Abstract:
A plunger machine for molding reinforced polymer compositions is provided. The plunger machine has particular application in molding polymer that is reinforced with particles having an aspect ratio that is greater than 1:1. The plunger machine includes a barrel housing with a smooth walled barrel with longitudinal fins projecting inwardly towards the center of the bore that defines a main melt chamber. A plunger housing, having a plunger bore, defines an initial melt chamber and is in communication with the main melt chamber. A plunger resides in the plunger bore and is reciprocatable therein. The barrel bore is continuously inwardly tapered and cooperates with the longitudinal fins to provide a shortened melt period and a smooth transition and alignment of reinforcing members within the polymer mixture during the melt process. The smooth bore and cooperating fins ensure substantial alignment of the reinforcement members with the longitudinal axis of the bore in the direction of the composition flow to avoid excessive breakage of the reinforcing particles and prepare the polymer mixture for extrusion into a mold assembly.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a divisional of U.S. Ser. No. 10/213,177, filed on Aug. 6, 2002, which is related to and claims priority from earlier filed provisional patent application No. 60/316,484, filed Aug. 31, 2001. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    The present invention relates generally to an improved injection molding machine and method of using the machine to form net shape molded parts. More specifically, the present invention relates to a plunger molding machine for use in molding reinforced polymer compositions, particularly, polymers loaded with thermally conductive filler media, such as carbon, ceramics and metallic material in the form of fibers and flakes.  
           [0003]    In the molding industry, it has been well known to injection mold plastic materials into various articles of commerce. In particular, it has become well known to load such plastics or polymer-based compositions with filler materials to form a reinforced polymer composition. Reinforcing a polymer composition with other media is done for many different purposes. For example, a reinforced polymer composition may be employed to provide a thermally conductive plastic where the reinforcing material is highly thermally conductive, such as is the case with carbon fiber or aluminum flakes. Another example includes an application where the polymer is loaded with copper fiber to provide an electrically conductive polymer composition. Still further, aluminum flakes may be loaded in the polymer composition to provide a composition that includes EMI shielding properties. Also, glass, carbon or other structural fibers may be employed to add strength and/or stiffness.  
           [0004]    In general, the loading of a polymer base matrix, with a reinforcing material, raises many concerns regarding the ability to successfully injection mold such a composition due to the presence of the additional suspended reinforcing material. For example, if the reinforcing material that is loaded into the polymer matrix is long carbon fiber, there is a greatly increased potential for strand and/or filament breakage during the melting and molding process. During the molding process, the competing issues of thorough mixing of the loaded polymer composition and the concern of excessive breakage of the delicate reinforcing media must be balanced to achieve the desired product. Prior art molding machines typically create high turbulence and/or grinding of the polymer material for the purposes of mixing the composition. These prior art machines commonly included a torpedo-shaped member or spreader located in the center of the injection molding bore to increase the level of turbulence as the composition passes through the bore to cause the polymer to melt in a uniform manner and to improve the mixing of the composition. However, such turbulence and grinding of the polymer composition under pressure during the molding process results in increased reinforcing fiber breakage and greatly reduced reinforcement media length.  
           [0005]    As a result, it can be clearly seen that these known molding processes are incompatible with the molding of thermally conductive polymer compositions as described above. In particular, a thermally conductive composition that employs carbon fiber reinforcing requires that the breakage or damage to the reinforcing fibers be kept to a minimum to ensure that the desired properties of the resulting composition are maintained. In the above example, if the lengths of the carbon fibers loaded within the polymer composition are ground up into much shorted lengths, it is clear that the overall thermal conductivity of the composition will be degraded as a result.  
           [0006]    In an attempt to address the problems with breakage of reinforcing fibers, compression molding has been attempted where there is a manual lay-up of material and the reinforcing media thereon. As can be understood, such manual assembly is expensive and is far too slow for mass production. Thus, compression molding is inadequate and impractical for molding reinforced material and suffers from economic and geometry-related limitations.  
