Patent Application
20170348924
The invention relates to the general field of the fabrication of composite material structures by impregnating fibrous preforms with resin, notably using the LRI (liquid resin infusion) or RTM (resin transfer molding) process. It relates more particularly to the monitoring of proper impregnation of the preform by the resin.
The invention relates to the field of the characterization of the injection of fibrous preforms by the LRI or RTM process.
An industrial injection system may be reduced to a piston and a mold. The latter may of constant volume (RTM mold with rigid punch and die) or variable volume (RTM-VAP or LRI mold with rigid punch and flexible membrane with or without compensation plate (or “caul plate”)).
Where the impregnation of the preform with the resin inside the mold is concerned, monitoring the completeness of the impregnation phase entails good control of the quantity of resin with which the fibrous preform is actually impregnated at a given time.
The study of the flow of the impregnation fluid (resin) entails determining the permeability of the reinforcement that is part of the composite structure.
This is determined by the injection at a constant flow rate of a liquid through a sample of the reinforcement to be characterized and measuring the pressure difference created by this injection, which measurement is stored. In this operation, the fibrous reinforcement is progressively saturated by the liquid as the resin front advances.
The basic model governing flow of this kind is governed by the DARCY law according to which:
Q=K×[A/μ]×[ΔP/L]
where here Q represents the volume flow rate of resin, A the flow section, μ the viscosity, and L the flow length.
The
As can be seen in the figure, a system of this kind includes a resin injection monitoring device 13 positioned at the inlet of the mold 13 and an excess resin recovery monitoring device 15 at the exit from the mold. Here by excess resin is meant resin that has not been captured by the preform during its transit in the mold, for whatever reason.
The inlet device 13, connected to the fitting 11 mounted at the inlet of the mold 14, essentially includes means 131 for injecting resin at a controlled pressure and means 132 for producing a vacuum of given value in the device, the production of a vacuum in this way notably being for the purpose of impregnating the fibrous preform.
For its part, the outlet device 15 includes an inlet circuit connected to the fitting 12 mounted at the outlet of the mold 14 including an inlet valve 151 connected to a settling tank 152 and means for degassing the recovered resin, said means notably including a vacuum pump 153.
At present, there is no method of effectively quantifying the flow of resin entering or leaving the mold. The porosity introduced by incorrect degassing or leakage is not clearly identified.
In the systems generally used at present, the inflow parameters that govern the flow of the resin and its absorption by the preform (flow rate, pressure, temperature of injection, . . . ) are set points merely introduced into the system that controls the injection pressure and no further monitoring is carried out downstream of that system.
Thus, various insufficiencies are found in the systems generally used at present:
What is more, merely installing a thermocouple just ahead of the inlet into the mold does not make it possible to distinguish the thermal front from the real front of the resin as it advances inside the mold, so that there exists a real uncertainty concerning the geographical presence of the resin.
Also, the flow sensors generally installed in the body of the mold detect well the presence of resin but give no information as to its degassing state.
Also, in the context of a (variable volume) LRI structure, the lifting of the caul plate or the of the membrane of the mold is evaluated only through the variation with time of the ratio of the injected volume to the theoretical (predicted) volume. This variation is subject to errors inherent to the parameters cited above.
To alleviate these insufficiencies the injection time is generally lengthened, which in practice is reflected in a lengthening of the for which the resin is allowed to flow into the recovery device. The effect of this lengthening is to lengthen the impregnation operation and to increase the quantity of resin injected unnecessarily into the mold, which resin is in the end poured into the recovery device.
It is known from the document US2007/145622 A1 to install ultrasound type sensors in a resin injection mold but those sensors are arranged inside the mold and therefore do not make it possible to monitor and to analyze the entry flow and the exit flow of the resin.
This also applies to the document US2002/155186 A1 in which the sensors are positioned inside the mold.
