Abstract:
The present invention is directed to the structure of an inline runner for injection molding which uniformly increases the temperature of the injected compound as the compound flows from the injection port to single or multiple drop gates at the entrance to single or multiple injection mold cavities. The inline runner has two opposing series of parallel channels formed into the runner for the purpose of mixing the compound as it flows through the runner.

Description:
RELATED APPLICATIONS 
     The present application is a divisional of patent application Ser. No. 09/659,012, filed Sep. 8, 2000, now abandon. 
    
    
     FIELD OF THE INVENTION 
     The present invention is directed towards the structure of an inline runner of an injection molding machine. The inline runner has opposing series of parallel grooves to provide for mixing of an injected compound as it flows through the runner, uniformly increasing the compound temperature. 
     BACKGROUND OF THE INVENTION 
     Traditionally, hot runners have been used in injection molding to distribute rubber from top or side injection ports to small injection gates located at the entrance to single or multiple injection mold cavities. A traditional hot runner is designed to minimize pressure losses through the runner so that the maximum pressure drop and corresponding temperature increase occurs at the drop gate entrances to an injection mold cavity. Rubber flow through a traditional hot runner design is laminar because the high viscosity of the rubber prevents turbulence. Rubber is instead heated as it flows through the runner by conduction and by shear heating at the boundary layer between the rubber flow profile and the surface of the metal mold. Thus the outside layer of rubber in contact with the mold surface is heated, but the rubber flowing through the center of the runner is not heated uniformly by the mold during the short time that a part is being filled. 
     Attempts have been made to increase the mixing of a fluid material prior to injection into a mold. U.S. Pat. No. 5,262,119 discloses mixing thermoplastic material by placing static mixers in the flow channels leading to a mold. The static mixers act to disperse degraded material or the wrong color material back into the stream for purposes of uniformity. The static mixer is comprised of a fixed twisted metal blade. U.S. Pat. No. 5,688,462 discloses static mixers as part of the cold runners leading into a mold block. The static mixers are a series of baffles, resembling bent fingers, in the runner about which the thermoplastic must flow. 
     U.S. Pat. No. 3,924,989 discloses an after-mixer in an apparatus for molding thermoset urethane materials. After the components of the material are combined in a mixing chamber, the material is sent into an after-mixer to ensure complete mixing of the components. One disclosed after-mixer is defined by a series of cross channels, the cross-sectional width of the channels increasing and decreasing. 
     U.S. Pat. No. 4,027,857 discloses a static mixing nozzle for injection molding thermoplastic. The disclosed four way branch static mixer is employed for improved blending of materials. 
     None of the disclosed after mixers are suitable for rubber molding, nor do any of the teachings appreciate the use of such mixers in thermoelastic or thermoset rubber molding. In the thermoplastic injection molding methods and apparatus disclosed in the prior art, it is a goal to achieve greater color and compound uniformity in the mixing of the thermoplastic or thermoset urethane flow. Heating of the flow stream to its highest optimum temperature is achieved prior to the introduction of the flow stream into the mixer or after-mixer, and to achieve a molded product, the flow stream is cooled after it is injected into the mold to form a solid article. Thus, while it is desired to maintain the flow stream at a defined temperature, it is not a desire of the prior art to increase the temperature of the flow stream as the flow stream travels toward the mold as greater cooling in the mold would be then required if the flow stream entered the mold at a higher temperature. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a method and an assembly for injection molding thermoelastic and thermoset rubbers. The disclosed invention provides for quicker, more efficient, and more uniform heating of the elastomer prior to entering the mold cavity. 
     The disclosed assembly for injection molding rubber is comprised of a mold cavity, a gate located immediately adjacent to the mold cavity, and a hot runner in communication with the gate. A portion of the hot runner has a first and a second plurality of spaced flow channels disposed at intersecting angles to each other to create cross directional flow of the rubber. The cross directional flow of the rubber provides for heating of the rubber as it flows through the non-linear, non-planar path created for mixing of the rubber along the channels and at the junctions of the cross directional flows; the rubber streams thermally mixing and uniformly increasing the temperature of the rubber. 
