Filament Winding- Materials & Engineering
D. Shaw-Stewart, Director, Pultrex Limited, Brunel Road, Clacton-on-Sea, Essex C015 4LT. Abstract Topics covered include an introduction to filament winding, the range of materials used, how these are handled, types of filament winding, applications and the future for filament winding. Introduction The basic components of a filament winding system are shown in Fig. 1. The mandrel on which the fibres are to be wound is rotated at a constant speed. Fibres come from a creel through a resin impregnation stage and a pay-out eye, which is mounted on a moving traverse carriage. The carriage motion is controlled relative to the mandrel rotation to give the required fibre orientation. The traverse carriage will move backwards and forwards until the required amount of fibres have been applied.
TRAVERSE CARRIAGE ._ _
PAY OUT EYE
Materials and their handling Many types of fibres can be used. The most common are glass fibres; others include carbon, often in a pre-preg form and aramid fibres such as Kevlar. Where large pipes or tanks are being wound, a high number of fibres need to be used in order to get the production times down. The easiest means of handling the glass fibre packages is with a centre pull, where the fibre unwinds internally. The disadvantage of the system is the amount of twist put into the fibres. A better way of handling fibres is to mount them on a spindle and unwind from the outside. The fibres pass up over a roller and then around two rollers mounted on a dancer arrrL The latter rotates through an arc and is being constantly pushed by an air cylinder. Variation of the air pressure with the regulator changes the fibre tensiorL The spindle has an electric brake on it to control the back tension. This type of device can apply tensions in the range of 300 to 3000 grams. The second part of the filament winding process is resin impregnation. The most popular types for filament winding are polyester, vinyl-ester and
FILAMENT WINDING SYSTEM
Filament winding systems
Drum type impregnation
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epoxy resins. Others include acrylic and phenolic resins. Resins can be applied during the winding process, (ie wet winding), or to the fibres as a prepreg, Pre-preg fibres can then be wound straight from the fibre-package. A drum type resin impregnation system is often used in wet winding. A closer view of such a system is shown in Fig. 2. The drum about 8" in diameter is free running, Its lower part is immersed in a resin bath. The bath is located on a temperature controlled platten, so that the resin temperature and viscosity, can be controlled if required. Lower viscosity of resin will improve the wet out of the fibres. The amount of resin being applied to the drum surface is controlled by a doctor blade. The fibres pass over the drum and the resin is forced through the fibres. As the fibres leave the drum they take the resin with them. Subsequent working of the fibres ensures good wet
CIRCUMFERENTIAL OR HOOP WINDING
Circumferential or hoop winding
Aluminium cylinder being overwound
So far only thermoset resins have been mentioned. However, considerable development work is being done in thermoplastics, such as nylon, polypropylene and the L C.I. Polyetheretherketone (Peek) material At present these materials are supplied as preimpregnated fibres. In the case of Peek, the fibres are carbon filaments.
Types of Filament Winding Windings are put into the categories of hoop or circumferential, helical, and multi directional. These different types are concerned with how the fibres are applied and the orientation & winding angles that are used. Many components are made with a combination of wind angles. For a cylindrical mandrel the fibres can be wound on at an angle. For every rotation of the mandrel the winding pattern will move a given distance- the winding pitch- There is therefore a direct relationship between the diameter of the mandrel, the wind angle and the pitch. Fibres wound with zero pitch and fibres laid axially along a mandrel are at 90 and zero degrees respectively. For circumferential or hoop winding, the fibres can be used as a single end or a number of ends giving a band-width of fibres. In this mode of winding it is usual to lay fibres side by side, to give complete coverage as the winding progresses. Therefore, the winding pitch will be the same dimension as the fibre band width ( Fig, 3). An application of hoop winding is shown in Fig. 4. This shows an aluminium cylinder that is being overwrapped with glass fibres as the band is starting
