a mixing element that is held in alignment with and communicates with a melt channel. As the non-homogeneous melt flows through the mixing element it is mixed and homogenized thereby reducing melt stream imbalances. 
     Further objects and advantages of the present invention will appear hereinbelow. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 a - 1   c  are simplified cross-sectional views of an exemplicative embodiment of the present on; 
     FIG. 2 is an enlarged cross-sectional view of an exemplicative embodiment of the present invention; 
     FIG. 2 a  is an end view of the elongated torpedo; 
     FIG. 3 is a simplified cross-sectional view of a second exemplicative embodiment of the present invention; 
     FIG. 4 is a simplified cross-sectional view of a third exemplicative embodiment of the present invention; 
     FIG. 4 a  is a simplified cross-sectional view of a fourth exemplicative embodiment of the present invention; 
     FIG. 4 b  is a simplified cross-sectional view of a fifth exemplicative embodiment of the present invention; 
     FIG. 5 is a simplified cross-sectional view of a sixth exemplicative embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring first to FIGS. 1 a - 1   c , cross-sectional views of an exemplicative embodiment of the present invention are shown. A mixer assembly  10  is sealingly inserted into a manifold bore  26  which is formed in a hot runner manifold  12 . Mixer assembly  10  is comprised of a mixer insert  18 , which in a preferred embodiment is comprised of a metallic cylindrical bushing with optional flanges  19  protruding from a top surface of the insert  18 . An insert passageway  24  is formed in the mixer insert  18  perpendicular to its longitudinal axis for receipt of a mixing element  13 . The insert passageway  24  aligns with and communicates with a melt channel  16  when the mixer assembly  10  is fully seated in the manifold  12 . It should be noted that while the embodiments described herein are directed at cylindrically shaped mixer inserts  18 , one skilled in the art could easily provide myriad alternative embodiments comprising various shapes, attachment means and mixing elements therein. All such variations are fully contemplated by the present invention. 
     As shown in FIG. 1 b , the insert passageway  24  is a stepped bore, with one portion sized to receive and retain a mixing element  13 . For illustrations purposes only and not by limitation, the mixing element  13  in this embodiment is comprised of a torpedo  20  which is co-axially inserted in a mixer bushing  22 . The mixer bushing  22  is also retained in the insert passageway  24 . In a preferred embodiment, the torpedo  20  and the mixer bushing  22  are press fit in the insert passageway  24 . This helps to reduce leakage around the mixer, however, such a retaining means may not be necessary due to the manner in which the mixer bushing and torpedo are already retained inside the mixer insert  18 . Mixing element  13  could easily be modified by one skilled in the art to be any of the known static melt mixers. 
     An optional seal  40  may be provided around the periphery of the mixer insert  18  to reduce or eliminate the leakage of any molten material. An optional fastener  30  is provided to retain the insert  18  in the manifold  12 . In a preferred embodiment the fastener  30  is threaded into a threaded bore  28  located in the mixer insert  18  to rigidly affix the mixer assembly  10  in the manifold  12 . An optional alignment feature  42  is provided to maintain the alignment of the entrance  20   a  with the melt channel  16 . In a preferred embodiment, the alignment feature  42  is a pin press fit into the manifold  12  that interfaces with one of the flanges  19 . Alternatively, flat edges on the flanges  19  could be used for alignment through insertion of the flanges into a appropriately shaped pocket in the manifold  12 . 
     As shown in FIG. 1 a , the mixer assembly  10  is placed in various locations in the hot runner manifold  12 . The melt enters the manifold  12  at melt inlet  14  and splits into melt channels  16 . Melt channel  16  communicates with an entrance  20   a  of the mixer assembly  10  and the molten material is forced through the mixer bushing  22  where exit  20   b  further communicates with a second melt channel  32 . Second melt channel  32  further splits into a plurality of third melt channels  34 . Plugs  36  and  38  are affixed in manifold  12  to direct the molten material through the manifold  12 . Preferably, as shown in FIG  1   a , the mixer assembly  10  is installed just before the melt channel splits. This placement helps reduce the melt flow imbalances that adversely impact the quality of a molded part. 
