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RTM technology development: new methodologies for process control improvement

News International-French

5 Aug 2011

This article was presented at the 25th Jubilee International SAMPE Europe Conference 2004, that took place in Paris from March 30th to April 1st 2004.

(Published on May 2005 – JEC Magazine #17)

 

ARNAUD ALIX,
PROCESS DEVELOPMENT ENGINEER

EMMANUELLE BENAZZI,
HEAD OF THE PHYSICAL/
CHEMICAL TESTING
LABORATORY
DASSAULT AVIATION

HERVÉ TRÉTOUT,
HEAD OF THE NON DESTRUCTIVE TESTING
LABORATORY

 

The interest of the RTM process in the manufacturing of new composite parts is now well known. At Dassault Aviation, the RTM process has been developed for several years through the manufacturing of more and more complex feasibility parts for structural applications. These developments have lead to industrial applications as the horizontal stabiliser spars for Falcon business jets with now more than 1400 pieces produced in the plant at Biarritz.

 

-The goal of this article is to show how the development phases, the process control improvement and new methodologies like process simulation contribute to optimise the industrialisation of new complex parts for future aircraft.

 

Constant process control improvement:

 

The process control includes the material knowledge, the injection parameters and process window validation, the mould design, the production feedback for the existing industrial parts and so on, without neglecting the constant improvement coming from the development phases on more and more complex parts. So what we call “process control” put together the experience of both development and industrial phases, so as to improve the process robustness and optimise the process window.

 

Knowledge coming from development phases

 

The first thing to be considered is that the RTM parts targeted are more and more complex and integrated. First, the development phases are aimed to take into account these increasing difficulties. Through the state of the art of the RTM process, it can be shown how the increasing complexity of RTM parts has been taken into account by Dassault Aviation. First the horizontal stabiliser spars made it possible to integrate the ribs. The following developments on fuselage part demonstrators such as fuselage frame, box structures… made it possible to validate the process on bigger and more complex parts but with only one material. After that came demonstration parts with “multi-material” applications such as the M2000 strake or the F900 landing gear door with bi-axial fabrics for the two skins, foam and carbon inserts. Then the complexity increased with more and more integration. Two examples are the feasibility parts of an RTM elevon and to thicker parts representative of a cockpit frame (thickness evolution from 12 around 25mm). For these applications curing difficulties and woven deformation during draping were new problems to deal with. Today, the new coming technologies such as multi-axial woven, 3D-reinforcement technologies, integrated parts by preform assembling (stitching, rods…) are studied.

 

 

Concerning material knowledge, the development phase is aimed at defining how the reinforcement can be well preformed, manipulated and impregnated for a targeted part. The fabric structure, the binder and the resin for impregnation have to be perfectly known in order to specify the best material to the supplier. The following graph presents some relevant pieces of the methodology for material comprehension.

 

Moreover, the material knowledge (especially for resin modelling), must take into account “industrial constraints” such as:
- the fibre volume tolerances due to the industrial mold…,
- the thermal history of resins and binders before injection : storage at -18°C, at room temperature, debulking…,
- the thermal history of resins and binders in the mould: preheating, injection and curing,
- fabric deformation in the mould,
- the thermal history of resins and binders after injection: preform cooling within a massive industrial mould,
- and a lot of other parameters…

 

This kind of parameters could be ignored before but become very important with the coming bigger and more complex parts manufactured with the RTM process, because of the increasing manufacturing cycle time.

 

Figure 1 illustrates the optimisation work on the material so as to be able to inject it correctly (fibre volume optimisation, carbon and stitching yarn density validation, fabric permeability…) and the necessity to associate the NDT (Non Destructive Testing) process as soon as possible in the material development phase. A comparison with already known fabrics based on the following data (US absorption depending on the material thickness) is then possible:

 

 

A new NDT approach is developed further.
Figure 2 shows a test based on glass transition temperature measurement along a part aimed at validating the type of binder. Indeed, it has been demonstrated that a inappropriate binder could migrate within the preform during the injection and disturb the thermo-mechanical properties (Tg) :

 

 

The glass transition temperature measured at the injection gate is similar to the Tg value of the injection resin alone. The Tg value decreases regularly till the event (around 7% loss). These figures demonstrate the binder migration within the fabrics during the injection by identifying the thermo-mechanical properties evolutions of the final part. The consequence is a non-homogeneous binder ratio in the preform. The following table shows the impact of the binder ratio on the injectability time:

 

Injection resin(%) Binder(%) Minimum
viscosity(mPa.s)
Injectability
time *(min.)
100 0 7 30
95.4 4.6 12 25
90 10 26 17
87 13 36 12

* The injectability time is defined as the time the resin viscosity remains under 0.1 Pa.s

 

A binder ratio evolution on the preform could lead to injectability problems. The conclusion is that this type of binder can not be selected. The third example shows kinetical and rheological laws for resin modelling which could be used as input for process simulation. The parameters of these laws are defined thanks to experimental tests such as DSC, rheology tests… at different temperatures. The new developed models intend to take into account the thermal history of the resin, especially to deal with typical “industrial problems” like resin storage and debulking cycles before injection, injectability time for big parts, exothermy within thick parts… Before having a complete and validated thermal model of resins, a law based on experimental tests can also be used to evaluate the injectability time with industrial constraints :
T = T0 - f(x0, x1, x2, x3 x4)
where :
To: injectability time at 160°C for a new resin in minute
x0: resin storage time at -18°C (day)
x1: resin storage time at room temperature (hour)
x2: resin storage time at 40°C (hour)
x3: resin storage time at 75°C mini debulking temperature (hour)
x4: resin storage time at 90°C maxi debulking temperature(hour)

