System and process for manufacturing a towpreg material

Abstract

A method for applying resin to a fibre 10 to form a pre-impregnated fibre (e.g. a towpreg), comprises directing a nozzle 25 towards a first surface of a fibre, the nozzle delivers fluid across a dista

Description

Field of invention The present invention relates to the production of pre-impregnated composite materials, and in particular to the production of pre-impregnated composite materials having a relatively narrow width (e.g. towpregs). Background art Composite materials having a resin matrix and reinforcing fibres as the primary constituents are finding ever increased applications owing to their advantageous properties (e.g. high strength and light weight) compared to conventional materials. Pre-impregnated composite material (prepreg) is a common intermediate product in the manufacture of composite components. Prepreg is typically composed of fibres which have been impregnated with uncured resin. After impregnation, protective layers are typically applied to either side of the prepreg, and the product is then rolled up for storage or transport to a manufacturing facility. As the resin is uncured, the prepreg remains malleable and can be shaped and moulded as required prior to curing into the final composite product. The prepreg process facilitates accurate control of the fibre / resin ratio and the distribution of resin within the fibre network, which (together with the arrangement and type of fibres, and the type of resin) can affect the structural properties of the final product. Prepreg is typically composed of multiple fibres which are either arranged unidirectionally (i.e. extending parallel to each other) or are woven into a multidirectional web. Generally, each “fibre” is made up of a bundle of numerous individual fibres or filaments. Such bundles are often referred to as a “tow”. In certain applications, fibres or tows may be chopped and randomly oriented within the resin to form a non-woven mat. Examples of suitable fibrous materials include glass fibre, carbon fibre, polyethylene and aramid. The resin typically used in prepreg materials depends on the application and the required properties of the material (e.g. strength, plasticity, temperature / moisture resistance, fire retardancy). Examples of commonly used resins include phenolic resins, epoxy resins, bismaleimides and polyimides. Uncured resins typically comprise reactive components (such as the resin material (e.g. epoxy) itself, and curing agents or hardeners). Other reactive and non-reactive additives may also be present such as tougheners, accelerators (to reduce cure time / temperature) or flame retardants. Prepregs are typically supplied in roll format and have a width of up to 1.5 metres, and are thus suitable for the manufacture of larger composite components. However there is an increasing need for smaller scale composite products where accurate fibre placement is important; or products having a highly contoured shape which is difficult to achieve without creating wrinkles when using a wide roll . Towpregs are composite materials composed of fibres impregnated with a resin. Towpregs are thus similar to prepreg materials, but typically have only a small number (1-3) of fibre tows (bundle of individual filaments or fibres) arranged unidirectionally. Given the small number of fibre tows, towpregs typically have a width of under 50 mm. The narrow width enables towpregs to be layered by automated fibre placement technology and thus to comply with highly contoured surfaces; or by winding to form components with cylindrical geometry such as pressure vessels (e.g. gas cannisters), tubes, piping and vehicle drive shafts. Like prepregs, towpregs are typically supplied with the resin in an uncured state such that the material is malleable and can be shaped as required prior to curing. Existing methods of manufacturing towpreg materials include the splitting and slitting (cutting) of a wider web of fibres which have been pre-impregnated with a resin (e.g. a prepreg). These are also referred to as “slit tape”. However this approach can expose dry fibres and damaged fibre around the cutting or splitting site. Dry fibres tend to be frangible, thus exposed fibres on the edge of a towpreg can lead to structural defects within the towpreg material and any assembly usi™towpreg material. Debris from the cut or split may also contaminate the uncured resin, leading to defects in the resin matrix once cured. Where the final product is intended to be subject to significant forces (for example when used in a driveshaft or a pressure vessel) then these structural defects may decrease the material lifetime or increase the risk of failure. The manufacturing process for slit tape is also more complex (given the need to cut the fibre web) which results in higher cost. An alternative method of manufacturing towpreg is to use a dry tow or multiple tows having a (combined) fibre weight equal to the desired fibre weight of the towpreg (e.g. one 24K tow produces a 24K towpreg, or two 24K tows produces a 48K towpreg). Thus no splitting or slitting is required. This alternative method typically uses a resin bath to apply the resin. Resin within the resin bath is often diluted in a solvent to prevent premature ageing and to facilitate impregnation (which can cause increased wastage and risk of exothermic reactions). The use of solvents lowers the viscosity allowing homogenous resin coverage of the filaments. These solvents are later removed from the process by heating. However this heating step requires accurate control of temperature and duration to avoid ageing the resin contained within the solvent, and can if not optimised result in incomplete solvent removal. Any residual solvent / volatiles can result in porosity in moulded parts. The use of solvents also has significant downsides with regards to a sustainability and environmental impact of having to create, handle and dispose of solvents. The frangible nature of the dry fibres also makes it advisable to minimise contact with the dry tow, as any contact could damage the fibre structure. Additionally, damaged fibres can also impact the ability to unwind towpreg from a spool, as dry filaments adhere to adjacent material, and may need to be cut apart before unwinding is possible. These defects are referred to as “stringers” and can result in entire spools of material being scrapped. Manufacturing towpreg materials without cutting is challenging due to the need to accurately apply resin to a tow having a relatively narrow width. Resin can be applied by dipping or immersing the fibre tow into a resin bath, or by using process paper to transfer a resin film onto the fibre. Depending on the thickness of the towpreg, it may be necessary to apply resin to both sides to ensure that there is sufficient impregnation / wetting of the fibres by the resin. Where the resin is applied using a process paper, applying resin to both sides of the fibre increases the complexity of the manufacturing line, increases the amount of waste paper produced and also increases required contact with the dry fibre tow. When process paper is used as a substrate for a film of resin, which can then be transferred to one or both sides of the tow. This “filming” typically limits the viscosity of the resin: it must be capable of being applied to process paper and thus the viscosity cannot be too low. Pressure (i.e. contact) must be applied to the process paper and fibre tow to transfer the resin from the process paper to the fibre tow and to cause the resin to migrate into the fibres / filaments of the tow. This results in the production of significant amounts of waste process paper which has adverse environmental and cost impacts associated with disposal, and increases the contact required with the dry fibres. Although submersion in a resin bath can apply resin to both sides of the tow, this requires a large standing volume of resin. It is difficult to monitor and control ageing of the resin within the resin bath, which can result in a degradation in product quality or increased wastage through frequent resin bath changes. Summary of the Invention The present invention seeks to provide a system and method for applying resin to a fibre which overcomes these problems of towpreg manufacture. Viewed from a first aspect the present invention provides a method for applying resin to a fibre to form a pre-impregnated fibre (e.g. a towpreg), comprising: directing a nozzle towards a first surface of a fibre, wherein a tip of the nozzle and the first surface of the fibre are separated by a separation distance and the nozzle is configured to deliver fluid across the separation distance to the fibre; delivering a resin through the nozzle to the