           [0007]    In addition to the problems associated with the reduction of the length of reinforcing media, the alignment of the reinforcing fibers within the composition is also a concern. In the examples above, a highly aligned and oriented loading of reinforcing material along the path of conductivity is preferred to obtain higher performance of the molded composition. For example, a highly oriented array of carbon fiber within a polymer base would yield higher thermal conductivities than a composition that included randomly oriented fibers, because the number of transitions from carbon to polymer to carbon within the composition would be greatly reduced. Further, packing densities are higher when the fibers or filaments are well-aligned. The foregoing alignment and breakage problems become even more important where the aspect ratio of the reinforcing media becomes larger and larger.  
           [0008]    Tapered bore injection molding machines have also been used to overcome the above noted deficiencies. However, while tapered bore machines preserve reinforcing fiber length and alignment, since there is no transfer of heat to the center of the bore the polymer melts in an uneven fashion and requires extended melt time within the injection molding bore.  
           [0009]    In view of the foregoing, there is a demand for an improved injection molding machine and method that is well suited for accommodating polymer compositions loaded with reinforcing media having aspect ratios greater than 1:1 while enhancing the melt uniformity of the composition. Further, there is a demand for a molding machine that is capable of greatly decreasing the amount of breakage of reinforcing media during the molding process while enhancing the speed at which the polymer reaches its molten state. There is also a demand for a molding machine and method of using the machine that can better align reinforcing media along the line of melt flow to provide a better oriented reinforced composition.  
         BRIEF SUMMARY OF THE INVENTION  
         [0010]    In this regard, the present invention provides a novel molding machine and method of using the machine to injection mold a reinforced polymer composition. The present invention results in a reduction in the amount of damage to the reinforcing particles loaded in the polymer molding composition while providing a increased uniformity in the heat transfer from the injection bore to the composition that results in a reduced residence time, the time required for heating the polymer to achieve the desired melt viscosity. The plunger injection machine of the present invention has particular application in molding polymer compositions that are reinforced with particles having an aspect ratio that is greater than 1:1.  
           [0011]    The plunger machine includes a barrel housing with an interior barrel bore that defines a main melt chamber. A plunger housing, having a plunger bore, defines an initial melt chamber that is in communication with the main melt chamber. A plunger resides in the plunger bore and is reciprocatable therein. The barrel bore is continuously tapered inwardly to provide a smooth transition of the melted polymer composition while causing an alignment of reinforcing members in the polymer mixture during the melt process. The inner wall of the barrel bore is substantially smooth with a plurality of longitudinal fins extending along the length of the bore. The fins on the interior of the bore are in thermal communication with the bore and provide thermal transfer paths that allow the heat from the melt element to be transferred to the interior of the flow and therefore more uniformly throughout the polymer media. In addition, the configuration of the smooth bore walls and longitudinal fins cooperate to ensure substantial alignment of the reinforcement members with the longitudinal axis of the bore to avoid excessive breakage of the reinforcing particles and prepare the polymer mixture for extrusion into a mold assembly. Compression of the polymer composition via the plunger is minimized to avoid unwanted breakage of the reinforcement particles, which is deleterious to the integrity of the reinforcing media.  
           [0012]    In accordance with the method of the present invention, a mixture of polymer, reinforcing particles, such as carbon fibers of an aspect ration greater than 1:1, are fed into a feed port with the assistance of an auger through a hopper. The mixture is gently fed into an initial melt chamber where the mixture is melt and then urged by a plunger into a main melt chamber. The main melt chamber includes smooth walls and a plurality of longitudinal fin sections. The walls and fins of the main melt chamber are heated by heater bands, or the like, and gradually and inwardly tapered to gradually and gently melt the mixture and to gradually align the reinforcing particles with the polymer base matrix without causing excessive breakage of the reinforcing particles. The fins assist in transferring heat into the path of the flow increasing the speed at which the media is melted, thus reducing the required residence time of the polymer composition within the melt chamber. At the exit port of the main melt chamber, the reinforcing members are substantially aligned lengthwise along the direction of flow of the melt within the chamber to provide a highly oriented melt mixture for subsequent injection into a mold for an article.  
