Abstract:
A runner system and runner component particularly adapted for use in multiple cavity injection molding systems is disclosed. The runner system and runner component utilize particular structural features and geometrical configurations that have been found to result in superior molding practices and molded components in multiple cavity injection molding systems. A corresponding multiple cavity injection molding system utilizing the noted runner system is described. And, a related method for promoting uniform filling of multiple mold cavities is also disclosed.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an apparatus and method for forming molded parts from a polymeric material by injecting the material into a multiple cavity mold. More specifically, the present invention relates to an apparatus and method for promoting uniform flow of polymeric material and uniform filling of all mold cavities in a multiple cavity injection molding apparatus. Another significant aspect of the invention is that it eliminates a requirement of supplemental or external heating of runner systems used in conventional multiple cavity injection molding operations. Moreover, the invention eliminates conventional trial and error methods of adjusting flow parameters in order to achieve uniform filling of multiple molding cavities. 
     BACKGROUND OF THE INVENTION 
     Multiple cavity injection molding offers several advantages over single mold cavity injection molding. A significant advantage is the increased number of molded articles produced for every injection molding cycle. Consequently, an increase in the number of parts produced has the potential to increase overall productivity and reduce production costs. 
     However, potential problems also arise with the use of multiple cavity injection molding. These problems stem from the flow of a polymeric material through the injection molding system. Ideally, the flow and state of the polymeric material is uniform as it travels from a feed source or through a runner system, to each respective mold cavity. However, flow paths or runner systems become increasingly complex as the number of mold cavities increases. This complexity is detrimental for the following reasons. Complex runner systems associated with multiple cavity injection molding may contribute to pressure changes which can alter the flow rate. Temperature changes or variances thereof along such relatively long runner systems may increase the viscosity leading to a reduced flow rate. Premature curing of thermoset polymeric material or solidification of thermoplastic polymeric material can also potentially reduce flow rates and cause eventual blockage of flow. 
     Such flow problems directly affect the production of molded parts. Cavities that are not sufficiently filled may produce unacceptable parts. As a result, productivity decreases and operating costs may increase in order to produce a desired number of acceptable products. 
     Nonuniform curing of the molded articles affects the physical characteristics of the articles. Different cure states of a polymer give rise to different physical properties. If the polymer is not in a uniform state as the mold cavities are filled or, if the polymer cures at different rates at various locations within the mold cavities or the runners, there exists a potential that the physical characteristics of the molded articles will vary from part to part. Some characteristics may be undesirable leading to defective molded parts which can not adequately function in a manner for which they were designed. 
     Conventional multiple cavity injection molding systems generally produce a great deal of scrap. Rejected parts, underfill material, overfill material, flash material and polymer material frozen in runners or gates are not commercializable and thus become scrap. A large amount of scrap typically increases production costs, as more material is required to produce the desired articles. Productivity may also decrease, as longer periods of time are needed to clean and prepare the system between injection cycles. 
     Various approaches have been employed in an attempt to avoid the problems associated with multiple cavity injection molding. A common approach to ensuring that the polymer is in a proper melt state is the use of thermally insulated or hot runner systems, along with trial and error adjustment of molding conditions such as temperature, pressure, and injection speed. 
     A means for controlling melt flow and reducing scrap is described in U.S. Pat. No. 5,945,139. An apparatus and method is disclosed in that patent for introducing an uncured thermoset material through a series of runners and into a plurality of cavities. Material in the cavities is allowed to cure and material remaining in the runners is kept live in order to be used in a subsequent injection cycle. 
     Techniques for ensuring sufficient filling of mold cavities have also been disclosed. These methods may involve devices, which monitor the amount of material supplied to a cavity, such as that described in U.S. Pat. No. 5,149,547. 
     U.S. Pat. No. 4,120,921 describes a method of sequentially filling mold cavities by supplying ultrasonic energy to a gating device. Material is injected into a cavity to which the ultrasonic energy has been supplied until the cavity is filled. After the material is frozen, ultrasonic energy is supplied sequentially to the gates of the remaining cavities. The material in each cavity is allowed to freeze before energy is supplied to the next gate. 
     Flow control is addressed in U.S. Pat. No. 5,849,236 which discloses an apparatus employing control members that may be inserted into a conduit to varying degrees to provide balanced flow of thermoplastic material into each of a plurality of cavities. 
     Although satisfactory in some respects, many of the prior art techniques and equipment used for multiple cavity injection molding fail in varying degrees, to adequately address the problems associated with multiple cavity injection molding. Also, prior art techniques cover thermoset polymeric materials, which employed compression and/or transfer molding methods. In transfer and compression molding procedures, longer periods of time, eg. up to 50 minutes, are afforded for the melt flow to travel to the cavity and for the curing time inside the cavity, whereas thermoplastic polymeric materials can only be afforded relatively short travel and fill times, eg. 5 to 30 seconds. Therefore, external heating devices and ultransonic methods were found to be costly and ineffective. For instance, external heating of runners or control devices, as discussed in the prior art, involve additional costs in both the design and operation of the molding apparatus. 
     Accordingly, there still exists a need for an improved approach suitable for remedying at least some of the difficulties associated with multiple cavity injection. 
     SUMMARY OF THE INVENTION 