fibre; and moving the fibre relative to the nozzle in a direction substantially parallel to a longitudinal axis of the fibres to apply resin to the fibre along its length to form a resin coated fibre. By providing a nozzle to deliver fluid (e.g. resin) to the fibre, physical contact of the dry fibre by anything other than the resin is essentially eliminated for the resin application step of towpreg manufacture. This significantly reduces the risk of fibre damage (fraying, breaking) as a result of physical contact. The nozzle may be configured to deliver the fluid (e.g. resin) directly to the fibre (i.e. without any intermediate layers between the fibre and the resin, and without the resin contacting any other solid substrate between the nozzle and the fibre). The step of delivering the resin may therefore comprise delivering the resin through the nozzle directly to the fibre. This advantageously eliminates the need for process paper. It will be appreciated that the amount of resin delivered to the fibre will depend on both a resin flow rate through the nozzle and a fibre transit rate (a speed of movement of the fibre). The total resin content of the resultant resin coated fibre (expressed as a weight percentage relative to the overall weight of the resin coated fibre) will also depend on the properties (width, thickness, density) of the fibre. Such fibre properties will be known or are readily measurable for any given fibre, thus it is possible to control resin content in the resin coated fibre by adjusting the resin flow rate through the nozzle and / or the fibre transit rate. One or both of a fibre transit rate (a speed of movement of the fibre) or a resin flow rate through the nozzle may be such that the resin coated fibre has a resin content of between 20 wt.% and 60 wt.%; or preferably between 25 wt.% and 50 wt.%; or even more preferably between 30 wt.% and 40 wt.%. The method may further comprise the step of controlling one or more of a rate of resin delivery from the nozzle and a rate or velocity of fibre movement to provide a resin coated fibre with a resin content of between 20 wt.% and 60 wt.%; or preferably between 25 wt.% and 50 wt.%; or even more preferably between 30 wt.% and 40 wt.%. The nozzle may be an atomising nozzle configured to deliver fluid (e.g. resin) as an atomised spray. The step of delivering the resin may therefore comprise delivering the resin through the nozzle to the fibre as an atomised spray. Where the nozzle is an atomizing nozzle configured to deliver resin as an atomised spray, the nozzle may have a spray pattern selected from a group consisting of a flat fan, a full cone, a hollow cone, or a mist. Preferably the spray pattern of the atomizing nozzle is a flat fan or a full or hollow cone. The step of delivering the resin may therefore comprise delivering the resin through the nozzle to the fibre as an atomised spray having a flat fan, a full cone, a hollow cone or a mist pattern. These spray patterns advantageously reduce resin waste by directing the spray towards the fibre. Using an atomising spray nozzle advantageously allow for high pressure spraying of a resin. This increases the force with which the resin contacts the fibre, and may enable some resin to permeate into the fibre beneath the surface filaments (i.e. enabling at least partial wetting / impregnation of internal filaments without requiring an additional heating step). Atomising spray nozzles also facilitate high fluid / resin flow rates, enabling a larger area of the resin to be coated simultaneously and thus enabling higher fibre transit rates. Atomising spray nozzles advantageously provide high uniformity in terms of coverage of the fibre with resin. Where the nozzle is configured to have a flat fan spray pattern, the nozzle is preferably configured such that a plane of the spray pattern extends laterally (i. e. substantially perpendicular to the longitudinal axis of the fibre. Thus the step of delivering the resin may comprise delivering the resin through the nozzle to the fibre as an atomised spray having a flat fan pattern extending laterally. Preferably the separation distance between the nozzle tip and the fibre is selected such that the lateral extent of the spray pattern in the plane of the first surface (i. e. the diameter of the cone defined by the hollow or full conical spray pattern in the plane of the first surface; or the width of the flat fan spray pattern in the plane of the first surface) is approximately equal to the width of the fibre. This advantageously means that there is no (or a negligible) amount of resin waste caused by overspray passing to the side of the fibre). For example the lateral extent of the spray pattern in the plane of the first surface may be between 80% and 120% of the width of the fibre; preferably between 90% and 110% of the width of the fibre; even more preferably between 95% and 105% of the width of the fibre. Alternatively the nozzle may be configured to deliver fluid (e.g. resin) as a continuous bead in a spiral pattern, and the step of delivering the resin may comprise delivering the resin through the nozzle to the fibre as a continuous bead in a spiral pattern. By delivering resin to the fibre as a continuous bead in a spiral pattern, it is advantageously possible to more accurately control the amount of resin delivered to the fibre by adjusting one or more of the fibre transit rate (the speed of movement of the fibre) or the resin flow rate through the nozzle. When a continuous flow rate and fibre speed is maintained then the amount of resin delivered to the fibre is consistent. The size and shape of the bead can also be varied by air temperature, fibre temperature, resin temperature and the temperature of the nozzle. Thus these parameters can be monitored and controlled to provide delivery of an accurate and controlled amount of resin to the fibre. The position, size and shape of the nozzle can also be altered, as can the amount / pressure of carrier fluid (e.g. pressurised air) delivered to the nozzle. Delivering the resin as a continuous bead in a spiral pattern advantageously reduces the amount of overspray (i.e. wasted resin which is sprayed outside the fibre width or which fails to contact or remain on the fibre), and thus eliminates resin wastage or reduces resin wastage to a negligible amount. As overspray is reduced, delivering the resin as a continuous bead thus also reduces the air extract!on / purification requirements to prevent operatives inhaling potentially harmful resin spray. The force exerted on the fibre by the continuous resin bead contacting the fibre is also significantly reduced compared to atomised spray systems. A reduction in overspray (overspray can be hard to quantify) also enables a more accurate determination of the amount of resin delivered to the fibre. The nozzle may be configured to deliver fluid in a cylindrical spiral / helix bead or a divergent conical spiral / helix bead from the nozzle tip. The step of delivering the resin may comprise delivering the resin through the to the fibre as a continuous bead in a spiral pattern as a cylindrical or divergent conical spiral / helix bead. Preferably the nozzle is configured to deliver fluid in a divergent conical spiral / helix bead from the nozzle tip. A divergent conical spiral / helix advantageously allows the method to be adapted to different widths of fibre by adjusting the separation distance. Where the nozzle is configured to deliver fluid in a cylindrical spiral / helix bead then a diameter of an imaginary cylinder defined by the helical bead path is preferably greater than or equal to a width of the fibre. Preferably a diameter of the imaginary cylinder defined by the helical bead path is between 100% and 150% of the width of the fibre, or more preferably between 110 % and 140 % of the width of the fibre, or between 115 % and 130 % of the width of the fibre. Where the nozzle is configured to deliver fluid in a divergent conical spiral / helix bead, then a diameter of an imaginary cone defined by the divergent helical bead path in the plane of the first surface is preferably approximately equal to the width of the fibre. Preferably a diameter in the plane of the first surface of the imaginary cone defined by the divergent helical bead path is between 100% and 150% of the width of the fibre, or more preferably between 110 % and 140 % of the width of the fibre, or between 115 % and 130 % of the width of the fibre. By providing a diameter of the cylindrical or conical spiral resin bead within the range of between 100% and 150% of the width of the fibre, adequate coverage of the fibre with resin is achieved with minimal wastage of resin (e.g. by overspray beyond the edges of the fibre). Particularly advantageously, it has been discovered