Abstract:
The present invention is directed towards the hybrid combination of a drop gate injection molding system and a lattice gate. The lattice gate is located prior to the orifice of the drop gate. The lattice gate is comprised of a series of cross-direction flow channels through which the rubber flows, with mixing of the rubbers at the intersections of the flow channels. This hybrid molding system permits shorter cure times, and improved process cycle time for curing rubbers.

Description:
This application is a Continuation-in-Part of U.S. patent application Ser. No. 09/445,522, having a §102(e) date of Dec. 7, 1999, now U.S. Pat. No. 6,379,603, issued on Apr. 30, 2002, which was based upon PCT application PCT/US97/09916, filed Jun. 9, 1997. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to an improved gate design for drop gate injection molding of rubber compounds to form rubber articles. More particularly, the present invention relates to the unexpected combination of a drop gate with a lattice gate for increased gate heating efficiency and reduced cycle time while the rubber part is being injection molded. 
     BACKGROUND OF THE INVENTION 
     In a typical elastomer injection molding process, uncured viscous elastomeric compound is introduced into an elongated barrel of an injection molding machine at ambient temperatures. The compound is advanced through the barrel towards a mold connected to the downstream end of the barrel, usually by either a rotating screw conveyor or a reciprocating ram or piston disposed in the barrel. As the elastomeric compound advances, it is heated by heat conduction and mechanical shear heating in the barrel to reduce its viscosity and render the elastomer more flowable and amenable to subsequent injection into the mold. Typically, the less viscous the compound, the more easily it flows through a conventional gate system and the more easily it fills a mold cavity to produce a satisfactorily molded object. 
     One type of conventional gate for injection molded products is a “drop gate” design. This type of gate is used when a side injection system is not feasible depending upon the mold design used for certain injection molded products. With a drop gate, the elastomer is forced through a small diameter orifice and into the molded part. The majority of the shear heating occurs at the orifice area of the gate. FIG. 10 is a schematic drawing of a drop gate design  100  used to mold an article. A sprue pad  102  feeds elastomer to drop gate runners  104 . From the runners  104 , the elastomer flows through the drop gate orifices  106 , which have a smaller diameter than the drop gate runners  104 , and fills the mold cavity  108 . 
     WO 98/56559 discloses another gate design for injection molding rubber compounds. The gate design is a lattice gate. The lattice gate 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 teaches replacement of the conventional flat gate of the prior art with the inventive lattice gate. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an improved method and apparatus for a drop injection gate for molding rubber. The inventive method reduces curing time and improves the heat characteristics of the rubber as it is injected. 
     One aspect of the invention is an improved method of drop gate injection molding rubber. The method comprising injecting a rubber into a drop gate and through drop gate runners and into a mold cavity. The rubber flows at cross angles after the rubber is injected into the gate and before the rubber enters into the drop gate runners. 
     Another aspect of the invention is an improved drop injection gate for injection molding rubber into a mold cavity. The gate is comprised of drop gate runners. The gate has a region adjacent to the drop gate runners comprising a first and a second plurality of spaced flow channels disposed at intersecting angles to each other to create cross direction flow of the rubber before the rubber enters into the drop gate runners. 
     In another aspect of the invention, the drop injection gate has more than one separate region adjacent to drop gate runners comprising the first and second plurality of spaced flow channels disposed at intersecting angles to each other. 
    