     In one aspect, the present invention provides a multiple cavity injection molding system that remarkably and consistently produces molded parts from injecting molten polymeric material simultaneously into a collection of mold cavities. The system comprises a feed source of molten polymeric material, a collection of mold cavities, a primary sprue for transferring polymeric material from the feed source, and a runner system for distributing the polymeric material from the primary sprue to the collection of mold cavities. The runner system utilizes a unique configuration and at least two turbulence inducing components that maintain a particular shear rate within the polymer as it exits the runner system and enters the mold cavities. 
     In another aspect, the present invention provides a runner system adapted for multiple cavity injection molding in which the runner system comprises a primary runner, first and second secondary runners, and a plurality of tertiary turbulence inducing runners, arranged in a particular configuration. Upon introducing a flowable material into a feed opening of the primary runner at a pressure sufficient to cause the material to flow through the runner system and exit through exit ports defined in the tertiary turbulence inducing runners, the material while in the tertiary turbulence inducing runners, is in a pseudo-turbulent state. 
     In a further aspect, the present invention provides a turbulence inducing runner adapted for incorporation in a runner system for a multiple cavity injection molding system. The turbulence inducing runner comprises a primary runner, first and second secondary runners in communication with the primary runner, and a plurality of turbulence inducing steps in communication with the first and second secondary runners. 
     In yet another aspect, the present invention provides a method for promoting uniform filling of multiple mold cavities. The method comprises a step of providing a collection of runner components, and also selecting at least two turbulence inducing components that are combined with the runners. The method further comprises the step of configuring together the runners and turbulence inducing components in such a manner as to form a runner system having a particular geometry that has been found to maintain a particular shear rate within the polymeric material when flowing through the runner system. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a process schematic of a conventional injection molding unit; 
     FIG. 2 is an illustration of a first preferred embodiment turbulence inducing runner in accordance with the present invention; 
     FIG. 3 is a top view of a runner system employing a plurality of turbulence inducing runners of the first preferred embodiment; 
     FIG. 4 is a side view of the runner system employing a turbulence inducing runner of the first preferred embodiment taken along plane  4 — 4  in FIG. 3; 
     FIG. 5 is an end view of the runner system employing a turbulence inducing runner of the first preferred embodiment taken along plane  5 — 5  in FIG. 3; 
     FIG. 6 is a top view of the runner system employing a turbulence inducing runner of the first preferred embodiment depicting polymer flow through a portion of the system; 
     FIG. 7 is an illustration of a second preferred embodiment of a turbulence inducing runner according to the present invention; 
     FIG. 8 is a top view of a runner system employing a plurality of turbulence inducing runners of the second preferred embodiment; 
     FIG. 9 is a side view of the runner system employing a turbulence inducing runner of the second preferred embodiment taken along plane  9 — 9  in FIG. 8; 
     FIG. 10 is an end view of the runner system employing a turbulence inducing runner of the second preferred embodiment taken along plane  10 — 10  in FIG. 8; 
     FIG. 11 is a top view of the runner system of FIG. 8 employing a turbulence inducing runner of the second preferred embodiment depicting polymer flow through a portion of the system; and 
     FIG. 12 is a perspective view of the runner system illustrated in FIG.  8 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will now be described more fully hereinafter. The invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. The embodiments described herein are provided so that this disclosure will be thorough and complete and will fully convey the invention to those skilled in the art. 
     The term “runner” as used herein generally refers to nearly any pipe, conduit, or channel typically used for transferring flowable materials such as by pumping or gravity flow therethrough. All runners as described herein have a hollow or open interior through which the flowable material may travel. 
     FIG. 1 is a schematic of a conventional injection molding system  5  employing two mold cavities. The injection molding system  5  generally comprises an injection unit  10 , a main sprue  12  for transfer of flowable molding material to a runner  14 , which in turn directs the material to one or more gates  16  and mold cavities  18 . Injection unit  10  comprises a source of polymeric material and a transfer or pumping device such as a reciprocating screw. Melted polymer is heated and fed under pressure from injection unit  10  and into the main sprue  12 . When the screw turns, the polymeric material is pumped from unit  10  toward sprue  12 . Polymer flows through sprue  12  until reaching an opening at the end of sprue  12 , which branches into runner  14 . Polymer material travels in both directions of runner  14  and through gates  16  at each end of runner  14 . The gates are in communication with mold cavities  18 . Polymer material flows through the gates and into each respective mold cavity. 
     A wide array of pressures may be used in an injection molding system. Typically in the case of thermoplastic polymeric molding systems, system pressures are within the range of from about 5000 psi to about 30,000 psi. Polymer material leaves the injection unit  10  typically heated to a temperature of about 400 to 450° F. in the case of ABS polymer. Polymeric material may be heated when fed into the injection unit. Typically, the temperature of the polymeric material increases further as the reciprocating screw turns and forces material into the sprue  12  of the molding system  5 . Screw action and injection speed generate shear and subsequently a rise in the temperature of the polymer. The viscosity, or resistance to flow, of the polymeric melt decreases as the temperature rises. The lower the viscosity value of a material, the more readily the material flows. An excessive amount of heat produced from the shear may lead to degradation or breakdown of the polymeric material. 
     Polymer flow may be analyzed by considering the relationship between the viscosity of the polymer and shear rate imparted to the polymer. Specifically, for most polymeric systems, viscosity is a function of shear rate. Often, such relationship is measured by extruding polymer through a fixed orifice at different shear rates. Table 1 shows the effects of shear rate on viscosity for an acrylonitrile-butadiene-styrene terpolymer (ABS) and a polyphenylene oxide and polystyrene blend glass filled polymer. 
     