           [0013]    Accordingly, one of the objects of the present invention is the provision of an injection molding device for molding a polymer composition that includes high aspect ratio reinforcing particles while minimizing the breakage of the particles. Another object of the present invention is the provision of an injection molding device for the molding of a reinforced polymer composition that produces a high degree of axial alignment of the reinforcing material during the melting and injection process. A further object of the present invention is the provision of an injection molding device that preserves the length of the reinforcing particles in a polymer composition while providing enhanced heat transfer from the device to the composition to reduce the residence time of the polymer composition within the injection bore. It is yet another object of the present invention is the provision of an injection molding method where a polymer composition is reinforced with high aspect ration filler so that the length of the filler particles is preserved and a substantial alignment of the particles is achieved.  
           [0014]    Other objects, features and advantages of the invention shall become apparent as the description thereof proceeds when considered in connection with the accompanying illustrative drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    In the drawings which illustrate the best mode presently contemplated for carrying out the present invention:  
         [0016]    [0016]FIG. 1 is a cross-sectional view of the molding machine of the present invention illustrating the first step of injection molding a part in accordance with the method of the present invention;  
         [0017]    [0017]FIG. 2 is cross-sectional view the preferred embodiment of the molding machine of the present invention illustrating the step of packing the main melt chamber in accordance with the present invention;  
         [0018]    [0018]FIG. 3 is a cross-sectional view through the line  3 - 3  of FIG. 2;  
         [0019]    [0019]FIG. 4 is a cross-sectional view through the line  4 - 4  of FIG. 2;  
         [0020]    [0020]FIG. 5 is an alternative embodiment of the present invention with angled feed port; and  
         [0021]    [0021]FIG. 6 is a cross-sectional view of the molding machine of the present invention being used as a pelletizer. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]    Referring now to the drawings, the present invention injection molding barrel  10  shown in conjunction with a molding machine  12  and corresponding method of using the machine  12  is shown and generally illustrated in FIGS. 1-4. The machine  12  is suitable for accommodating a wide array of compositions of different materials loaded with reinforcing media of different shapes in the form of fibers, flakes, ribbons and rice. For example, the present invention  12  is suitable for molding a thermally conductive polymer composition loaded with carbon fibers as well as a polymer composition loaded with aluminum flakes tailored for EMI shielding applications. Further, an aluminum base material may be loaded with steel flakes to enhance the physical tensile strength of the resultant molded part. For simplicity and ease of illustration, the molding machine  12  and corresponding method will be described in detail below in connection with a thermally conductive composition with a polymer base material loaded with carbon fiber reinforcing. This is one example of the many applications of the machine  12  and method of the present invention where a base material is loaded with a reinforcing media that needs to be aligned but not broken during the molding process. This example is not intended to be limiting, as the present application has broad applications relating to the general concepts described herein.  
         [0023]    Referring both to FIGS. 1 and 2, cross-sectional views of the injection molding barrel  10  of the present invention in connection with a plunger molding machine  12  is shown. A plunger housing  14  that contains a plunger  16  or piston is generally illustrated. The plunger  16  is movable between a retracted position, as shown in FIG. 1, and a forward position, as shown in FIG. 2, with the assistance of a hydraulic pump  18  or other similar reciprocating apparatus via linkage  19 . The plunger housing  14  is mated with a barrel housing  20  of the injection molding barrel  10  that has a barrel bore  22  located therein. The bore  22  is configured in accordance with the present invention as will be further described below. In addition, a feed port  24  is provided, which communicates with the plunger housing  14  and provides a means by which the dry polymer mixture  26  and reinforcing fibers  28  can be fed to the molding machine  12  for melting and subsequent extrusion. The extruded material may be extruded directly into a cavity in a mold assembly to form a molded part or extruded as a rod and cut into pellets for later use in future molding operations. Details of the molding process in accordance with the present invention will be further described below.  
