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Implementation of the Liquid infusion in the
Manufacturing of Aerospace Structures

Periodic Report Summary 1 - ILMAS (Implementation of the Liquid infusion in the<br/>Manufacturing of Aerospace Structures)

Project Context and Objectives:
The use of light weight solutions based on fiber reinforced polymer composites (FRPC) has been more and more increased in the recent years in the whole area of mobility, especially automotive and aerospace [i-v] . FRPC are a relatively new group of materials offering various possibilities to combine the properties of the constituents, namely high stiffness and strength of reinforcing fibers and ductile fracture behavior of polymer matrices. Synergistic effects can result in a composite performance on a high level and not reachable when using each of the constituents separately. In addition to the properties of each constituent, the reinforcement architecture in the composite and the fiber-matrix interface are affecting the final component behavior. In contrast to other materials, in most cases the composite is generated when manufacturing the component in the final geometry. Thus, the manufacturing process has a pronounced effect and is a key factor regarding the composite component performance in service.


Figure 1a. Basics of composite manufacturing

Starting with reinforcing fibers and matrix resin, the reinforcing architecture has to be created, the fibers have to be impregnated with the resin and the whole system has to be consolidated (Figure 1a).
With particular reference to the aerospace industry, hand lay-up of thermoset based prepregs in addition with an autoclave cycle is the traditional procedure. These methods have been used for decades to manufacture, for example, composite wing and fuselage parts and, generally speaking, they allow to reach very complex shapes and highest component quality.
However there are also several disadvantages terms of efficiency associated to them. In particular:
1. The working conditions for personnel are not optimal. A considerable amount of people working in contact with liquid or b-staged epoxies build-up an allergy to specifically the epoxy hardeners.
2. Autoclave moulding and its associated tooling, machinery and equipment is costly, partly due to the costs of prepregs for which conditioned transport and storage are obligatory.
3. The production rate is very limited since placing the thin layers (typically the thickness ranges from 100µm to 300µm) and autoclave cycle are very time consuming.
Accordingly, there is great interest to avoid the autoclave cycle. Out-of-autoclave processing applying low compaction pressure, e.g. by using vacuum bagging, is studied intensively. The main challenge here is the void formation in combination with the through thickness air permeability and resin viscosity [i-iv] . Due to heating, compressed cavities, especially if containing water vapor (e.g. due to environmental humidity during placement), will build-up an internal pressure. Depending on relative humidity and curing temperature the resulting pressure level will exceed the vacuum compaction level already at temperatures slightly above 100°C
As a result of lower compaction pressure when using vacuum bag only curing, the fiber volume content decreases significantly since a power law relationship between compression pressure and fiber volume fraction is given [v] . To enable higher compaction pressures, conventional pressing systems can be used. But, this will limit the part complexity and size.
During the last decade there has been an increasing interest for the liquid composite molding (LCM) techniques and in particular for the vacuum infusion process. In the LCM techniques the reinforcing structure is first build-up completely and afterwards infiltrated by the resin system. The procedure can be done in many different variants, starting with vacuum driven infusion using simple open molds and ending with vacuum assisted but high pressure driven injection using closed molds. Kissinger prepared an overview about different process variants and their advantages and disadvantages [vi]. For higher efficiency, manufacturing of a preform and the successive LCM process are done in completely separated steps.
Liquid composite molding has several advantages. For example:
1. it allows manufacturing of up to 50,000 parts per year (in some cases even up to 100,000 parts per year) and so, it is an excellent processing route for series production [vii, viii] .
2. it is offering very high process flexibility and enables to manufacture complex shapes, highly integrated structures, and a wide range regarding part size.
Over the years a wide variety of different infiltration, mold, and preform technologies have been developed [vi,ix]. Some examples are: Pressurized “Resin Transfer Molding” (RTM), “Vacuum Assisted RTM” (VARTM), “Vacuum infusion” (VI), infusion techniques with through thickness impregnation known as “Seemann Composites Resin Infusion Molding Process” (SCRIMP) [x], or infiltration before compacting to the final fiber volume content by “Compression Liquid Composite Molding” (CLCM) [xi-xiii]. Furthermore, a lot of combined techniques have been developed, e.g. "Resin Infusion under Flexible Tooling" RIFT [xiv], "Differential Pressure Resin Transfer Moulding" DP-RTM [xv], or "Single-Line-Injection" SLI [xvi].
All LCM-techniques can be characterized by fluid flow through a porous structure, i.e. infiltration. As fluid low viscosity resin systems are used. Uncured thermoset resin systems are most commonly used, but thermoplastic systems are also possible, e.g. if they are available as low viscosity in-situ polymerizing systems [xvii-xx]. The porous structure is the reinforcement required for the component. During the infiltration the direction of fluid flow, whether 1d, 2d, or 3d flowing takes place, the driving pressure gradient (evacuated porous structure and/or pressurized resin injection), and some further process parameter (e.g. consolidation pressure and temperature) govern the process [viii, xxi]. A lot of optimization work over the last two decades resulted in LCM processes already challenging the prepreg technology and allowing to manufacture high performance structural components even for aerospace applications [xxii].
It is worth mentioning that LCM manufacturing techniques are widely used in the production of marine structures, like boats or yachts, while limitedly used in the aerospace industry.
The main objective of the present program is to develop a detailed plan, from technical feasibility to manufacturing, for the successful liquid infusion manufacturing process useful for the fabrication of aeronautical components with a market appeal from environmental, economic and technical points of view. In particular the major aim is to transfer all the above mentioned advantages of the LCM techniques to the fabrication of aeronautical components by developing a detailed plan, from technical feasibility to manufacturing, for the successful liquid infusion manufacturing process useful for the fabrication of aeronautical components with a market appeal from environmental, economic and technical points of view.
The following activities have been performed by the involved partners:
1. Feasibility study for the implementation of the liquid infusion manufacturing process in the real industrial environment;
2. Definition of the path to bring the process to the production phase;
3. Evaluation of the industrial repercussion during its implementation in manufacturing plant;
4. Study of the technical and economical impact deriving from its introduction and assessment of the minimum conditions required to make this introduction viable;
5. Evaluation of the relevant parameters of its impact on the environment during the production cycle with reference to the healthy human oriented working;
6. Assessment of the risk assessment plan;
7. Preparation of the manufacturing plan and of the standard manual for the Industrial application.
Although the project addresses a relatively mature manufacture technique for composites, i.e. liquid composite moulding (LCM), it will provide highly increased cost-efficiency in manufacture of composite aircraft components. Building on the partners know-how in LCM techniques from working with the automotive, marine and wind power industries the Consortium will propel technology transfer in efficient composites manufacture to the aircraft industry.


