Field of Invention

Fibre-reinforced plastic composites are being used more and more in various technical fields as construc¬ tional material. The more advanced types of fibre-rein- forced plastics, such as inter alia carbon-fibre resin composites, have made inroads primarily within the aeronautical and space industries and have presented therewith clear advantages over traditional types of materials, such as aluminium. One obstacle which has prevented more general use of. such composites within, for example, the workshop industry is that the techniques employed in their manufacture are still undeveloped. For example, there is a lack of developed manufacturing processes which provide an end product of good quality coupled with a high degree of accuracy and a high produc¬ tion rate.

Background Art

One available method of manufacture at present under- going vigorous development is the fibre winding technique. This technique is used primarily in the manufacture of bodies which are rotationally symmetrical, such as tubes, shafts, reinforcing rings and cylindrical pressure vessels. The fibres are wound in a given pattern around a core, which determines the appearance of the end product. Special machines are available with which rotational speeds and feed speeds can be adjusted so as to obtain a desired pattern or fibre pitch or angle.

The choice of reinforcing material and matrices used are factors of the uttermost importance to the properties and quality of the final product. In addition hereto, win¬ ding parameters such as filament tension, winding speed, wetting rate, resin temperature and wetting method often play a fully decisive role. The quality of the final

product often depends upon the amount of ingoing fibres and the amount of air inclusions present in the laminate. That which has previously been said with regard to winding parameters is primarily applicable in so-called wet winding methods using matrix material consisting of epoxy, polyesters and vinyl esters. In the performance of such methods a dry fibre bunch is.normally passed down in¬ to a bath of liquid resin so as to be impregnated there¬ with, whereafter the fibre bunch is passed through a number of rollers, partly to ensure that the fibres are satisfactorily impregnated and partly to check the amount of ingoing resin.

The wet winding method has many weaknesses. It is, above all, highly unreliable and produces diverse results, primarily because of the large number of parameters to be checked and controlled. Since the method employs the use of liquid thermosetting resins, which may be harmful to the health, the method is also difficult to carry out.

There has long been a desire to exchange the liquid epoxy resin for a material form in which the resin is already initially bound to the fibre bunch. This material form is generally designated "pre preg" and is primarily used in the manufacture ofplanar panels in the aeronau¬ tical industry. Much of the uncertainty in the manufacture can be eliminated by applying this technique and winding with narrow pre-impregnated strips. It is also a desire to replace the thermosetting resin type matrix, for example the epoxy matrix, with a thermoplastic type matrix. Among other things this is because of the risks entailed when handling epoxy and the changing properties of the epoxy matrix when used at elevated temperature and in humid atmospheres.

Suitable thermoplastic type matrix materials are commercially available. Examples of such materials are polysulphone, PS, polyethersulphone, PES, polyphenylsul- phone, PPSu, polyphenylenesulphide, PPS, and polyether- etherketone, PEEK. These materials have properties which correspond to epoxy and in many cases exceed said proper-

ties. The problem, however, is to find suitable methods of manufacture suited to this type of material.

Object of the Invention The object of the present invention is thus to provide a method in which composite bodies of fibre- reinforced plastics material can be manufactured in a rational and highly qualitative manner, said method enabling the bodies to be manufactured by winding in several layers around a core a strip material which com¬ prises said plastics material and fibres bound thereto, wherewith the superposed layers are joined together by pressing said layers against one another in a heated condition.

Brief Disclosure of the Invention

The method according to the present invention is characterized in that there is used as fibre material a material capable of conducting electric current; and in that the material strip is heated in conjunction with said winding operation by passing an electric current through the electrically conductive fibre material in said strip.

Thus, the solution afforded by the invention is based on the use of a fibre material, suitable for reinforcement purposes, for example carbon fibres, which has a resistivi¬ ty which when current passes therethrough gives rise to losses which result in the heating of the fibre filaments and the surrounding matrix material.

This heating of the fibre filaments and surrounding matrix material, departing from the fibres, i.e. from the interior of thematrix,affords important advantages over previously known heating methods, such as heating with hot air for example. When hot air is used it is difficult to control the supply of heat accurately, and hence there is a risk of damaging the material by burning the same, since all heat which is to be supplied to the interior of the material must pass through the outer layer. This problem

does not occur when heating from the interior in accor¬ dance with the invention, and neither are any losses experienced as a result of heat conduction to the surroun¬ dings, but that all heat generated is usefully accounted for by the matrix material. The heating process can also be controlled relatively simply by controlling the current intensity or amperage. Since the current is also distri¬ buted substantially uniformly over all the fibres present, heating is effected smoothly over the whole width of the material strip.

