Apparatus for manufacturing composite parts produced by resin transfer molding

An apparatus for manufacturing composite parts produced by resin transfer molding (RTM) is configured to operate in controlled flow-rate mode or controlled pressure mode to use the RTM technique for manufacturing large-sized composite parts with a low fiber content. In both modes, control is by means of a fuzzy logic control card. In controlled flow-rate mode the resin is injected alternately by two injection cylinders (40a and 40b) controlled by motors whose speed is controlled. In controlled pressure mode the resin is injected directly from resin supply vessels (16) subjected to pressure controlled by an air regulator (30).

DESCRIPTION 
 1. Field of the Invention 
 The present invention relates to an apparatus for manufacturing composite 
 parts produced by resin transfer molding (RTM). 
 More precisely, the apparatus according to the invention is designed for 
 producing large-size composite parts that may either have a high fiber 
 content, i.e. 58% or greater, or a lower fiber content, i.e. below 54%. 
 This type of apparatus is particularly used for applications in the 
 aerospace industry in which large-size parts with a high fiber content are
 used in conjunction with large-size sandwich parts in which the fiber 
 content of the skins covering the core of the sandwich structure is 
 relatively low. 
 2. Background Art 
 The RTM technique has been used for many years in fields such as the 
 automobile and producer's goods industries to produce parts with a 
 relatively low fiber content, i.e. lower than 54%. It consists of 
 inserting a dry fibrous preform into a mold and then using low pressure to
 inject a liquid organic resin into the mold under negative pressure, 
 thereby impregnating the fibrous preform. The impregnated preform is then 
 heated in order to polymerize it before removing it from the mold. 
 The RTM technique has a number of advantages over other known composite 
 materials techniques: overall operating time is reduced, the investment 
 cost of the production line is low due to the absence of cooling and 
 autoclave systems, part measurements, particularly thickness, can be 
 accurately calibrated, the technique yields excellent surfaces, and health
 and safety conditions for operators are very good. 
 In order to produce the type of parts with a high fiber content greater 
 than 58% required by the aerospace industry, the dry preform onto which 
 the resin is injected in the mold must be strongly compressed. However, 
 the RTM procedures in the known art do not allow for the manufacture of 
 large-sized parts with this degree of compression. 
 The considerable loss of pressure introduced by the compressed textile 
 fibers in the preform inhibits the penetration of resin into the preform. 
 Moreover, the resin injection pressure must remain low enough for the 
 orientation of the textile fibers in the preform to remain unaffected 
 during the injection procedure; the mechanical characteristics of the part
 thereby obtained are essentially determined by the orientation of the 
 textile fibers in the resin matrix. 
 Consequently, when large parts are being manufactured, the time required 
 for the resin to penetrate into the entire fibrous preform exceeds the 
 time the resin takes to polymerize (normally known as the gel time). The 
 highly compressed textile fibers also constitute an obstacle to the 
 penetration of the resin and may leave entire areas of a part 
 unimpregnated as well as leaving bubbles. 
 For these various reasons the RTM technique is currently little used for 
 the manufacture of large parts with a high fiber content. 
 Moreover, existing apparatuses that use the RTM technique are usually 
 designed for series production of parts with relatively uniform 
 characteristics, particularly as concerns their measurements and their 
 fiber content. This characteristic of the existing apparatuses also tends 
 to make them very unsuitable for the aerospace industry in which parts 
 with widely differing characteristics are produced in limited series. For 
 example, large parts such as structural components, require a high fiber 
 content, i.e. greater than 58%. In contrast, the manufacture of sandwich 
 parts comprising a core made of a honeycomb material, or foam covered on 
 both sides with a skin, generally uses a relatively low fiber content i.e.
 less than 54% for the skins. 
 In the manufacture of resin parts obtained by injection molding, various 
 techniques have been developed to give improved control over the various 
 injection parameters such as flow-rate, pressure, temperature, etc. while 
 taking the shape of the part produced into consideration. Documents U.S. 
 Pat. Nos. 5,178,805, 5,316,707, 4,850,217 and 5,551,486 all disclose 
 certain of these techniques as examples. 
 However, the specific drawbacks of the RTM technique described above have 
 hitherto prevented the techniques used for RTM injection molded production
 being adapted to produce large parts with high or low fiber contents as 
 required. 
 DISCLOSURE OF THE INVENTION 
 The invention relates precisely to an apparatus for manufacturing composite
 parts using the RTM technique that will allow relatively simple, cheap 
 production of large parts with a high or low fiber content while ensuring 
 that the parts thereby obtained have a particularly low porosity level 
 (maximum 1 to 3%) without the risk of premature gelling of the resin or 
 the presence of unimpregnated areas within the parts. 
 According to the invention, this result is obtained by means of an 
 apparatus for manufacturing composite parts by resin transfer molding, 
 characterized by the fact that it comprises: 
 at least one mold able to accept a fibrous preform, 
 means for compressing the preform in the mold, 
 at least one resin supply vessel, 
 at least one injection cylinder for injecting the resin into the mold, 
 control means capable of operating the apparatus in controlled flow-rate 
 mode in which the resin is transferred from the resin supply vessel to the
 injection cylinder before being injected into the mold by the cylinder at 
 a generally constant rate, and in which the compressing means generally 
 ensure that the fibrous preform is highly compressed, and in a controlled 
 pressure mode wherein the resin is injected directly into the mold by the 
 resin supply vessel at a generally constant rate and wherein the 
 compressing means generally ensure weak compression of the fibrous 
 preform, and 
 means of switching the control means between the controlled flow-rate and 
 controlled pressure modes. 
 In the apparatus thus defined, the controlled flow-rate mode is used to 
 produce large parts with a fiber content greater than 58% while controlled
 pressure mode is used to produce large-size sandwich parts in which the 
 skins have a fiber content less than 54%. 
 In a preferred embodiment of the invention, the control means act on a 
 three-way valve comprising one inlet connected to the resin supply vessel,
 a first outlet connected to the injection cylinder and a second outlet 
 connected to the mold. The inlet of the three-way valve is connected to 
 the first outlet in controlled flow-rate mode and to the second outlet in 
 controlled pressure mode. 
