Filament winding is a technique primarily used to manufacture hollow, circular, or prismatic parts such as pipes and tanks. It is performed by winding continuous fiber tows onto a rotating mandrel using a specialized winding machine. Filament wound parts are commonly used in the aerospace, energy, and consumer product industries.

## Filament Winding Process

Continuous fiber tows are fed through a fiber delivery system to the filament winding machine, where they are wound onto a mandrel in a predetermined, repeating geometric pattern. The tow location is guided by a fiber delivery head, which is attached to a movable carriage on the filament winding machine. The relative angle of the tow to the mandrel axis, called the winding angle, can be tailored to provide strength and stiffness in the desired directions. When sufficient layers of tow have been applied, the resulting laminate is cured on the mandrel. The overall size and shape of the finished part are determined by the mandrel shape and thickness of the laminate.

The winding angles will determine the mechanical properties of the composite part, such as strength, stiffness, and weight. The density of the laminate is the result of the tension of the tows during winding. The composite parts made through these methods generally have good strength-to-weight properties.

## Materials Options

Resins: Thermoset resins, e.g. epoxy, polyester, vinyl ester, phenolic.

Fibers: Any. The fibers are used straight from a creel and not woven or stitched into a fabric form.

Cores: Any, although components are usually single skin.

## Types of filament winding

Filament winding can be found in two different variations.

Wet winding

Dry winding

### Wet winding

In wet winding, the fibers are unwound from roving and passed through a bath of resin mixture i.e. impregnation, before it is wound on a mandrel of defined orientation. The pattern of placement is controlled by the rate of rotation of the mandrel and the feeding or metering mechanism.

### Dry winding

The dry method uses fibers in their pre-impregnated form (towpreg composite fabrics).

When the right layer thickness is achieved, the assembly is cured in an oven. After curing, the core can be removed or used as part of the finished part. During curing, crosslinking occurs and thus forms 3D network fibers.

## Fiber winding angle

### The Clairault Relation:

There are other interesting properties of axisymmetric bodies, as discovered by the prominent French mathematician Alexis Claude de Clairault. Clairault’s relation is a formula in classical differential geometry. The relation is valid for any point on a geodesic path on an arbitrary surface of revolution and gives:

r .sin(α) = Constant

Where r is the radial distance of any point on a geodesic path from the axis of revolution, and α is the angle between the tangent vector and the latitudinal circle, or, to non-mathematicians, α is the winding angle.

BANDWIDTH For a simple axisymmetric case such as a pipe, the number of circuits required to cover a pipe is determined by considering a fiber band of true width, B, at a winding (helix) angle of α. By applying some simple trigonometry, the width of the band in the circumferential direction is B/cos(α), and dividing this into the pipe circumference, the number of circuits (N) required to cover a pipe of diameter D is given by

N = Pi. D.Cos(α)/B

The number of cycles is always an integer! With a given diameter D of the mandrel and the bandwidth B which is defined by e. g. the type and the number of the rovings, the winding angle can not be freely chosen. The better way to get the possible winding angle possible given all other variables would be

α = Cos-1 (N.B /Pi. D )

With the equation above, now N can be varied to achieve the closest fiber orientation to match the FEM simulation.

## Main Advantages

This can be a very fast and therefore economic method of laying material down.

Resin content can be controlled by metering the resin onto each fiber tow through nips or dies.

Fiber cost is minimized since there is no secondary process to convert fiber into the fabric prior to use.

The structural properties of laminates can be very good since straight fibers can be laid in a complex pattern to match the applied loads.

## Main Disadvantages

The process is limited to convex-shaped components.

Fiber cannot easily be laid exactly along the length of a component.

Mandrel costs for large components can be high.

The external surface of the component is unmoulded