For the purpose of supplying current to the strip it is preferred that, in conjunction with winding the strip onto the core, the strip is passed over two contacts which comprise an electrically conductive material and which are connected to different poles of a voltage source, wherewith heating current will flow through the strip between said contacts. Preferably, also the nearest underlying layer of the strip is heated when winding a further layer. Heating of this underlying layer can also be effected with the aid of current in the fibre material in said layer, or alterna¬ tively with the aid of heated air, since in this case the purpose is mainly to maintain the layer at an elevated temperature subsequent to being wound in a heated state. The plastics material suitably comprises a thermoplastic resin which is heated to melting point during the winding operation, while the fibre material used is suitably carbon fibre. Other electrically conductive fibre material may also be used.

The invention also relates to apparatus for anufac- turing a composite body of fibre-reinforced plastics materia by winding a material strip in several layers on a core. The apparatus is characterized in that it includes electri¬ cally conductive means in contact with the material strip and connected to a voltage source, said means being arranged to cause an electric current to pass through the conducting fibre material in the material strip so as to heat said strip in conjunction with said winding operation.

In accordance with a preferred embodiment the afore¬ said means comprise electrically conductive rollers or like devices. In this respect, one of said rollers suit¬ ably serves as a pressing roller for pressing the strip against the underlying layer, and also as a pole connection, said roller co-acting with another roller or the like abutting the underlying layer and serving as a counterpole in order to cause a heating current to also pass through the conductive fibre material in said underlying layer.

Description of an Embodiment of the Invention

The invention will now be described in more detail with reference to the accompanying drawing, which illustra¬ tes highly schematically an embodiment of apparatus accor- ding to the invention.

In the drawing the reference 1 identifies a winding core comprising an electrically insulating material. A fibre-reinforced plastics body is to be formed on the core 1 , by winding a strip 2 of plastics material having fibres bound thereto around the core 1 in a plurality of super¬ posed layers, to form a homogenous body. The strip 2 is taken in a dry state from a storage reel (not shown) and suitably comprises a thermoplastic resin, for example PS, PES; PPSu, PPS or PEEK, reinforced with carbon fibres. The incoming, dry strip 2 passes over a pole roller 3 and a guide roller 4, to a further pole roller 5, which in this embodiment also serves as a pressure roller for pressing the strip 2 against underlying winding layers on the core 1. As illustrated in the drawing, the pole rollers 3 and 5 are connected to opposite poles of a current source 6, for example via trailing contacts. The rollers 3 and 5 are made of a conductive material, suitably copper, resul¬ ting in a uniform supply of current from the source 6 over the whole width of the strip, this current passing substan- tially uniformly through the whole length of the strip, from the roller 3 to the roller 5, which serves as a collector and returns the current to the source 6.

E85/00094

By suitable selection of the values for the current source 1 in dependence upon the total conducting area in the strip 2 represented by the carbon fibres and the strip speed, the current in the strip 2 can be regulated so that the melting point of the plastics material is reached substantially when the strip is in contact with the com¬ bined pole and pressure roller 5. The strip 2 will then be bonded effectively to an underlying layer, adhered thereto. The aforesaid values in respect of the current source 1 and the strip speed are entirely dependent upon the nature of the strip 2, particularly its fibre content. One of normal skill in this art, however, will be able to establish suitable values for each type of strip, by calculation and/or by empirical experiment. In order to improve adhesion and to reduce further the risk of air inclusions in the plastics body, the underlying layer on which the strip is to be wound is preferably maintained at an elevated temperature or is re-heated immediately prior to contact with the strip 2. In accordance with the illustrated embodiment, this can be achieved by also passing a current through the under¬ lying layer, which can be effected by connecting a second current source 7 between the combined pole and pressure roller 5 and a further pole roller 8. In this case, the two material webs are in a substantially molten state at the point of contact, thereby providing an extremely good bond between the various layers, resulting in a substan¬ tially homogenous body. The temperature of the underlying layer may also be maintained, or said underlying layer may also be re-heated with the aid of hot air, since the precision to which this heating is effected is less impor¬ tant than the precision required when heating the strip 2. As an alternative to the illustrated coupling of the accumulators 6 and 7, there can be used a common accumula- tor which is coupled between the pole rollers 3 and 8 in a manner to pass a current between said rollers via the pressure roller 5. The combined pole and pressure roller 5 may also be replaced with a separate pole roller and a separate pressure roller.

The apparatus for carrying out the method according to the invention can be modified in several respects while retaining the central concept of the invention, in which heating of the strip to be wound is effected with the aid of a current passing through the electrically conductive reinforcing material of the strip. For example, the described rollers can be replaced with other contact means, such as brushes, trailing contacts or like devices. As will be understood, fibres of a different material to carbon may be used, for example metal fibres, provided that the resistivity is sufficiently high to achieve the requisite heating. Fibres which are not electrically conductive may also be used, provided that the fibres are provided with an electrically conductive outer layer or core. The fibre material may be provided with additives or treated in a manner which renders the material sufficiently conductive.