Abstract:
A system for in-situ and on-line monitoring of a preform layup process for liquid composite molding allows for gathering of permeability measurements of a fabric preform for use in a liquid composite molding process after situation of the preform within the mold wherein the molding process is to occur. The system uses a plurality of pressure sensors located within one of the mold sections of the liquid composite molding process mold. The sensors take pressure measurements which are processed to obtain a permeability profile of the preform. From such data, any local permeability variations that are caused from defects in the preform, deformation of the preform, or from mold misfit can be noted and acted upon before resin flow. The system is relatively simple to use and is easy to implement.

Description:
[0001]    This application claims the benefit of provisional patent application number 60/176,326 filed on Jan. 18, 2000. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to that gathers permeability measurements of a fabric preform for use in a liquid composite molding process after situation of the preform within the mold wherein the molding process is to occur.  
           [0004]    2. Background of the Prior Art  
           [0005]    Fiber reinforced composite materials are an important class of engineering materials that offer outstanding mechanical properties and unique design flexibility. Such materials are lightweight, corrosive resistant, impact resistant and exhibit excellent fatigue strength. Composite materials are used in a wide variety of applications including automotive parts, aviation, marine vessels, offshore structures, containers and piping, and sporting goods among others. Liquid composite molding, which includes resin transfer molding, reaction injection molding, and resin infusion, is one of the most attractive manufacturing solutions to producing high quality, affordable, and environmentally friendly composite materials.  
           [0006]    Recently, a considerable amount of progress has been achieved for liquid composite molding techniques such as constituent material development, tooling, reinforcement preform development, curing control, and process simulation. These advances have lifted the liquid composite molding process to new heights.  
           [0007]    However, a major barrier has retarded the advancement of liquid composite molding, the barrier being the reproducibility of the finished part, which tends to be lower than expected resulting in higher than expected manufacturing costs of the finished products actual manufactured and accepted.  
           [0008]    In the liquid composite molding process, resin flow induced defects are recognized as the largest source of quality and reproducibility problems. The resin flow pattern is strongly related to fiber preform layup and mold fit. In a liquid composite molding process, poor raw material (fiber) quality or improper fiber preform preparation often result in local permeability variations. These permeability variations can lead to unbalanced resin flows, which in turn, produce defect parts.  
           [0009]    Permeability, which characterizes the flow resistance of the fiber preform to the liquid resin in the liquid composite molding process, is widely used in liquid composite molding applications as one of the most important process design and control parameters. If serious permeability variations and misfit between the preform and the mold exist, defects will result in the resin low field, which leads to defective finished parts.  
           [0010]    Current methods for testing the permeability of the preform use specially designed molds that measure the pressure and flow rate or flow distance of the resin to obtain an average permeability using Darcy&#39;s law. The problems for arriving at the permeability of the preform using the existing method are twofold. First, the method only delivers the average permeability of the preform. It does not measure local permeability variations of the fiber preform due to inherent raw material variations as well as improper preform loading. Second, the permeability measurements are conducted off-line. As the preform is a flexible structure, it is easy to deform and restructure during the handling process. When the preform is laid up in the production mold, significant changes in the preform permeability can be realized due to fiber deformation and mold misfit. Accordingly, the measured permeability may not represent the true value of the permeability of the preform in the production mold.  
           [0011]    Therefore, there is a need in the art for a system for use in the liquid composite molding process that overcomes the above-stated problems in the art. Specifically, such a system must allow for a high level of reproducibility of the finished part manufactured using the liquid composite molding process. In order to achieve this high level, the permeability of the preform needs to be measured across the entire preform after situation of the preform within the actual mold wherein the liquid composite molding process upon the preform will occur so that preform fabric defects and/or mold misfits can be detected prior to the commencement of resin flow. Ideally, such a system will be of relatively simple construction and will be relatively easy to use and will not have adverse effects upon the finished product.  