that within this range, overspray of the resin beyond the edges of the fibre may not break off as waste, but can wrap around the fibre to coat both the edges and a second (underside) surface of the fibre. This is likely due to the angular momentum present in the helical bead path. This advantageously both reduces (or even eliminates) resin waste, and facilitates the delivery of resin to both sides (and the edges) of a fibre without requiring multiple nozzles or multiple passes of the fibre under the nozzle. This in turn reduces the required contact of the fibre, reducing the risk of damage to the dry fibres. This advantageously reduces the risk of incomplete resin coverage of the edge filaments, and thereforo ^uces the likelihood of subsequent filament damage and detachment from the fibre tow during the manufacturing process (e.g. through the formation of “stringers”) or after manufacture. This is advantageous to improving the mechanical properties of the fibre, reducing width variability of the resulting towpreg and reducing contamination (for example as a result of stringers) of process components in the manufacturing line. The nozzle is preferably positioned with its tip approximately central with respect to the width (i.e. the lateral extent) of the fibre. This ensures that the resin delivery is correctly aligned to ensure consistency in coverage and minimal wastage from overspray. For example, the nozzle tip may be directed towards a point on the fibre within the central 20% of the fibre width, preferably within the central 10% of the fibre width, even more preferably within the central 5% of the fibre width (e.g. within the central 1%). An axis of the fluid / resin delivery (i.e. the rotation axis of the helical / spiral bead) may be at an angle of between 45 ° and 135 ° with respect to the first surface of the fibre. Preferably the axis of the fluid / resin delivery is at an angle of between 60 ° and 120 ° with respect to the surface of the fibre (e.g. at an angle of between 65 ° and 115 °), more preferably at an angle of between 80 ° and 100 °. Even more preferably, the axis of the fluid / resin deliver is substantially perpendicular to the first surface of the fibre (i.e. the nozzle is preferably oriented substantially perpendicularly with respect to the first surface of the fibre). In this context, “substantially perpendicular” is intended to mean that where the first surface of the fibre defines an x,y plane, the axis of the fluid / resin delivery is at an angle of <5 ° (e.g. <1 °) with respect to the z axis (perpendicular axis) in both the x andj directions. Alternatively, the axis of fluid / resin delivery may be substantially parallel to a plane of the first surface of the fibre (i.e. the resin may be delivered to the fibre from the edge). Where the axis of the fluid-resin delivery is substantially parallel to the plane of the first surface, the axis is preferably substantially coplanar with the plane of the first surface. In this context, “substantially parallel” is intended to mean that the axis of the fluid / resin delivery is at an angle of <5 ° (e.g. <1°) with respect to the x,y plane of the first surface. The method may further comprise supplying a carrier fluid {e.g. a carrier gas) to the nozzle to facilitate the formation of an atomised spray or a helical / spiral bead of resin. The carrier fluid is preferably compressed air. The method may comprise one or both of heating the carrier fluid prior to supplying the carrier fluid to the nozzle; and drying (i.e. removing moisture from) the carrier fluid prior to supplying the carrier fluid to the nozzle. A transit rate of the fibre may be adjusted as required to adjust the amount of resin delivered to the fibre. Preferably the transit rate is greater than or equal to 30 metres / minute (m / min), particularly preferably between 30 m / min and 100 m / min inclusive. Alternatively, in certain embodiments the transit rate may be less than or equal to 20 m / min {e.g. about 2 m / min, about 5 m / min, or about 10 m / min). An appropriate fibre transit rate may be selected dependent on the required resin content of the resin coated fibre. The resin may be delivered through the nozzle at any appropriate flow rate dependent on the required resin content of the resin coated fibre. The resin may have a viscosity of between 0.5 and 500 pascal-seconds (Pa s) at the point of delivery from the nozzle ( / . e. the point at which the resin exits the nozzle). Viscosity measurement from the nozzle in real time is not practical. However by measuring viscosity of the resin under the conditions (temperature and shear rate) exhibited at the point of delivery from nozzle, the viscosity of the resin at the point of delivery from the nozzle can be determined. Preferably the resin has a viscosity of between 1 and 100 Pa s at the point of delivery from the nozzle. Even more preferably, the resin has a viscosity of between 10 and 50 Pa.s {e.g. between 10 and 30 Pa.s) at the point of delivery from the nozzle. In this specification, all viscosity measurements and values are isothermal tests at varying shear rates as measured using a cone and plate rheometer (Anton Paar MCR 92 API). The resin may be heated before delivery through the nozzle. Heating the resin advantageously controls its viscosity, improving distribution of the resin on the fibre and facilitating the wrapping round of the resin onto the underside of the fibre. When working with certain resins, heating of the resin may be required in order to facilitate delivery of the resin through the nozzle to the fibre, and sv^^^'T'ent infiltration into the fibres (referred to as “wetting”). Where the resin has a viscosity at ambient temperature (herein defined as 25 °C) which is outside an operable or optimal range (e.g. 0.5 - 500 Pa s; or 1 - 100 Pa s; or 10 - 50 Pa s)) then the method preferably includes the additional step of heating the resin to reduce its viscosity to within the operable or optimal range prior to the step of delivering the resin through the nozzle. The temperature to which the resin is heated will depend on the specific resin and should not be so high that the resin begins to cure (for thermally cured resins) or otherwise degrade. For example, the resin may be heated to a temperature of between 30 °C and 200 °C; preferably between 40 °C and 200 °C; more preferably between 60 °C and 160 °C; even more preferably between 70 °C and 140 °C (e.g. about 90 °C) before and / or at the point of delivery through the nozzle. Compared to other resin delivery systems such as a resin bath or a reverse roll coater, heating resin to higher temperatures as it passes through the nozzle or upstream components advantageously minimises / reduces time that the resin spends at elevated temperatures (due to the relatively small volume of fluid travelling through the nozzle and upstream components). The resin may be any conventional resin which is used in prepreg and composite material manufacturing. The resin may be a thermoplastic resin, a thermosetting resin, a mixture of thermoplastic and / or thermosetting resins, or a thermoformable resin. The resin may be an epoxy resin, a phenolic resin, a bismaleimide resin or a polyimide resin. The resin may be a mixture of two or more different resin components, for example a mixture of two or more of trifunctional, tetrafunctional or solid epoxy resins. The resin may further comprise one or more active or non-active ingredients such as a catalyst, a curing agent, a hardener, a toughener, an accelerator, or a flame retardant. The active or non-active ingredient such as the catalyst may be present within the resin in a homogeneous form (e.g. as a miscible liquid or a solute) or a heterogeneous form (e.g. as a dispersion of powder within the liquid resin or an emulsion within the liquid resin). The method may further comprise directing one or more additional nozzles towards the first surface or a second surface of the fibre, wherein a tip of the or each additional nozzle and the first surface (or second surface) of the fibre are separated by a second separation distance and the (or each) additional nozzh ic Q,co configured to deliver an additional fluid across the separation distance to the fibre; delivering the (or each) additional fluid through the (or each) additional nozzle to the fibre; and moving the fibre relative to the (or each) additional nozzle in a direction substantially parallel to a longitudinal axis of the fibres to apply the additional fluid(s) to the fibre along its length. These steps relating to the additional nozzle(s) may be before the step of directing the nozzle towards the first surface of a fibre; or after the step of moving the fibre relative to the nozzle in a direction substantially parallel to a longitudinal