Abstract:
A combination rubber injection gate system ( 24 ) and method of injection molding rubber to produce desired flow patterns in the rubber is disclosed. The combination gate ( 24 ) directs rubber through a first region ( 26 ) to flow at cross angles ( 38, 40 ) through the injection gate ( 24 ), relative to the direction of flow of the rubber ( 22 ) through the injection gate ( 24 ). The rubber ( 22 ) is then directed through an adjacent region ( 28 ) to flow through the injection gate ( 24 ) at a direction parallel to the direction of flow of the rubber ( 22 ) through the entire injection gate ( 24 ). The combination gate ( 24 ) yields a highly mixed and uniform flow of rubber ( 22 ) that is particularly useful when injection molding fiber-loaded rubber where it is desired to orient the fibers in a particular direction.

Description:
TECHNICAL FIELD 
     The present invention relates to an improved gate design for injection molding of rubber compounds to form rubber articles. More particularly, the present invention relates to an improved gate design for injection molding of fiber loaded rubber compounds with increased gate heating efficiency, reduced cycle time during the rubber part being injection molded, and improved fiber orientation in the rubber article. 
     BACKGROUND ART 
     In a typical rubber injection molding process, see FIG. 9, the uncured viscous rubber compound is introduced into the elongated barrel  12  of an injection molding machine  10  at ambient temperatures. It is advanced through the barrel  12  towards a mold  20  connected to the downstream end of the barrel  12 , usually by either a rotating screw conveyor or a reciprocating ram or piston  14  disposed in the barrel  12 . As the rubber compound advances, it is heated by heat conduction and mechanical shear heating in the barrel  12  to reduce its viscosity and render the rubber more flowable and amenable to subsequent injection into the mold  20 . Typically, the less viscous the rubber compound, the more easily it flows through the conventional gate system  16  and more easily it fills the mold cavity  18  to produce a satisfactorily molded object. 
     Composite articles formed of an elastomeric base and reinforcing fibers are known in the art. The reinforcing fibers impart improved mechanical properties, such as abrasion resistance, tensile strength, compression resistance, and the like to the article. The fiber distribution and orientation are important factors which affect such properties. Controlling fiber orientation, therefore, is an important consideration to provide a reinforced article having the desired mechanical properties. 
     One known composite article is a tire tread. It is known that short-fiber reinforced treads with fibers oriented perpendicular to the tread surface can provide improved wear resistance and have excellent cornering resistance. Unfortunately, treads prepared by a conventional extrusion process have fibers oriented in the extrusion direction, that is, the tire circumferential direction. This orientation direction actually worsens the wear property because fibers oriented parallel to the rotational direction easily come off the tread surface. 
     An expanding die technology was developed to alter the fiber orientation direction. This technology is used to prepare short fiber reinforced tread extrusions with fibers oriented perpendicular to the tread surface. This technology is disclosed in WO 98/13185. WO 98/13185 is hereby fully incorporated by reference. 
     There are two steps involved in the expanding die technology. First a flat gate is used to orient fibers in the extrusion direction (or parallel to the tread surface). Then, the orientation direction of the fibers is changed to normal direction (or perpendicular to the tread surface) due to the folding action of the expanding die. Results showed that this technology prevented the fibers from orienting in the extrusion direction due to the folding action of the expanding die. However, it has been determined that the fibers are not exclusively oriented perpendicular to the tread surface, but there is also a lateral orientation of the fibers in the width direction of the tread. 
     The main reason for the lateral orientation in the width direction is due to the flat gate design combined with the expanding die. The pressure drop through the center path of the die is smaller than at the side path due to the additional pressure drop through the runner. This results in faster rubber flow at the center of the gate that creates a slight width direction extensional flow and lateral fiber orientation. 