 The control means advantageously act upon means for pressurizing the resin 
 supply vessel so that constant pressure is applied to the vessel in 
 controlled flow-rate mode while variable pressure is applied to the vessel
 in controlled pressure mode. 
 In order to manufacture large-size parts while reducing the risk of the 
 resin starting to polymerize in the resin supply vessel (this risk is 
 increased the greater the volume and the diameter of the vessel), several 
 resin supply vessels are preferably used. These vessels are connected in 
 parallel in a tank connected to pressurizing means. Each resin supply 
 vessel is then connected in turn to the inlet of the three-way valve via a
 valve commanded by the control means. 
 The injection cylinder is normally controlled by an electric motor fitted 
 with a speed regulator. The control means act on the speed regulator in 
 controlled flow-rate mode so that the injection cylinder injects a 
 controlled flow of resin into the mold. 
 In the preferred embodiment of the invention a first pressure sensor 
 supplying a first instantaneous signal is located on the mold inlet. The 
 control means also comprise a first comparator means that supply a first 
 input signal representative of the deviation between a maximum pressure 
 setting and the first instantaneous signal, a second comparator means that
 supplies a second input signal representative of the difference between 
 two consecutive first input signals, and at least one fuzzy logic control 
 card that, in controlled flow-rate mode, uses the first and second input 
 signals to supply a flow-rate control signal. 
 A second pressure sensor is preferably located on the outlet of each 
 injection cylinder and supplies a second instantaneous signal. The control
 means comprise a third comparator means that supply a third input signal 
 representative of the difference between a maximum permissible pressure on
 the outlet of the injection cylinder and the second instantaneous signal, 
 the flow-rate control signal being generated by the fuzzy logic control 
 card, also using the third input signal. 
 The control means thus comprise first limiting means that compare the 
 flow-rate control signal with a reference flow-rate signal to supply a 
 flow-rate control signal to the speed regulator that is equal to the 
 reference flow-rate signal when said signal is less than the flow-rate 
 control signal, and equal to the flow-rate control signal if not. 
 Flow-rate measuring means are also located between the second outlet of the
 three-way valve and the mold and supply a third instantaneous signal. 
 Control means comprising a fourth comparator means supplying a fourth 
 input signal flow-rate control signal representative of the difference 
 between a maximum permissible flow-rate and the third instantaneous signal
 and a fifth comparator means supplying a fifth input signal representative
 of the difference between two consecutive fourth input signals, the fuzzy 
 logic control card using the fourth and fifth input signals to supply a 
 pressure-regulating signal in controlled pressure mode. 
 The control means thus comprise second limiting means that compare the 
 pressure control signal with a reference pressure signal to supply 
 pressurizing means with a pressure control signal equal to the reference 
 pressure when the latter is less than the pressure control signal and 
 equal to the pressure control signal if not. 
 In the preferred embodiment of the invention two injection cylinders are 
 mounted in parallel between a second three-way valve comprising a inlet 
 connected to the resin supply vessel and a third three-way valve 
 comprising an outlet connected to the mold. In controlled flow-rate mode, 
 the control means actuate these second and third three-way valves so that 
 one of the injection cylinders is connected to the resin supply vessel and
 isolated from the mold while the other injection cylinder is connected to 
 the mold and isolated from the resin supply vessel, and conversely. This 
 arrangement makes it possible to inject resin continuously into the mold 
 from each injection cylinder alternately. It is therefore particularly 
 well suited to the manufacture of large-sized parts and contributes to 
 reducing the risks of the resin starting to polymerize in the cylinders. 
 In this situation a second pressure sensor is located on the outlet of each
 injection cylinder and the third input signal injected on the fuzzy logic 
 control card is representative of the difference between the maximum 
 permissible pressure on the outlet of the cylinders and the second 
 instantaneous signal supplied by the second pressure sensor associated 
 with the injection cylinder connected to the mold.

DETAILED DISCLOSURE OF A PREFERRED EMBODIMENT 
 In the preferred embodiment shown schematically in FIG. 1, the apparatus 
 according to the invention comprises one or more molds 10 suitable for 
 manufacturing composite parts using resin transfer molding. The cavity 
 (not shown) of each mold 10 is constructed so that its shape and 
 measurements correspond to the external characteristics of the part to be 
 produced. The cavity is designed to accept a fibrous preform 12 made of 
 preassembled dry organic fibers. Each mold 10 is fitted with heating means
 (not shown) designed to maintain the mold at the required temperature 
 during injection of the resin and to subject it to a polymerization cycle 
 when the injection phase has been completed. 
 Each mold 10 is also fitted with compression means indicated by arrows 14 
 in FIG. 1. These compression means 14 are designed to apply a controlled 
 compression force with an adjustable value to fibrous preform 12. As a 
 non-limitative example, said compression means 14 may comprise one or more
 inflatable bladders placed with fibrous preform 12 in the cavity of mold 
 10 as disclosed in document FR-A-2 720 028. 
 For the purposes of clarity, FIG. 1 shows the simplest embodiment of the 
 invention with only one mold 10. When several molds 10 are used, i.e. when
 several parts are manufacture simultaneously, the molds are disposed in 
 parallel so that the resin is injected simultaneously into each mold. 
 According to the invention, the apparatus can operate in two different 
 modes, enabling it to be used for producing parts with different 
 characteristics as required. The first mode, known as controlled flow-rate
 mode, is used to produce large parts with a high fiber content, i.e. 
 greater than 58%. A second mode, known as controlled pressure mode, is 
 used to produce large parts, normally sandwich-type components, with a 
 relatively low fiber content, i.e. less than 54%. When several parts are 
 produced simultaneously in several molds 10 the operation of the apparatus
 in one or other of these two modes requires that all the parts be of the 
 same type, i.e. either large parts with a high fiber content or sandwich 
 parts with a low fiber content. 