An object of the invention is to propose means for and a method for:
To this end the invention consists in an ultrasound device for characterizing the flow of resin entering and leaving an injection mold during the phase of impregnation by said resin of a preform contained in said injection mold. The device includes two ultrasound sensors arranged respectively in the vicinity of the inlet port where the resin enters the injection mold, outside said mold, and in the vicinity of the outlet port where the resin leaves said mold. Each sensor emits an ultrasound wave towards the end of the mold in the vicinity of which it is positioned and receiving the ultrasound wave reflected by the medium.
According to one feature of the invention, the ultrasound sensors are devices integrated into the structure of the fittings for connecting the injection mold to means for injecting resin into said mold and means for recovering resin evacuated from the mold.
According to another feature, the device according to the invention also includes means for effecting the temporal analysis and/or the spectral analysis of the ultrasound wave received by each of the sensors and determining the stabilization of the flow of resin through the mold.
In one particular embodiment suited to the situation in which the injection mold includes means for monitoring inside the mold the material health of the part formed from the preform, the device according to the invention further includes means for determining the completeness of the step of impregnating the preform by combining the information supplied by the means for analyzing the reflected ultrasound waves received by the ultrasound sensors and the information supplied by the material health monitoring means.
The invention also consists in a method for determining the completeness of the operation of impregnating a preform with a resin, said preform being positioned in an injection mold into which the resin is introduced, said method employing the device according to the invention. The method includes the following steps:
The invention also consists in a method for determining the completeness of the operation of impregnating a preform with a resin, said preform being positioned in an injection mold into which the resin is introduced, said method employing the device according to the invention. The method includes the following steps:
According to one particular embodiment of the above method, the third step of monitoring the injection operation also employs an operation of analyzing the material health monitoring effected inside the module, the injection process being stopped if the flow of resin leaving the mold is stabilized and if the material health monitoring is positive.
According to an additional feature of the method according to the invention in its preceding form, the third step takes account of the elapsed time so that if, during the execution of the third step the material health monitoring remains negative while the flow of resin leaving the mold has stabilized since at least a given time lapse, the injection process is continued and then stopped at the end of a limit time lapse if, despite the lengthening of the injection operation, the material health indicator remains negative.
The features and advantages of the invention will be better understood thanks to the following description, which description relies on the appended figures, which show:
It is to be noted that in the figures appended to the present text the same reference corresponds to the same functional element or to the same function.
As
According to the invention, the ultrasound sensors employed are piezoelectric components able to function at a high temperature, typically at the resin injection temperature. The operating principle of these sensors is that of ultrasound echography based on relative measurements of amplitude (attenuation) and measurements of time delay (flight time) as well as on a frequency (phase-shift) analysis of the echoes of an emitted acoustic wave reflected by the various structures encountered during its passage within the thickness of the material being produced, placed in the mold.
Also in accordance with the invention, the ultrasound sensors are disposed as close as possible to the inlet and outlet ports of the injection mold so that no pressure or temperature drop can affect the pertinence of the measurements of the state of the flow of resin at the inlet and at the outlet of the mold.
According to the invention, the ultrasound sensors 21 and 22 are positioned as close as possible to the inlet 23 and the outlet 24 of the mold. In a preferred but non-exclusive embodiment of the device according to the invention illustrated by
A fitting according to the invention is generally made of steel. However, it can equally well be made of a refractive polymer, for example polyimide charged with graphite.
Refractory polymer fittings favor the propagation of the emitted ultrasound signal because of the closeness of their acoustic characteristics to those of the material to which the event to be quantified relates, notably epoxy resin. However, these fittings, although reusable nevertheless have a limited service life (because of wear). The low cost of manufacture/fitting is a material choice criterion here.
Thus the analysis of the reflected echoes as seen from the side of the inlet port 23 of the mold 13 and captured by the sensor 21 positioned at the level of the port 23 makes it possible to detect and to timestamp the events such as the presence of a flow of resin at the inlet of the mold 13 and the absence of porosity (i.e. of gas bubbles) in that flow of resin. This time stamping notably makes it possible to define a synchronization pulse triggering the starting up of the monitoring system responsible for the evaluation of the material-health of the impregnated object during the operation.
