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As the wind market matures, there is increasing focus on the cost of quality for blade manufacturing, and more attention is being paid to the efficiency of blade construction and asset/mould utilization. This market need has driven the development of new advanced materials that address these key issues.
(Published on May 2008 – JEC Magazine #40)
DAMIAN BANNISTER, DIRECTOR OF INNOVATION, PRODUCTS AND SOLUTIONS, GURIT
The manufacturing process of a wind turbine blade involves four key components: the root section; the structural spar or spar cap; the aerodynamic fairing or shell; and the surfacing solution. Each component has a distinct set of technology requirements and a wide variety of engineering and material solutions. The incorporation of these four elements into a finished blade also varies considerably from one blade manufacturer to another. However, all blade manufacturers have the common goal to increase quality and productivity, whilst simultaneously engineering the blades to reduce their cost. This article briefly outlines the composite processes used to manufacture these components, and highlights new material developments at Gurit that address these market requirements using advanced materials technology (Figure 1).
The surfacing of a wind blade typically consists of an in-mould gelcoat, or an in-mould primer followed by a subsequent painting operation. In-mould gelcoats have no post-production treatment and therefore must provide full environmental stability and durability. These coatings have been used extensively on smaller blades (
The painting solution has the advantage of separating the surfacing process from the shell manufacturing, which reduces the technical complexity of the operation. The painting process is also a wellestablished technology that enables the manufacturer to draw on a larger pool of resources and expertise. However, the painting process still requires some form of in-mould primer to provide a suitable base for the painting operation. The primer provides a sandable surface which serves as a mechanical key for the paint, while reducing the number of surface defects such as pinholing and sink marks.
To overcome these issues, Gurit has developed a number of products to provide a primer surface as an integrated component of the shell reinforcing material. This has the advantage of eliminating the priming step by using a wet system that has an associated application and tack-off time. For blades manufactured using prepreg and/or SPRINT™ materials, this product is known as SPRINT™IPT and is discussed in more detail below.
Shells typically contain triaxial glass and core materials with localized biaxial and UD reinforcements. They can be broadly categorized into two types according to their engineering design: the shell is either structural or non-structural. Structural shells include two UD spar caps running from the root to the tip of each half shell, which are later connected by bonded and over-laminated shear webs. Non-structural shells are reinforced by a structural spar component that has been assembled in a parallel production operation. This spar structure is normally completed with caps, shear webs and a root section. It is then bonded into the lightweight shell structure in the final blade assembly. In addition to this categorization, the type of shell component can then be further defined by the two common material processing routes: infusion or prepreg.
The first stage of this process is the application of an in-mould primer or gelcoat into the mould. The coat provides either the final 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).
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:
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.
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.
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.
Spar caps for structural shells
Material developments for spar manufacture
With the current market focus on the cost of quality for blade manufacturing, there has been an increasing demand for products that produce consistent quality for a competitive overall blade cost. One of the key areas for improvement was that of the spar cap, where the associated difficulties in processing had led to variability in part quality and/or expensive engineering solutions.
Using Gurit’s experience in air breathe materials and solutions, namely SPRINT™ and Airstream™, a new unidirectional material has been developed known as SparPreg™ (Figure 3). This product has been specially formulated to facilitate the removal of interply air within a UD stack using standard vacuum bag processes, and without intermediate debulking stages. SparPreg™ is also designed to be fully compatible with SPRINT™ multiaxial materials, providing a very versatile material solution for all structural spar concepts.
The root section of the blade has the primary function of transferring the loads from the composite structure to the hub and main drive shaft of the wind turbine via metal inserts. The metal inserts are normally bonded into the composite using infusion or adhesive resins, or using mechanical fasteners at 90° to the laminate. To accommodate the metal inserts, the laminate section thickness is typically 50-100 mm depending on the root design. As with the spar caps, this laminate needs to be of high quality with a high fibre volume fraction and low void content, but as the root load requirements are different to that of the spar, it is normally made from both unidirectional and multiaxial materials.
The root section of a blade is incorporated into the blade structure using many engineering solutions. It may be integrated into the structural shell, integrated into a structural spar or spar cap, or manufactured separately to both spar and shell and bonded into the structure during blade assembly. Whatever the format of the root section, the key requirement is to produce a thick laminate section of high dimensional tolerance, with a high fibre volume fraction and minimum void content.
Material developments for root manufacture
One of the major constraints to root productivity is the exothermic reaction of the laminate during the cure due to the inherent thickness of the component. This is also a major constraint for shell and spar manufacturing leading to the need for increased tooling performance (and therefore investment) to maintain efficient cycle times. To overcome this fundamental issue, Gurit has used its extensive knowledge of resin chemistry to develop a range of products with significantly reduced exothermic characteristics.
The exothermic characteristics of the new resin matrix for PrepregLE and SPRINT™LE are illustrated in Figure 4 for a worst-case thick matrix casting (no reinforcements or core). This illustrates the significant potential to reduce or eliminate intermediate dwell temperatures in a component cure cycle, reducing the overall cycle time.