 When the part or parts produced by the apparatus are large parts with a 
 high fiber content, fibrous preform 12 fills virtually the whole of the 
 cavity inside each mold 10, as shown schematically in FIG. 1. 
 In contrast, when the part or parts produced by the apparatus are sandwich 
 parts, fibrous preform 12 comprises a central core made of a honeycomb 
 material or foam, both sides of which are covered with a dry fiber 
 preform. In this type of production precautions are taken such as 
 inserting a leaktight film to prevent the resin penetrating the honeycomb 
 or foam core when it is injected into the dry fiber preforms designed to 
 form the skins of the parts. 
 In the embodiment of the invention shown in FIG. 1, in which large-size 
 parts are produced, the apparatus comprises several resin supply vessels 
 16. The resin supply vessels 16 are usually relatively small (e.g. 5 
 liter) cylindrical vessels in which the transfer pipes 18 are immersed 
 vertically. These transfer pipes carry the resin when pressure is applied 
 to the surface of the resin. 
 The use of several resin supply vessels 16 is justified by the fact that 
 the risk of polymerization of the resin starting to occur in the recipient
 containing it increases the greater the volume and diameter of the 
 recipient. 
 All the resin supply vessels 16 required to produce one or more parts are 
 placed in the same tank 20. This tank is leaktight and capable of 
 withstanding an internal pressure of several bar. The bottom of the tank, 
 as well as the area surrounding the vessels 16, is fitted with means (not 
 shown) for heating the resin contained in the vessels. Said heating means 
 may be of any type, for example electrical resistors. 
 Weighing means (not shown) are preferably fitted inside tank 20 to measure 
 the individual weight of each vessel 16. 
 As shown schematically in FIG. 1, a stirring apparatus 22 is placed inside 
 each resin supply vessel 16. Each stirring apparatus 22 is driven by an 
 electric motor 24 located above tank 20 via a vertical shaft 26 that 
 passes through the cover of tank 20. 
 A compressed air pipe 28 opens into tank 20 in order to pressurize the 
 tank. Said pipe 28 is fed by a compressed air source (not shown) such as 
 the standard supply of the plant in which the apparatus is installed. 
 An air regulator 30 fitted with a proportional valve is fitted to the 
 compressed air pipe 28 so that the pressure applied to resin supply 
 vessels 16 inside tank 20 can be controlled instantaneously and 
 accurately. Compressed air pipe 28 and its air regulator 30 constitute the
 means for pressurizing resin supply vessels 16. 
 Outside tank 20 each transfer pipe 18 is fitted with a solenoid valve 32 
 that makes it possible to use each resin supply vessel 16 in turn during 
 operation of the apparatus. Downstream of solenoid valves 32, all the 
 transfer pipes 18 are connected to a single resin supply pipe 34. 
 Resin supply pipe 34 is connected to the inlet of a first three-way valve 
 36 that modifies the routing of the resin in the apparatus depending on 
 which operating mode is being used. 
 Three-way valve 36 comprises a first outlet to which is connected a pipe 38
 that supplies two resin injection cylinders 40a and 40b alternately via a 
 second three-way valve 42. More precisely, pipe 38 is connected to the 
 inlet of three-way valve 42 which comprises a first outlet connected to 
 the pressure chamber of cylinder 40a via pipe 44a and a second outlet 
 connected to the pressure chamber of cylinder 40b via pipe 44b. 
 The two cylinders 40a and 40b are identical and each has a limited 
 capacity, for example approximately 10 liters. The use of low-capacity 
 cylinders helps reduce the risk of resin starting to polymerize inside the
 components. 
 As will be seen in more detail below, cylinders 40a and 40b are only 
 actuated when the apparatus operated in controlled flow-rate mode. The 
 cylinders 40a and 40b are used alternately to inject resin into mold or 
 molds 10. More precisely, when a mold requires a quantity of resin greater
 than 10 liters, the cylinder not being used for injection is being filled 
 and vice versa. 
 Each cylinder 40a and 40b is controlled by an electric motor 46a, 46b 
 fitted with a speed regulator 48a, 48b respectively. 
 Pipe 44a connecting the first outlet of three-way valve 42 to cylinder 40a 
 is also connected to a first inlet of a third three-way valve 50. 
 Similarly, pipe 44b connecting the second outlet of three-way valve 42 to 
 cylinder 40b is connected to a second inlet of three-way valve 50. 
 Three-way valve 50 has a single outlet connected to the inlet of the mold 
 or molds 10 by an injection pipe 52 via a heat exchanger 54. Heat 
 exchanger 54 is used to heat the resin to the temperature required on the 
 inlet of mold 10 by means, for example, of controlled temperature heat 
 exchange. 
 The first three-way valve 36 comprises a second outlet connected directly 
 to injection pipe 52, between three-way valve 50 and heat exchanger 54 via
 a shunt pipe 56. This shunt pipe 56 is fitted with a flow-rate meter 58 
 that produces an instantaneous electric signal QE representative of the 
 value of the flow rate of resin in shunt pipe 56. 
 A vacuum pump 60 is connected to each mold 10 via a suction pipe 62 that 
 opens into each mold on the opposite side to injection pipe 52. A pipe 64 
 also connects a suction aperture of vacuum pump 60 to tank 20. This acts 
 to create negative pressure in the tank during the initial debulking phase
 of the resin contained in the resin supply vessels 16. Pipe 64 is fitted 
 with a valve 65 that is normally in the closed position during transfer of
 resin. 
 Vacuum pump 60 may also be used to bleed cylinders 40a and 40b into an 
 interchangeable recovery trough 66 once an injection cycle in controlled 
 flow-rate mode has been completed. A similar bleeding operation may also 
 be effected when incipient polymerization accidentally occurs in one or 
 other of the cylinders. This is effected by means of a suction aperture of
 vacuum pump 60 connected to injection pipe 52 between the third three-way 
 valve 50 and heat exchanger 54 via a bleed pipe 68 connected to trough 66.
 Said bleed pipe 68 is fitted with a solenoid valve 70 that is normally 
 closed upstream of trough 66. 