         SUMMARY OF THE INVENTION  
         [0012]    The system for in-situ and on-line monitoring of a preform layup process for liquid composite molding of the present invention addresses the aforementioned needs in the art. The system allows for gathering of permeability measurements of a fabric preform for use in a liquid composite molding process after situation of the preform within the mold wherein the molding process is to occur. The system can be used to create a pressure profile, a contour profile, or can be processed in any other fashion desired by the user. From such data, any local permeability measurements that are caused from defects in the preform, deformation of the preform, or from mold misfit can be noted and acted upon before resin flow. The system is relatively simple to use and is easy to implement.  
           [0013]    The system for in-situ and on-line monitoring of a preform layup process for liquid composite molding of the present invention is comprised of a mold that has an upper section and a lower section forming a cavity therebetween and has an inlet port and an outlet port. A plurality of openings are located within the lower section while a plurality of pressure sensors are each attached to a respective one of the plurality of openings. A source of compressed gas is fluid flow connected to the inlet port. Each pressure sensor can be attached to its respective opening via an adapter which adapter can be operable between an open position wherein fluid flow communication between the pressure sensor and its respective opening is present and a closed position wherein fluid flow communication between the pressure sensor and its respective opening is absent. The pressure sensors may be MEMS pressure sensors. Each pressure sensor is connected to a data acquisition and processing device. which can be a computer. A reject hole located within the mold and a pin hole may each be located within the mold. A flow meter may be fluid flow connected with the inlet port, the flow meter also be connected to the data acquisition and processing device. A resin pump fluid flow is connected to the inlet port and may be fluid flow connected with the flow meter. The gas within the source of pressurized gas may be nitrogen.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 is a perspective view of the system for in-situ and on-line monitoring of a preform layup process for liquid composite molding of the present invention.  
         [0015]    [0015]FIG. 2 is a top plan view of the lower mold section.  
         [0016]    [0016]FIG. 3 is a bottom plan view of the lower mold section.  
         [0017]    [0017]FIG. 4 is a sectioned view of the lower mold section taken along line  4 - 4  in FIG. 2.  
         [0018]    [0018]FIG. 5 is a sectioned view of the lower mold section taken along line  5 - 5  in FIG. 2.  
         [0019]    [0019]FIG. 6 is a side view of the first type of adapter used with the system of the present invention.  
         [0020]    [0020]FIG. 7 is a sectioned view of the first type of adapter received within a hole in the lower mold section.  
         [0021]    [0021]FIG. 8 is a side view of the sealing plug used with the system of the present invention.  
         [0022]    [0022]FIG. 9 is a sectioned view of the sealing plug received within a hole in the lower mold section.  
         [0023]    [0023]FIG. 10 is a side view of the second type of adapter used with the system of the present invention.  
         [0024]    [0024]FIG. 11 is a sectioned view of the second type of adapter received within a hole in the lower mold section and in an open position.  
         [0025]    [0025]FIG. 12 is a sectioned view of the second type of adapter received within a hole in the lower mold section and in a closed position.  
         [0026]    [0026]FIG. 13 is a graph of the measured pressure profile of the gas flow for a defect free and uniform permeability preform.  
         [0027]    [0027]FIG. 14 is a graph of the simulated pressure profile of the gas flow for a defect free and uniform permeability preform.  
         [0028]    [0028]FIG. 15 is a graph of the measured pressure profile of the gas flow for a preform having a central cutout defect.  
         [0029]    [0029]FIG. 16 is a contour profile of the graph of FIG. 15.  
         [0030]    [0030]FIG. 17 is a graph of the measured pressure profile of the gas flow for a preform having a race tracking defect.  
         [0031]    [0031]FIG. 18 is a contour profile of the graph of FIG. 16.  
