Lightweight Structures B.V. | Vacuum infusion of rotorblades | 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
The Netherlands

Tel +31 15 278 20 99
Fax +31 15 278 72 99

aldert.verheus@lightweight-structures.com

Chamber of commerce nr 27280593

Vacuum infusion of rotorblades

1. Introduction

There are in the Netherlands three companies that build large composite rotorblades for windmills for electrical energy generation. One of them is Rotorline, currently a daughter company of LM Glasfiber in Denmark, probably the world’s largest rotorblade builder with approximately 1500 employees. The strict competition forces Rotorline to highly optimize engineering and production procedures. The time-to-market of new types of rotorblades is very short. Every year the blade’s length is increased by several meters reacting on the market demands for higher energy output. Current rotorblades are shown in fig. 1. The turbine has three rotorblades with a length of approximately 20 m. Fig. 2 shows a worker inspecting the joint of the root to the axle.

Fig. 1 A current wind turbine with Rotorline rotorblades (length approx. 20 m)

Fig. 2 A worker inspecting the root of the rotorblade

The rotorblades shown in fig. 1 and 2 are made of two shells consisting of 1200 kg vinylester resin, 1300 kg of glass fibre reinforcement (approx. 75% unidirectional and 25% ±45° fabrics) and PVC foam core. The shells are bonded together with adhesives. Optimization of the production processes is necessary due to required short cycle times and the protection of workshop personnel and the environment; the allowed emission of styrene from vinylester resins (conventionally processed with open mould techniques) has recently been decreased in the Netherlands. So, sound and fast production is the driving force for innovative production methods like vacuum injection. In 1998 Rotorline and Centre of Lightweight Structures TUD-TNO co-operated in a project that aimed at adjusting the practical injection strategies using vacuum injection techniques for boat building, as developped by CLS, to the requirements of Rotorline. Underneath in general terms the approach towards implementation of vacuum injection into the workshops of composites processing companies is shown. Some results of this approach in case of LM are shown in the last chapter.

2. Project scheme

In this short outline a description will be given of the different activities necessary for the implementation of the vacuum injection technology for the manufacture of large composites structures, like boat hulls, rotorblades etcetera.These activities will be carried out in a joint effort of the company and the Centre of Lightweight Structures TUD-TNO (CLS). During the project the technology is transferred from CLS to the company involved.

Starting-point
The starting-point is an existing product. This implies that the design (including materials to be used, fibre lay-up, etcetera) is already known. A mould for the manufacture of the product (for hand lay-up or spray-up techniques) is (mostly) already available.

Activities

Setting up a description of the product
Identify the problems which will be encountered and define final product characteristics

GO / NO GO
The next activities depend on the choices made under 2.

Define reinforcement lay-up
A vacuum injection lay-up need to be determined based on the lay-up used for hand lay-up or spray-up manufacture. Both the type fibre reinforcement as well as the core materials will be determined.
Define resin properties and select resin
Define injection strategy
Including small scale experiments, supported by flow simulations if necessary.
Investigate and solve specific problems (identified under 2.)
Selection of additional means, tools and equipment necessary for production
Mould modifications
Training employees
Manufacture of a typical segment of the product
Evaluation of the production process and optimise the process
Manufacture of the product

Some of the above mentioned activities can be carried out simultaneously. The different activities are described in more detail below.

Setting up a description of the product
The product will need to be described in full detail (including materials to be used and reinforcement lay-up). The currently used manufacturing technique need to be described as well.
Identify the problems which will be encountered and define final product characteristics
Based on the description of the product the requirements for the vacuum injection technique are defined. This might include:
- Surface finish
- Integration of parts (stiffeners, etceteras)

Based on these requirements and the description of the product possible problems for using the vacuum injection technology can be identified. Based on these identified problems a GO / NO GO decision can be taken. If a GO is decided the necessary activities can be further elaborated.

Define reinforcement lay-up
Based on the lay-up used for hand lay-up or spray-up manufacture the lay-up for the vacuum injection technology will be determined. The preference of the moulder for specific suppliers can be taken into account. The type fibre reinforcement, the core materials and possible surface veils will be determined.
Define resin properties and select resin
A listing of necessary resin properties will be determined. Based on the currently used resin and the preference for a certain supplier a resin type and formulation will be selected (in close co-operation with the resin supplier). Important resin properties include geltime, peak-exotherm and cure.
Define injection strategy
Based on the overall product shape and reinforcement lay-up a basic injection strategy will be determined. Successively this injection strategy can be optimised in detail. When bottlenecks or problems are foreseen small scale experiments will be carried out, supported by flow simulations if necessary.
Investigate and solve specific problems
These activities strongly depend on the choices made and problems identified under 2.
Selection of additional means, tools and equipment necessary for production
All necessary tools and equipment necessary for production will be selected (varying from tubes, clamps, resin pots and vacuum pumps).
Mould modifications
Based on the selected injection strategy the necessary modification for the mould can be determined. Possible modifications can include vacuum channels, resin outlet ports, etceteras.
Training employees
Employees will be trained and instructed about the vacuum injection technology.
Manufacture of a typical segment of the product
A typical segment of the product will be manufactured. This will ensure a final check of the injection strategy and the different steps in the production process. In addition this can be used to further train the employees (the segment will be manufactures in close co-operation between CLS and the company’s employees).
Evaluation of the production process and optimise the process
All manufacturing aspects will be evaluated. If necessary adjustments will be suggested to improve the process.
Manufacture of the product
Finally the complete product is manufactured.

3. Some results

Fig. 3 shows the root of the rotorblade (the part nears to the axle) just before the vacuum injection process is started.

Fig. 3 The root of the rotorblade just before the vacuum injection process starts

The glass fibre reinforcement (the white material along the edges) lies in the mould; it is covered sucessively with a release fabric (not visible), a flow enhancing knitted fabric, a runner and a nylon film. The film is sealed on the mould flanges with tacky tape. A vacuum is applied between mould and film, so the complete lay-up is compressed in the mould. A liquid vinylester resin will be flowing from a resin container though the runner in the length of the mould. The resin then flows through the flow enhancing knitted fabric into the reinforcement while impregnating the fibre bundles completely. This process is shown in fig. 4 in which the two flow fronts are visible on both sides of the runner. In the left bottom corner the sealing is visible. The filling of the mould takes approximately 1 hour. After the cure of the resin the rotorblade halve can be demoulded.

Fig. 4 One rotorblade halve during vacuum injection.

Fig. 5 shows a detail of the sandwich part in the rotorblade. The sandwich structure is left in the rotorblade of fig. 4, between de flow front and the left mould edge. So, the resin is just entering the foam core. The core consists of small square blocks of PVC foam on a fabric back. Consequently, small channels are present between the blocks in which the resin can flow rapidly. These channels thus support fast mould filling.

Fig. 5 Detail of the sandwich part in the rotorblade during injection