     Referring now to FIGS. 2 and 2 a , which shows an enlarged cross-sectional view in accordance with one preferred embodiment in accordance with the present invention where like features have like numerals. The mixer bushing  22  has at least one helical groove  50  formed therein running from an inlet  60  to the outlet  62  for communication of the fluid through the mixer assembly  10 . An elongated torpedo  20  is inserted into the mixer bushing  22  and is maintained in a preferably coaxial position by at least one land  54  formed between the helical groove  50 . Adjacent the flow inlet  60 , the torpedo  20  is comprised of an annular disk  58  which abuts against one end of the mixer bushing  22 . A plurality of spokes  64  extend from the center of the torpedo  20  to annular disk  58 , thereby creating space for the flowing melt as it enters the mixer assembly  10 . As the helical groove  54  and lands  56  travel along the direction of the melt flow, a gap  51  which increases in the direction of the melt flow, is formed between the elongated torpedo  20  and the mixer bushing  22 . The cross-sectional area of the helical groove  50  also decreases in the direction of the melt flow. 
     As the melt travels through mixer bushing  22 , more and more of the melt gradually spills out of the helical groove  50  and over lands  54  such that the melt flow transitions from all helical to all annular flow. This mixing action has been shown to substantially eliminate flow imbalances that occur inside a melt stream. 
     Referring to FIG. 3, (where like features have like numerals) a second embodiment  100  of the mixer assembly in accordance with the present invention is generally shown. In this embodiment, the mixer insert  18  is attached to the side of a typical hot runner manifold  12  after a 90-degree turn of melt channel  16 . In a preferred embodiment, a plurality of fasteners  30   a  and  30   b  are inserted through a respective hole in flange  19  and affixed to manifold  12  for retention of the mixer insert  18 . 
     Referring to FIG. 4 (where like features have like numerals), a third preferred embodiment  200  in accordance with the present invention is generally shown. In this embodiment, and similar to second embodiment  100 , the mixer insert  18  is placed in the manifold bore  26  which is formed through a side of the manifold  12 . The mixer insert  18  has an additional melt passageway  25  formed therein at 90 degrees from the insert passageway  24  thereby forming a 90 degree corner in the mixer insert  18  downstream from the mixer bushing  22 . Optionally, a plurality of fasteners  30   a  and  30   b  are used to affix the mixer assembly  200  in the manifold  12 . 
     Referring now to FIG. 4 a  (where like features have like numerals), a fourth embodiment  200   a  in accordance with the present invention is generally shown. In this embodiment, the insert melt passageway  25  is in fluid communication with multiple second melt channels  32 . As such, the branching of the melt channel  16  occurs within the mixer insert  18  rather than in the manifold  12 . 
     Referring now to FIG. 4 b  (where like features have like numerals), a fifth embodiment  200   b  in accordance with the present invention is generally shown. In this embodiment a spring element  39  abuts the mixer insert  18  and is held thereon by a cap  41  which is affixed to the manifold  12 . In the preferred embodiment, the cap  41  has a flange  19  and an optional seal  40  to reduce leakage. The spring element  39  in the preferred embodiment is a belleville type disc spring, but could easily be made from any suitable resilient material. The use of the spring element  39  reduces the need for tight tolerance parts that would normally be required to provide a reliable seal against the high pressure melt. The spring element  39  allows for the cap  41  to sealing seat on a surface of the manifold  12  while also providing a compressive force between the mating surfaces, (for example surface  60   a ,  60   b  and  60   c ) to prevent or substantially reduce leakage of the high pressure melt therebetween. 
     Referring now to FIG. 5 (where like features have like numerals), a sixth preferred embodiment  300  in accordance with the present invention is generally shown. In this embodiment, the mixer insert  18  is inserted from a top surface of manifold  12  and provides a 90 degree turn just upstream of the mixer entrance  20   a  where melt passageway  25  interfaces with torpedo  20 . The annular disk  58  of the elongated torpedo is retained between the mixer insert  18  and the mixer bushing  22 . It should be noted that in this embodiment, the mixer bushing  22  is not retained in the mixer insert  18  but rather is seated in the manifold bore  26  and abuts against the annular disk  58  of the torpedo  20 . Again, an optional plurality of fasteners  30   a  and  30   b  are provided to retain the mixer insert  18  in the manifold  12  which in turn secures the torpedo  20  and mixer bushing  22  in alignment with the melt channel  16 . 
     It should be noted that while the foregoing description provided only a single description for a mixing element, one skilled in the art could easily envision alternative mixing element arrangements, and as such, all such mixing element embodiments are fully contemplated within the scope of the present invention. 
     As can be seen, a mixer assembly is provided in accordance with the present invention that may easily and reliable be inserted at various points along a melt channel. Various configurations have been shown that allow insertion of a mixer into a hot runner subsystem that may be replaced or allow for insertion of alternate mixer bushing types to accommodate various molding parameters. 
     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.