GB2089761A - Heat shrinking plastics member onto filament/resin composite - Google Patents

Abstract

It is known to improve the surface finish of wound filament reinforced plastics material tubes by wrapping the uncured article in shrink-wrapping film and then shrinking the film progressively onto the tube. Herein, instead of film, a preformed heat-shrink sleeving is used to actually consolidate the tube as well as improve its surface finish. It is also proposed that preformed heat-shrinkable plastics material members be used to similarly consolidate filament-reinforced plastics articles other than tubes.

Description

SPECIFICATION Composite articles This invention relates to composite material articles and is more particularly but not exclusively concerned with articles made by the process known as "filament winding".

In the filament winding process, an article such as a tube is made by winding resin impregnated filamentary material, for example tow or tape comprising glass or carbon filaments, onto a mandrel.

The resin is then cured and the article removed from the mandrel. To improve the surface finish of such an article it may have applied to it a machining process such as surface grinding after the resin has cured and/or it is also known, for tubes, to wrap around the wound but uncured article a sheet of shrink wrap film. It has been further proposed to shrink such a film onto the tube progressively, from one end to the other, so that as well as improving the surface finish it also, to some extent, helps to remove any excess surface resin.

According to one aspect of this invention, there is provided a method of making a composite article comprising filamentary material and resion for example, epoxy, polyester in which, before the resin is cured, there is applied to the article a pre-formed, heat-shrinkable plastics material member having a shape appropriate to the article and then the member is shrunk onto the article.

According to a second aspect of the invention there is provided a method of making a composite article comprising filamentary material and resin in which, before the resin is cured, there is applied to the article a heat-shrinkable plastics material member which is such that, when the member is shrunk onto the article, it applies thereto a substantial pressure which squeezes out resin, either excess, or by design.

According to a third aspect of the invention, there is provided a method of making a generallytubular article comprising filamentary material and resin, in which the article is partly formed by winding filamentary material onto a mandrel and partly by laying in place pre-formed sheet material comprising filamentary material and in which, before the resin is cured, a tubular heat-shrinkable plastics material member is placed over the article and shrunk down onto it so as to consodidate the walls thereof.

The heat shrinkable member may be a length of what is known as heat-shink sleeving or a shaped member made by extrusion or moulding for exam ple and of the same kind of material as heat shrink sleeving. Preferably the wall thickness of the member before it is shrunk is at least around 0.5 mm., or better still at least 1.0 mm. or even better 2.00 mm.

or more.

According to a fourth aspect of the invention there is provided a method of making a generallytubular article comprising filamentary material and resin, in which the article is partly formed by a process known as filament winding, or hand lay-up, or a combination of both and in which, before the resin has gelled, a tubular heat shrinkable plastics material member is placed over the article and shrunk down onto it, after curing, the heat shrinkable member is removed, this then improving the external surface finish asto alleviate the necessity for some machining processes.

For a better understanding of the invention and to show how the same way may be put into effect reference will now be made, by way of example, to the accompanying drawing the single figure of which is a diagrammatic view of a length of heat shrink sleeving being shrunk onto a filament wound tube.

In the drawing, a tube 1 has been made by winding a resin impregnated glass or carbon filament onto a mandrel 2. Then, while the resin is still fluid, a length of heat-shrink sleeving 3 which, as supplied, is a fairly loose fit on the tube 1 is positioned over the tube. Then the mandrel along with the tube and sleeving are slowly rotated while a heat source 4 is moved along the mandrel so as to shrinkthe sleeving progressively, i.e. from one end to the other, down onto the tube. As this happens, the tube is consolidated and air and excess resin are squeezed out of the windings and pushed along to the end of the tube. In addition, the action of the sleeving gives the tube a smooth surface finish. The tube is cured, during which process, the heat shrink sleeving further acts on the tube until gelation takes place.

The heat source 4 can be of any suitable form, for example a gas burner or a catalytic or infra-red heater but it is preferred, for safety, cleanliness and ease of use, for it to comprise a heated air blower.

This is advantageously designed to provide a substantial flow of quite hot air, at around 5000C say to apply fast heating of an area opposite the source and also extending around the periphery of the sleeving (i.e. the heating should not be too localised) with a view to shrinking the sleeving rapidly before the resin too has time to become heated and hence perhaps prematurely gelled.The hot air blower might be an electrically heated air blower having a heating element power of say 1850 Watts and providing air at a temperature of around 600 C. In order to give more even heating all round the sleeving, the air may be guided or constrained near the sleeving, for example by providing a box-like housing having a hole right through it so that it can be placed around the mandrel and moved therealong and an entry for receiving hot airfrom the source 4. Using such a housing, it may be possible to reduce the heating air temperature from the 500 600 C found appropriate where no housing is used.

After the sleeving 3 has been shrunk onto the tube 1,the resin is cured, in an oven for example, and then the sleeving may be removed, say by carefully slitting it along its length. Alternatively, for some applications, it may be desirable to leave the sleeving in place for example to provide protection for the composite article.

It will be appreciated that heat-shrink sleeving is not the same as the shrink wrap film of which the use for filament wound tubes is already known. Shrink film is a material generally made from thermoplas tics, such as polyethylene, P.V.C. and polyester.

Normally, the shrinkage effect is due to the randomisation, accelerated by the application of warmth, of the molecules which make up the material and which, during manufacture of the film, were aligned in a particular direction. Thus, the shrinkage takes place in this same direction. The film is supplied in such form that, for its previously mentioned use in relation to filament wound tubes, it has to be wrapped around the tube. This may mean that, in the final article, non-uniformity of the tube surface may be apparent, particularly at the place where the two edges of the film met or overlapped.

