Lightweight Structures B.V. | Vacuum infused composite parts for small aircraft | lightweight, structures, engineering, vacuum, infusion, composites, bridge, thermoplastic aerospace, maritime, building, construction, truck bodies, trailers, road furniture, automotive, wind, energy

Lightweight Structures B.V.
Rotterdamseweg 380
2629 HG DELFT

Tel 015 - 278 2099
Fax 015 - 278 7299

aldert.verheus@lightweight-structures.com

Kamer van Koophandel nr 27280593

Vacuum infused composite parts for small aircraft

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ENAER's Eaglet with infused rudder

Lightweight Structures B.V. has developed a vacuum infusion process for manufacturing composite small aircraft parts that:

eliminates exposition of workshop personnel to harmful components in epoxy resins during open-mould manufacturing techniques;
reduces costs of composite parts due to lower investments in tooling and machinery, and lower labour costs;
improves reproducibility of composite part quality due to less dependency on personnel’s skills;
renders material properties in compliance with JAR-22/23/25/27/29 requirements;
bears only limited consequences for the design, production and test methods for aircraft parts in compliance with JAR-21/22/23/25/27/29 requirements.

Novelty and degree of innovation

Existing manufacturing techniques for composite parts of small and large aircraft are wet lay-up combined with vacuum bagging and autoclave moulding of prepregs, respectively. These open-mould and manual methods have been used for decades to manufacture, for example, composite wing and fuselage parts. Glass and carbon woven fabrics and epoxy resins, as raw materials or as prepregs, have been certified for these applications.

However, the disadvantages of vacuum bagging and autoclave moulding are essential in terms of efficiency:

The working conditions for composite workshop 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. Eventually working with open-mould techniques is prohibitive for them.
The composite properties (mechanical performance, fibre volume fraction) are dependent on (variation in) the practical skills of the workshop personnel.
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.

RTM and vacuum infusion

During the last decade large aircraft manufacturers and material suppliers developed a growing interest in a manufacturing technique called resin transfer moulding (RTM). It is a closed-mould technique in which a (preformed) reinforcement of glass or carbon fabrics is positioned into a matched die mould. A liquid resin is then injected into the mould cavity. After curing of the resin the component is demoulded. The injection pressure is applied by pressurising the resin or by creating a vacuum in the mould; the combination is known as vacuum assisted resin injection.

In the non-aerospace market another RTM version is becoming popular. A growing number of glass/polyester processing companies in Europe and USA have introduced vacuum infusion as an alternative for their open-mould processing techniques like spray-up and hand lay-up. A single (composite) stiff mould is used with a plastic film or flexible mould as a countermould. In this way vacuum infusion is turned into a technique which can be used efficiently and effectively for composite structures larger than 10 m². Applying relatively cheap plastic films makes vacuum infusion economically feasible for very small product series. Vacuum infusion using plastic films is currently being used world-wide for manufacturing glass reinforced polyester and vinylester rotorblades (typical length 30-60 m) and sailing yacht hulls and decks (typical length 20-30 m). (Please take a look at the Vacuum infusion page in Technologies tab in the righthand sidebar of this website for further information.)

In table 1 the expected characteristics of vacuum infusion for small aircraft are shown compared to the state-of-the-art RTM technology for large aircraft.

Table 1 Comparison of RTM vs. vacuum injection, projected on aerospace industry

RTM – large aircraft (state-of-the-art) (state-of-the-art)	Vacuum infusion – small aircraft
heavy, metal matched die moulds	one-sided composite moulds, with plastic film
one-component epoxies	conventional epoxy resins or novel styrene based hybrid resins
(vacuum assisted) pressure injection	vacuum infusion
injection at high temperature	infusion at low or room temperature
preforming necessary	preforming not necessary
surface area typically 1 m²	surface area typically >10 m²
total costs: still expensive	total costs: relatively cheap

Innovative aspects
The innovative aspects of the Lightweight Structures approach of vacuum infusion are:

A validated vacuum infusion process for composite parts for small aircraft as an alternative for wet lay-up/vacuum bagging and prepregging/autoclave moulding laid down in process rules.
The determination of material properties in compliance with JAR-22/23/25/27/29 requirements, resulting from the vacuum injection process.
The determination of the consequences of the vacuum infusion process for the design and calculation methods for aircraft parts in compliance with JAR-21/22/23/25/27/29 requirements, laid down in design rules.

Outline of the Lightweight Structures approach

The methodology of the approach is to start with analysing/defining the requirements (the commercial and aviation rules) and their constraints on further developments. Based on the outcome a recursive (theoretical and experimental) investigation is foreseen into:

making an inventory and selecting composite raw materials (resins, fabrics, cores) in terms of both (resulting) mechanical properties and flow behaviour;
(re)-designing the (composite or aluminium) airplane part in terms of structural performance, costs, weight, assembling et cetera based on the inventory made in the previous step;
defining, characterising and selecting vacuum infusion processes in terms of infusion strategies, flow materials, tooling, auxiliaries et cetera, including mould filling simulation for a selected redesigned part.

The recursive approach is due to the nature of composites engineering and manufacturing, i.e. an (advanced) composite structure is produced while the material is composed. The consequence is that production technique, structural design and material choice are highly inter-linked.

Materials, coupons and components will be mechanically tested (static & dynamic) to investigate if they comply with the aviation rules. The manufacture of the components is also used for technical, environmental and economical evaluation of the process concept.

In a next second stage this more basic knowledge is used to manufacture demonstrator parts. Parallel to the development of these demonstrators, process rules (describing the preferred ways of vacuum infusion for aerospace structures) and design rules (describing the influence of the choice for vacuum infusion process on the structural design) are developed and laid down in handbooks/standards according to JAR regulations.

Currently Lightweight Structures B.V. is involved in the development of a low cost composites manufacturing route with DSM Composite Resins, Stork Fokker and Airbus in which vacuum infusion and the application of styrene based hybrid resins are combined.

Please let us know if you are interested in our capabilities!