Abstract:
A process for resin transfer molding (RTM) with staggered injection of a resin is provided that injects resin into a plurality of injection ports of a mold. The temperature and pressure applied to the mold are controlled during injection to limit promote rapid filling of the mold cavity. The injection ports are activated for injecting the resin in any order of individually, in groups, or pairings. Fibers are readily added to the mold separately or within the resin. Cycle times of from 1 to 5 minutes are provided for the process.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims priority benefit of U.S. Provisional Application Ser. No. 61/910,974 filed 3 Dec. 2013; the contents of which are hereby incorporated by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention in general relates to resin transfer molding and in particular to an improved resin transfer molding process with a staggered injection of resin by time and position in a multipoint injection system. 
       BACKGROUND OF THE INVENTION 
       [0003]    Resin transfer molding (RTM) is a method of fabricating composite structures with high strength, complex geometries, tight dimensional tolerances, and part quality typically required for automotive and aerospace applications. RTM uses a closed mold commonly made of aluminum with a fiber “lay-up” such as graphite placed into the mold. The mold is closed, sealed, heated, and placed under vacuum. A room temperature (20° C.) or heated resin is then injected into the mold to impregnate the fiber layup. Having the mold heated and under vacuum assists the resin flow. The mold is then held at a temperature sufficient to cure the resin. Typically, a resin experiences three distinct states during the RTM process; a viscous state during injection, a jelling state during which the viscosity of the resin changes to a higher viscosity, and a cure time when the resin materials chemically crosslink and the resin hardens. Molds used for RTM have one or more injection ports for introducing the resin, and at least one exhaust port for gas and excess resin to leave the mold. Multiport injection molds are typically used for larger parts that have an increased processing time. 
         [0004]    A common problem encountered during the RTM process is a non-uniform flow of the resin, where the resin experiences a change in viscosity as the resin travels away from the injection point. Furthermore, as shown in the graph of  FIG. 1  as the time of processing progresses the pressure and temperature experienced by the resin decreases as a function of time. The aforementioned problems experienced during the RTM process contribute to defects in formed parts. 
         [0005]    Thus, there exists a need for an improved RTM multiport injection process that minimizes both production defects and cycle time. 
       SUMMARY OF THE INVENTION 
       [0006]    A process for resin transfer molding (RTM) with staggered injection of a resin is provided that injects resin into a plurality of injection ports of a mold. The temperature and pressure applied to the mold are controlled during injection to limit promote rapid filling of the mold cavity. The injection ports are activated for injecting the resin in any order of individually, in groups, or pairings. Fibers are readily added to the mold separately or within the resin. Cycle times of from 1 to 5 minutes are provided for the process. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a graph showing the typical response curve as the time of processing progresses the pressure and temperature experienced by the resin decreases as a function of time; a 
           [0008]      FIG. 2  is a graph showing the response curve of pressure and temperature as a function of time for resin with staggered injection in a multiport RTM mold according to an embodiment of the inventive process; 
           [0009]      FIG. 3  is a plan view of a multiport RTM mold with four injection ports for staggered resin injection according to embodiments of the invention; 
           [0010]      FIG. 4  is a plan view of a multiport RTM mold with six injection ports for staggered resin injection according to embodiments of the invention; and 
           [0011]      FIG. 5  is a flowchart of a production process of the staggered multiport RTM process according to embodiments of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0012]    The present invention has utility as an improved resin transfer molding (RTM) process with a staggered injection of resin by time and position in a multipoint injection system. The staggered introduction of resin in a mold provides for an improved pressure and temperature profile versus time for the resin used for forming parts versus existing RTM processes that simultaneously inject resin. The staggered introduction of resin in embodiments of the inventive process provide for improved product yields with fewer formed part defects resulting from non-impregnated portions, thereby improving the quality of the molded product. Furthermore, product cycle time is also reduced by staggering the injection of resin in a multiport mold versus the existing process of simultaneous injection of resin in a multiport mold. It has been observed that cycle time for forming parts have been reduced from approximately 10-60 minutes to three to five minutes with the inventive process of staggering the injection of resin in a multiport mold. 