axis of the fibres to apply resin to the fibre along its length. The (or each) additional fluid may also be a resin and may be the same resin as delivered through the nozzle or a different resin. Alternatively the (or each) additional fluid may comprise a reactive or non-reactive additive such as a curing agent, hardener, toughener, accelerator or a flame retardant. The (or each) additional fluid may comprise one or more reactive or non-reactive additives and a solvent. By providing these additives in separate process streams to the resin until they are mixed on the surface of the fibre, premature resin hardening or curing can be limited / avoided. Different components of the resin can also be incorporated into different process streams and thus form distinct layers on the fibres. Preferably the fibre is in the form of a tow (a bundle of individual filaments). The fibre may comprise a single tow (i.e. a single bundle of filaments). Alternatively, the fibre may be an assembly of multiple tows, typically 2, 3 or 4, arranged in parallel and spaced apart laterally (such that a first surface of each tow is coplanar) or vertically (such that a first surface of each tow is not coplanar). Each tow may contain from 3000 to 50000 filaments (referred to as 3K to 50K tows). For example, each tow may have between 5000 and 25000 (5K to 25K) filaments, e.g. 6000, 12000 or 24000 (6K, 12K or 24K). The tow (or each tow) is preferably spread into a substantially flat, tape-like reinforcement prior to the step of directing a nozzle towards a first surface of the fibre. The fibre or spread fibre (whether composed of a single tow or multiple tows) may have a total width of 50 mm or less, preferably 30 mm or less (e.g. 25 mm or less). The fibre may be any of the conventional fibre types and configurations that are used in the prepreg, towpreg and composite material industry. Examples of suitable fibres include fibreglass, carbon fibres, polyethylene and aramid (aromatic polyamide) fibres. Preferably the fibre is carbon fibre. The method may further comprise one or more conventional dry fibre processing steps before the step of directing the nozzle towards the first surface of the fibre, including unwinding the fibre from a spool, aligning the fibre, pre-heating the fibre, and / or spreading the fibre. Pre-heating the fibre may relax cohesion between the fibres and thus aid spreading. The method may further comprise one or more conventional coated fibre processing steps after the steps of delivering the resin to the fibre and moving the fibre. These steps may include heating the coated fibre to reduce the viscosity of the resin and promote wetting (impregnation) of the fibres with resin, aligning the fibre, applying a protective coating to one or both sides of the resin coated fibre, and / or winding the coated fibre onto a drum for transport and / or storage. Viewed from a second aspect the present invention provides a system for applying resin to a fibre to form a pre-impregnated fibre (e.g. a towpreg) according to the method hereinbefore described, comprising: a conveyor configured to convey a fibre from a first point to a second point in a direction substantially parallel to its longitudinal axis; a nozzle directed towards a third point between the first and second points, wherein the nozzle is configured to deliver a fluid to the fibre at or around the third point; and a fluid source in fluid communication with the nozzle. The nozzle may be an atomising nozzle configured to deliver fluid (e.g. resin) as an atomised spray. Alternatively the nozzle may be configured to deliver fluid (e.g. resin) as a continuous bead in a spiral pattern. Where the nozzle is an atomizing nozzle configured to deliver resin as an atomised spray, the nozzle may have a spray pattern selected from a group consisting of a flat fan, a full cone, a hollow cone, or a mist. Preferably‘’"’•ay pattern of the atomizing nozzle is a flat fan or a full or hollow cone. These spray patterns advantageously reduce resin waste by directing the spray towards the fibre. The nozzle may have a carrier fluid inlet connected or connectable to a carrier fluid supply to supply a carrier fluid (such as compressed air) to the nozzle. The carrier fluid supply may include a heater configured to heat the carrier fluid prior to supply to the carrier fluid inlet. The carrier fluid supply may include a dryer configured to remove moisture from the carrier fluid prior to supply to the carrier fluid inlet. The dryer may be a column comprising a common drying agent such as silica, calcium hydride etc, through which the carrier fluid is passed. Where the nozzle is configured to have a flat fan spray pattern, the nozzle is preferably configured such that a plane of the spray pattern extends laterally (i. e. substantially perpendicular to the longitudinal axis of the fibre. The fluid source may include a pump and a reservoir, where the pump is operable to convey fluid from the reservoir to the nozzle. The pump may be a variable speed pump to enable the rate of fluid delivery to the nozzle to be adjusted as required. The pump and pump speed may be controlled manually or by an automated control system (e.g. a computer running control software). The reservoir may be provided with a tank heat source operable to heat the fluid prior to delivery to the nozzle. The tank heat source may be any conventional heat source and may be inside the reservoir (e.g. an immersed heating element) or may be outside the reservoir and configured to deliver heat to the walls of the reservoir (e.g. a heat jacket). Preferably the reservoir includes one or more tank temperature probes to monitor the temperature of the fluid within the reservoir. The system may further comprise piping to convey the fluid from the reservoir to the nozzle (optionally via the pump). The piping may be provided with one or more hose temperature probes to monitor the temperature of the fluid at one or more locations along the piping (e.g. between the reservoir and^"mp, and between the pump and the nozzle). The piping may also be provided with a piping heat source (e.g. an external heat jacket) to heat the fluid within the piping and to prevent the fluid cooling as it passes between the heated reservoir and the nozzle. The system may further comprise a nozzle temperature probe provided at or adjacent the nozzle to measure the fluid temperature at the nozzle. The nozzle may be provided with a nozzle heater operable to heat fluid as it passes through the nozzle. The tank temperature probe(s), the piping temperature probe(s) and / or the nozzle temperature probe may provide data to a display to allow an operator to manually monitor the temperature of the fluid within the reservoir, piping and / or nozzle and to manually operate or adjust the tank and / or piping heat source as required. Alternatively (or additionally) the tank temperature probe(s), the piping temperature probe(s) and / or the nozzle temperature probe may provide data to an automated control system which may be operable to control the tank and / or piping heat source to maintain the fluid within the reservoir, within the piping and / or at the nozzle at a certain temperature or within a temperature range. By providing temperature probes and heat sources to monitor and control the temperature of the fluid at various points, the viscosity of the fluid can be controlled as required (by adjusting the temperature). Furthermore higher temperatures required to spray the resin, can be used only at the nozzles, while the resin can be maintained at lower temperature in the feeding system. This minimises volume and time / temperature exposure at temperature, allowing to use highly reactive resin formulation without impacting their chemical stability The system may comprise one or more additional nozzles directed towards one or more additional points between the first and second points, wherein the (or each) additional nozzle is configured to deliver a fluid to the fibre at or around the (or each) additional point and is in fluid communication with either the fluid source or an additional (i.e. different) fluid source. One or more of the additional points may be laterally adjacent to the third point (i. e. at the same longitudinal position between the first and second points, but spaced apart laterally from the third point). The conveyor may be any conventional conveyor system used for conveying elongate tapes, ribbons or fibres. For example the conveyor system may comprise two or more rollers at each of the first and second points. Preferably the conveyor is configured to maintain tension in the fibre between the first point and the second point such that a first surface of the fibre between the first point and the second point is substantially