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Effect of Shear Rate on Dynamic Viscosity 
               
             
          
           
               
                   
                   
                 Dynamic Viscosity 
               
               
                   
                   
                 (poise) 
               
               
                   
                 Dynamic Viscosity 
                 Polyphenylene 
               
               
                 Shear Rate 
                 (poise) 
                 oxide/polystyrene blend 
               
               
                 (1/second) 
                 ABS polymer 
                 (20% glass filled) 
               
               
                   
               
             
          
           
               
                 1 
                   1 × 10 5   
                 1 × 10 6   
               
               
                 10 
                   9 × 10 4   
                 4 × 10 5   
               
               
                 100 
                 2.5 × 10 4   
                 0.5 × 10 5     
               
               
                 1000 
                 0.5 × 10 4   
                 3 × 10 4   
               
               
                   
               
             
          
         
       
     
     Table 1 shows that increasing the shear rate generally reduces the viscosity value of a polymeric material, thereby yielding a more flowable material. 
     Referring to FIG. 1 again, the problems associated with conventional injection molding systems, and particularly those using multiple mold cavities, will now be described. 
     After traveling through the runner system, e.g., sprue  12  and runner  14 , the polymer contacts “cold” mold cavities  18  having a temperature of about 100° F. The large decrease in temperature of about 350° F. causes a severe reduction in the flowability of the polymeric material. Accordingly, the viscosity value of the polymer increases dramatically. An increase in the viscosity value indicates the material having reduced flowability. The dramatic increase in viscosity value of the polymeric material therefore produces a front-end flow that is essentially frozen. As a result, the flow speed of polymeric material throughout the runner system is greatly reduced. The polymer does not flow through the runners at the same rate and the front end of the flow does not reach the cavities at the same rate. Another consequence of reduced viscosity or frozen front end flow is block flow in which the polymeric material is otherwise restricted from flowing to a desired cavity. Both occurrences greatly increase the likelihood that mold cavities will be insufficiently filled. Insufficient filling of mold cavities is not desirable because unacceptable or defective molded parts are produced by such cavities. 
     Additionally, pressure changes throughout, or variances across, a runner system contribute to reduced or non-uniform flow. Comparing the pressure at a first location in a runner system to the pressure at a second location, downstream from the first location during a molding operation, there will be, by definition, a pressure drop across the runner from the first to the second location. Such pressure differential, as will be appreciated, causes flow of material between these locations. In a typical runner system utilized in a conventional injection molding system, the pressure drop is usually about 1200 psi across primary runners, about 850 psi across secondary runners, and about 400 psi across tertiary runners. Examples of primary, secondary, and tertiary runners are provided herein in conjunction with the description of the preferred embodiment turbulence inducing runners and systems utilizing such. As will be further understood by those skilled in the art, the amount of a pressure drop may vary according to runner size and length and other variables. 
     Complex runner systems, such as those used for multiple cavity injection molding systems, typically exhibit an array of pressure variances within different regions of the system. These pressure variances lead to differing pressure drops across similar regions of the system, which in turn lead to differences in flow rate and flow rate fluctuations. A reduction in pressure drop causes the polymer melt travel speed to decrease, thereby reducing the melt front temperature and increasing non-mixing or “pseudo-laminar” flow behavior in the polymeric material to the point that flow is almost stagnant. These aspects are discussed in greater detail herein. Consequently, the speed at which the polymer melt front reaches the gates and cavities varies, which may result in a failure to sufficiently fill all the mold cavities. 
     In terms of shear rate, in a conventional injection system, a polymeric material may experience a shear rate of about 8000 (1/sec) in a feed line or sprue, but after entering a first branched line, may experience a significant reduction in shear rate, for instance to about 4000 (1/sec). It will be recalled that for most polymeric materials, as the shear rate decreases, the viscosity increases. As the flowing polymeric material travels through a series of branches or split flows, the shear rate continues to drop. Accordingly, the viscosity further increases. This phenomenon is further compounded by the previously noted reductions in temperature and pressure drop across the runner system. 
     The present invention is based on the discovery that by promoting turbulence in the polymer material in a specific manner as the material is administered to a plurality of mold cavities, the noted flow problems may be avoided or at least significantly reduced. More specifically, by particularly agitating or forcing polymeric material through runners, sprues, nozzles etc., continuous action or turbulent mixing of the material occurs at least at the polymer melt front if not throughout the entire flow path. 
     Polymer melt viscosities, as measured by pressure, were compared in an injection molding system that did not employ the present invention and also in an injection molding system that did utilize the present invention. A monitoring device, manufactured by RJG Technology, Inc., was used to measure the pressure changes inside an injection unit. Pressure transducers were installed at the gate entrance (post gate pressure) and the end of the cavity (end of fill pressure). The injection molding system pressure was measure by monitoring the hydraulic pressure, which represents the total injection pressure exerted on the molding operation. 
     Table 2 compares the system pressures of an injection molding unit that does not utilize a turbulence inducing system of the present invention, to an injection unit that does employ a turbulence inducing system of the present invention. The data was obtained from an ABS injection molding system. The values would be expected to change for other polymers and molding parameters. 
     
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Effect of Turbulence Inducer on Viscosity 
               
             
          
           
               
                   
                   
                 Injection 
                   
                 Injection 
                   
               
               
                   
                   
                 Unit Without 
                   
                 Unit With 
                   
               
               
                   
                   
                 Turbulence 
                   
                 Turbulence 
                   
               
               
                   
                 System Parameter 
                 Inducer 
                   
                 Inducer 
                   
               
               
                   
                   
               
             
          
           
               
                   
                 Hydraulic Pressure 
                 13,340 
                 psi 
                 13,430 
                 psi 
               
               
                   
                 Post Gate Pressure 
                 4112 
                 psi 
                 4110 
                 psi 
               
               
                   
                 End of Fill Pressure 
                 *3500 
                 psi 
                 3470 
                 psi 
               
               
                   
                   
               
               
                   
                 *Due to underfill, end of fill pressure was not always detected.  
               
