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

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a Continuation-in-Part of co-pending application Ser. No. 09/733,349 entitled “Flow Deflector Apparatus and Method”, filed Dec. 8, 2000 by the present inventor and is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     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. 
     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 multicavity 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. 
     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. 
     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. 
     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. 
     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. 
     Reference should also be made to the following reference: “Extrusion Dies for Plastics and Rubber” by W. Michaeli, Carl Hanser Verlag, Munich, ISBN 3-446-16190-2 (1992). 
     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 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 
     A flow deflector in a melt channel is provided, preferably around a valve stem or other flow obstruction, where the melt flow is converted from circular flow to annular flow. One preferred embodiment comprises a cylindrical body with a gradually constricting channel disposed on its outer surface. The channel is formed to be decreasing in depth and width, so as the melt flows into the channel, it gradually spills out of the channel. The gradual restriction of the channel helps direct the melt around the back of the cylindrical body which helps to eliminate stagnation points behind the flow obstruction while also providing uniform annular flow of the melt. 
     Further objections and advantages of the present invention will appear hereinbelow. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simplified view showing the basic principle of a coat-hanger manifold; 
     FIG. 2 is a partial sectional view of a preferred embodiment of the present invention in a co-injection hot runner nozzle; 
     FIG. 2A is a simplified isometric view of a preferred embodiment of the present invention; 
     FIG. 2B is a partial sectional view of another preferred embodiment of the present invention in a co-injection nozzle comprising two melt flow inlets; 
     FIG. 3 is a sectional view of another preferred embodiment of the present invention comprising a valve-gated nozzle in an injection molding system; 
     FIG. 4 is a sectional view of another preferred embodiment of the present invention comprising a valve-gated nozzle assembly; 
     FIG. 5 is a cross-sectional view of another preferred embodiment of the present invention comprising a nozzle tip assembly of a hot runner nozzle; 
     FIG. 6 is a partial cross-sectional view of another preferred embodiment of the present invention comprising a nozzle tip with two melt flow inlets; 
     FIGS. 7 a  and  7   b  are partial cross-sectional views of the flow deflector in accordance with the present invention; 
     FIG. 8 is a partial cross-sectional view of the flow deflector in accordance with a preferred embodiment of the present invention; 
     FIG. 9 is a partial sectional view of another preferred embodiment of the present invention comprising an injection nozzle assembly having a tapered surface; 
     FIG. 10 is a partial sectional view of another preferred embodiment of the present invention comprising a flow deflector formed in a bushing. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring first to FIG. 1, a simplified flat construction is shown which depicts the basic principles behind the present invention. Similar to coat hanger manifold principles well known in the extrusion arts, the melt flow will enter at a predetermined angle to a channel  19  at a flow inlet  18 . The melt will then split and flow equally down each side of the symmetrical channel  19  till it reaches an end  16  of the channel. The channel  19  is formed to have a decreasing cross section so as the melt travels down the channel  19 , more and more of the melt will spill over and out of the channel  19  over lip  35  into annular area  20  toward exit  17 . In this arrangement, the melt will reach exit  17  exhibiting substantially uniform flow V as shown by the arrows on the figure. In order to maintain a constant pressure drop as the melt travels through the channel  19 , the volumetric flow rate in the channel  19  from the inlet  18  to the end  16  must fall off to zero in a linear fashion. To maintain uniform volumetric flow, annular area  20  is defined by a uniform cross-sectional area along its longitudinal axis. 
     Now referring to FIGS. 2,  2 A and  2 B, a co-injection hot runner nozzle  11  according to a preferred embodiment of the present invention is generally shown. This preferred embodiment is comprised of the device as shown in FIG. 1, which has been wrapped around the circumference of a deflector body  26 . Deflector body  26  is concentric to and inserted into a nozzle body  24  and aligned with a second melt passage  30  such that the melt enters the flow inlet  18  substantially perpendicular to deflector body  26 . This alignment is fixed by a locating pin  34 . Locating pin  34  could be any suitable alignment means known in the art including (but not limited to) screws, rivets, spring pins, dowel pins, etc. Deflector body  26  further comprises a first melt passage  28  which is aligned with second melt passage  30  for communication of a first melt from an injection molding machine (not shown) or hot runner manifold (also not shown). 
     A valve stem  32  extends through a third melt passage  33  that is located inside of and runs the length of deflector body  26 . Third melt passage  33  is provided to communicate the flow of a second melt into the mold cavity. Valve stem  32 , as well known in the art, is selectively positioned through an up and down motion to start and stop the flow of the two melt streams through a nozzle outlet  36 , thereby controlling the filling of the mold cavity. In this arrangement, popularly known as co-injection, a mold cavity may be filled with two or more different melts for effects such as multiple colors, different melt materials and the like. 