    
     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 schematic representation of the inventive hybrid gate system; 
     FIG. 2 is an overhead view of the bottom plate of the lattice portion of the hybrid gate system; 
     FIG. 3 is a perspective view of the bottom plate; 
     FIG. 4 is an overhead view of the top plate of the lattice portion of the hybrid gate system; 
     FIG. 5 is the gate after the top and bottom plates have been assembled; 
     FIGS. 6 and 7 are section views of the gate along lines  6 — 6  and  7 — 7  of FIG. 5, respectively; 
     FIG. 8 is another embodiment of the bottom plate; 
     FIG. 9 is another embodiment of the bottom plate; 
     FIG. 10 is schematic representation of a conventional drop gate design used to mold an article. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is related to the design of an improved gate system for improving mixing and temperature uniformity and decreasing mold times in a drop gate mold injection system. The improved gate system is the unexpected combination of a lattice gate and a drop gate. FIG. 1 is a schematic representation of the inventive hybrid gate system  2 . The hybrid gate system  2  incorporates a lattice gate  4  after the sprue inlet  6 . Elastomeric material flows into the hybrid gate  2  through the sprue inlet  6 , and then flows through the lattice gate  4 . An outlet distribution channel  8  from the lattice gate  4  feeds the elastomer from the lattice gate  4  to the drop gate runners  10 . From the runners  10 , the elastomer flows through the drop gate orifices  12  and fills the mold cavity  14 . 
     The plates comprising the lattice gate  4  are illustrated in FIGS. 2 to  7 . FIGS. 2 and 3 illustrate the bottom plate  16 , also known as the gate plate. Offset from the center of the plate  16  is a distribution area  18 . Adjacent to the distribution area  18  is a plurality of flow channels  20 . At the opposing end of the flow channels  20  is a U-shaped outlet distribution channel  8 . The U-shaped outlet distribution channel  8  has a central portion  22  coincident with the ends of the flow channels  20 . At the terminal ends  24  of the side portions  26  of the outlet distribution channel  8  are ports  28  to the drop gate runners  10 . 
     The top plate  30 , also known as the sprue plate, is illustrated in FIG.  4 . Centrally located in the top plate  30  is the sprue bore  32 . Adjacent to the sprue bore  32  is a plurality of flow channels  34 . 
     When the flow channels  20 ,  34  of the top and bottom plates  16 ,  30  are formed into the respective plates  16 ,  30 , as seen from overhead, the inclination direction of the channels  20 ,  34  is identical. The plurality of flow channels  20 ,  34  formed in the plates  16 ,  30  are parallel to each other and inclined at angles β of about 30° to about 70°, preferably at angles of about 45° to about 60°, with respect to a centerline  36 . As the angle β of the parallel flow channels  16 ,  30  increases with respect to the centerline  36  of each plate  16 ,  30 , the time required for any elastomeric compound to flow through the channels  20 ,  34  also increases, and vice versa. The flow channels  20 ,  34  are illustrated with a semi-circular cross-section; however, it is within this invention to form the flow channels  20 ,  34  with other cross-sections, such as elliptical, triangular, or square as desired. 
     FIGS. 5,  6 , and  7  illustrate the lattice gate  4  after the top and bottom plates  16 ,  30  have been assembled. The sprue inlet bore  32  of the top plate  30  is aligned along a same axis with the drop gate ports  28 , while being offset from the center of the distribution area  18  of the bottom plate  16 . Also, the plurality of flow channels  20 ,  34  are coincident with the edge of the distribution area  18  at one end, while the opposing end of the flow channels  20 ,  34  are coincident with the central portion  22  of the outlet distribution channel  8 . As the elastomeric compound flows into the lattice gate  4  through the sprue bore  32 , the compound fills the distribution area  18  due to the injection pressure. The compound then flows into the cross directional flow channels  20 ,  34 , exiting the flow channels  20 ,  34  into the central portion  22  of the outlet distribution channel  8 . The injection pressure of the elastomer forces the compound to then flow through the side portions  26  of the outlet distribution channel  8  and through the drop gate runner ports  28 . 