         [0032]    Similar reference numerals refer to similar parts throughout the several views of the drawings.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0033]    Referring now to the drawings, it is seen that the system for in-situ and on-line monitoring of fiber preform permeability variation for liquid composite molding (LCM) (which includes resin transfer molding, reaction injection molding, and resin infusion, etc.,), generally denoted by reference numeral  10 , is comprised of a standard mold  12  for liquid composite molding having an upper mold section  14  and a lower mold section  16  (the lower mold section  16  being a one section design or as illustrated a two section design), having reject holes  18  and pin holes  20 , wherein a fabric F or preform is loaded into the cavity within the mold  12  and the upper mold section  14  and lower mold section  16  are brought together in order to close the mold  12 . Once the mold  12  is closed, a resin pump  22  pumps an appropriate resin into the closed mold  12  through at least one inlet port  24  which is fluid flow connected to the pump  22  via an appropriate conduit  26 . A flow meter  28  is disposed between the inlet port  24  and the pump  22  so that the resin flow into the mold  12  can be monitored. Excess resin exits the mold through at least one outlet port  30 .  
         [0034]    Several holes  32  are located in the lower mold section  16 . A pressure sensor  34  is attached to each hole  32  and each pressure sensor  34  is also connected to an appropriate data acquisition and processing device such as a computer  36 , as is the flow meter  28 . Advantageously, Micro Electro Mechanical Systems (MEMS) pressure sensors  34 , such as those manufactured by Lucas NovaSensor Company of Fremont California, as well as others, are used due to their relatively low cost and high sensitivity and long term stability compared to other pressure sensors. An appropriate external excitation and offset circuit (not illustrated) can be constructed using operation amplifiers and resistors and be connected to each pressure sensor  34 . Alternately, MEMS pressure sensors with built-in amplification circuits can be used such that the external excitation and offset circuits can be eliminated. Additionally, the MEMS pressure sensor  34  requires a relatively small hole  32  in the lower mold section  16 . thereby allowing a robust density of holes  32  and pressure sensors  34  to populate the mold  12 .  
         [0035]    The pressure sensor  34  can be attached directly to its respective hole  32 , however, it is more advantageous to connect the pressure sensor  34  to the hole  32  via an adapter (reference number  38 , although not in the figures being used to describe the adapter generally, reference number  38   a  being used to designate the first type of adapter and reference number  38   b  being used to reference the second type of adapter) wherein the adapter  38  is attached to the lower mold section  16  and any pressure measurements pass through a channel  40  within the adapter  38  to the pressure sensor  34  via a conduit  42 . The use of an adapter  38  removes the pressure sensor  34  from direct contact with resin during the LCM process, which contact can damage or destroy a pressure sensor  34 . The adapter  38  can be of the type illustrated in FIGS. 6 and 7, wherein a portion of the adapter  38   a  is inserted through the hole  32  such that the end of the adapter  38   a  is generally flush with the inner surface of the lower mold section  16 . The conduit  42  connects the adapter  38   a  with the pressure sensor  34 . When use of the pressure sensor  34  is not needed, the adapter  38   a  is removed from the hole  32  and a sealing plug  44  is inserted into the hole  32  in order to seal the hole  32 . Alternately, the adapter  38  can be of the type illustrated in FIGS.  10 - 12  wherein the adapter  38   b  is operable between two positions. In FIG. 11, the adapter  38   b  is in an open position wherein the channel  40  is open and the pressure sensor  34  can take readings from the mold  12 . Rotation of the adapter  38   b  causes it to move from the open position to the closed position, illustrated in FIG. 12, wherein a shank  46  on the adapter  38   b  is received within the upper section of the hole  32 , the end of the shank  46  being generally flush with the inner surface of the lower mold section  16 . Counterrotation of the adapter  38   b  will return the adapter  38   b  to its open position. The adapter  38  and the plug  44  are each made from an appropriate material such as engineering plastics such as PP and PET for LCM systems that have a curing temperature lower than about  120  degrees Celsius, while for systems that have a higher cure temperature, high performance plastics such as PPS or metal should be used to make the adapter  38  and the plug  44 . By using an adapter  38  or plug  44  (described below), the adapter  38  or plug  44  is in contact with the resin and if one  38  or  44  should become damaged, or if a conduit  42  becomes clogged or otherwise inoperable, the damaged part (which is relatively less expensive than the pressure sensor  34 ) is discarded and replaced.  
         [0036]    The inlet port  24  (at least one inlet port in a multiple inlet port  24  system) is connected to a source of pressurized gas  48 , such as nitrogen (N 2 ) or compressed air. The pressurized gas source  48  can be connected to the same conduit  26  through which the resin pump  22  introduces the resin into the mold  12 , via an appropriate valve  50 .  
