Wind turbines are becoming larger and increasingly sophisticated, so that they are able to operate under highly demanding conditions in offshore locations around the world and in both cold and warm climates. The production technologies and high performance materials now available to wind energy equipment manufacturers can be used at all points of a wind turbine's product life cycle – from master models, through mould production, to blade part production and assembly. Many can also now be formulated to meet specific customer or project requirements.
Composite materials, particularly epoxy composites, are now playing a major role in most of the world's manufacturing industries, including wind energy generation. As a result, manufacturing processes too are changing, with direct processes such as resin infusion becoming much more widely used. These are delivering a range of production, cost and environmental advantages to manufacturers as well as improved composite properties such as shorter cure cycles, greater durability and a higher quality finish.
Direct processes (sometimes called liquid moulding processes) differ from prepreg-based technologies because the resin and reinforcement are combined and cured in the same operation. This makes direct processing a very cost effective way of producing composite parts with much shorter processing cycles.
Infusion
With the length of wind blade turbines now reaching around 60 m, infusion technology is proving a popular choice for wind turbine blade manufacturers. Standard resin transfer moulding (RTM) processes involving resin injection directly into the fabric stack would take too long compared to the gelation time of the resin.
The infusion process overcomes this problem by injecting the system (about 5 tons of resin for a 60 m blade) via a special medium placed on the surface of the fabric stack. Injection speed in this medium is very fast and impregnation of the structural reinforcement (usually glass fibre) takes place, perpendicular to the thickness of the stack, reducing the length of the path through which the system has to travel.
The formulated resin infusion systems developed by Huntsman Advanced Materials combine high performance characteristics with high processability. For example, the system combining the resin Araldite® LY1564 and hardeners Aradur® 3486 and Aradur® 3487, approved by Germanischer Lloyd (GL), provides low viscosity, adjustable reactivity and high levels of flexibility. The reactivity can be easily adjusted as required by varying the combination of both hardeners.
The long pot life of Aradur 3486 (3-10 hours at 23°C) facilitates the production of very large parts. Initial viscosity of this system is very low, about 250 mPa.s at 23°C (±2°C). In order to reduce injection time and to maintain ambient manufacturing conditions throughout the year, the system should be injected at 30-35°C. The viscosity is then lower than 100 mPa.s and injection time for a 60 m blade lasts 60 ±15 minutes depending on the fibre volume fraction. Glass transition temperatures between 80-90°C with G1c of about 250-300 J/m2 are typical properties in line with expected performance criteria for this application.
Toughened adhesives
Some new structural adhesive bonding technologies based on two patented concepts have been developed that provide both high mechanical performance and easy processing characteristics. This makes them ideal for wind energy applications where bonded assembly joints have to withstand enormous and prolonged stress.
A new concept in chemical thixotropy has been specifically developed to allow easy pump dispensing of low viscosity products without slump on vertical surfaces. The second concept, based on a synergistic combination of nanomaterials with selected curing agents provides an exceptional level of toughness without compromising thermal performance or causing significant impact on initial product viscosity. (No further information is available on these products yet.)
High temperature SMP
A high temperature tolerant seamless modelling paste (SMP) with a heat deflection capacity of up to 200°C is also now available. This two-component epoxy paste is used in a patented process with aluminium honeycomb sub-structures. It can be used for the rapid production of large-scale master models and high temperature resistant tools. It is particularly appropriate where good surface quality is a critical factor, such as wind blade plugs.
RenPaste® Seamless Modelling Paste has improved the manufacturing processes for one of leading tooling and prototyping companies in the Netherlands, Vostermans Tooling and Prototyping BV. Using RenPaste SV/HV 4503, combined with the latest CAD/CAM systems and CNC machines, Vostermans has been able to produce the models to make the moulds for wind turbine blades measuring up to 46 m in length, faster and more cost effectively than has been previously possible.
RenPaste SV/HV 4503 is used for the face layer of the model – the critical surface that will define the dimensions and surface quality of the mould tool, and then the final composite part. Working from CAD/CAM data, the paste is machine applied to a backing structure made from low cost substrate blocks that have been CNC milled to the required dimensions. This gives a virtually void-free layer of paste, applied to the right thickness – 15-20 mm provides the best balance of cost efficiency and performance.
After application of the paste, the model assembly is allowed to cure at room temperature for one day. In applications where the model is to be used at elevated temperature a stepwise post cure up to the working temperature is recommended.
After curing, the SMP layer is milled to its final dimensions. The milling is controlled by the original CAD model data to ensure accuracy, which in this case was critical as the maximum tolerances for the model were ±5 mm over the entire 46 m length.
Minimal hand polishing of the pattern then gave the fine, smooth surface finish required. The final step of the model building process was a spray coating of sealer as a base for application of the release agent prior to production of the composite mould tool.
“These wind blade models had to meet a range of demanding criteria and be produced to a tight timescale,” explains Rob Luijten, Location Manager at Vostermans Tooling & Prototyping BV. “The best time to test wind blades is in the Spring. This meant that the model had to be finished, the mould completed and the rotor blades produced in time for the test period ‘window’."
“Using RenPaste materials rather than traditional methods enabled us to cut overall production time of the model by up to 20%. The model was also of a high and sustained quality throughout its 46 m length, providing a smooth and seamless working surface. This in turn reduced the labour time required to complete the project.”
“The combination of RenPaste materials with CAD/CAM and CNC technology not only delivers benefits to our customers," continues Luitjen. “It has enabled us to differentiate the services Vostermans provides from those of our competitors and also opened up new markets we were unable to service before.”