planar (e.g. the fibre does not sag between the first point and the second point). The conveyor may be a variable speed conveyor to enable the rate of fibre movement between the first and second points to be controlled. The speed of the conveyor may be controllable manually, and / or may be controlled by an automated control system. The conveyor may be mechanically or electronically (e.g. via the automated control system) coupled to the pump such that, in use, variation in the speed of the conveyor automatically varies the speed of the pump (and vice versa). This ensures that any minor variations in conveyor or pump speed do not affect the amount of fluid (e.g. resin) delivered to the fibre. The nozzle is preferably oriented to point substantially perpendicularly towards an imaginary plane containing the first, second and third points. Where the nozzle is configured to deliver fluid as a single continuous bead in a spiral pattern, the nozzle may comprise an inlet connected (or connectable) to a supply of compressed air, operable to supply compressed air as a carrier fluid to a cyclone to create a spiralling vortex. The supply of compressed air may be regulated either by manual or automated pressure regulation systems, and may also be preheated to further manipulate viscosity of the resin. The resin is delivered to the vortex of air prior to exiting the nozzle, enabling the resin to be carried in the air stream and adopt the spiralling trajectory of the air. A nozzle exit determines the shape of the spray pattern, and may be (for example) circular for a circular spiral, or may be elliptical to change the aspect ratio of the spiral pattern. The required diameter of the nozzle exit depends on the required application and flow rate. For a spray width of up to 25 mm, the nozzle exit may be between 0.3 mm and 0.5 mm, (e.g. 0.4mm). The system may include conventional fibre processing apparatus before and after the first and second points; e.g. de-spooling, alignment, and / or spreading apparatus before the first point; and fibre heating, alignment, protection and / or spooling apparatus after the second point. Viewed from a third aspect the present invention provides a pre-impregnated fibre material, comprising a fibre having a first side and a second side, wherein the first side is at least partially coated with a resin arranged in a pattern of ovals which partially overlap and are spaced apart in a longitudinal direction. The ovals may have a width (in the lateral direction) of greater than 90% of the width of the fibre. The ovals may have a width greater than (more than 100%, e.g. about 110% of) the width of the fibre, such that the oval shape on the first side is incomplete (i.e. it is cut by the edges of the fibre). At least a portion of the second side of the fibre may also be coated with the resin. The pre-impregnated fibre material may comprise between 20 wt.% and 60 wt.% of the resin; or preferably between 25 wt.% and 50 wt.% of the resin; or even more preferably between 30 wt.% and 40 wt.% of the resin. Preferably the fibre is in the form of a tow (a bundle of individual filaments). The fibre may comprise a single tow (i.e. a single bundle of filaments). Alternatively, the fibre may be an assembly of two to four tows arranged in parallel and spaced apart laterally (such that a first surface of each tow is coplanar) or vertically (such that a first surface of each tow is not coplanar). Each tow may contain from 3000 to 50000 filaments (referred to as 3K to 50K tows). For example, each tow may have between 5000 and 25000 (5K to 25K) filaments, e.g. 6000, 12000 or 24000 (6K, 12K or 24K). The tow (or each tow) is preferably spread into a substantially flat, tape-like reinforcement prior to the step of directing a nozzle towards a first surface of the fibre. The fibre or spread fibre (whether composed of a single tow or multiple tows) may have a total width of 50 mm or less, preferably 30 mm or less (e.g. 25 mm or less). The fibre may be any of the conventional fibre types and configurations that are used in the prepreg, towpreg and composite material industry. Examples of suitable fibres include fibreglass, carbon fibres, polyethylene and aramid (aromatic polyamide) fibres. Preferably the fibre is carbon fibre. The resin may be any resin as hereinbefore described with reference to the first aspect, including any additives as hereinbefore described with reference to the first aspect. Brief Description of Drawings A specific implementation of the present invention will now be described, by way of example only, and with reference to the accompanying drawings in which: Figure lisa simplified diagram showing a system for applying resin to a fibre to form a pre-impregnated fibre according to the present invention. Figure 2 is a simplified diagram showing the system of Figure 1 as part of a system for manufacturing a towpreg. Figure 3 is a detailed view of the nozzle of the system of Figure 1. Figure 4 is a simplified illustration of the method of applying resin to a fibre to form a preimpregnated fibre according to the present invention. Figure 5 is a schematic of a control system for the system of Figure 1. Figure 6 is a photograph showing the resin bead pattern on a first surface of a fibre according to a particular embodiment of the present invention. Figure 7 is a simplified diagram showing an alternative system for applying resin to a fibre to form a pre-impregnated fibre according to the present invention. Figure 8 is a simplified diagram showing a test at different spray inclines. Figure 9 shows the results of a reproducibility test in relation to the amount of resin (resin content) of resin coated fibres. Figures 10A to 10D show modelled spray deposition patterns for various linespeeds and spiral rates. Detailed Description Example 1: 10.5 mm spread T700-24k carbon fibre tow with M9.6G epoxy resin Referring initially to Figure 1, an example system 1 for applying resin to a fibre 10 to form a pre-impregnated fibre according to the present invention is shown in a simplified form (not to scale). The fibre 10 is an elongate fibre which (in this particular embodiment) comprises a single fibre “tow”, i.e. a single bundle of individual filaments aligned in a longitudinal direction (X). In this particular embodiment, the fibre 10 is a T700-24k carbon fibre tow with a filament count (number of individual carbon filaments) of 24000. Other types of fibre and other filament counts can also be used in the system 1. The fibre tow has been spread (see Figure 2, by a mechanical spreader 5 consisting of a set of rollers) into a substantially flat, tape-like structure having a longitudinal axis (X) and a width along a lateral axis (Y, see Figure 4). In this particular embodiment, the spread width (i.e. the lateral extent) of the fibre 10 is approximately 10.5 mm. The spread fibre 10 thus has a substantially planar first surface 11 (shown in Figure 1 as an upwards facing surface) and a substantially planar second surface 12 (shown in Figure 1 as a downwards facing or underside surface). The system 1 includes a conveyor 20 (not shown in Figure 1) which is operable to convey the fibre 10 in a longitudinal direction (X) from a first point 21 to a second point 22 through a third point 23. With reference to Figure 2, the conveyor 20 may comprise one or more of the tension controllers 3, a dry fibre aligner 4, the fibre spreader 5, a wet fibre aligner 6, a tow creel 2 and a rewinder 9. Alternatively the conveyor 20 may comprise one or more distinct components (not shown). The conveyor 20 and / or the tension controllers 3 act to maintain tension in the fibre 10 between the first point 21 and the second point 22 such that there is minimal (or preferably no) sagging of the fibre 10 between the first point 21 and the second point 22 and the first and second surfaces 11, 12 are thus substantially planar. The system 1 includes a nozzle 25 which is positioned such that it is directed towards the first surface 11 of the fibre 10 (when the fibre 10 is in place) at the third point 23. The nozzle 25 is spaced apart in a vertical (Z) direction from the fibre 10 by a distance A. With reference to Figure 3, the nozzle 25 in this particular embodiment is configured to deliver a continuous bead of fluid in a spiral or helical pattern. The nozzle 25 is supplied with the resin through a fluid inlet 49 and a carrier fluid (in this case compressed air) through a carrier fluid inlet 42. The carrier fluid inlet 42 is in fluid communication with a cyclone chamber 43 within a central area of the nozzle 25. A plunger 44 is provided in a centre of the cyclone chamber 43 and is moveable in a Z direction to open and close a nozzle exit 47 at the tip of the nozzle 25. The cyclone chamber 43 is generally cylindrical