         [0024]    Still referring to FIGS. 1 and 2, the construction of the barrel  10  of the molding machine  12  of the present invention is shown. The inner construction of the barrel housing  20  is arranged to provide a substantially tapered bore  22  where the entry port  30  is larger than the exit port  32 . Further, the entry port  30  of the bore  22  is substantially equal to the dimension of the exit  34  of the plunger housing  14  and preferably, at least a first portion of the bore  22  of the barrel housing  20  is, essentially, identical to the dimension of the bore  34  of the plunger housing  14  so as to receive the reciprocating plunger  16  therein. The barrel bore  22  gradually tapers inwardly from a diameter of, for example, approximately 2.0 inches to an exit port  32  of, for example, approximately 0.25 inches and extends, for example, to a length of approximately 12.0 inches. The stroke length of the plunger  16  is, for example, approximately 7.0 inches. The interior surface of the barrel bore  22  is generally a smooth and polished surface to allow a smooth and even flow of the extrusion material  36 . Further, on the surface of the bore  22 , several fins  38  are provided. The fins  38  are generally linearly shaped ribs that align substantially in alignment with the longitudinal axis of the bore  22 , extending between the entry port  30  and the exit port  32 .  
         [0025]    The fins  38  of the present invention have a height, or protrusion into the bore  22 , that is proportionally tapered relative to the taper of the barrel bore  22 . More specifically, the fins  38  have a deeper profile  40  at the entry port  30  of the bore  22  and a shallower profile  42  at the exit port  32 , generally maintaining the same clearance distance from the center line of the bore  22  along the length of the bore  22 . FIGS. 3 and 4, cross-sectional views through lines  3 - 3  and  4 - 4  of FIG. 2, respectively, further illustrate the inward taper of the bore  22  of the molding machine  12  of the present invention while also illustrating the proportional taper of the fin  38  profile. FIG. 3 shows the larger inner diameter of the bore  22  proximal to the entry port  30  of the bore  22  where the fins  30  have a pronounced depth and profile  40 , while FIG. 4 shows a reduced inner diameter of the bore  22  proximal to the exit port  32  of the bore  22  with the fins  38  having a reduced profile  42  that approaches nearly flat and flush with the inner surface of bore  22 . It is possible to adjust the degree of taper and size of the entry port  30  and exit port  32  as well as the overall depth of the fins  38  to the application at hand and the composition of the material to be processed by the present invention.  
         [0026]    The fins  38  serve two general purposes in the present invention. The first purpose of the fins  38  is to facilitate heat transfer into the extrusion material  36 . The fins  38  provide increased surface area to provide an increased rate of thermal transfer from the bore  10  to the extrusion material  36 . In the prior art, a torpedo was placed within the bore and supported on wings that extended from the bore surface. However, this configuration caused a high degree of turbulence within the barrel in addition to providing several locations where the linear flow of material collided with the wing supports resulting in a high degree of broken fiber reinforcing material. The fins  38  of the present invention allow heat to be transferred closer to the center of the bore  22  while also slightly increasing the overall turbulence within the bore  22  in addition to reducing the number of locations for potential flow collisions. In this manner, effective mixing and melting of the extrusion material  36  can be achieved while preserving the length of the reinforcing fibers  28  and maximizing fiber length in the finished product. The second purpose of the fins  38  is to generally direct the flow within the bore  22  into a substantially aligned linear direction. In this manner, the fins  38  generally cause the fibers  28  within the extrusion material  36  to align linearly along the axis of the flow. This effect is pronounced as the fins  38  operate in cooperation with the tapered bore  22  as will be fully described in the method below.  