         [0037]    In order to use the system for in-situ and on-line monitoring of preform layup process of liquid composite molding, the mold  12  is opened and the fabric or preform F is loaded into the cavity of the mold  12  in the usual way. The mold  12  is closed. Each plug  44  is removed from its respective hole  32  and is replaced with an adapter  38 , the adapter  38  being connected to a pressure sensor  34  via a conduit  26 . Alternately, if the second type of adapter  38   b  is used, the adapter  38   b  is rotated so as to place the adapter  38   b  into its open position. Thereafter the compressed gas source  48  is activated so as to introduce the compressed gas into the mold  12  through the inlet port  24 . Each of the pressure sensors  34  takes a pressure reading and the data from the pressure sensor  34  is collected and processed. The collected data can be used to create a pressure profile, a contour profile, or can be processed in any other fashion desired by the user. The data, either in raw or processed form, is used by the user to determine if the preform F has any defects and/or if there is a misfit between the preform F and the mold  12  giving the user in-situ and on-line information regarding the preform layup, prior to actual liquid injection. If a defect or a misfit is detected, appropriate corrective action is taken. If there are no defects or misfits, then the LCM process continues. Accordingly, the adapters  38   a  are removed from their respective holes  32  and each is replaced with a plug  44  (or the adapter  38   b  is counterrotated to the closed position if the second type of adapter  38   b  is used). The compressed gas source  48  is deactivated and the valve  50  is articulated so that the resin flows from the resin pump  22  to the mold  12  in the usual way.  
         [0038]    In order to demonstrate use of the system of the present invention, a multiple layer knitted fabric having a fiber volume content of about 50 percent and a uniform gas permeability, the flow resistance of preform to the gas flow, in the cavity of about 1.82E-6 cm 2  was prepared with accurate dimensions and clear edge cuts and was carefully loaded into the cavity of the mold  12 . A theoretical pressure profile of the gas flow for this preform was calculated using a simulation tool developed at the Florida State University, Tallahassee, Florida, the profile is illustrated in FIG. 14 with each circle within the profile representing a point of calculated data. The pressure profile was measured using the system  10  of the present invention, the profile illustrated in FIG. 13, each circle in the profile representing a point of measured (pressure sensor  34 ) data.  
         [0039]    Thereafter, several layers of the preform F were cut out from a central portion of the preform, thereby reducing the fiber volume content of the preform at the cut out area to about 30 percent and the gas permeability to about 15.35E-6 cm 2 . The pressure profile was measured using the system  10 , the profile is illustrated in FIG. 15, each circle in the profile representing a point of measured (pressure sensor  34 ) data, while the contour of the pressure profile is illustrated in FIG. 16 wherein the arrows represent flow direction and the dotted lines delimit the cut out area. The pressure profile and the contour profile each show a gas permeability variation due to the cut out area and allow corrective action to be taken.  
         [0040]    In another example, race tracking was induced by a misfit between the preform F and the mold  12 . Several layers were cut out along one edge of the preform to simulate the low flow resistance channel induced by misfit between the preform F and the mold  12 . The pressure profile was measured using the system  10 , the profile is illustrated in FIG. 17, each circle in the profile representing a point of measured (pressure sensor  34 ) data, while the contour of the pressure profile is illustrated in FIG. 18 wherein the arrows represent flow direction and the dotted lines delimit the cut out area. The pressure profile and the contour profile each show a gas permeability variation due to the cut out area (simulating preform and mold misfit) and allow corrective action to be taken.  
         [0041]    Furthermore, the detected gas permeability correlated with the liquid permeability variation for the same preform used in an LCM process Therefore, the in-situ and on-line detection of permeability variation of a preform for an LCM process can be conducted before actual liquid resin injection.  
         [0042]    While the invention has been particularly shown and described with reference to an embodiment thereof, it will be appreciated by those skilled in the art that various changes in form and detail may be made without departing from the spirit and scope of the invention.