Abstract:
A plunger machine for molding reinforced polymer is provided. The plunger machine has particular application in molding polymer that is reinforced with particles having an aspect ratio greater than 1:1. The plunger machine includes a barrel housing with a smooth 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 inwardly to provide a smooth transition and alignment of reinforcing members in the polymer mixture during the melt process. The smooth bore ensures 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.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to an improved injection molding machine and method of using the machine to form parts. More specifically, the present invention relates to a plunger molding machine for reinforced polymer compositions, particularly, polymers loaded with thermally conductive media, such as carbon and aluminum in the form of fibers and flakes. 
     In the molding industry, it has been well known to injection mold plastics into various articles of commerce. In particular, it has become well known to load such plastics or polymer-based compositions with other media to form a reinforced polymer composition. Reinforcing a polymer composition with other media is done for many different purposes. For example, reinforced polymer may be employed to provide a thermally conductive plastic where the reinforcing media is highly thermally conductive, such as carbon fiber or aluminum flakes. Another example, is 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 with EMI shielding. Also, glass, carbon or other fiber may be employed to add strength and/or stiffness. 
     In general, the loading of polymer, with a reinforcing media, raises many concerns as to the ability to successfully injection mold such a mixture because of the presence of the additional reinforcing media. For example, the loading of long carbon fiber into a polymer composition raises concerns as to strand and/or filament breakage during the melting and molding process. There is present the competing issues of the concern of thorough mixing of the loaded composition with the concern of excessive breakage of the delicate reinforcing media. Prior art molding machines typically create high turbulence and/or grinding of the polymer for the purposes of mixing the composition. These prior art machines commonly included a torpedo-shaped member or spreader to increase the level of turbulence to improve turbulence. However, such turbulence and grinding under pressure results in greatly reduced reinforcement media length. 
     However, these known processes are incompatible with the examples above, particularly the thermally conductive composition with carbon fiber, where it is critical that the breakage or damage to the reinforcing media be kept to a minimum to ensure that the desired properties of the 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. 
     To address these problems, 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. 
     In addition to the problems associated with the reduction of the length of reinforcing media, the alignment of such media is also a concern. In the examples above, a highly aligned and oriented loading of reinforcing media 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. 
     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. 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. 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. 
     SUMMARY OF THE INVENTION 
     The present invention preserves the advantages of prior art molding machines and methods for molding reinforced plastic. In addition, it provides new advantages not found in currently known machines and methods and overcomes many disadvantages of such currently available machines and methods. 
     The invention is generally directed to the novel and unique molding machine and method of using the same to molding reinforced polymer into articles. The molding machine and method of using the machine of the present invention enables reinforced polymer to be molded with minimal damage to the reinforcing particles loaded in the polymer molding composition. 
     The plunger machine of the present invention has particular application in molding polymer that is reinforced with particles having an aspect ratio greater than 1:1. The plunger machine includes a barrel housing with a smooth 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 inwardly to provide a smooth transition and alignment of reinforcing members in the polymer mixture during the melt process. The smooth bore ensures 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 is minimized to avoid unwanted breakage of the reinforcement members which is deleterious to the integrity of the reinforcing media. 
     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 walls 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 without causing excessive breakage to the reinforcing particles. 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 so as to provide a highly oriented melt mixture for subsequent injection into a mold for an article. The surface area of the bore is minimized versus the volume of the bore to reduce friction within the bore. 
