Abstract:
A method for forming a plurality of composite parts via resin transfer molding in a single molding cycle. The method includes the steps of using a mold having first and second mold members that cooperate to house a plurality of shims and placing a plurality of preform workpiece into the mold cavity such that at least one of the shims is disposed between each of an adjacent pair of the preform workpieces. A liquid resin is then injected into the mold cavity and subsequently cured to thereby form a cured workpiece matrix that is composed of a plurality of semi-finished composite parts and at least one shim separating the plurality of semi-finished composite parts from one another.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to a closed molding process that produces cured composite parts, and more particularly, to a method for producing multiple composite parts via resin transfer molding in a single cycle. 
     BACKGROUND OF THE INVENTION 
     Resin Transfer Molding (RTM) is a process that uses a closed mold to produce cured composite parts. A dry fabric preform is placed in the mold cavity, the mold is closed and a resin in injected into the mold. The mold is exposed to an elevated curing temperature, and after a predetermined curing cycle has elapsed, a finished part is removed from the mold. The part is then ready for final trim. 
     Prior RTM processes are only capable of producing one composite part for each cycle. Therefore, after each cycle the RTM molding tool must be cleaned or reprocessed for the next injection. 
     Another disadvantage of the known RTM processes is the cost of low-tolerance tooling Typical aerospace applications for RTM, for example, will require a part to be formed to a predetermined thickness within about ±0.025 mm (±0.001 in.). As those skilled in the art will readily understand, the fabrication of the tooling having this capability is relatively expensive, often costing 20-35% more than similar tooling having more open tolerances. 
     Accordingly, there remains a need in the art for an RTM tool and method that is capable of producing a plurality of highly consistent molded parts in a single molding cycle that overcomes the aforementioned drawbacks. 
     SUMMARY OF THE INVENTION 
     The Resin Transfer Molding Multi-Part/Shim Tooling (RTM-MPST) process overcomes this disadvantage by injecting resin into the mold cavity thus flooding multiple parts and shims, the pressure is distributed hydrostatically (evenly in all directions). This distributes the tolerance across each of the parts and with more shims in the cavity, the closer the parts will be to nominal thickness in a cost effective manner. 
     In one preferred form, the present invention provides a method for forming a plurality of composite parts via resin transfer molding in a single molding cycle. The method includes the steps of providing a mold having first and second mold members that cooperate to define a mold cavity; providing a plurality of shims; loading a plurality of preform workpieces into the mold cavity such that at least one of the shims is disposed between each of an adjacent pair of the preform workpieces; injecting a liquid resin into the mold cavity; curing the liquid resin to thereby form a cured workpiece matrix that is composed of a plurality of semi-finished composite parts and the at least one shim; and separating the plurality of semi-finished composite parts and the at least one shim from one another. 