             
          
         
       
     
     Table 2 demonstrates that an injection molding system employing a turbulence inducing system exhibits pressures comparable to an injection molding system without a turbulence inducing system of the present invention. Because melt flow viscosity may be viewed as a measure of pressure, Table 2 demonstrates that the present invention turbulence inducer does not significantly alter the polymer melt viscosity, but rather allows the melt viscosity of the polymer to be maintained all the way along the flow path. 
     The “pseudo-turbulent” flow created by the present invention serves to “refresh” the polymer melt by shear heating. Thus the present invention device and/or system changes the flow characteristics of the polymer material such that it appears the viscosity has been reduced, i.e. the polymer is more flowable. 
     Table 3, set forth below, lists typical melt temperatures and mold temperatures for other polymeric systems. 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Examples of Melt Temperatures of some Polymers for 
               
               
                 Injection Molding 
               
             
          
           
               
                   
                 Melt 
                 Mold 
               
               
                   
                 Temperatures, ° F. 
                 Temperature, ° F. 
               
               
                   
               
               
                 Acrylonitrile-Butadiene-Styrene 
                 380-500 
                 120-160 
               
               
                 (ABS) 
               
               
                 Terpolymer 
               
               
                 Polycarbonate (PC) 
                 510-600 
                 180-240 
               
               
                 ABS/PC Blend 
                 410-525 
                 120-180 
               
               
                 Poly(butyleneterephthalate) 
                 440-500 
                 160-190 
               
               
                 (PBT) 
               
               
                 Polyamide (Nylon) 
                 480-570 
                 100-200 
               
               
                 Poly(phenylene oxide)/ 
                 450-520 
                 150-180 
               
               
                 polystyrene Blend 
               
               
                   
               
             
          
         
       
     