with an inward taper towards the nozzle exit 47. As the carrier fluid enters the cyclone chamber 43, it passes around the plunger 44 to generate a vortex 46. Resin flows from the fluid inlet 49 into an annular channel 48 which surrounds the cyclone chamber 43. The annular channel 48 is provided in this embodiment with a heat jacket 45 to provide heat to the resin within the annular channel 48 as required to control the viscosity of the resin. The temperature of the resin within the annular channel 48 is monitored by a nozzle temperature probe 35 (not shown in Figure 3). The annular channel 48 and the cyclone chamber 43 both open into a mixing region 50 adjacent the nozzle exit 47. In use, when the plunger 44 is in an open position (as shown in Figure 3) then resin from the annular channel 48 flows into the mixing region 50 and mixes / converges with the carrier fluid. The vortex 46 generated in the cyclone chamber 43 causes the resin and carrier fluid to exit through the nozzle exit 47 as a continuous bead 24 in a spiral pattern. When the plunger 44 is in a closed position, the cyclone chamber 43 and the annular channel 48 are both isolated fr^ mixing region 50 and the nozzle exit 47. Referring again to Figure 1, the nozzle 25 is connected to a fluid source (reservoir 29) via an upstream pipe 28, a pump 27, and a downstream pipe 26. The reservoir 29 is capable of storing a volume of fluid (e.g. resin) and may be filled via a separate inlet (not shown). The nozzle is also connected to a carrier fluid supply 51 to supply compressed air as a carrier fluid to the nozzle 25. The compressed air in this particular embodiment is heated to a temperature of between 30 °C and 90 °C prior to being supplied to the nozzle 25 by a heater (not shown), and is dried by passing the compressed air through a column (not shown) of silica drying agent prior to being supplied to the nozzle. The reservoir 29 includes a tank heater 30 which in this particular embodiment takes the form of an immersed electric heating element. It will be appreciated that other heating means may be employed equally effectively (e.g. immersed piping pumping heated fluid; an external heat source such as a burner; or a heat jacket around the reservoir walls). The tank heater 30 is operable to heat fluid within the reservoir 29. In this particular embodiment, the upstream and downstream pipes 28, 26 are provided with a hose heater 31 in the form of a heated jacket surrounding the pipes 28, 26. The hose heater 31 is operable to heat the pipes 28, 26 and the fluid within. The upstream section of the pipe 28 and the downstream section of the pipe 26 may be heatable independently. The upstream and downstream sections of the pipe 28, 26 are separated by a pump 27. The pump 27 is operable to pump fluid from the reservoir 29 through the upstream section of the pipe 28 and the downstream section of the pipe 26 to the fluid inlet 49 of the nozzle 25. The reservoir 29 is provided with a tank temperature probe 32 to measure the temperature of the fluid within the reservoir 29. The upstream and downstream pipe sections 28, 26 are also provided with upstream hose temperature probe 33 and downstream hose temperature probe 34 respectively, to measure the temperature of the fluid within each pipe section. The nozzle 25 is provided with a nozzle temperature probe 35 to measure the temperature of the fluid as it passes through the annular channel 48 of the nozzle 25. With reference now to Figure 5, each of the temperature probes 32, 33, 34, 35 are in communication with a control system 40. In use, the control system 40 monitors the temperature of the fluid at each point (reservoir 29, upstream and downstream pipes 28, 26, and nozzle 25) to determine whether fluid temperature is within an acceptable temperature range. Where the fluid temperature is outside the acceptable temperature range, the control system activates or deactivates the tank heater 30, heat jacket 45 on the nozzle 25 and / or each hose heater 31 as required to adjust the fluid temperature. The control system 40 may also provide data to a user interface (not shown) to allow an operative to monitor the temperatures and heating states and to manually adjust if necessary. In this particular example embodiment, the fluid is an M9.6G epoxy resin (manufactured by Hexcel Composites) which has a glass transition temperature (sub Tg) of 120 °C. The complex viscosity of the resin is between 1 and 100 Pa s at a temperature of between about 30 °C and 90 °C (although the resin is thermally curing and will begin to cure if maintained at these temperatures for extended periods). The acceptable temperature range is therefore between 30 °C and 90 °C, and the control system 40 controls the tank heater 31, the nozzle heat jacket 45 and each hose heater 31 to maintain the resin temperature at the nozzle temperature probe 35 within this range, whilst ensuring that the temperature of the resin at the hose temperature probes 33, 34 and the tank temperature probe 32 does not exceed the acceptable temperature range. Referring now to Figure 4, the nozzle 25 is operable to deliver the resin as a continuous fluid bead 24 in a generally helical or spiral pattern. Figure 4 shows the fluid bead 24 being expelled from the nozzle 25 in a divergent conical helix (i.e. a helix with an increasing diameter), however in other embodiments the nozzle 25 may provide a continuous bead 24 as a cylindrical helix (i.e. a helix with a constant diameter). The shape of the fluid bead path (e.g. cylindrical or conical) depends on the shape of the nozzle exit 47. The distance A between the tip of the nozzle 25 (at the nozzle exit 47) and the first surface 11 of the fibre 10 is such that the diameter D of the fluid bead 24 is between 100% and 150% of the width of the fibre 10 at the point where the fluid bead 24 reaches the plane of the first surface 11. As shown in Figure 4, where the diameter D of an imaginary cone defined by the fluid bead 24 is between 100% and 150% of the width of the fibre 10 at the point where the fluid bead 24 reaches the plane of the first surface 11, at least some of the overspray 16 (fluid which passes through the plane of the first surface 11 beyond the edges 13 of the fibre 10) advantageously wraps around the edges 13 of the fibre 10 as shown in Figure 4 due to its angular momentum, to adhere to and partially cover an underside 12 of the fibre 10. Thus both surfaces 11, 12 of the fibre 10 can advantageously be coated with resin in a single pass, with minimal wastage of resin. Figure 6 shows the resin pattern 14 formed on the first surface 11 of the fibre 10 as a result of the helical fluid bead 24 produced by the nozzle 25. As the fibre 10 is conveyed in a longitudinal direction X, the helical fluid bead 24 deposits resin on the first surface 11 as a series of overlapping ovals. Any overspray 16 beyond the edges 13 of the fibre 10 can coat the underside 12 of the fibre 10 (not shown in Figure 6) to form a different resin pattern 15 (see Figure 1). The fibre 10 is conveyed in a longitudinal direction X by the conveyor 20. As shown in Figure 5, in this embodiment the conveyor 20 is coupled to the pump 27 by an encoder 41. The encoder 41 ensures that any variation in the speed of either the conveyor 20 or the pump 27 (for example caused by varying proportions of the fibre 10 on the tow creel 2 and the rewinder 9 as the reel of fibre is processed) results in a corresponding adjustment to either the conveyor speed or the pump speed to maintain the flow of resin through the nozzle 25 consistent with respect to the fibre transit speed, thus maintaining consistent coating of the fibre 10 with the resin. In an alternative embodiment, the relative speeds of the conveyor 20 and the pump 27 may be monitored and controlled in real-time by the control system 40 with or without a separate encoder 41. The flow of resin through the nozzle 25 in this particular embodiment and the conveyor speed are set to give a resin content on the coated fibre 10 of 40%. After applying the resin to the fibre 10, the coated fibre 10 (i.e. the pre-impregnated fibre or towpreg) may optionally be subjected to a further heating step to facilitate penetration of the resin between the filaments in the fibre ^“wetting”). As shown in Figure 2, the coated fibre 10 in this particular embodiment has a protective paper or polymer layer 7 applied to the second surface 12. In other embodiments then no protective layer may be applied to the coated fibre 10, or a protective layer may be applied only to the first surface 11 or to both the first and second surfaces 11, 12. The protective paper layer 7 is fed from a paper drum 8. The protected, coated fibre 10 can then be wound onto a drum by a rewinder 9 for storage or transport. Example 