 

The validation of this experimental law has been realised on different cycles:

  Cycle
description
Initial
injectability time
Experimental
injectability time
Calculated
injectability time
cycle 1 12 months at -18°C
3 days at room
temperature
2 days at 40°C
10 hours at 75°C
25 minutes 10 minutes 11 minutes
cycle 2 6 months at -18°C
3 days at room
temperature
16 hours at 40°C
45 minutes at 65°C
27 minutes 24 minutes 22,5 minutes

* The injectability time is defined as the time the resin viscosity remains under 0.1 Pa.s

 

These 3 examples are good illustrations of new methodologies developed to improve the process control at the material level but also at the industrial level.

 

Knowledge coming from process simulation

 

Concerning the process parameter and process window validation, and the mould design, a new tool as the process simulation enables to decrease the high costs of real feasibility iterative tests. That is why RTM process simulation has an increasing role both on the development and industrialisation steps to help the manufacturer from the feasibility to the mould design and the process window validation.

 

This kind of simulation has been developed in parallel with the process developments and is not only a filling simulation anymore but takes into account all the steps of the process. The following example is based on a representative part of a cockpit frame for business jets:
- preheating of the mould:
   - preventing from cold area,
   - heating mean choice,
   - optimised mould heating without preform temperature disturbing.

 

 

- draping:
  - shear prediction of bi-axial fabrics during the draping phaseinfluence on permeability, fibre volume ratio…,
  - filling strategy optimisation (injection gates, vent…),
  - process parameter optimisation (pressure, flow rate, temperature…).

 

 

- curing:
   - cure cycle definition and process window validation,
   - preventing from problem of exothermy in thick area,
  - cure rate homogeneity.

 

All the information coming from the simulation has an impact on the development costs by avoiding a lot of iterative tests.

 

Knowledge coming from production feedback

 

With more than 1400 horizontal stabiliser spars for Falcon produced in the plant at Biarritz, Dassault Aviation has good feedback of the production with the RTM process. Of course, it contributes a lot to the process knowledge and this experience must be taken into account during the development phases. This can be illustrated by the NDT approach chosen on this part. At the beginning, this part was 100% ultrasonically inspected. With the experience and some correlations between NDT, visual inspection and micrographies, it has been proven that for this kind of geometry and this kind of thickness (up to 3mm thick), each porosity type defect detected with NDT methods (with defined criteria for defect diameter) could have been detected by visual inspection. So, the NDT has been drastically decreased and represents now only 4% of the part price.

 

For future structural parts, the same kind of methodology should enable to reduce the NDT costs. The proposed approach is early correlation work between NDT, visual control and micrographies to minimise NDT as soon as possible during the production phase. This kind of correlation is based on micrographies of relevant areas (no porosity till dry area, no US attenuation till maximum US absorption) and can be summarised with such tables:

 

 

The limit of the visual detection can be evaluated in terms of percentage of porosity as well as in terms of US attenuation. If all that we cannot see during a visual inspection corresponds to an acceptable percentage of porosity (that is to say acceptable mechanical properties), the methodology can lead to decrease the NDT process. Of course, mechanical tests have to be performed with material in the limit area of the visual inspection.

 

This methodology deals with thin parts : it has been proven for thicknesses up to 3mm and extension to higher thicknesses is under development. For really thicker parts, this rule is not correct anymore because of possible relevant dry areas in the material thickness without detecting them by a simple visual inspection.

 

Abstract

The interest of the RTM process in the manufacturing of new composite parts is now well known. At Dassault Aviation, the RTM process has been developed for several years through the manufacturing of more and more complex feasibility parts for structural applications. These developments have lead to industrial applications as : the horizontal stabiliser spars for business jets with now more than 1400 pieces produced in the plant at Biarritz. The presentation shows how the development steps, the process control and new means like process simulation contribute to the industrialisation of new complex parts for future aircraft.

 

The process control includes the material knowledge, the injection parameters and the process window validation, the mould definition, the production feedback on the industrial parts, without neglecting the constant improvements coming from the development steps on complex parts (multiaxial woven, new 3D preform technologies, integrated parts, always more complex geometry…). Therefore the process simulation has an increasing role both on the development and industrialisation steps to help the manufacturer from the feasibility to the mould design and the process window validation.

 

Moreover it makes it possible to reduce the high costs of real feasibility iterative tests. Finally, the last complex step is to control and certificate such RTM structural parts at acceptable costs. The proposed approach is an early correlation work between NDT, visual control and micrographies to minimize NDT during production phase. The example of the horizontal stabiliser spar will be given. To conclude it can be said that the RTM process control has drastically improved at Dassault Aviation and reached a satisfying maturity to go further. That is why new complex structural RTM parts will result soon from this progress.

 

Conclusion

 

To conclude it can be said that the RTM process control has drastically improved at Dassault Aviation and reached a satisfying maturity to go further with new complex structural RTM parts. This progress is based both on constant development activities and production feedback. Thanks to this input, new methodologies such as material modelisation including “industrial needs” (e.g.: thermal history of resins, process window…), process simulation, new NDT approaches… have been developed and enable to reduce costs in development, industrialisation and production phases. The final interest of the process control is the process optimisation both for manufacturer needs and mechanical property respect.

7 Sep 2011

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