 The apparatus of the invention is fitted with a first pressure sensor 
 referred to as CPE. Pressure sensor CPE is fitted in injection pipe 52 
 between heat exchanger 54 and mold or molds 10. It supplies an electric 
 signal representative of the instantaneous pressure on the inlet of mold 
 or molds 10. This signal will be referred to as the "first instantaneous 
 signal PE". 
 The apparatus also comprises two pressure sensors CPVa and CPVb, located 
 respectively in pipes 44a and 44b, between cylinders 40a and 40b and the 
 third three-way valve 50. Each of these pressure sensors CPVa and CPVb 
 supplies an electric signal representative of the instantaneous pressure 
 on the outlet of cylinders 40a and 40b respectively. These signals will be
 referred to as the "second instantaneous signals PVa and PVb". 
 The apparatus shown in FIG. 1 also comprises a negative pressure sensor 
 CPP, located on suction pipe 62 immediately upstream of vacuum pump 60. 
 Negative pressure sensor CPP supplies an electric signal representative of
 the instantaneous pressure on the suction aperture of vacuum pump 60. 
 The apparatus according to the invention also comprises control means 72 
 that are fitted with switching means 74. Control means 72 receive the 
 electric signals supplied by the various sensors and transmitters with 
 which the apparatus is equipped. Dotted lines have been used in FIG. 1 to 
 illustrate how control means 72 receive instantaneous signals supplied by 
 flow-rate meter 58 and sensors CPE, CPVa, CPVb and CPP. 
 Moreover, control means 72 act on the various active components of the 
 apparatus, depending on the status of switching means 74. These effects 
 are shown schematically by unbroken arrows in FIG. 1. The active 
 components particularly affected by control means 72 include compression 
 means 14, air regulator 30, first three-way valve 36 and speed regulators 
 48a and 48b. They also include stirring apparatus motors 24, solenoid 
 valves 32, vacuum pump 60 and second and third three-way valves 42 and 50.
 The switching means 74 are composed of a switching device that can be 
 actuated by an operator in order to switch control means 72 between 
 controlled flow-rate mode and controlled pressure mode depending on the 
 type of parts being produced. 
 The main components of control means 72 and their operation in both 
 controlled flow-rate and controlled pressure mode will now be described in
 detail with reference, successively, to FIGS. 2 and 3. 
 More precisely, FIG. 2 shows the effect of control means 72 alternately on 
 speed regulators 48a and 48b of injection cylinders 40a and 40b during 
 injection of the resin into mold 10 at a flow rate that is generally 
 constant when the apparatus is in controlled flow-rate mode. FIG. 3 shows 
 the effect of control means 72 on air regulator 30 in order to inject the 
 resin directly into mold 10 at a generally constant pressure when the 
 apparatus is in controlled pressure mode. 
 In controlled flow-rate mode (FIG. 2), resin is injected alternately by 
 injection cylinders 40a and 40b, as explained above. To simplify matters, 
 speed regulators 48a and 48b of injection cylinders 40a and 40b, that are 
 controlled alternately, are shown as a single rectangle, 48a, b. 
 The control means 72 shown in FIGS. 2 and 3 comprise a programmable logic 
 controller 76 that, depending on the operating mode selected, controls the
 flow-rate or pressure regulation function by means of one or more fuzzy 
 logic control cards 78. The various parameters of the fuzzy logic control 
 card 78 may be configured as required via programmable logic controller 
 76. 
 FIGS. 2 and 3 show schematically that programmable logic controller 76 is 
 fitted with input components such as thumbwheels that can be used to enter
 various settings into the system. 
 In controlled flow-rate mode (FIG. 2), a first maximum pressure setting 
 PEmax is the maximum pressure permissible on the inlet of the mold or 
 molds 10 used. This PEmax setting is determined essentially by the 
 characteristics of the parts to be produced. A second setting Qref is the 
 reference flow-rate to which injection of the resin is to be set. This 
 reference flow-rate is selected according to the volume of the mold or 
 molds to be filled and the gel time of the resin at the injection 
 temperature selected. 
 Other settings determined by the intrinsic characteristics of the apparatus
 are permanently stored in programmable logic controller 76. When the 
 apparatus operates in controlled flow-rate mode (FIG. 2) these settings 
 include, in particular, pressure setting PVmax which is the maximum 
 pressure permissible on the outlet of each of the injection cylinders 40a 
 and 40b, given their mechanical characteristics. They also include a 
 setting Cmax which is the maximum torque permissible for each electric 
 motor 46a and 46b. 
 As shown schematically by reference 80 of FIG. 2, the programmable logic 
 controller 76 computes in real time a first input signal S1(t) that is 
 representative of the difference or deviation .DELTA.PE between the 
 maximum pressure setting PEmax and the first instantaneous signal PE 
 supplied by sensor CPE located on the inlet of mold 10. This first input 
 signal S1(t) constitutes one of the input magnitudes injected into the 
 fuzzy logic control card 78 in controlled flow-rate mode. 
 As shown by reference 82 of FIG. 2, programmable logic controller 76 also 
 generates in real time a second input signal S2 that is representative of 
 the difference ERPE between two consecutive signals S1, i.e. the rate of 
 variation of first input signal S1. More precisely, this second input 
 signal S2 is equal to the difference between the last input signal S1, 
 calculated at time t, and the input signal S1 immediately preceding it, 
 calculated at time t-1. The second input signal S2 also constitutes an 
 input magnitude injected into the fuzzy logic control card 78 in 
 controlled flow-rate mode. 
 In the embodiment shown in FIG. 2 a third input magnitude is also injected 
 into the fuzzy logic control card 78. This magnitude consists of a third 
 input signal S3. This third input signal S3 is representative of the 
 difference or deviation .DELTA.PV between the maximum pressure setting 
 PVmax permissible on the injection cylinders 40a and 40b and the 
 instantaneous value of pressure PVa, PVb measured by sensor CPVa and CPVb 
 on the outlet of injection cylinders 40a and 40b during injection. 