     It is therefore an object of the present invention to provide a molding machine and method of molding that is suitable for molding reinforced polymer compositions. 
     It is an object of the present invention to provide a molding machine and method of molding that can mold reinforced polymer compositions while substantially decreasing the amount of breakage of the reinforcing media. 
     It is a further object of the present invention to provide a molding machine that aligns reinforcing media in a polymer composition with the melt flow for optimal results during injection molding. 
     Another object of the present invention is to provide a molding machine that can directly injection feed a mold or produce highly oriented reinforced polymer pellets for later use in an injection molding process. 
     It is a further object of the present invention to provide an injection molding machine that minimizes friction, shear and length degradation of reinforcing media while optimizing mixing of the reinforcing media with the polymer base and the alignment of the media with the melt flow. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features which are characteristic of the present invention are set forth in the appended claims. However, the inventions preferred embodiments, together with further objects and attendant advantages, will be best understood by reference to the following detailed description taken in connection with the accompanying drawings in which: 
     FIG. 1 is a cross-sectional view the preferred embodiment of the molding machine of the present invention illustrating the first step of injection molding a part in accordance with the present invention; 
     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; 
     FIG. 3 is a cross-sectional view through the line  3 — 3  of FIG. 2; 
     FIG. 4 is a cross-sectional view through the line  4 — 4  of FIG. 2; 
     FIG. 5 is an alternative embodiment of the present invention with angle feed port; 
     FIG. 6 is a first alternative bore configuration of the molding machine of the present invention; 
     FIG. 7 is a second alternative bore configuration of the molding machine of the present invention; and 
     FIG. 8 is a cross-sectional view of the preferred embodiment of the molding machine of the present invention being used as a pelletizer. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the present invention, molding machine  10  and corresponding method of using the machine  10  is suitable for accommodating a wide array of compositions of different materials loaded with reinforcing media of different types, such as in the form of fibers, flakes, ribbons and rice. For example, the present invention is suitable for a thermally conductive polymer composition loaded with carbon fibers as well as polymer composition loaded with aluminum flakes for EMI shielding applications. Further, an aluminum base may be loaded with steel flakes to enhance the physical tensile strength of the molded part. For simplicity and ease of illustration, the molding machine  10  and corresponding method will be described in detail below in connection with a thermally conductive composition with a polymer base loaded with carbon fiber. This is one example of the many applications of the machine  10  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. 
     Referring both to FIGS. 1 and 2, cross-sectional views of the plunger molding machine  10  of the present invention is shown. A plunger housing  12  contains a plunger or piston  14  therein. The plunger  14  reciprocates between retracted position, as shown in FIG. 1, and a forward position, as shown in FIG. 2, with the assistance of a hydraulic pump  16  or other similar reciprocating apparatus. The plunger housing  12  is mated with a barrel housing  18  that defines a barrel bore  20  therein. The bore  20  is configured in accordance with the present invention. In addition, a feed port  22  is provided which communicates with the plunger housing  12  and supplies the dry polymer mixture  24  to the molding machine  10  for melting and subsequent extrusion either into a cavity in a mold assembly  26  to form a molded part or cut into pellets for later use. Details of the molding process in accordance with the present invention will be described in detail below. 