     Another type of known gate is the lattice gate, disclosed in WO 98/56559. The lattice gate of WO 98/56559 minimizes differences in temperature and pressure that result in a parabolic rubber flow through the gate. This is achieved by a series of crossed flow channels. WO 98/56559 is fully incorporated herein by reference. However, for molding fiber-loaded compounds with a particular fiber orientation, the lattice gate fails to provide any particular orientation of the fiber. Due to the inclination angle of the flow channels and the flow of the rubber through the channels, the fibers are oriented at angles corresponding to the flow channels. The fiber-loaded ribbons coming out of the channels tangle with each other in a random structure that result in a random orientation of the fiber. Thus, the lattice gate alone cannot be used to prepare compounds with a specific fiber orientation. 
     The present invention provides an improved method and apparatus for injection molding rubber and, preferably, orienting fibers in a composite article, which overcomes the limitations of the known gate systems. 
     SUMMARY OF THE INVENTION 
     The present invention is an improved method of injection molding rubber. The method includes injecting a rubber through an injection gate to produce desired flow patterns in the rubber. The rubber is first directed to flow at cross angles through the injection gate, relative to the direction of flow of the rubber through the injection gate. The rubber is then directed to flow through the injection gate at a direction parallel to the direction of flow of the rubber through the entire injection gate. 
     In one aspect of the disclosed invention, the rubber is a fiber-load rubber. When the fiber-load rubber travels through the injection gate in accordance with the invention, the fibers are first oriented at the cross flow angles and then re-oriented to the direction of flow parallel to the flow direction of the rubber through the entire injection gate. 
     In another aspect of the invention, the distance of the cross-directional rubber flow, relative to a centerline of the gate, relative to the parallel-direction rubber flow, relative to the centerline of the gate, is within the ratio of 2:1 to 1:2. 
     Another aspect of the invention includes the step of directing the rubber through an injection gate exit into a mold cavity to form a series of folding planes perpendicular to the direction of flow through the injection gate exit. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The invention will be described by way of example and with reference to the accompanying drawings in which: 
     FIG. 1 is a plan view of rubber formed in a combination gate in accordance with the present invention; 
     FIG. 2 is a schematic illustration showing the inventive combination gate; 
     FIG. 2A is a view through line  2 A— 2 A of FIG. 2; 
     FIG. 3 is a plane view of half the combination gate at the sprue, or fixed mold, side; 
     FIG. 4 is a view through line  4 — 4  of FIG. 3; 
     FIG. 5 is a view through line  5 — 5  of FIG. 3; 
     FIG. 6 is a plane view of the other half of the combination gate at the moving mold side; 
     FIG. 7 is a view through line  7 — 7  of FIG. 6; 
     FIG. 8 is a perspective cross-section view of an exemplary mold showing the introduction of molding compound into the mold; and 
     FIG. 9 is a cross-sectional view of a conventional rubber extruder and gate system. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is related to the design of an improved gate system for improving the mixing uniformity, temperature uniformity, and the fiber orientation of the rubber exiting the gate system. FIG. 1 is a shaped portion of rubber  22  that corresponds to the internal flow passage through the inventive gate system  24 . The inventive gate system  24  is a combination of a lattice region  26  and a flat region  28 . The entrance section  30  of the gate  24  is a lattice region  26  that provides a more uniform distribution of rubber compound. The exit section  32  is a flat region  28  that orients fibers in the injection direction. This unique combination gate system  24  provides a uniformly distributed rubber flow to the mold cavity  18  and, when using a fiber-loaded compound, improves the degree of fiber orientation in a fiber-loaded compound. 
     The improved gate system  24 , illustrated in FIG. 2, incorporates a sprue channel  34 , a first distribution channel  36 , a lattice region  26  of intersecting rubber flow channels  38 ,  40 , a second distribution channel  44 , and a flat region  28 . The rubber  22  flows from the sprue channel  34 , into the first distribution channel  36 , and into the intersecting flow channels  38 ,  40 . The intersecting rubber flow channels  38 ,  40  force the rubber  22  flowing through the lattice region  26  of the gate system  24 . Because of this structure, rubber  22  exiting the lattice region  26  is uniformly distributed when it enters channel  44 . Upon entering the flat region  28 , because of the flow direction of the rubber  22  towards the gate exit  46 , any fibers present in the rubber  22  are reoriented to be parallel to the rubber flow direction. 
