JEC COMPOSITES MAGAZINE - Issue #40 - May 2008 - 42

FEATURE “Wind energy” Prepreg shells As with the infusion process, the first step in shell manufacturing with prepreg materials is to use an in-mould coating as either a gelcoat or a primer for painting. Once this has cured sufficiently, the prepreg, which has been cut to kit form off-line, is laid up into the mould. This is followed by structural foam, also pre-kitted, and subsequent layers of prepreg. The vacuum stack is then applied and the component is cured at elevated temperature, typically between 90 and 130°C. g…/… Materials technology for the wind energy market exterior surface or a priming coat for subsequent painting. For infused blades, it also ensures a secondary function of sealing the tool against air leaks. The coat is then heated to cure to a level where operators can walk on its surface. The fabrics, core, and spar caps are then positioned before application of a vacuum stack. The vacuum stack will vary from one blade to another and will utilize various types of mesh to facilitate resin flow during the infusion process. The core and reinforcement lay-up are also optimised to facilitate fast and even resin infusion whilst maintaining their primary structural functions. Material developments for infused shells One of the key variables of the infusion process is the consistency of the resin flow from blade to blade, and the subsequent variation in blade weight and quality. To overcome these phenomena, Gurit has developed customized cutting patterns in the core material to optimise resin flow and distribution, whilst maintaining low resin uptake within the structure and eliminating the need for sacrificial meshes (Figure 2). Material developments in prepreg shells In 2007, Gurit launched a new product to provide an integrated primer using the SPRINT™ technology. SPRINT®IPT combines the experience gained in the automotive market to produce “Class A” carbon body panels, with the industrial requirements of the wind market for high deposition rates, heavy weight fabrics and an appropriate surface quality for subsequent painting. SPRINT™IPT combines the standard SPRINT™ triaxial technology with a surfacing film to produce Integrated Primer Technology (IPT). The result is a significant reduction in the shell manufacturing time and cost by removal of the in-mould priming process, and an increased surface quality that needs minimal preparation before painting. The surfacing film provides both a pinhole-free surface that is easy to sand for removing trace release coats, and a mechanical key for the painting operation. The SPRINT™ sub-structure prevents defects such as voiding at overlaps and sink marks from core details. Fig.2: Core-cell optimised infusion format Spars The main load-bearing structure of a wind turbine blade is the spar component. As discussed above, this component is either integrated into a structural shell as a spar cap, or constructed in parallel production to the shell as a separate spar structure complete with shear webs. What is common to both approaches is the use of unidirectional fibre (UD) – glass or carbon – to provide bending strength and stiffness. Significant quantities of UD material are required, resulting in laminate sections up to 50 mm thick towards the root section. This provides some technical challenges when considering fibre alignment, resin content, void content, deposition rate, exotherm control, and connection to the shear web using multiaxial materials. Furthermore, as the blade size has increased and carbon fibre is used more widely, the strength requirements have become the main design driver, placing even more focus on fibre alignment and high-quality laminates with low void content. This cut system uses a thin knife cut (typically 0.8mm) with a cut spacing of 30 mm on one side of the sheet, coupled with the same pattern but in the perpendicular orientation on the opposing side. Using the cuts in this way provides excellent flow characteristics, while providing the conformability required to suit difficult mould shapes and curvatures. The low cost characteristics of this type of pattern are due to the fact that the material requires no glass scrim (often used to provide stability to heavily cut infusion core patterns), and that no drilling or perforating is required – the depth of the cuts and the regular 30 mm by 30 mm intersection provide a perfect infusion flow between the laminate skins. In wind turbine blade manufacturing, this cut configuration has been proven to: - reduce overall resin consumption, - provide consistently faster infusion times (as much as 20%), - provide more repeatable infusion quality, - reduce dry areas caused by scrim cloth, - improve flow from perforations created by cross-cuts rather than drilled holes. Spar caps for structural shells The main load-bearing structure of a wind turbine blade is the spar component. As discussed above, this component is JEC Composites Magazine / No40 May 2008

JEC COMPOSITES MAGAZINE - Issue #40 - May 2008

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #40 - May 2008