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to build up. The fibres used here are' S' glass, and the resin is epoxy. By adding only 10% more weight of the fibres, the pressure capacity can be doubled. This type of material was used in the American Everest Expedition. To get better production rates, multiple spindle filament winding machines can be used. In the case of helical winding, the winding pitch is greater than the band width of the fibres (Fig. 5 ). Accordingly, the pattern is 6pen as the winding starts and gradually fills in to cover the mandrel. Fig. 6 shows an actual part being wound with a band of four fibres. The winding is nearing the end of its cycle. Helical winding of this sort is probably the most common way of making pipes. The winding angle is designed to suit the application, so that the fibres take the stresses in the hoop direction and axial direction in the correct proportions. Machines to do helical winding can be fairly simple. A machine with capacity for parts up to 7 ft. long is shown in Fig, 7. The controls consist of start and stop, a speed control for the mandrel rotation and an automatic counter to stop the machine at the end of a wind. Polar winding is done on a different sort of machine from the hoop and helical winding, Fig. 8. It is used to wind almost axial fibres on domed pressure vessels. The arm rotates about a vertical axis, and can be set to different angles of tilt to suit the configuration of the part. The mandrel is mounted on a shaft on the arm. Gearing gives the mandrel rotation relative to that of the arm. On vessels with parallel sides a subsequent circumferential wind would be done. The advent of lower cost and more reliable computer control systems have lead to the development of multi-axis filament winding machines. The various axes of movement and control are shown in Fig. 9. The spindle is located in the headstock. For the types of winding that we have discussed so far, full control over spindle rotation is not necessary. The spindles rotate in one direction only. To be able to lay zero degree axial fibres along a mandrel, the spindle must be able to index in discreet increments and becomes a fully controlled axis 'A'. The traverse carriage differs from that in the previous types of machine for helical and hoop windin~ it is large, carries other axes and is designated ' X ' . A vertical slide ' Z ' moves on the ' X ' carriage and a horizontal ram moves in and out; this is the 'Y' axis. Mounted on the 'Y' ram is a rotary unit, 'B' axis. On this can be
Helical winding of part
Helical winding machine
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mounted a rotating pay out eye. Fig. 10 shows a Modwind 5 axis filament winding machine. The headstock with control panel is shown here with the tailstock mounted on the traverse beam. The carriage can be seen on the back of the machine. The carriage runs on the back of the traverse beam, Fig 11. The ' Z ' slide is shown in mid position and the 'Y' ram is in its back position. A 5 axis control system is used, which can be interfaced with external systems. Control of the machine slides is to a 1/1000 inch increment and speeds of up to 50 metres/minute can be achieved.
TRAVERSE B E A ~
Future for Filament Winding Tubes and tanks will continue to
I IJ I
front elev. HEADSTOCK
Applications Pipes and tanks represent the largest proportion of filament wound applications. Tubes are also used for mechanical applications such as lamposts, fishing rods, sailboard masts etc. More sophisticated tubes are used to make structures in which a series of filament wound tubes are joined to make a space frame. Each tube is wound with a metal end fitting, As the fibre orientation can be tailored to suit its application, these tubes are made with mostly unidirectional fibres with a few hoop fibres. This gives optimum strength and stiffness for the tension and compression loads. A recent structure for space work has been made in carbon fibre. This gives the added advantage of the zero expansion of carbon fibres in elevated temperatures producing a dimensionally stable structure. In the automotive field a similar type of tube is used for lightweight drive shafts. For this application, the properties are changed to give high torsional stiffness. For this the fibre orientation needs to have 45 degrees for torsion and zero degree for bending with some hoop winding to give strength at the end fittings. It is the ability to place fibres in the most suitable load carrying direction, that makes composites and in particular filament winding, a most efficient way of making things. The automotive industry has been developing a number of filament wound components such as the steering wheel and leaf spring, Fig. 12 shows a skeletal bumper frame, that has been wound with continuous glass fibres and ployester resin. Such a structure could then be moulded with urethane, or similar material. Polypropylene is used for the main structure for ease of release and cleaning.
MULTI - AXIS
Fig. 10 'Modwind' 5-axis machine
Fig. 11 Traverse carriage on traverse beam
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dominate the market. Pressure vessels made by polar winding are being increasingly used for many applications, including motor casings. The use of new materials, in particular thermoplastics, will bring many fresh applications. Prototype work on multi-axis machines will lead to the development of specialised, high volume, production equipment for automotive applications. Filament winding and its associated tape placement will provide aerospace with its automated systems. Computer controls and a range of new fibres and materials will enable the technology to advance rapidly.
Fig. 12 Skeletal bumper system
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