     The components of the combination gate system  24  are more clearly illustrated in FIGS. 3-7. FIG. 3 illustrates a plan view of the inner surface  48  of the gate plate  50 , which is half of the combination gate  24  at the sprue side. The plate  50  includes a sprue bore  52  that extends from the outer surface  54  to the inner surface  48 . The plurality of flow channels  38  formed into the inner surface  48  of the plate  50  are parallel to each other and inclined at angles of about 30° to about 70°, preferably at angles of about 45° to about 60°, with respect to a centerline  56 . As the angle of the parallel flow channels  38  with respect to the centerline  56  increases, the time required for the rubber  22  to transverse between the inlet  58  and outlet  60  sides of the plate  50  also increases, and vice versa. The flow channels  38  are illustrated with a semi-circular cross-section; however, it is within this invention to form the flow channels  38  with other cross-sections, such as elliptical, triangular, or square as desired. 
     The flow channels  38  terminate in the distribution channel  61 . Adjacent to the distribution channel  61  is an indented flat region  62 , as seen in FIGS. 4 and 5. Rubber flows directly from the channels  38  to the distribution channel  61  and into the flat region  62  towards the outlet end  60  of the sprue plate  50 . The depth of the distribution channel and the depth of the flat region may be equal or different as illustrated. 
     Referring now to FIG. 6, there is shown a plan view of the flat inner surface  64  of the other half of the combination gate plate  66  having an inlet end  68  and an outlet end  70 . A sprue inlet counterbore  72  extends into the inner surface  64  and is positioned between the inlet end  68  and the outlet end  70  of the plate  66 . As illustrated in FIG. 7, the sprue inlet counterbore  72 ; is in flow communication with an elongated distribution channel  74  extending partially across the length of the plate  66  and in parallel relation to the outlet end  70 . A plurality of flow channels  40  are formed in the inner surface  64  of the plate  66 . The flow channels  40  are formed similar to the channels  38  formed on the plate  50 . 
     Adjacent to the flow channels  40  is a second elongated distribution channel  76  that extends parallel to the first elongated distribution channel  74 . An indented flat region  78  is formed extending from the second distribution channel to the outlet end  70  of the plate  66 . Mixed rubber flows from the channels  40  to the second distribution channel  76  towards the outlet end  70  of the plate  66 . Also, as seen in FIG. 7, the distribution channels  74 ,  76  have a depth greater than the depth of the channels  40  or the indented flat portion  78 . 
     Referring to FIGS. 2 and 2A, there is shown the inventive gate system  24  with the flat inner surface  48  of the sprue side gate plate  50  abutted against the flat inner surface  64  of the other side gate plate  66  and secured thereto by conventional means such as bolting one to the other. After the plates  50 ,  66  are secured to each other, the sprue bore  52  and the sprue inlet counterbore  72  intersect to form sprue channel  34 . Also, the inner surface  48  of plate  50  lo abuts against the elongated distribution channel  74  to form first distribution channel  36 . 
     Channels  38  and  40  abut against each other. Because the channels  38 ,  40  are inclined in the same direction in the plates  50 ,  66 , when one plate is flipped over to abut the two inner surfaces  48 ,  64  of the plates  50 ,  66 , the channels are then oriented in cross-directions to form the lattice region  26 . The distribution channel  61  of the plate  50  abuts against the distribution channel  76  of plate  66  to form the second distribution channel  44 . The indented flat region  62  of the plate  50  abuts against the indented flat region  78  of the plate  66  to form the flat region  28  with a constant thickness t. 
     An important aspect of the invention relates to the configuration of the flow channels  38 ,  40  and the flat region  28  after the gate system  24  is assembled. The flow channels  38 ,  40  are disposed to intersect each other at an angle of about 60° to about 140° with respect to each other, preferably about 90° to about 120° with respect to each other. Also, portions of the flow channels  38 ,  40  of plate  50  and plate  66 , respectively, are partially formed, typically with a half circle or an elliptical shape, resulting from being abutted against the flat inner surface  48 ,  64  of the opposing plates  50  or  66 . The remaining portions of the flow channels  38 ,  40  are formed at the intersections  42  of the flow channels and are illustrated in FIG. 2A as having an elliptical shape. The lattice portion  26  of the inventive gate system  24  effectively creates more physical mixing, rubber-to-rubber shear heating, and thermal mixing than in the flat design portion  28  of the gate system  24 . 