     In one aspect of the invention, the first and second plurality of spaced flow channels are located in a distribution runner adjacent to the gate. 
     In another aspect of the invention, the first and second plurality of flow channels are inclined at angles of 15° to 70° relative to the centerline of the runner, preferably 30° to 60° relative to the centerline of the runner. 
     In another aspect of the invention, the first and second plurality of flow channels have a cross-sectional configuration selection from among the following shapes of semicircular, elliptical, triangular, trapezoidal, square, polygonal, and curvilinear. 
     The disclosed method of producing a molded rubber article comprises injecting a rubber into a hot runner, passing the rubber through a mold gate and into a mold cavity. The process is characterized by the rubber flowing at cross angles after the rubber is injected into the runner and before the rubber enters into the mold gate to uniformly increase the temperature of the rubber as it flows through the runner. 
     In one aspect of the disclosed method, the runner is a branched distribution runner leading to a plurality of molds. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described by way of example and with reference to the accompanying drawings in which: 
     FIG. 1 is a mold with an insert modified in accordance with the invention; 
     FIG. 2 is a modified runner plate; 
     FIG. 3 is a section view of the modified runner plate along line  3 — 3 ; 
     FIGS. 4-7 are embodiments of runners in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is an overview of a mold plate  10 . The plate  10  has an injection runner  12  leading to at least one drop gate  14 . The runner  12  illustrated is a branched runner comprising a main runner  16  and a set of branched distribution runners  18 . At the end of each branched distribution runner  18  is a drop gate  14  leading to a mold  20 . While a branched runner  12  is illustrated, the present invention may also be used in a system wherein the runner  12  leads directly to a single drop gate or injection port. 
     In accordance with the present invention, at least a portion  22  of the injection runner  12  is modified for mixing of the material flowing through the runner  12  and into the mold  20  by creating non-linear directional flow of the rubber as it flows through the runner  12 . It may be the main runner  16  that is modified, as illustrated, or the branched distribution runners  18  may be modified. In whatever location selected, the modified portion  22  of the runner  12  will be located at a parting surface of the mold  10  for ease of cleaning and removal. The modified portion  22  of the runner  12  is schematically illustrated, as FIG. 2 illustrates the actual modified runner portion  22 . 
     FIG. 2 illustrates a runner plate  24  forming either the top or the bottom of the modified portion  22  of the runner  12 . Instead of a single bore defining the runner  12 , the runner plate  24  has a plurality of inclined flow channels  26 . The channels  26  are inclined at an angle β of 15° to 70° relative to the centerline  28  of the runner  12 , preferably 30° to 60°. As the angle β of the channels  26  increases with respect to the centerline  28 , the time required for any viscous compound to flow through the channels  26  also increases, and vice versa. The channels are illustrated with a semi-circular cross-section having a radius R, see FIG. 3; however, it is within this invention to form the flow channels with other cross-sections, such as elliptical, triangular, trapezoidal, square, or any other polygonal or curvilinear shape. The end points  27  of the channels  26  at the end of the modified runner portion  22  are located on the centerline  28  to be aligned with the remainder of the runner  12 . While FIG. 2 illustrates only four complete cross channels  26 , the number of cross channels  26  in either the top or bottom mold plate  24  is selected based upon the desired thermal mixing and temperature increase of the rubber as it flows through the modified runner portion  22 . 
     Both the top and the bottom plate  24  forming the modified portion  22  of the runner  12  are formed in an identical configuration, such that when the top plate is positioned over the bottom plate, the flow channels  26  are inclined in opposing directions and form a series of junctions  32  along the length of the modified portion  22  of the runner  12 . 