 The third input signal S3 is also generated by the programmable logic 
 controller 76 as shown by reference 84 in FIG. 2. 
 When the apparatus operates in controlled flow-rate mode, the fuzzy logic 
 control card 78 permanently generates a flow-rate control signal SRQ using
 the three input signals S1, S2 and S3. The essential characteristics of 
 the rules governing the generation of signal SRQ will be described briefly
 below as an example. 
 In practice, flow-rate control signal SRQ is a speed signal that 
 establishes at any time the speed at which electric motor 46a or 46b of 
 the cylinder being used to inject must be actuated to comply with the 
 rules contained in fuzzy logic control card 78. 
 As shown by reference 86 of FIG. 2, when the apparatus operates in 
 controlled flow-rate mode, the programmable logic controller of the 
 control means compares the flow-rate control signal SRQ with a reference 
 flow-rate signal SQref that is representative of the reference flow-rate 
 Qref entered previously on the programmable logic controller. The control 
 means then generate a flow-rate control signal SCQ that is equal to signal
 SQref as long as the flow-rate control signal SRQ is greater than or equal
 to said signal SQref, and equal to flow-rate control signal SRQ when 
 signal SRQ is less than signal SQref. The flow-rate control signal SCQ is 
 a speed signal that is injected into the speed regulators 48a or 48b of 
 injection cylinders 40a or 40b during resin injection. 
 As can be seen from FIG. 3, when the apparatus is operated in controlled 
 pressure mode a setting QEmax for the maximum permissible flow-rate on the
 inlet of mold 10, together with a setting Pref that is the reference 
 pressure at which injection is to be effected, is entered on programmable 
 logic controller 76. 
 As shown schematically by reference 90 of FIG. 3, the programmable logic 
 controller 76 computes in real time a fourth input signal S4(t) that is 
 representative of the difference or deviation .DELTA.QE between the 
 maximum flow-rate setting QEmax and the instantaneous signal QE supplied 
 by flow-rate meter 58. This fourth input signal S4(t) constitutes one of 
 the input magnitudes injected into the fuzzy logic control card 78 in 
 controlled pressure mode. 
 As shown by reference 92 of FIG. 3, programmable logic controller 76 also 
 generates a fifth input signal S5 that is representative of the difference
 ERQE between two consecutive signals S4, i.e. the rate of variation of 
 fourth input signal S4. More precisely, this fifth input signal S5 is 
 equal to the difference between the last input signal S4, calculated at 
 time t, and the input signal S4 immediately preceding it, calculated at 
 time t-1. The fifth input signal S5 also constitutes an input magnitude 
 injected into the fuzzy logic control card 78 in controlled pressure mode.
 When the apparatus operates in controlled pressure mode, the fuzzy logic 
 control card 78 permanently generates a pressure control signal SRP using 
 input signals S4 and S5. This pressure control signal SRP permanently 
 determines the pressure to be applied to the resin supply vessel 16 being 
 used to inject in order to comply with the rules contained in fuzzy logic 
 control card 78. 
 As shown by reference 94 of FIG. 3, when the apparatus operates in 
 controlled pressure mode, the programmable logic controller of the control
 means 72 compares the flow-rate control signal SRP with a signal SPref 
 that is representative of the reference pressure Pref entered previously 
 on the programmable logic controller. The control means 72 then generate a
 pressure control signal SCP that is equal to signal SPref as long as the 
 pressure control signal SRP is greater than or equal to said signal SPref 
 and equal to pressure control signal SRP when signal SRP is less than 
 signal SPref. The pressure control signal SCP is injected into the air 
 regulator 30 in order to adjust the opening of the proportional valve it 
 contains. 
 Once the one or more molds 10 have been connected between injection pipe 52
 and suction pipe 62 (FIG. 1), the operator can select either controlled 
 flow-rate mode or controlled pressure mode depending on the 
 characteristics of the parts being produced. As has already been explained
 above, controlled flow-rate mode is used to produce large-sized parts with
 a high fiber content, typically greater than 58%. Controlled pressure 
 mode, on the other hand, is used to produce sandwich parts in which the 
 skins have a relatively low fiber content, typically less than 54%, in 
 order to limit the force due to the injection pressure on the honeycomb or
 foam core. Switching means 74 are actuated to set the apparatus to the 
 required mode. 
 When controlled flow-rate mode is selected, the first three-way valve 36 is
 automatically set to the state in which resin supply pipe 34 is connected 
 to cylinder supply pipe 38. The proportional valve of air regulator 30 is 
 also set to its maximum open state, giving a pressure of several bar. 
 Finally, control means 72 act on compression means 14 to ensure that 
 fibrous preform 12 is compressed strongly enough to obtain a fiber content
 greater than 58% when said compression means 14 are actuated. 
 Use of the apparatus in controlled flow-rate mode also has the effect of 
 initially opening at least one of the solenoid valves 32 while maintaining
 the other solenoid valves 32 closed. At the same time the second and third
 three-way valves 42 and 50 are actuated so that one of the cylinders, for 
 example cylinder 40a, is initially connected to the resin supply vessel 16
 that corresponds to the open solenoid valve 32 while remaining isolated 
 from mold 10, while the other cylinder, for example cylinder 40b, is, in 
 contrast, isolated from the resin supply vessels 16 and connected to mold 
 10. 
 During a preliminary operating phase, the cylinder, for example cylinder 
 40a, that is initially connected to one of the resin supply vessels 16, 
 fills with resin due to the effect of the compressed air introduced into 
 tank 20 by pipe 28. Given the relative volumes of the resin supply vessels
 16 and injection cylinders 40a and 40b, cylinder 40a is usually filled 
 using approximately the contents of two resin supply vessels 16. This may 
 be achieved either by opening two solenoid valves 32 simultaneously or by 
 opening one solenoid valve, then the other once the first has been closed.
 Once this preliminary stage is complete, the control means 72 
 simultaneously command the reversal of the states of second and third 
 three-way valves 42 and 50. Consequently, a cylinder, for example cylinder
 40a that has just been filled, is isolated from the resin supply vessels 
 16 and connected to mold 10. Conversely, the other cylinder, for example 
 cylinder 40b, is in turn connected to one or more resin supply vessels 16 
 for filling. 