     In another preferred form, the present invention provides a mold apparatus for performing a resin transfer molding operation on a plurality of preform workpieces to substantially simultaneously mold a plurality of semi-finished composite parts in a single molding cycle. The mold apparatus includes a first mold member, which defines a first mold line, a second mold member, which defines a second mold line, and at least one shim. The second mold member cooperates with the first mold member to define a mold cavity that is bounded by the first and second mold lines. The shim(s) are sized to fit within the mold cavity and are configured to be spaced in relation to the first and second mold lines to thereby segregate the mold cavity into a plurality of sub-cavities. Each of the sub-cavities is configured to house one of the perform workpieces. With the present invention, (RTM-MPST) produces multiple composite parts in one cycle thereby reducing the frequency that the tool is reprocessed and reduces resin waste. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Additional advantages and features of the present invention will become apparent from the subsequent description and the appended claims, taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a side elevation view of a molding apparatus constructed in accordance with the teachings of the present invention; 
     FIG. 2 is a sectional view of a portion of the molding apparatus of FIG. 1 illustrating the mold assembly in greater detail; 
     FIG. 3 is an exploded perspective view illustrating the arrangement of the composite preforms and shims as they would be loaded into the mold cavity prior to the injection of a liquid resin; and 
     FIG. 4 is an exploded perspective view illustrating the semi-finished composite parts and shims as separated from one another after their removal from the mold cavity. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to FIG. 1 of the drawings, a molding apparatus  10  is illustrated to include a mold assembly  12  that is constructed in accordance with the teachings of the present invention. The molding apparatus  10  is also illustrated to include a base  14 , an upper platten  16 , a lower platten  18 , a ram  20 , a control unit  22  a plurality of upright guides  24   a ,  24   b  and a mold assembly  12 . The base  14 , the upper and lower plattens  16  and  18 , the ram  20 , the control unit  22  and the upright guides  24   a ,  24   b  comprise a conventional molding press  10  of the type that is suited for resin transfer molding. Since such presses are well known in the art and as such, a detailed discussion of their construction and operation need not be provided herein. Briefly, in the example provided, the upright guides  24   a ,  24   b  are coupled to the base  14  and serve to fix the upper platten  16  in a predetermined spaced relation from the base  14 . The ram  20  is illustrated to be coupled to both the base  14  and the lower platten  18 . The ram  20  is conventionally hydraulically actuated via a hydraulic power source (not shown) to permit the lower platten  18  to be raised and lowered relative to the upper platten  16  in a desired manner. The upright guides  24   a ,  24   b  conventionally guide the lower platten  18  as it is being raised and lowered. The control unit  22  is coupled to and operable for controlling the source of hydraulic power and the heating elements (not shown) within each of the upper and lower plattens  16  and  18 . The control unit  22  may be configured to control the source of hydraulic power such that the ram  20  positions the lower platten  18  at a predetermined distance from the upper platten  16 , or may be simply configured to exert a predetermined force or pressure onto the upper and lower plattens  16  and  18 . When actuated by the control unit  22 , the heating elements generate heat that is transmitted through the upper and lower plattens  16  and  18  to the mold assembly  12 . 
     With additional reference to FIG. 2, the mold assembly  12  includes a first mold member  32 , a second mold member  34 , a resin inlet conduit  36 , a resin outlet conduit (not shown) and at least one shim member  40 . The first and second mold members  32  and  34  are illustrated to define a mold cavity  42  having a predetermined size and thickness. The resin inlet  36  provides a means for facilitating the introduction of a liquid resin into the mold cavity  42 , while the resin outlet provides a means for facilitating the evacuation of air from the mold cavity  42 . 
     The shims  40  are disposed within the mold cavity  42  and operably segregate the mold cavity  42  into a plurality of sub-cavities  44 . The shims  40  are preferably “free-floating” within the mold cavity  42  and as such, are formed to a size (e.g., length and width) that are relatively smaller than the size of the mold cavity  42 . In the example provided, the shims  40  are perishable, being disposed of after one or more molding cycles. The quantity of shims  40  in the mold cavity  42 , as well as their thickness, are dependant upon several factors, which may include, for example, the actual depth of the mold cavity, the quantity of composite parts that are to be formed in a single molding cycle and the thickness tolerance of the finished composite parts. 
     In the particular example provide, the finished composite parts that are to be formed in the mold assembly  12  are substantially flat panels having a thickness tolerance of about +/−0.0254 mm (+/−0.001 inch). As noted above, the fabrication of tooling that would reliably and repeatably meet this thickness tolerance is extremely expensive, whereas tooling having “standard”, more open tolerances, e.g., ±0.127 mm (±0.005 inch), often costs as much as 35% less. As, in the example provided, both improved processing efficiency and reduced tooling costs are desirable, a decision is made prior to the construction of the mold assembly  12  as to what tolerances are to be used with regard to the overall depth of the mold cavity  42 . For illustrative purposes, the example provided herein will assume a tolerance of ±127 mm (±0.005 inch) with regard to the overall depth of the mold cavity  42 ; this tolerance will hereinafter be referred to as the mold cavity tolerance. 