     Molten polymer has a high viscosity value compared with most other fluids. Turbulence creates a shear force, or rather increases shear force, which for most polymeric materials, results in increased flowability of the material. The increased shear force may also generate an increase in temperature in the polymeric material. The temperature rise leads to a further decrease in the viscosity value, producing more flowable and fresher polymeric material. Thus, the present invention is directed toward providing a “pseudo-turbulent” melt front to reactivate or rejuvenate the front end of the flow, thereby serving to equalize and promote polymeric flow to each mold cavity. As previously noted, in the absence of such mixing or agitation, the material exhibits a non-mixing or “pseudo-laminar” flow. Although conventional flow terminology such as “turbulence” and “laminar” is not particularly applicable in describing the flow of most polymeric materials, these terms are used herein to denote the respective states of flow. Restated, although a polymeric material in a pseudo-turbulent state may not necessarily exhibit a Reynolds Number of greater than about 2000, eg. the Reynolds Number being a traditional indicator of fluid states being laminar or turbulent, the material will exhibit properties and characteristics similar to a turbulent state. For instance, polymeric flow in a pseudo-turbulent state according to the present invention exhibits relatively high degrees of mixing and agitation as compared to the flow in a pseudo-laminar state. It is believed that the lines of flow of polymeric material in a pseudo-turbulent state resemble the flow lines of a conventional liquid in a state of turbulence and thereby having a Reynolds Number of greater than about 2000. 
     The present invention provides a turbulence inducing runner that may be directly incorporated in a runner system in a multiple cavity injection molding system. The runner is designed to mix and tumble polymeric material flowing therethrough, thereby generating a “pseudo-turbulent” melt front. The turbulence inducing runner and its particular geometry and configuration described herein, and the incorporation of turbulence inducing steps as described herein, are all aspects of the present invention which enable pseudo-turbulence to be maintained throughout an injection system. At a minimum, these aspects result in a turbulent state of the liquid or flowable material as it exits the runner system and is introduced into a mold cavity. 
     The preferred embodiment turbulence inducing runners as described herein cause or impart pseudo-turbulent flow to the polymeric material flowing through the runners. Such mixing creates a shear force within the material that contributes to an apparent reduction in the viscosity of the polymeric material thereby creating a refreshing melt front all the way to the mold cavities, as if the viscosity of the material were reduced. A refreshed melt front promotes an equalized flow of polymer throughout a runner system. 
     FIG. 2 depicts, in schematic form, a preferred embodiment turbulence inducing runner  22  according to the present invention. It will be understood that FIG.  2  and all other figures referenced herein are in schematic form and not necessarily to scale. Turbulence inducing runner  22  provides turbulence inducing steps  24  and  26 , which serve to agitate and mix the polymeric material flowing therethrough. Polymeric material enters the runner  22  through a port or entrance between the opposing pairs of steps  24 . Polymer material that flows through turbulence inducing runner  22  is agitated and mixed by turbulence inducing steps  24  and  26  and continues to flow to openings  28 . Each opening  28  preferably leads to a gate (not shown) which communicates with a mold cavity (not shown). 
     The first preferred embodiment runner  22  employs two turbulence inducing steps  24  and  26 . There is no specific limit regarding the number of steps employed by the preferred embodiment turbulence inducing runners. The primary limiting factor with regard to the number of steps employed in a turbulence inducing runner is the overall turbulence produced by a runner. Preferably, a turbulence inducing runner in accordance with the present invention should impart a shear force and induce turbulence in the polymer, i.e. mix and agitate the polymer, such that an increase in the viscosity of the polymeric material does not occur, as compared to the original melt viscosity, as the polymeric material enters a runner from a nozzle. Therefore, the polymeric material more readily flows to a mold cavity downstream, but not to such a degree that the shear force and turbulence lead to excessive energy losses or generate an excessive temperature rise that leads to a breakdown of the polymeric material. 
     A turbulence inducing runner according to the present invention preferably comprises one or more turbulence inducing angles. These are illustrated in FIG. 2 as angles x and y. As will be appreciated, turbulence inducing angles may result from turbulence inducing steps, i.e. turbulence inducing angle x results from turbulence inducing step  26 . The turbulence inducing angles agitate and mix the polymeric material, which as previously explained, promote flowability of the polymeric material and generally offset the noted detrimental viscosity altering effects, thereby allowing the viscosity of the polymer to be maintained. No limitation is placed on the shape or curvature of the turbulence inducing angles. Turbulence inducing angles are preferably rounded or linear, eg. sharp at the region at which the direction of flow changes. Additionally, turbulence inducing angles may be stepped i.e., in the form of a series of continuous curves of varying angles. No limit is placed on the magnitude of a given angle. Preferably, turbulence inducing angles are less than or equal to 90° and more preferably, turbulence inducing angles are within the range of from about 30° to about 60°. No limit is placed on the number of turbulence inducing angles employed in a turbulence inducing runner. It is preferred, however, that the turbulence inducing runner be symmetrical, which is described in greater detail herein. The preferred symmetrical configuration of the runner dictates that the total number of turbulence inducing angles in the runner be an even number. It is also preferred that the sum of angles of all turbulence inducing angles in a turbulence inducing runner is 90° or a multiple thereof, such as 180°, 270°, or 360°. 
     FIGS. 3-6 illustrate aspects relating to a first preferred embodiment injection molding runner system utilizing a plurality of the first preferred embodiment turbulence inducing runners of the present invention. In FIG. 3 the preferred embodiment runner system  30  is viewed from the top i.e., the viewer is looking down on the runner system. Flowable polymer material such as from a feed source (not shown), is forced under pressure into sprue  32  which defines a sprue channel  34 , and into primary runner  36 . Primary runner  36  branches off of sprue  32 . Polymer material travels through sprue  32  and into branches  36   a  and  36   b  of runner  36 . Runners  36   a  and  36   b  are equivalent to one another and herein are referred to collectively as primary runner  36 . 