         [0013]    Resins used in embodiments of the inventive staggered RTM process include thermosetting resins such as epoxy; urethanes; polyesters, and vinylesters; that are low in viscosity and easy to be impregnated into reinforcing fibers. These resins illustratively include epoxy resin, an unsaturated polyester resin, a polyvinylester resin, a phenolic resin, a guanamine resin, a polyimide resin such as bismaleimide triazine resin, a furan resin, a polyurethane resin, a polydiarylphthalate resin, a melamine resin, a urea resin, an amino resin, etc. Fibers may be introduced to the resin used in certain embodiments of the inventive staggered RTM process to strengthen formed parts including glass, carbon, and other synthetic fibers, as well as natural fibers. Natural fibers may include coconut fibers, bamboo fibers, sugar cane fibers, banana skin fibers, etc. 
         [0014]    Referring now to  FIG. 2 , a graph  20  showing the response curve of pressure and temperature as a function of time for resin with staggered injection in a multiport RTM mold according to an embodiment of the inventive process. As can be seen as compared to the graph  10  of  FIG. 1  for a simultaneous injected part, the rapid decline of pressure and temperature during the processing of a molded part in the mold is avoided compared to conventional RTM process, and the temperature and pressure is maintained during the RTM process of embodiments of the invention to increase cycle time of molding a part. 
         [0015]      FIG. 3  is a plan view of a multiport RTM system  30  with a mold  32  with four injection ports  34 A- 34 C for staggered resin injection and an exhaust port  36  according to embodiments of the invention. In embodiments of the staggered RTM process the injection ports may be activated in any order both individually, and in groups, or pairings. For example, injection ports  34 A and  34 B may be turned on as a pair and alternated with injection ports  34 C and  34 D as a second pair, where the adjacent pairings ( 34 A- 34 B,  34 C- 34 D) inject resin at alternating times. In certain embodiments, cross-pairings of injection ports ( 34 A- 34 D) and ( 34 B- 34 C) may inject resin at alternating times. In certain embodiments, the injection ports  34 A,  34 B,  34 C, and  34 D are each individually turned on and off in sequential clockwise or counter clockwise order. While four such ports  34 A- 34 D are shown it is appreciated that the number of injection ports ranges from 1 to 10 in various inventive embodiments, while in other embodiments, from 2 to 6 ports and in still other embodiments 2 or 3 ports are used. 
         [0016]      FIG. 4  is a plan view of a multiport RTM system  40  with a mold  42  with six injection ports  44 A- 44 F for staggered resin injection and an exhaust port  46  according to embodiments of the invention. As was discussed with respect to the inventive embodiment of  FIG. 3 , the injection ports  44 A- 44 F may be activated in any order both individually, and in groups, or pairings. As should be apparent the number possible combinations and configurations for activating and injecting resin through the various injection ports increases as the number of available injection ports increases for a given mold. 
         [0017]    In embodiments of the inventive staggered RTM system and process; the firing or activation order and patterns of usage of the resin injectors are preprogrammed into a numerical controller or computing device with a processor and a storage medium for storing and executing the programs. In an embodiment of the inventive staggered RTM system, the numerical controller may also control a carousal with multiple injection mold fixtures that can be automatically loaded into position for the staggered multiport injection process, and then removed and another mold fixture can be moved into position while a formed part is removed from the first fixture. 
         [0018]      FIG. 5  is a flowchart of a production process  50  of the staggered multiport RTM process according to embodiments of the invention. The process starts with an operator programming a production controller (step  52 ) with process parameters for a particular molded part to be formed with the RTM process using a multi resin injection port mold. Process parameters including pressure, temperature, and firing sequence of the resin injectors are set by the operator based on the resin material and part to be formed in the RTM process. The operator loads the resin for forming the part into the reservoirs of the resin injectors, as well as the layup into the mold (step  54 ). The multiport mold fixture resin input lines are hooked up to the resin injectors (step  56 ). The RTM process is started (step  58 ), and the completed part is removed from the mold (step  60 ). 
         [0019]    The foregoing description is illustrative of particular embodiments of the invention, but is not meant to be a limitation upon the practice thereof. The following claims, including all equivalents thereof, are intended to define the scope of the invention.