How wind turbine blades are made: the process in outline

Most blades are built as two aerodynamic shells, each laid up and infused in its own open mould. Each shell carries a spar cap that takes the flapwise bending load. Shear webs are bonded between the caps, the shells are closed and bonded along the leading and trailing edges, and the root is machined for the hub bolts.

The dominant process for multi-megawatt blades is vacuum-assisted resin infusion, which NREL's blade cost model (2019) describes as the most common method for modern blades: dry fabric and core are sealed under a vacuum bag, liquid thermoset resin is drawn through the stack, and the assembly is cured with heat. How wind turbine blades are made determines where the weak points are: the process is largely manual, the structure is inspected mostly from outside, and a defect built into the laminate stays there for the life of the blade.

What happens in the blade mould: gelcoat, fabric layup, core and spar caps

Mould preparation and gelcoat

The blade mould is a heated composite tool that defines the aerodynamic surface. Many manufacturers spray in a gelcoat that becomes the blade's outer surface. Gelcoat defects, pinholes, sagging and cracks, are cosmetic on their own but hide what is underneath. An auditor checks that the mould heating has been thermally mapped, so that the cure temperature on the process sheet is the one the laminate sees.

Layup of glass and carbon fabrics

The shell laminate is built from layers of dry fabric: biaxial and triaxial glass for the skins, unidirectional glass or carbon for the load-carrying bands. Each ply has a position, orientation and drop-off taken from the design's ply book, and operators place them by hand. This is where fibre misalignment, missing or duplicated plies and misplaced ply drops enter the structure. It is also where wrinkles begin: fabric that bridges a tight radius, is pulled during placement, or is compacted unevenly under the bag folds out of plane. An auditor asks to see the ply book on the shop floor, checks that each ply stage is signed off against the current revision, and tests whether fabric traceability is real rather than nominal.

Core: balsa, PET and PVC

Away from the spar caps the shell is a sandwich: thin glass skins either side of end-grain balsa, PET foam or PVC foam, cut into kits of panels. Core defects are gaps and steps between panels and poorly wetted core that the skin can separate from. A core gap becomes a heavy, brittle resin-filled channel, and damp balsa bonds poorly to the skins. An auditor looks at how core is stored, whether its moisture is measured, and whether gap and step tolerances are inspected after placement rather than assumed.

Spar caps: infused in the mould or pultruded planks

The spar cap is the structural heart of the blade: a thick stack of unidirectional glass or carbon fibre running most of its length.

In the traditional method the plies are laid into the shell mould and infused with it. The stack is thick, the plies are stiff, and any disturbance during layup or bagging can produce an out-of-plane wave that persists through infusion. The Blade Reliability Collaborative identified fibre waves and porosity as the defects the industry most needed to understand (Nelson, Riddle and Cairns, 2017), and earlier coupon tests by the same group, cited in Riddle, Nelson and Cairns (2018), showed laminate strength falling exponentially with wave severity.

Spar cap pultrusion moves this work out of the mould. Planks are pulled through a heated die continuously, giving a cured, well-aligned element with far less scope for waviness. The planks are cut, stacked and placed in the mould, then infused with the shell or bonded. Pultrusion removes one class of defect and introduces another: a contaminated plank surface leaves a kissing bond, and a gap between planks that is too wide or not fully wetted leaves a resin-rich layer or a void between elements that should act as one. Plank ends and the tapered transitions towards the tip are the places to look.

For either method the auditor asks whether the full cap length is scanned ultrasonically by someone trained to recognise waviness, and whether the acceptance limits for wave amplitude and wavelength are written down and applied.

Blade resin infusion and cure: where dry fibres and voids come from

The finished layup is sealed under a vacuum bag, which must hold a stated vacuum with a stated leak rate before resin may flow. Mixed and degassed resin is then fed through a network of inlets, following a flow plan designed so that the last point to fill is a vent, not a dead end.

Blade resin infusion is sensitive to nearly everything. A slow leak produces porosity. A resin batch that is too warm, or mixed too early, thickens before it reaches the tip and leaves dry fibres. Flow that races along a core gap or plank edge reaches the vent early and starves the region behind it. Under-cure leaves a laminate that creeps and absorbs moisture.

The auditor's checklist here is about records: the leak test and the vacuum held through infusion; resin batch and mix records; infusion times against the flow plan; the cure log from thermocouples on the laminate, not only the mould controller; and glass transition tests on coupons cut from the blade's own material. Where any of these is missing, the blade has been made on trust.

Shear web bonding, shell closing and the adhesive bond line

With both shells cured, the shear webs are bonded onto the spar cap of one shell with a paste adhesive. The second shell is lowered onto the first, and the leading edge, trailing edge and web tops are bonded as the mould halves close.

Shear web bonding and shell closing are the steps where the least is visible. The blade adhesive bond line sits inside the closed structure, formed by parts made separately that do not fit perfectly. Adhesive beads should spread as the shells close to fill the gap and squeeze out at the edges. Where the gap is wider than planned, a bond-line gap results. Where the bead is too small, adhesive starvation leaves the flange partly unbonded. Where a bonding surface is contaminated or has aged too long after peel ply removal, the adhesive may touch it without bonding to it: a kissing bond, invisible to most inspection methods. Trapped air gives voids.