     The flat region  28  provides for a preferred orientation of the fiber in the rubber compound  22 . Due to the flow of the rubber  22  through the lattice channels  38 ,  40 , the fibers in the compound exiting the channels  38 ,  40  and entering the second distribution channel  44  have an orientation corresponding to the channel inclination angle, relative to the centerline  56  of the gate plates  50 ,  66 . In order to achieve a rubber flow, and thus fiber orientation, parallel to the centerline  56  of the gate plates  50 ,  66 , the flow direction of the rubber must be reoriented 60° to 20°, or 45° to 30° if the channels  38 ,  40  are at the preferred inclination angles. The necessary reorientation of the rubber and fibers is less than any required reorientation of the fibers for known flat gate designs. 
     After the plates  50  and  66  are assembled, the flat region  28  has a thickness t and a length l F  associated with it, see FIGS. 2 and 2A. Both the thickness t and the length l F  are optimized to allow for the reorientation of the fibers carried within the rubber  22  flowing through the flat region  28  of the gate system  24 . Because of the range of fiber length, the thickness t of the flat region  28  of the gate  24  must be comparably narrow with respect to the inlet ports known in the prior art in order that a majority of the fibers are aligned with the flow direction F of the rubber  22  (see also FIG.  8 ). Also, if the length l F  is too long, the rubber  22  may scorch or cure in the gate. If the length l F  is too short, then the fibers may not become fully oriented in the direction of flow F before entering the mold cavity  18 . Since the fibers in the rubber  22  are entering the second distribution gate requiring a reorientation of only 70° to 30°, the length l F  can be reduced from that known disclosed in WO 98/13185. As the rubber  22  flows the direction of flow F through the flat region  28  of the gate  24 , the fibers become oriented parallel to the centerline  56  of the sprue and gate plates  50 ,  66 . 
     After passing through the gate system  24  into the mold cavity  18 , the flow direction of the rubber  22  is altered. As illustrated in FIG. 8, at the gate exit  80 , located at the junction of the gate system  24  with the mold cavity  18 , the opening for the rubber  22  is significantly increased in the direction parallel to the thickness of the gate system  24 . The rubber compound  22  folds over onto itself, creating a series of planes  82  generally perpendicular to the initial direction F as the rubber  22  fills the mold cavity  18 . 
     The offset distance d between the gate exit  80  and the interior walls  84 ,  86  of the mold cavity  18  can also influence the orientation of the fibers. If the offset distance d between the gate exit  80  and the interior walls  84 ,  86  is too small, the rubber  22  may get hung up or temporarily attached to the nearest interior wall  84 ,  86 . While some rubber compounds  22  can be successfully run in some conditions where the offset distance d is equal to zero, generally the offset distance d should be greater than one-fourth of the mold cavity width w. However, in some particular applications, unless the offset distance d is between one-fourth and one-half the mold cavity width w, the type and number of folds necessary to achieve the desired fiber orientation may not occur. For more details regarding the various parameters of different applications, reference is made to the incorporated WO 98/13185. 
     Comparison Test 
     Comparisons between a flat gate and the inventive combination gate  24  were prepared. Samples of a Keviar pulp loaded rubber compound were prepared using both a flat gate and the inventive combination gate  24 . The combination gate  24  had a lattice entrance structure  30  of 45/20/0.031″/0.51″ (channel angle/number of channels/channel radius/length) and a flat gate exit structure  32  of 0.010″/0.5″ (thickness/length). The flat gate had a structure of 0.010″/1.0″ (thickness/length). The barrel temperatures, mold temperatures, and injection speed for both samples. Both 5″×5″×⅞″ block and 5″×5″×{fraction (1/10)}″ sheet samples were prepared. 
     Test Sample 1 
     Five samples were taken from sheet samples 1.0″ from the gates. The five samples were circular samples, spaced across the width of the sheet sample. The x direction is the lateral direction of the sheet, and y is the injection direction of the sheet; the desired orientation is y. The following chart shows the results of the orientation of the fibers in the samples, in comparison to the sample location. The solvent swell ratios given in the table are average of three samples and obtained by dividing the length in the y direction by the length in the x direction. The swelling ratio is defined as a short axis divided by a long axis when a circular fiber loaded rubber sample is swelled into an oval shape in toluene to equilibrium state. The short axis direction is parallel to the fiber orientation direction. Since fibers were oriented in the short axis, the orientation direction is x if the swell ratio is greater than 1.0 and y if the swell ratio is less than 1.0. The smaller the swelling ratio, the higher the degree of fiber orientation. 
     