2: IM5-24K fibre with M21E resin This embodiment uses the same system 1 as shown in Figures 1-5 and described above with respect to Example 1. Thus the features of the system will not be described again in detail. However this embodiment uses a different fibre and a different resin. As such, some parameters are different from those in Example 1. In this particular embodiment, the fibre 10 is a IM5-24k carbon fibre tow with a filament count (number of individual carbon filaments) of 24000. The resin is an inert form of M2 IE resin (manufactured by Hexcel Composites) which is a blend of epoxy resins. The inert form has a curative substituted with calcium carbonate. As in Example 1 above, the fibre tow has been spread (see Figure 2, by a mechanical spreader 5 consisting of a set of rollers) into a substantially flat, tape-like structure having a longitudinal axis (X) and a width along a lateral axis (Y, see Figure 4). In this particular embodiment, the spread width (i.e. the lateral extent) of the fibre 10 is approximately 12.7 mm. The spread fibre 10 thus has a substantially planar first surface 11 (shown in Figure 1 as an upwards facing surface) and a substantially planar second surface 12 (shown in Figure 1 as a downwards facing or underside surface). The liquid resin formulation is stored in reservoir 29 and is heated by the tank heater 30 to a temperature of 70 °C. The hose heater 31 on the upstream and downstream pipes is also configured to heat the resin to a temperature of 70 °C. The nozzle heat jacket 45 is configured to heat the nozzle to a temperat"ro 105 °C. The pump 27 is configured to supply the resin to the nozzle 25 at a flow rate of 29.5 ml / min. These temperatures and this flow rate results (for this particular resin) in a resin viscosity of 30 Pa- s at the point of exiting the nozzle. Compressed air at a pressure of 1 bar to 4 bar is supplied from the carrier fluid supply 51 to the nozzle 25, resulting in the formation of a regular spiral bead 24 of resin extending in a divergent conical helix from the nozzle 25. In this particular embodiment, the planar first surface 11 of the spread fibre 10 is separated from the nozzle by 45 mm. This separation distance results in the regular spiral bead 24 of resin from the nozzle 25 having a diameter D (spray width) of 15 mm when it reaches the plane of the first surface 11. This equates to around 118% of the width of the spread fibre 10. In this embodiment, the axis of the spiral bead 24 is substantially perpendicular to the plane of the first surface 11. As shown in Figure 4, at this particular spray width relative to the fibre, the overspray 16 advantageously wraps around the edges 13 of the fibre 10 due to angular momentum to adhere to and completely cover an underside 12 of the fibre 10 with no or a negligible amount of resin wastage. Thus both surfaces 11, 12 of the fibre 10 can advantageously be completely coated with resin in a single pass, with negligible wastage of resin. This not only simplifies the manufacturing process but also allows for more precise control of the amount of resin added to the fibre 10, as the amount of lost overspray 16 negligible. The linespeed in this particular embodiment is 10 m / min. With these temperature settings, flow rate, carrier fluid pressure, separation distance and linespeed, the inert M2 IE resin is deposited on the fibre 10 in regular spirals at a frequency of approximately 100 Hz (i.e. 100 spirals are deposited on the fibre 10 over a length of fibre 10 corresponding to 1 second of travel, in this around 16.67 cm). This results in a resin content in the resin coated fibre of 30 wt.%. Various tests were conducted with the system of Example 2 to determine the effects of altering certain parameters. Resin Temperature It is desirable to maintain the resin at the lowest temperature possible whilst having a viscosity sufficiently low to ensure adequate spray performance. To determine whether a lower nozzle temperature could still achieve adequate spray performance, a number of tests were carried out varying the heater set temperature (i.e. the temperature to which the heater thermostat is set) of the nozzle heat jacket 45 and the hose heater 31. The results are indicated in Table 1. Tank Heater Temperature / °C Pump Temperature / °C Hose Heater Temperature / °C Nozzle Heater Temperature / °C Performance 70 70 70 80 Sprayed but inconsistent 70 70 70 90 Sprayed but inconsistent 70 70 90 90 Sprayed well 70 70 70 100 Sprayed well As shown in Table 1, the M21E inert resin has adequate performance if the nozzle temperature is reduced from 115 °C to 100 °C. It is also possible to further reduce the nozzle temperature to 90 °C, but the hose temperature must then be increased to 90 °C to achieve adequate spray performance. At lower temperatures the spray performance is inconsistent for this resin. It will be appreciated that other resins have different viscosity and temperature performance characteristics, however by carrying out tests similar to those shown in Table 1 it is possible to determine optimum temperatures for a given resin. All examples shown in Table 1 maintain a lower temperature (70 °C) in the tank and pump, and then heat the resin to a higher temperature downstream (i.e. in the hose or the nozzle itself). This advantageously minimises the residence time of the resin at higher temperatures to prevent or limit premature ageing or curing. Spray Incline Referring now to Figure 8, the angle of spray deposition (i.e. the angle between the axis of the spiral bead 24 and the plane of the first surface 11 of the fibre 10) was varied to determine the effect on deposition performance. This was achieved by increasing the angle B between a horizontal axis and the longitudinal direction X of the fibre 10, whilst maintaining the nozzle 25 in a vertical axis Z. The angle between the axis of the spiral bead 24 (i.e. the vertical axis Z) and the plane of the first surface 11 (extending along longitudinal axis X) is therefore 90 ° - B. Results of this test indicated that in the range -25 ° <B <+25 °C; the spray and deposition performance was not substantially affected. Controlling Resin Content A test was carried out to determine the reproducibility of resin content. Various lengths of fibre 10 were coated with M2 IE inert using the system 1 as described above for Example 2, with a linespeed of 10 m / min and a pump speed of 18 RPM. Resin was delivered through the system with the tank, pump and hose set temperatures at 70 °C and the nozzle set temperature at 100 °C. The resin content (in wt.%) was then determined for each length of coated fibre. Five samples were obtained for each tested length, and the results are shown in Figure 9. The error bars show the standard deviation. Figure 9 indicates that for fibre lengths >Im, a good reproducibility and consistency in resin content is achieved. In practice, fibres processed through the system 1 are likely to be much longer than 1 m, thus the system shows high precision in the amount of resin added to the fibre. Effect of Linespeed and Spiral Frequency on Deposition Pattern The effect of changing linespeed and spiral frequency on the observed deposition pattern was modelled. Spiral frequency can be controlled by changing the pressure of the compressed air supplied to the nozzle 25 from the carrier fluid supply 51. As will be appreciated, a higher carrier fluid pressure results in a higher frequency. Figures 10A to D show modelled spray deposition patterns for linespeeds of 50 m / min and 100 m / min and a spiral frequency of 100 Hz and 50 Hz. It should be noted that the spray height is fixed at 15 mm for these models, and the whole spiral deposition pattern is shown, although for fibres with a spread width narrower than 30 mm it will be appreciated that there will be some overspray 16. Figures 10A to 10D show how changing line speed and / or spiral frequency can be used to control the amount of resin deposited on the fibre 10, and thus the resin content of the resin coated fibre. Example 3: Atomising Nozzle In an alternative embodiment shown in Figure 7, a system 101 is provided. The system 101 is similar to that shown in Figures 1 and 2, with like features numbered at an increment of +100 and not described again in detail. The nozzle 25 replaced with an atomising nozzle 125. Atomising nozzle 125 is configured to deliver resin as an atomised spray 124 to a first surface 111 of the fibre 110. The atomising nozzle 125 is configured to deliver resin in a hollow cone spray pattern. This advantageously provides a substantially uniform coating on the first surface 111 of the fibre 110. Unlike the system 1, the resin delivery in the system 101 does not include the resin “wrapping around” the fibre to coat an underside 112 in a single step.