The auditor asks how the bond gap is measured on each blade before closing, whether adhesive thickness after cure is recorded along the full length, and what happens when it is outside tolerance. The answer to the last question, in our experience, says more about a plant's quality culture than any certificate on the wall.

Trimming, root machining, inserts and finishing

After demoulding the blade is trimmed along the flash line, and over-cutting with hand tools can expose fibre or notch the bond line.

The root is where composite meets steel. The bolted joint uses either T-bolts, where a cross bolt sits in a hole drilled through the laminate, or inserts, threaded steel bushings embedded during layup or bonded into holes machined after cure. Root defects are among the most consequential because the root carries the whole blade load and cannot be inspected once bolted: inserts displaced during infusion, adhesive that has not filled the annulus around a bonded insert, delamination around drilled holes, and thread damage. An auditor checks fixture calibration, insert traceability, how insert bonding is verified, and the record of root face flatness and hole position on each blade.

Finishing follows: filling and sanding, paint, leading edge protection, lightning receptors, drain holes, and weighing so that three blades can be matched for one rotor. Finishing defects are mostly cosmetic, but paint can hide an unrepaired defect.

Where the wind turbine blade manufacturing process is checked: quality control, NDT and the factory audit

Blade quality control in a good plant is layered: signed in-process checks at each step, non-destructive inspection after cure, and a final inspection that releases the blade with a documentation package that should follow it for life. Non-destructive inspection is mostly ultrasonic scanning of spar caps and bond lines, supported by tap testing and an internal visual inspection that catches core gaps, adhesive shortfalls and foreign objects. Acceptance limits draw on work such as the effect-of-defect tests on intentionally flawed blades under the Blade Reliability Collaborative, but the limits themselves are the manufacturer's decision.

IEC 61400-5:2020 sets the minimum requirements for manufacture and its quality management, and DNV says the 2024 edition of DNV-ST-0376 gives greater weight to design for manufacture and to the link between manufacturing quality and reliability. A type certificate says that the design and the manufacturing system were assessed; it does not say that the blade on the lorry was made correctly. Blade manufacturing defects usually arise at a plant and in a batch, though a design that is hard to build invites them, and they are why a blade factory audit exists.

What an auditor looks for: a short checklist

  • Ply book and process sheets at the current revision, in use on the shop floor
  • Batch traceability of fabric, core, resin, adhesive and inserts, with storage records
  • Leak tests, vacuum levels, mix ratios, infusion times and cure logs for every blade
  • Ultrasonic coverage of spar caps and bond lines, and who reads the scans
  • Bond gap before closing, adhesive thickness after cure, root machining and insert records
  • The non-conformance register: what was found, how it was repaired and who approved it

Why plant audits matter most during replacement-blade production

Replacement blades, whether under warranty, after a failure or to close out a serial defect, are produced on a compressed schedule, sometimes at a plant that did not build the originals. A pre-production audit establishes that the plant and its quality system can make the blade to the design; in-production surveillance confirms that they are doing so, blade by blade; and inspection before installation is the last point at which a defect can be found and dealt with on the ground. For a lender or an insurer, the audit report is the evidence that the replacement has closed the risk rather than moved it.

Frequently asked questions

Which step of the wind turbine blade manufacturing process introduces the most serious defects?

In our judgement, the spar cap layup and the bond lines. A wrinkle in the unidirectional laminate of the spar cap, and a gap, starvation or kissing bond in a shear web or trailing edge joint, are the manufacturing defects most often found at the origin of a structural failure.

What is a kissing bond in a wind turbine blade?

A kissing bond is a joint where the adhesive touches the surface but has not bonded to it, usually because the surface was contaminated, too smooth, or had aged after preparation. The joint looks complete and may pass a thickness check, but it carries little load. Ultrasonic methods can miss it because there is no air gap to reflect the sound, which is why surface preparation records matter.

Does a type certificate guarantee that a blade was made correctly?

No. Certification assesses the design, the materials and the manufacturing quality system, but it does not inspect each blade. Whether a particular batch was made to the design depends on process control at the plant on the day, which is what a factory audit and in-production surveillance are for.

What should an owner ask for from the blade factory?

The documentation package for each blade: material traceability, infusion and cure records, NDT results with acceptance criteria, bond line measurements and the non-conformance register. If the manufacturer will not release it, that is itself information, and the contract for the next batch should require it.

How Apex Wind can help

We are an independent blade engineering consultancy in Denmark, with a background in the design, manufacture, testing and certification of production blades. For manufacturers, repair companies and developers, our blade design and structural assessment service reviews the gap between what a design intends and what production will deliver. For buyers, lenders and asset managers, our blade technical due diligence reviews manufacturing documentation and inspection records, and we audit plants and witness production when replacement blades are being made. We sell no repairs, hardware or inspection products and take no referral fees. Contact us to discuss a factory audit, a pre-installation inspection or a design review.