     For the illustrated lattice gate configuration, spacing must be maintained between the flow channels  20 ,  34  and the side portions  26  of the outlet distribution channel  8 . If this spacing is insufficient, the elastomeric compound may not travel completely through the lattice gate  4 , but it may enter the distribution channel  8  prior to complete mixing of the compound through the flow channels  20 ,  34 . This spacing can be seen in FIG.  6 . 
     In the illustrated lattice gate configuration, the distribution area  18  has a predominantly circular configuration and depth greater than the flow channels  20 ,  34  so that the injection pressure may be used to exert pressure on an essential component about which the elastomeric compound is to be molded; e.g. a central steel tube in a molded motor mount. The illustrated distribution area  18  is specific to a particular application. The shape of the distribution area  18  is designed to meet the needs of each specific molding application. For example, the distribution area  18  of a bottom plate  38  may be configured as a single portion flow channel  18 ′ connecting the sprue bore of the top plate to the flow channels  20 ,  34 ; such a variation is illustrated in FIG.  8 . 
     Additionally, the outlet distribution channel  8  may have a configuration other than a U-shape. A U-shape outlet distribution channel addresses the need to have two spaced drop gate ports  28  feeding two drop gate runners  10 . If the specific molding application or injection machine permits, the outlet distribution channel  8  may have another configuration, such as a straight line flow channel  8 ′ with a central drop gate port  40 , as illustrated in FIG.  8 . 
     If the production of the molded article requires more than the illustrated two drop gate runners  10 , the shape of the outlet distribution channel  8  may be further modified to accommodate more drop gate orifices. Another alternative for an injection molding apparatus employing plural drop gate runners  10  is to form the top and bottom plates with multiple sprue inlets and lattice gate portions, FIG. 9 illustrates a configuration that may be employed for the bottom plate  42 . The hybrid injection gate  2  has two separate lattice gates  44 . Each lattice gate has a distribution area  46 , flow channels  48 , a distribution channel  50 , and drop gate ports  52 . Such a hybrid gate  2  permits the use of four drop gate runners  10  to feed to mold cavity  14 . 
     The use of the lattice gate in combination with the drop gate, i.e. the inventive hybrid gate  2 , results in a faster cycle time for molding articles compared to the conventional drop gate system  100 . This is illustrated in the comparison tests set forth below. 
     Comparative Cure Time Tests 
     40-ton vertical injection molder was used for the test. The mold employed was a production engine mount mold with exchangeable top plates. The first set of top plates had a drop gate design delivery system only (schematically illustrated in FIG.  10 ). The second set of top plates had the lattice gate followed by an identical drop gate delivery system (schematically illustrated in FIG.  1 ). The drop gate design delivery system had a 0.229 cm (0.090″) injection orifice. The lattice gate had a structure of 45/5/0.099 cm/3.175 cm (channel angle/number of channels/channel radius/length) (45/5/0.039″/1.25″), and had a construction similar to the lattice gate  4  illustrated in FIG.  5 . For all of the runs, the mold temperature was 154° to 163° C. (310° to 325° F.), depending upon the location within the mold. The rubber temperature at the time of injection was 127° to 130° C. (260° to 265° F.). The top plates with the lattice gate design were not treated with any release coatings. 
     Engine mount bushings were injected on the 40-ton injector using bushing metals without adhesive to permit easy removal of the metal insert from the molded rubber. To determine the extent of cure, the rubber was cut open at the point of least cure. The size of the uncured region at the point of least cure gives a relative state of cure. The bushing was considered sufficiently cured if there was no sign of uncured or doughy rubber. While many of the parts that had uncured rubber inside immediately after injection would eventually have cured completely from the latent heat within the bushing, the intent of the comparative tests was to establish a baseline for comparison of the two gate designs. Thus, all comparisons were done by cutting the part open immediately after removal from the mold. 
     The minimum cure time for each type of gate was determined by reducing the cure time in successive steps until such time at which uncured regions were seen in the part. The same elastomeric compound was used for all the trials. The compound used was a standard commercial product. 
     