     Still referring to FIGS. 1 and 2, the construction of the bore  20  of the molding machine  10  of the present invention is shown. The inner construction of the barrel housing  18  is dimensioned to provide a substantially tapered bore  20  where the entry port  28  of the bore  20  is substantially equal to the dimension of the exit of the plunger housing  12 . Preferably, a first portion of the bore  20  of the barrel housing  18  is, essentially, identical to the dimension of the bore  30  of the plunger housing  12  so as to receive the reciprocating plunger  14  therein. The bore  20  gradually tapers inwardly from a diameter of, for example, approximately 2.0 inches to an exit port nozzle end  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  14  is, for example, approximately 7.0 inches. FIGS. 3 and 4, cross-sectional views through the line  3 — 3  and  4 — 4 , respectively, further illustrate the inward taper of the bore  20  of the molding machine  10  of the preferred embodiment of the present invention. FIG. 3 shows the an inner diameter of the bore  20  proximal to the entry port  28  of the bore  20  while FIG. 4 shows an inner diameter of the bore  20  proximal to the exit port  32  of the bore  20 . It is possible to adjust the degree of taper and size of the entry port  28  and exit port  32  to the application at hand and the composition of the material to be processed by the present invention. 
     Referring back to FIGS. 1 and 2, the method of using the preferred embodiment of the molding machine  10  of the present invention is shown. In FIG. 1, a dry blend mixture  24  of base material  34 , such as polymer, and reinforcing material  36 , such as carbon fiber, is introduced into the plunger housing  12  via a feed port  22  with the assistance of a non-destructive auger  38  that gently feeds the material  24  in a downward direction. The nature of this sample composition is and of a dry and feathery consistency. Due to the low bulk density of this sample composition  24 , an auger  38  is needed; however, a heavier composition may be gravity feedable and may not need an auger. A hopper (not shown) may also be provided to further assist in the feeding of the material  24 . The plunger housing  12  and barrel housing  18  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  24  is fed into the plunger housing  12  and begins to meld and flow toward the exit port of the bore  20  of the barrel housing  18 . Due to heat applied to the plunger housing  12  and barrel housing  18 , the mixture  24 , particularly the polymer component  34  of the mixture, begins to melt. 
     Turning now to FIG. 2, filling and pre-packing the bore  20 , in preparation for extrusion, is shown. Preferably, a volume of melted or partially melted  40  composite material, with reinforcing members  36  loaded therein is packed into the bore  20  by blocking the exit port  32  of the bore  20 . The plunger  14  is actuated forward to urge melted or partially melted composite material  40  from the plunger housing  12  into the barrel housing  18 . Retraction of the plunger  14  permits the further loading of dry material  24  via the feed port  22 . Actuation forward and back of the plunger  14  is preferably carried out to remove all air pockets in the bore  20  and to ensure smooth flow of material  40 . It is preferred that the stroke length of the plunger  14  be from just rear of the feed port  22  to a location just prior to the entry port  28 . 
     In accordance with the present invention, as melted or partially melted material  40  travels down the bore  20  toward the exit port  32 , the polymer  34  is gradually heated to become fully melted. Due to the smooth taper of the bore  20 , loaded reinforcing media  36 , such as carbon fibers are naturally aligned with the downward flow of melt material  40  along the length of the bore  20 . In FIG. 3, at a location proximal to the entry port  28  of the bore  20 , the fibers  36  in the composition  40  are somewhat randomly oriented with the base matrix of polymer  34 . However, in accordance with the present invention, the fibers  36  become highly oriented further down the bore  20 , namely proximal to the exit port  32  of the bore  20 . As a result, the smooth taper of the bore  20  effective orients the fiber  36  within the composition  40 . In addition, the overall length of the bore  20  enables the mixture  40  to be properly mixed without using turbulent mixers of the prior art which would damage the delicate carbon fibers  36 . The gradual inward taper of the bore  20  gently increases compression without creating additional turbulence with less friction. 
     Once the bore  20  is pre-packed, flow of the composition  40 , with the highly oriented fiber  36  therein, can be started. The exit port  32  is opened and the appropriate molding assembly  26  is connected to the machine for the injection of the composite  40  material therein. At the exit port  32 , the composition  40  will be free of clumps of polymer  34  and will be fully wetted out with fibers  36  aligned and evenly distributed therein. Further dry mixture  34  (prior to melting) may be fed through the feed port  22  and, with the assistance of the auger  38 , routed into the plunger housing  12  and into the bore  20  for extrusion via the exit port  32 . The plunger  14  actuates back and forth to maintain a constant flow of melting mixture  40  through the bore  20  to provide the extrudate out of the exit port  32 . 
     Below is an example of an article formed by the molding machine  10  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. 
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
                   