         [0027]    Referring back to FIGS. 1 and 2, the method of using the molding machine  12  of the present invention is shown. In FIG. 1, a dry blend mixture of base material  26 , such as polymer, and reinforcing material  28 , such as carbon fiber, is introduced into the plunger housing  14  via a feed port  24  with the assistance of a non-destructive auger  44  that gently feeds the material  36  in a downward direction. The nature of this sample composition  36  is of a dry and feathery consistency. Due to the low bulk density of this sample composition  36 , an auger  44  is needed; however, a heavier composition may be gravity feedable and may not need an auger  44 . A hopper (not shown) may also be provided to further assist in the feeding of the material  36 . The plunger housing  14  and barrel housing  20  is heated or pre-heated prior to the start of mixture feeding process with heater bands (not shown), or the like. As shown in FIG. 1, the mixture  36  is fed into the plunger housing  14  and begins to melt and flow toward the entry port  30  of the bore  22  of the barrel housing  20 . Due to heat applied to the plunger housing  14  and barrel housing  20 , the mixture  36 , particularly the polymer component  26  of the mixture  36 , begins to melt.  
         [0028]    Turning now to FIG. 2, filling and pre-packing the bore  22 , in preparation for extrusion, is shown. Preferably, a volume of melted or partially melted composite material  36 , with reinforcing members  28  loaded therein is packed into the bore  22  by blocking the exit port  32  of the bore  22 . The plunger  16  is actuated forward to urge melted or partially melted composite material  36  from the plunger housing  14  into the barrel housing  22 . Retraction of the plunger  16  permits the further loading of dry material  36  via the feed port  24 . Actuation forward and back of the plunger  16  is preferably carried out to remove all air pockets in the bore  22  and to ensure smooth flow of material. It is preferred that the stroke length of the plunger  16  be from just rear of the feed port  30  to a location just prior to the entry port  24 .  
         [0029]    In accordance with the present invention, as melted or partially melted material  36  travels down the bore  22  toward the exit port  32 , the polymer  26  is gradually heated to become fully melted. To enhance the heating of the polymer material  26  heat transfer into the partially melted material  25  is further enhanced by conducting heat from the housing walls  20  of the bore  22  into the fins  38  where there is increased surface area available for thermal transfer. The smooth taper of the bore  22  and the fins  38  cooperate together to cause the loaded reinforcing media  28 , such as carbon fibers to become naturally aligned with the axis of the downward flow of melt material  36  along the length of the bore  22 . In FIG. 3, at a location proximal to the entry port of the bore  22 , the fibers  28  in the composition are randomly oriented within the base matrix of polymer  26 . However, in accordance with the present invention, the fibers  28  become highly oriented as they travel further down the bore  22  and are particularly aligned proximal to the exit port  32  of the bore  22 . As a result, the smooth taper of the bore  22  and the fins  38  located therein effectively orient the fiber  28  within the composition  36  while providing an increased surface area for thermal transfer thereby decreasing the required residence time of the composition  36  within the bore  22 . In addition, the overall length of the bore  22  enables the mixture to be properly mixed without using turbulent mixers of the prior art, which would damage the delicate carbon fibers  28 . The gradual inward taper of the bore  22  also provides a gentle increase in compression without creating additional turbulence or an increase in friction.  
         [0030]    Once the bore  22  is pre-packed, flow of the composition  36 , with the highly oriented fiber  28  therein, can be started. The exit port  32  is opened and the appropriate molding assembly is connected to the machine  12  for the injection of the composite material  36  therein. At the exit port  32 , the composition  36  will be free of clumps of polymer  26  as the fins  38  enhance the overall consistency of the polymer  26  melt. Further, the polymer  26  and will be fully loaded with fibers  28  that are completely wetted out, aligned and evenly distributed therein. The process can then continue by feeding additional dry mixture  36  (prior to melting) through the feed port  24  and, with the assistance of the auger  44 , routed into the plunger housing  14  and into the bore  22  for extrusion via the exit port  32 . The plunger  16  actuates back and forth to maintain a constant flow of melting mixture  36  through the bore  22  to provide the molten extrudate out of the exit port  32 .  