     As shown in FIG. 3, disposed at each end of runner  36  is one or more secondary runners  40 . Preferably, each secondary runner  40  branches off of primary runner  36 . Runners  40   a  and  40   b  branch off of runner  36   a , and runners  40   c  and  40   d  branch off of runner  36   b . Runners  40   a-d  are equivalent to one another and herein are referred to collectively as secondary runner  40 . The runner system  30  depicted in FIG. 3 utilizes one primary runner  36  and two secondary runners  40  disposed at opposite ends of the primary runner  36 . The polymer flows through primary runner  36  until reaching openings leading to secondary runner  40  wherein the polymer can flow into runner  40 . At the same time, some of the polymer may flow through the openings and into an extension of runner  36  at which it will encounter a curved end region  38 . Curved end regions  38   a  and  38   b  are located at the end of runners  36   a  and  36   b , respectively. Curved end regions  38   a  and  38   b  are equivalent to one another and herein are referred to collectively as curved end region  38 . Curved end region  38  redirects the polymer into runner  40 . Although the runner system  30  shown in FIG. 3 utilizes curved end regions, it is not necessary that such systems employ these curved end regions. Clearly, runner systems in accordance with the present invention may avoid the use of curved end regions. 
     Disposed at the open ends of runner  40  are a corresponding number of preferred embodiment turbulence inducing runners in accordance with the present invention. As shown in FIG. 3, four of the preferred embodiment turbulence inducing runners, such as runner  22  shown in FIG. 2, are utilized in the runner system  30 . These are designated in FIG. 3 as runner  44 . The preferred embodiment turbulence inducing runners are incorporated into the runner system  30  such that the turbulence inducing runners constitute tertiary runners. That is, they are located downstream from primary and secondary runners. Hence, the preferred embodiment turbulence inducing runners are referred to as tertiary runners  44  upon incorporation in the runner system  30  depicted in FIG.  3 . 
     Specifically, the runner system according to FIG. 3 shows tertiary turbulence inducing runners  44   a  and  44   b  branching off of secondary runner  40   a . Tertiary runners  44   c  and  44   d  branch off of secondary runner  40   b . Tertiary runners  44   e  and  44   f  branch off of secondary runner  40   c . And, tertiary runners  44   g  and  44   h  branch off of secondary runner  40   d . Tertiary turbulence inducing runners  44   a-h  are equivalent to one another and herein are referred to collectively as tertiary runner  44 . The runner system  30  illustrated in FIG. 3 utilizes four tertiary runners  44 . Tertiary turbulence inducing runner  44  provides a series of turbulence inducing steps and angles that serve to agitate, mix, and induce turbulence in the polymeric material. Additionally, a curved end region is encountered at the end of each member of secondary runner  40 . Thus curved end region  42   a  forms the end of runner  40   a , curved end region  42   b  forms the end of runner  40   b , curved end region  42   c  forms the end of runner  40   c , and curved end region  42   d  forms the end of runner  40   d . Curved end regions  42   a-d  are equivalent to one another and herein are referred to collectively as curved end regions  42 . As polymer flows through secondary runner  40 , curved end region  42  redirects the polymer flow into tertiary runner  44 . 
     An opening or exit port  46  is located at each end of runner  44 , leading to a gate (not shown) which communicates with a mold cavity (not shown). Openings  46   a-h  are located at the end of runners  44   a-h , respectively. Openings  46   a-h  are equivalent to one another and herein are referred to collectively as opening  46 . Tertiary runner  44  provides a series of turbulence promoting bends causing the polymer to change direction and be agitated as it flows through opening  46 , into a gate and eventually into a mold cavity. 
     As previously explained, inducing turbulence in the polymer produces a shear force that promotes flowability of polymeric material. An increase in shear force also tends to increase the internal temperature of the polymer. This promotes the polymer to remain “fresh.” More specifically, such induced turbulence results in the leading region or front end of the polymer to be continuously reactivated or rejuvenated. Thus, the polymer generally maintains its viscosity or flowability thereby promoting an equal flow of the polymer to all mold cavities. 
     FIGS. 4 and 5 illustrate the runner system  30  when viewed in different planes. These figures show the relationship of sprue  32  to the runner system  30 . These figures also illustrate extension  48  of sprue  32 . Extension  48  serves as a stepped portion. Namely, the polymer flowing through sprue channel  32  may flow directly into runner  36  or may flow through and past the end of the runner  36  where it encounters stepped portion  48 . The stepped portion  48  serves to mix and redirect the polymer back into runner  36 . 
     FIG. 6 illustrates flow of polymer through a portion of the runner system  30  of the first preferred embodiment. After leaving sprue channel  34  the polymer flows into primary runner  36 . From runner  36 , a portion of the polymer branches into secondary runner  40  and another portion flows into curved end region  38 . The polymer flowing into curved end region  38  is redirected toward secondary runner  40  and mixed with polymer flowing from primary runner  36  into secondary runner  40 . Material flows through runner  40  until reaching the point at which it may branch off and flow into tertiary runner  44  or to another curved end region  42 . Material flowing to curved end region  42  is redirected to tertiary turbulence inducing runner  44 . The material is agitated and subsequently mixed with material flowing directly from secondary runner  40  into tertiary turbulence inducing runner  44 . Tertiary turbulence inducing runner  44  comprises a series of bends that redirect and mix the polymer as it travels toward opening  46 . Opening  46  leads to and is in communication with, a gate (not shown) which gate is in communication with a mold cavity (not shown). Accordingly, polymer material flows through opening  46  into the gate and then fills a mold cavity. 
     An injection molding runner system utilizing one or more turbulence inducing runners of the present invention, preferably employs a balanced configuration. The term “balanced configuration” as used herein refers to a runner system comprising one or more turbulence inducing runners, in which the runner system utilizes an arrangement of runners, conduits, or flow channels such that the physical properties and flow characteristics of polymeric material exiting the system through exit openings or ports, are equivalent, as measured at each of the exits. Typically, this aspect is accomplished by utilizing similar or equivalent flow paths between each individual exit opening and the main or primary sprue that supplies polymeric material to the runner system. Similar or equivalent flow paths may be realized by using similar geometrical configurations and dimensions for the various runners constituting each flow path from a main sprue to each of the exit ports. 
     The previously described balanced configuration is preferably symmetrical. The term “symmetrical” as used herein refers to a balanced runner system, as previously described, with an additional feature that the arrangement of runners, conduits, and/or flow channels is symmetrical with respect to a single plane bisecting the runner system through the feed sprue and perpendicular to the plane in which the runners, conduits, and flow channels primarily extend. Accordingly, it is preferred that the various groups of runners, eg. the primary, secondary, and tertiary runners, in a runner system be oriented perpendicular to each other. Referring to FIG. 3, this symmetrical aspect will be further described. The runner system  30  is symmetrical with respect to a plane bisecting the runner system through sprue  32  and perpendicular to either or both planes  4 — 4  and  5 — 5 . Such symmetry is evident when the bisecting plane, passing through sprue  32 , is parallel to secondary runners  40 . Such symmetry is also evident if the bisecting plane, passing through sprue  32 , also bisects primary runner  36 . 