 As soon as injection of resin from one of the injection cylinders, for 
 example cylinder 40a, into mold 10 begins, the speed of the cylinder's 
 electric motor 46a is continuously regulated by speed regulator 48a 
 governed by the flow-rate control signal SCQ supplied by control means. 
 The injection flow-rate of the resin into mold 10 remains constant and 
 equal to the reference flow-rate Qref as long as the flow-rate control 
 signal SRQ is greater than the signal SQref representing this flow-rate 
 setting. In contrast, when the value of the flow-rate control signal SRQ 
 is less than that of the signal SQref, the speed of electric motor 46a is 
 regulated so that injection of resin into mold 10 occurs at a flow-rate q 
 lower than the flow-rate setting Q. Injection flow-rate q thus varies with
 the value of signal SRQ. As will be seen below, this flow-rate control 
 signal SRQ is determined by fuzzy logic control card 46, mainly using 
 instantaneous pressure PE measured on the inlet of mold 10. 
 In practice, it should be noted that the instantaneous pressure on the 
 inlet of the mold increases progressively as injection progresses until it
 reaches pressure setting PEmax. This pressure increase is due to the 
 presence of the compressed preform 12 that causes resistance to the 
 progress of the resin to increase as it fills the mold. Injection 
 continues at a constant flow-rate until pressure setting PEmax is reached.
 When pressure setting PEmax is reached, return to injection at constant 
 flow-rate is impossible unless at least one other injection characteristic
 is modified. In the apparatus described this is achieved by temporarily 
 reducing the pressure applied to preform 12 by compression means 14. This 
 reduction in pressure causes the instantaneous pressure PE on the inlet of
 the mold to drop immediately below pressure setting PEmax. Consequently 
 control means 72 cause the injection flow-rate of resin into the mold 
 immediately to increase to return to the value of setting Qref. 
 Compression means 14 are then actuated to restore the high pressure applied
 to preform 12. 
 When the apparatus is operated in controlled flow-rate mode, injection of 
 the resin into the mold or molds is performed alternately by cylinders 40a
 and 40b. This alternation is due to the change of state of the second and 
 third three-way valves 42 and 50 that is automatically commanded by 
 control means 72 as soon as one of the cylinders is nearly empty. This 
 data is sent automatically to the control means 72 by the sensors (not 
 shown). Working within the limits of the resin gel time, large parts can 
 thus be produced without having to use the sort of large-size cylinders 
 that are costly and tend to cause premature polymerization of the resin. 
 This operating mode is made possible by the fact that injection of resin 
 into the mold by one of the cylinders is accompanied simultaneously by the
 filling of the other cylinder from one or more resin supply vessels 16. 
 To conclude the description of the apparatus operating in controlled 
 flow-rate mode, it will be seen from FIG. 2 that speed regulators 48a and 
 48b also receive a signal SC that represents the maximum torque Cmax 
 permissible for each electric motor 46a and 46b. This signal SC is derived
 directly from the maximum torque value Cmax entered on the programmable 
 logic controller of control means 72 after passing through scaling circuit
 88. 
 When the operator decides to produce the type of parts that require the 
 apparatus to operate in controlled pressure mode by using switching means 
 74, the first three-way valve 36 is automatically set to its second state 
 in which resin supply pipe 34 is connected directly to shunt pipe 56, 
 which connects it to mold 10 without using the cylinders; the cylinders 
 are excluded from the circuit. 
 In controlled pressure mode, the pressure applied to fibrous preform 12 by 
 compression means 14 is automatically set to a relatively low value, 
 giving a fiber content lower than 54%. In addition, control means 72 are 
 regulated to command the opening of the proportional valve built into air 
 regulator 30 in order to ensure that the pressure of the resin entering 
 mold 10 is generally constant. 
 In controlled pressure mode, the resin is continuously injected directly 
 into the mold or molds 10 from one or more resin supply vessels 16 in 
 succession due to the successive opening of solenoid valves 32. The 
 injection pressure is set by the value of the pressure in tank 20, i.e. by
 the degree to which the proportional valve of air regulator 30 is opened. 
 In order to eliminate any risk of interruption in resin injection while 
 switching from one resin supply vessel 16 to another by means of solenoid 
 valves 32, the solenoid valves are automatically controlled by control 
 means 72 using data supplied, for example, by weighing means (not shown) 
 associated with the resin supply vessels 16, or by the flow-rate meter 58 
 before the resin supply vessels are completely empty. 
 When the resin is injected into the mold or molds 10 in controlled pressure
 mode, the proportional valve of air regulator 30 is continuously regulated
 by the pressure signal SCP supplied by control means 72. The injection 
 pressure of the resin in the mold or molds 10 thus normally remains 
 constant and equal to the reference pressure Pref as long as the pressure 
 regulation signal SRP is greater than the signal SPref representing this 
 pressure setting. In contrast, the control means 72 actuate the 
 proportional valve of air regulator 30 so that the injection of resin into
 the mold or molds 10 takes place at a pressure p that is less than the 
 pressure setting Pref when the value of the pressure control signal SRP is
 lower than that of the Pref signal. Injection pressure p thus changes 
 according to the value of the signal SRP. 
 In this controlled pressure mode, the pressure control signal SRP is set by
 the fuzzy logic control card 78 only using the instantaneous value QE of 
 the flow-rate measured by flow-rate meter 30 on the inlet of the mold or 
 molds 10. 
 The flow-rate control signal SRQ and pressure control signal SRP are 
 generated in the fuzzy logic control card or cards 78 in virtually the 
 same way. The essential difference lies in the fact that the flow-rate 
 control signal SRQ is generated using the three input signals S1, S2 and 
 S3 while the pressure control signal SRP is only generated using the two 
 input signals S4 and S5. The description that will now be given to explain
 the operation of fuzzy logic control card 78 in generating the flow-rate 
 control signal SRQ using the three input signals S1, S2 and S3 can 
 therefore easily be extended to understand how fuzzy logic control card 78
 generates the pressure control signal SRP. 