         [0031]    Below is an example of an article formed by the molding machine  12  and corresponding method of the present invention. In this example, the molded article is a plastic heat sink where carbon fibers therein provide the article with high thermal conductivity, particularly in the direction of the length-wise orientation of the carbon fibers. The following table also provides a comparison with a prior art process employing a known screw machine to illustrate the advantages of the present invention. The chart below illustrates that use of the present invention results in longer fiber lengths in the molded part, which in turn results in higher overall thermal conductivity of the finished part.  
         [0032]    Comparison  
                                                                 Present Invention   Prior Art                                    Base   Polymer Resin   Polymer Resin       Matrix   Polyetherimide (ULTEM)   Polyetherimide           Liquid Crystal Polymer   (ULTEM)           (XYDAR)   Liquid Crystal           Others   Polymer (XYDAR)               others       Reinforced   Carbon Fiber   Carbon Fiber       Media   BP Amoco   BP Amoco           ermaGraph ™ CKDX   ThermaGraph ™           pitch-based ultrahigh   CKDX pitch-           modulus graphite fiber   based ultrahigh           iber Length: 0.25-2.0   modulus graphite           inches   fiber           Fiber Diameter: 10   Fiber Length:           microns   0.25-2.0 inches               Fiber Diameter:               10 microns       Loading of   10-80 weight %   10-80 weight %       Reinforced       Media       Machine   Smooth Tapered Bore   Reciprocating Screw           Bore Length: 12 inches   Injection           Entry Port Size:   Molding Machine           2 inches           Exit Port Size: 0.25           inches       Barrel Melt   Polymer Dependent:   Polymer Dependent:       Temperature   450-700° F.   450-700° F.           (for liquid crystalline   (for liquid crystalline           polyester)   polyester)       Fiber Length   0.040-0.200 inches   0.015-0.040 inches       in Molded Part   or greater       Thermal   120 Watts/m-°K   28 Watts/m-°K       Conductivity                  
 
         [0033]    Referring now to FIGS. 5 and 6, two alternative embodiments of the present invention are shown. In FIG. 5, an alternative configuration of the feed port  24  is shown to be angled relative to the longitudinal axis of the bore  22  of the barrel housing  20 . In the preferred embodiment above, the dry mixture  36  of polymer  26  and carbon fiber  28  is routed through the feed port  24  and into the plunger housing  14  necessitating that the material  36  make a 90 degree turn in direction. The alternative embodiment of FIG. 5 lessens the severity of the angle of entry of the polymer  26  with delicate reinforcing fibers  28  therein by “pre-aligning” the fibers  28 . As a result, the initial flow of the mixture  36  is less turbulent with less trauma to the fibers  28 , causing less breakage of fibers  28  in the mixture  36 . In addition, the auger  44  feed thread size can be made larger to further reduce breakage of the fibers  28 .  
         [0034]    It has been described above that the molding machine  12  extrudes a melted composition  36  for injection into a cavity of a mold for forming a reinforced part or article. Appropriate nozzles (not shown) are attached to achieve this transition. As shown in FIG. 6, the molding machine  12  and method can be employed as a pelletizer to form composite pellets  48  for later use in a molding machine. In FIG. 6, a mechanical cutter  50 , such as a radial cutter  50 , is employed to cut extruded material  36  into pellets  48  for ejection into a collection bin  52 . The cutter  50  may be driven by rack and pinion linkage, gears and other mechanical assemblies and would be fully adjustable to control the length of the pellet  48  and synchronization with the plunger  16 , if required. Each of the pellets  48  include fiber strands therein (not shown) running along the entire length of the pellet  48  thus maintaining the integrity of the fiber  28  within each pellet  48 . This pelletizing process of the present invention is greatly superior to prior art pultrusion methods. The pellets  48  can then be stored for further processing by later melting them and forming them into a molded part using an injection molding machine such as the one described above in accordance with the present invention.  
         [0035]    While there is shown and described herein certain specific structure embodying the invention, it will be manifest to those skilled in the art that various modifications and rearrangements of the parts may be made without departing from the spirit and scope of the underlying inventive concept and that the same is not limited to the particular forms herein shown and described except insofar as indicated by the scope of the appended claims.