     FIGS. 4 to  7  illustrates a compound as it would appear as it flows through various embodiments of the modified runner  22 , excluding the end portion of the rubber as it flows through the modified runner portion  22 , it being understood that the crossed ends will terminate at the centerline  28  of the runner portion  22  as shown in FIG.  2 . The compound  30  alternatively flows through the channels  26  formed in a top plate of the mold  10  or through the channels  26  formed in a bottom plate of the mold  10 . The flowing compound meets and mixes at the various junctions  32  spaced along the length of the runner. At the end of each channel  26 , the rubber flow changes direction when it enters the opposing channel. The energy input from the flow redirection also increases the rubber temperature. 
     In FIG. 4, the channels  26  form one of the simplest configuration in accordance with the invention. Junctions  32  are formed between the bottom plate channels  34  and the top plate channels  36  along the lengths of the channel. At each junction  32 , the compound flowing from both the upper and lower channels meet and mix, transferring heat absorbed by the mold and increasing the temperature of the compound  30 . For the illustrated embodiment, the thirteen junctions  32  occur along the runner centerline  28 . The channels  26  are inclined at an angle of about 30° relative to the runner centerline  28 . 
     The radius R of the channels  26  may be selected so that the cross sectional area created by the channels  26  is equivalent to the cross sectional area of a conventional single bore runner. Alternatively, the total cross sectional area of the channels may be greater than that of a conventional single bore runner. In such a construction, a larger volumetric flow rate of rubber through the gate is realized; however, the increase in rubber temperature as it flows through the modified runner will be reduced in comparison to a modified runner with an equivalent cross sectional area. The total cross sectional area may also be selected to be less than that of a conventional single bore runner. In such a construction, a greater temperature increase is realized, however, the volumetric flow rate through the runner is decreased in comparison to a conventional runner. 
     In FIG. 5, the same inclination angle β is used, however, instead of a single channel defining the end location  38  of each modified mold half, a pair of channels  26 ′,  26 ″ are located at the end location  38  of each modified mold half The increase in the number of channels  26  at the end location  38  results in an increased width of the modified runner portion  22  and more junctions  32  along the runner length; there being a total of 40 junctions spaced along the length of the illustrated runner. As discussed above, the total cross sectional area of the channels  26  is optimized to achieve the desired increase in rubber temperature and volume of rubber flow through the runner. 
     In FIG. 6, the channels  26  are inclined at an angle of about 15° relative to the runner centerline  28 . By forming the channels  26  with a decreased inclination angle, the runner  22  has fewer junctions  32 , and rubber flows through the runner in a shorter distance. FIG. 7 illustrates channels  26  formed at the same angle as the channels  26  of FIG. 6, but with two channels  26 ′,  26 ″ at each end location of each modified mold half, resulting in a greater number of junctions. 
     Alternatively, the top and bottom mold plates  24  may be fixed into the mold  10  such that the channels  26  do not intersect to form junctions  32 , but instead only connect at the end portions to form cross-directional flow patterns resembling a zig-zag pattern through the modified portion of the runner. 
     The modified portion  22  of the runner  12  mixes the rubber flow boundary layer into the flow profile of the rubber at each junction  32  of the channels  26 . Through use of the modified runner  12 , heat absorbed by the rubber boundary layer at the mold surface is more efficiently transferred into the center of the rubber flow profile. As a result, the average temperature of the rubber entering the mold at the drop gates is increased and is more uniform. Thus, cure times are decreased for injection molded parts without a major increase in injection time and without rubber scorch at the drop gate entrance. 
     The diameter of a typical gate entrance can be increased at the drop gate entrance because part of the rubber heating currently accomplished by the drop gate will now be accomplished by the modified runner. The modified runner provides shorter cure and cycle times than currently possible by present rubber injection and drop gate designs. 
     Additionally, the present invention can be used to retro fit older injection systems with a minimal infusion of capital since the single bore runners can be machined out of the mold plate and replaced with modified runner plates in accordance with the present invention. 
     Variations in the present invention are possible in light of the description of it provided herein. While certain representative embodiments and details have been shown for the purpose of illustrating the subject invention, it will be apparent to those skilled in this art that various changes and modifications can be made therein without departing from the scope of the subject invention. It is therefore, to be understood that changes can be made in the particular embodiments described which will be within the full intended scope of the invention as defined by the following appended claims.