What is filament winding in composite manufacturing

Filament winding is an automated composite manufacturing process for hollow, axisymmetric parts. Continuous resin-impregnated rovings or tapes are wound under tension onto a rotating mandrel along mathematically defined helical or hoop patterns; the part is cured (often on the mandrel) and the mandrel is then removed.

The process produces very high-fiber, strong structures with optimized hoop and axial properties for cylinders, but is restricted to convex, axisymmetric profiles. Inside that envelope, filament winding is the dominant manufacturing process for high-pressure composite tanks, rocket-motor casings, and tubular structures.

How filament winding works#

A filament winding machine consists of three coordinated motions: a rotating mandrel that defines the part's inner surface, a carriage that traverses parallel to the mandrel axis, and a fiber delivery system that pays out resin-impregnated tow under controlled tension. The coordination of mandrel rotation rate to carriage traverse rate sets the winding angle: equal rates yield a helical wrap at a fixed angle; high mandrel speed with slow carriage traverse yields a near-circumferential hoop wrap.

The resin can be applied in two ways. Wet winding runs dry fiber through a resin bath immediately before the fiber lands on the mandrel; this is the historic and still dominant method for cost-sensitive applications. Towpreg winding uses pre-impregnated fiber that is partially cured before winding, giving tighter control of resin content at the cost of preform handling complexity and cold storage. Most aerospace and high-pressure tank winding has shifted to towpreg for the consistency and the lower void content it produces.

After the winding pattern is complete, the part is cured. Many filament-wound parts cure on the mandrel itself, with the mandrel functioning as a heat-transfer surface and a structural support during the cure cycle. The mandrel is then removed: collapsible mandrels for closed-end vessels, soluble or sacrificial mandrels for complex internal geometry, simple slip-out mandrels for open-end tubes.

The winding-angle mathematics#

The winding angle is the geometric variable that controls the part's mechanical response.

The 54.7-degree isotensoid angle. For a closed-end cylindrical pressure vessel under internal pressure, the hoop stress is twice the axial stress (the thin-wall biaxial-stress result). For a helical-only single-angle wrap to carry both stresses with equal fiber strain, the winding angle must satisfy tan²(angle) = 2, which solves to about 54.7 degrees. This is the netting-analysis isotensoid angle. It is the analytic ideal for a helical-only wrap but not the production solution for real tanks.

Geodesic paths. On a doubly curved surface (the domed end of a pressure vessel), the fiber must follow a geodesic path, the most stable path where the fiber will not slip during winding. The geodesic winding angle is the inverse sine of the ratio of polar-opening to cylinder diameter. A small polar opening requires a steeper helical angle near the dome; a large polar opening allows a shallower angle.

Pattern winding. Production pressure vessels combine hoop windings close to 90 degrees with helical windings transitioning between 20 and 85 degrees. The hoop layers carry the dominant circumferential stress; the helical layers carry the axial stress and provide the dome continuity. A typical 350 bar Type IV hydrogen tank using Teijin ITS50 carbon fiber requires about 6.26 mm of hoop and 3.39 mm of helical winding to meet a 2.25 factor of safety against burst.

Applications#

Four application categories cover most filament-wound production.

Rocket-motor cases. Solid rocket motors and many liquid rocket pressure vessels are filament-wound. The Trident D5 motor cases, the Atlas V solid boosters, and the Vulcan GEM 63 are all carbon-filament-wound on the case body. The high specific strength of wound carbon over wound steel or aluminum drives the design choice.

High-pressure gas tanks. Type III (metal liner with full composite overwrap), Type IV (polymer liner with full composite overwrap), and Type V (linerless all-composite) pressure vessels for compressed natural gas and hydrogen. Toyota Mirai, Hyundai Nexo, fuel-cell trucks and buses, and most stationary hydrogen storage installations use filament-wound Type IV tanks at 350 or 700 bar working pressure.

Industrial tanks, pipe, and process equipment. FRP (fiberglass-reinforced polymer) storage tanks for chemicals, fuel, and water; high-pressure pipe for oil and gas; scrubber towers, separator vessels, and process equipment. Filament winding dominates the corrosion-resistant tank-and-pipe market.

Drive shafts, tubular structures, and pressure hulls. Automotive driveshafts, mast and pole structures, ROV and AUV pressure hulls, mast components, and tubular structural members. The high torsional and axial efficiency of a wound tube is the design driver.

Confusion points#

Filament winding versus pultrusion. Both are automated continuous processes. Filament winding produces hollow axisymmetric parts with fiber at the wound angle. Pultrusion produces straight constant-cross-section profiles with fiber parallel to the pull axis. A wound pressure vessel is a wound part; a pultruded carbon rod is a pulled part.

Filament winding versus AFP (automated fiber placement). AFP places narrow prepreg tows onto a contoured mold under heat and a compaction roller, producing complex 3-D laminated parts. Filament winding wraps continuous tows under tension onto a rotating mandrel, producing hollow axisymmetric parts. A 787 fuselage barrel is AFP-laid; a Toyota Mirai hydrogen tank is filament-wound.

Winding angle versus ply angle. Filament-wound parts do not have plies in the laminated sense. The wound layers have a winding angle relative to the mandrel axis. The terminology is sometimes used loosely (calling a layer a ply), but the underlying construction is continuous wound tow, not stacked plies of finite extent.

Wet winding versus towpreg winding. Both are filament winding. The difference is whether the resin is applied at the moment of winding (wet) or pre-impregnated into the tow before winding (towpreg). Towpreg winding gives tighter resin control and lower void content at higher material and storage cost.

The mandrel is not part of the finished part. The mandrel is a forming surface, removed after cure. Many designs use a thin metal or polymer liner as a permanent inner surface that doubles as the winding mandrel during fabrication (the Type III metal liner, the Type IV polymer liner). The liner is structural in the sense that it carries permeation and provides a leak path under pressure; the composite overwrap carries the structural pressure load.

Related terms#

• Pultrusion: the other major continuous composite manufacturing process, restricted to straight constant-cross-section profiles.
• Fiber volume fraction (Vf): the consolidation metric that filament-wound parts typically reach 60 to 70 percent.
• Void content: the porosity metric that towpreg winding is run to minimize.
• Autoclave cure cycle: the cure regime sometimes used for high-pressure wound vessels after winding.