A monitoring system of this kind is for example a system constituted in known manner of various ultrasound sensors disposed inside the injection mold and the function of which is to determine by echography if the composite material being fabricated inside the mold features any structural anomaly. The published French patent application FR 2995556 filed by the applicant notably describes a “material health” monitoring system of this kind.
For its part, the analysis of the reflected echoes as seen from the side of the outlet orifice 24 of the mold 13 and captured by the sensor 22 positioned at the level of the port 24 makes it possible to evaluate the volume of resin injected (mold of fixed volume and volume of fibers determined as a percentage of that volume) as well as its quality (degassing) and thus to define the end of injection operation pulse as accurately as possible.
This end pulse is moreover intended to be compared with the information supplied by the sensors constituting the monitoring system situated on the upstream side of the mold 13 to determine as accurately as possible the time at which it can be considered that the impregnation of the preform is completed in order to stop the impregnation operation and to launch the operation of polymerization of the resin impregnating the preform.
In addition to the two fittings equipped with ultrasound sensors, the device according to the invention also includes means for analysis of the signals transmitted by the sensors, these means carrying out the analysis of the received signals to determine if the flow of resin at the location concerned is a stabilized flow. By a stabilized flow is meant a continuous flow of resin with no gas bubbles present. As indicated above, the analysis of the received signals consists mainly in a measurement of the variation over time of the amplitude of the echo received by the sensor concerned.
The determination of these amplitude variations notably makes it possible to determine at the level of the inlet of the mold the time at which the resin begins to pass through the fitting and the time at which the resin flows in a continuous stream (without bubbles) through the fitting (time T0). In the same way it makes it possible to determine at the outlet of the mold the time at which the resin leaving the mold begins to pass through the fitting and the time at which the flow of resin through the latter becomes continuous (complete impregnation).
The principle of determination of the above times is illustrated by the
In this diagram, the curve 71 shows the variation over time of the travel time of the soundwaves in the fitting at the level of the inlet fitting 11 for times before and after the arrival of resin in the fitting and the curve 72 shows the variation over time of the amplitude of the soundwaves for the same times before and after the arrival of resin in the fitting.
For these two curves, the arrival of resin is characterized by a sudden inflection (zone 73 of diagrams 71 and 72). After stabilization of a flow of resin without bubbles each of the two curves has an easily discernible and substantially constant amplitude or propagation time value (zone 74 of diagrams 71 and 72).
As can be seen in
From a functional point of view, and with the aim of optimum control of the impregnation process, as much in terms of impregnation quality (homogeneity, fiber content, etc.) as in terms of operation duration and quantity of resin used, the device according to the invention can advantageously be employed to carry out fine control of the process of impregnation of the preform.
To this end, the device according to the invention may be employed on its own or in association with the internal monitoring system equipping the injection mold and intended mainly to determine the material health of the composite material component produced inside the mold. The state of material health is a criterion generally resulting from echography measurements carried out by means of ultrasound sensors installed in the mold the echoes from which are analyzed and the analysis results compared to reference values, the agreement with the reference values making it possible to declare the part in a good state of health.
In this variant, the monitoring method includes the following steps:
It should be noted that because the time of starting counting is determined by the device according to the invention, a more accurate measurement is available of the real duration of the injection of resin into the mold. Usually, in the absence of the device according to the invention, the duration of the injection generally has to be measured taking as the starting time the time of starting up the resin injection device 13.
Then, when the measured injection time reaches the reference value tinjection, it is generally obligatory to allow the injection to continue for a given further time lapse to take account of the difference that may exist between the time of starting injection (starting up of the device 13) and the real time of the beginning of penetration of the resin into the mold. In contrast, the use of the device according to the invention makes it possible to shorten if not eliminate this time lapse. This therefore achieves optimization of the injection time and economizes on resin.