[i] Stauber, R.: Vorwort. In ‚Kunststoffe im Automobilbau‘, VDI-Gesellschaft Kunststofftechnik. Düsseldorf: VDI-Verlag 1999, Seiten 1–2.
[ii] N.N.: BMW: Composites to be mass-produced soon. Composites International, No. 46, Juli-August 2001, S. 48-50
[iii] Lorenz T.: Kosteneffektive CFK Fertigungsverfahren der nächsten Generation. 7. Nationales SAMPE Symposium 2001, Erlangen, 22.-23. Februar 2001, S. III.2.1-III.2.10
[iv] Banhardt V.: Scrimp-Verfahren für Großbauteile im Schienenfahrzeugbau. Duroplaste im Schienenfahrzeugbau, Halle, 9.- 10. Mai 2000, S. K/1-K/14
[v] Russell, J.D. Shenk, B., Holzwarth, R., Swanson, M., Paige, D., Tresnak, M., Ames, S., Neumeier, P.: Advanced composite cargo aircraft, Proc. 2009 SAMPE Fall Technical Conference and Eshibition, Wichita, KS, USA, 19.-22. October, 2009
[vi] Hughes, J.C. Arai, N., Haro, A.P. Satterwhite, J.A.: Studies for the development of OOA prepreg used in aircraft applications, Proc. Intern. SAMPE Tech 2011, Fort Worth, TX, 17.-20. October, 2011
[vii] Grunenfelder, L.K. Nutt, S.R.: Air removal in VBO prepreg laminates: Effects of breathe-out distance and direction, SAMPE Tech 2011, Fort Worth, TX, 17.-20. October, 2011
[viii] Tavares, S.S. Michaud, V., Manson, J.-A.E.: Through thickness air permeability of prepregs during cure, Composites Part A, 40(10), October 2009, 1587-1596
[ix] Stringer, L.G.: Optimization of the wet lay-up/vacuum bag process for the fabrication of carbon fibre epoxy composites with high fibre fraction and low void content, Composites, 20 (5), September 1989, 441-452
[x] Saunder, R.A. Lekakou, C., Bader, M.G.: Compression and micriostructure of fibre plain woven cloths in the processing of polymer composites, Composites Part A, 29A (1998), 443-454
[xi] Kissinger, C.: Ganzheitliche Betrachtung der Harzinjektionstechnik – Messsystem zur durchgängigen Fertigungskontrolle, IVW-Schriftenreihe Bd. 28, Institut für Verbundwerkstoffe, Kaiserslautern, (2001).
[xii] Wallentowitz, H.; Adam, H.; Bröcking, J.: Potential of fiber reinforced plastic space frame structures. VDI Tagung Entwicklung im Karosseriebau, Hamburg, 14-15.Mai 1996, Seite 477-495
[xiii] Beckwith, S.W.: RTM, VARTM, and SCRIMP processing infusin technologies. 'SAMPE 44th ISSE / SAMPE '99', Long Beach, CA, 24.05.1999 Beckwith Technology Group, Murray, Utah (1999)
[xiv] Chavka, N.G.; Dahl, J.-S.: P4: Glass Fiber Preforming Technology for Automotive Applications. In: Resin Transfer Molding SAMPE Monograph No. 3. SAMPE Publications. Covina. USA. 1999.
[xv] Pachalis J.: SCRIMP Technology Overview. The Second Workshop on Liquid Composite Molding, Columbus, Ohio (1996), June 13
[xvi] Young W.B.; Chiu C.W.: Study on compression transfer molding. Journal of Composite Materials, 29 (1995), 2180–2091
[xvii] Meyer-Noack, S.: Konzeption und Erprobung des Spaltimprägnierformens. Dissertation RWTH Aachen, 2006
[xviii] Michaeli, W., Fischer, K.: Analysis of the gap-impregnation process. SAMPE 2009 Technical Conference Proceedings: Changing Times. New Opportunities. Are You Ready?, Baltimore, MD, May 18-21, 2009. Society for the Advancement of Material and Process Engineering, CD-ROM—17 pp.
[xix] Williams, C.; Summerscales, J.; Grove, S.: Resin infusion under flexible tooling (RIFT): a review. Composites Part A, Vol. 27 A (1996), pp. 517-524.
[xx] Sigle, C.: Neu kostengünstige Fertigungsverfahren für CFK-Strukturen. '4. Nationales SAMPE Symposium', Braunschweig, 12.-13.3. 1998, S. 1-15.
[xxi] Sigle, C.: Ein Beitrag zur kostenoptimierten Herstellung von großflächigen Hochleistungsverbundbauteilen. Dissertation. TU Braunschweig 1998.
[xxii] Jiang, Z.; Siengchin, S.; Zhou, L.-M. et al.: Poly (butylene terephthalate)/silica nanocomposites prepared from cyclic butylene terephthalate. Composites Part A: applied science and manufacturing, 40, 3 (2009), pp. 273-278