               
               
                   
                 Present Invention 
                 Prior Art 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Base Matrix 
                 Polymer Resin 
                 Polymer Resin 
               
               
                   
                 Polyetherimide (ULTEM) 
                 Polyetherimide (ULTEM) 
               
               
                   
                 Liquid Crystal Polymer 
                 Liquid Crystal Polymer 
               
               
                   
                 (XYDAR) 
                 (XYDAR) 
               
               
                   
                 others 
                 others 
               
               
                 Reinforced Media 
                 Carbon Fiber 
                 Carbon Fiber 
               
               
                   
                 BP Amoco ThermaGraph ™ 
                 BP Amoco ThermaGraph ™ 
               
               
                   
                 CKDX pitch-based ultrahigh 
                 CKDX pitch-based ultrahigh 
               
               
                   
                 modulus graphite fiber 
                 modulus graphite fiber 
               
               
                   
                 Fiber Length: 0.25-2.0 inches 
                 Fiber Length: 0.25-2.0 inches 
               
               
                   
                 Fiber Diameter: 10 microns 
                 Fiber Diameter: 10 microns 
               
               
                 Loading of 
                 10-80 weight % 
                 10-80 weight % 
               
               
                 Reinforced Media 
               
               
                 Machine Used 
                 Smooth Tapered Bore 
                 Reciprocating Screw Injection 
               
               
                   
                 Bore Length: 12 inches 
                 Molding Machine 
               
               
                   
                 Entry Port Size: 2 inches 
               
               
                   
                 Exit Port Size: 0.25 inches 
               
               
                 Barrel Melt 
                 Polymer Dependent: 450-700° F. 
                 Polymer Dependent: 450-700° F. 
               
               
                 Temperature 
                 (for liquid crystalling polyester) 
                 (for liquid crystalling polyester) 
               
               
                 Fiber Length in Molded 
                 0.040-0.200 inches or greater 
                 0.015-0.040 inches 
               
               
                 Part 
               
               
                 Thermal Conductivity 
                 120 Watts/m-° K. 
                 28 Watts/m-° K. 
               
               
                   
               
             
          
         
       
     
     Referring now to FIGS. 5-8, a number of alternative embodiments of the present invention are shown. In FIG. 5, an alternative configuration of the feed port  22  is shown to be angled relative to the longitudinal axis of the bore  20  of the barrel housing  18 . In the preferred embodiment above, the dry mixture  24  of polymer  34  and carbon fiber  36  is routed through the feed port  22  and into the plunger housing  12  necessitating a 90 degree turn in direction. The alternative embodiment of FIG. 5 lessens the severity of the angle of entry of the polymer  34  with delicate reinforcing fibers  36  therein by “pre-aligning” the fibers  36 . As a result, the initial flow of the mixture  24  is less turbulent with less trauma to the fibers  36  causing less breakage of fibers  36  in the mixture. In addition, the auger  38  feed thread size can be made even larger to further avoid breakage of the fibers  36 . 
     FIGS. 6 and 7 illustrate bore configurations as alternatives to the continuously inwardly tapered bore  20  of the preferred embodiment shown in FIGS. 1 and 2. FIG. 6 shows a tapered bore  120  in a barrel housing  118  where the entry port  128  of the bore  120  is more tapered that the exit port  132  of bore  120 . In this embodiment, the angle of taper is less and less extending from the entry port  128  to the exit port  132  of the bore  120 . In this configuration, a more severe wall transition is provided to reduce the volume of composite material within the bore  120 . Also, FIG. 7 provides for another smoothly bore configuration where the inner wall of the bore  220  in barrel housing  218  has no taper at all. The bore, in FIG. 7, is cylindrically shaped and is well suited for optimum alignment of the fibers  36  within the composition with little breakage; however, mixing is not as effective as the tapered bores  20  and  120  discussed above. Any one of the foregoing bore geometries  20 ,  120 ,  220 , or a combination thereof, may be used to accommodate the application at hand to provide the appropriate volume and taper according the particular composition. These geometries may be adjusted to avoid pack out of the bore and ensure consistent flow down the bore. 
     It has been described above that the molding machine  10  extrudes a melted composition  40  for injection into a cavity of a mold  26  for forming a reinforced part or article. Appropriate nozzles (not shown) are attached to achieve this transition. As shown in FIG. 8, the molding machine  10  and method can be employed as a pelletizer to form composite pellets  42  for later use in a molding machine. In FIG. 8, a mechanical cutter  44 , such as a radial cutter, is employed to cut extruded material into discrete pellets  42  and for ejection into a collection bin  46 . The cutter  44  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  42  and synchronization with the plunger  14 , if required. Each of the pellets  42  include fiber strands therein (not shown) running along the length of the pellet  42  thus maintaining the integrity of the fiber  36  with each pellet  42 . This pelletizing process of the present invention is greatly superior to prior art pultrusion methods. The pellets  42  are later melted and formed into a molded part using an injection molding machine such as the one described above in accordance with the present invention. 
     It would be appreciated by those skilled in the art that various changes and modifications can be made to the illustrated embodiments without departing from the spirit of the present invention. All such modifications and changes are intended to be covered by the appended claims.