     As the melt flows from second melt passage  30  to flow inlet  18 , it strikes the outside wall of the deflector body  26  substantially perpendicular to valve stem  32  longitudinal axis (However, non-perpendicular flow impingement could easily be accomplished). If channel  19  was not present, the melt would tend to flow down along the face of deflector body  26  closest to flow inlet  18 , thereby causing stagnation points behind deflector body  26 . However, in this preferred embodiment, the melt flows into channel  19  and is directed to flow around the deflector body  26 , thereby eliminating the formation of stagnation points. As the melt flows through channel  19 , the depth and width of the channel decreases so as to force more and more of the melt out of the channel  19  over lip  35 . This gradually transitions the flow to annular flow through annular area  20  which has a uniform cross-section so that by the time the melt reaches the exit  17 , a uniform velocity profile has been established which results in the formation of a high quality molded part. 
     In FIG. 2B, a dual inlet co-injection nozzle similar to that shown in FIG. 2 is shown. The significant difference between these two preferred embodiments is the use of an additional first melt passage  28   a  that is diametrically opposed to the other first melt passageway. It should be noted that the melt channels are not required to be diametrically opposed. In this embodiment, identical channels  19  and  19   a  are provided. In this arrangement, elimination of stagnation points and the creation of a uniform annular velocity is also achieved. 
     Referring to FIG. 3 (where like features have like numerals), another preferred embodiment in accordance with the present invention is generally shown. A hot runner valve gate system  100  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 . 
     As the melt flows from melt channel  142  to flow inlet  18 , it strikes the outside wall of the deflector housing  130  substantially perpendicular to valve stem  126  longitudinal axis (However, non-perpendicular flow impingement could easily be accomplished). If channel  19  was not present, the melt would tend to flow down along the face of deflector housing  130  closest to flow inlet  18 , thereby causing stagnation points behind deflector housing  130 . However, in this preferred embodiment, the melt flows into channel  19  and is directed to flow around the deflector housing  130 , thereby eliminating the formation of stagnation points. As the melt flows through channel  19 , the depth and width of the channel decreases so as to force more and more of the melt out of the channel  19  over lip  35 . This gradually transitions the flow to annular flow so that by the time the melt reaches the exit  17 , a uniform velocity profile has been established which results in the formation of a high quality molded part. 
     As shown in FIG. 3, 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. 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 . 
     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. 
     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. 3, the nozzle heater  116  surrounds a portion of the nozzle body  112 . 
     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 in the deflector 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 . 
     The piston head  131  is housed within a cylinder housing which comprises the upper distal end of deflector 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 . 
     The hot runner system of this preferred embodiment also includes a manifold/deflector arrangement  110  consisting of the manifold  138  and the deflector housing  130  inserted into bore  143  therein. A locating pin  129  fixes the alignment of the deflector 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. 
     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 deflector 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 . 
     The deflector 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. 
     The deflector 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. 3, the deflector housing channel  19  mates with the melt channel  142  in the manifold  138  and the axial channel  120  in the nozzle assembly  108 . 
     As the melt flows from melt channel  142  to flow inlet  18 , it strikes the outside wall of the deflector housing  130  substantially perpendicular to valve stem  126  longitudinal axis (However, non-perpendicular flow impingement could easily be accomplished). If channel  19  was not present, the melt would tend to flow down along the face of deflector housing  130  closest to flow inlet  18 , thereby causing stagnation points behind deflector housing  130 . However, in this preferred embodiment, the melt flows into channel  19  and is directed to flow around the deflector housing  130 , thereby eliminating the formation of stagnation points. As the melt flows through channel  19 , the depth and width of the channel decreases so as to force more and more of the melt out of the channel  19 . This gradually transitions the flow to annular flow so that by the time the melt reaches the exit  17 , a uniform velocity profile has been established which results in the formation of a high quality molded part. 
     It should be noted that even though the preceding embodiments describe a deflector body  26  (FIG. 2) that is separate from the nozzle body  24  (FIG.  2 ), a single bushing could easily be fabricated that incorporates all the required features. 
     Referring now to FIG. 4 (where like features have like numerals), another preferred embodiment in accordance with the present invention is generally shown. In this embodiment, the deflector body  26  is a singular bushing that is inserted in the nozzle body  24  for a single-melt nozzle. 
     Here again, the valve stem  32  is inserted through the deflector body  26 , thereby supporting and guiding the valve stem  32  while also directing the melt around the back of the valve stem. Similar to the previous embodiments, melt flows from melt channel  142  through the first melt passage  28  which is located in the upper flange of the deflector body  26 . Alignment between melt channel  142  and first melt passage  28  is maintained by locating pin  34 . The melt then flows through second melt passage  30  which is located inside nozzle body  24 . 