In this variant, the monitoring method includes the following steps:
Then, as soon as the flow of resin leaving the mold is considered as stabilized, the injection of resin is stopped and the impregnation process is considered as finished. The operation of polymerization of the preform impregnated with resin can then begin.
It should be noted that, in this variant, the determination of an event, other than a time measurement, characterizing the fact that the flow of resin leaving the mold is stabilized makes it possible to provide a stronger guarantee that, the mold being filled with resin, the preform housed in the mold is completely impregnated with resin. A variant of this kind, which exploits the information supplied by the two ultrasound sensors of the device according to the invention, therefore proves advantageously appropriate for ensuring the proper impregnation of a preform intended to produce a composite material component the fabrication quality of which is highly critical.
Without using the device according to the invention the impregnation of the preform to fabricate a component of this kind necessitates extending the injection time well beyond the theoretical time concerned.
In this variant, the monitoring method includes the following steps:
It also includes simultaneously collecting from the material health monitoring system integrated into the mold information making it possible to determine if the structure of the component contained in the mold conforms to what is expected.
It then consists in effecting the merging 631 of the information relating to the stabilization of the flow of resin leaving the mold obtained by the operation 432, and to the good material health of the component.
Then, if the “stabilized flow” and “good material health” conditions are both satisfied in combination (operation 632), the injection of resin is stopped and the impregnation process is considered as finished. The operation of polymerization of the preform impregnated with resin can then begin.
It should be noted that the material health information delivered by the monitoring system integrated into the mold can take various forms that it is therefore necessary to consider and to process so as preferably to make available a simple indicator, for example of the “go-no go” or “0/1” kind, easily exploitable in the context of the method described here, as
It should also be noted that this variant constitutes a sophisticated variant of the previous variant illustrated by
This is in particular the case if the material health monitoring system detects an area of the preform into which the resin is not able to penetrate. In a situation of this kind the material health indicator will assume a permanent “no-go” or “0” state indicating a structural anomaly of the component.
Then, although the flow of resin appears stabilized, the injection operation will be continued until, the zone concerned being finally impregnated, the material health indicator assumes a “go” or “1” state leading to stopping the injection operation. If despite the extension of the injection operation the zone concerned remains non-impregnated, then the material health indicator remains in a “no-go” or “0” state that can lead to stopping the injection operation at the end of a limit time lapse determined elsewhere.
Although the foregoing description elements rely on an application example concerning an RTM process involving a mold of fixed volume, the device according to the invention as has just been described can advantageously be used with various systems for injection of dry textile preforms of RTM and LRI type equally at a high temperature for the usual industrial applications or at room temperature as in the context of permeability benches. The functional characteristics of the device according to the invention are not commensurately modified, however.
Variants of the position of the device on a mold can nevertheless be induced by the nature of the mold or its operating principle, which variants can lead to modification/adaptation of the injection fittings.
This is in particular the case when fitting the device according to the invention to a mold of variable volume, such as LRI injection molds. In a context of this kind the resin outlet port may for example coincide with the vacuum suction port and the outlet fitting constituting the device be placed on that port.
Thus, the sensors can be installed so as to operate them in send/receive mode or in transmission mode.
Similarly, the fittings can be modified in terms of design as much where this concerns the material used (metal in the standard manner or refractory polymer (more costly)), the presence of zones machined or modified specifically to favor the propagation of longitudinal or transverse waves, or a structural optimization linked to the configuration of the injection tooling used (for example multipoint injection).
Moreover, the installation of the ultrasound sensor in the fitting can have various specific features. Accordingly:
The sensitive component, the sensor, can furthermore be mounted on a machined area and fixed by gluing.
The sensitive component may further consist in a deposition of material by spraying (piezo-spraying), the material being cured on the fitting.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1463321 | Dec 2014 | FR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2015/080606 | 12/18/2015 | WO | 00 |