Project Results:
Liquid infusion process has been proved by recent researches to be suitable for the manufacturing of structural aeronautical components. The aim of this project is to develop a detailed analysis of the technical, economical and environmental impact of the extrapolation to industrial condition of the liquid infusion manufacturing technique.
This analysis is deemed to be a necessary step in the successful scaling-up of the process for the fabrication of aeronautical components. The activity carried out to achieve the aim of the proposal has been the following ones:
1. Preliminary conceptual analysis of the process, where the Raw Materials to be used have been identified, as well as the main phases of the process in order to clarify the applicability of the process in the real industrial environment and to identify the requirements to implement the process and to start the production phase
2. Path to bring process to production phase: A production line for the industrial manufacturing of a reference part, which has been introduced by ALA during the kick-off meeting, has been conceptually designed by means of a production plan as well as a production layout. For that purpose, the individual working steps of the entire production process, i.e. from the arrival of the required materials to quality checks on the final component, have been investigated. Moreover, a production layout has been developed which addresses the entirety of working steps required for manufacturing a single type of reference part in a cyclic manner along a single processing route.
3. Technical and economical impact. Several activities were carried out in this phase. In particular:
• Breakeven point analysis. A breakeven point analysis was carried out which showed that the automated fiber placement solution is preferable by an economic point of view after a breakeven point of 73 manufactured panels per year
• Manual work force rough-cut capacity estimation. A rough-cut capacity estimation of the manual work force necessary to produce the reference panel was carried out.
• Oven and robot utilization curves.
• The layout and energy consumptions. An estimation of the floor spaces required in the plant was made.
4. Impact of manufacturing process defects into mechanical performances and costs. An analysis of the of manufacturing process defects into mechanical performances and costs was carried out
5. Evaluation of the economic impact. The cost elements were divided by category and by their being variable or fixed.
6. Life Cycle Assessment. The SimaPro software was used in order to carry out the LCA.
7. Risk assessment. A risk assessment was carried out to identify and minimize risks during manufacturing and provide recommendations for particular issues. The major outcome of the risk assessment was the introduction of a job card.
8. Manufacturing plan. A manufacturing plan was developed, with indications about optimal equipments to be used and their utilization.
9. Standard manual. A standard manual has been established. It provides a set of recommendations for the development of process specifications for the industrialisation of the manufacturing of the wing stiffened panel in composite material.