     More preferably, the balanced configuration is a mirror image arrangement. The term “mirror image arrangement” as used herein refers to a balanced configuration that is symmetrical, and which exhibits a feature of additional symmetry. In a mirror image arrangement, the arrangement of runners, conduits, and/or flow channels is symmetrical with respect to not only one plane, but two planes. The arrangement is symmetrical with respect to a first plane bisecting the runner system through a feed sprue (or other source) and perpendicular to the plane in which the runners, conduits, and flow channels primarily extend. The arrangement is also symmetrical with respect to a second plane that bisects the runner system through the feed sprue and which is perpendicular to both the first plane and the plane in which the runners, conduits, and flow channels primarily extend. Referring to FIG. 3, that runner system  30  not only exhibits a symmetrical configuration, but also a mirror image configuration. Specifically, the runner system  30  exhibits a mirror image configuration since the system  30  is symmetrical with respect to (i) a plane bisecting sprue  32 , perpendicular to planes  4 — 4  and/or  5 — 5 , and parallel to runners  40 ; and (ii) a plane bisecting sprue  32  and runner  36 , and perpendicular to planes  4 — 4  and/or  5 — 5 . 
     Employing a balanced configuration, preferably a symmetrical configuration, and most preferably, a mirror image configuration, promotes the likelihood that a polymer material at any given location in the system is in the same physical state and/or is subject to forces equal to those in an equivalent location within the runner system. This leads to uniform flow. 
     A second preferred embodiment turbulence inducing runner  50  is illustrated in FIG.  7 . In this embodiment, the turbulence inducing runner provides a plurality of turbulence inducing angles, in conjunction with one or more turbulence inducing steps. The turbulence inducing steps and angles effect a change in the direction of flow of liquid material passing through that particular region of the turbulence inducing runner. The turbulence inducing runner system  50  comprises a first runner  52 . The first runner  52  utilizes a pair of turbulence inducing steps  53  and corresponding turbulence inducing angles w and z. The first runner  52  is in communication with two runners  56  via openings  54  in the opposite ends of the runner  52 . Each runner  56  preferably provides at least one turbulence inducing step  58  as the material flows through runner  56 . Polymeric material flows through runner  56  and towards turbulence inducing steps  58 . Polymeric material flows through the turbulence inducing steps  58  and is agitated and mixed as it flows into another runner (not shown). A more detailed description of the second preferred embodiment turbulence inducing runner  50  and steps  58  is provided as it relates to an injection molding runner system comprising such. 
     A runner system  60  employing the second preferred embodiment turbulence inducing runner  50  is illustrated in FIGS. 8-12. Runner system  60  employs a balanced symmetrical configuration. FIG. 11 illustrates the flow path of polymer material through a portion of the runner system  60 . Referring to FIGS. 8-12, polymer material is forced under pressure into sprue  62 , and through sprue channel  64 . Primary runner  66  branches off of sprue  62 . Polymer material travels through sprue channel  64  and into branches  66   a  and  66   b  of primary runner  66 . Runners  66   a  and  66   b  are equivalent to one another and herein are referred to collectively as runner  66 . 
     A secondary runner system  70  branches off of primary runner  66 . Secondary runners  70   a  and  70   b  branch off of primary runner  66   a . Secondary runners  70   c  and  70   d  branch off of primary runner  66   b . Runners  70   a-d  are equivalent to one another and herein are referred to collectively as runner  70 . Polymer flows through primary runner  66  until reaching openings leading to secondary runner  70  wherein polymer material may flow into runner  70 . Some of the polymer material may flow into curved end regions  68  at the end of runner  66 . Each end of runner  66  is provided with a curved end region  68 . Curved end region  68   a  is located at the end of runner  66   a . Curved end region  68   b  is located at the end of runner  66   b . Curved end regions  68   a-b  are equivalent to one another and herein are referred to collectively as curved end regions  68 . These regions redirect the polymer back towards secondary runner  70 . As previously noted, these regions are optional for the runner system. 
     At the end of each secondary runner  70  there is an opening  72  leading to a branched tertiary runner  74 . Openings  72   a ,  72   b ,  72   c  and  72   d  are defined at the end of runners  70   a ,  70   b ,  70   c  and  70   d , respectively. Openings  72   a-d  are equivalent to one another and herein are referred to collectively as opening  72 . Opening  72   a  leads to tertiary runners  74   a  and  74   b . Opening  72   b  leads to tertiary runners  74   c  and  74   d . Opening  72   c  leads to tertiary runners  74   e  and  74   f . Opening  72   d  leads to tertiary runners  74   g  and  74   h . Tertiary runners  74   a-h  are equivalent to one another and herein are referred to collectively as tertiary runner  74 . Polymer material flows through secondary runner  70  into opening  72  and into tertiary runner  74 . 
     Secondary runner  70  of the runner system  60  preferably comprises stepped turbulence inducing angles, providing agitation and mixing of the polymer. The stepped turbulence inducing angles defined along secondary runner  70  correspond to turbulence inducing steps  53  and turbulence inducing angles w and z illustrated in FIG.  7 . Secondary runner  70  preferably extends between the plane in which primary runner  66  and the plane in which tertiary runner  74  extends. These planes are described in greater detail below. 
     Referring to FIG. 8, polymer material flowing through tertiary runner  74  encounters stepped turbulence inducing angle  76 . Stepped turbulence inducing angles  76   a-h  are located at the end of tertiary runners  74   a-h , respectively. Stepped turbulence inducing angles  76   a- h are equivalent to one another and herein are referred to collectively as stepped turbulence inducing angle  76 . Stepped turbulence inducing angle  76  agitates and mixes the polymer as it flows to a quaternary runner  78 . It is preferred that quaternary runner  78  resides in the same plane as that of primary runner  66 . Accordingly, stepped turbulence inducing angles  76  extend between the plane in which tertiary runner  74  extends and the plane in which quaternary runner  78  extends. Stepped turbulence angles  76   a-h  lead to quaternary runners  78   a-h  respectively. Quaternary runners  78   a-h  are equivalent to one another and herein are referred to collectively as quaternary runner  78 . Quaternary runner  78  preferably resides in a horizontal plane above or below that of runner  74 , thus stepped turbulence inducing angle  76  carries polymer material out of the plane of runner  74 . However, the present invention includes arrangements and configurations in which quaternary runner  78  could reside in the same plane relative to tertiary runner  74 . FIG. 9 shows the preferred relationship of quaternary runner  78  to tertiary runner  74 , secondary runner  70 , and primary runner  66 . There is no limit with respect to the number of vertical changes employed by the present invention structures nor is there a limit with respect to the actual change in height by a branched runner. 