 In controlled flow-rate mode, the use of fuzzy logic is justified by the 
 need to handle two pressure settings simultaneously, namely PE on the 
 inlet of the mold and PV on the outlet of the cylinders. Irrespective of 
 the operating mode, fuzzy logic is also useful for controlling injection 
 without the need to know the exact mathematical modeling of each factor: 
 speed regulators, electric motors, reducers associated with motors, 
 cylinders, pipework, heat exchanger, air regulator, resin supply vessels, 
 valves, resins, etc. 
 The fuzzy logic processing carried out by the control card or cards 78 can 
 be roughly broken down into three stages: 
 the transformation of each instantaneous input signal S1, S2 and S3 (or S4,
 SR in controlled pressure mode) into a fuzzy part; this is the input 
 stage, 
 the transformation of the fuzzy parts derived from the input signals into a
 fuzzy part that is representative of the output signal; this is the 
 inference stage, and 
 the transformation of the fuzzy part produced during the inference stage 
 into an instantaneous output signal SRQ (or SRP in controlled pressure 
 mode); this is the output stage. 
 This processing operation makes it possible to model the know-how of 
 operators skilled in using resin transfer molding systems. In practice, 
 this modeling is obtained on the basis of logic rules that describe the 
 possible conditions of the system and the actions appropriate to those 
 conditions in terms of fuzzy logic. 
 The transformations brought about during the input and output stages are 
 effected by allocating membership functions to each input magnitude and 
 the output magnitude. These membership functions are fuzzy sets defined 
 mathematically by functions that are generally triangular or trapezoidal. 
 For example, in fuzzy logic control card 78 the instantaneous value of each
 signal S1, S2 and S3 (or S4 and SR in controlled pressure mode) is 
 compared with membership functions defined beforehand for each of the 
 following input magnitudes: .DELTA.PE, ERPE, .DELTA.PV (or .DELTA.QE and 
 ERQE in controlled pressure mode). 
 In the embodiment described, the membership function allocated to the 
 deviation .DELTA.PE (or .DELTA.QE in controlled pressure mode) between 
 pressure setting Pemax (or flow-rate QEmax) and instantaneous pressure PE 
 (or instantaneous flow-rate QE) on the inlet of mold or molds 10 are: high
 negative (Ng), medium negative (Nm), low negative (Np), zero (Ze), low 
 positive (Pp) medium positive (Pm) and high positive (Pg). 
 Similarly, the membership function allocated to the deviation ERPE (or ERQE
 in controlled pressure mode) corresponding to the difference between two 
 consecutive first signals S1 (or between two consecutive fourth signals 
 S4) are: high negative (Ng), medium negative (Nm), low negative (Np), zero
 (Ze), low positive (Pp), medium positive (Pm) and high positive (Pg). 
 Finally, the only membership function allocated to the input magnitude 
 .DELTA.PV that is representative of the difference between the maximum 
 pressure permissible on the outlet of the injection cylinders 40a and 40b,
 PVmax and instantaneous pressure PV measure on the outlet of the cylinder 
 is: bad (this magnitude has no equivalent in controlled pressure mode). 
 Membership functions are also allocated to the output magnitude in the 
 fuzzy logic control card 78. These consist of changes .DELTA.SRQ in the 
 rotation speed of electric motor 46a or 46b (or .DELTA.SRP in the opening 
 of the proportional valve of air regulator 30 in controlled pressure 
 mode), represented by flow-rate control signal SRQ (or SRP in controlled 
 pressure mode). In the embodiment described, the functions allocated to 
 the output magnitude are: high negative (Ng), medium negative (Nm), low 
 negative (Np), zero (Ze), low positive (Pp), medium positive (Pm) and high
 positive (Pg). 
 The membership functions of the input magnitudes are related to the 
 membership functions of the output magnitude by a table of rules. If the 
 influence of input magnitude .DELTA.PV, which only rarely has any effect, 
 is ignored, the table of rules has two inputs, .DELTA.PE and ERPE, and one
 output, .DELTA.SRQ. This table of rules is drawn up experimentally and may
 take the following form: 
 
 TABLE OF RULES 
 ERPE 
 .DELTA.SRQ Ng Nm Np Ze Pp Pm Pg 
 .DELTA.PE Ng Ng Ng Ng Ng Ng Ng Ng 
 Nm Ng Ng Ng Ng Nm Nm Nm 
 Np Nm Nm Nm Np Ze Ze Ze 
 Ze Ze Ze Ze Ze Ze Ze Ze 
 Pp Ze Ze Ze Pp Pm Pm Pm 
 Pm Pm Pm Pm Pg Pg Pg Pg 
 Pg Pg Pg Pg Pg Pg Pg Pg 
 Moreover, the only rule that relates input magnitude .DELTA.PV to the 
 output magnitude .DELTA.SRQ is: 
 if .DELTA.PV=BAD, then .DELTA.SRQ=Ng. 
 This rule has priority, i.e. it is imposed irrespective of the values of 
 the other input magnitudes. 
 The membership functions allocated to deviation .DELTA.PE, variation rate 
 ERPE, deviation .DELTA.PV and modification .DELTA.SRQ to be made to the 
 rotation speed of electric motor 46a or 46b are shown in FIGS. 4A, 4B, 4C 
 and 4D respectively. 
 For input magnitudes .DELTA.PE and ERPE (FIGS. 4A and 4B), the membership 
 functions are, for example, in the shape of identical isosceles triangles 
 that partially overlap with one another, whose bases lie on the axis of 
 the abscissas and that are regularly offset in relation to the other 
 triangles. The instantaneous value of the corresponding magnitude is 
 plotted or read on the axis of the abscissas depending on whether the 
 magnitudes are inputs or outputs. The axis of the ordinates determines the
 degree of membership of the magnitude with each function; the height of 
 the isosceles triangles represents the functions corresponding to the 
 unit. The measurements and angles of the bases of the triangles are 
 established experimentally. 