Claims

1. A method for applying resin to a fibre to form a pre-impregnated fibre (e.g. a towpreg), comprising:directing a nozzle towards a first surface of a fibre, wherein a tip of the nozzle and the first surface of the fibre are separated by a separation distance and the nozzle is configured to deliver fluid across the separation distance to the fibre;delivering a resin through the nozzle to the fibre; andmoving the fibre relative to the nozzle in a direction substantially parallel to a longitudinal axis of the fibres to apply resin to the fibre along its length to form a resin coated fibre.

2. The method of claim 1, wherein the nozzle is configured to deliver fluid as a continuous bead in a spiral pattern.

3. The method of claim 1, wherein the nozzle is configured to deliver fluid as an atomised spray having a flat fan, a full cone or a hollow cone spray pattern.

4. The method of claim 3, wherein the separation distance between the nozzle tip and the fibre is selected such that the lateral extent of the spray pattern in the plane of the first surface is between 80% and 120% of the width of the fibre.

5. The method of claim 2, wherein the nozzle is configured to deliver fluid in a divergent conical spiral / helix bead from the nozzle tip.

6. The method of claim 5, wherein a diameter of an imaginary cone defined by the divergent helical bead path in the plane of the first surface is between 100 % and 150 % of the width of the fibre.

7. The method of any preceding claim, wherein the nozzle is positioned with its tip directed towards a point on the fibre within the central 20% of the fibre width.

8. The method of any preceding claim, wherein an axis of the fluid / resin delivery is substantially perpendicular to the first surface of the fibre.

9. The method of any one of claims 1 to 7, wherein an axis of the fluid / resin delivery is substantially parallel to the first surface of the fibre.

10. The method of any preceding claim, wherein the resin has a viscosity of between 1 and lOOPa.s as it is delivered through the nozzle to the fibre.

11. The method of any preceding claim, further comprising the step of heating the resin to a temperature of between 30 °C and 150 °C before delivery through the nozzle.

12. The method of any preceding claim, further comprising the step of controlling one or more of a rate of resin delivery from the nozzle and a velocity of fibre movement to provide a resin coated fibre with a resin content of between 20 wt.% and 60 wt.%.

13. The method of any preceding claim, further comprising the steps of: directing one or more additional nozzles towards the first surface or a second surface of the fibre, wherein a tip of the or each additional nozzle and the first surface (or second surface) of the fibre are separated by a second separation distance and the (or each) additional nozzle is also configured to deliver an additional fluid across the separation distance to the fibre;delivering the (or each) additional fluid through the (or each) additional nozzle to the fibre; andmoving the fibre relative to the (or each) additional nozzle in a direction substantially parallel to a longitudinal axis of the fibres to apply the (or each) additional fluid to the fibre along its length.

14. The method of claim 13, wherein the (or each) additional fluid comprises a reactive or non-reactive additive.

15. A system for applying resin to a fibre to form a pre-impregnated fibre (e.g. a towpreg) according to the method of claims 1 to 14, comprising:a conveyor configured to convey a fibre from a first point to a second point in a direction substantially parallel to its longitudinal axis;a nozzle directed towards a third point between the first and second points, wherein the nozzle is configured to deliver a fluid to the fibre at or around the third point; anda fluid source in fluid communication with the nozzle.

16. The system of claim 15, wherein the nozzle is an atomizing nozzle configured to have a spray pattern selected from a group consisting of a flat fan, a full cone and a hollow cone.

17. The system of claim 15, wherein the nozzle is configured to deliver fluid as a continuous bead in a spiral pattern.

18. The system of any one of claims 15 to 17, wherein the fluid source includes a pump and a reservoir, where the pump is a variable speed pump and is operable to convey fluid from the reservoir to the nozzle.

19. The system of claim 18, wherein the reservoir is provided with a tank heat source operable to heat the fluid prior to delivery to the nozzle, and one or more tank temperature probes to monitor the temperature of the fluid within the reservoir.

20. The system of claim 18 or 19, further comprising piping between the reservoir, the pump and the nozzle to convey the fluid from the reservoir to the nozzle via the pump, wherein the piping is provided with one or more hose temperature probes to monitor the temperature of the fluid at one or more locations along the piping; and a piping heat source.

21. The system of any one of claims 15 to 20, further comprising a nozzle temperature probe provided at or adjacent the nozzle to measure the fluid temperature at the nozzle; and a nozzle heater.

22. The system of any one of claims 19 to 21, wherein the tank temperature probe(s), the piping temperature probe(s) (if present) and / or the nozzle temperature probe (if present) each provide data to an automated control system operable to control the tank heat source, nozzle heater and / or piping heat source (if present) to maintain the fluid within the reservoir, within the piping and / or at the nozzle within a temperature range.

23. The system of any one of claims 15 to 22, wherein the conveyor is coupled to the pump by an encoder such that, in use, variation in the speed of the conveyor automatically varies the speed of the pump (and vice versa).

24. A pre-impregnated fibre material, comprising a fibre having a first side and a second side, wherein the first side is at least partially coated with a resin arranged in a pattern of ovals which partially overlap and are spaced apart in a longitudinal direction.

25. The pre-impregnated fibre material of claim 24, wherein the ovals may have a width (in a lateral direction) of greater than 100% of the width of the fibre.

26. The pre-impregnated fibre material of claim 24 or claim 25, wherein at least a portion of the second side of the fibre is also coated with the resin.

Images