     The melt is then directed against deflector body  26  at inlet  18  where the flow is diverted around to the back of the valve stem  32  by channel  19 . The melt flow is diverted through the channel  19  and gradually spills out of channel  19  over lip  35  into annular area  20  such that when it reaches exit  17  of the deflector body  26 , it has been transformed from circular flow to uniform annular flow which exits nozzle outlet  36  to form a high quality, molded part. 
     Referring now to FIGS. 5 and 6, (where like features have like numerals) another preferred embodiment of the present invention is shown comprising an injection molding nozzle tip assembly  200 . In this embodiment, the principles of coat hanger manifolds previously discussed have been applied to the tip of an injection nozzle assembly. Commonly referred to as a “hot tip” or “pin point”, this preferred embodiment comprises a nozzle without the valve stem as shown in the previous embodiments. 
     An elongated first melt passage  28  is located in a sleeve  40  for the communication of a melt to a tip  44 . The sleeve  40  is rigidly affixed inside the nozzle body  24  and traps the tip  44  co-axially in the nozzle body  24 . In the preferred embodiment, the sleeve is threaded into the nozzle body  24  and abuts against a top flange of tip  44 . A heater  116  is wrapped around the outside of nozzle body  24  for maintaining the temperature of the melt as it flows through the nozzle assembly. 
     Melt flows through first melt passage  28  and is further communicated to flow inlet  18  through a tip passage  46 . The flow is thus communicated to channel  19 . In this arrangement, the melt flow exits nozzle outlet  36  as a uniform annular flow. Elimination of stagnation points behind the tip  44  is accomplished by forcing the melt to flow around to the back of the tip  44 . 
     Referring to FIG. 6, a nozzle assembly similar to FIG. 5 is shown, except for the addition of a second tip passage  46  which communicates the melt flow to two sides of the tip  44 . In addition, a second symmetrical channel  19  is provided. Here again, the melt flows into the channel  19  and gradually spills over lip  35  into all annular flow by the time it reaches exit  17 . 
     Referring to FIGS. 7 a  and  7   b,  another preferred embodiment in accordance with the present invention is shown. In these embodiments, the channel  19  is not formed from a groove having a curved profile but instead is a square groove profile. In FIG. 7 a,  the channel  19  slopes downward at a fixed angle whereas in FIG. 7 b,  the channel  19  has a radius which defines the path of the channel  19  along the deflector body  26 . 
     Referring to FIG. 8, another preferred embodiment in accordance with the present invention is shown. In this embodiment, an opposing channel  19 ′ is formed in the manifold  138  for further directing the melt flow around the deflector body  26 . Channel  19  and  19 ′ in combination form a deeper channel to direct the melt around the back of the deflector body  26 . Here too, the melt gradually spills out of the channels  19  and  19 ′ to convert the flow to uniform annular flow while eliminating stagnation points. Due to melt flow principles, this embodiment will exhibit the least amount of variations in melt properties and will produce molded parts with the least amount of variation. 
     Referring now to FIG. 9 (where like features have like numerals), another preferred embodiment of the present invention is shown which comprises a deflector body  26  which has a tapered flow surface. Coat hanger manifold principles have shown that a tapered flow surface, especially in the area of the channel  19 , helps to substantially reduce the pressure drop that occurs in the melt as it is diverted around a flow obstruction. The tapered deflector body  26  is inserted into a tapered receiving hole in the manifold  138 , and alignment is maintained by the abutting tapered surfaces. A locating pin, similar to that shown in previous embodiments may also be used to further maintain the deflector body  26  alignment with the melt channel  142 . 
     This tapered channel arrangement could also be utilized in the all aforementioned embodiments. These embodiments could all incorporate the use of the tapered flow surface to reduce the melt pressure drop as it flows around obstacles. 
     While the previous embodiments all show the use of the channel  19  on a deflector body  26  that is wrapped around a valve stem  32 , the channel  19  could easily be placed directly on the outside surface of the valve stem  32 . A disadvantage to this approach however is the reduction in the valve stem support provided by the deflector housing that may lead to accelerated wear of the valve stem. In addition to this drawback, it would also be necessary to incorporate an alignment feature to maintain alignment of the valve stem with the manifold channel. 
     Referring now to FIG. 10, another preferred embodiment in accordance with the present invention is shown wherein the channel  19  is formed integral to a bushing  131 . In this embodiment, the channel  19  directs the melt to flow around the valve stem  126  rather than a deflector body. This embodiment reduces the additional valve stem support as shown in previous embodiments. 
     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.