 In the case of the input magnitude .DELTA.PV (4C), the membership function 
 BAD has the shape of a rectangular trapezoid whose height corresponds to 
 that of the unit. 
 Finally, as shown in FIG. 4D, the membership functions of the corresponding
 to the output magnitude .DELTA.SRQ are regularly spaced right-hand 
 segments that are parallel to the axis of the ordinates. The length of 
 each segment is a unit. Each segment thus corresponds to a precise speed 
 increase (+direction) or decrease (-direction). 
 All the parameters corresponding to the table of rules and membership 
 functions are introduced in a once-only operation into the fuzzy logic 
 control card 78. 
 Assuming that the instantaneous pressure PV on the outlet of injection 
 cylinder 40a or 40b is more or less lower than the maximum permissible 
 pressure PVmax, as is usually the case, fuzzy logic control card 78 
 establishes the change .DELTA.SRQ to be made to signal flow-rate control 
 signal SRQ using the instantaneous values of input magnitudes .DELTA.PE 
 and ERPE given by signals S1 and S2. 
 For example, the fuzzy logic control card 78 establishes on the input the 
 degree of compliance of each input magnitude .DELTA.PE and ERPE with the 
 membership functions related to these magnitudes. 
 More precisely, it is assumed, for example, that the value of the first 
 signal S1, plotted on the axis of the abscissas of the membership 
 functions related to deviation .DELTA.PE, is located in both the isosceles
 triangles representing membership functions Pm and Pp associated with this
 magnitude (FIG. 4A). This indicates that the deviation between the 
 pressure setting PEmax and the instantaneous pressure PE measured on the 
 inlet of the mold or molds is positive and that its amplitude is between 
 medium and low. The degree of membership 0.4 and 0.6 of this deviation 
 .DELTA.PE to each of the membership functions Pm and Pp respectively is 
 given by the ordinate of the intersection of the sides of the triangle 
 representing it with the height given by the point representing the first 
 signal. 
 It is also assumed, again as an example, that the value of the second 
 signal S2, plotted on the axis of the abscissas of the membership 
 functions related to variation rate deviation ERPE of the first signal, is
 located in both the isosceles triangles representing membership functions 
 Ze and Np associated with this magnitude (FIG. 4B). This indicates that 
 the pressure on the inlet of the mold or molds reduces very slightly. The 
 degree of membership 0.1 and 0.9 of the variation rate ERPE to each of the
 membership functions Ze and Np respectively is given as for deviation 
 .DELTA.PE. 
 In the above hypotheses, four of the rules in the table of rules given as 
 an example are involved: 
 if .DELTA.PE=Pm and ERPE=Ze, then .DELTA.SRQ=Pg (1st rule), 
 if .DELTA.PE=Pm and ERPE=Np, then .DELTA.SRQ=Pm (2nd rule), 
 if .DELTA.PE=Pp and ERPE=Ze, then .DELTA.SRQ=Pp (3rd rule), 
 if .DELTA.PE=Pp and ERPE=Np, then .DELTA.SRQ=Ze (4th rule). 
 For each of these four rules the fuzzy logic control card 78 compares the 
 degree of compliance of each input magnitude .DELTA.PE and ERPE with the 
 membership functions concerned by the rule in question, and the lowest 
 degree of compliance is selected. 
 For example, for the first of the above rules, the membership functions 
 concerned are function Pm for the deviation .DELTA.PE and the function Ze 
 for variation rate ERPE. 
 In the case of signals S1 and S2 whose values are given as examples in 
 FIGS. 3A and 3B, the degrees of compliance corresponding to membership 
 functions Pm and Ze are respectively 0.4 and 0.1. It is therefore this 
 last value of 0.1 that is selected. The same procedure leads to selection 
 for the second, third and fourth rules respectively of degrees of 
 membership of 0.4, 0.1 and 0.6. 
 The degree of compliance thus selected for each of the rules under 
 consideration is applied to the membership function corresponding to that 
 rule for output magnitude .DELTA.SRQ. 
 As can be seen from FIG. 4D, this leads to respective allocation for 
 membership functions Pg, Pm, Pp and Ze of the output magnitude .DELTA.SRQ 
 degrees of compliance of 0.1, 0.4, 0.1 and 0.6 for the signals S1 and S2 
 given as examples in FIGS. 4A and 4B. 
 On output, the fuzzy logic control card 78 establishes the position of the 
 center of gravity of all the membership functions Pg, Pm, Pp and Ze 
 weighted by the degrees of compliance 0.1, 0.4, 0.1 and 0.6. It then 
 modifies the previous values of the flow-rate control signal SRQ by 
 determining the change .DELTA.SRQ to be made to this value using the 
 abscissa of this center of gravity. In the example under consideration, 
 the signal SRQ is increased by a value slightly superior to that 
 corresponding to membership function Pp. 
 The same fuzzy logic control card 78, or another card if the capacity of 
 the first card is insufficient, determines, on the basis of a table of 
 rules linking the two input magnitudes ERQE and .DELTA.QE to the output 
 magnitude .DELTA.SRP, the modifications to be made to pressure control 
 signal SRP when the apparatus is operating in controlled pressure mode. 
 More precisely, the table of rules is identical or comparable with that 
 linking the two input magnitudes ERPE and .DELTA.PE to the output 
 magnitude .DELTA.SRP in controlled flow-rate mode. Similarly, the 
 membership functions allocated to deviation .DELTA.QE, to variation rate 
 ERQE and the change .DELTA.SRP to be made to the pressure control signal 
 are identical or comparable to those allocated to magnitudes .DELTA.PE 
 (FIG. 4A), ERPE (FIG. 4B) and .DELTA.SRQ (FIG. 4D). 
 The invention is clearly not limited to the embodiment that has been 
 described with reference to the figures. For example, where the apparatus 
 comprises injection cylinders suitable for withstanding all the pressures 
 likely to reached in all the production ranges concerned, measuring the 
 instantaneous pressure on the outlet of the cylinders is not necessary 
 during operation in controlled flow-rate mode.