       
         
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
               
               
                 Comparison Cure Times 
               
             
          
           
               
                   
                   
                   
                 Cure 
                   
                   
                   
               
               
                   
                   
                 Injection 
                 Time 
                 Total 
                 Com- 
               
               
                 Part 
                 Gate 
                 time 
                 (sec- 
                 Cycle 
                 pletely 
               
               
                 No. 
                 Type 
                 (seconds) 
                 onds) 
                 time 
                 Cured? 
                 Comments 
               
               
                   
               
             
          
           
               
                 1 
                 Hybrid 
                 18 
                 270 
                 298 
                 Yes 
                 Fully cured 
               
               
                 2 
                 Hybrid 
                 18 
                 240 
                 258 
                 Yes 
                 Fully cured 
               
               
                 3 
                 Hybrid 
                 18 
                 180 
                 198 
                 Yes 
                 Fully cured 
               
               
                 4 
                 Hybrid 
                 18 
                 120 
                 138 
                 Yes 
                 Fully cured 
               
               
                 5 
                 Hybrid 
                 18 
                  60 
                  78 
                 No 
                 Part looks fully 
               
               
                   
                   
                   
                   
                   
                   
                 cured, point of least 
               
               
                   
                   
                   
                   
                   
                   
                 cure occurring at 
               
               
                   
                   
                   
                   
                   
                   
                 center metal insert 
               
               
                 6 
                 drop 
                 5 
                 360 
                 365 
                 Yes 
                 ˜½″-¾ − D glossy 
               
               
                   
                   
                   
                   
                   
                   
                 spot but no dough 
               
               
                 7 
                 drop 
                 6 
                 300 
                 306 
                 No 
                 Process 
               
               
                   
                   
                   
                   
                   
                   
                 conditioning part, 
               
               
                   
                   
                   
                   
                   
                   
                 but had ˜1″ × ¾″ 
               
               
                   
                   
                   
                   
                   
                   
                 uncured area 
               
               
                   
               
             
          
         
       
     
     Observations from Trials 
     In Part No. 5, the bushing was almost completely cured at 60 seconds, but rubber porosity occurred at the interface between the smaller metal insert and the rubber. The metal insert acted as a heat sink, cooling the rubber at its surface and slowing the cure rate. A conservative estimate for Part No. 5&#39;s minimum cure time can be estimated at 90 seconds, this being extrapolated from the 60 second and 120 second cure times although the actual minimum cure time could be closer to 60 seconds. 
     In Part Nos. 1 to 5, there was no significant flashing, i.e., all of the rubber traveled through the injection gate prior to entry through the drop gate runner orifice. It was noted that for Part Nos 1 to 5 the injection time may be reduced by the use of release coatings on the lattice gate plates. 
     The use of the lattice gate in combination with the drop gate reduced the part cure time from 360 seconds to 90 seconds (see above discussion), the total cycle time for the part from 365 to 108 seconds. This is a 75% reduction in cure time and a reduction of over 70% in the total cycle time. Even if the minimum cure time for Part No. 5 is not extrapolated out to 90 seconds and the cure time of Part No. 4 is used instead to determine the cure time improvement, the cure time for Part No. 4 is reduced by 66%, and total cycle time is reduced by over 60%. 
     As discussed previously, the majority of rubber shear heating occurs at the orifice area of the gate. By incorporating a lattice gate prior to the drop gate, the hybrid injection gate is more efficient in heating the rubber prior to the rubber entering the mold cavity. This can be clearly seen by the substantially reduced cure times of the trial runs. 
     Using the hybrid concept permits the incorporation of the lattice gate into the gate design where the known plane of entry lattice gate design alone could not be used. The flow from the lattice gate is instead channeled through outlet distribution channels acting as mold runners to move the rubber to the drop gate and into the mold cavity. 
     The present invention of incorporation of a lattice gate with a drop gate is capable of reducing cure times required to mold parts. Such reduced cure times are especially beneficial for molded parts with very thick cross-sections. The mixing of the molding compound as it flows through the channeled entry lattice gate also improves the mixing and temperature uniformity of the molding compound prior to entry of the compound into the mold. 
     While the invention has been described in combination with embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing teachings. Accordingly, the invention is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and scope of the appended claims.