     It should be noted that the mold cavity tolerance exceeds the thickness tolerance for the finished composite parts, i.e., ±0.0254 mm (±0.001 inch). The segregation of the mold cavity  42  by the shims  40  into a plurality of sub-cavities  44 , however, permits the mold cavity tolerance to be substantially equally distributed to each of the sub-cavities  44 . With the mold cavity tolerance (MCT) and the thickness tolerance (TT) of the finished composite parts being known quantities, the quantity (Q) of finished composite parts that are to be formed in a single molding cycle is determined from the equation: 
     
       
           Q ≧( MCT÷TT ). 
       
     
     Accordingly, the mold cavity  42  must be sized for a minimum quantity of five (5) sub-cavities  44  to ensure that each composite part that is formed in the mold assembly  12  meets the desired thickness tolerance. As those skilled in the art will understand, the mold cavity  42  may, of course, be sized for a greater quantity of sub-cavities  44  to thereby achieve improved processing efficiency and improved process capability. 
     With concerns for tolerances, processing efficiency and process capability being accounted for when determining the number of sub-cavities  44  that are to be formed within the mold cavity  42 , the overall depth of the mold cavity  42  is then determined. This is determined with reference to the number of finished composite parts that are to be formed in a single molding cycle (i.e., the number of sub-cavities  44 ), the nominal thickness of each of the finished composite parts, and the nominal thickness of the shims  40  that form the sub-cavities  44 . 
     The mold assembly  12  thus formed is loaded with the shims  40  and a plurality of composite preform workpieces  46  as shown in FIG.  3 . 
     After the preforms and shims are loaded into the mold cavity, the mold members are closed and a liquid resin is injected into the mold cavity via the resin inlet conduit. The liquid resin is pressurized to a predetermined pressure. As the resin outlet conduit permits air within the mold cavity to escape, a pressure differential across the mold cavity is generated, permitting the liquid resin to fully infiltrate and surround each of the sub-cavities, thereby coating and/or penetrating each of the composite preform workpieces with resin. 
     Once the air has been completely evacuated from the mold cavity and resin is exiting the mold assembly from the resin outlet conduit, the supply of liquid resin to the mold assembly is halted and the heating elements within the upper and lower plattens are actuated by the control unit to heat the mold assembly and cure the resin within the mold cavity. 
     When the resin within the mold cavity has cured sufficiently after the mold assembly&#39;s exposure to a predetermined elevated temperature and the elapse of a predetermined amount of time, the mold assembly is opened and a cured workpiece matrix  50  is removed from the mold cavity, as shown in FIG.  4 . 
     While the finished composite parts that are formed via the single molding cycle of the present invention are described herein as being substantially flat panels, those skilled in the art will understand that composite parts having other contours may also be molded. For example, the construction of a panel that conforms to a spherical radius may be readily undertaken assuming, of course, that the tolerance on the radius is sufficiently large to permit the deviations that would occur from the successive layering of one or more shims and composite preform workpieces onto the lower mold member. In this regard, it is preferable to form the lower mold member such that it will form the semi-finished composite part that contacts it to a radius that is at or just slightly greater than the minimum spherical radius to which the panels are to be formed. Construction of the mold assembly in this manner permits the maximization of the number of spherical panels that are formed in the single molding cycle. Those skilled in the art will understand that the contour of the upper mold member will be different from that of the lower mold member, as the radius to which it is formed is greater than that for the lower mold member by an amount that is related to the number of shims and composite preform workpieces that are disposed between the upper and lower mold members. 
     While the invention has been described in the specification and illustrated in the drawings with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention as defined in the claims. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment illustrated by the drawings and described in the specification as the best mode presently contemplated for carrying out this invention, but that the invention will include any embodiments falling within the foregoing description and the appended claims.