     Referring further to FIGS. 8 and 9, an opening  80  in each quaternary runner  78  leads to another branched runner hereinafter referred to as a discharge runner  84 . Openings  80   a-h  communicate with runners  78   a-h , respectively, leading into discharge runners  84   a-h , respectively. Openings  80   a-h , are equivalent to one another and herein are referred to collectively as opening  80 . Discharge runners  84   a-h  are equivalent to one another and herein are referred to collectively as discharge runner  84 . Additionally, curved end regions  82   a-h  are provided at the ends of quaternary runners  78   a-h , respectively. Curved end regions  82   a-h  are equivalent to one another and herein are referred to collectively as curved end region  82 . 
     Polymer material flows through quaternary runner  78  from which it may flow directly into discharge runner  84  via opening  80  or into curved end region  82 . Curved end region  82  redirects polymer material back toward opening  80  and into discharge runner  84 . Redirected polymer is agitated and subsequently mixed with polymer flowing directly into discharge runner  84 . 
     The end of each discharge runner  84  is provided with a rounded curved end stop  86 . Thus rounded curved end stops  86   a-h  reside at the end of discharge runners  84   a-h , respectively. Curved end stops  86   a-h  are equivalent to one another and herein are referred to collectively as curved end stop  86 . Each discharge runner  84  is also in communication with an exit port  88 , leading to a mold cavity (not shown). Thus exit ports  88   a-h  are in communication with discharge runners  84   a-h , respectively. Exit ports  88   a-h  are equivalent to one another and herein are referred to collectively as exit port  88 . 
     Polymer material flows through discharge runner  84  and may either flow directly through exit port  88  into a mold cavity or may flow into curved end stop  86 . Curved end stop  86  agitates and mixes polymer material, redirecting it back toward exit port  88 . Redirected polymer material is then mixed with material flowing directly into exit port  88  (and into a mold cavity). FIG. 8 illustrates a runner system utilizing various aspects of the present invention, eg. incorporation of two (2) turbulence inducing runners (generally comprising runner components  70  and  74 ), and eight (8) turbulence inducing steps (component  76 ). It will be understood that the present invention encompasses runner systems using a greater or lesser number of turbulence inducing runners and turbulence inducing steps than those illustrated in FIG.  8 . Moreover, it will be appreciated that the present invention encompasses runner systems using turbulence inducing runners and turbulence inducing steps, as described herein, independently of one another. 
     In the preferred embodiment runner systems in accordance with the present invention, as a result of having a balanced, symmetrical, or mirror image configuration, the various groups of runners are typically oriented at right angles with respect to other groups of immediately adjacent runners. And, the runners within a single group are typically oriented parallel to each other. For example, in the previously described runner systems  30  and  60 , the secondary runners are all oriented parallel to each other, and perpendicular to the primary runner and all tertiary runners. 
     The present invention is not limited to any particular number of splits or branched runners as long as the runner system utilizes a balanced configuration. The actual number of branches depends on mold size and the number of mold cavities in a runner system. 
     There is no limitation regarding the number of turbulence inducing angles employed by an apparatus such as a turbulence inducing runner of the present invention. The number of steps in a step turbulence inducing angle system is also not limited to a specific number. The present invention encompasses any shape for a turbulence inducing angle, so long as the shape imparts a sufficient amount of turbulence to the polymeric material flowing therethrough. Restrictions associated with turbulence inducing angles are determined by, but not limited to, the mold size, the number of cavities, and the degree to which turbulence can be induced such that the viscosity value of the polymer is sufficiently low enough to prevent blockage of flow but not to the point where there is too much turbulence and shear resulting in excessive energy losses or an excessive generation of heat resulting in a breakdown of the polymeric material. Furthermore, all references to flow states of polymeric material, eg. pseudo-turbulent, are made with regard to the material within the runner system at temperatures, pressures, and flow rates that are typically employed in injection molding operations. 
     A turbulence inducing runner system of the present invention is preferably installed prior to any branched runners, which split into two flow paths other than the sprue. This is evident in FIG. 8 for example where a turbulence inducing runner such as runner  50  depicted in FIG. 7 is incorporated within the runner system  60  prior to the branched arrangement of runners  78  and  84 . And, such system as shown in FIG. 8, comprises two (2) turbulence inducing runner devices. Turbulence inducing runners may be employed in any multiple cavity injection molding system. For example, an eight (8) cavity mold typically employs, but is not limited to, one (1) turbulence inducing runner system. Turbulence inducing steps, however, may be introduced at any point where the runner splits. No restriction is placed on runner dimensions. Runner dimensions should be selected and used according to known and recommended dimensions based on the size of parts to be molded and polymeric material chosen to form a molded part. 
     Although not wishing to be bound to any particular theory or numerical limits, it is believed that the present invention runner system maintains a minimum shear rate within a polymeric material flowing through the system under conditions suitable for injection molding. More particularly, it is believed that the present invention runner system maintains the noted shear rate such that the shear rate of polymer at the region of discharge into the mold cavities, i.e. at the gates, is preferably, at least about 40% of the shear rate of the polymer entering the runner system through a feed port or sprue. It is more preferred that the shear rate of polymer exiting the system be at least 50%, more preferably at least 60%, and most preferably, at least about 70% of the original shear rate of the polymer at the point of introduction of the polymer into the feed system. 
     The present invention runner system requires no additional devices, thereby allowing the costs of construction and maintenance of such systems to be kept low. 
     The foregoing description is, at present, considered to be the preferred embodiments of the present invention. However, it is contemplated that various changes and modifications apparent to those skilled in the art, may be made without departing from the present invention. Therefore, the foregoing description is intended to cover all such changes and modifications encompassed within the spirit and scope of the present invention, including all equivalent aspects.