       
         
               
               
               
             
               
               
               
               
               
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Sample 
                 Combination Gate 
                 Flat Gate 
               
             
          
           
               
                 Location 
                 Average 
                 Std Dev 
                 Direction 
                 Average 
                 Std Dev 
                 Direction 
               
               
                   
               
               
                 left 
                 0.880 
                 0.024 
                 y 
                 0.918 
                 0.063 
                 y 
               
               
                 mid-left 
                 0.854 
                 0.021 
                 y 
                 1.057 
                 0.052 
                 x 
               
               
                 center 
                 0.901 
                 0.017 
                 y 
                 1.280 
                 0.096 
                 x 
               
               
                 mid-right 
                 0.868 
                 0.035 
                 y 
                 1.080 
                 0.043 
                 x 
               
               
                 right 
                 0.857 
                 0.029 
                 y 
                 0.950 
                 0.031 
                 y 
               
             
          
           
               
                 Overall 
                 0.872++/− 0.019 
                 1.057 +/− 0.142 
               
               
                   
               
             
          
         
       
     
     It can be seen that samples made with the combination gate  24  had relatively uniform fiber orientation. The solvent swell ratios ranged from 0.86 to 0.90. On the other hand, samples made with the flat gate, with solvent swell ratios of 0.918 to 1.280, had relatively strong orientation in the lateral direction (x) at the center. The orientation gradually changed to the injection direction (y) toward the sides of the sample, indicating non-uniformity in the fiber orientation. The standard deviations of solvent swell ratios are 0.019 and 0.142 for the combination and flat gates, respectively. The results of the sheet samples show that the combination gate  24  is much better in preparing injection molded parts with uniform fiber orientation and, thus, more uniform physical properties. 
     Test Sample 2 
     From block samples prepared by the combination gate  24  and a flat gate, a thin slice was cut at a location one inch from the gate and five samples were taken to test the fiber orientation in the thickness direction (z) of the block. The five samples were circular samples, spaced across the width of the thin slice. Three block samples prepared under identical conditions were used to obtain variation in solvent swell data. As noted above, the smaller the swelling ratio, the higher the degree of fiber orientation. 
     
       
         
               
               
               
             
               
               
               
               
               
               
               
             
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Sample 
                 Combination Gate 
                 Flat Gate 
               
             
          
           
               
                 Location 
                 Average 
                 Std Dev 
                 Direction 
                 Average 
                 Std Dev 
                 Direction 
               
               
                   
               
               
                 left 
                 0.675 
                 0.024 
                 z 
                 0.824 
                 0.063 
                 z 
               
               
                 mid-left 
                 0.679 
                 0.021 
                 z 
                 0.725 
                 0.052 
                 z 
               
               
                 center 
                 0.688 
                 0.017 
                 z 
                 0.779 
                 0.096 
                 z 
               
               
                 mid-right 
                 0.654 
                 0.035 
                 z 
                 0.748 
                 0.043 
                 z 
               
               
                 right 
                 0.691 
                 0.029 
                 z 
                 0.714 
                 0.031 
                 z 
               
             
          
           
               
                 Overall 
                 0.678 +/− 0.015 
                 0.758 +/− 0.044 
               
               
                   
               
             
          
         
       
     
     The results show that the block samples produced from the combination gate  24  had a high degree of fiber orientation in the thickness direction. The average solvent swell ratios are 0.678 and 0.758 for the combination and flat gates, respectively. The standard deviation of the solvent swell ratios for the combination gate is also lower, 0.015 vs. 0.044, indicating more uniform fiber orientation within the sample. 
     The illustrated gate has a lattice region  26  and a flat region  28  that are of substantially the same length l L , l F . While this is the preferred length ratio of the lattice region  26  and the flat region  28 , the ratio of the lattice region  26  to the flat region  28  may vary from 2:1 to 1:2 and still achieve the desired high degree of fiber orientation in the rubber exiting the gate system  24 . 
     The fibers in the rubber  22  injected into the combination gate system  24  may be any conventional fiber used in manufacturing fiber reinforced rubber articles. This includes short fibers have a length ranging from 0.1 microns to 10 3  microns and fibers have a length up to and including 0.5 inch (1.2 cm). To properly orient fibers of the longer lengths, the actual length of the gate  24 , the diameter of the flow channels  38 ,  40 , and the thickness t of the flat region  28  may be increased to achieve the necessary mixing and reorientation discussed above. 
     Additionally, while the disclosed invention illustrates a closed cavity mold, it will be appreciated by those skilled in the art that the mold may be an open ended mold. In such a mold, the defined relationships between the gate exit  80  and mold walls  84 ,  86  remain as described above; however, there is no end wall to limit the movement of the uncured rubber through the mold. Uncured rubber flow through the gate exit  80 , forming the folding planes  82 , and continues through the cavity to form a continuous strip of rubber defined by folded planes  82  creating a rubber with oriented fibers. 
     The inventive lattice/flat combination gate  24  offers both the advantages of the lattice and flat gates and an unexpected benefit of a higher degree of fiber orientation in the thickness direction and more uniform fiber orientation distribution. The achieved higher degree of orientation can not be achieved by using only the lattice gate or only the flat gate; nor would such a eater degree of orientation be expected by the mere combination of the two gate designs.