Heat shrink sleeving on the other hand may have a wall thickness of around 0.1 mm. or more e.g. up to or more than 2.5 mm. After shrinkage, the thickness may be say 0.23 to 5.0 mm. The material is relatively strong and able to exert substantial pressure on the tube 1. Hence the sleeving is capable of actually consolidating the tube and squeezing resin out from within the windings of which the tube is formed rather than just assisting in the removal of excess surface resin. Generally the sleeving is made by extrusion and then it is stretched to give it a larger diameter and stabilised at that diameter for example by being irradiated by an electron beam. Then, when it is heated, it returns or attempts to return to its original size. During manufacture it generally becomes thermoset and its molecules cross-linked.

The material of which it is made may be a polyolefine, P.V.C., a fluoro-elastomer such as Viton, F.E.P.

(fluoro-ethylene-polypropylene), silicone rubber, modified P.T.F.E., Kynar (polyvinylidene fluoride) or other elastomeric material.

The invention may be applied to the manufacture of composite articles other than generally cylindrical filament wound tubes. For example, it may be applied to non-cylindrical tubes such as tubes hav ing an oval cross-section orto conical composite structures whether or not formed by winding, by the use of a shrinkable member made of the same kind of material as heat-shrink sleeving but having a pre-shrinkage shape adapted to the article being made. Such a member may be made by extrusion if it is required to have a constant cross-section whether or not circular, or by moulding, for example injection or blow moulding.

Machinery for making filament wound tubes is often such that the operator can vary the angle of the wound filament, i.e. the angle between the tube axis and the filament. The tube can then be made up of layers in which the filament lies in different direc tions with the object of giving the desired properties in these different directions. It may be required to have one or more layers in which the filaments are parallel or nearly parallel to the tube axis. Such layers are termed "longitudinally wound" layers in the art although the process is more a case of laying the filament along the tube than of winding it.

Although there does exist machinery which can perform 0 longitudinal winding as well as winding angles above 0 , it is inevitably expensive and may not be available. Accordingly this invention further provides a method of making a composite tube which method is useful, for example where it is desired to include one or more layers in which the filament direction is other than can be obtained with the available machinery. In this method some of the layers are wound normally while others are made up of filaments pre-formed, into such forms as woven, knitted, unidirectional fabrics.Such sheet material is commercially available sometimes already impregnated with resin in which case it is known as "pre'preg" and a piece of it can be cut so that its length equals that of the tube and its width such as to enable it to be laid, by hand, around the tube with its edges butting together. The piece is of course cut and laid so that the filaments lie in the required direction. After the tube is formed a length of heat-shrink sleeving is placed over it and shrunk as before, the shrinking properties only being effective when the resin is in a mobile condition, and then the tube is cured. It may be appreciated that normally a tube forming method in which some layers are made by laying pieces of pre-preg on by hand is not very desirable because, thereby, accurate control of the tube diameter and such is lost, and has to be regained by a machining process.However, the use of heat-shrink sleeving to consolidate the tube, and provide an outer surface which can alleviate the need to machine can be done so as to regain that control.

In the manufacture of tubes where a machining step is normally done between the application of different layers, e.g. where a hoop layer is to be followed by a longitudinal layer, it may be necessary to smooth the hoop layer to avoid crimping of the subsequent longitudinal layer, the use of heat shrink sleeving may avoid the need for that machining step.

Namely, the hoop layer or the like may be smoothed by the use of heat shrink sleeving and this avoids the fibres of the hoop layer being broken by the machining process.

Filament wound articles may be made by a wet process, in which filamentary material is passed through a bath of resin just before being wound onto a mandrel or by a process in which filamentary material pre-pregnated with resin, i.e. pre-preg, is placed on a mandrel in the desired pattern, the pre-impregnated material being heated either just before laying or while it is on the mandrel. In the process using pre-preg, the heat shrink sleeving does not necessarily act to remove resin from the article (since the resin may always remain in a viscous state) but may nevertheless still be useful to consolidate the article and improve its surface finish.

Claims (6)

1. A method of making a composite article comprising filamentary material and curable plastics or resin material wherein, before the curable mate rial is cured, there is applied to the article a pre-formed, heat-shrinkable plastics material mem ber having a shape appropriate to the article and then the member is shrunk onto the article.

2. A method of making a composite article comprising filamentary material and curable plastics or resin material wherein, before the curable mate rial is cured, there is applied to the article a pre-formed heat-shrinkable plastics material member which is such that, when the member is shrunk onto the article, it applies thereto a substantial pressure and squeezes a substantial amount of the curable material out from between the filaments of the article.

3. A method of making a generally tubular article comprising filamentary material and curable plastics or resin material wherein the article is partly formed by winding filamentary material onto a mandrel and partly by laying in place pre-formed sheets comprising filamentary material and wherein, before the curable material is cured, a tubular heat-shrinkable plastics material member is placed over the article and shrunk down onto it so as to consolidate the walls thereof.

4. A method according to claim 1, 2 or 3, wherein the wall thickness of said heat-shrinkable plastics material member, before it is shrunk, is at least around 0.5 mm, or at least 1.00 mm, or 2.00 mm or more.

5. A method of making a composite material article substantially as hereinbefore described with reference to the accompanying drawing.

6. A composite article made by the method of any preceding claim.