Potential Impact:
The topic addressed by the call was the " Extrapolation to industrial condition of the liquid infusion manufacturing process". On this basis, all the activities carried out within the project have been planned with the aim to answer to two basic questions:
1) Is the liquid infusion process feasible in an industrial environment for the manufacturing of aeronautical parts?
2) Is the change from the conventional technology (autoclave) to the new one (liquid infusion) providing environmental, economic and technical benefits and which these benefits are?
The expected impact of the project is therefore to provide decisional tools and results for the effective and successful implementation of liquid infusion in the aeronautical industry manufacturing.
The steps required to produce these tools and results are those described in detail in the previous section, which can briefly summarized in a deep and thorough investigation on the technical feasibility, the industrial plant and manufacturing requirements, and the technical, economical and environmental impact of the implementation of the liquid infusion. As evident from the previous section, the majority of the analyses done in the project involved the application of technical and economical analytical tools.
As mentioned before, hand lay-up of thermoset based prepregs in addition with an autoclave cycle have been used for decades to manufacture, for example, composite wing and fuselage parts and, generally speaking, they allow to reach very complex shapes and highest component quality.
However there are also several disadvantages in terms of efficiency associated to them. In particular:

1. The working conditions for personnel are not optimal. A considerable amount of people working in contact with liquid or b-staged epoxies build-up an allergy to specifically the epoxy hardeners.

2. Autoclave moulding and its associated tooling, machinery and equipment is costly, partly due to the costs of prepregs for which conditioned transport and storage are obligatory.

3. The production rate is very limited since placing the thin layers (typically the thickness ranges from 100µm to 300µm) and autoclave cycle are very time consuming.

4. Due to the high pressures and the high curing temperatures of prepreg materials the energy consumption of the autoclave process might be much high
It is evident that some of these disadvantages have a direct social impact, especially point 1 which deals with workers' health and point 4 which deals with energy consumptions.

As far as the dissemination activities are concerned the following dissemination activities have been carried out:

1. A website dedicated to the project has been implemented containing details of the project;
2. Results obtained by the partners have been circulated within the Consortium and presented during the periodic meetings;
3. Part of the results were presented to some scientific conferences. In particular:
• E. Fauster, C. Schillfahrt, R. Schledjewski. Automated preforming of profiles for efficient LCM-processing of structural components in aeronautic applications, presented at the Reinforced Plastics International BALATON Conference, May 20th-22nd 2014, Keszthely (Hungary)
• L. Maragoni, P.A. Carraro, A. Dallavia, M. Quaresimin. Influence of manufacturing-induced voids on the mechanical properties of carbon/epoxy laminates to be presented at the XLIII National Conference of the Italian Association for Stress Analysis, September 9th-12th 2014 Rimini (Italy)
4. In the next future the partners planned to further disseminate results by presenting them to scientific conference or through publications on international journals.

List of Websites:
http://static.gest.unipd.it/ILMAS